-Dicycloheylcarboimide Dcm

Dichloromethane Dmf

Dimethylaminopyridine Dmso

Dimethylsulfoxide Dma

Hydroxyuracil Ohu

Paa

A Frequency factor AAC Acrylic acid CAN Acrylonitrile AE

Keywords: functionalization, adsorption of heavy metal ions, catalytic activity, green conditions Oxyde de graphène, fonctionnalisation, adsorption d'ions de métaux lourds, activité catalytique, conditions vertes

Graphene oxide-based materials : from water depollution to the catalysis of oganic reactions

First, the presented Ph.D. research has been carried out within the framework of the scientific cooperation program, between Claude Bernard University, Lyon1, France, and Monastir University, Tunisia. This Ph.D. study was carried out in the Laboratory of Polymer Materials Engineering (IMP)-France and Unit of Materials and Organic Synthesis Research-Tunisia.

For the best management, help, and valuable advice, but also for the psychological support, I would like to address my special thanks to my supervisor in France Prof.

Emmanuel BEYOU. His unwavering support and confidence have made this whole process more interesting and rewarding.

I also want to thank my supervisor in Tunisia Prof. Mohamed Hassen V BAOUB, who has systematically provided me with invaluable guidance throughout my doctoral studies. I owe my loving thanks to my mother "Faiza" and my father "Lotfi" for their endless love, support, patient, during my research abroad. Thanks to my sisters, Jalila, Marwa, Safa, my brother Bilel, and my lovely girlfriend Rose, they inspired me to work hard.

vii

Résumé

De nos jours, le rejet des eaux usées chargées en métaux lourds, dans le milieu environnant est une préoccupation majeure compte tenu des effets indésirables engendrés sur la santé humaine et sur l'environnement. Des nombreuses méthodes peuvent être mises en oeuvre pour l'élimination des métaux lourds présents dans l'eau. Parmi elles, la technologie d'adsorption est reconnue par de nombreux auteurs comme l'une des meilleures techniques de traitement des eaux, en termes de coût initial, de flexibilité et de simplicité de conception. Des matériaux carbonés comme le graphène sont utilisés dans les nombreuses applications et en particulaire pour éliminer les métaux lourds des solutions aqueuses en raison de ses propriétés physico-chimiques exceptionnelles, notamment une grande surface spécifique (2600 m².g -1 ), une flexibilité, une bonne stabilité chimique, et conductivité électrique élevée. De plus, le graphène peut être facilement modifié chimiquement avec un processus d'oxydation permettant de générer des groupes fonctional de type époxy, hydroxyle et acide carboxylique sur sa surface. La fonctionnalisation de graphène oxydé (GO) peut améliorer ses propriétés et en particulier sa capacité maximale d'adsorption des ions métalliques. L'objectif de ce travail de thèse est de développer des approches simples et à faible coût qui soient rapides à mettre en oeuvre ainsi que contrôlables et qui permettent de produire en grande quantité de l'oxyde de graphène modifiée et décoré avec des nano-métaux comme (la magnétite) pour l'élimination des ions métallique tels que Cu (II), Ni (II) et Co (II) des solutions aqueuses. L'étude de l'influence de quelques paramètres expérimentaux tels que le pH, le temps de contact et la concentration initiale sur la fixation de ces ions sur le support graphénique modifié a été effectuée. Dans un deuxième temps, nous avons modélisé les résultats expérimentaux avec divers modèles mathématiques permettant de dégager les différents paramètres afin de comparer la performance des matériaux testés. Des expériences de régénération ont également été menées pour explorer la recyclabilité des adsorbants à base de feuillets d'oxyde de graphène (GO). Enfin, nous avons utilisé les matériaux à base de graphène et des métaux adsorbés comme catalyseurs supportés pour les réactions d'homo-couplage et de couplage A3 dans les conditions expérimentales vertes.

xi Résumé substantiel de la thèse L'objectif majeur de ces travaux de thèse est de développer des techniques simples et à faible coût qui soient rapides à mettre en oeuvre ainsi que contrôlables, et qui permettent de produire en grande quantité de l'oxyde de graphène modifiée et décoré avec des nanométaux (comme la magnétite) pour l'élimination des ions métalliques tels que Cu (II), Ni (II) et Co (II) des solutions aqueuses. En plus, nous avons utilisé les matériaux à base de graphène et les métaux adsorbés comme catalyseurs supportés pour les réactions d'homo-couplage et de couplage-A3 dans des conditions expérimentales 'vertes '.

La première partie de ce mémoire présente une revue bibliographique dans laquelle un premier chapitre décrit sur la préparation, les propriétés et les applications des matériaux à base de graphène. Dans la deuxième partie de l'étude bibliographique a été consacré sur la possibilité d'utiliser des matériaux carbonés tels que des matériaux à base de graphène comme des adsorbants pour l'élimination des métaux lourds dans l'eau. En plus, l'étude de l'influence de quelques paramètres expérimentaux tels que le pH, le temps de contact et la concentration initiale sur la fixation de ces ions sur les divers supports qu'a été effectués. Enfin, un rappel bibliographique sur les méthodes catalytiques développées pour les réactions organiques utilisant la catalyse à base de graphène sera rapporté dans le dernier chapitre de cette partie.

La deuxième partie est une description des différents matériaux utilisés et des protocoles expérimentaux qui ont été mis en jeu tout en discutant l'intérêt des techniques de caractérisation spectroscopique et microscopique qui ont été utilisées. D'habitude il est difficile de séparer rapidement les adsorbants à partir d'un grand volume de solution, tandis que les adsorbants magnétiques peuvent contourner ce problème parce qu'ils permettent de remplacer les techniques lourdes de centrifugations, de sédimentation, et de filtrations, par une simple séparation de phase par application de champ magnétique via l'utilisation d'un électroaimant ou d'un aimant permanent. En plus leurs surfaces spécifiques élevées, leurs permettent d'avoir une grande efficacité à enlever les polluants de l'eau, pour améliorer encore leurs efficacités à retenir les différents polluants. C'est pourquoi des adsorbants combinant la nanotechnologie et la technique de séparation par un champ magnétique peuvent éliminer les polluants moléculaires et les métaux lourds de l'eau avec une très grande performance. En effet, nous avons modifié des feuilles de GO avec des nanoparticules de magnétite (Fe3O4NPs) grâce à une méthode de coprécipitation pour l'élimination des ions Cu (II) des solutions aqueuses. Sa valeur de capacité d'adsorption à l'équilibre pour les ions Cu (II) était 3,808 ±0,125 mmol.g -1 à une concentration en ions métalliques de 250 mg.L -1 , pH = 7, et T = 293 K.

xiii

De plus, il a été montré que les processus de comportement d'adsorption des ions Cu (II) sur les deux adsorbants à base de GO magnétique correspondent à un modèle cinétique pseudo-second ordre et un modèle isotherme de Jossens. Enfin, la recyclabilité des adsorbants à base de GO magnétique a été vérifiée et l'élimination des ions Cu (II) de la solution aqueuse n'a pas changé de manière significative même après dix cycles.

Enfin, un nouvel adsorbant à base d'oxyde de graphène zwitterionique a été réalisée selon un procédé en deux étapes comprenant le greffage successif de groupes N,N-bis(2-pyridylméthyl)amino (BPED) et 1,3-propanesultone (PS) sur le graphène oxydé (GO). L'effet des paramètres expérimentaux tels que le pH, la concentration initiale et le temps de contact a été estimé. Les meilleures capacités d'adsorption maximales de l'adsorbant GO-BPED-PS étaient 4,174 ± 0,098 mmol.g -1 pour les ions Ni (II) et 3,902 ± 0,092 mmol.g -1 pour les ions Co (II). L'étude de la cinétique de rétention de Ni(II) et Co(II) par les supports préparés a été examiné via quatre modèles mathématiques et les résultats ont monté que le processus de sorption est décrit par une cinétique du second ordre qui peut être contrôlé par une diffusion intraparticule. xiv De plus, le catalyseur supporté [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) a été recyclé consécutivement pendant quinze fois sans aucune perte de son activité. D'autre part, le GO-BPED-PS@NiNP préparé a été utilisé comme catalyseur hétérogène pour la réaction de couplage à trois composants (réaction de couplage-A3) de l'aldéhyde, de la morpholine et des alcynes terminaux dans la γ-valérolactone comme solvant vert. Le faible temps de réaction, les excellents rendements des propargylamines, la bénignité du catalyseur et la recyclabilité magnétique du catalyseur sont les principaux avantages de cette recherche. De plus, le catalyseur GO-BPED-PS@NiNPs pourrait être réutilisé jusqu'à onze fois sans perte considérable d'activité catalytique.

En enfin, une conclusion générale rassemblera les principaux résultats obtenus au cours de ce travail et quelques perspectives sont aussi avancées.

Healthy peoples require clean drinking water as one of the essential life requirements. Toxicities of water sources due to the discharging of industrial effluents is a worldwide environmental problem. Toxic metals are one of the most persistent pollutants because of different industrial mining, and agriculture wastewater that has adverse effects on the ecological environment and human health due to their toxicity, bioaccumulation, and non-biodegradable when discharged into water bodies. In particular, several heavy metal ions such as Cu(II), Ni(II), Co(II) ions are used in the battery manufacturing, electroplating, and light industries, and accordingly, the aqueous effluents often contain high metal ion concentrations. The discharges of effluents containing these heavy metal ions have serious risk to both Earth's ecological cycle and human beings. Today, the removal of metal ions from wastewater before discharged into aquatic systems is still one of the biggest challenges. Nowadays, several technologies have been developed for removing metal ions from water in large scale and industrial application, including coagulation, reverse osmosis, filtration, and adsorption. However, most of these methods have some disadvantages and limitations, such as low efficiency and sludge production. As a physicochemical treatment method, the adsorption technique is a more environmentally friendly, low cost, and has relatively high efficiency with respect to metal ion removal. The adsorption process also depends on the physicochemical characteristics of the aqueous solutions, such as pH, temperature, the concentration and speciation of metallic ions, and the presence of other ions (anions and cations) in the aqueous solution.

Over the years, many adsorbents have been designed and used for removing heavy metal ions including polymeric materials, bio-materials, and carbonaceous materials.

Among them, carbonaceous materials such as graphene is an ideal material for removing heavy metals from aqueous solution due to its exceptional physicochemical properties including high surface area (a huge specific surface area over 2600 m².g -1 ), flexibility, good chemical stability, chemical ineffectiveness, and high electrical conductivity. In addition, graphene can be easily modified due to the presence of oxygen groups on its surface with the oxidation process. The oxidation of graphite powder using the Hummer's method introduces many functional groups on the surface of graphene oxide (GO) sheets including carboxyl, hydroxyl, and epoxy, which is suitable for further functionalization process.

Moreover, the functionalization of GO may enhance its properties and in particular its maximum adsorption capacity. It has been established that adsorbent containing nitrogen chelating ligands could be effective in adsorption of heavy metal ions.

However, the separation of GO-based material from aqueous solution did not permit us to recover it easily, due to its higher dispersibility in aqueous solutions. Therefore, the magnetic GO-based material is the appropriate adsorbent, which makes the separation easier and faster from aqueous solution (without filtering) at the end of the adsorption process. In addition, in many studies, it has proven to be a good adsorbent for metal ions removal after functionalization. The mechanism of adsorption is usually investigated by fitting the experimental data obtained for adsorption with kinetic models. Commonly used kinetic models for describing solid-liquid adsorption systems are the pseudo-first-order model, the pseudo-second-order model, the Elovich model, and the intraparticle diffusion model. In addition, the isotherm and thermodynamic studies are distributed between the solution and the surface of the adsorbent at equilibrium concentration and at different temperatures. There are three well-established types of adsorption isotherm: the Langmuir, the Freundlich, and the Jossens ones.

By now, the immobilization of homogeneous catalysts or bio-catalysts into materials is one of the most effective and convenient ways for the preparation of heterogeneous catalysts. Graphene and graphene oxide (GO) sheets are important components of many synthetic heterogeneous catalysts. They have been used as effective catalysts or as supports of other catalysts. Recently, graphene-based materials have been widely explored as catalysts for the synthesis of different organic compounds via different organic reactions. As indicated, a variety of organic reactions such as oxidation, cycloaddition as well as the synthesis of various organic compounds have been considered in the presence of GO sheets. In addition, GO sheets have been considered as potential supports for metal-catalyzed C-C coupling reactions. This thesis is split into three parts.

The first part ''literature review" is divided into three chapters. The first chapter focuses on graphene derivatives, and their functionalization as well as their properties and applications. In the second chapter an overview of the heavy metal ions pollution and the utilized treatment methods, including adsorption, is presented. Moreover, a detailed study was done in order to compare the efficiencies of several available adsorbents either natural or synthetic. These adsorbents include carbonaceous materials such as activated carbon, carbon nanotubes, graphene, and graphene-based materials. In addition, the kinetic, isotherm, and thermodynamic studies have highlighted in accessing the quality of fits and adsorption performance because of their wide applicability in a variety of adsorption data. The third chapter evidenced the possibility to use graphene-based materials as heterogeneous catalysts for chemical and photochemical reactions.

Then, the second part deals with the different materials used and the experimental protocols that were involved. Characterization techniques of adsorbents and their application for the removal of metal ions from aqueous solutions are also described.

Finally, the third part is a scientific discussion of the experimental results obtained, which are presented in the form of five publications associated with two different chapters. The first chapter described the modeling and comparative studies on the adsorption of metal ions from aqueous solutions using various functionalized graphene oxide sheets as promising adsorbents. In the second chapter entirely devoted the possibility to use graphene-based materials as supported catalysts for carboncarbon coupling reactions.

II. LITERATURE STUDY

Graphene based materials

Graphene or a single layer (monolayer) of graphite is defined as a single layer of Sp 2 carbon atoms arranged in a hexagonal honeycomb lattice. The adjacent graphene sheets in graphite are separated from each other by 0.335 nm, which is half the crystallographic spacing of hexagonal graphite (Figure 1) [1]. Many researchers have already demonstrated that graphene is an exceptional material due to its various properties. Indeed, it exhibits a very high electrical conductivity of 2,000,000 cm 2 .V -1 .s -1 , hence 45,000 times higher than copper and it is the strongest material ever tested, with a Young's Modulus of 1 TPa and a Tensile strength of 130 GPa [2,3]. Due to its 2D dimension, graphene has a surface area of 2,630 m 2 .g -1 , which provides an increase in the electrical power that can be stored in the capacitor. This would help batteries to be recharged in few minutes instead of hours [4]. Besides, graphene is unbelievably light weighing, about only 0.77 mg.m -2 [5]. Despite its exceptional properties, this material is still challenging to produce at an industrial scale. Over 20 methods have been proposed and used in the last years for the production of graphene with different dimensions, shapes, and thicknesses [6].

Production of single graphene sheets

Many researchers have been working on graphene synthesis, thus, several methods have been performed to produce "different qualities and characteristics" of graphene.

They can be divided into two main procedures: the top-down technique where graphene sheets from natural graphite exfoliation and the bottom-up route in which a graphene sheet is grown from carbon atoms (Figure 2). Top-Down methods

Micromechanical exfoliation

This technique was first introduced by researches from Manchester University (UK) [7]. They used an adhesive tape to split the graphene layer from graphite flakes.

However, multiple exfoliation steps are required to obtain single layers. After exfoliation, these layers were deposited onto a silicon wafer using a "dry deposition".

This technique is also known as 'Scotch tape' or 'drawing' method (Figure 3). The obtained nanosheets show high quality with few defects and remarkable fundamental electronic and optoelectronic properties. Nevertheless, this method is seriously restricted to wide applications due to the low-throughput and difficulty of control [8].

Liquid-phase exfoliation

Liquid-phase exfoliation is another top-down method that can be performed by ultra-sonication in organic solvents such as dimethylformamide and N-methylpyrrolidone or through intercalations in presence of butyllithium and metal naphthalenide. After exfoliation, the interaction between solvent and graphitic flakes needs to balance the inter-sheet attractive forces. The ideal solvents for graphitic flake dispersion are those that minimize the interfacial tension (the force that minimizes the area of the surfaces in contact) between the flakes and the surrounding liquid [9][10][11].

The advantages of the liquid-phase exfoliation methods for preparing graphene sheets are their low cost and their high yield. However, the crystal quality of graphene sheets is often degraded and many functional groups or residual solvent and ions resulting affect the electrical properties.

Electrochemical exfoliation

Recently, electrochemical exfoliation of graphite has attracted attention due to its fast, and environmentally friendly nature to produce high-quality graphene sheets.

Electrochemical exfoliation of natural graphite has been performed mainly in two different ways in presence of electrolytes, i.e., ionic liquids and aqueous acids (e.g., H2SO4 or H3PO4) [12].

For example, Lu et al., [13] have performed a one-pot method to synthesize graphene sheets, by exfoliation of graphite in ionic liquids. They studied the incorporation of 1-butyl-3-methylimidazolium tetraflouroborate [BMIm][BF4] in water, used as an electrolyte for the electrochemical exfoliation of natural graphite.

Static potentials of 1.5 to 15 V were applied. However, exfoliation in ionic liquids results in only a low yield of graphene sheets with a small size and it often leads to a functionalization, with the ionic liquids, which disturb the electronic properties [14].

On the other hand, exfoliation in acidic electrolytes can yield graphene exhibiting a better quality and a larger size, but a significant amount of oxygen-containing functional groups cannot be avoided due to the over oxidation of graphite by the acid [15]. Therefore, a proper electrolyte system that can lead to a high-quality and largequantity synthesis of exfoliated graphene sheets is highly demanding.

Bottom-up preparation methods

Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) was first introduced by Hagstrom in 1965 [16].

In this method, the graphene layer growth on a surface is due to a thermal decomposition hydrocarbon molecules such as methane, acetylene or propane, etc.

catalyzed by a metal surface or through the precipitation of carbon atoms from the bulk metal [17]. Transition metals such as Copper (Cu), Platinum (Pt), Nickel (Ni), and Cobalt (Co) are used as catalysts and are particularly appealing for obtaining a highquality graphene and for developing a process ready to be integrated into the existent semiconductor industry. The main drawback of the CVD method is the transfer of the graphene sheet from the metal to a more suitable substrate [18].

Epitaxial growth

The formation of crystalline graphene layers on SiC was first observed by Van Bommel et al., in 1975 [19]. In this method, graphene layers growth were achieved on silicon carbide (SiC) substrates by a heat treatment at 1300 °C under ultrahigh vacuum conditions [20]. In addition, Berger et al., [21] demonstrated that single-crystal graphene could be grown on SiC with a controlled number of atomic layers, depending only on the annealing temperature and time.

Functionalization of graphite derivatives

The chemical modification of "graphene" not only maintains its excellent characteristics but also introduces new functional groups. Presently, the methods for the chemical modification of graphene mainly include covalent and non-covalent functionalization.

Non-covalent functionalization

The non-covalent bond functionalization of graphene results in the formation of a composite material based on interactions between graphene and functional molecules, the greatest advantage of which is maintaining the bulk structure and excellent properties of graphene. For example, non-covalent bonds mainly include П-П bond interaction, ionic bonding, and Van der Waals force [22].

For example, perylene tetracarboxylate aromatic (PTCA) interact with the graphene surface via П-П stacking to trigger the exfoliation in water, whereas the tetra-anionic state provides the necessary inter-flake repulsive electrostatic forces for stable dispersions (Figure 4) [23].

1.2.2.

Covalent functionalization Pristine graphene does not display functional groups on its surface and its aromatic rings are very stable and thus it is very hard to obtain in covalent linkages. However, a few chemical reactions can be used for combining graphene with newly introduced groups in the form of covalent bonds to improve its performance. Indeed, graphene sheets can be modified with radicals, nitrenes, carbenes, and arynes. These reactive species covalently modify graphene through free radical addition, CH insertion, or cycloaddition reaction (Figure 5) [24]. Graphite oxide and graphene oxide (GO) Graphite oxide (GO) also known as graphitic oxide or graphitic acid is usually prepared by the treatment of graphite flakes with oxidizing agents (Figure 6). It was first prepared by Brodie in 1859 through the reaction of graphite flakes with potassium chlorate (KClO3) and nitric acid (HNO3) [25]. In 1958, Hummers and Hoffman [26] developed a faster and more efficient method for the preparation of GO using anhydrous sulfuric acid (H2SO4), sodium nitrate (NaNO3) and potassium permanganate (KMnO4). The structure of graphite oxide (GO) contains epoxide, hydroxyl, carbonyl, and carboxyl groups, thereby widening the interlayer spacing of the graphene sheets (Figure 4) [27]. Therefore, graphite oxide can be easily dispersed in water and exfoliated into single layered graphene oxide by ultrasonication [28].

1.3.2.

Graphite oxide structure Aside from the operative oxidation procedures, the chemical structure of graphite oxide has been the subject of extensive research over the years. Consequently, several structural models of graphite oxide have been described from time to time [29][30][31][32][33][34][35][36][37] (Figure 7). Among these models, the Lerf-Klinowski (LK) model has been the most widely used due to the excellent interpretability over the majority of experimental observations, and easiness of further modification. Lerf and Klinowski characterized their GO by the elemental analysis, transmission electron microscopy, X-ray diffraction, diffuse reflectance; infrared Fourier transform spectroscopy, X-ray photoelectron spectroscopy, and electron spin resonance and NMR.

Accordingly, they proposed a model featuring the majority of oxygen-functional groups in graphene oxide with epoxide and hydroxyl groups located in the middle of the graphene plane, while the small amount of carboxylic and carbonyl groups are located on the edges [33]. More recently, Liu et al., [37] experimentally observed oxygen bonding and evidenced the presence C=O groups on the edge and plane of GO, confirming parts of earlier proposed models, especially the Lerf-Klinowski (LK) model. Furthermore, Rourke et al., [35] found that GO decomposed into slightly oxygenated graphene part and strongly graphene-bound oxidative debris (OD) upon suffering a base washing, which was much different from those previously proposed, upgrading the way we used to understand about GO (Figure 7).

1.3.3.

Functionalization of graphite oxide Due to the nature of GO, surface functionalization is probably the most popular and oldest technique in material science, in order to add other functional groups onto the surface of the materials by chemical methods. Functionalization of the graphene oxide surface is possible due to the presence of different oxygen-containing functional groups. Non-covalent and covalent methods are the two ways for the functionalization of graphene oxide based materials with different advantages and drawbacks [38].

Non-covalent functionalization

The non-covalent functionalization of GO sheets mainly ncludes electrostatic and hydrogen bond interaction with porphyrins or biomolecules [39][40][41]. Since this method contains no bonding cooperation, it is easy to carry out without changing the chemical structure of the GO sheets it and could effectively tailor the electronic properties, dispersity, and solubility of the GO-based nanosheets.

Functionalization via hydrogen bonding

Hydrogen bonding, occurs between the oxygen-containing groups of GO sheets and polar groups of other molecules or polymers. For example, Zhao et al., [42] obtained GO-DEPA through interactions of graphene oxide (GO) sheets with diethyl-N,N-bis(2-hydroxyethyl) phosphoramide (DEPA) (Scheme 1).

Scheme 1. Illustration for the interactions of graphene oxide sheets (GO) with diethyl-N,N-bis(2-hydroxyethyl) phosphoramide (DEPA) [42].

Functionalization via coordination bonds

Generally, the transition metals can be adsorbed onto the oxygen-containing functional group of the graphene oxide (GO) sheets through coordination bonds [43].

For example, interactions of graphene oxide with Ag(I) , Cu(II), Fe(II), Fe(III), and Bi(III) ions under ultrasonic treatment were studied and reported by Laure et al., [44].

Based on the obtained results, the authors have suggested the potential utility of GO sheets as an efficient sorbent of metal ions from aqueous solutions through the coordination of the metal ions onto oxygen-containing groups of GO sheets. A possible model of coordination of the metal ions to oxygen containing groups of GO was proposed (Figure 8). On the other hand, Park et al., [45] manufactured GO papers with high mechanical properties through chemical cross-linking of GO sheets with divalent alkaline earth metal ions such as Mg(II) Ca(II), etc. Similarly, Yamada et al., [46] prepared functionalized graphene oxide (GO) through coordination bonding. They investigated the coordination force of serval metal cations with ammonia-treated GO.

It was found that Co(II), Ni(II) and Cu(II)-based divalent catalyst performed best, leading to stable bonding which enables to prevent aggregation of the treated GO sheets.

Functionalization via electrostatic interaction

Another functionalization of GO can be realized through electrostatic attraction between modified GO sheets and other charged molecules. In addition, the electrostatic interactions are used as a promising facile and scalable strategy for the preparation of graphene composites materials [47].

For instance, Xu et al., [48] used the electrostatic interactions to prepare a novel hybrid material based on negatively charged sulfonated-reduced graphene oxide and positively charged poly(3,4-ethyldioxythiophene) (PEDOT), which exhibited good flexibility (4.2 GPa), and excellent electrical conductivity (2 × 10 2 S/m) (Figure 9).

Covalent Functionalization

In general, covalent functionalization makes covalent linkage between molecules, polymers and the the functional groups located onto the GO surface. The covalent modification improves both the physical and chemical properties of GO such as solubility, catalytic activity, and introduces novel functionalities to the material [49].

Covalent functionalization of GO is based on the use of well know organic reactions such as nucleophilic substitutions, electrophilic substitutions, condensation reactions, and so on. Additionally, the covalent functionalization of GO may be accompanied by the re-hybridization of C-atom from sp 3 to sp 2 which partly recover the electrical conductivity of graphite.

Nucleophilic substitutions

According to the widely accepted Lerf-Klinowski (LK) model [33], reactive functional groups on GO are epoxy, hydroxyl, carbonyl, carboxyl, and ester moieties.

The epoxy groups are mainly located on the basal plane of GO and can be easily Scheme 3. Reactivity of the epoxy groups of the GO surface [52].

Moreover, Xue et al., [53] studied the grafting of tris(hydroxymethyl)aminomethane (TRIS) onto GO via a nucleophilic SN2 reaction and they obtained a very high grafting efficiency (27 .wt% from TGA measurements).

Interestingly, using a similar approach, the crosslinking of GO with polyallylamine was reported for the mechanical enhancement of 'paper-like' materials that are generated by simple filtration of colloidal suspensions [54].

Electrophilic substitutions

Electrophilic substitution is the preferred method for the controlled formation of functional groups onto the GO surface with minimum damage. In particular, Gao et al., [55] performed the direct grafting of diazonium-based molecules onto GO in order to retain the available hydrophilic groups and to modify the remaining sp 2 carbon domains.

They also performed XPS and solid-state 13 C-NMR to assess the covalent nature of the grafting. Moreover, Rubuttini et al., [56] reported the grafting of 4-Carboxyphenyl groups onto GO via diazonium chemistry for studying their role on the adsorption of iron oxide nanoparticles (Scheme 4).

*

Scheme 4. Schematic representation of the chemical modification procedure performed through diazonium chemistry [56].

Raman spectroscopy showed a schift of the D and G bands to higher wavenumbers with respect to GO after grafting. The up-shift might be an effect of the electron transfer from the П states of the C network to the 4-carboxyphenyl functionalities introduced through the chemical modification process.

Condensation reactions

Another approach for the modification of GO is based on the ue of condensation reactions. The condensation reactions occur via two reactive oxygenated groups (hydroxyl and carboxyl functionalities) through several mechanisms such as esterification, amidation, and silianization.

Esterification

The reaction involves the reaction between the carboxylic groups located on the edges of GO and the alcoholic groups of the molecules to be bonded [57][58][59][60][61][62][63]. For instance, Ramasamy et al., [57] used an esterification reaction to crosslink GO using 3thiophene acetic acid (TAA) and they obtained with a grafting efficiency of 7 wt% (determined by TGA). Moreover, Kumara et al., [58] covalently achieved Meta Toluic acid through an esterification reaction on GO. They also demonstrated that the Toluic acid modified graphene oxide (TGO) was completely soluble in methanol, propan-1ol, dimethylformamide (DMF), acetone, ethanol and dimethyl sulfoxide (DMSO)

which may find its application in various fields like gas and humidity sensors.

Amidation

Introduction of substituted amines is one of the most common methods for covalent functionalization of GO, and the final products have been investigated for various applications in polymer composites [64], drug-delivery vehicles [65], and optoelectronics [66]. In the literature, the native edge carboxylic groups on GO react with various amine terminated polymers, chromophores, biomolecules, and ligands.

For example, Mallakpour et al., [67] reported the covalent functionalization of GO with biocompatible, natural aromatic-aliphatic amino acids (phenylalanine and tyrosine) and aliphatic amino acids (alanine, isoleucine, leucine, methionine, and valine) by a simple and green procedure. The resulting materials exhibit improved nonlinear optical properties.

To increase the reactivity of carboxylic acid groups towards -NH2 groups, the carboxylic acid groups are first activated by coupling agents such as thionyl chloride However, modifying the surface exposed epoxy and carboxylic acid groups on GO with naturally occurring amino acids, occurred easily through nucleophilic substitution and condensation reactions.

Silanization

The silanization reaction allows the functionalization of hydroxyl groups located on the surface of GO. Indeed, chloro and alkoxysilane are very reactive toward protic groups [69][70][71][72]. For example, Mejias et al., [70] functionalized GO with N-[ (3trimethoxysilyl)propyl]-ethylenediamine triacetic acid (TEDTA). The final compound was studied through FTIR and XPS analysis to assess its structure. It is believed that the silanization is a two-step reaction. First, there is the hydrolysis of the trialkoxy groups of the silane and then the reaction between the Si-OH groups and the hydroxyl groups of GO ensure the formation of Si-O-C bonds.

Similarly, Zhang et al., [71] used silanized GO through the use of amine-terminated silanes to prepare primary and tertiary amine-bifunctional graphene oxides with excellent catalytic activities and 100% selectivity.

1.3.4.

Reduction of graphene oxide (rGO) In order to recover the electrical proprieties of graphene sheets the removal of oxygen functional groups of GO is obtained through a reduction reaction. The most widely applied techniques for preparing reduced graphene oxide (rGO) are chemical [73], solvothermal/hydrothermal [74], thermal [75], photocatalyst, and photoreduction [76]. Among these, chemical reductions of graphene oxide with chemical reagents is one of the most conventional procedure to recover the sp 2 hybridization in large quantities [77]. For example, Stankovich et al., [78], used hydrazine hydrate (N2H4.H2O) as the reductant. The reduction mechanism for the removal of epoxide groups in the presence of hydrazine is described in Scheme 6. Scheme 6. A proposed reaction pathway for epoxy reduction by hydrazine [78].

The mechanism of reduction was postulated to proceed via a direct nucleophilic attack of hydrazine on an epoxide group to resulting in a hydrazine alcohol moiety, which released a water molecule and lead to the formation of an aminoaziridine.

Finally, a thermal elimination of di-imide allowed for the formation of a double bond. Furthermore, Gao et al., [79] further elucidated the effect of hydrazine treatment on different functional groups by DFT simulation. Their results show that the hydrazine reduction can only result in reducing epoxy groups, while no reaction path was found for the reduction of the carbonyl and carboxyl groups of GO. Other reductants, such as sodium hydride, sodium borohydride, and lithium aluminium hydride or more environmentally friendly tryptophan, hydroiodic acid, pyrogallol, urea, and thiourea have also been employed to prepare RGO [80][81][82].

1.3.5.

Hybridizing of graphene and its derivatives with metallic nanoparticles The binding or loading of metal and metal oxide nanoparticles on graphene or its derivatives for the preparation of graphene metal@based nanocomposites is generally realized through both in-situ (e.g., growing the metal nanoparticles on the graphene surface) and ex-situ (e.g., attaching formed metal nanoparticles to the graphene surface) methods. GO and rGO are especially promising templates for this purpose as the presence of defects and oxygen functional groups on their surfaces allows for the nucleation, growth, and attachment of various metal nanoparticles (e.g., Au [83], Ag [84], Pt [85], etc.) and metal oxide nanoparticles (e.g., Fe3O4 [86], TiO2 [87], CuO2 [88],

MnO2 [89], NiO [90], etc.). In this section, we will give a comprehensive of the different methods that are available for the preparation of graphene@metal nanoparticles including various in-situ (e.g., reduction and hydrothermal ) and ex-situ methods.

1.3.5.1.

In-situ methods

Reduction method

Methods for the preparation of graphene metal-based composites by in-situ chemical reduction consist in the use of metal precursors such as HAuCl4, AgNO3, K2PtCl4, and H2PdCl6 and reductants like hydrazine hydrate, ethylene glycol, and sodium borohydride [91]. More specifically, the functionalities that exist on the GO and rGO surface, such as alcohols, carbonyl, and carboxyl groups are responsible for the adsorption of free metal ions through electrostatic interactions. Subsequently, the addition of a reducing agent promotes the reduction of the attached metal ions, thereby enabling the growth of metal nanoparticles onto the GO and rGO surfaces [92].

Noble metal nanoparticles such as gold (AuNPs) and silver nanoparticles (AgNPs) are of particular interest because of their unique and unusual properties such as high biocompatibility and optical properties, which can easily be tuned to the desired wavelength according to their shape, size, and composition [93]. In addition, GO@noble metal nanoparticles are able to exhibit SERS (Surface Enhance Raman Scattering) as well as enhanced catalytic potential [94].

For instance, GO@AuNPs can be attained by mixing HAuCl4 precursor with exfoliated GO and sodium citrate, resulting in gold precursors anchored to the surface of GO via electrostatic interactions [95].

On the other hand, graphene@bimetallic nanoparticles can also be obtained utilizing a two-step reduction process. For example, Guo et al., [96] TEM image of the Graphene-PVP@Pt-on-Pd nanoparticles [96].

Apart from metallic and bimetallic NPs, the composites of metal oxides with rGO have been synthesized by in-situ chemical reduction [97]. For example, Kim et al., [98] prepared rGO@Co3O4NPs, by the reduction of GO and cobalt acetate ((C2H3O2)2Co.4H2O) in deionized water with NH4OH and hydrazine as reductants, and it was demonstrated that the resulting rGO@Co3O4NPs material was efficient as a reversible anode for lithium rechargeable batteries with a reversible capacity, as high as 990 mA h g -1 and 778 mA h g -1 in the 1st cycle and 42nd cycle, respectively.

Hydrothermal method

The hydrothermal method is also commonly used to synthesize inorganic nanoparticles that have high crystallinity and narrow size distribution on graphene and its derivatives. In general, the process includes the use of high temperatures and pressures, which induce the growth of nanocrystals as well as the reduction of GO.

However, while the high temperature and long reaction times can partially or completely reduce GO, in most cases, reducing agents are added to ensure the complete reduction of GO [99]. Graphene@metal oxide nanoparticle composites (e.g., ZnO [100], Fe3O4 [101], TiO2 [102]) are the most common hybrids synthesized using the hydrothermal method.

In order to have better control over the morphology of NPs on the surface of rGO sheets during the reduction process, ionic liquids (ILs) that are commonly used as solvents, reactants, and templates for the synthesis of inorganic nanomaterials with novel morphologies and improved properties were applied. For example, Shen et al., [103] described a one-pot hydrothermal method to prepare rGO@TiO2NPs using GO, tetrabutyl titanate and 1-butyl-3-methylimidazolium hexafluorophosphate

[BMIM][PF6] ionic liquid as starting materials. The hydrothermal conditions not only favored the reduction of GO but also facilitated the generation of TiO2NPs in the presence of ILs. In addition, some researchers have also used hydrothermal methods for the preparation of rGO@noble metal nanoparticles [104]. In the case of rGO@AuNPs, a solution of HAuCl4.3H2O and NaOH was mixed with GO.

Subsequently, the solution was sonicated at a frequency of 40 kHz and heated to 180 °C. The resulting rGO@AuNPs material had AuNPs with a narrow size distribution in the range of 2 or 18 nm, with and without sonication, respectively [104].

Ex-situ methods

Graphene@metal nanoparticles can also be produced by the ex-situ assembly of nanoparticles onto the graphene surface. In this method, the metal nanoparticles are pre-synthesized and then later attached onto the graphene surface via linking agents that can utilize either covalent or non-covalent interactions [105]. For the covalent attachment of metal nanoparticles, GO is more favourably used than rGO due to the higher amount of oxygen-containing groups on its surface, which can facilitate linkage with other functional groups.

For example, Fuan et al., [106] modified GO sheets with Fe3O4 nanoparticles (NPs) through a two-step process for removing methylene blue (MB) from aqueous solutions. In the first step, Fe3O4NPs was modified by tetraethyl orthosilicate and (3aminopropyl)triethoxysilane to introduce amino groups on its surface. In the second step, amino groups of Fe3O4NPs were reacted with the carboxylic groups of GO with the help of 1-ethyl-3-(3-dimethyaminopropyl)carbodiimide and N-hydroxysuccinnimide to form a GO@Fe3O4NPs hybrid. The attachment of Fe3O4NPs onto the GO sheet surface was evidenced by transmission electron microscopy (TEM) and Fouriertransform infrared spectroscopy (FTIR).

Alternatively, metal nanoparticles can be attached to graphene sheets via noncovalent bonds including van der Waals interactions, hydrogen bonding, π-π stacking, and electrostatic interactions. Among these non-covalent bonds, π-π stacking and electrostatic interactions have been the most widely used. For π-π stacking, generally, aromatic compounds are attached to the nanoparticle surface, which enables their attachment to graphene via π-π stacking. For instance, Wang et al., [107] fabricated GO-DNA@AuNPs and GO-DNA@AgNPs by first functionalizing AuNPs or AgNPs with DNA via di-dentate capping ligands and then assembling them onto GO via π-π stacking interactions. In addition, the scanning electron microscopy (SEM) images of (a) GO-DNA@AuNPs and (b) GO-DNA@AgNPs showed the presence of sphere-like structures with average particle sizes of 15.9±1.8 nm for AuNPs and 16.0±2.8 nm for AgNPs (Figure 11). As mentioned previously, electrostatic interactions are also commonly used to modify graphene with various metal nanoparticles. GO and rGO have a negative charge as a result of the ionization of the oxygen functional groups on their surface which can be decorated with positively charged metal nanoparticles through electrostatic interactions. For instance, GO@MnO2NPs were formed by mixing positively charged manganese dioxide nanoparticles (MnO2NPs) with negatively charged GO [108]. By electrochemical analysis, the specific capacitance of GO@MnO2NPs was found to be about 40% larger than the manganese dioxide one [108].

1.3.6.

Applications of graphene and its derivatives Since 2004, many interesting properties of graphene and its derivatives have been discovered, which include high thermal conductivity, high charge carrier mobility, large theoretical specific surface area, and excellent mechanical properties. Herein, we have focused on the properties of graphene and its derivatives that are most relevant for its biomedical and sensor applications.

Biomedical

Graphene has been at the forefront of research for the past few years. The development of new methods for graphene synthesis has made this material easily available in the market. Along with graphene-based materials, its derivatives such as GO and rGO have been carried out for biomedical applications, including drug delivery, imaging, antibacterial and anti-cancer activities, and so on [109].

For example, when GO is injected into mices (immunocompetent) having CT-26 colon cancer cells [110] it prevented from the progression of the tumor but also induced autophagy [111], immune responses and cell death [112]. In addition, superparamagnetic magnetite nanoparticles (Fe4O3NPs) were applied in magnetic resonance imaging (MRI), biological separation, and hyperthermia therapy [113]. In this regard, GO@Fe3O4NPs have attracted attention to achieve enhanced MRI contrast, improved biocompatibility, and physiological stability [114]. Thus, Chen et al., [115] prepared Fe3O4NPs via thermal decomposition followed by surface modification with aminodextran (AMD). Then, AMD@Fe3O4NPs were finally grafted onto the GO surface via the formation of an amide bond between amine groups of the AMDfunctionalized Fe3O4NPs and carboxylic acid groups of the GO sheets. The final magnetic material exhibited significantly improved T2 weighted agents for cellular magnetic resonance imaging (MRI) and did not affect the cellular viability and proliferation. Furthermore, GO and rGO exhibited more prominent antibacterial activity [116] thoroughly a membrane stress. The stress is generated by the pointed uneven corner of graphene sheets, which leads to cell membrane damage with a RNA (ribonucleic acid) and membrane integrity loss [117]. When the concentration, incubation time and the conditions are the same, GO shows the highest antibacterial activity than rGO [117]. In another hand, it is well known that silver nanoparticles (AgNPs) present antimicrobial activity and have been used as biocide agents in health, food, and textile applications [118]. Due to this property, the silver nanoparticles assembled on graphene oxide sheets (GO@AgNPs) have been exploited as novel antibacterial systems [119,120]. For example, Ma et al., [119] demonstrated antibacterial activity for GO@AgNPs against the gram-negative Escherichia coli (ATCC 4758) strain.

The presence of the proteolytic enzymes present in the cytoplasm often interferes with the drug delivery process. GO is used in the case of effective gene and drug delivery carriers (Figure 11) [121].

Figure 12. GO as a carrier for target (gene or small molecular drug) delivery.

The different oxygen-containing functional groups of GO allow it to conjugate with various polymers and biomolecules (ligand, DNA and protein). One of the approaches for use of GO in gene delivery includes its functionalization with a cationic polymer such as polyethylenimine (PEI) [121].

It was used as a non-viral gene vector due to its strong electrostatic interactions with negatively charged phosphates of RNA (ribonucleic acid) and DNA (Deoxyribonucleic acid). It makes transfection easy and efficient, improves cell selectivity, and reduces cellular toxicity [122]. Thus, Feng et al., [123] and Chen et al., [124] used GO-PEI for gene delivery using different molecular weights of PEI. Both studies showed significantly lower cytotoxicity of GO-PEI complex compared to PEI alone and successful use of GO as a novel nano-gene delivery vector with high transfection efficiency. Although the research on graphene and its derivatives is at its early stage, in the up-coming days it will provide wide applications for biomaterial science and regenerative medicines [125][126][127]. Despite its wide range of applications, the toxic effect of graphene has been recently reported from various studies. The future approaches may be to abolish the toxic effect without affecting the physical and chemical properties [128].

Sensor

Recently, the trend of applying graphene and its derivatives materials for the fabrication of electrochemical sensors and biosensors has gained great importance [129,130]. So far, many researchers have employed GO-based material to detect humidity [130], hydrogen peroxide [131], trimethylamine [132], ammonia [133], and nitrogen dioxide [134]. Several graphene-based inorganic nanocomposites have been reported as enzyme-free biosensors for glucose and other biomolecules. They exhibited improved sensitivity and selectivity during the oxidation of target molecules [135]. For instance, Lu et al., [136] reported an electrochemical biosensor based on rGO@PdNPs modified electrode for the detection of glucose. In another study, Yola et al., [137] employed GO@AuNPs involving 2-aminoethanethiol (2-AET) for the detection of tyrosine (Tyr) in milk, which makes a superior sensor with a low detection limit (1.5 × 10-10 M) (Figure 13) [137].

Figure 13. The procedure for fabrication of the Tyr imprinted film on (GO-2-AET)@AuNPs sensor [137].

Conclusion

This section has highlighted the preparation, properties, and applications of graphene-based materials. Graphene, as one of the advanced carbon nanomaterials, is a 2-dimensional single sheet of carbon atoms arranged in a hexagonal network. It has been prepared by thermal decomposition of SiC wafer under ultrahigh vacuum (UHV) conditions or by Chemical vapor deposition (CVD) growth on metal substrates (Ni and Cu) or by substrate-free CVD. This is a potential mass production method with the aim of producing graphene for electronics applications. In addition, graphene can also be prepared at a larger scale by liquid-phase exfoliation to form a graphene oxide (GO) intermediate, followed by reduction to restore the graphene structure (rGO). It is also considered the most economical way to produce graphene. Therefore, the graphene oxide must be chemically modified or functionalized to improve its stability and dispersity as well as prevent its irreversible aggregation. The functionalization and chemical modification of the graphene surface using various covalent and noncovalent interactions can provide multiple strong interfacial bonding sites to the resin matrix. In addition, the loading of metal and metal oxide nanoparticles onto graphene sheets or its derivatives are generally performed using both in-situ and exsitu methods which produce a series of novel materials with highly specific functionalities and enormous potential for targeted applications.

Wastewater treatment

Many of the major problems that humanity is facing in the twenty-first century are related to water quantity and water quality issues [START_REF]The United Nations World Water Development Report 3: Water in a Changing World[END_REF]. These problems are going to be more aggravated in the future by climate change, resulting in higher water temperatures, melting of glaciers, and an intensification of the water cycle [START_REF] Huntington | [END_REF], with potentially more floods and droughts [140] Therefore, wastewater treatment is critical to the safety of a human being.

Metal pollutants in water

Pollutants, such as heavy metals, are serious threats to the environment. They get introduced to aquatic streams due to industrial activities, i.e. mining, refining ores, fertilizer industries, tanneries, batteries, paper industries, and pesticides [142]. In addition, the heavy metals cannot be biodegraded and can enter and accumulate in the human body through the food chain and drinking water, resulting in a series of irreversible physiological diseases, such as kidney damage, nervous-system disorders, and bone necrosis [143]. Heavy metals of chromium (Cr), iron (Fe), selenium (Se), vanadium (V), copper (Cu), cobalt (Co), nickel (Ni), cadmium (Cd), mercury (Hg), arsenic (As), lead (Pb), and zinc (Zn) represent the major toxic hazardous materials to humans and other forms of life [143]. Some metals (cadmium, chromium, lead and, mercury) are considered toxic even at low concentrations, while other micronutrients such as cobalt, copper, and zinc are essential at low concentrations, but may become harmful with increased exposure [144].

Techniques for removing metal ions

In recent years, many methods that are being used to remove heavy metal ions include chemical precipitation, ion-exchange, adsorption, membrane filtration, electrochemical treatment technologies, etc. The following section examines the effectiveness, problems, and advantages associated with some of these techniques.

2.2.1.

Chemical precipitation Chemical precipitation is a physicochemical process and far the most widely used process in the industry because it is relatively simple and inexpensive to operate [145].

In this process, pH adjustment (basic pH) of heavy metals and reaction with chemical reagents (for example, lime, hydroxides, and sulfides) result in the formation of insoluble particles, which are then separated by simple sedimentation or filtration. For example, researchers found that 70% removal of Cd, Cu, and Pb from electroplating, textile, and leather tanning industry wastewater was possible by using a coupled system of hydroxide precipitation [146]. The major disadvantage is that the heavy metal concentration does not reach ranges acceptable for discharge, mandating additional post-treatments. Large sludge generation, disposal of sludge, and possible secondary pollution are additional major disadvantages of this process [147].

2.2.2.

Ion-exchange Historically, ion exchange is an ancient technique documented more than a hundred years ago. Since then, this technique has been used for softening water to an incomparable wider scale of applications and has become an integral part of new technical and industrial processes [START_REF] Dardel | Ullmann's Encyclopedia of Industrial Chemistry[END_REF]. There is a vast diversity of ion-exchange materials. They have many appearances like natural and synthetic, organic and inorganic, cationic, anionic, and amphoteric [START_REF] Azimi | [END_REF]. These resins are categorized depending on their available functional groups as either strong or weak exchangers and may be acidic or basic in nature [START_REF] Letterman | Water Quality and Treatment[END_REF].

Nowadays, the inorganic ion-exchange materials such as zeolites represent the most conventional inorganic ion exchangers, occurring naturally, but also synthesized in a wide diversity of variations. For example, Lu et al., [START_REF] Lu | [END_REF] studied the adsorption behavior of Pb(II) and Cu(II) on thermally treated zeolite. The maximum adsorption capacities of Pb(II) and Cu(II) on zeolite were 1.273 and 1.817 mmol.g -1 , respectively.

2.2.3.

Membrane separation Membrane filtration technologies with different types of membranes have great promise for heavy metal removal due to their high efficiency, easy operation, and space-saving [152]. Examples of membrane processes used to remove metals from the wastewater are: reverse osmosis [153], ultrafiltration [154], nanofiltration [155], and electrodialysis [156].

Among these processes, the reverse osmosis process is considered as the best technique to remove metal ions from wastewater. In addition, it is an increasingly popular wastewater treatment option in chemical and environmental engineering. For example, Mohsen-Nia et al., [157] demonstrated that Cu(II) and Ni(II) ions were successfully removed by the reverse osmosis process and the removal efficiency of the two ions increased up to 99.5% by using disoduim salt ethylene diamine tetraacetic acid (Na2EDTA). Although membrane separation is cost-effective, its application for removing metal ions is constrained by short membrane half-life, fouling, and low selectivity [158].

2.2.4.

Adsorption Adsorption process has become a well-known, effective, and economical method to remove heavy metal ions from aqueous solutions due to the high removal efficiency, the possibility of regenerating of different adsorbents and the flexibility in design and operation [START_REF] Hu | Nanotechnology for Water Treatment and Purification[END_REF]. In addition, the adsorption phenomenon is one of the physicochemical treatment processes, describing the accumulation of heavy metal ions (adsorbate) from an aqueous solutions to the surface of a solid (adsorbent) (Figure 14) [START_REF] De Sa´ | [END_REF]. However, adsorption is often accompanied by the desorption process, which represents the transfer of adsorbate ions from the adsorbent surface to the solution [START_REF] Anjum | [END_REF].

Types of adsorption

According to the nature of the interaction between adsorbate and adsorbent, there are two types of adsorption; namely:

 Physical adsorption (physisorption)
 Chemical adsorption (chemisorption)

Physical adsorption (physisorption)

Physical adsorption or physisorption is based on electrostatic forces; including Vander Waals interactions, and hydrogen bonding, which do not involve a significant change in the electronic orbital patterns between the surface of solids (adsorbent) and heavy metal ions (adsorbate) [162][163][164][165][166]. Furthermore, physical adsorption is relatively non-specific and reversible in nature due to the weak attraction forces between the adsorbate and the adsorbent and resulting multilayer adsorption. In this case, the adsorbate is not adsorbed to a specific site on the solid surface and it is free to move over the entire surface.

Chemical adsorption (chemisorption)

Chemical adsorption or chemisorption involves the formation of chemical bonds between the surface of solids (adsorbent) and heavy metal ions (adsorbate) [START_REF] Somasundaran | HANDBOOKOF Surface and Colloid Chemistry[END_REF]. In chemisorption, the attraction between adsorbent and adsorbate is a covalent or ionic bond between atoms, with shorter bond length and higher bond energy [START_REF] Kong | [END_REF][START_REF] Lee | [END_REF].

In addition, high temperatures favor this type of interaction. Indeed, the chemical adsorption process depends on the surface area [170] and it increases by increasing the surface area because the heavy metal ions are linked to the surface of solids by covalent bonds in which the adsorbate is not considered free to move from one surface site to another one [171,172]. Sometimes, both physisorption and chemisorption may occur on the surface at the same time that a layer of adsorbate may be physically adsorbed on the top of an underlying chemisorbed layer [173].

Adsorption kinetics

The dynamic or mechanism of adsorption is usually investigated by fitting the experimental data obtained for adsorption with kinetic models. Commonly used kinetic models for describing solid-liquid adsorption systems are the pseudo-firstorder model [174], the pseudo-second-order model [175], the Elovich or Roginsky-Zeldowitsch model [176], and the intraparticle diffusion model [177].

Pseudo-first-order model

Lagergren et al., [178] presented a pseudo-first-order model to describe the kinetic process of liquid-solid phase adsorption of oxalic acid and malonic acid onto the charcoal, which is believed to be the earliest model related to the adsorption rate based on the adsorption capacity.

It can be expressed in the nonlinear form with Eq. (1):

dQ t dt = K 1 (Q e -Q t ) (1)
Where Qe and Qt (mmol.g -1 ) are the adsorption capacities at equilibrium and at test time t (min), respectively; K1 (min -1 ) is the rate constant of the pseudo-first-order adsorption.

Integrating Eq. (1) with the boundary conditions t = 0 at Qt = 0 and Qt = Qt at t = t, the linear form of pseudo-first-order model is expressed as Eq. (2):

ln( Q e Q e -Q t ) = K 1 t (2)
Which can be rearranged to:

ln(Q e -Q t ) = ln Q e -k 1 t (3)
or

log(Q e -Q t ) = logQ e - K 1 2.303 t (4)
The slope and intercept of plots of log(Qe-Qt) versus t are then used to determine the rate constant k1 and Qe.

Pseudo-second-order model

In 1998, Ho et al., [179] described a kinetic process of the adsorption of Cu 2+ ions onto peat, in which the chemical bonding among Cu 2+ ions and polar functional groups on peat, such as aldehydes, ketones, and acids are responsible for the cation-exchange capacity of the peat.

Therefore, the peat-metal reaction may be presented according to Eqs. ( 5) and (6):

2HP + Cu 2+ CuP2 + 2H + (5)
2P -+ Cu 2+ CuP2 (6)
Where HP and P -are active sites on the peat surface.

The main assumptions for the above two equations were that the adsorption may be second-order, and the rate-limiting step may be chemical adsorption involving interactions through sharing or exchange of electrons between the peat and Cu 2+ ions.

The rate of adsorption is described by Eqs. ( 7) and (8) and it is dependent upon the amount of divalent metal ions on the surface of peat at time t and the one adsorbed at equilibrium.

dP t dt = K 2 (P 0 -P t ) 2 (7)
or

dP t dt = K 2 (P 0 -P t ) 2 (8)
where P0 and HP0 (mmol.g -1 ) are the amount of equilibrium sites available on the peat, Pt and HPt denote the amount of active sites occupied on the peat at time t, and K2 (g.mmmol -1 .min -1 ) is the rate constant of pseudo-second-order adsorption.

Moreover, the driving force, (Qe-Qt) is proportional to the available fraction of active sites according to Eq. ( 9).

dQ t dt = K 2 (Q e -Q t ) 2 (9)
Integrating Eq.( 9) with the limits t = 0 at Qt = 0 and Qt = Qt at t = t, the pseudosecond-order model can be expressed in the linear form as Eq. (10):

Q t = t 1 k 2 .Q e 2 + t Q e (10)
Eq. ( 10) has been rearranged in different forms according to Eqs. ( 11)-( 14):

t Q t = ( 1 k 2 Q e 2 ) + t Q e ( 11
)
1 Q t = ( 1 k 2 Q e 2 ) 1 t + 1 Q e ( 12
)
Q t = Q e -( 1
K 2 Q e ) Q t t (13)
Q t t = k 2 Q e 2 -k 2 Q e Q t ( 14
)
Among the linearized equations of pseudo-second-order, Eq. ( 11) gives better fitting results compared to other forms [179]. Hence, the values of k2 and Qe can be calculated from the slopes (1/Qe) and intercepts (1/k2Qe 2 ) of the plots of t/Qt versus t, respectively.

Elovich or Roginsky-Zeldowitsch model

In 1934, Zeldowitsch proposed a modified model to describe the sorption of solutes on highly heterogeneous sorbents, which could not be described by the firstorder kinetic model [180]. The application of this model was first used to describe the rate of adsorption of carbon monoxide on manganese dioxide that decreases exponentially with an increase in the amount of adsorbed gas [180]. In addition, Elovich or Roginsky-Zeldowitsch model was used to determine the nature of adsorption on the heterogeneous surface of the adsorbent, whether chemisorption or not.

The Elovich or Roginsky-Zeldowitsch model can be presented as:

dQ t dt = α e (-βQt ) (15)
Where α (mmol.g -1 .min -1 ) is the Elovich initial adsorption rate. β (g.mmol -1 ) is the desorption constant.

Integrating Eq. ( 15) with the limits t = 0 at Qt = 0 and Qt = Qt at t = t, the Elovich model can be linearized as Eq. ( 16):

Q t = 1 β ln (t + 1 αβ ) + 1 β ln(αβ) (16)
As the system approaches equilibrium t ≫ (1/αβ), thus, Eq. ( 16) is modified according to Eq. ( 17):

Q t = 1 β ln(αβ) + 1 β ln(t) (17)
The constants α and β can be obtained from the slope and the intercept of the plot of Qt versus ln(t).

Intra-particle diffusion model

Intra-particle diffusion model based on the theory proposed by Weber and Morris (1963) was described to examine the rate-limiting step during adsorption [181], which can be expressed with the following Eq. ( 18):

Q t = k i √t + C (18)
Where ki (mmol.g -1 .min -1/2 ) is the rate constant of intra-particle diffusion and C (mmol.g -1 .min -1/2 ) is a constant correlated to the thickness of the binding layer. If the plot of Qt versus t 1/2 is intercepted at zero, the rate of adsorption will be divided into many stages; these stages correspond to different mechanisms that control the adsorption process. In most cases, these lots give general features of three stages [182].

The first stage is called mass transfer of adsorbate up to which the adsorbent is dropped into the solution. This process is too fast, thus it is not considered during the design of kinetic systems. The second mechanism is called film diffusion; it involves the slow mobility of adsorbate from the binding layer to the adsorbent's surface. The final stage involves the rapid adsorption attachment of the adsorbate on the active sites of the pores; being a rapid process [183]. If the system is characterized by a poor mixing, a small adsorbate size, and low concentration, therefore film diffusion will become the rate-controlling step.

Adsorption isotherm

Adsorption isotherms are known as equilibrium data suggest how the adsorbate (metal ions in aqueous solutions) is distributed between the solution and the surface of the adsorbent at equilibrium concentration and at different temperatures [184]. A plot of the amount of the adsorbate on the adsorbent (Qeq) versus the equilibrium concentration of adsorbate in solution (Ceq), produces a curve to understand the mechanism of the adsorption process [185]. There are three wellestablished types of adsorption isotherm the Langmuir, [186], the Freundlich [187],

and the Jossens [188] ones (Figure 15).

Langmiur isotherm

The Langmuir isotherm model is one of the commonest models used for describing the adsorption of adsorbates in solution onto the homogeneous sites of the adsorbent [186].

In addition, this model is based on three assumptions:

-Adsorption is limited to monolayer coverage.

-All surface sites are alike and only can accommodate one adsorbed atom.

-The ability of a molecule to be adsorbed on a given site is independent of its neighboring site occupancy.

By applying these assumptions, the Langmuir isotherm model can be expressed as Eq. ( 19):

Q e = QbC e 1+bC e (19)
Where, Ce (mmol.L -1 ) is the equilibrium concentration of metal ions, Qe (mmol.g -1 )

is the equilibrium adsorption capacity, Q (mmol.g -1 ) is the Langmuir monolayer saturation capacity and b (L.mmol -1 ) is a constant related to the adsorption energy.

The Langmuir isotherm model can be linearized into four different types according to

Eqs. ( 20)-( 23):

C e Q e = C e Q + 1 bQ (20)
1

Q e = 1 bQC e + 1 Q (21)
Q e = Q - Q e bC e (22)
Q e C e = bQ -bQ e (23)
Among the four linear forms, which will result in the calculation of different parameters, Eq. ( 20) is one of the most popular linear forms used in literature, due to the minimal deviations from the fitted equation [189,190]. Hence, by plotting Ce/Qe against Ce it is possible to obtain the values of b and Q.

The Langmuir isotherm model can be also expressed in terms of a dimensionless constant called separation factor (RL), giving information about the feasibility of the adsorption process.

In 1966, Vermeulan et al., [191] proposed a dimensionless separation factor (RL), as an essential feature of the Langmuir isotherm to predict if an adsorption system is "favorable" or "unfavorable", and expressed as Eq. ( 24)

q = X R L (1-X)+X (24)
with

X = C e C ref (25) and q
= Q e Q ref ( 26
)
When the adsorption is in a monolayer, Cref and Qref are defined as being the concentration of the residual adsorbent in the equilibrium bath and the maximum concentration of adsorbent onto the surface of the adsorbent, respectively.

By substituting Eqs. ( 25) and (26) in Eq. ( 24), it leads to Eq. ( 27)

R L = C e (Q ref -Q e ) Q e (C ref -C e ) (27)
And by replacing Qe with its value in Eq. ( 19), Eq. ( 31) is obtained:

R L = ( C e Q e ) × ( Q ref -( QbC e 1+bC e ) C ref -C e ) (28)
R L = (1 + bc e ) × ( C ref 1+bC ref - C e 1+bC e C ref -C e ) (29)
R L = 1 (1+bC ref ) × ( C ref 1+bC ref - C e 1+bC ref C ref -C e ) (30)
R L = 1 1+bC ref (31)
According to the value of RL indicates four types of adsorption behavior are known:

 (RL>1) unfavorable  (RL =1) linear  (0 <RL<1) favorable  (RL =0) irreversible

Freundlich isotherm

Freundlich isotherm model is commonly used to describe the adsorption characteristics for the heterogeneous surface [START_REF] Freundlich | Adsorption in solids[END_REF]. It represents initial surface adsorption followed by a condensation effect resulting from strong adsorbateadsorbate interaction. Hence, the Freundlich isotherm model describes adsorption for both monolayer and multilayer adsorption. The Freundlich isotherm model is described by Eq. ( 32):

Q e = K F × (C e ) 1 n (32)
Where, KF and n are the Freundlich constants representing the adsorption capacity and adsorption intensity, respectively.

It can also be expressed in the linearized logarithmic form as Eq. ( 33):

lnQ e = ln(K F ) + 1 n ln(C e ) (33)
The constants KF and 1/𝑛 can be obtained from the slope and the intercept of the plot of ln(Qe) versus ln(Ce). Adsorption is favorable when the value of n lies between 1 and 10 [193]. Additionally, the surface of the adsorbent is assumed to become more heterogeneous as the value of n is close to zero [194].

Josens isotherm

The Jossens model is an empirical isotherm incorporating three parameters. It represents a combination of the two previous models and describes a simple equation based on the energy distribution of adsorbate/adsorbent interactions at adsorption sites [188].

This model is defined by Eq. (34):

Q e = i×C e 1+j×(C e ) m (34)
Where, i (L.g -1 ), j (L.mmol -1 ) are Jossens isotherm constants and m is in the range values 0-1.

At high liquid-phase concentrations of the adsorbate which j(Ce) m ˃˃ 1; so 1+j(Ce) m = j(Ce) m , Eq. (34) leads to the Freundlich isotherm model:

Q e = i j C e 1-m (35)
Where i/j = KF and (1-m) = 1/n of the Freundlich isotherm model.

Where m = 1, Eq. (34) leads to Langmuir isotherm model:

Q e = i × C e 1+j×C e (36)
With i = KL = Q×b and j = b

The equation of the Jossens isotherm contains three unknown constants; their determination requires an iteration by a computer software (Orgin Pro 9; 2019)

Adsorption thermodynamic

In order to fully understand the nature of adsorption, the Langmuir isotherm constant can be used to estimate the thermodynamic parameters such as Gibb's free energy (ΔG°; kJmol -1 ), enthalpy (ΔH°; kJmol -1 ), and entropy (ΔS°; kJmol -1 K -1 ). They can be calculated using Eqs. (37-39) [195]:

ln(K T ) = ΔS°R + ΔH°R T (37)
ΔG°= ΔH°-TΔS°= -R × T × ln(K T ) (38)
K T = Q × b (39)
Where, T (K) is the absolute temperature in Kelvin, R (8.314 J.mol -1 .K -1 ) is the universal gas constant and KT is thermodynamic constant. The values of ΔS° and ΔH° can be geometrically calculated from the slope and intercept of the Van't Hoff plot of ln KT against 1/T.

The value of ΔH° is used to identify the nature of adsorption [196]. A positive value of ΔH° indicates that the reaction is endothermic which the negative value of ΔH° is realated to an exothermic reaction [197,198]. Moreover, a positive value of ΔS° indicates increased randomness of adsorbate molecules on the solid surface in comparson with the solution [199]. In addition, the adsorption nature can be classified as chemisorption or physisorption process when the values of ΔG° are in the ranges -80 to -400 KJ.mol -1 and 0 to -20 kJ.mol -1 , respectively [200].

Adsorbents

Adsorbents are solid materials that have affinities for attracting substances in solution (pollutants) onto their surface. The attraction of pollutants may proceed via electrostatic, ion-exchange, hydrogen, or π-π interactions to the active sites on the adsorbents. The affinity for easy interaction largely depends on the chemical properties, porosity, and surface area of the adsorbent [201]. Over the years, many adsorbents have been designed and used for removing heavy metal ions including polymeric materials, biomaterials, and carbonaceous materials [202,203]. It is clear that polymeric materials are the most widely considered substrates due to their commercial availability and their regenerating [204]. However, polymeric materials such as ion exchange resins are relatively expensive. Historically, thousands of research focused on the use of low-cost biomaterials such as cow bone [205], wood bark [206], bentonite clay [207], and so on. Despite the low-cost of these materials, they exhibited low adsorption capacity and took a long time to achieve equilibrium in heavy metal ions adsorption [208]. The advances in material engineering have assisted in preparing and developing various new adsorbents such as carbonaceous materials.

Carbonaceous materials

Nowadays, there is an increasing tendency to employ carbonaceous materials, such as activated carbon [209], and especially, carbon nanotubes [210] and graphene sheets [211], in many fields, including the removal of heavy metals from wastewater effluents, due to their unique physical, chemical and mechanical properties. They have good mechanical resistance, high electrical conductivity, and high thermal stability.

Besides, the high specific surface area of these materials combined with the various types of intermolecular interactions after the functionalization allow them to be effectively used in various adsorption systems.

Activated carbons

Powdered activated carbons were first produced commercially in Europe in the early 19th century, using wood as a raw material [START_REF] Mantell | Carbon and Graphite Handbook[END_REF]. Adsorbent based on activated carbons is widely used to remove heavy metal ions from wastewater due to their welldeveloped porous structure and a high internal surface area, as well as different surface functional groups, including carboxyl, carbonyl, phenol, quinone, and lactone [START_REF] Anirudhan | [END_REF]. Since coal-based activated carbons are expensive, their use has been limited nowadays, and further efforts have been made to convert cheap and abundant sources into activated carbon [START_REF] Anirudhan | [END_REF]. In this regard, activated carbons are now prepared from various agricultural [214][215][216][217].

For example, Karthikeyan et al., [214] removed Cr(VI) ions from aqueous solutions using activated carbon derived from rubber wood sawdust and the maximum adsorption capacity was as high as 0.846 mmol.g -1 at pH 2.0. In addition, the maximum adsorption capacity obtained in their work was higher than the one of other adsorbents such as coconut tree sawdust [218], coconut shell carbon [219],

sugarcane bagasse [220] and treated sawdust of Indian rose wood [221], with a maximum adsorption capacity of 0.069 mmol.g -1 , 0.258 mmol.g -1 and 0.192 mmol.g -1 ,

respectively.

The ability of activated carbon prepared from apricot stones for removing Cu(II), Co(II), Ni(II), Cd(II), Pb(II), Cr(III) and Cr(VI) from aqueous solutions by adsorption was investigated by Kobya et al., [222]. Apricot stones were carbonized and activated after treatment with H2SO4 at 200 °C for 24 h. The optimum pH for an efficient removal (99.99%) was 1.0 for Cr(VI), and it varied from 3.0 to 6.0 for other heavy metal ions with a removal efficiency ranging from 97.48% to 99.68%. The maximum adsorption capacity for the metal ions decreased in the following sequence:

Cr(VI)> Cd(II)> Co(II)> Cr(III)> Ni(II)> Cu(II)> Pb(II).
Another study was reported by Alslaibi et al., [223] for activated carbon from olive stone prepared via microwave irradiation (radiation power of 565 W) with KOH to remove heavy metal ions such as Cu(II), Ni(II), Fe(II), Pd(II), Zn(II), and Cd(II) from aqueous solutions using a batch process. The removal efficiency was found to be 98.55% for Cu(II), 95.32% for Cd(II), 98.19% for Ni(II), 98.83% for Pb(II), 99.32% for Fe(II), and 98.36% for Zn(II) at a metal ion concentration of 20 mg.L -1 in water at pH 5.0 for 3h with a stirring rate of 200 rpm. The kinetic adsorption described was a pseudo-second-order rate equation.

Carbon nanotubes

Carbon nanotubes are members of the fullerene family, which were discovered by Iijima in 1991 [223,224]. They are made of single or multi-walled long carbon cylinders in the form of rolled graphite sheets with a diameter from one to several tens of nanometers and a length of up to several centimeters (Figure 16). Carbon nanotubes possess extraordinary chemical stability, large specific surface area (150-1500 m 2 /g), excellent mechanical and electrical properties that make their efficient adsorbents for removing heavy metal ions from wastewater [224].

However, carbon nanotubes are hydrophobic which may not favor the adsorption of metal ions present in aqueous solution [224]. Nevertheless, one famous route to make carbon nanotubes hydrophilic consists in their oxidation [225]. For this purpose, carbon nanotubes oxide are usually obtained from natural carbon nanotubes by acid or air oxidation [226]. Carbon nanotubes oxide display a highly oxygenated surface with carboxyl, aldehyde, and hydroxyl groups. These functional groups provide a more number of active sites on the carbon nanotubes surface and enhanced their adsorption capacity. For example, Li et al., [227] studied the adsorption of Pb(II), Cu(II), and Cd(II) ions from aqueous solutions on single-walled carbon nanotubes (SWCNTs) oxidized with concentrated nitric acid (HNO3). The maximum adsorption capacities of Pb(II), Cu(II), and Cd(II) ions by oxidized carbon nanotubes, calculated from Langmuir isotherm, were 0.594, 0.375 and 0.097 mmol.g -1 , respectively.

Compared with chemically oxidized carbon nanotubes, SWCNTs oxide showed a higher adsorption capacity than MWCNTs due to the addition of functional groups containing oxygen to the carbon nanotubes.

Similarly, Wang et al., [228] demonstrated that the adsorption capacity of multiwalled carbon nanotubes (MWCNTs) treated with concentrated nitric acid (HNO3) can be significantly increased mainly due to oxygen functional groups created on the surface of acidified nanotubes that can react with Pb(II) ions to form a complex on the surface. The adsorption equilibrium time for the acidified nanotubes was found to be 20 min at pH 2.0, which is much less than for activated carbons (about 120 min).

Moreover, the functionalization of carbon nanotubes may enhance their properties and in particular their maximum adsorption capacity. They have been established that adsorbents containing nitrogen or sulfur chelating groups could be effective in adsorption of heavy metal ions. For example, Li et al., [229] reported that dithiocarbamate groups functionalized MWCNTs oxide prepared by ethylenediamine and carbon disulfide exhibit a very high adsorption capacity for Cd(II), Cu(II), and Zn(II) in aqueous solutions. The adsorption capacity of Cd(II), Cu(II) and Zn(II) were 0.259, 0.586 and 0.165 mmol.g -1 , respectively. Adsorption kinetic and isotherm processes for three metal ions were found to correlate with the pseudo-second-order model and Langmuir isotherm model, respectively.

Furthermore, Gibb's free energy (ΔG°) was -23.429, -27.957, and -27.197 KJ.mol - 1 for Cd(II), Cu(II) and Zn(II), respectively at 25 °C, which revealed that the adsorption processes including chemisorption and physisorption were spontaneous.

In another study, Zazouli et al., [230] demonstrated that acrylonitrile (AN) was successfully grafted onto multiwalled carbon nanotubes (MWCNTs) by plasma techniques, and then converted it to amidoxime by the reaction of cyano group with neutralized hydroxylamine hydrochloride (Scheme 7).

Scheme 7. Preparation of amidoximated MWCNTs [230].

Meanwhile, they reported that AO-g-MWCNTs were used to remove U(VI) ions from nuclear industrial effluents. The U(VI) ions removal capacity of AO-g-MWCNTs decreased with the decrease of pH and full equilibrium was reached in 4 hours. Kinetic and thermodynamic studies suggested the sorption process was fast, endothermic and spontaneous [230].

Graphene and graphene-based materials

Recently, graphene has become an interesting research topic due to its exceptional electrochemical, thermal and mechanical properties and its very high specific surface area (2600 m 2 .g -1 ). Graphene in pure form is seldom used as adsorbent [231]. A major drawback for water treatment is that it tends to agglomerate or restack to form graphite through π-π stacking and Van der Waals' interactions during processing.

Nevertheless, one famous route to make graphene hydrophilic consists in its oxidation [232]. For this purpose, graphene oxide (GO) is usually obtained from natural graphite by chemical oxidation following either the Hummers, Brodie

Staudnmaier or modified methods [233]. Oxygen-containing functionalities in the resulting GO sheets include carboxylic, carbonyl, hydroxyl and, epoxy functional groups that can alter Van der Waals' interactions leading to a range of solubilities in water and organic solvents [234]. In addition, GO has attracted special attention not only as a precursor for large-scale production of graphene but also for removing heavy metal ions from wastewater [235].

For exemple, Yang et al., [236] found that GO adsorbed Cr(III) effectively onto the surface of GO with a maximum adsorption capacity of 1.782 mmol.g -1 at an optimum pH of 5. In addition, the adsorption of chromium onto graphene oxide was found to be endothermic and the Langmuir model was applicable in describing the adsorption. Moreover, Gopalakrishnan et al., [237] proved that GO had much higher Cr(III) adsorption capacity (2.321 mmol.g -1 ) when comparing to carbon nanotubes (CNTs) (0.954 mmol.g -1 ). Indeed, the oxygen atoms of carboxyl acid groups are categorized as hard ligand groups while the adsorption capacities of an adsorbent depend on the nature of the functional groups and their affinity for pollutants such as heavy metal ions [238].

Moreover, the functionalization of GO may enhance its excellent properties and increase its maximum adsorption capacity [239]. One of the attractive ways for modifying GO is the use of multifunctional organic materials such as polymers and multidentate chelating ligands. For instance, Alimohammady et al., [240] modified graphene oxide sheets with 3-aminopyrazole (APL) for removing Cd(II), Hg(II), and As(III) ions (Scheme 8). Scheme 8. Schematic illustration for the preparation of GO-APL [240].

The maximum adsorption capacities were 2.541, 1.133, and 1.756 mmol.g -1 for Cd(II), Hg(II), and As(III) ions, respectively. Thermodynamic analysis revealed that the adsorption of Cd(II), Hg(II), and As(III) ions were spontaneous and endothermic.

Moreover, the adsorption kinetics could be well explained by the pseudo-second-order kinetic equation. In another study, Fang et al., [241] modified GO sheets with aromatic diazonium salt (ADS) for removing Co(II) ions from aqueous solutions, and they obtained a maximum adsorption capacity of 1.974 mmol.g -1 . The equilibrium adsorption data showed an excellent fit to the Langmuir isotherm model and it was exothermic in nature.

In addition, Polymeric adsorbents (such as polypyrrole, chitosan, etc.) improve both the physical and chemical properties of GO. For example, Li et al., [242] modified GO sheets with polypyrrole (PPY) for the removal of Cr(VI) ions from aqueous solution. They showed that the maximum adsorption capacities of GO-PPy were 9.212 mmol.g -1 for Cr(VI) ions which were higher than the GO one (⁓1.357 mmol.g -1 ). In addition, in the field of radioactive wastewater treatment associated with environmental remediation, a big challenge is to achieve the simultaneous elimination of toxic cations and anions metals. So, Huang et al., [243] functionalized GO with poly(ethyleneimine) (PEI) for the simultaneous removal of cationic U(VI) ions and anionic Re(VII) ions from aqueous solution. The maximum adsorption capacity for GO-PEI composites at pH 5.0 for U(VI) ions and at pH 3.5 for Re(VII) ions was 2,644 and 1,410 mmol.g -1 , respectively (Figure 17). Based on the Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS)

analyses, and density functional theory (DFT) calculations, the adsorption of U(VI)

ions was dominantly attributed to the coordination with abundant amino and oxygencontaining groups anchored on the hydrogel. In contrast, the removal of Re(VII) ions was correlated with the anion-exchange mechanism.

Chitosan (CS) is a basic polysaccharide polymer with active amine and carboxyl functional groups that are commonly combined with GO to form GO-CS composite, with excellent properties and enhanced adsorption capacity [244]. Kyzas et al., [245] studied the performance of two adsorbents, GO and GO-CS for the removal of Hg(II)

ions from aqueous solution. The maximum adsorption capacities were 0.932 and 1.899 mmol.g -1 for GO and CS-GO, respectively.

In another study, Zhang et al., [246] prepared GO-CS and further modified it with disodium ethylenediaminetetraacetate (EDT) as a chelating agent, for the adsorption of Cr(VI) ions. The maximum adsorption capacity of this composite was found to be 1.657 mmol.g -1 with a maximum removal capacity of 90% at the optimum pH 2.0. In addition, they showed that the adsorption capacity of GO-EDT-CS for Cr(VI) ions decreased by 5% after 7 times of reuse (Figure 18).

Figure 18. Regeneration study of GO-EDT-CS within seven cycles [246].

In order to separate the GO-based material from the aqueous solution, its magnetization with magnetic nanoparticles such as magnetite nanoparticles (Fe3O4NPs) followed by the use of an external magnetic field can be a suitable pathway [247]. Indeed, it is will known that magnetite nanoparticles (Fe3O4NPs) can be combined with GO sheets by various methods such as co-precipitation, solvothermal, and ionothermal [248]. The co-precipitation method is the most used technique to synthesize Fe3O4NPs because of its eco-friendly procedure [249].

For instance, Wang et al., [250] prepared a magnetic-GO modified chitosan composite (GO-CS-Fe3O4NPs) through a facile and fast process for the adsorption of Pb(II) ions. The modified adsorbent exhibited an adsorption capacity of 0.749 mmol.g - 1 , retaining 90% of its capacity after five cycles.

In another study, Liu et al., [251] prepared a magnetite nanoparticles@GOsupported (3-mercaptopropyl)trimethoxysilane (MPTES) composite (GO@Fe3O4NPs-MPTES) and studied the effect of single and multi-organic acid ligands on the adsorption of Cd(II) ions. The maximum adsorption capacity of (GO@Fe3O4NPs-MPTES) for Cd(II) ions obtained from the Langmuir isotherm was 1.112 mmol.g -1 . The high-performance of the adsorbent toward Cd(II) ions was attributed to the synergistic effects of thiol groups, Fe3O4NPs and graphene sheets (Scheme 9). Scheme 9. Proposed adsorption mechanism of Cd(II) ions onto GO@Fe3O4NPs-MPTES [251].

GO-CS-Fe3O4NPs

Pb(II) 0.749 6.0 Pseudo-Second-order Langmuir-Freundlich [250] GO@Fe3O4NPs-MPTES Cd(II) 1.112 5.0 -Langmuir [251] MWCNTs-EDA-CD: Oxide prepared by ethylenediamine and carbon disulfide, GO: graphene oxide, GO-APL: Graphene oxide sheets with 3-aminopyrazole, GO-PEI: Graphene oxide with poly(ethlenimine), GO-CS-Fe3O4NPs: Graphene oxide modified chitosan composite, GO@Fe3O4NPsMPTES: magnetic-GO-supported-(3-mercaptoproyl)trimethoxysilane

Conclusion

This study was provided in-depth information on the sources and health effects associated with the exposure of heavy metals to the human body, and the environment. Effective removal of toxic heavy metal ions from aqua sources is important for the environment and public health protection. Numerous technologies have been developed to remove heavy metal ions aqueous solutions, such as adsorption, ion exchange, precipitation, membrane separation, etc. Among these, adsorption was considered relatively superior because it is versatile and widely used, very efficient because of its high removal capacity, inexpensive, and applicable at very low concentrations. Despite the important role of adsorption in water treatment, a low regeneration rate is one of the major restrictive obstacles in adsorption processes.

The use of carbonaceous materials such as activated carbon and especially, carbon nanotubes and graphene sheets makes the adsorption process more environmentally friendly. Their performance for the removal of heavy metals from aqueous solutions has been widely reported. With the advent of nanotechnology, graphene oxide-based materials have been studied intensively for their potential applications in the removal of heavy metals. In addition, the kinetic, isotherm, and thermodynamic studies have highlighted in accessing the quality of fits and adsorption performance because of their wide applicability in a variety of adsorption data.

Supported catalysis for organic chemical reactions

The term ''catalysis'' was coined in 1835 by the Swedish chemist Berzelius, but a suitable definition was introduced only many years later by Ostwald who wrote in 1894: ''Catalysis is the acceleration of a slow chemical process by the presence of a foreign material'' [252]. According to IUPAC (1976), catalysis is the process where a substance (catalyst) increases the rate of a reaction without modifying the overall standard Gibbs energy (ΔG) [START_REF]Principles and Practice of Heterogeneous Catalysis[END_REF]. Today catalysts play an important role in the main technologies for the development of the world economy, including petroleum refinery, energy production, chemical production, as well as the fine chemicals and pharmaceutical industry. Catalysts are also essential to environmental protections, reducing emissions of modern cars, monitoring and removing pollutants from the air, soil or water, etc [START_REF] Kimura | [END_REF].

Catalysts Classification

The classification of catalysts falls into three main groups as homogeneous, biological, and heterogeneous as shown in Figure 17.

3.1.1.

Homogenous catalysts Homogeneous catalysis usually uses molecular catalysts such as organometallic compounds (Transition metal ions with ligands coordinated), acid/base molecules, and salts [START_REF] Beller | Transition Metals for Organic Synthesis[END_REF]. In a homogeneous catalytic system, the reactant and active centers of catalysts are present in the same phase (generally liquid) and, due to easy accessibility of active centers to the reactant it results in high activity performance. The homogeneous catalytic system also has other advantages such as high selectivity, etc. [START_REF] Bhaduri | Homogeneous Catalysis: Mechanisms, Industrial Applications[END_REF].

Although the large numbers of homogeneous catalysts are widely applied in industries, these systems are associated with some drawbacks such as low stability, difficulties in isolation and separation of the final product from reaction mixture and recycling of priced noble metals or expensive ligands, and thus making the overall process complicated [START_REF] Shende | [END_REF]. The complete recovery of the transition metal catalysts from the reaction mixture is challenging as catalyst remains in the overall product (at ppm or even at ppb level). In the drug and pharmaceutical industry, it is important to remove completely the leached metal catalyst from the final purified product as it can lead to serious metal contamination issue thereby affecting the human health [258].

3.1.2.

Biocatalysts Bio-catalysis underpins some of the oldest chemical transformations known to humans. Bio-catalysis is the use of natural catalysts, such as protein enzymes to perform chemical transformations on organic compounds [259]. The active site of a typical enzyme contains amino acids, known as catalytic groups, which simultaneously participate in localizing, binding, and catalyzing transformations of reactant (substrate) molecules [260]. Enzymatic catalysis is formally a type of homogeneous catalysis because it is molecular, single-site, and not based on physical properties.

Although biocatalysts are often highly active and extremely selective, there are still drawbacks associated with bio-catalysis. The major weakness of biocatalysts is their very limited range of stability with respect to temperature, solvents, pH value, ionic strength, and salt type [261]. In addition, they are not usually commercially available which hinders the diffusion of this technology.

3.1.3.

Heterogeneous catalysts For heterogeneous catalysis, the catalyst and the reactants are in different phases.

During the reaction, the active sites of the solid surface are available for the reactant molecules to adsorb them and to take part in the reactions [262]. Heterogeneous catalysis offers advantages such as ease of separation and recycling of catalysts, continuous operations, and easy purification of products, but heterogeneous catalysts are more complex, so that their active sites and relevant mechanisms are not well understood in most cases [263]. One of the straightforward ways is to immobilize homogeneous catalysts on solid supports to take the advantages of both homogeneous and heterogeneous catalysis. By now, the immobilization of homogeneous catalysts or biocatalysts onto a substrate is one of the most effective and convenient way for the preparation of heterogeneous catalysts [START_REF] Sheldon | Fine Chemicals Through Heterogeneous Catalysis[END_REF].

Carbon nanomaterials including Carbon Black, carbon nanotubes (CNTs), graphene, and their derivatives are important components of many synthetic heterogeneous catalysts. They have been used as effective catalysts or as supports of other catalysts [START_REF] Compton | [END_REF]. Among the carbon materials described above, graphene and their derivatives such as graphene oxide (GO), reduced graphene oxide (rGO) have recently attracted the most intensive attention. Indeed, graphene-based materials have been widely explored as catalysts for chemical, photochemical, electrochemical, or optical reactions, or as carbonaceous supports for loading catalysts such as metals, metal oxides, and enzymes [266].

Graphene-based materials as catalysts

The performance of a catalyst is influenced by the concentration, the nature, and the accessibility of the active sites that are capable of chemisorbing the reactants and form surface intermediates [267]. Unfunctionalized graphene may not have a sufficient number of reactive sites to be a viable catalyst for many reactions, while functional group-rich materials, such as GO, exhibit high a reactivity than unfunctionalized ones, under mild conditions [268]. In addition, GO is moderately acidic (pH = 4.5 at 0.1 mg.mL -1 ) and powerful oxidant, which makes it particularly attractive for catalytic applications [269].

The oxidation of various functional groups is probably the most important routine operation in organic synthesis. Accordingly, a wide range of oxidants and catalysts are available for selective transformations. In particular, the strongly oxidizing properties of GO have been long recognized. For example, Dreyer et al., [270] have identified this material as a powerful catalyst to be used in the oxidation of benzylic and aliphatic alcohols to their respective ketones and aldehydes in the absence of an inert atmosphere. Thus, when neat benzyl alcohol (1) was heated to 100 °C in the presence of GO (200 mg), benzaldehyde (2) was selectively formed in >98% yield (Scheme 10).

Scheme 10. Oxidation of benzyl alcohol into benzaldehyde in the presence of GO [270].

Similarly, Mirza-Aghayan et al., [271] have investigated the oxidation of aromatic, heterocyclic, and aliphatic alcohols to their respective aldehydes and ketones using GO sheets under ultrasonic irradiation. Given the propensity of GO sheets to oxidize various alcohols, Jia et al., [272] examined the ability of GO sheets to oxidize cis-stilbene (3) to benzil (4), a transformation that is similar to Wacker oxidation. Under optimal conditions, the benzil (4) was isolated in 68% yield (Scheme 11).

Scheme 11. Oxidatgion of Cis-stilbene into benzil in presence of GO [272].

Furthermore, GO sheets have been used as a strong oxidant for the oxidative aromatization of 1,4-dihydro pyridines (5) into their corresponding pyridine derivatives (6) in excellent yields (Scheme 12). The reaction was performed in toluene at 100 °C [273].

Scheme 12. Aromatization of 1,4-dihdro pyridines into pyridines in presence of GO [273].

The multi-component Strecker reaction of carbonyl compounds (aldehydes and ketones) was reported using GO as a heterogeneous catalyst in neat and open-air conditions (Scheme 13). In this research, GO sheets (50 mg) was added to a mixture of ketones or aldehydes (7), amine (8) and trimethylsilyl cyanide (TMSCN) (9) at 50 °C under solvent-free conditions to produce α-aminonitriles (10). The desired products (10) were obtained in 50-97% yield within 1-24 h. Under these experimental conditions, different types of amines (8) including aromatic and heterocyclic units and aldehydes were readily coupled [274].

Scheme 13. One-pot Strcker reaction of carbonyl and amine compounds using GO [274].

Moreover, Bhattacharya et al., [275] reported an efficient synthesis of functionalized 1,4-benzothiazines (13) from the reaction of 2-aminothiophenol (11) and 1,3-dicarbonyl compound (12) derivatives in the presence of 25 mg of GO at room temperature under solvent-free conditions (Scheme 14). Scheme 14. One-pot synthesis of α-aminophosphonates using GO [275].

They optimized the reaction conditions for the reaction of 2-aminothiophenol with acetylacetone, and the desired product was obtained in 88% yield within 8 h. The synthesized compounds (13) exhibited biological properties such as anti-HIV, antifungal, antimalarial, and antioxidant activities. In addition, Khodabakhshi et al., [276] reported a green synthesis of biscoumarins (16) in water using GO sheets by the treatment of different aromatic aldehydes (14) with 4-hydroxycoumarin (15) in the presence of GO sheets in EtOH/H2O under reflux conditions for 10-25 min, with 90-95% yields (Scheme 15). Scheme 15. Synthesis of biscoumarins using GO nanosheets [276].

Moreover, it was demonstrated that aromatic aldehydes (14) with both electronwithdrawing and electron-donating groups have a crucial effect on the product yields and reaction time. Khalili et al., [277] reported a new methodology for the one-pot synthesis of 2-amino-3-cyanopyridines (21) using GO (12 mg) as a reusable and heterogeneous catalyst (Scheme 16). Reactions were performed by the multicomponent reaction of ketones (17), aldehydes (18), ammonium acetate (64), and malononitrile (19) in the presence of 10 mg of GO sheets at 80 °C in water.

Scheme 16. Synthesis of 2-amino-3-cyanaopyridines using GO in water [277].

The corresponding products (21) were produced in 75-97% yields within 5 h.

Interestingly, GO sheets showed a good reusability and a recovery for six catalytic cycles.

The Friedel-Crafts reactions are one of the most important Lewis acid-mediated reactions [START_REF] Heaney | Comprehensive Organic Synthesis[END_REF]. The Friedel-Crafts addition of indoles (22) to α,β-unsaturated ketones and nitro styrenes (23) in the presence of heterogeneous GO catalyst (20 mg) was described by Rao et al., [START_REF] Kumar | [END_REF] (Scheme 17).

Scheme 17.

Friedel-Crafts addition of indoles [START_REF] Kumar | [END_REF] The desired products (24) were obtained in 92-96% yields within 2.5-36 h. The catalyst could be easily recovered and recycled up to five times without loss of activity.

In another work, rGO obtained by a GO reduction using hydrazine has been used as a catalyst for the synthesis of 5-substituted 1H-tetrazoles by a [2 + 3] cycloaddition of nitriles in presence of sodium azide (26) and DMF, as solvent (Scheme 18) [280]. In general, the reaction smoothly occurred for the formation of the corresponding 5-substituted 1H-tetrazoles derivatives (27) in 70-89% yields.

Scheme 18. Synthesis of 5-Substituted 1H-Terazoles from Benzonitrile and sodium azide with rGO as catalyst [280].

Metal supported graphene-based materials as catalyst

The formation of new carbon-carbon bonds is of great importance in organic chemistry and a prerequisite for all life on earth. Cross-coupling reactions, such as Mizoroki-Heck and Suzuki-Miyaura, have revolutionized the chemistry through the bonds between carbon atoms. These coupling reactions have been typically performed under homogeneous conditions using Pd catalysts [281,282]. Unfortunately, catalyst recovery and recyclability remain a challenge, with several attempts being made to overcome this difficulty. Recently, graphene and graphite oxide (GO) have been considered as potential supports for Pd-catalyzed C-C coupling reactions.

For example, Bannwarth et al., [283] extended the scope of application of GO@ Pd(II), previously synthesized in their research lab, to Mizoroki-Heck coupling. In this way, the reaction of butyl acrylate (28) with (29) in the presence of 2mg of GO@Pd(II) at 140 °C in N-Methyl-2-Pyrrolidone (NMP), yielded the corresponding coupling products (30) in 73-93% yields together with >99% of (E) selectivities (Scheme 19).

Scheme 19. GO@Pd(II) mediated Mizoroki-Heck coupling [283] In a similar work, Bannwarth et al., [283] explored the versatility of GO@Pd(II) catalysts for Sonogashira coupling. Accordingly, the reaction of aryl iodides (31) and phenyl acetylene (32) in the presence of 6 mg of GO@Pd(II) gave the corresponding coupling products (33) (Scheme 20).

Scheme 20. GO@Pd(II) mediated Sonogashira coupling [283] In another work, Moussa et al., [284] reported the synthesis of palladium nanoparticles (PdNPs) catalysts, supported on partially RGO sheets (PRGO@PdNPs), by pulsed laser irradiation without using any capping agent as well as chemical reduction. The Sonogashira coupling reactions of iodobenzene (34) with phenyl acetylene (35), and potassium carbonate in H2O/EtOH solvents were investigated in the presence of 3 mg of (PRGO@PdNPs), under microwave at 180 °C for 10 min (Scheme 21). The heterogeneous (PRGO@PdNPs) catalysts showed a high catalytic activity with 100% conversion.

Scheme 21. Heck and Sonogashira coupling reraction using the PRGO@PdNPs catalyst [284] Arylated imidazo [1,2-a] pyridine derivatives are an important group of heterocyclic systems and there are found in numerous natural products, biologically significant molecules and functional materials. These heterocyclic skeletons are found in various drugs, bioactive scaffolds, and other functional materials or therapeutic agents [285].

Kilic et al., [286] reported the use of a monodisperse Ni@Pd core@shell nanoparticles supported onto rGO. The nanocomposite (rGO@Ni@PdNPs) (15 mg) was utilized as a highly efficient and reusable heterogeneous catalyst for the C-H In another work, Lu et al., [287] reported the synthesis of Cu@Cu2O nanoparticles catalysts supported on reduced graphene oxide (RGO) sheets (Cu@Cu2ONPs@RGO) for Glaser homo-coupling reaction (Scheme 23).

activation of imidazo[1,2-a]pyridine (
Scheme 23. Homo-coupling reaction using Cu@Cu2ONPs@RGO.

The homo-coupling reactions were investigated in the presence of 1 mmol of phenylacetylenes which contain electron-donating groups and electron-withdrawing groups (40), 4 mg of Cu@Cu2ONPs@RGO and 1.2 mmol of Cs2CO3 in EtOH at 80 °C under O2 for 8 h. The corresponding products (41) were produced in 99-53% yields.

Moreover, Huo et al., [288] used GO@Fe3O4NPs hybrid nanostructures for the Furthermore, this catalyst could be reused up to 8 times with essentially no loss of activity.

Scheme 24. A3-coupling reraction using the GO@Fe3O4NPs catalyst.

In addition, heterogeneous photocatalysis is a suitable way to use solar light as a renewable source of energy for photocatalytic degradation of insecticides, organic dyes, and other carcinogenic chemicals [289]. For example, several inorganic nanoparticles such as Ag, Cu, CuO, ZnO, TiO2, BiOCl, Bi2O3/BiOCl, CdS, CdO, SnO2, ZrO2, ZnWO4, NiFe2O4, MnFe2O4, ZnO@ZnWO4, zero valent metals (Fe 0 , Cu 0 , Al 0 ), bimetallic nanoparticles (Pd@Fe), etc. have been proposed for photocatalytic water disinfection [290][291][292]. In a photocatalytic system, a reaction takes place at the surface of the catalyst. The generation of an electron-hole pair plays a significant role in the mechanism of a photocatalytic reaction. When a photocatalyst is exposed by a light stronger than its band gap energy, electron-hole pairs diffuse out of the surface of the photocatalyst and participate in a chemical reaction. Here, the valence band (VB)

electrons (e -) of the photocatalyst is excited and transferred to the conduction band (CB), creating holes (h + ) in the valance band (VB) [293]. Those free electrons and holes transform the surrounding oxygen or water molecules into hydroxyuracil (OHU) free radicals with super-strong oxidation [294]. These free radicals are then used to decompose the organic pollutant into carbon dioxide and water [295]. A highly efficient visible light photocatalysis should have a high quantum efficiency resulting from low recombination of the photogenerated electron-hole pair and a wide light response range because of the narrow band gap [293]. Because of the excellent mechanical strength, low density, high catalytic activity, high surface area, and superior electron-transporting properties, graphene oxide (GO) can be exploited as a an efficient electron acceptor to boost the photoinduced charge transfer for amended photocatalytic activity (Figure 18) [296]. TiO2NPs anchored to Graphene oxide (GO) is one of the most common nanocomposites to be employed as a photocatalyst, because TiO2NPs is well known for photocatalytic decontamination.

For example, Liu et al., [297] reported the use of GO@TiO2 NPs for the degradation of methylene blue under UV light irradiation. They observed an effective reduction in charge recombination because of improved contact between graphene and TiO2NPs, increasing the photocatalytic activity. In addition, GO@SnO2NPs, and GO@ZnONPs have also been used as efficient photocatalysts for decomposition of different pollutants such as pesticides and dyes in water [297,298].

In

Conclusion

This section has highlighted the catalytic methods developed for the organic reactions using graphene-supported catalysts. The application of these catalysts serves as a versatile tool in the synthesis of different organic compounds via different organic reactions. As indicated, a variety of organic reactions such as oxidation, Strecker, Friedel-Crafts, and cycloaddition as well as the synthesis of various organic compounds have been considered in the presence of GO. Moreover, it was shown that the presented catalytic systems gain special interest as recoverable and reusable catalytic systems in the organic reactions. Notably, in some organic reactions, the suggested catalysts formed a new bond between carbon and carbon or heteroatom under metal-free conditions. Moreover, the study of the mentioned reactions with different substrate scope could be of interest. In the years ahead, the use of carbocatalysts as high stability support is expected to grow rapidly. Besides, the investigation of other functionalities supported on GO sheets can be some of the future of the carbocatalysts based on graphene sheets.

Conclusion of the literature study

Graphene has attracted the interest of chemists, physicists, and materials scientists due to its extraordinary structural, mechanical, and electronic properties.

Evidently, despite having several intriguing properties, the potential of graphene for various applications is limited, due to difficulties encountered during physical handling. Numerous attempts have been made to stabilize graphene through oxidation/reduction processes. Yet, materials prepared through this way usually present many defects like the disruption of the conjugated П-system leading to inferior physical and mechanical properties. Currently, numerous works have been conducted

to modify/functionalize the surface of graphene oxide sheets. The two most common methods adapted toward this purpose involved covalent and non-covalent chemical modifications... In addition, the loading of metal and metal oxide nanoparticles onto graphene sheets or its derivatives are generally performed through the use of both insitu and ex-situ methods which produce a series of novel materials with highly specific functionalities and enormous potential for targeted applications. Several studies have shown the great potential of modified graphene oxide (GO) sheets for removing heavy metal ions from wastewater. Kinetic, isotherm and thermodynamic studies have been frequently used in accessing the quality of fits and adsorption performance because of their wide applicability in a variety of adsorption data. In addition, the adsorption process is a more versatile and widely used method for the treatment of wastewater due to its simplicity, insensitivity, easy operation, high efficiency, and large industrial processing capacity. The last section evidenced the possibility to use graphene-based materials as heterogeneous catalysts for chemical and photochemical reactions. This growing interest in graphene materials, in particular, GO and its composites are mainly due to their valuable acidic properties combined with specific surface area, catalytic supports, adsorption capacities, benign environmental characteristics, and reusability that offer the design and development of countless combinations of composite materials for catalysis. The forthcoming chapters will deal mostly with functionalization strategies to develop functionalized graphene oxide for the removal of heavy metal ions from aqueous solutions and displaying catalytic activities.

Characterization and analytical techniques

Nuclear Magnetic Resonance (NMR)

Nuclear magnetic resonance (NMR) spectrometry is a non-destructive technique generally used to predict the structure of an organic compound. The physical, chemical and structural properties of a molecule can be determined based on the chemical shifts of the sample. 1 H NMR (600 MHz) and 13 C NMR (150 MHz) spectra were recorded at Bruker AVANCE II (Instrument FEI, USA) spectrometer at 600 MHz using a 5 mm BBFO. All chemical shifts were reported as δ values (ppm) relative to the residual nondeuterated solvent.

Mass spectrometry (MS)

Mass spectrometry was used to determine the molecular mass of compounds, to detect fragmented species and possible sites of fragmentation and identify analytes based on their mass-to-charge (m/z) ratio. This technique employs the bombardment of the atoms in a molecule with electrons to obtain highly energetic positively and negatively charged ions (molecular ion). The molecular ion is fragmented and separated based on the m/z ratio of the fragments and these are detected in proportion to their abundance. The molecular mass of the compound represents the molecular ion, which appears at the highet value of m/z ratio. EI-MS measurements were determined with an Agilent 5975N mass spectrometer.

Fourier-Transform Infra-Red Spectroscopy (FTIR)

Fourier-Transform Infra-Red Spectroscopy (FTIR) was a largely used technique to identify the functional groups in the materials (qualitative analysis) by using the beam of infrared radiations. FTIR spectra of the dried samples were recorded on a Nicolet FTIR 460 spectrometer (Thermo Nicolet Corporation, USA) using the KBr pellets.

Specimens for the measurements were prepared by mixing 1 mg of the sample powder with 150 mg of KBr and by pressing the mixture into pellets. The FTIR spectra were obtained at a resolution of 6.0 cm -1 at room temperature in a wavenumber range between 4000 and 400 cm -1 and averaged over 64 scans.

X-Ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is also known as electron spectroscopy for chemical analysis (ESCA). XPS is a surface characterization technique that utilizes

Vibrating Sample Magnetometer (VSM)

Vibrating Sample Magnetometer (VSM) systems were used to measure the magnetic properties of materials as a function of magnetic field, temperature, and time.

Magnetic at 300 K on powdered samples packed in polycarbonate capsules using a Quantum Design MPMS-XL SQUID magnetometer. The magnetic properties measurements were performed by a vibrating sample magnetometer (VSM, Lake Shore, 7304, France) at 300 K and in a magnetic field varying from -60000 to +60000 (Oe).

The dependence of magnetization on the magnetic field and temperature were recorded in zerofield-cooled (ZFC) and field cooled (FC) regimes. Additionally, FC and ZFC measurements were performed in the temperature range of 10-300 K.

Scanning Electron Microscopy (SEM)

The scanning electron microscopy (SEM) technique was used worldwide in many disciplines. It can be regarded as an effective technique to adopt the production of images on a nanometer (nm) to micrometer (µm) scale by focusing a beam of electrons onto the solid materials. Hence, generating information on the surface morphology and topography of the materials. Scanning electron microscope (SEM) images were taken by a commercial 250 FEI Company, Netherlands. The powder was deposited onto carbon tape and observed with an accelerating voltage of 15 Kv. Composities were fractured in liquid nitrogen and deposited on carbon tape and the observation was made at 15kv.

UV-visible spectroscopy (UV-Vis)

UV-vis spectroscopy is a physical technique of the optical spectroscopy that uses light in the visible, ultraviolet, and near-infrared ranges and it is based on Beer-Lambert law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species in the solution and path length. UV-vis experiments were recorded using a Perkin Elmer Lambda UV-Vis 950 spectrophotometer in quartz cuvettes with an optical path of 10 mm. The adsorption specctra was recorded in the 200-900 nm range with a scan speed of 240 nm/min.

Themogravimetric analysis (TGA)

Thermogravimetric analysis or thermal gravimetric analysis (TGA) is used for the measurement of changes in the physical and chemical properties of materials whilst increasing the temperature constantly. TGA experiments were analyzed using thermogravimetry on a Perkin Elmer thermal analyzer instrument (Nicolet, TA SDT Q600, USA). The heating rate of 5 °C.min -1 was used and the atmosphere in the TGA was nitrogen. The amount 10-15 mg of ground sample was placed directly into the crucible for TGA testing.

Experimental procedures

Synthesis of N,N-Bis (2-pyridymethyl)ethylenediamine (BPED)

The synthesis of N,N-bis(2-pyridylmethyl)ethylenediamine (BPEN) was arranged in scheme 1.

Scheme 1. General synthetic route for the target compound BPED.

Synthesis of N-(2-aminoethyl)carbamic acid tert-butyl ester (1)

Compound 1 was prepared according to the literature [1]. In a typical experiment, Di-tert-butyl dicarbonate (DTBT, 3.05g, 14mmol) was dissolved in 10mL of dichloromethane (DCM) and slowly added to a solution of ethylenediamine (EDA, dimethyl sulfoxide (DMSO, 5mL) and compound (1) (1.60g, 10 mmol) were added and stirred at 80°C for 24h, following the experimental procedure described in the literature [2]. Then, the oily residue was dissolved in dichloromethane and washed five-times with a saturated aqueous solution of sodium chloride (NaCl). The organic layer was dried over anhydrous sodium sulfate (Na2SO4) and the solvent was removed to yield a yellow viscous oil, which was purified by silica gel column chromatography

Preparation of graphene oxide (GO)

Hummers and Hoffman, in 1958, [3] proposed one of the most widely used methods for the synthesis of graphene oxide (GO), which consists of the oxidation of graphite to graphite oxide and subsequently an exfoliation to GO [4][5][6][7], as shown in GO was prepared from graphite sheets (4 µm in size, 99.99% purity; provided by TIMCAL Graphite and Carbon, France) using the improved Hummers method reported in the literature and taking into account changes that have been suggested by several authors to perform a procedure with good performance and laboratory safety, in which the main process consists of oxidation. 7.00 g of natural graphite powder were added into an optimax glass reactor with 140 mL of sulfuric acid (H2SO4) and 3.00 g of sodium nitrate (Na2NO3). The mixture was maintained at 0 °C for 1 h under mechanical stirring. Then, 18.22 g of potassium permanganate (KMnO4) were added slowly. The solution was then heated to 35 °C for 30 min and 200 mL of distilled water were added dropwise.

After heating at 90 °C for 15 min the reaction was quenched by a quick addition of 750 mL of distilled water and 66.23 mL of hydrogen peroxide (H2O2). The brown suspension was then repeatedly washed with 5 % hydrochloric acid (HCl) solution and distilled water until a neutral pH was reached. The dark brown product was then dried under vacuum at 80 °C for 16 h.

Grafting of ethylenediamine onto GO: (GO-EDA)

1 g of graphite oxide (GO) was sonicated for 1 h in 100 mL of N,N-dimethylformamide (DMF) in a Schlenk-type reactor using an ultrasonic bath. Then, 150 mL of ethylenediamine (EDA) and 10 mg of HATU were added and the grafting reactions was carried out under constant stirring and heating at 60 °C for 6 h. At the end of the reaction, ethylenediamine grafted GO (GO-ED) was centrifugated and further purified by centrifugation with EtOH as solvent.

Grafting of chloroacetyl chloride onto GO-EDA: (GO-EDA-CAC)

1 g of GO-EDA was sonicated in 100 mL of anhydrous DMF and Et3N (3 mL) was added. Afterwards, 20 mL of chloroacetyl chloride was placed in around bottom flask and heated at 70 °C for 72 h. At the end of the reaction, the product was collected by centrifugation and washed with diethyl ether (Et2O) several times and finally during under vacuum at 70 °C overnight.

Grafting of BPED onto GO-EDA-CAC: (GO-EDA-CAC-BPED)

N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) was grafted onto the GO-EDA-CAC according to the experimental conditions described in the literature [8] and in a typical experiment, 1.00 g of GO-EDA-CAC was dispersed in 30 mL anhydrous DMF by ultrasonication for 30 min followed by the addition of Et3N (4 mL). Then, BPED (3.63 g, 15mmol) was added in the mixture and stirred under reflux at 80 °C for 24h.

After the reaction, the solution was cooled to room temperature, centrifugated, and washed with ethanol to remove the excess of BPED. This purification step was monitored by UV-Visible analysis until there is no remaining free BPED in the mixture (Figure 8). Finally, GO-EDA-CAC was dried under vacuum at 50 °C overnight.

Absorbance (%)

Wavelength (nm) 2.6. Preparation of the (GO-EDA-CAC)@Fe3O4NPs material 1.50 g of FeCl3.6H2O and 0.75 g of FeCl2.4H2O were added to a GO-EDA-CAC dispersion in 10 mL of deionized water followed by a sonication at 60 C for 1 h. Subsequently, the pH of the corresponding aqueous suspension was adjusted to 10-11 by dropping 5.46 g of NH4OH [9]. At the end of the reaction, the (GO-EDA-CAC)@Fe3O4NPs material was separated from the aqueous solution with the help of a magnet and washed repeatedly with deionized water and ethanol and finally dried under vacuum at 80 C overnight.

Grafting of BPED onto the (GO-EDA-CAC)@Fe3O4NPs substrate

1 g of GOEDA-CAC@Fe3O4NPs was dispersed in 40 mL anhydrous DMF and sonicated for 30 min. Then, 3.63 g of BPED and 3.95 g of Et3N were added in the suspension and stirred at 80 °C for 24 h. At the end of the reaction, the suspension was centrifugated and washed with ethanol to remove the free BPED molecules. The latter purification step was monitored by UV-Visible analysis (Figure 9). Finally, the powder was dried under vacuum at 80 °C overnight.

Grafting of BPED onto GO: (GO-BPED)

1.00 g of GO was sonicated in 30 mL of anhydrous DMF for 30 min followed by the addition of 10 mg of HATU and 3.63 g (15 mmol) of BPED. Then, the reaction mixture was stirred at 70 °C for 24 h. At the end of the reaction, the GO-BPED material was isolated by centrifugation and further purified by using a washing procedure in presence of ethanol as solvent for removing the free BPED molecules. This washing procedure was repeated until complete removal of the non-reacted BPED molecules as checked by Uv-Visible analysis of the supernatant solution. (Figure 10). Finally, the recovered BPED grafted GO was dried under vacuum at 80 °C overnight.

Grafting of 1,3-propanesultone onto GO-BPED

1 g of GO-BPED was sonicated in 30 mL of ethanol for 30 min followed by the addition of 2 g of 1,3-propanesultone. Then, the corresponding suspension was stirred at 60 °C for 3h under a nitrogen atmosphere and the resulting GO-BPED-PS material was isolated by centrifugation and purified as described above.

Removal of metal ions using batch method

The effect of the functionalization of GO sheets on their metal ion adsorption capacities was studied by using GO-based adsorbents to remove Cu(II), Ni(II) and Co(II) metal ions in aqueous solutions.

Zero point Charge (pHZPC)

The zero point charge is the pH at which the electrical charge density of an adsorbent is equal to zero. In adsorption studies, thi concept provides information to the ability of an adsorbent to remove either a cation or an anion from aqueous solutions. The Zero point charge (pHZPC) for the GO-based adsorbents was evaluated by preparing 5x50 mL of 0.01 M NaCl aqueous solutions with pH varying from 2 to 10 by adding either a volume of an aqueous solution of HCl 0.1M or NaOH O.1M. Then, 0.50 g of the adsorbent was added into the corresponding solutions and stirred for 48h.

The initial and the final pH were measured using a pH meter.

Heavy metal ions adsorption experiments

The adsorption behavior of the GO-based adsorbents towards the different metal ions (Cu(II), Ni(II) and Co(II)) were studied in details. Indeed, the effect of pH (3.0-8.0), contact time (0-24h)), initial metal ion concentration (50-500 mg.L -1 )) on metal ion adsorption were invertigated. The effect of pH on the different metal ion (Cu(II), Ni(II) and Co(II)) adsorption was studied in the range 4-8 and adjusted with a Clark-Lub's buffer solutions [10]. In a typical experiment, 100 mg of adsorbent and 100 mL of a metal ion concentration of 250 mg.L -1 in water were stirred at room temperature for 24 h to achieve adsorption equilibrium.

Then, the concentration of metal ions in the solution before and after adsorption by the GO-based adsorbents was measured by UV-Vis spectrometry using calibration curve (Table 1). of the metal ion solution within the range of 50-500 mg.L -1 . In addition, all the isotherm experiments were performed in the temperature range 293 K-353 K and the adsorption thermodynamic parameters were calculated.

The equilibrium adsorption capacity (Qe; mmol.g -1 ) was calculated with the following equation (Eq. 1):

Q e = 𝑉(𝐶 0 -𝐶 𝑒 ) 𝑚 (𝟏)
Where C0 and Ce are the initial and equilibrium concentrations (mmol.L -1 ) of the metal ions of each remaining solutions, respectively. V is the volume (mL) of the heavy metal ion solution and m (mg) is the mass of adsorbent.

Effect of other ions on uptake of heavy metal ions

100 mg of adsorbent and 100 mg of competing species such as KCl, NH4Cl, NaCl, CaCl2, and MgSO4, were added in 100 mL of an aqueous solution of metal ions (250 mg. L -1 ) at a pH of 7.0, 293 K and for a contact time of 6 h. Then, the reaction mixture was filtered off and the concentrations of (Ni(II) and Co(II)) metal ions in the supernatant were calculated as described in section 2.5. The experiments were repeated three times and the average values were computed.

Regeneration and recycling studies of adsorbents

After the equilibrium study, the metal-loaded adsorbent was collected from the suspension by centrifuging, washed with deionized water several times, and finally dried under vacuum at 60 °C. Then, 0.1 g of each adsorbent loaded with heavy metal ions was placed in a series of glass vials containing 10 mL of different eluents (0.2 M EDTA, 0.2 M HCl, and 0.2 M HNO3) and the mixture was agitated at 20 °C for 10 h.

After filtration, the remaining metal ions concentrations in the filtrates were measured by UV-Vis spectrometry using the calibration curve and the adsorbents were then used for repeated adsorption-desorption cycles as described above to study their recyclability. In a typical reaction, the terminal alkyne (1.00 mmol) and Na2CO3 (1.20 mmol) were dissolved in 2 mL of glycerol. Then, the supported copper catalyst [GO-EDA-CAC@Fe3O4NPs]-BPED-Cu(II)] (10 mg) was added in the mixture and stirred at 80 °C for 30 min to 320 min. The reaction was monitored by TLC using EtOAc and n-hexane

Exprimental procedures for organic chemical reactions in

(1:4) as eluent and gas chromatography analysis. After completion of the reaction, the reaction mixture was cooled to room temperature and diluted with water (5 mL), followed by the recovering of the magnetic supported copper catalyst from the reaction mixture by using an external magnet. Then, the [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) supported catalyst was washed with methanol and dried carefully before use in the next run. The remaining products in the supernatant mixture was extracted with EtOAc (4×5 mL) and finally purified by column chromatography.

Preparation of the GO-BPED-PS@NiNPs

100 mg of GO-BPED-PS was dispersed in 100 mL of deionized water for 20 min to form a homogeneous dispersion, then 250 mg of NiCl2.6H2O was added in the corresponding suspension under magnetic stirring at room temperature for 4h to ensure Ni(II) completely attached to GO-BPED-PS sheets. Subsequently, 200 mg of NaBH4 was added to the above mixture followed by addition 50 mL of NaOH (1.0 M), and the reaction was conducted at room temperature for 180 min. At the end of the reaction, the suspension was cooled to room temperature and then centrifugated and washed with sufficient deionized water to remove adsorbed ions and excess reactants.

Finally, the powder was dried under vacuum at 90 °C for 24 h. It was shown that the pseudo-second-order model was the most appropriate one to describe the adsorption of heavy metal ions by the GO-based materials. Finally, it was demonstrated that their desorption-regeneration capacities were higher than 10 cycles, opening the path to the removal of metal ions from wastewater solutions.

Introduction

Nowadays, human activities and industrial development increasingly contributes to the pollution of air, soil and water [1]. Among these, water contaminants especially heavy metals ions, have become worldwide issues due to their extreme toxicity, nonbiodegradability and bioaccumulation in the living environments [2]. In particular, several heavy metal ions such as Cu(II), Ni(II) and Co(II) are used in the electroplating, battery manufacturing and light industries and accordingly the aqueous effluents often contains high metal ion concentrations [3]. The discharges of effluents containing these heavy metal ions have serious risk to both Earth's ecological cycle and human beings [4]. For exemple, some health effects associated to human exposure to copper ions are known for causing cramps, hypoglycemia, nervous system damage and

Wilson disease [5]. Today, the removal of metal ions from contaminated environment is still one of the biggest challenges. In the last several years, various methods such as chemical precipitation, reverse osmosis, photo-catalytic reduction, membrane separation and adsorption have been used to remove heavy metals from wastewater [6][7][8]. However, most of these methods have some disadvantages and limitations, such as low efficiency and sludge production [9]. Adsorption is a more versatile and widely used method for the treatment of wastewater in industry due to its simplicity, high efficiency and easy scale up [10][11][12]. Over the years, a large number of adsorbents have been designed and used for removing heavy metal ions including active carbon, polymer resins and biomass materials [13,14]. It is clear that polymeric materials and activated carbon are the most widely considered substrates due to their commercial availability [15]. Historically, thousands of research focused on the use of low-cost adsorbents such as cow bone [16], wood bark [17], bentonite clay [18] and so on.

Despite the low-cost of these materials, they exhibit low adsorption capacity and it takes a long time to achieve equilibrium in heavy metal ions adsorption on their surfaces [19]. Recently, various new adsorbents based on modified magnetic iron oxide [20], carbon nanotubes [21] and graphene sheets [22,23] have been developped. For example, Chen et al., [24] used oxidized carbon nanotubes as adsorbent for the adsorption of Ni(II) and they obtained a removal efficiency of 80%.

By Now, graphene has received particular interest due to its exceptional electrochemical, thermal and mechanical properties and its very high specific surface area (2600 m 2 .g -1 ) [25,26]. However, graphene is hydrophobic which may not favor the adsorption of metal ions from aqueous solutions [27]. Nevertheless, one famous route to make graphene hydrophilic is based on its oxidation [28]. For this purpose, graphene oxide (GO) is usually obtained from natural graphite by using the Hummer's method [29]. GO sheets display a highly oxygenated surface with carboxyl, epoxy and hydroxyl groups [30]. These functional containing oxygenous groups are expected to increase the adsorption capacity of metal-based clusters [26][27][28][29]. Yang et al., [31] showed that the maximum adsorption capacity of GO was 0.733 mmol.g -1 for Cu(II)

ions which was higher than the active carbon one. Indeed, it is well known that oxygen atoms of carboxyl acid groups are efficient ligand groups while the adsorption properties of an adsorbent depend on the nature of its functional groups and their affinity for pollutants [32]. Moreover, the functionalization of GO may enhance its properties and in particular its maximum adsorption capacity. It has been established

that adsorbent containing nitrogen chelating ligands such as amino, imidazole and amidoxime groups could be effective in adsorption of heavy metal ions [33][34][35]. For instance, Chen et al., [36] modified GO sheets with 3-aminopropyltriethoxysilane (APTS) and 4-aminothiophenol (ATP) respectively, for removing Cu(II) ions. The maximum adsorption capacities were 1.625 mmol.g -1 and 1.560 mmol.g -1 for APTS and ATP respectively, which was 10 times higher than GO. Moreover, Fang et al., [37] prepared GO-supported aromatic diazonium salt and then they studied the adsorption of Co(II) ions from aqueous solution and they obtained a maximum adsorption capacity of 1.974 mmol.g -1 . The detection and analysis of such heavy metal ions in water is still considered as a challenge. Many modern methods such as anodic stripping voltammetry, neutron activation analysis, flame atomic adsorption spectrometry and inductively coupled plasma-mass spectromertry have been developed for the rapid analysis and detection of heavy metal ions from aqueous solutions [38][39].

Regrettably, most of these methods are not appropriate for the determination of metal ions concentrations, because of the use of expensive equipment, high cost of maintenance and use of complicated experimental procedures [40].

Spectrophotometry and UV-Vis absorption spectrometry are known as the most appropriate techniques for the development of low-cost, rapid and simple analytical methods [41]. Unfortunately, the absence of chromophore groups on metal ions makes their corresponding solutions weak colored or colorless so that it is necessary to add a color reagent [42]. For instance, Moghadam et al., [43] To understand the adsorption behavior of these materials towards heavy metal ions, the influence of pH, initial metal ion concentrations and contact time onto their adsorption onto the GO-based surfaces were studied and discussed, thoroughly.

Additionally, the experimental data were fitted to the pseudo-first order, pseudosecond order, Rorginsky-Zeldovich (or Elovich) and intraparticle diffusion kinetic models. Furthermore, adsorption data were analyzed by the Freundlich, Langmuir and Jossens isotherms and the thermodynamic parameters were also systematically calculated. Finally, the regeneration experiments of adsorbed metal ions in presence of EDTA as a base were explored to evaluate the reusability of these GO-based adsorbents.

Results and discussion

The synthesis of the new adsorbent material requires a multistep procedure and is based on the introduction of nitrogen and oxygen chelating groups within chemically functionalized graphite oxide sheets (GO). First, the graphite oxide sheets (GO) were produced by the oxidation of a natural graphite powder using a modified

Hummers' method, as described in our previous work [30]. Then, the covalent functionalization of GO sheets was performed in three steps (Scheme 1).

Scheme 1. Functionalization of GO sheets with BPED in three steps.

First, GO was modified with ethylenediamine to introduce some amino and amide groups onto the GO surface by converting epoxy and carboxylic groups, respectively.

Then, the treatment of the amino groups with chloroacetyl chloride lead to the formation of the GO-EDA-CAC adsorbent. Finally, the GO-EDA-CAC adsorbent reacted with N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) to yield the GO-EDA-CAC-BPED adsorbent. Nevertheless, as discussed before, the main interest in functionalizing GO sheets was highlighted by studying the heavy metal ion adsorption capacities of Cu(II), Ni(II) and Co(II) ions after each functionalization step.

Characterization of GO-based materials

As described in a previous article [44], we synthesized GO-EDA by the direct grafting of ethylenediamine onto GO in DMF. The introduction of the chloromethyl groups onto the GO-EDA surface through an amination reaction was qualitatively evidenced by FT-IR analysis (Fig. 1c). It can be observed, that the FTIR spectrum of the GO-EDA-CAC (Fig. 1c) displays nearly the same absorption peaks as the GO-EDA ones (Fig. 1b) while the peak in the range 600-650 cm -1 may be attributed to both the stretching vibration of amide groups and the stretching vibrations of the C-Cl groups.

In addition, by comparing the spectra of GO-EDA-CAC (Fig. 1c) and GO-EDA-CAC-BPED (Fig. 1d), it can be observed that the stretching vibration bands of the C-Cl groups in GO-EDA-CAC disappear while a new absorption peak located at 767 cm -1 appears that can be attributed to the out-of-plane bending vibration of the pyridine groups. The presence of both CAC and BPED groups onto the GO-EDA surface was confirmed by using the X-ray photoelectron spectroscopy (XPS). The XPS spectra of C 1s sand N 1s of the GO-EDA-CAC and GO-EDA-CAC-BPED samples are presented in Fig. 2 and Fig. 3, respectively. Based on the XPS results, the peak area ratios of carbon and nitrogen containing groups were calculated (Table 1 andTable 2). eV and C=O at 288.82 eV. After grafting BPED, C-N species ratio content increase from 17.0% to 31.4% for GO-EDA-CAC and GO-EDA-CAC-BPED, respectively, as expected (Table 1). Furthermore, the high resolution N 1s spectra of GO-EDA-CAC (Fig. 3a)

show three types of nitrogen atoms with three peaks located at 399.01 eV (C=N), 400.16

eV (C-NR2) and 401.76 eV (N-C=O). Moreover, it can be observed that the amount of O=C-N (11.2%) and C=N-(10.2%) significantly increase when compared to those of the GO-EDA-CAC confirming the successful grafting of BPED onto GO-EDA-CAC.

Studying the stability of GO-based suspensions in a solvent is also usual after a grafting step, so GO-EDA (a), GO-EDA-CAC (b) and GO-EDA-CAC-PBED (c) were dispersed at a concentration of 0.2 mg.mL -1 in various organic solvents and water. Fig. 4a shows that highly stable suspensions were obtained in water and some organic solvents such as DMF and DMSO for GO-EDA, as reported earlier literature [44]. After the grafting of chlororacetyl groups onto the GO-EDA surface (Fig. 4b), the resulting material precipitates in water and DMF which may be attributed to an increase of its hydrophobicity. Finally, after the chemical modification of GO-EDA-CAC with BPED, the corresponding GO-EDA-CAC-BPED sheets (Fig. 4c) display a better dispersibility in polar protic solvents (water and ethanol) and DMSO. This behavior may be explained by the stronger hydrogen bond interactions in presence of amino and pyridine groups. Then, all the GO-based materials were analyzed by thermogravimetric analysis (TGA). Indeed, it is well known that TGA permit to determine of the grafting density of the organic moieties. The TGA curve of GO (Fig. 5a) displays two main weight loss in the temperature range 50-120°C and 150-300°C, respectively. The loss of mass below 100 °C corresponds to physically adsorbed moisture while the major mass loss at about 200°C may be attributed to the pyrolysis of oxygen containing groups [46][47][48]. After EDA grafting, a new weight loss is observed in the temperature range 230-550 °C, which corresponds to an EDA grafting density of 19 mmol.g -1 .

The grafting of chloroacetyl groups onto the GO-EDA surface was suggested by an increase of the weight loss (~5 wt%) on the TGA curve (Fig. 5c) in comparison with the one of GO-EDA (Fig. 5b). Finally, it can be observed that the TGA curve of the GO-EDA-CAC-BPED (Fig. 5d) does not display the same thermal decomposition behavior with the appearance of a weight loss at a higher temperature which may be attributed to the presence of BPED groups. Moreover, the higher residual weight content at 700 °C in comparison with the TGA curves of the other GO-based materials suggests that the BPED groups participated to the formation of char during burning. The effect of the chemical modification of GO sheets onto the morphology of the resulting materials was studied by scanning electron microscopy (SEM). As shown in Fig. 6a, the GO sheets are stacked together and they exhibit wrinkled areas due to oxidation process while their edges seem crooked. After reaction with ethylenediamine (Fig. 6b), the GO-EDA surface shows the presence of randomly crumpled thin large flakes [49]. Moreover, all the functionalized GO-based sheets display a higher surface roughness (Fig. 6b-6d) than the GO one (Fig. 6a). The UV-Vis absorption spectra of aqueous suspensions of GO, GO-EDA and GO-EDA-CAC-BPED were also studied (Fig. 7). The GO sample exhibits a broad absorption band at 260 nm which is related to the (π→π*) transitions of aromatic ring (C=C) (Fig. 7a) while this main absorbance band is shifted to 243 nm, after EDA grafting (Fig. 7b) [44]. After BPED grafting, Fig. 7c shows the appearance of a new absorption band at 340 nm which is related to (n→π*)

transitions of the C=N bonds, suggesting a successful functionalization of GO-EDA-CAC with BPED.

Influence of pH, contact time and initial metal ion concentration on heavy metal ion adsorption

Effect of pH on the adsorption capacities

The pH of metal ion containing aqueous solutions may play an important role in their adsorption by adsorbents. Indeed, adsorption of metal ions relates mostly on the deprotonation and protonation of binding sites of the chelating molecules located onto the surface of an adsorbent and on the metal species present in solution [50]. For As it can be seen, the adsorption capacities of these materials towards different metal ions increase with increasing pH in the range 4-6. This behavior may be attributed to the protonation of the active sites of the adsorbents in a too much acidic media that alter the metal ion adsorption. Indeed, the concentration of H + ions is high at very low pH which leads to the development of positive charge on the active metal ion binding sites and thereby a competition between the adsorption of metal ions and H + ions. Therefore, a less competition is expected by increasing the pH.

To understand the underlying phenomena, the pH point zero charge (pHPZC) of the various adsorbents were measured (plots of pH(Final) vs pH(Initial), (Fig. 9)) and we obtained 3.9, 5.6, 5.7, and 6.8 for GO, GO-EDA, GO-EDA-CAC and GO-EDA-CAC-BPED, respectively. It is well known that the adsorbent surface is positively charged at pH<pHPZC and it is negatively charged at pH>pHPZC [50] so the active metal ion binding sites located onto the GO-based adsorbent will be protonated at pH<pHPZC which should compete successfully with the adsorption of metal ions, as discussed above. Therefore, for the GO-EDA-CAC-BPED adsorbent exhibiting a pHPZC of 6.8, it is expected that its adsorption capacity will be favored at pH higher than 6.8, confirming the obtained experimental values (Fig. 8).

We consider that the higher maximum adsorptions (QpH) is observed at pH=7 for the GO-EDA-CAC-BPED adsorbent with QCu(II)=3.464±0.112 mmol.g -1 , QNi(II)=3.254 ±0

.095 mmol.g -1 and QCo(II)=3.054±0.091 mmol.g -1 because the removal of metal ions is facilitated at pH˃7 due to the precipitation of metal hydroxides in the aqueous solution which leads to increased removal efficiencies, as discussed by Samantary et al., [51].

Effect of the initial metal ion concentration on their adsorption

The effect of the metal ion concentration (50 to 500 mg.L -1 ) on their adsorption by the GO-based adsorbents (GO, GO-EDA, GO-EDA-CAC and GO-EDA-CAC-BPED) at 293 K, keeping all the other parameters constant, is shown in Fig. 10. K, t=24h and using optimal pH conditions: pH=5 for GO, pH=6 for GO-EDA, pH=6

for GO-EDA-CAC and pH=7 for GO-EDA-CAC-BPED); the error bars correspond to one standard deviation n=3.

It is clear that the equilibrium adsorption capacities of the various adsorbents increase with increasing the metal ion concentrations. As discussed by Zhang et al., [52], the increase in metal ion removal efficiency may be explained by the higher interaction between metal ions and the active sites located onto the surface of the adsorbents. Besides, when the initial concentration of metal ions increases from 50 to 250 mg.L -1 , the maximum adsorption capacities become nearly constant for a metal ion concentration higher than 250 mg.L -1 . This latter value is probably related to the saturation of the available binding active sites of the adsorbent.

Effect of contact time on the adsorption capacities of GO-based adsorbents

To understand the adsorption behaviors of GO, GO-EDA, GO-EDA-CAC and GO-EDA-CAC-BPED towards heavy metal ions, the equilibrium adsorptions capacities of Cu(II), Ni(II) and Co(II) ions at different contact time were investigated (Fig. 11). The measurements were performed using a metal ion concentration of 250 mg.L -1 at pH=5 for GO, pH=6 for GO-EDA and GO-EDA-CAC and pH=7 for GO-EDA-CAC-BPED. At the first stage of the adsorption of heavy metal ions, a huge increase in the adsorption capacity is observed whatever the metal ion nature and then it reaches a plateau for content time higher than 4 hours (Fig. 11). The time required to reach equilibrium depends on the adsorbent so that it is 9 h for GO, 8 h for GO-EDA, 6 h for GO-EDA-CAC and 4 h for GO-EDA-CAC-BPED. Thus, GO-EDA-CAC-BPED can achieve an equilibrium adsorption faster than GO-EDA-CAC, GO-EDA and GO. By comparing these adsorption kinetics, it can be concluded that GO-EDA-CAC-BPED displays the highest ability to adsorb of Cu(II), Ni(II) and Co(II) with equilibrium adsorption capacities of 3.464±0.100 mmol.g -1 , 3.254±0.098 mmol.g -1 and 3.054±0.093 mmol.g -1 , respectively. These results indicate that the presence of various oxygenated and nitrogenated groups onto the surface of GO favor the adsorption of heavy metal ions. Additionally, the equilibrium adsorption capacities of the various metal ions onto all the adsorbents followed the order: Cu(II)>Ni(II)>Co(II). As discussed by Su et al., [53], the behavior of the adsorption of heavy metal ions is explained by their intrinsic properties such as atomic number, atomic weight, electronegativity and density. Therefore, we have compared the metal ion characteristics of Cu(II), Ni(II) and Co(II) (Table 3).

Table 3. Metals ordered according to ionic properties and adsorbent

The higher equilibrium adsorption capacity of Cu(II) ions onto all adsorbents may mainly relies on its higher electronegativity and density (Table 3).

Adsorption kinetics of the metal ions

The kinetic data were analyzed with four used kinetic models, including the pseudo first order model [54], the second order model [55], the Elovich or Roginsky-Zeldovich model [56] and the Weber and Morris model [57], more recently called intraparticle diffusion model [58].

The pseudo-first-order equation can be expressed in the linear form as follows:

𝐿𝑛(𝑄 𝑒 -𝑄 𝑡 ) = 𝐿𝑛𝑄 𝑒 -𝐾 1 × 𝑡 (2)
Where, Qe and Qt (mmol.g -1 ) are the adsorption capacity at equilibrium and at test time t (min), respectively; K1 (min -1 ) is the rate constant of pseudo-first-order adsorption.

The slope and intercept of plots of log (Qe-Qt) versus t were used to determine the rate constant k1 and Qe.

The linear form of pseudo-second-order equation could be expressed as follows:

𝑡 𝑄 𝑡 = 1 𝐾 2 ×𝑄 𝑒 2 + 𝑡 𝑄 𝑒 (3)
Where, K2 (g.mmol -1 .min -1 ) is the rate constant of pseudo-second-order adsorption. The values of k2 and Qe can be calculated from the slopes (1/Qe) and intercepts (1/k2Qe 2 ) of the plots of t/Qt versus t, respectively.

Metal characteristics Metal order according to its property Atomic number

Cu (II) (29) The linear form of the Elovich or Roginsky-Zeldovich equation is given as:

𝑄 𝑡 = 1 𝛽 𝑙𝑛(𝛼 × 𝛽) + 1 𝛽 𝑙𝑛(𝑡) (4)
Where, 𝛼 (mmol.g -1 .min -1 ) is the Elovich initial adsorption rate. 𝛽(g.mmol -1 ) is the desorption constant. The constants α and β can be obtained from the slope and the intercept of the plot of Qt versus ln (t).

The mechanism of adsorption can be explained by Weber and Morris [57] model or by the intraparticle diffusion model, which can be written as follows:

𝑄 𝑡 = 𝐾 𝑖 × 𝑡 1/2 + 𝐶 (5)
Where, ki (mmol.g -1 .min -1/2 ) is the rate constant of intra-particle diffusion and C (mmol.g -1 .min -1/2 ) is a constant correlated to the thickness of the binding layer.

If the plot of Qt versus t 1/2 is intercepted at zero, the rate of adsorption will be divided in many stages in the adsorption process while if the slope of the first part is not zero, it will suggest that the binding diffusion layer controls the adsorption rate at the beginning of adsorption. The values of the kinetic parameters calculated from both pseudo-first-order and pseudo-second-order models are summarized in table 4.

According to R 2 values, the pseudo-second-order model fits better with the kinetic data than the pseudo-first-order model. Moreover, the calculated values (Qe,cal) obtained from the pseudo-second-order kinetic model are greatly closer to the experimental data (Qe,exp) than those obtained from the pseudo-first-order model (Table 4). In order to understand the adsorption mechanism of Cu(II), Ni(II) and Co(II) onto the different adsorbents, the experiment data were fitted with the intra-particle diffusion model. The first order rate constants and the corresponding correlation coefficients were calculated by using Eq. 5 (Table 6) and plotted in Figure 12. The second stage may be attributed to the gradual adsorption of metal ions onto adsorbents (Fig. 12a, 12b and12c). It is clear that ki1 (first stage) is higher than ki2 (second stage) indicating that the second stage corresponds to a slower adsorption process. In addition, the intra-particle diffusion of the GO-EDA-CAC-BPED adsorbent displays three curves (Fig. 12d). The first stage is quick and it may be related to the diffusion of Cu(II), Ni(II) and Co(II) ions through the solution to the boundary layer of GO-EDA-CAC-BPED. The intra-particle diffusion in the second stage exhibit a lower diffusion rate due to further diffusion of metal ions passing through the surface to the internal layer of GO-EDA-CAC-BPED.

Finally, the third stage corresponds to a well-known slowdown of the diffusion rate when the binding sites are saturated. As a result, the order of constants Ki for the adsorbent GO-EDA-CAC-BPED is: ki1 (first stage) > ki2 (second stage) > ki3 (third stage). As discussed by Vu et al., [60], the rate of adsorption is possibly explained by the concentration of the adsorbate, the metal ions properties and its affinity with the adsorbents.

Adsorption isotherms of the metal ions

The study of the adsorption isotherms gives a good understanding of the adsorption mechanisms of metal ions onto adsorbents. Herein, Langmuir [61],

Freundlich [62] and Jossens [62] models were applied to simulate and to fit the adsorption isotherms of Cu(II), Ni(II) and Co(II) ions onto the GO-based adsorbents.

Langmuir model is commonly used to describe the adsorption on a homogeneous surface assuming the uniformity of the adsorption sites and no interaction between the adsorbate and the surface of the adsorbent.

The linear form of the Langmuir isotherm model can be expressed as:

𝐶 𝑒 𝑄 𝑒 = 𝐶 𝑒 𝑄 + 1 𝑄×𝑏 (6)
Where, Ce (mmol.L -1 ) is the equilibrium concentration of metal ions, Qe (mmol.g - 1 ) is the equilibrium adsorption capacity, Q (mmol.g -1 ) is the Langmuir monolayer saturation capacity and b (L.mmol -1 ) is a constant related to the adsorption energy.

The Langmuir isotherm model can be also expressed in terms of a dimensionless constant called separation factor (RL), giving information about the feasibility of the adsorption process. It can be expressed as:

𝑅 𝐿 = 1 1+𝑏×𝐶 𝑅𝑒𝑓 (7)
Where, CRef (mmol.L -1 ) is the metal ions concentration and b (L.mmol This isotherm can be written as follows:

𝑄 𝑒 = 𝑖×𝐶 𝑒 1+𝑗×(𝐶 𝑒 ) 𝑚 (9)
Where, i (L.g -1 ), j (L.mmol -1 ) and m are Jossens isotherm constants which can be obtained from adsorption data by an iterative procedure.

The Langmuir and Freundlich isotherm models were used to examine the relationship between the metal ions adsorption onto different adsorbents at different temperatures [63]. A parameters comparison between the two models is shown in Tables 7a-7d and Tables 8a-8d. 3.508±0.009 mmol.g -1 and QCo(II)= 3.401±0.008 mmol.g -1 ) may be also explained by its higher binding energies values. In addition, the Q values of GO and GO-EDA-CAC (Table 7a and Table 7c) increase as the solution temperature increases confirming that the adsorption of metal ions onto the adsorbents are favorable at higher temperatures, while the Q values of GO-EDA and GO-EDA-CAC-BPED decrease with an increase of temperature suggesting that a better adsorption performance can be obtained at low temperature (Table 7b and Table 7d). These results show that the different functional groups onto adsorbents exhibit different behaviors that are temperature dependent when combining with metal ions. Furthermore, the RL values are in the range 0-1 whatever the GO-based adsorbent (Tables 7a-7d) confirming that the metal ion adsorption is favorable.

Moreover, the highest values of adsorption capacity Freundlich (KF) are observed at 293 K for GO-EDA and GO-EDA-CAC-BPED adsorbents (Tables 8b and8d) and at 353 K for GO and GO-EDA-CAC adsorbents (Tables 8a and8c). In addition, the n values of the Freundlich isotherm model at all studied temperature are greater than 1 for all adsorbents, indicating that the adsorption of metal ions onto different adsorbents is favorable under our experimental conditions.

In addition, the Jossens model represents a combination between Langmuir and

Freundlich isotherms and it can be applied to both homogenous and heterogeneous systems. The calculated parameter values as well as the correlation coefficients at different temperatures ranging from 293 to 353 K are presented in Tables 9a-9d. It can be observed that the Jossens isotherm model shows better correlation coefficients (R 2 ) than both Freundlich and Langmuir models. The i and j values for GO and GO-EDA-CAC adsorbents increase with increasing temperature demonstrating that a better adsorption performance can be obtained at high temperatures (Tables 9a and9c). In contrary, for GO-EDA and GO-EDA-CAC-BPED adsorbents, the i and j values of decrease as the solution temperature increases suggesting that the adsorption of metal ions onto these adsorbents are favorable at low temperatures (Tables 9b and9d). The latter results also confirm that the Jossens model is the most suitable one in simulating the adsorption of Cu(II), Ni(II) and Co(II) onto the GO-based adsorbents.

Thermodynamic adsorption parameters

The determination of the thermodynamic adsorption parameters may favor the understanding of the nature and feasibility of the adsorption process at different temperatures. The study of the thermodynamic adsorption of metal ions (Cu(II), Ni(II)

and Co(II)) onto the GO-based adsorbents was investigated in the temperature range 293K-353K (Tables 10a-10d). The thermodynamic parameters such as enthalpy (ΔH°;

kJ.mol -1 ), entropy (ΔS°; J.mol -1 .K -1 ) and Gibb's free energy (ΔG°; kJ.mol -1 ) were calculated using the following equations (Eqs. 10-12):

ln(K T ) = ΔS°R + ΔH°R T ( 10
) ΔG°= ΔH°-TΔS°= -R × T × ln(K T ) (11)
K T = i = Q × b (12)
Where, T (K) is the absolute temperature in Kelvin, R (8.314 J.mol -1 .K -1 ) is the universal gas constant and KT is thermodynamic constant correlated to Jossesn constant "i". The values of ΔS° and ΔH° can be geometrically calculated from the slope and intercept of the Van't Hoff plot of ln KT against 1/T. As it can be observed in Tables 10a-10d, the negative values of the Gibb's free energy (ΔG°) suggest that the adsorption of Cu(II), Ni(II) and Co(II) ions onto the GObased adsorbents is thermodynamically spontaneous and feasible. In addition, the higher negative values of ΔG° of GO and GO-EDA-CAC (Table 10a and10c) with increasing temperature confirming that metal ions adsorption is favorable at high temperature which could be caused by the lower electrostatic repulsions between the adsorbate and adsorbent surface and increasing mobility of metal ions to active sites.

On the other hand, the negative values of ΔG° of GO-EDA and GO-EDA-CAC-BPED (Table 10b and10d) decrease by increasing temperature, which also supported that the elevated temperature would reduce the adsorption capacity of metal ions.

According to these results, an increase temperature may lead to lower binding interactions between metal ions and adsorbents. In addition, the ΔG° values reflects the nature of adsorption process. As discussed by Rodrigues et al., [64] the adsorption nature can be classified as chemisorption and physisorption process when the values of ΔG° are in the ranges -80 to -400 kJ.mol -1 and 0 to -20 kJ.mol -1 , respectively. Thus, the adsorption of the metal ions onto the GO-based adsorbents is a physical adsorption process. The positive values of ΔH° for GO and GO-EDA-CAC adsorbents (Table 10a and 10c) indicate the endothermic nature of the adsorption process. The interpretation to the endothermicity of the adsorption process can be explained that the adsorption efficiencies of metal ions onto GO and GO-EDA-CAC adsorbents are favorable at high temperature, because all metal ions are dehydrated and their solubility in the aqueous solution increases with increasing temperature, as discussed by Peer et al., [65] The positive values of ΔS° reflect the affinity of GO and GO-EDA-CAC adsorbents toward metal ions in aqueous solutions and an increase of the disorderliness at the solid/solution interface during the adsorption process. In contrary, the negative values of ΔH° were shown for GO-EDA and GO-EDA-CAC-BPED (Table 10b and10d) which indicated the exothermic process. Indeed, the decrease in adsorption capacity of metal ions with rise in temperature may be due to the desorption caused by an increase in the available thermal energy, as discussed by Mekonnen et al., [66].

Moreover, Higher temperature induces higher mobility of the adsorbate causing desorption and the negative values of ΔS° revealed that the order degree increased at the solid/solution interface during the adsorption process and some structure changes in the adsorbents.

Desorption and re-adsorption of metal ions

Desorption and re-adsorption studies of the GO-based adsorbents are two key parameters when assessing for commercial applications. The desorption studies of metal ions from the adsorbents show that the metal ions desorbed with a recovery above ~99.5% in presence of EDTA (Fig. 11). The desorption-readsorption was reiterated for ten cycles (Fig. 12) and the higher amount of metal ion adsorption after this procedure was obtained for the GO-EDA-CAC-BPED adsorbent with a decrease of the adsorption capacity from 3.464±0.100 mmol.g -1 , 3.254±0.098 mmol.g -1 and 3.054±0.093mmol.g -1 to 3.245±0.062 mmol.g -1 , 3.0521±0.054 mmol.g -1 and 2.858±0.082 mmol.g -1 , for Cu(II), Ni(II) and Co(II) ions, respectively. Nevertheless, all these results suggest that the GO-based adsorbent exhibit a high enough chemical stability and could be used as adsorbent for wasterwater treatment. and contact time conditions : pH=5 and t=8h for GO, pH=6 and t=8h for GO-EDA, pH=6 and t=6h for GO-EDA-CAC and pH=7 and t=4h for GO-EDA-CAC-BPED).

Comparison of the adsorption capacities of different adsorbents toward Cu(II), Ni(II) and Co(II) ions

A comparison between the maximum adsorption capacities of GO, GO-EDA GO-EDA-CAC and GO-EDA-CAC-BPED adsorbents and various adsorbents previously used for the adsorption of Cu(II), Ni(II) and Co(II) ions is listed in Table 7.

For example, Zhang et al., [67] prepared a biomass soybean dregs-poly(acrylic acid) (SESD-PAA) adsorbent for removing Cu(II) ions and they obtained a maximum adsorption capacity of 1.186 mmol.g -1 , which is slightly higher than that of poly(vinyl alcohol) grafted GO sheets (GO-PVA) [73] but lower than that of chemically modified cellulose with hydroxyethyl methacrylate and acrylic acid (Cell-g-HEMA-Co-ACC) [71]. ions. One limitation of the GO-EDA-CAC-BPED adsorbent is the multistep synthetic procedure to graft BPED groups onto the surface of GO sheets so future work will consist in simplifying the synthetic scheme.

SESD

Functionalization of graphene oxide sheets with magnetite nanoparticles as a

tool for the adsorption of copper ions from aqueous solutions.

 Abstract

This paper deals with the preparation of graphene oxide sheets (GO) modified with magnetite nanoparticles (Fe3O4NPs) for removing Cu(II) ions from aqueous solutions.

The adsorption behavior of magnetic GO-based materials towards Cu(II) ions from aqueous solutions was studied as a function of the pH value and the contact time and it was observed that the magnetic GO-based absorbent can be separated quickly from its aqueous solution by using an external magnetic field. Its equilibrium adsorption capacity value for Cu(II) ions was as high as 3.808±0.125 mmol.g -1 at a metal ion concentration of 250 mg.L -1 , pH=7 and T= 293 K. In addition, it was shown that the adsorption behavior processes of Cu(II) ions onto both the GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbents correspond to a pseudo-secondorder kinetic model and Jossens isotherm model. Nevertheless, the calculated thermodynamic parameters indicated that the adsorption behavior of Cu(II) ions onto the [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbent was spontaneous and exothermic process contrary to the adsorption process onto the GO-EDA-CAC)@Fe3O4NPs adsorbent which was endothermic. Finally, the recyclability of the magnetic GO-based adsorbents was checked and the removal of the Cu(II) ions from aqueous solutions did not change significantly even after ten adsorption/desorption cycles.

Introduction

With the rapid growth of industrial activities, water contamination by heavy metals ions has enhanced undesirable effects on both human health and environment considering their extreme toxicity, persistence, non-biodegradability, and bioaccumulation in the food chain [1][2][3]. One of the most diffuse toxic metal ions are copper ions (Cu (II)) coming from several industries including battery manufacturing, fertilizer industry, mining, and electroplating [4][5][6][7]. Nowadays, various methods and processes such as flocculation, ultrafiltration, electro-dialysis, reverse osmosis, and adsorption have been used to remove toxic metal ions from wastewater and water [8][9][10][11]. However, most of these methods require high operating costs and produce toxic sludge while the use of absorbents do not allow an easy recovering [12,13]. Moreover, traditional adsorbents such as active carbon, biomass materials, and polymer resins suffer from poor adsorption sites and consequently display a low adsorption capacity [14][15][16]. Recently, various novel adsorbents based on functionalized carbon nanotubes and graphene oxide (GO) sheets have been developed [17].

In particular, GO displays a good adsorption capacity towards heavy metal ions due to the presence of epoxy, carboxyl, and hydroxyl functional groups onto their surface [18][19][20][21][22] and its functionalization can improve it [9,23]. One of the attractive ways for modifying GO is the use of multifunctional organic molecules containing strong chelating groups [24,25]. For instance, Laroussi et al., [25] modified GO with N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) for removing Cu(II), Ni(II) and Co(II) ions from aqueous solutions. However, the obtained GO-based materials, having high dispersibility in aqueous solutions, did not permit to revover it easily. In this context, a few authors [26][27][28] studied the modification of GO with magnetic nanoparticles which allow its recovering by using an external magnetic field. Indeed, it is well known that magnetite nanoparticles (Fe3O4NPs) can be combined with GO sheets by various methods such as co-precipitation, solvothermal, and ionothermal [29]. The co-precipitation method is the most used technique to synthesize Fe3O4NPs because of its eco-friendly procedure [30].

For instance, Liu et al., [31] prepared a magnetic-GO modified 1,2diamineocyclohexanetetraacetic acid composite (GO-Fe3O4NPs-DCTA) and they studied the effect of multi-functional organic acid ligands on the adsorption of Cu(II)

ions. The incorporation of metal ions onto magnetic GO-based materials provided a new strategy for the fabrication of atomic level-distributed metal catalysts.

In the present study, we have designed a novel multi-functional magnetic GObased adsorbent for high-efficiency removal of Cu(II) ions from aqueous solutions.

First, the magnetite nanoparticles (Fe3O4NPs) were successfully adsorbed onto GO sheets through a co-precipitation method after the grafting of ethylenediamine (EDA)

and chloroacetyl chloride (CAC) onto the GO sheets. Then, the grafting of N,N-bis(2pyridylmethyl)ethylenediamine (BPED) onto (GO-EDA-CAC)@Fe3O4NPs was characterized by FTIR, XPS, TGA, SEM, EDX, XRD, and VSM and the corresponding GO-based absorbent was used as an adsorbent to remove Cu(II) ions from aqueous solutions. The influence of the pH and contact time onto the Cu(II) ions adsorption capacities of the adsorbents were investigated, thoroughly. Additionally, the kinetics of the adsorption process and the adsorption isotherms were studied to identify the mechanism and to provide information on the atteractions between adsorbates and adsorbents. The regeneration experiments were also conducted to explore the reusability of the magnetic GO-based adsorbents.

Results and discussion

As described in our previous work, GO-EDA-CAC was easily synthesized in two steps from GO sheets. However, the synthetic procedure involves a complementary step based on the doping of GO-EDA-CAC with magnetite nanoparticles (Fe3O4NPs) through the co-precipitation method (scheme 1). Indeed, it is expected that the presence of Fe3O4NPs onto the GO-based surface will facilitate its recovering after the Cu(II) adsorption. Finally, the grafting of N,N-bis(2-pyridylmethyl)ethylenediamine groups (BPED) onto (GO-EDA-CAC)@Fe3O4NPs was realized in order to ensure higher Cu(II) ions adsorption (Scheme 1).

Scheme 1. Preparation of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED absorbent followed by the adsorption of Cu(II) from aqueous solutions.

Synthesis and characterization of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED absorbent

Doping GO-based materials with magnetite nanoparticles

As discussed before, the synthesis and the characterization of GO-EDA-CAC are described in our previous work. Then, the magnetite nanoparticles (Fe3O4NPs)

were in situ produced onto the GO-EDA-CAC surface through the well-known coprecipitation method, which requires the use of both ferric and ferrous ions in basic conditions (Scheme 2) [29].

Scheme 2. Preparation of the (GO-EDA-CAC)@Fe3O4NPs through coprecipitation method.

First, the formation of the Fe3O4NPs onto the GO-EDA-CAC surface was qualitatively evidenced by FTIR analysis (Fig. 1a). Indeed, the FTIR spectrum of the (GO-EDA-CAC)@Fe3O4NPs (Fig. 1a) displays a peak at 635 cm -1 which may be attributed to the stretching vibrations of the CH2-Cl groups and another one located in the range 620-598 cm -1 that is characteristic of octahedral and tetrahedral stretching vibrations of the Fe-O bonds in Fe3O4NPs [5]. The immobilization of Fe3O4NPs onto the GO-based material surface was also demonstrated by analyzing both the elemental composition of (GO-EDA-CAC)@Fe3O4NPs by EDX (Fig. 2a) and its morphology by SEM (Fig. 2b). Fig. 2a shows the EDX spectrum of GO-EDA-CAC@Fe3O4NPs, which mostly displays the presence of carbon, nitrogen, oxygen, chlorine, and iron, as expected. In addition, the SEM image of (GO-EDA-CAC)@Fe3O4NPs (Fig. 2b) shows the presence of sphere-like structures with diameters ranging from 6 nm to 20 nm. A few larger aggregates with an average diameter centered at 80 nm are also observed. Then, the weight content of magnetite nanoparticles in the (GO-EDA-CAC)@Fe3O4NPs was estimated by thermogravimetric analysis (TGA) (Fig. 3a) and it was calculated to be around 28wt % when compared to the curve of GO-EDA-CAC [25].

Grafting of BPED onto (GO-EDA-CAC)@Fe3O4NPs

The grafting of BPED groups onto the GO-EDA-CAC@Fe3O4NPs surface was performed according to the experimental procedure described by Beyou et al., [25] and it was characterized by X-ray photoelectron spectroscopy (XPS). The XPS spectra of C 1s and N 1s for (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED materials are shown in Fig. 4 and Fig. 5, respectively. Each individual peak of carbon and nitrogen functional groups was fitted with a Gaussian function and their peak area ratios were calculated (Table 1 and Table 2). bonds, respectively, (Fig. 4) while the peak intensity of carbon atoms bonded to nitrogen increases after BPED grafting, as expected (Fig. 5). In addition, the C-N species ratio content increases from 18.7% for GO-EDA-CAC@Fe3O4NPs to 32.5% for [(GO-EDA-CAC)@Fe3O4NPs]-BPED (Table 1). Furthermore, the high-resolution N 1s spectrum of (GO-EDA-CAC)@Fe3O4NPs (Fig. 5) displays three peaks centered at 400.11 eV (C=N), 400.18 eV (C-NR2) and 401.74 eV (N-C=O) and the amounts of C=N-(13.7%) and O=C-N (12.9%) (Table 2) increase when compared to those of the (GO-EDA-CAC)@Fe3O4NPs, which attest for the successful grafting of BPED groups onto the (GO-EDA-CAC)@Fe3O4NPs.

Moreover, the presence of BPED groups onto the (GO-EDA-CAC)@Fe3O4NPs was quantitatively studied by TGA and it was observed an increase of the weight loss of 10% in comparison with the TGA curve of (GO-EDA-CAC)@Fe3O4NPs (Fig. 3).

To obtain more understanding of the effect of the grafting of BPED groups onto the morphology of the (GO-EDA-CAC)@Fe3O4NPs surface, scanning electron microscopy (SEM) was also conducted. However, no significant change was observed upon BPED grafting (Fig. 6).

Material AC=N/AT (%) AC-NR2/AT (%)

AN-C=O/AT (%) (GO-EDA-CAC)@Fe3O4NPs 3.5 12.2 6.7 [(GO-EDA-CAC)@Fe3O4NPs]-BPED 13.7 9.2 12.9 The XRD patterns of [(GO-EDA-CAC)@Fe3O4NPs]-BPED is shown in Fig. 7. 220), ( 311), ( 400), ( 422), (511), and (440) crystal plane, respectively [5].

Then, the size of Fe3O4NPs was estimated from the full-width at half maximum of the most instance diffraction peak by Scherrer's equation (Eq.2) [44]:

𝐷 = 𝐾 𝜆 𝛽 𝑐𝑜𝑠𝛳 (2)
Where K is the grain shape factor (k= 0.89), λ is the wavelength of CuKα radiation (λ = 0.15406 nm), β is the Full-Width at half maximum of the most instance diffraction peak (311) in the 2ϴ scale and ϴ is the Bragg's angle. Using Eq.2, we obtained a diameter of 6.00 nm, which is in the same size range that the one observed by the SEM image.

In addition, the magnetization proprieties of [(GO-EDA-CAC)@Fe3O4NPs]-BPED were studied at 300 K and the plots of magnetization (M) versus magnetic field (H) are shown in Fig. 8a. The maximum of the ZFC curve is called the blocking temperature (Tb) of Fe3O4NPs that can be also calculated by using Eq. 4 [47]:

𝑇 𝑏 = 𝛽×𝐾×𝑉 𝑚 𝑘 𝑏 ×ln ( 𝜏 𝑚 𝜏 0 ) (4)
Where β is a constant depending on the crystal size distribution (β=9. A value of 87.67 K was calculated for Tb, which is close to the experimental one (88 K; Fig. 9). The blocking temperature (Tb) of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED (88 K) is lower than that of the Fe3O4NPs (226 K) [48] which may be explained by the increase of inter-distance between Fe3O4NPs and consequently a decrease of dipoledipole interactions.

Effect of the pH and the contact time on Cu(II) ions adsorption by (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED

Effect of the pH onto Cu (II) adsorption capacities and zero-point charge (pH zpc) studies

The pH of the metal ions containing aqueous solutions is a key parameter for the obtained adsorption capacity, as reported by several authors in the literature [49].

Herein, we investigated the pH range 4.0-8.0 and a blank experiment without the GObased absorbent was also performed in order to check the precipitation of Cu(II) ions, as illustrated in Fig. 10. As it can be observed, the Cu(II) ions adsorption capacities increase with increasing the pH from 4 to 8 whatever the GO-based absorbent and the adsorption capacities are higher for the [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

he low Cu(II) adsorption capacities obtained in acidic experimental conditions may be explained by the positive surface charge of the absorbents at low pH and the protonation of the BPED groups for the [(GO-EDA-CAC)@Fe3O4NPs]-BPED absorbent which partly prevent Cu(II) ions from adsorption. Indeed, the pH point zero charge (pHpzc) of the adsorbents were determined to evaluate their surface charges and the experimental curves (plots of pH(Final) vs pH(Initial)) are shown in Fig. 11.

Figure 11. Plot for the determination of the point zero charge of (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

It is expected that the surface of adsorbent becomes negatively charged when the pH of the aqueous solution is higher than pHpzc leading to favorable electrostatic interactions with Cu(II) ions [49,50] and we obtained a pHpzc of 6.6 for the (GO-EDA-CAC)@Fe3O4NPs absorbent and a pHpzc of 6.9 for the [(GO-EDA-CAC)@Fe3O4NPs]-BPED one. Accordingly, the higher adsorption capacity (QCu(II)) is achieved at pH=7

for the [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbent with a value of 3.808±0.125 mmol.g -1 which is slightly higher than the one of GO-EDA-CAC-BPED (Q(Cu(II))= 3.464±0.100 mmol.g -1 ).

Effect of the contact time on the Cu(II) ions adsorption capacities

The adsorption of Cu(II) ions at different contact times were performed with a Cu(II) ion concentration of 250 mg.L -1 at pH=7 for (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED absorbents (Fig. 12). =250 mg.L -1 , T = 293K, [Adsorbent] =0.100 g/100mL and using optimal pH conditions: pH=7 for (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED; the error bars correspond to one standard deviation n=3.

As it can been seen, the optimal contact time depends on the nature of the adsorbent and the optimal contact time is 5h for (GO-EDA-CAC)@Fe3O4NPs and 3h

for [(GO-EDA-CAC)@Fe3O4NPs]-BPED. Thus, the [(GO-EDA-CAC)@Fe3O4NPs]-BPED absorbent achieves an equilibrium adsorption faster than the (GO-EDA-CAC)@Fe3O4NPs one and other GO-based adsorbents (GO, GO-EDA, GO-EDA-CAC, and GO-EDA-CAC-BPED) that were studied in our previous work [25]. This could be ascribed to a higher number of active sites (e.g. BPED groups) located onto the [(GO-EDA-CAC)@Fe3O4NPs]-BPED sheets.

Adsorption kinetics

Four kinetics models, including, the pseudo-first-order [51], the pseudo-secondorder [52], the Roginsky-Zeldovich [53] and the intra-particle diffusion [54] ones were used to fit the experimental data and to understand both the adsorption process and the diffusion of the Cu(II) adsorbate through adsorbent pores.

The linear form of the pseudo-first-order and the pseudo-second-order models are generally given by the following equations Eq. ( 5) and Eq. ( 6), respectively:

𝐿𝑛(𝑄 𝑒 -𝑄 𝑡 ) = 𝐿𝑛𝑄 𝑒 -𝐾 1 × 𝑡 (5)
𝑡 𝑄 𝑡 = 1 𝐾 2 ×𝑄 𝑒 2 + 𝑡 𝑄 𝑒 (6)
Where Qe and Qt (mmol.g -1 ) are the amount of the metal ions adsorbed at equilibrium and at any time t (min), respectively; K1 (min -1 ) and K2 (g.mmol -1 .min -1 ) are the rate constant of pseudo-first-order and pseudo-second-order, respectively. In the pseudo-first-order model, the rate constants k1 and Qe can be obtained from the slope and the intercept of the plot of log (Qe-Qt) versus t. The values of k2 and Qe can be calculated from the slopes and intercepts of the plots of t/Qt versus t for the pseudo-second-order model.

The linear form of the Roginsky-Zeldovich is described in the Eq. ( 7):

𝑄 𝑡 = 1 𝛽 𝑙𝑛(𝛼 × 𝛽) + 1 𝛽 𝑙𝑛(𝑡) (7)
Where 𝛼 (mmol.g -1 .min -1 ) is adsorption rate constant and 𝛽 (g.mmol -1 ) is desorption rate constant. The rate constants 𝛼 and 𝛽 can be calculated from the slope and intercept of the plot of Qt versus ln(t)

The intraparticle diffusion model is expressed in the Eq. ( 8):

𝑄 𝑡 = 𝐾 𝑖 × 𝑡 1/2 + 𝐿 (8)
Where ki (mmol.g -1 .min -1/2 ) is the rate constant of intra-particle diffusion and L (mmol.g -1 .min -1/2 ) is a constant related to the boundary layer thickness. A plot of Qt vs 𝑡 1/2 should give a linear line where the slope and intercept correspond to the value of ki and L, respectively. However, if the linear plot does not pass through the origin, the rate-determining step is multi-step, hence, controlled by two or more processes.

Linear fitting plots of pseudo-first-order and pseudo-second-order models are shown in Fig. 13 and the usual calculated parameters from these models are summarized in table 3. According to the correlation coefficient values, it is demonstrated that the adsorption data fit better with the pseudo-second-order model than with the pseudofirst-order model while the calculated Qe (Table 3) are close to the experimental ones (in the range 3.5-3.8 mmol.g -1 ). Thus, the adsorption of Cu(II) ions onto magnetic adsorbents is controlled by a physical sorption which includes exchange electrons between adsorbent and Cu(II) ions [55].

Table 3. Usual calculated parameters calculated from the pseudo-first-order kinetic, the pseudo-second-order kinetic, the Roginsky-Zeldovich model and the intraparticle diffusion kinetic models for the adsorption of Cu (II) ions onto (1) (GO-EDA-CAC)@Fe3O4NPs and (2) [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

The experimental data were also fitted with the Roginsky-Zeldovich model by using Eq. 7, and the corresponding calculated parameters are summarized in Table 3.

In particular, the low values of α suggest a fast equilibrium adsorption and a low activation energy for the chemsorption [56]. In order to investigate the mechanism of the adsorption of Cu(II) ions onto the different adsorbents, the kinetic results were also fitted with the intra-particle diffusion model by using Eq. 8, and the corresponding calculated parameters are listed in Table 3. Considering these latter results, Fig. 14 shows a multi-linear fit of the intraparticle diffusion model for the adsorption of Cu(II) ions onto (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED. For both the adsorbents, the first linear stage corresponds to the diffusion of the Cu(II) ions onto the surface of the magnetic adsorbent. The second linear stage describes the gradual adsorption stage which may be attributed to the diffusion of Cu(II) ions inside the active sites of the adsorbent and the beginning of saturation.

Models

Moreover, the intra-particle diffusion constants show Ki1 (first stage) is higher than Ki2

(second stage) (Table 3), indicating that the second stage corresponds to a slower transport of Cu(II) ions from the liquid phase up to the active sites of the adsorbent during the adsorption process.

Adsorption isotherms

The adsorption isotherm models describe the relationship between the concentration of Cu(II) ions in solution at equilibrium (Ce) and the amount of metal ion adsorbed onto the different adsorbents (Qe). Theherefore, to understand the adsorption mechanism of the Cu(II) ions onto (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED, the linear Langmuir, Freundlich, and Jossens models were used to fit the equilibrium experiment data. The Langmuir model is based on the monolyer coverage and homogenous sorption of metal ions with no interaction amog the adsorbed ions and with the equivalent energy of the whole surface of the adsorbent [57]. The linear form of the Langmuir isotherm model can be expressed as:

𝐶 𝑒 𝑄 𝑒 = 𝐶 𝑒 𝑄 + 1 𝑄×𝑏 (9)
Where Ce (mmol.L -1 ) and Qe (mmol.g -1 ) are the Cu(II) concentration and adsorption capacity at equilibrium, Q (mmol.g -1 ) is the Langmuir monolayer saturation capacity and b (L.mmol -1 ) is the equililbrium constant related to the adsorption energy.

A further analysis of the Langmuir isotherm model can be also expressed in terms of a dimensionless constant called separation factor (RL), giving information about the feasibility of the adsorption process. It can be expressed as:

𝑅 𝐿 = 1 1+𝑏×𝐶 𝑅𝑒𝑓 (10)
Where, CRef (mmol.L -1 ) is the metal ions concentration and b (L.mmol -1 ) is the Langmuir constant related to the energy of adsorption. The value of RL indicates the adsorption behavior = RL>1: unfavorable; RL =1: linear; 0 <RL<1: favorable; RL =0: irreversible. In contrast, the Freunlich model is appropriate to adsorption processes that take place on multilayer coverage and confirm the heterogeneous nature of the adsorbent [58].

The linear form of the Freundlich equation is given as:

𝑙𝑛Q e = 𝑙𝑛(𝐾 𝐹 ) + 1 𝑛 𝑙𝑛(𝐶 𝑒 ) (𝟏𝟏)
Where, KF and n are the Freundlich constants representing the adsorption capacity and adsorption intensity, respectively

The Jossens model also combines both the postulation of the Langmuir and

Freundlich models, indicating that the adsorption mechanism is a mix of the postulation of mono-and multi-layer adsorption [59].

This isotherm can be written as follows:

𝑄 𝑒 = 𝑖×𝐶 𝑒 1+𝑗×(𝐶 𝑒 ) 𝑚 (12)
Where, i (L.g -1 ), j (L.mmol -1 ) and m are Jossens isotherm constants which can be obtained from adsorption data by an iterative procedure.

First, the Langmuir and Freundlich isotherm models were exploited to examine the adsorption behaviors of Co(II) and Ni(II) ions onto at a defined temperature and the corresponding calculated parameters are compared in Table 4 and Table 5, respectively. Considering the calculated of the isotherm constants and the R 2 values, it is found that Langmuir model fits the experimental data better than the Freundlich one, which may be attributed to a homogeneous distribution of adsorption active sites onto the (GO-EDA-CAC)@Fe3O4NPs and the [(GO-EDA-CAC)@Fe3O4NPs]-BPED surfaces. Thus, it can be concluded that the adsorption process of Cu(II) ions onto the GO-based adsorbents is based on a monolayer. It should be also noted that the Q values of (GO-EDA-CAC)@Fe3O4NPs (Table 4) increase as the solution temperature increases confirming that the adsorption of metal ions onto the adsorbents are favorable at higher temperatures, while the Q values of [(GO-EDA-CAC)@Fe3O4NPs]-BPED decrease with an increase of temperature suggesting that a better adsorption performance can be obtained at low temperature. In addition, the calculated adsorption capacities are close to the experimental ones and follow the order [(GO-EDA-CAC)@Fe3O4NPs]-BPED ˃ (GO-EDA-CAC)@Fe3O4NPs for temperatures lower than 353K (Table 4). Finally, analyzing the separation factor values (RL), it is observed that they are all between 0 and 1 confirming that the adsorption process is favorable (Table 4). For all the samples and whatever the solution temperature (ranging from 293 K to 353 K), the R 2 values are very close to 1 and the "m" values are higher than 1 so it may be concluded that the Jossens model fits better with the experimental data than both Langmuir and Freundlich models (Table 5).

Thermodynamic study

The effect of temperature on the adsorption isotherm was investigated under isothermal conditions in the temperature range of 293-353 K. The Gibb's free energy (ΔG°; kJ.mol -1 ), enthalpy (ΔH°; kJ.mol -1 ), and entropy (ΔS°; J.mol -1 .K -1 ), associated with the adsorption process, can be calculated by using the following equations (Eqs. 12-14) [60] :

ΔG° = ΔH°-TΔS° = -R×T×ln (KT) (12)
ln(K T ) = ΔS°R + ΔH°R T (13)
K T = i (14)
With R is gaz constant (8.314 J.mol -1 .K -1 ); T is the absolute temperature (K); KT = thermodynamic constant related to Jossens constant "i" (Table 5). The values of ΔS° and ΔH° were calculated from the slope and the intercept of the plot of ln (KT) versus 1/T.

The thermodynamic parameters were calculated and listed in Table 6. The negative values of all ΔG° implied that the adsorption process of Cu(II) ions onto (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED occurs favorably and spontaneously. In addition, the values of ΔG° of (GO-EDA-CAC)@Fe3O4NPs (Table 6)

increase with increasing temperature confirming that metal ions adsorption is favorable at high temperature which could be caused by the lower electrostatic repulsions between the adsorbate and adsorbent surface and increasing mobility of metal ions to active sites. In contrary, the values of ΔG° of [(GO-EDA-CAC)@Fe3O4NPs]-BPED (Table 6) decrease by increasing temperature indicating that the higher temperature would reduce the adsorption capacity of Cu(II) ions. In addition, the adsorption nature can be classified as physisorption and chemisorption process when the values of ΔG° are in the ranges 0 to -20 kJ.mol -1 and -80 to -400 kJ.mol -1 , respectively. Thus, the adsorption of Cu(II) ions onto the GO-based adsorbents is a physical adsorption process.

The negative value of ΔH° was shown for [(GO-EDA-CAC)@Fe3O4NPs]-BPED (Table 6) which indicated the exothermic process. Indeed, the decrease in adsorption capacity of metal ions with rise in temperature may be due to the desorption caused by an increase in the available thermal energy, as discussed by Laroussi et al., [25]. In contrary, the positive value of ΔH° for (GO-EDA-CAC)@Fe3O4NPs adsorbent (Table 6) confirm the endothermic nature of the adsorption process, indicating that the adsorption efficienciy of Cu(II) ions onto (GO-EDA-CAC)@Fe3O4NPs adsorbent is favorable at high temperature, because Cu(II) ions are dehydrated and their solubility in the aqueous solution increases with increasing temperature.

Moreover, the positive values of ΔS° is reflected to a good affinity of (GO-EDA-CAC)@Fe3O4NPs adsorbent toward Cu(II) ions in aqueous solutions and an increase of the disorderliness at the solid-solution interface during the adsorption process. In contrary, the negative value of ΔS° for [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbent (Table 6) revealed that the order degree increased at the solid-solution interface during the adsorption process and some structure change in the adsorbent [60].

Desorption and regeneration of GO-based adsorbents

Desorption and regeneration performance of the (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbents are erxtremely important in consideration their practical application. The saturated EDTA (0.02 mg.L -1 ) has been chosen as the eluent in the study. After desorption, the (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbents were treated with deionized water to wash away the residual EDTA solution and explored for Cu(II) ions adsorption in the succeeding cycles (Figure 15). As demonstrated in Fig. 15, the adsorption capacities of the (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbents for Cu(II) ions only decreased 2.54% and 1.18% after successive adsorption-desorption cycles, respectively. Consequently, the GO-based adsorbents with the good regeneration performance (10 cycles) and high stability could be effectively and economically used for the selectively treatement of Cu(II) contamined wasterwater.

Compariason of the adsorption capacities of various adsorbents for Cu(II) ions

The maximum adsorption capacity of the (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbents for the romval of Cu(II) ions were compared with others adsorbents reported in the literature and the values are given in Table 8. For instance, Chen et al., [61] prepared Polyvinyltetrazole-grafted polystyrene resin (PVT-g-PS) adsorbent for removing Cu(II) ions from aqueous solution and they obtained a equilibrium adsorption capacity of 2.65 mmol.g -1 , which is slightly higher than Magnesium titanate nanorods (Ti-Mg Nanorod) [62] but lower than that of GObased materials. In addition, our findings for Cu(II) ions adsorption by [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbent was faster than the graphene oxide (GO) sheets adsorbent (540 min) [25], and the GO-EDA-CAC-BPED adsorbent (240 min).

Furthermore, our magnetic GO-based adsorbents can be regenerated serveral times (10 cycles), suggesting that these adsorbents are excellent candidates for commercial applications involving the effective removal of Cu(II) ions from aqueous solution.

Conclusion

The results of the present investigation showed that the graphene oxide (GO)

sheets was successfully modified with magnetite nanoparticles (Fe3O4NPs) through a co-precipitation method for high-efficiency removal of Cu(II) ions from aqueous solutions. The effects of pH and contact time on the adsorption of Cu(II) ions by the different adsorbents were systematically studied and it was demonstrated that the [(GO-EDA-CAC)@Fe3O4NPs]-BPED absorbent exhibited the highest Cu(II) ions adsorption capacity (e.g. 3.808± 0.125 mmol.g -1 ) at a pH of 7. Moreover, the pseudosecond-order model was the most appropriate one to describe the adsorption of Cu(II) ions from aqueous solutions and it was found that the Jossens isotherm model was the most appropriate one in simulating the adsorption of Cu(II) ions onto both (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbents. Furthermore, the calculated thermodynamic parameters indicated that the adsorption behavior of Cu(II) ions onto the [(GO-EDA-CAC)@Fe3O4NPs]-BPED adsorbent was spontaneous and exothermic process contrary to the adsorption process onto the (GO-EDA-CAC)@Fe3O4NPs adsorbent which was endothermic. Finally, the recyclability of the magnetic GO-based adsorbents was checked and the removal of the Cu(II) ions from aqueous solution did not change siginificantly even after 10 cycles. These results are very encouraging for the industrial production of functionalized magnetic GO sheets with application in the field of water purification and separation treatment.

Introduction

Water is often polluted with heavy metal ions such as Pb(II), Cu(II), Ni(II) and Co(II) which are considered to be dangerous for the human beings and ecological systems due to their high toxicity [1][2][3]. In particular, Ni(II) and Co(II) ions are among the most common heavy metal ions in industry effluents because they are widely used in pigments, paints, stabilizers, metal plating, battery, alloy industries and catalysts [4,5] and they may cause gastrointestinal distress, pulmonary fibrosis and cancer when penetrating human body [6]. Therefore, wastewater treatment is challenging and various conventional techniques have been performed for the removal of metal ions from aqueous solutions including chemical precipitation, oxidation, ion exchange, filtration, biological treatment and adsorption [3,7]. The adsorption technique is often used due to its low cost and its economical process with a high removal efficiency [8][9][10]. Nowadays, there is an increasing attention for the use of carbonaceous materials as adsorbents [11,12]. Among the carbonaceous-based adsorbents, graphene has received a particular interest because it exhibits a very high specific surface area (2600 m 2 .g -1 ) [13]. Nevertheless, a major drawback for a water treatment with graphene sheets as adsorbents is their hydrophobic character so that an acidic chemical treatment is necessary to make its surface hydrophilic [14]. Indeed, oxidation of natural graphite by using the Hummers method leads to the formation of graphene oxide (GO) sheets whose oxygen-containing functionalities increase the hydrophilicity of graphene promoting the interfacial interaction with heavy metal ions [15].

Therefore, GO and its derivatives have been examined for water depollution [16,17].

Moreover, it was demonstrated that the functionalization of GO sheets with polymers and chelating ligands could significantly enhance their adsorption capacities [18].

Considering the removal of cobalt and nickel ions from aqueous solutions, the literature is scarce. For example, -Cyclodextrin modified graphene oxide sheets were synthesized by Song et al., [19] as adsorbents for Co(II) ions and they obtained an adsorption capacity under optimal experimental conditions which was higher than that of Co(II) on the neat GO surface (12.228 mmol.g -1 and 0.804 mmol.g -1 , respectively). This behavior was ascribed to the abundant oxygen-containing functional groups located onto the -Cyclodextrin modified graphene oxide sheets.

Moreover, Fang et al., [20] studied the effet of the amination of GO sheets onto the removal of Co(II) ions from an aqueous solution containing 0.3 g.L -1 of GO-NH2 and 90% of the Co(II) ions were removed within 5 min. The corresponding high Co(II) adsorption capacity of GO-NH2 (1.974 mmol.g -1 ) was attributed to both the high surface area of GO-NH2 ( 320 m 2 .g -1 ) and the presence of amino groups onto the GO surface. In addition, the adsorption behavior of Ni(II) onto GO modified with multidentate chelating agents such as N-(trimethoxysilylpropyl) ethylenediamine triacetic acid (EDTA-GO) and 3-tri-methoxysilyl-propyl-diethylenetriamine was investigated by Zhao et al., [21] and they obtained a maximum adsorption capacity of 1.755 mmol.g - 1 within 30-45 min. More recently, Beyou et al., [17] reported maximum adsorption capacities of 1.789±0.115 and 1.510±0.095 mmol.g -1 for Ni(II) and Co(II) ions, respectively, by using GO sheets modified with bis(2-pyridylmethyl)amino groups. In conclusion, the chemical modification of the GO surface with organic modifiers remarkably influence its corresponding adsorption behavior, as expected.

Nevertheless, the use of a GO-based adsorbent containing with zwitterionic functionalities is not usual to remove heavy metal ions from aqueous solution and to the best of our knowledge, only Ibrahim et al., [22] reported the grafting of a polyampholyte onto the GO surface by combining the grafting through procedure and the free radical polymerization in presence of a zwitterionic monomer. Then, the asprepared nano-hybrid was incorporated into a polysulfone hollow fiber membrane for the effective decolorization of wastewater containing dye. In the present study, we describe the synthesis of a novel zwitterionic GO-based material, which was prepared via a three-step procedure, and we have investigated its adsorption behavior toward toxic heavy metal ions in comparison to the GO-BPED one. The synthetic GO-based adsorbents were characterized by using FTIR, XPS, TGA, and SEM.

Moreover, to highlight the effect of each GO functionalization step onto the metal ion adsorption performance, the adsorption capacities of GO-BPED and GO-BPED-PS were compared. The effects of pH, contact time, metal ion concentrations, and temperature on their adsorption capacity were studied. Additionally, the kinetics of the adsorption process and the adsorption isotherms were studied to identify the mechanism and to provide information on the interactions between adsorbates and adsorbents. Finally, the regeneration and reusability of GO-BPED and GO-BPED-PS were studied.

Results and discussion

Scheme 1 illustrates the multistep procedure to prepare the zwitterionic GO sheets. It is well known that the presence of oxygen containing groups on both the surface and edges of GO allows for the use of different functionalization methods. Herein, GO sheets were first functionalized with BPED followed by a chemical treatment with 1,3propanesultone (PS) yielding the zwitterionic GO adsorbent.

Adsorbents characterization

First, both GO-BPED and GO-BPED-PS adsorbents were characterized by FTIR spectroscopy. The FTIR spectra are presented in Fig. 1 and it should be noted that the two spectra exhibit a broad absorption band around 3400 cm -1 that can be assigned to stretching vibrations of hydroxyl groups while the absorption bands in the range 2800-2900 cm -1 correspond to C-H streching vibrations. Moreover, the presence of BPED onto the GO surface was confirmed by the presence of absorption bands at 1655 and 759 cm -1 that may be attributed to the vibrations of amide groups and pyridine groups, respectively. (Fig. 1a) onto the surface of GO. In addition, the comparison of the FTIR spectra of GO-BPED (Fig. 1a) and GO-BPED-PS (Fig. 1b) highlights the appearance of new absorption bands after the propanesultone (PS) grafting with novel peaks at 1395 and 1208 cm -1 which are characteristic of S=O as well as the presence of an adsorption peak at 1043 cm -1 assigned to the stretching vibrations of O=S=O (Fig. 1b) [23,24].

To confirm the presence of both N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) and 1,3-propanesultone (PS) groups onto the GO sheets, the GO-BPED and the GO-BPED-PS adsorbents were analyzed by X-ray photoelectron spectroscopy (XPS)

and the corresponding XPS spectra of C 1s and N 1s are shown in Fig. 2 and Fig. 3, respectively. After PS grafting, it can be observed the appearance of a new peak located at 286.33 eV which may be attributed to C-N + species thus confirming the conversion of pyridine groups of BPED into quaternary ammonium ions, as expected (Fig. 2b).

Additionally, the comparison of the XPS spectra of GO-BPED and GO-BPED-PS in the nitrogen region (Fig. 3) confirms the presence of ammonium and pyridinium species at 399.53 eV (-N + R2) and 400.23 eV (C=N + ), respectively.

Then, presence of sulfonate species (SO3 -) onto the GO surface was also demonstrated by an XPS analysis in the sulfur region (S 2p) with a peak at 168.62 eV (Fig. 4). The sonication treatment of 10 minutes allows for the dispersion of all the GObased adsorbents in water giving stable homogeneous black suspensions even after one hour (Fig. 5). However, after 15 days GO almost entirely precipitates while GO-BPED and GO-BPED-PS keeps a high affinity for water. One month later, only GO-BPED-PS gives a stable suspension when placed in water, which may be due to the zwitterionic character of this adsorbent.

Then, the grafting density and thermal stability of the GO-based materials were evaluated by thermogravimetric analysis (TGA) (Fig. 6). The TGA curve of GO (Fig. 6a) is well known and displays two stages: the first weight loss in the temperature range 50-120 °C is usually ascribed he loss of physically adsorbed water while the second one corresponds to the pyrolysis of oxygen containing groups [26,27]. In contrast, the TGA curve of GO-BPED (Fig. 6b) exhibits a low weight loss in the temperature range 50-120 °C due to the decreasing amount of physisorbed water after BPED grafting and a higher weight loss in the temperature range 200-500 °C in comparison with the GO one. grafting density of about 1.5 mmol.g -1 (Fig. 6c). For the GO-BPED-PS sample, the weight loss increases to 63% with a PS-based grafting density similar to that of the BPED-based one, as expected for a full conversion of the GO-BPED adsorbent to the zwitterionic one.

Furthermore, insights to the morphology of the GO sheets after grafting were obtained through scanning electron microscopy (SEM) analysis (Fig. 7). The SEM image of GO-BPED shows a few clusters/agglomerates on the surface of graphene sheets with lateral dimensions of a few micrometers which may be attributed to the successful BPED coating with some -stacking interactions between the modified sheets (Fig. 7a). Nevertheless, the GO-BPED surface is mostly smooth.

After PS grafting, a morphological change is observed with the disappearance of the biggest agglomerates and the increase of the roughness due to the presence of thin large flakes with wavy wrinkles (Fig. 7b). These as-prepared GO-based materials were then used as adsorbents for the removal of Co(II) and Ni(II) ions from aqueous solutions.

Effect of pH, initial metal ion concentration, contact time and the presence of competing cations on metal ion adsorption capacities

For a better understanding of the adsorption proprieties of the adsorbents GO-BPED and GO-BPED-PS towards metal ions, adsorption experiments were performed with Ni(II) and Co(II) ions as models for heavy metal ions and varying solution pH, metal ion concentration and contact time. Moreover, the influence of the presence of competing cations in the aqueous solution on the maximum adsorption capacity was examined.

Effect of the pH on the adsorption capacities of the GO-based materials and zero-point charge (pH zpc) studies

A key parameter in the metal ions adsorption process is the solution pH, as described by several authors in the literature [28]. In this study, the effect of solution pH on the adsorption capacities of Ni(II) and Co(II) ions by the GO-based materials was explored in the pH range 4.0-8.0. Indeed, the protonation of both remaining hydroxyl and carboxyl on the GO surface as well as the grafted pyridine groups is observed at pH<4 which may successfully competes with the adsorption of the free cations in aqueous solution due to electrostatic repulsions while alkaline pH (>8) will ensure the hydroxide precipitation of the free cations [29].

Therefore, a blank experiment without any adsorbent in the aqueous solution was studied to confirm the precipitation of both Ni(II) and Co(II) ions in alkaline experimental conditions. The obtained results are shown in Fig. 8 for an initial metal concentration of 250 mg.L -1 , which was chosen according to our previous work [17]. The adsorption capacities of GO-BPED and GO-BPED-PS adsorbents towards Co(II) and Ni(II) metal ions increase with increasing the pH from 4 to 8 (Fig. 8a and8b) with maximum adsorption capacities of about 4 mmol.g -1 and in the range 2.5-3 mmol.g -1 for the GO-BPED-PS and the GO-BPED adsorbents, respectively. In addition, the blank experiment (without adsorbent) suggests some metal ion precipitation at a pH of 8. The increasing adsorption capacity with increasing pH is not surprising because a less competition between the protonation of the oxygen-based adsorption sites onto the GO surface and the adsorption of cations is expected by decreasing the concentration of H + ions in the aqueous solution.

Nevertheless, in order to strengthen this hypothesis, the pH zero point charge (pHzpc) of each GO-based adsorbent was measured to evaluate their surface charges (Fig. 9). Indeed, the point of zero charge is the pH for which the surface charge of adsorbent is equal to zero so, if pHzpc<pH, the binding of metal ions onto the adsorbent surface will not favorably occurs because of the positively charged surface of GO [30]. Herein, we obtained pHzpc values of 6.5 and 6.9 for GO-BPED and the zwitterionic GO-BPED-PS, respectively which is in accordance with the maximum adsorption capacities obtained at pH=7 for both the zwitterionic GO-BPED-PS (Q(Ni(II) = 4.174±0.102 mmol.g -1 ; Q(Co(II)=3.902 ± 0.099 mmol.g -1 ) and the GO-BPED adsorbents (Q(Ni(II)) =3.254±0.098 mmol.g -1 ; Q(Co(II)) = 3.054±0.093 mmol.g -1 ). The higher maximum adsorption capacities of the GO-BPED-PS adsorbent in comparison with the GO-BPED ones may be due to the presence of sulfonate groups that can act as efficient binding sites for cations.

Effect of the initial metal ion concentration

The effect of the initial metal concentration on the maximum adsorption capacity of GO-BPED and GO-BPED-PS adsorbents was estimated by using an adsorbent dose of 1mg.mL -1 and a metal ion concentration varying from 50 to 500 mg.L -1 at 293 K (Fig. 10). As expected, the adsorption capacities increase by increasing the initial metal ion concentrations whatever the adsorbent due to the high viability of the adsorption sites and the presence of a plateau for higher metal ion concentration (250 mg.L -1 ) suggests the saturation of the binding sites. As it can be observed in Fig. 12, the maximum adsorption capacities of the GObased adsorbents towards different metal ions slightly decrease in the presence of monovalent cations (Na(I) and K(I)) while the effect is more pronounced for divalent cations such as Mg(II) and Ca(II). These results can be correlated to the selectivity of Ca(II) and Mg(II) ions that may bind with two active functional groups onto the GObased adsorbent surfaces while monovalent cations react with only one active site.

Adsorption kinetics

The kinetics investigations are a huge importance to understand both the dynamics and mechanism of the metal ion adsorption process. Four well known kinetic models were exploited to analyses the experimental data, including the pseudo-first-order kinetic model [31], the pseudo-second-order kinetic model [32], the Elovich kinetic model [33] and the Weber and Morris model [33], more recently called intra-particle diffusion kinetic model.

The corresponding linear forms of the governing equations are given by equations 1-4, respectively.

log(Q e -Q t ) = logQ e - K 1 2.303 t (1) t
Q t = ( 1 k 2 Q e 2 ) + t Q e (2)
Q t = 1 β ln(t) + 1 β ln(αβ) (3)
Q t = K i × t 1/2 + L (4)
where t (min) is the contact time, Qe and Qt (mmol.g-1) are the amount of metal ions adsorbed at equilibrium and t time, respectively, K1 (min -1 ) is the equilibrium rate constant of the pseudo-first-order model, k2 (g.mmol -1 .min -1 ) is the equilibrium rate constant of the pseudo-second-order model ; β (g.mmol -1 ) is the desorption constant, α (mmol.g -1 .min -1 ) is the initial adsorption rate constant, ki (mmol.g -1 .min -1/2 ) is the intra-particle diffusion rate constant and L (mmol.g -1 .min -1/2 ) is the constant related to the binding layer thickness.

First, to evaluate the change in adsorption process with time, the experimental data were fitted with the pseudo-first-order and the pseudo-second-order models (Fig. 13 and Fig. 14) and the calculated results of the constants for the two models are tabulated in Table 1. According to the linear correlation coefficient (R 2 ) values and the calculated Qe in comparison with the experimental ones, we can consider that the pseudo-second-order model is well adapted to describe the experimental data whatever the adsorbent and the metal ion. Therefore, we can admit that the adsorption of both Co(II) and Ni(II) ions onto GO-BPED and GO-BPED-PS is controlled by a chemical sorption related to electron exchanges between adsorbent and metal ions [32].

Moreover, the K2 values show that the adsorption rates for Ni(II) and Co(II) ions onto the zwitterionic GO-BPED-PS adsorbent are higher than that of the GO-BPED adsorbent, especially in he case of Co(II) confirming that the zwitterionic GO-BPED-PS adsorbent could remove the metal ions faster than the GO-BPED adsorbent (see section 3.3.2.3) and the adsorption rate followed the order Ni(II)>Co(II).

Furthermore, the Elovich model is related to adsorbents whose surfaces are heterogeneous [34], so this model was implemented to our experimental results and the corresponding calculated constants are summarized in table 2. In particular, it is observed that the initial sorption rate () is higher for the Ni(II) ions than for the Co(II) whatever the adsorbent while the desorption constant () is higher of the Co(II) ions, as expected, considering the higher affinity of Ni(II) ions for both GO-BPED and GO-BPED-PS adsorbents.

Table 2. Kinetic parameters calculated from the Elovich model for the adsorption of Ni(II) and Co(II) ions onto a) GO-BPED and b) GO-BPED-PS.

The higher values obtained for the zwitterionic GO-BPED-PS adsorbent may be due to its higher surface area. To gain more insight into the adsorption mechanisms, the experimental results were also fitted with the intra-particle diffusion model (Table 3). to the GO-based surface followed by a low diffusion rate of metal ions into the pores of the GO-based sheets [34]. In addition, the break point (B.P) values (Fig. 15) confirm that the zwitterionic GO-BPED-PS adsorbent can achieve an equilibrium adsorption faster than the GO-BPED one, which may be attributed to its higher number of adsorption sites.

Adsorption isotherms

Adsorption isotherms provide information about the distribution relationship of the adsorbed metal ions between the liquid and the solid phases at the equilibrium adsorption state [35]. Herein, the Langmuir [36], Freundlich [37] and Jossens [38] models isotherms were fitted with the experimental data by using Eq. 5-8. Moreover, both the adsorption energy "b" and Qmax of GO-BPED-PS decrease by increasing the solution temperature suggesting that the combination of the functional groups located onto the GO surface with Co(II) is temperature dependent (Table 4). Finally, the separation factor values (RL) are in the range 0-1 confirming that the adsorption process is favorable ( zwitterionic GO-BPED-PS adsorbent is a better adsorbent than GO-BPED at 293K, which may be due to the high affinity between the metal ions and the sulfonate groups located onto the GO-BPED-PS sheets (Table 5).

Even if the regression coefficient values (R 2 ) are higher for the Langmuir model, it is believed that adsorption of Co(II) and Ni(II) ions onto GO-BPED and GO-BPED-PS involve a GO-based surface that is not completely homogeneous so the Jossens model was also exploited and the results are listed in Table 6. In contrary, the use of the zwitterionic GO-BPED-PS adsorbent leads to a decrease of the absolute values of ΔG° by increasing the temperature suggesting that the electrostatic attractions between sulfonate groups and cations decrease with increasing temperature. The obtained negative values of ∆S° suggest that the adsorption process involves an associative mechanism with the formation of an activated complex between the GO-BPED-PS adsorbate and the metal ions while the adsorption process is exothermic, as suggested by the negative ∆H° values [41]. Furthermore, the activation energy of the adsorption process which is defined as the minimum energy needed by the adsorbate to interact with the adsorbent surface was calculated by using the Arrhenius equation (Eq. 12) [44].

Support

ln K = ln A - E a RT ( 12
)
Where K is Jossens constant "i" (L.g -1 ) which obtained from Table 6, A is the frequency factor, Ea (KJ.mol -1 ) is the activation energy of the adsorption, R (0.008314 KJ.mol -1 .K -1 ) is the universal gas constant and T (K) is the solution temperature in Kelvin. By plotting ln K versus 1/T, the activation energy values (Ea) are calculated from the slope and the intercept.

The results are summarized in Table 8. The positive activation energies obtained by using the GO-BPED adsorbent suggest an endothermic nature of the adsorption process and their magnitudes are in the range of energy for chemisorption process, which agrees well with earlier conclusion based on the values of the enthalpy. Indeed, it is well known that the chemisorption increases with increasing temperature because of a higher surface coverage [41][42][43]. Nevertheless, some authors reported a critical activation energy value of 40 kJ.mol -1 for which the adsorption is a diffusion-controlled process (below 40 kJ.mol -1 ) or not (chemisorption) [45,46]. In case of the GO-BPED-PS adsorbent, we obtained negative Ea values suggesting a potential barrier corresponding to a physisorption process. Karaoglu et al., [44] reported an Ea value of -15.65 kJ.mol -1 for the biosorption of Cr(III) ions on vineyard pruning waste while Padmavathy, 2008 obtained an Ea value of -13.9 kJ.mol -1 for the biosorption of nickel(II) ions onto deactivated protonated yeast.

Regeneration and reusability of GO-based adsorbents

The recovering of adsorbates is of crucial importance for the regeneration and reuse of adsorbents and it can be carried out by various methods including solvent washing, chemical, electrochemical, biological and thermal treatments [47]. The efficiency of an adsorbent is expected to decrease with successive adsorption-desorption cycles [47,48]. For example, Kulkarni al., [47] used a thermal treatment to recover the adsorption capacity of a carbon-based adsorbent. Herein, we have studied the desorption of the metal ions from the GO-BPED and GO-BPED-PS adsorbents by using different experimental conditions.

In particular, we focused our attention on the use of EDTA-, HCl-, and HNO3based aqueous solutions (0.02 mg.L -1 ). Figure 16 shows that the EDTA-based aqueous solution is the most suitable one to recover the metal ions from the GO-BPED adsorbent while the HCl one also displays an efficiency close to 100% for the GO-BPED-PS adsorbent. After the desorption procedure, the GO-based adsorbents were re-used in numerous adsorption-desorption cycles (Fig. 17). Fig. 17 shows that the maximum adsorption capacities of the GO-based adsorbents slightly decrease as the number of cycles increases whatever the metal ion.

Futhermore, after eighteen adsorption-desorption cycles, the highest adsorption capacities for Co(II) and Ni(II) ions were obtained by using the GO-BPED-PS adsorbent (3.52±0.08 mmol.g -1 and 3.89±0.05 mmol.g -1 , respectively).

Comparison of the adsorption capacities of various adsorbents for Co(II)

and Ni(II) ions. For comparison, the maximum adsorption capacities, adsorption equilibrium times, pH, and exploited kinetic and isotherm models to GO-BPED, GO-BPES-PS and various popular adsorbents used for the removal of Co(II) and Ni(II) ions from aqueous solutions were compared (Table 9). Table 9 highlights that the adsortion capcities of Co(II) and Ni(II) ions obtained by most of the GO-based materials are much higher than that of conventional adsorbents such as peel of Artocarpus nobilis and Humic acid-immobilized-amine modified polyacrylamide/bentonite (HA-Am-PAA-B). Moreover, it should be noted that the grafting of aminoethyl groups and cellulose onto reduced graphene oxide (AERGO and GO-cellulose, respectively, (Table 9) did not permit to increase the adsorption capacity of GO contrary to the use of BPED and PS groups (this work) which ensured an increase of the GO adsorption capacity by approximately a factor 2. In addition, the zwitterionic GO-BPED-PS adsorbent can be regenerated several times (18 cycles), much more than the conventional adsorbents. * HA-Am-PAA-B: Humic acid-immobilized-amine modified polyacrylamide/bentonite, AERGO: The grafting of aminoethyl onto reduced graphene oxide, GO: graphene oxide, GO-EDA-CAC-BPED: The grafting of ethylenediamine (EDA), chloroacetyl chloride (CAC) and bis(2pyridylmethyl)ethylenediamine (BPED) onto GO, PFO: Pseudo-first-order kinetic model, and PSO: Pseudo-second-order kinetic model.

Conclusion

The chemical modification of graphene oxide sheets (GO) with bis respectively, at a metal ion concentration of 250 mg.L -1 , pH=7 and T= 293K.

Futhermore, the adsorption kinetics highlighted a chemical sorption process for both metal ions whatever the adsorbent while the adsorption isotherms suggested that the adsorption of Co(II) and Ni(II) ions onto GO-BPED and GO-BPED-PS involve a GObased surface that is not completely homogeneous. Nevertheless, the calculated thermodynamic parameters indicated that the adsorption behavior of Ni(II) and Co(II) ions onto the zwitterionic GO-BPED-PS adsorbents was spontaneous and exothermic process contrary to the adsorption process onto the GO-BPED adsorbent which was endothermic. Finally, the recyclability of the GO-based adsorbents was checked and the removal of the Ni(II) and Co(II) ions from aqueous solutions did not change significantly even after eighteen adsorption/desorption cycles.

Introduction

By now, graphene oxide (GO) sheets as one of the most intriguing carbon-based nanomaterials, has received particular interest due to its large surface area with abundant oxygen-containing functional, such as carboxylic, epoxide and hydroxide groups. Moreover, these characteristics make GO a good building block for novel composites by integrating it with other functional materials [1,2]. Meanwhile, magnetite nanoparticles (Fe3O4NPs) have been reported to be excellent adsorbent and catalyst [3,4]. Moreover, the magnetic property of Fe3O4NPs makes it easy to recover the Fe3O4-based composites by applying an external magnetic field [5].

Thus, to harness the superior properties of GO, and Fe3O4NPs, one proposed powerful strategy is to incorporate them into composites for achieving synergetic properties of the multiple components, making it more suitable for acting as a support material of surface imprinting. For example, Huang et al., [6] synthesized a graphene oxide-mesoporous silica-based ion-imprinted polymer and employed to remove chromium (VI) from aqueous solution. More recently, Laroussi et al., [7] prepared a magnetic-GO modified with N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) groups and they studied the effect of multi-functional organic acid ligands on the adsorption of Cu(II) ions. Today, ineffective disposal of waste GO and the recovered toxic metal ions often pose serious health and environmental risks [8][9][10]. On the other hand, many studies reported that heavy metal ions can be introduced onto GO sheets to catalyze various chemical reactions [11,12]. Furthermore, the recyclability of GObased absorbents containing metal ions is of great interest in particular for the catalysis of some organic reactions such as the homocoupling of alkynes. Indeed, homocoupling of alkynes is one of the conventional methods for the synthesis of symmetric 1,3-diynes [13]. This reaction is usually carried out in presence of copper salts, as catalyst [14].

For example, Wang et al., [15] studied the homocoupling of terminal alkynes in the presence of CuCl2 and Et3N as the base under solvent-free conditions.

Moreover, Singh et al., [16] reported the homocoupling of terminal alkynes to symmetric 1,3-diynes using CuI, benzotraizole as the metal ligand and K2CO3 as the base in dimethylformamide (DMF). However, the heterogeneous catalysis is often preferred with regard to facile catalyst separation and recycling [17]. Recently, several copper-based heterogeneous catalytic systems for the oxidative homocoupling of terminal alkynes have been reported, such as CuI immobilized on 3-(2aminoethylamino)propyl-functionalized (MCM-41) [18] and copper(I)-modified zeolites [19]. However, these catalytic systems require the use of high reaction temperatures and stabilizing ligands while displaying a low selectivity [20]. Herien, we investigated the development of a noval heterogeneous copper catalyst based on [(GO-EDA-CAC)@Fe3O4NPs]-BPED material purposing an easy magnetic recoverability for the homocoupling of a series of terminal alkynes under green experimental conditions.

Results and discussion

As described in our work (See section 2, page 156), the magnetite nanoparticles (Fe3O4NPs) were successfully adsorbed onto GO sheets through a co-precipitation method after the grafting of ethylenediamine (EDA) and chloroacetyl chloride (CAC) onto the GO sheets. Then, the grafting of N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) onto (GO-EDA-CAC)@Fe3O4NPs was exhibited the highest Cu(II) ions adsorption capacity (e.g. 3.808± 0.125 mmol.g -1 ) at a pH of 7.

Homo-coupling study of terminal alkynes in the presence of reused [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II), as supported catalyst

The recyclability of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) material is of great interest in particular for the catalysis of organic reactions such as homocoupling of alkynes. Indeed, the synthesis of 1,3-diynes through the homocoupling of alkynes requires the use of metal catalysts such as palladium which is efficient at low loading but the price of this precious metal has led to the use of a low-cost metal catalyst such as copper [21]. Therefore, we investigated the use of [(GO-EDA-CAC)@Fe3O4NPs]-Cu(II) and [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) as supported catalysts for the homocoulping of phenylacetylene. The latter behavior may be due to the high Cu(II)-GO-based suspension stability in glycerol and especially to the easier coordination of the penylacetylene reactant onto the Cu(II) ions adsorbed onto the GO-based surface. Moroever, it is observed that the time necessary to produce the 1,4-diphenylbuta-1,3-diyne depends on the amount of Cu(II) ions that is adsorbed onto the GO-based surface, so that it is 180 min for (GO-EDA CAC)@Fe3O4NPs-Cu(II) and 120 min for [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II). Then, the influence of the solvent onto the homocoupling reaction yield was studied and the results are summerized in Table 1. Table 1. Effect of the solvent on the time-yield plot for the homocoupling of phenylacetylene in presence of [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II).

a Experimental conditions: Phenylacetylene (1.0 mmol, 1.0 eq.), Et3N (1.2 mmol, 1.2 eq.), [GO-EDA-CAC@Fe3O4NPs]-BPED-Cu(II) (10 mg), and solvent (1.0 mL) at 80 °C under air for 120 min; the error bars correspond to one standard deviation n=2.

It can be concluded that the use of polar aprotic solvents do not permit to obtain an homocoupling reaction yield higher than 60% for reaction time higher than 100 min.

In contrast, the use of polar protic solvents, such as n-propanol and ethanol, leads to a homocoupling reaction yield increasing from 62% to 89%, respectively, for a reaction time of 120 min (Table 1). Moreover, to improve the greenness of the homocoupling reaction, it was also carried out in a biocompatible and non-flammable solvent such as glycerol, and we obtained an excellent yield of 94±0.62% for a reaction time of 120 min.

Therefore, to investigate the effect of temperature on the homocoupling reaction of phenylacetylene under the greener experimental conditions, glycerol was selected as the most suitable solvent and the reaction was performed in a low temperature range (40-80°C; Fig. 2). It should be also mentioned that the homocoupling reaction of alkynes requires the use of low temperatures to avoid the presence of side reactions that is why a temperature of 80°C is often used [20,21,22]. Fig. 2 shows that the reaction yield increases by increasing both the reaction time and the temperature and the highest yield is as high as 94% and was achieved at 80 °C for 120 min. Thus, 80 °C was selected as the optimum temperature for studying the effect of the base nature onto the reaction yield.

Solvents

Table 2 summarized the results obtained with the use of some common organic and inorganic bases under identical experimental conditions.

Table 2.

Effect of the base nature on the time-yield plot for the homocoupling of phenylacetylene. As expected, the reaction could not be performed without any base and most of the organic bases including tert-butylamine, diethylamine, and diisopropylamine gave poor yields (<65%) for a reaction time of 120 min. Triethylamine was the most efficient organic base with a yield of 94±0.62% for 120 min. In comparison, the inorganic bases including CaCO3 and K3PO4.3H2O lead to higher yields, especially Na2CO3 with a yield to 99%. Thus, the optimal greener experimental conditions for the homocoupling of phenylacetylene are based on the use of 10 mg of [GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) in glycerol at 80 °C, in presence Na2CO3 as a base.

Bases

In this regard, it is interesting to measure the green performance of the homocoupling reaction so that we have calculated some green metrics [23] such as:

The E-factor, defined by the ratio of the total mass of waste generated in the synthetic scheme to the mass of product isolated (Eq. 1)

𝑬~𝐟𝐚𝐜𝐭𝐨𝐫 =

∑ mass of wastes mass of isolated product

The Process Mass Intensity (PMI), defined as the ratio of the total mass of materials to the mass of the isolated product (Eq. 2)

𝐏𝐌𝐈 =

∑ mass of materials mass of isolated product = 𝐸 + 1

The Atom Economy (AE), defined as the number of atoms from the starting materials that are incorporated into the final product (Eq.

The results are summarized in table 3. According to these results, we can conclude that our experimental conditions can enable the synthesis of symmetrical conjugated 1,3-diynes at a large scale with an E-factor of 13.74 Kg/1 kg product, 97.13% of atom economy and a reaction mass efficiency of 99.0% (Table 3). Therefore, these results are more interesting than the ones published elsewhere (E-factor = 53.51 Kg.Kg -1 , PMI = 54.51 Kg.Kg -1 , AE = 78.85%, and RME = 99.0% (Table 4) [24]. All these results confirmed that the [GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) can be recycled as a supported catalyst for the homocoupling reaction of terminal alkynes

Mechanistic scheme for the homocoupling of terminal alkynes in the presence of [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II)

According to the literature [24][25][26][27] In the initial step, the acetylene group is deprotonated in presence of the alkaline conditions of Na2CO3. Then, it is expected that the high surface area of the GO-based adsorbent and the high number of coordination sites for Cu(II) ions will facilitate the in situ formation of complexes Cu(II)-acetylide (B).

In the second step, the air oxidation of Cu(II) (B) leads to the formation of the instable intermediate Cu(III) (C) whose decomposition allows the production of the 1,3-diyne product (D) through a spontaneous reduction [24]. In the last step, the reduced Cu(I) species are oxidized to Cu(II) and ensures the regeneration of the catalyst.

Regeneration/reusability

Regeneration of the catalyst is a key parameter when assessing for commercial applications. The reusability of the supported [GO-EDA-CAC@Fe3O4NPs]-BPED-Cu(II) catalyst for the homocoupling reaction was investigated with phenylacetylene as a model molecule under our optimal experimental conditions. After completion of the reaction, the catalyst was efficiently separated from the reaction media by using an external magnetic field after each experiment. Then, the recovered catalyst was washed with methanol several times and dried carefully before reuse in the next experiment (Figure 4). Experimental conditions: Phenylacetylene (1.0 mmol, 1.0 eq.), Na2CO3 (1.2 mmol, 1.2 eq.), and glycerol (1.0 mL) at 80 °C under air for 120 min.

As shown in Fig. 4, the recovered catalyst was consecutively reused for fifteen runs without a significant loss of its activity. It should be mentioned that the FTIR spectrum of the reused [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) after fifteen runs (Fig. 5) still contains the typical absorption band of the Cu-bond at 453 cm -1 while no significant differences are observed between the positions of both the stretching vibrations and the absorption bands of the samples before and after fifteen catalytic runs (Fig. 5). In another experiment, the homo-coupling reaction was performed under our optimal experimental conditions but the catalyst was quickly recovered from the hot solution after 60 min. After the removal of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) material, Figure 6 demonstrates that the homocoupling reaction do not show any significant progress, as expected.

In addition, the reaction mixture was analyzed using ICP-MS to determine the supernatant content of probably leached Cu(II) in the solution. Nevertheless, no significant amount of Cu(II) was detected in the mixture, demonstrating that the designed [(GO-EDA-CAC)@Fe3O4NPs]-BPED material exhibits active ligand sites for Cu(II) ions.

Influence of different catalysts onto the homocoupling of phenylacetylene

A comparison between the catalytic activity of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) supported catalyst and various catalysts for the homocoupling of phenylacetylene is highlighted in Table 5.

Table 5. Comparison of the experimental results obtained for the homocoupling of phenylacetylene in presence of different catalysts.

Narani et al., [20] reported a Cu(II) complex heterogenized on (N1-propylethane1,2-diamine)triethoxysilane (Cu(II)-N,N-SBA-15) for the homocoupling reaction of phenylacetylene in DMSO and they obtained a yield of 97% in 4 hours at 100 °C which is slightly higher than the one obtained with the homogenous Cu(OAC)2(H2O) catalyst [28]. This result may be explained by the use of a lower reaction temperature (90 °C). Nevertheless, these results are slightly lower than that of Cu/Cu2ONPs supported reduced graphene oxide sheets (Cu/Cu2ONPs@rGO) [31] and it we can also be concluded that our [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) supported catalyst exhibits the higher catalytic activity for the homocoupling of phenylacetylene in comparison with the results reported in the literature because the Moreover, it was regenerated several times, more than 15 cycles.

Conclusion

In conclusion, the recovered [GO-EDA-CAC@Fe3O4NPs]-BPED-Cu(II) was recycled as an efficient supported catalyst for the homocoupling of terminal alkynes and it lead to the highest catalytic performance for the homocoupling of phenylacetylene with a yield up to 99%, under green conditions. Furthermore, the [GO-EDA-CAC@Fe3O4NPs]-BPED-Cu(II) supported catalyst has been reused for fifteen times without a significant loss of its activity, making it acceptable for industrial-scale production.

Introduction

Propargylamines are key intermediates in the production of pharmaceuticals and therapeutics drug molecules and have been clinically used for the treatment of Parkinson's and Alzheimer's disease [1][2][3]. Conventionally, they are synthesized by nucleophilic attack of lithium-acetylides or Grignard reagents to imines or their derivatives [4][5][6][7]. However, these traditional methods for propargylamines synthesis have some disadvantages such as low atom economy and employing stoichiometric [8][9][10][11]. Recently, they can be atom-economically obtained from a one-pot threecomponent condensation between an alkyne, an aldehyde, and an amine, where water is the only theoretical product [12]. This Mannich-type reaction proceeds through activation of the terminal alkyne C-H bond in the presence of a metal catalyst [13,14].

Different salts and complexes of transition metals, such as Cu/Ru(II) bimetallic systems, copper [15], mercury [16], iridium [17], gold, [18] silver, [19] nickel [20], and copper [21] have been reported for A3-coupling reactions. Despite the advantages of these homogeneous catalysts, they suffer one or more serious limitations including difficulty in recovering these catalysts, possible contamination of the product with transition metals, and usage of expensive and toxic ligands [22]. The most important feature in many catalyst systems is recovery after reaction completion without a meaningful reduction of the catalytic efficiency [23]. Nowadays, heterogeneous magnetic nanoparticles catalytic systems are more popular due to the easy and costeffective separation of the catalyst from the reaction mixture by applying a strong magnetic field, which is facilitated the elimination of the long time separation procedures such as filtration or centrifugation [24]. More recently, nickel and nickel oxide nanoparticles are considered the most effective catalysts for the A3-coupling reactions due to their ability to activate the alkyne C-H bonds [25][26][27]. For example, the most drawbacks in the research field of the magnetic nanoparticles are their aggregation in the liquid phases and lead to a decrease in their specific properties such as catalytic activity [28][29][30].

The integration of magnetic nanoparticles with supports such as carbonaceous materials have become a popular strategy for these types of coupling reactions [31].

Among carbonaceous materials, graphene oxide (GO) sheets are widely used in various applications because they offer unique physical-chemical properties such as high specific surface area, electrical, and thermal conductivity [32][33][34]. In addition, GO sheets have shown unique properties in the field of catalysis and high potential adsorption of heavy metal ions due to the presence of hydroxyl carboxyl and epoxy functional groups onto the surface, thus making its processing and functionalization more suitable [35]. For example, Beyou et al., [36] modified GO sheets with BPED and followed by a chemical treatment with 1,3-propanesultone (PS) yielding the novel zwitterionic GO adsorbent for removing Co(II) and Ni(II) ions from aqueous solutions.

For the synthesis of excellent active catalysts, we combined both the advantages of the zwitterionic GO-BPED-PS sheets and nickel nanoparticles (NiNPs). Consequently, GO-BPED-PS@NiNPs were endowed with great anti-aggregation and excellent stability, and then further applied in the field of organic catalysis.

In this section, we reported the immobilization of Nickel nanoparticles (NiNPs) on the surface of the zwitterionic GO-BPED-PS sheets. The immobilization of magnetic nickel nanoparticles was conducted in three steps. First, graphene oxide (GO) sheets functionalized with N,N bis(2-pyridylmethyl)-ethylenediamine (BPED) followed by a chemical treatment with 1,3-propanesultone (PS) yielding the zwitterionic GO-BPED-PS sheets. Then, it doped with magnetic nickel nanoparticles (NiNPs) through the insitu reduction method, which requires the use of in the presence of sodium borohydride (NaBH4). The corresponding material was thoroughly characterized with help of FTIR, SEM, ATG, XRD, and VSM. Furthermore, we have shown the catalytic applications of GO-BPED-PS@NiNPs as a heterogeneous catalyst for a threecomponent coupling reaction of aldehyde, morpholine, and terminal alkynes in 𝛾-Valerolactone as a green solvent. The recyclability of GO-BPED-PS@NiNPs was also studied.

Results and descussion

As described in previous work (See section 3, page 197), we designed the zwitterionic GO-BPED-PS material by the grafting of N,N-bis(2pyridylmethyl)ethylenediamine (BPED) followed amination reaction of 1,3propanesultone (PS) onto graphene oxide (GO) sheets. After sodium borohydride (NaBH4) addition to the GO-BPED-PS@Ni(OH)2 aqueous suspension, Ni(OH)2 is expected to form Ni nanoparticles (Scheme 1). However, the magnetic GO-BPED-PS@NiNPs material was evaluated as a heterogeneous catalyst for the coupling A3 of benzaldehyde, morpholine, and terminal alkynes in 𝛾-Valerolactone as a green solvent.

Scheme 1. Immobilization of NiNPs onto the zwitterionic GO-BPED-PS material.

Characterization of the GO-BPED-PS@NiNPs

As discussed before, the zwitterionic GO-BPED-PS material was synthesized in two steps from GO sheets. Then, the resulting material was doped with Ni nanoparticles (NiNPs) through the well-known in-situ reduction method, which requires the use of Nickel ions (Ni (II)) in the presence sodium borohydride (NaBH4).

The presence of nickel nanoparticles (NiNPs) onto the zwitterionic GO-BPED-PS surface was qualitatively demonstrated by FTIR analysis (Fig. 1). Indeed, the FTIR spectrum of the GO-BPED-PS@NiNPs (Fig. 1) shows a peak at 657 cm -1 which may be attributed to the stretching vibrations of the Ni-O and confirmed the formation of NiNPs onto the zwitterionic GO-BPED-PS surface [37]. The coordination and immobilization of NiNPs onto the zwitterionic GO-BPED-PS surface were also investigated by analyzing the morphology of GO-BPED-PS@NiNPs by SEM analysis (Fig. 2).

Figure 2. SEM image of GO-BPED-PS@NiNPs.

The SEM image of GO-BPED-PS@NiNPs (Fig. 2) shows relatively protuberances onto the surface, confirming the presence of sphere-like structures of nickel nanoparticles with diameters ranging from 11 nm to 60 nm.

Then, thermogravimetric analysis (TGA) was carried out to determine the weight content of nickel nanoparticles (NiNPs) (Fig. 3) and it suggests that GO-BPED-PS@NiNPs contain 38 wt % weight nickel nanoparticles when compared to the curve of GO-BPED-PS [32]. The structure and size particles of Ni NPs onto the zwitterionic GO-BPED-PS surface were also demonstrated by using XRD analysis (Fig. 4). Where D is the average size of NiNPs, λ is the X-ray wavelength of CuKα radiation (λ = 1.5406 Å), K is Scherrer's constant (k= 0.89), βhkl is the Full-width at half maximum of the diffraction peak (111) in radian, and ϴhkl is Bragg's angle or the diffraction angle for (hkl) phase. Using Scherrer Eq. 1, the size of NiNPs was 11.327 nm and reasonably close to the mean size range determined by the SEM image.

To investigate the magnetization properties of the GO-BPED-PS@NiNPs material, magnetic measurements were performed at 300 K using a vibrating sample magnetometer (VSM) in terms of magnetic field (H) dependent magnetization measurement (M). The VSM results of GO-BPED-PS@NiNPs are shown in Fig. 5a.

It can be observed that the GO-BPED-PS@NiNPs material is superparamagnetic proprieties at 300 K, with no coercivity (Hc) and remanence (Mr). In addition, the saturation magnetization (Ms) of GO-BPED-PS@NiNPs is 19.48 emu.g -1 , which is significantly less than of bulk NiNPs (44.52 emu.g -1 ) [39]. It is ascribed that the saturation magnetization of NiNPs decrease with decreasing particle size, although the nature of the surfactant molecules onto the particle surface [40.

Thus, the excellent superparamagnetic behaviour of GO-BPED-PS@NiNPs fulfils our aim to develop more efficient and easily separable catalyst in catalysis reaction. The average size (Dm) of the Ni NPs can also be calculated from the initial susceptibility (xi) using Eq. 2 [41]: show moderate yields of the desirable product (propargylamine) for reaction times higher than 3h, whereas bulk Ni (commercially) gives a considerably low yield of the product. Moreover, the supported nickel nanoparticles' catalyst GO-BPED-PS@NiNPs shows superior catalytic properties to obtain the relatively high yield (90%) of propargylamine in a shorter time within 3h when compared to other Ni-based catalysts. This may be attributed to its larger surface area and other unique structure features, thus resulting in higher surface concentration of the reactive sites of Ni NPs.

𝐷 𝑚 = √
Furthermore, the reaction medium plays an important role in the A3 coupling reaction, hence, a variety nature of solvents were conducted to optimize the catalytic activity of the GO-BPED-PS@NiNPs and the results are summarized in Table 1.

Table 1. Effect of the solvent on the time/yield plot for the A3 coupling reaction of benzaldehyde, morpholine, and phenylacetylene in presence GO-BPED-PS@NiNPs. a Experimental conditions: benzaldehyde (1.0 mmol), morpholine (1.2 mmol), and phenylacetylene (1.5 mmol), GO-BPED-PS@NiNPs (20 mg, 1mol%) and solvent (3.0 mL) at 90 °C under air for 3h; the error bars correspond to one standard deviation n = 2.

Solvents

It was noted that the use of polar protic solvents gives A3 coupling reaction yields in the range 15-42%, owing to the poor dispersibility of the GO-BPED-PS@NiNPs catalyst in polar protic solvents. In contrast, polar aprotic solvents such THF, dichloromethane, acetone, DMF, and DMSO afforded an excellent A3 coupling reaction yields with 74±1%, 76±1%, 85%±2, 88±1%, and 90±1% respectively, for a reaction time of 3h. Moreover, γ-Valerolactone (GVL) has been used as a solvent to replace classic organic media. Considering its physical proprieties, we are investigating our hypothesis that γ-Valerolactone (GVL) could be a safe and efficient biomass-derived dipolar aprotic solvent. As expected, a superior yield (96±1%) of propargylamine was obtained when GVL was used as the solvent. This could be attributed to the high dispersibility of the GO-BPED-PS@NiNPs catalyst in GVL leading to a high surface area.

Therefore, γ-Valerolactone was selected as the most suitable solvent for studying the effect of the reaction temperature in the range from 60 to 100 °C onto reaction yield and the results are tabulated in Table 2. As it can be seen, the yield of propargylamine product increased with temperature and the highest yield up to 99% and was performed at 100 °C for 3h. On the other hand, the use of high temperatures at 100 °C did not influence negatively the reactivity of the system and constantly good A3 coupling reaction yields owing to the chemical-physical properties of γ-Valerolactone (GVL), which it has high boiling and overall good chemical stability. The results established that the most favorable reaction condition was achieved by using 20 mg GO-BPED-PS@NiNPs in γ-Valerolactone (3.0 mL) at 100 °C for 3h.

Scope and limitation of the A3 coupling of benzaldehyde, morpholine, and terminal alkynes

Having the optimized reaction condition for examining the scope of our methodology, we have applied the procedure to various substrates for this catalytic system. For this purpose, various types of substituent alkynes (electron-donating and electron-withdrawing) were studied and the results are summarized in Fig. 7. GO-BPED-PS@NiNPs catalyst, which suggests that the catalyst is heterogeneous in nature.

Figure 10. Effect of removal of the GO-BPED-PS@NiNPs catalyst onto the A3 coupling reaction of benzaldehyde, morpholine, and phenylacetylene yield after 2 h.

Comparison of catalytic activity of GO-BPED-PS@NiNPs catalyst with reported catalyst for the A3 coupling reaction

A comparative study of GO-BPED-PS@NiNPs catalyst with some other heterogeneous catalysts for the A3 coupling reaction of benzaldehyde, morpholine, and phenylacetylene is highlighted in Table 3. For example, Gholinejad et al., [45] described the use of a Copper (II) supported on periodic mesoporous organosilica (PMO) ionic liquid (PMO-PLs@Cu) for the A3 coupling reaction of benzaldehyde, morpholine, and phenylacetylene in chloroform as a solvent, and they obtained a yield of 96% at 60 °C which is higher the obtained with the Cu nanoparticles onto a silica aerogel (SiO2@CuNPs) and copper oxide nanoparticles coated onto graphene nanosheet (GNS@CuONPs).

However, Gold nanoparticles coated onto Monmorillonite (Monmorillonite@AuNPs) exhibit higher catalytic activity than other catalysts reported in the literature because the reaction time (e.g. 3 h) and some of these methods suffer from one or more of the drawbacks such as using a toxic solvent, and require longer reaction times to achieve reasonable yields. In particular, the GO-BPED-PS@NiNPs catalyst can be performed in green methodology for the A3 coupling reaction of benzaldehyde, morpholine, and phenylacetylene by using γ-Valerolactone (GVL) as a green solvent. Moreover, it is premiere over to other previously reported catalysts in terms of reaction times yield, and recyclability (e.g. 11 cycles). PMO-PLs@Cu = Copper (II) supported on periodic mesoporous organosilica (PMO) ionic liquid, SiO2@CuNPs = Cu nanoparticles onto a silica aerogel, GNS@CuONPs = copper oxide nanoparticles coated onto graphene nanosheet, Monmorillonite@AuNPs = Gold nanoparticles coated onto Monmorillonite, G@AgNPs = silver nanoparticle coated onto graphene nanocomposite (G-AgNPs), GVL = γ-Valerolactone.

Conclusion

In summary, we have designed a novel heterogeneous catalyst GO-BPED-PS@NiNPs for a three-component coupling reaction (A3 coupling reaction). The GO-BPED-PS@NiNPs was prepared through in situ generations of nickel nanoparticles (NiNPs) onto the zwitterionic GO-BPED-PS sheets. Results of X-ray diffraction (XRD) patterns and vibrating-sample magnetometer (VSM) indicated that the size of NiNPs was 11.32 nm with specific saturation magnetization (Ms) was 19.48 emu.g -1 . GO-BPED-PS@NiNPs was successfully applied as a magnetic and superior catalyst for the A3 coupling reaction of benzaldehyde, morpholine, and terminal alkynes for synthesis derivate of propargylamines in γ-Valerolactone at 100 °C for 3h. The magnetic GO-BPED-PS@NiNPs catalyst could the promote A3 coupling reaction to obtain up to 99% yield under optimized conditions. Furthermore, the magnetic GO-BPED-PS@NiNPs catalyst was effectively reduced up to eleven runs without a significant loss of its activity. The importance of the reaction such as its simple operations under mild conditions, compatibility, high atom economy, and good yields of the product, make this catalytic strategy a very attractive and eco-friendly process for the propargylamnies synthesis.

In this study, the bibliographic part highlighted the preparation, properties, and applications of graphene-based materials. Besides, several studies evidenced the possibility to use carbonaceous materials such as graphene-based materials as adsorbents for the removal of heavy metal ions from aqueous solutions. In addition, the kinetic, isotherm, and thermodynamic studies have also highlighted the quality of fits and adsorption performance because of their wide applicability in a variety of adsorption data. The last section has highlighted the catalytic methods developed for the organic reactions using graphene-based catalysis.

In the next part, the experimental protocols were involved and the characterization techniques of adsorbents and their applications for the removal of metal ions from aqueous solutions are described.

The third part is divided into two chapters. In the first chapter, the three studied materials herein were effective for removing metal ions from aqueous solutions. Then, in order to separate the GO-based material from the aqueous solution, its magnetization with magnetic nanoparticles such as magnetite nanoparticles (Fe3O4NPs) followed by the use of an external magnetic field can be a suitable pathway. Indeed, we have modified graphene oxide (GO) sheets with magnetite

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  His great support and commitment to the research have made the completion of this work possible. I cordially express my thanks to all the members of the jury for accepting to study my thesis. I would like to acknowledge Prof. Latifa BERGAOUI and Prof. Cécilia MENARD-MOYON for taking the time and effort necessary to review this manuscript. Many thanks to Prof. Eric DROCKENMULLER and Prof. Hatem MAJDOUB who accepted to be part of this jury. On the IMP side, I would like to thank Ms. Angés, Mr. Ali, Mr. Olivier, Mr. Florian, Ms. Sabine, Ms. Corine, Ms. Nadia, and Mr. Valentin for their friendly and everlasting help and attention, their moral support have established an atmosphere conducive to work. I would like to take the opportunity to thank my colleagues for their kind help and the nice days we spent together in IMP. Thanks to you Laura, Laetitia, Barabara and, Mathieu, my faithful cooffice works and thanks as well to Busra, Dimitri, Cyrielle, Hussein, Eupharsie, Soumaya, Chloé, Olivier, Hubert, Léa, Thomas, Yoanh, Benjmin, Félix, Aileen, Renaud, Omaima, Luisa, Noémie, Manon, Clémentine, Kilian, Fanny, Madalin, Anatole, Raphaél, Claire, Charlotte, Thibaut, Clémance, Mathilde, and Florian.

  La troisième partie concerne les travaux personnels dans laquelle un premier chapitre portera sur la préparation des nouveaux supports graphénique modifié pour l'élimination des ions métalliques tels que Cu(II), Ni(II), et Co(II) des solutions aqueuses. xii Tout d'abord, le graphène oxydé (GO) a été chimiquement modifié par des groupes N,N-bis(2-pyridylméthyl)amino (BPED) par une procédure en plusieurs étapes. L'influence des concentrations d'ions métalliques, du pH, du temps de contact et de la température sur leur adsorption sur les adsorbants à base de GO a été étudiée. L'adsorbant GO-EDA-CAC-BPED a montré la plus grande capacité à adsorber des ions métalliques tels que Cu (II), Ni (II) et Co (II) des solutions aqueuses. De plus, il a été montré que les processus de comportement d'adsorption des ions métalliques sur les adsorbants à base de GO correspondent à un modèle cinétique pseudo-second ordre et un modèle isotherme de Jossens. Les paramètres d'adsorption thermodynamique ont également été calculés et les valeurs négatives obtenues pour le changement d'enthalpie (ΔH°) et le changement d'entropie (ΔS°) ont démontré les caractéristiques exothermiques pour les adsorbants GO-EDA et GO-EDA-CAC-BPED. Enfin, il a été montré que tous les adsorbants à base de GO présentent une bonne recyclabilité et réutilisabilité pour les ions Cu (II), Ni (II) et Co (II).

L

  'allure des isothermes d'adsorption des ions Ni (II) et Co (II) sur des adsorbants zwitterioniques à base de GO obéit au modèle de Jossens, qui converge mieux avec les résultats expérimentaux que celui de Langmuir et Freundlich. Par ailleurs, le calcul des paramètres thermodynamiques a montré que les systémes d'adsorption des ions Ni (II) et Co (II) sur les adsorbants zwitterioniques GO-BPED-PS sont favorables, spontané et exothermique. La recyclabilité des adsorbants à base de GO a été vérifiée et l'élimination des ions métalliques des solutions aqueuses n'a pas changé de manière significative même après dix-huit cycles d'adsorption-désorption. Dans le deuxième chapitre, nous avons utilisé les matériaux à base de GO et es métaux adsorbés comme catalyseurs hétérogènes pour les réactions de couplage carbone-carbone. Dans ce contexte, le matériau résultant [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) a été utilisé comme catalyseur supporté dans la réaction d'homocouplage des alcynes terminaux dans des conditions expérimentales vertes.

Figure 1 .

 1 Figure 1. Layered structure of graphite showing the sp 2 hybridized carbon atoms tightly bonded in hexagonal rings.

Figure 2 .

 2 Figure 2. Overview of the main graphene production technique.

Figure 3 .

 3 Figure 3. Micromechanical exfoliation from natural graphite.

Figure 4 .

 4 Figure 4. Non-covalent functionalization of graphene by PTCA [23].

Figure 5 .

 5 Figure 5. Covalent modification paths for graphene.

Figure 6 .

 6 Figure 6. Synthesis of graphite oxide (GO) from graphite.

Figure 8 .

 8 Figure 8. Model of the interactions of graphene oxide (GO) with metal ions.

Figure 9 .

 9 Figure 9. Schematic representation of sulfonated-Rgo-PEDOT structure [48].

Scheme 2 .

 2 Scheme 2. Functionalization technique of the epoxy groups of the GO-N surface[51].

(Scheme 5 .

 5 Scheme 5. Synthesis functionalized GO sheets with ethylenediamine (EDA) in the presence of HATU[68].

Figure 10 .

 10 Figure 10. (A) Procedure to design Graphene-PVP@Pt-on-Pd nanoparticles. (B)

Figure 11 .

 11 Figure 11. SEM images of (a) GO-DNA@AuNPs and (b) GO-DNA@AgNPs [107].

Figure 14 .

 14 Figure 14. Adsorption process scheme of an adsorbent surface.

Figure 15 .

 15 Figure 15. Different types of adsorption isotherm.

Figure 16 .

 16 Figure 16. Carbon nanotubes. Top: molecular structure and the typical dimension of (left) single-walled carbon nanotubes (SWCNTs) and (right) multiwalled carbon nanotubes (MWCNTs). Bottom: Scanning electron micrograph images of (left) SWCNTs and (right) MWCNTs samples [224].

Figure 17 .

 17 Figure 17. Treatments of simulated radioactive water by the GO-PEI. [C 0] cations=0.42 mM, [C0] Re=0.54 mM, and m/V=0.4 g.L -1 [243].

Figure 17 .

 17 Figure 17. Catalysts classification.

1 )

 1 Scheme 22. (rGO@Ni@PdNPs) catalyzed C-H bond arylation of imidazole[1,2-a]pyridine[286].

  A3-coupling reactions. Reactions were performed by the multicomponent reaction of of aldehydes (42), piperidine (43), and terminal alkynes (44) to afford propargylamines (45) in the presence of 20 mg of GO@Fe3O4NPs at 80 °C in acetonitrile (Scheme 24).

Figure 18 .

 18 Figure 18. Electron scavenging mechanism of GO.

  another work, Li et al.,[299], synthesized rGO@AgNPs and studied the influence of rGO content, initial dye concentration, pH, and catalytic dose on decolorization of Rhodamine B (RhB) as a model contaminant. The result confirmed that as rGO content in the composite was raised, more silver nanoparticles (AgNPs) were able to attach onto the rGO sheets; therefore, extra charge carriers form exceedingly reactive species and boost the degradation of Rhodamine B. Based on the physical and structure characterization, a possible pathway for the photocatalytic degradation of RhB on rGO@AgNPs under visible-light irradiation was proposed (Figure19).

Figure 19 .

 19 Figure 19. Proposed mechanism for the photocatalytic degradation of RhB over rGO@AgNPs under visible-light irradiation [299].

X

  -rays to analyze the electronic configuration and chemical composition of the elements present on the surface of solid materials. XPS experiments were performed using a PHI physical electronic spectrometer (Quantra SXM instrument, USA) with a monochromated Al Kα radiation and 180° hemispherical electron energy analyzer assuring an energy resolution of 0.3 eV and 4 mA. The binding energy in the range -2 to 1400 eV. After subtraction of the Shirley-type background, the core-level spectra were decomposed into their components with mixed Gaussian-Lorentzian (30:70) shape lines using the CasaXPS software. 1.5. X-ray Diffraction (XRD) X-ray diffraction (XRD) is a powerful nondestructive technique that provides detailed information about the chemical composition and crystallographic structure of the samples. It gives details information about the chemical composition, lattice parameter, lattice strain, crystalline size (in case of metal nanoparticles) and the type of chemical bonding. XRD measurements were collected on a Siemens D500 diffractometer (Ni-filtered Cu Kα radiation, 1.5405 Ǻ, FEI, USA) at a voltage of 40 Kv with an intensity of 40 mA. d(0 0 2) basal spacings were calculated from the 2ϴ values using the Bragg's law.

Figure 1 .Figure 2 . 2 . 1 . 2 .

 12212 Figure 1. 1 H NMR spectra of N-(2-aminoethyl)carbamic acid tert-butyl ester (1) in

( 1 :

 1 1 v/v, EtOAc/MeOH). Yield: 3.12g (91.15%). 1 H NMR (600MHz, CDCl3, Figure 3) δ ppm: 8.51 (d, 2H, J = 4 Hz), 7.64 (t, 2H, J = 8Hz), 7.42 (d, 2H, J= 8Hz), 7.15 (m, 2H), 5.83 (s, 1H), 3.83 (s, 4H),3.20 (m, 2H), 2.68 (m, 2H), 1.41 (s, 9H). 13 C NMR (150MHz, CDCl3, Figure 4) δ ppm:28.72, 38.49, 48.59, 60.18, 78.61, 122.01, 122.56, 135.98, 148.50, 156.48, and 158.93.

Figure 3 .

 3 Figure 3. 1 H NMR spectra of N-(2-bis(2-pyridymethyl)amino)ethyl)carbamic acid tert-butyl ester (2) in CDCl3

Figure 4 .

 4 Figure 4. 13 C NMR spectra of N-(2-bis(2-pyridymethyl)amino)ethyl)carbamic acid tert-butyl ester (2) in CDCl3

Figure 5 .Figure 6 .

 56 Figure 5. 1 H NMR spectra of N,N-bis(2-pyridymethyl)ethylenediamine (BPED) (4) in CDCl3

Figure 7 .

 7 Figure 7.

Figure 7 .

 7 Figure 7. Scheme of the preparation of graphene oxide (GO) from graphite.

Figure 8 .

 8 Figure 8. UV-Vis spectra of the BPED containing surnageant during the purification of GO-EDA-CAC-BPED adsorbent (5 washing procedures with EtOH).

Figure 9 .

 9 Figure 9. UV-Vis spectra of the BPED containing surnageant during the purification of [(GO-EDA-CAC)@Fe3O4NPs-BPED)] (7 washing procedures with EtOH).

Figure 10 .

 10 Figure 10. UV-Vis spectra of the BPED containing supernatant during the purification of GO-BPED (5 washing procedures with EtOH).

4. 3 .I. 1 .

 31 Typical procedure for the synthesis of the one-pot synthesis of propargylamines in the presence of the GO-BPED-PS@NiNPs material 20 mg of GO-BPED-PS@NiNPs catalyst (1 mol %) was added to the mixture of aldehyde (1 mmol), morpholine (1.2 mmol), and the terminal alkyne (1.5 mmol) in 3 mL of γ-Valerolactone. The reaction mixture was stirred at 100 °C for the time indicated. At the end of the reaction, the mixture was cooled to room temperature and the solid catalyst was recovered by a strong magnet, rinsed with hot ethanol and deionized water, and dried carefully before use in the next run. The crude product was then extracted with 30 mL of dichloromethane and purified by short column chromatography (petroleum ether: EtOAc, 95:5) to acquire the desired product in high purity. In addition, the γ-Valerolactone phase from the previous run was also maintained for 30 min under a dynamic vacuum while stirring at 150 °C, and then the corresponding catalyst and reagents were added to the γ-Valerolactone phase and treated under the same reaction conditions as applied for the first run.A. Functionalization of graphene oxide sheets as a supported adsorbents for removal of metal ions from aqueous solutionsComparative studies on the adsorption of Cu(II), Ni(II), Co(II) ions from aqueous solutions using various functionalized graphene oxide sheets as supported adsorbents  Abstract Graphene oxide (GO) was chemically modified by N,N-bis(2pyridylmethyl)amino groups (BPED) through a multistep procedure. For comparison, and to justify the grafting of BPED groups onto the GO sheets, the GO-based material obtained after each step was used as a solid phase adsorbent for removing Cu(II), Ni(II)and Co(II) metal ions from aqueous solutions. The influence of metal ion concentrations, pH, contact time and temperature on their adsorption onto the GObased adsorbents was investigated and the GO-EDA-CAC-BPED adsorbent showed the highest ability to adsorb Cu(II), Ni(II) and Co(II) with a concentration of 250 mg.L-1 at pH=7. Additionally, it was demonstrated that the equilibrium adsorption capacities of these metal ions followed the order of Cu(II)>Ni(II)>Co(II) whatever the GO-based adsorbent. Moreover, to examine the underlying mechanism of the adsorption process, pseudo-first order, pseudo-second order, Elovich or Roginsky-Zeldovich and intraparticle diffusion models were fitted to experimental kinetic data.

  used bis-thiosemicarbazone as a color reagent for the spectrophotometric determination of Co(II), Ni(II) and Cu(II) ion concentrations in aqueous solution.This section aims to design a novel multi-functional GO-based adsorbent for high efficiency removal of Cu(II), Ni(II) and Co(II) ions from aqueous solutions. First, GO was modified by ethylenediamaine (GO-EDA) followed by the grafting of chloroacetychloride (GO-EDA-CAC). Then, the grafting of bis(2-pyridylmethyl)amino groups (BPED) onto GO-EDA-CAC was realized and producing the adsorbent (GO-EDA-CAC-BPED). All GO-based materials were characterized with the help of FTIR, TGA, XPS, SEM and UV-Vis. Then, the effect of the functionalization of GO sheets on their metal ion adsorption capacities was studied by using GO, GO-EDA, GO-EDA-CAC and GO-EDA-CAC-BPED as adsorbents to remove Cu(II), Ni(II) and Co(II) metal ions from aqueous solutions.

Figure 1 .

 1 Figure 1. FT-IR spectra of (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED.

Figure 2 .

 2 Figure 2. X-ray photoelectron spectroscopy binding energies of the C 1s in the (a) GO-EAD-CAC and (b) GO-EDA-CAC-BPED.

Figure 3 .

 3 Figure 3. X-ray photoelectron spectroscopy binding energies of the N 1s in the (a) GO-EAD-CAC and (b) GO-EDA-CAC-BPED.

  The high resolution C 1s spectra of GO-EDA-CAC (Fig.2a) can be deconvoluted into four peaks corresponding to C=C/C-C at 284.55 eV, C-N at 285.92 eV, O-C-O at287.18

Figure 4 .

 4 Figure 4. Digital images of (a) GO-EDA, (b) GO-EDA-CAC and (c) GO-EDA-CAC-BPED dispersions in water and various organic solvents at a concentration of 0.20 mg.mL -1 and 24 h after the ultrasonication treatment : (1) Water, (2) Ethanol, (3) Hexane, (4) Toluene, (5) DMF, (6) DMSO, (7) THF, and (8) Diethyl ether.

Figure 5 .

 5 Figure 5. TGA curves of (a) GO, (b) GO-EDA, (c) GO-EDA-CAC, (d) GO-EDA-CAC-BPED.

Figure 6 .

 6 Figure 6. SEM images of (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED.

Figure 7 .

 7 Figure 7. UV-Vis spectra of an aqueous (a) GO, (b) GO-EDA and (c) GO-EDA-CAC-BPED.

  comparison, the adsorption capacities of the GO-based materials to Cu(II), Ni(II) and Co(II) ions were investigated and the results are depicted in Fig. 8.

Figure 8 .

 8 Figure 8. Effect of the pH of the aqueous metal ion solution onto the adsorption capacities of GO, GO-EDA, GO-EDA-CAC and GO-EDA-CAC-BPED toward (a) Cu(II), (b) Ni(II) and (c) Co(II) ions ([metal ions] = 250 mg.L -1 , T = 293 K, [Adsorbent] = 0.100 g/100 mL, t =24 h); the error bars correspond to one standard deviation n=3.

Figure 9 .

 9 Figure 9. Plot for the determination of the point zero charge of GO, GO-EDA, GO-EDA-CAC and GO-EDA-CAC-BPED.

Figure 10 .

 10 Figure 10. Effect of the initial concentration of Cu(II), Ni(II) and Co(II) ions on their removal from aqueous solutions by the different adsorbents (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED. ([Adsorbent] = 0.100 g/100 mL, T = 293

Figure 11 .

 11 Figure 11. Effect of contact time on the adsorption capacities of (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED for Cu(II), Ni(II) and Co(II) ions ([metal ions] =250 mg.L -1 , T = 293 K, [adsorbent] = 0.100 g/100 mL and using optimal pH conditions : pH=5 for GO, pH=6 for GO-EDA, pH=6 for GO-EDA-CAC and pH=7 for GO-EDA-CAC-BPED); the error bars correspond to one standard deviation n=3.

  Co(II) onto adsorbents (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED.According to the correlation coefficient values, it can be concluded that the Elovich model is will adapted to describe the adsorption of Cu(II), Ni(II) and Co(II) ions onto the GO-based adsorbents in comparison with the pseudo-first-order kinetic model. In addition, the initial adsorption rates are in the order: GO-EDA-CAC-BPED>GO-EDA-CAC>GO-EDA>GO for removing Cu(II), Ni(II) and Co(II) ions and it is clear that these variations are dependent on the nature of the functional groups located onto the surface of the adsorbents.

Figure 12 .Table 6 .

 126 Figure 12. Adsorption capacities versus t 1/2 calculated with the intraparticle diffusion kinetic model for the adsorption of Cu(II), Ni(II) and Co(II) ions by the adsorbents : (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED.

  -1 ) is the Langmuir constant related to the energy of adsorption. The value of RL indicates the adsorption behavior: RL>1 : unfavorable ; RL =1 : linear ; 0 <RL<1 : favorable ; RL =0 : irreversible The Freundlich isotherm model is more suitable for describing the enthalpy of adsorption on a heterogeneous surface and it can be derived, assuming a binding affinity that decreases with the increase in adsorption degree. The linear form of the Freundlich equation is given as: 𝑙𝑛Q e = 𝑙𝑛(𝐾 𝐹 ) + 1 𝑛 𝑙𝑛(𝐶 𝑒 ) (𝟖) Where, KF and n are the Freundlich constants representing the adsorption capacity and adsorption intensity, respectively. The Jossens model represents a combination between the two previous models and describes a simple equation based on the energy distribution of adsorbate/adsorbent interactions at adsorption sites.

Figure 11 .

 11 Figure 11. Metal ions desorption at different concentrations of EDTA onto different adsorbents: (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED.

Figure 12 .

 12 Figure 12. Adsorption capacities of (a) GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED for Cu(II), Ni(II) and Co(II) after desorption-readsorption cycles. (desorption condition: [EDTA]=0.02 mg.L -1 ), (adsorption conditions: [metal ions] = 250 mg.L -1 , T = 293 K, [Adsorbent] = 0.100 g/100 mL and using optimal pH

  -PAA: soybean dregs-poly(acrylic acid), GO-PSF: Graphene oxide-polysulfone, WTRs: water treatment residuals, Cell-g-HEMA-Co-ACC: Cellulose (Cell) grafted by vinyl monomers hydroxyethyl methacrylate (HEMA) and acrylic acid (AAc), Cell-g-NIPAM-Co-GMA: Cellulose grafted by N-isopropylacrylamide (NIPAM) and glycidyl methacrylate (GMA) and GO-PVA: Graphene oxide grafted by poly(vinyl alcohol).Furthermore, our GO-based adsorbents can be regenerated several times, more than 10 cycles, confirming that they are excellent candidates for commercial applications involving the effective removal of Cu(II), Ni(II) and Co(II) ions from aqueous solutions.1.3. ConclusionFor the removal of Cu(II), Ni(II) and Co(II) ions from aqueous solutions, we have synthesized new graphite oxide (GO) based adsorbents through the chemical modification of GO. A multistep procedure was used to graft N,N-bis(2pyridylmethyl)ethylenediamine (BPED) groups onto the GO surface. Nevertheless, the adsorption capacities of Cu(II), Ni(II) and Co(II) of the GO-based materials were studied after each step and it was shown that the pH of the metal ion containing aqueous solution, the contact time and the initial metal ion concentration significantly affect their adsorption capacities. It was shown that the GO-EDA-CAC-BPED adsorbent exhibit the highest ability to adsorb of Cu(II), Ni(II) and Co(II) ions for a metal ion concentration of 250 mg.L -1 at pH=7 and their equilibrium adsorption capacities were as high as 3.464±0.100 mmol.g -1 , 3.254±0.098 mmol.g -1 and 3.054±0.093 mmol.g -1 , respectively. Then, considering the adsorption kinetics, the pseudo-secondorder model was the most appropriate one to describe the adsorption of these metal ions by the GO-based adsorbents. Furthermore, for the adsorption isotherm it was shown that the Jossens model has been the best one in simulating the adsorption of Cu(II), Ni(II) and Co(II) onto GO, GO-EDA, GO-EDA-CAC and GO-EDA-CAC-BPED.The thermodynamic adsorption parameters were also calculated and the obtained positive values for both enthalpy change (ΔH°) and entropy change (ΔS°) demonstrated the endothermic features of the adsorption process for GO and GO-EDA-CAC adsorbents. In contrary, the adsorption process was exothermic for the GO-EDA and GO-EDA-CAC-BPED adsorbents. Finally, it was shown that all the GObased adsorbents exhibit good recyclability and reusability for Cu(II), Ni(II) and Co(II)

Figure 1 .

 1 Figure 1. FTIR spectra of (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

Figure 2 .

 2 Figure 2. (a) EDX spectrum and (b) SEM image of (GO-EDA-CAC)@Fe3O4NPs.

Figure 3 .

 3 Figure 3. TGA of (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

Figure 4 .

 4 Figure 4. X-ray photoelectron spectroscopy energies of the C 1s in the (a) (GO-EDA CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

Figure 5 .

 5 Figure 5. X-ray photoelectron spectroscopy energies of the N 1s in the (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

Figure 6 .

 6 Figure 6. SEM image (a) of [(GO-EDA-CAC)@Fe3O4NPs-BPED)].

Figure 7 .

 7 Figure 7. XRD patterns of [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

Figure 8 . 1 )

 81 Figure 8. (a) Magnetization (M) versus magnetic field (H) recorded at 300K for (1) Fe3O4NPs and (2) [(GO-EDA-CAC)@Fe3O4NPS]-BPED. (b) Susceptibility of [(GO-EDA-CAC)@Fe3O4NPS]-BPED (2), when magnetic field is near zero.

  2), K is the anisotropy constant of the crystal (K= 5×104 J m -3 ), Vm is the crystal volume (1.125×10 - 26 m 3 ), kb is the Boltzmann constant (kb = 1,380 649 × 10 -23 J.K -1 ), τm is the measurement time and τ_0 is a characteristic relaxation time of crystal of 10 -11 s [47].

Figure 9 .

 9 Figure 9. ZFC (Zero field cooled) and FC (Field cooled) temperature dependence of magnetic susceptibility for [(GO-EDA-CAC)@Fe3O4NPS]-BPED.

Figure 10 .

 10 Figure 10. Effect of pH on the adsorption capacities of Cu(II) ions by (1) (GO-EDA-CAC)@Fe3O4NPs, (2) [(GO-EDA-CAC)@Fe3O4NPs]-BPED and (3) without adsorbent; conditions: [Cu(II)]=250 mg.L -1 , T=293 K, [Adsorbent]=0.100g/100 mL and t= 24h); the error bars correspond to one standard deviation n=3.

Figure 12 .

 12 Figure 12. Effect of contact time on the adsorption capacities of Cu(II) ions on (1) (GO-EDA-CAC)@Fe3O4NPs and (2) [(GO-EDA-CAC)@Fe3O4NPs]-BPED. [Cu(II)]

Figure 13 .

 13 Figure 13. (a) Pseudo-first-order and (b) Pseudo-second-order kinetics plots for adsorption of Cu(II) ions onto (1) (GO-EDA-CAC)@Fe3O4NPs and (2) [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

Figure 14 .

 14 Figure 14. Adsorption capacities versus t 1/2 calculated from the intra-particle diffusion kinetic model for the adsorption of Cu(II) ions by the adsorbents (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED.

Figure 15 .

 15 Figure 15. Adsorption capacities of (a) (GO-EDA-CAC)@Fe3O4NPs, and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED, for Cu(II) ions after desorption-readsorption cycles. (desorption condition: [EDTA]=0.02 mg.L -1 ), (adsorption conditions: [metal ions] = 250 mg.L -1 , T = 293 K, [Adsorbent] = 0.100 g/100 mL and using optimal pH and contact time conditions : pH=7 and t=5h for (GO-EDA-CAC)@Fe3O4NPs, and pH=7 and t=3h for [(GO-EDA-CAC)@Fe3O4NPs]-BPED; the error bars correspond to one standard deviation n=3.

  = Polyvinyltetrazole (PVT)-grafted polystyrene (PS) resin, Ti-Mg Nanorod = Magnesium titanate nanorods, GO = graphene oxide sheets, GO-EDA-CAC = The grafting of ethylenediamine (EDA) and chloroacetyl chloride (CAC) onto GO, GO-EDA-CAC-BPED = The grafting of ethylenediamine (EDA), chloroacetyl chloride (CAC) and N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) onto GO.

Scheme 1 .

 1 Scheme 1. Preparation of the zwitterionic GO-based material through the functionalization of GO sheets with bis(2-pyridylmethyl)-ethylenediamine (BPED) and 1,3-propanesultone (PS).

Figure 1 .

 1 Figure 1. FTIR spectra of (a) GO-BPED and (b) GO-BPED-PS.

Figure 2 .

 2 Figure 2. High-resolution X-ray photoelectron spectroscopy spectra of the carbon region (C 1s) for (a) GO-BPED and (b) GO-BPED-PS.

Figure 3 .

 3 Figure 3. High-resolution X-ray photoelectron spectroscopy spectra of the nitrogen region N 1s for (a) GO-BPED and (b) GO-BPED-PS.

Figure 4 .

 4 Figure 4. High-resolution X-ray photoelectron spectroscopy spectra of the sulfur region (S 2p) for GO-BPED-PS

Figure 5 .

 5 Figure 5. Influence of time onto the stability of GO derivatives dispersions in water with a concentration of 0.35 mg.mL -1 : (a) GO, (b) GO-BPED and c) GO-BPED-PS.

Figure 6 .

 6 Figure 6. TGA of (a) GO, (b) GO-BPED and (c) GO-BPED-PS.

Figure 7 .

 7 Figure 7. SEM images of (a) GO-BPED and (b) GO-BPED-PS.

Figure 8 .

 8 Figure 8. Effect of pH on the equilibrium adsorption capacities (Qe) of a) Ni(II) and b) Co(II) ions on the surface of GO-BPED and GO-BPED-PS, and without any adsorbent; [Adsorbent]=1mg.mL -1 , [Metal ion]=250 mg.L -1 , T=293 K, and t= 24h; the error bars correspond to one standard deviation n=3.

Figure 9 .

 9 Figure 9. Plot for the determination of the point zero charge of GO-BPED and GO-BPED-PS.

Figure 10 .

 10 Figure 10. Effect of the initial metal ion concentration on the maximum adsorption capacity of (a) GO-BPED and (b) GO-BPED-PS. [Adsorbent] =1mg.mL -1 , T = 293 K, and pH=7; the error bars correspond to one standard deviation n=3.

4. 1 . 1 . 10 .

 1110 Effect of the contact timeIn order to investigate the efficiency of the adsorption of Ni(II) and Co(II) ions by GO-BPED and GO-BPED-PS, we have studied the time to reach equilibrium in the time range 1 minute-7 hours (Fig.11).

Figure 11 .

 11 Figure 11. Adsorption kinetics of (a) GO-BPED and (b) GO-BPED-PS towards Ni(II) and Co(II) ions ([Adsorbent] =1mg.mL -1 , [Metal ion] =250 mg.L -1 , T = 293 K, and pH=7) ; the error bars correspond to one standard deviation n=3.

Figure 11 4 . 1 . 1 . 11 .

 1141111 Figure11shows a very fast increase of the adsorption capacities with time in the first few minutes whatever the metal ion and the adsorbent and then a plateau is observed for contact times higher than 1 hour. The maximum adsorption capacities are obtained in a lower contact time for the GO-BPED-PS adsorbent in comparison with the GO-BPED sample suggesting the presence of more efficient binding sites onto the GO surface and better stabilization and dispersion in the aqueous solution. The

Figure 12 .

 12 Figure 12. Effect of the presence of competing cations in the metal-based aqueous solutions on the maximum adsorption capacities of (a) GO-BPED and (b) GO-BPED-PS for Ni(II) and Co(II) ions, (Adsorption conditions: [Adsorbent] =1mg.mL -1 , [Metal ions]= 250 mg.L -1 , T = 293 K,[competing cations]= 100 mg.L -1 , and using optimal pH conditions and contact time conditions: pH=7 for GO-BPED and GO-BPED-PS with t = 4h and t =1h, respectively ; the error bars correspond to one standard deviation n=3.

Figure 13 .

 13 Figure 13. Pseudo-first-order kinetic plots for the adsorption of Ni(II) and Co(II) ions on (a) GO-BPED and (b) GO-BPED-PS.

Figure 14 .

 14 Figure 14. Pseudo-first-order kinetic plots for the adsorption of Ni(II) and Co(II) ions on (a) GO-BPED and (b) GO-BPED-PS.

Fig. 15

 15 Fig.15illustrates the multi-linear plots of Qt versus t 1/2 obtained from the intraparticle diffusion model whatever the adsorbent.

Figure 15 .

 15 Figure 15. Intra-particle diffusion kinetic for the adsorption of Ni(II) and Co(II) ions by (a) GO-BPED and (b) GO-BPED-PS. (B.P = break point).

Figure 16 .

 16 Figure 16. Metal ion desorption efficiency of EDTA-, HCl-and HNO3-based aqueous solutions (0.02 mg.L -1 ) onto a) sorbed GO-BPED and b) sorbed GO-BPED-PS.

Figure 17 .

 17 Figure 17. Maximum adsorption capacities of a) GO-BPED and b) GO-BPED-PS for Ni(II) and Co(II) ions after desorption/re-adsorption cycles. (Desorption experimental conditions: [EDTA]=0.02 mg.L -1 for GO-BPED and [HCl]= 0.02 mg.L -1 for GO-BPED-PS) ; Adsorption experimental conditions: [Adsorbent] = 1mg.mL -1 , [metal ions] = 250 mg.L -1 , T = 293 K, at pH=7 for both adsorbents and contact time of 4h and 1h, respectively; the error bars correspond to one standard deviation n=2.

( 2 -

 2 pyridylmethyl)ethylenediamine (BPED) and 1,3-propanesultone (PS) allowed for the preparation of efficient adsorbents for the removal of Co(II) and Ni(II) ions from aqueous solutions, using a batch process. The influence of pH, contact time, metal ion concentration, and temperature on the adsorption behavior of Co(II) and Ni(II) ions onto the GO-based adsorbents was investigated and compared. It was demonstrated that the zwitterionic GO-BPED-PS adsorbent exhibited very high adsorption capacities for both Ni(II) and Co(II) ions with 4.174±0.098 mmol.g -1 and 3.902±0.090 mmol.g -1 ,

Figure 1 .

 1 Figure 1. Effect of the different catalysts on the time-yield plot for the homocoupling of phenylacetylene: (1) without any catalyst, (2) with (GO-EDA-CAC)@Fe3O4NPs (10 mg), (3) with [(GO-EDA-CAC)@Fe3O4NPs]-BPED (10 mg), (4) with CuI (4 mg), (5) with CuBr2 (4 mg), (6) with CuCl2 (4 mg), (7) with (GO-EDA-CAC)@Fe3O4NPs-Cu(II) (10 mg ; 2.2 mg Cu(II)) and (8) with [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) (10 mg ; 2.4 mg Cu(II)). Experimental conditions: phenylacetylene (1.0 mmol, 1.0 eq.), Et3N (1.2 mmol, 1.2 eq.), and EtOH (1.0 mL) at 80 °C under air, the error bars correspond to one standard deviation n=2.

Figure 2 .

 2 Figure 2. Effect of temperature on the time-yield plot for the homocoupling reaction: (1) 40 °C, (2) 60 °C, and (3) 80 °C. Experimental conditions: Phenylacetylene (1.0 mmol, 1.0 eq.), Et3N (1.2 mmol, 1.2 eq.), [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) (10 mg), and glycerol (1.0 mL); the error bars correspond to one standard deviation n=2.

b

  Experimental conditions: Phenylacetylene (1.0 mmol, 1.0 eq.), base (1.2 mmol, 1.2 eq.), [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) (10 mg), and glycerol (1.0 mL) at 80 °C under air for 120 min; the error bars correspond to one standard deviation n=2.

3 ) 3 ) 4 )

 334 𝐀𝐭𝐨𝐦 𝐞𝐜𝐨𝐧𝐨𝐦𝐲 (%) = Molecular weight of desired product ∑ Molecular weight of reactants × 100 (Moreover, an alternative mass-based metric such as the Reaction Mass Efficiency is usually discussed and is defined as the percentage of actual mass of desire product to the mass of all reactants used. It takes into account both atom economy and chemical yield (Eq.

Scheme 1 .

 1 Scheme 1. A plausible mechanism for the [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) catalyzed homocoupling of terminal alkynes.

Figure 4 .

 4 Figure 4. Number of the catalytic runs for the homocouplig reaction of phenylacetylene in presence of [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II).

Figure 5 .

 5 Figure 5. FTIR spectra of (a) as-prepared [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) and (b) recovered [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) after 15 consecutive trials.

Figure 6 .

 6 Figure 6. Influence of the removal of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) onto the homocoupling yield of phenylacetylene; the error bars correspond to one standard deviation n=3.

Figure 1 .

 1 Figure 1. FTIR of GO-BPED-PS@NiNPs.

Figure 3 .

 3 Figure 3. TGA of (a) GO-BPED-Ps and (b) GO-BPED-PS@NiNPs.

Figure 4 .

 4 Figure 4. XRD patterns of GO-BPED-PS@NiNPs.

Figure 5 .

 5 Figure 5. (a) Magnetization (M) versus magnetic field (H) achieved at 300K for GO-BPED-PS@NiNPs. (b) Susceptibility of GO-BPED-PS@NiNPs, when magnetic field is near zero.

3 ( 2 ) 6 .Fig. 6 .( 5 )

 32665 Fig. 6. Effect of the Ni-based catalysts on the time/yield plot for the A3 coupling reaction of benzaldehyde, morpholine, and phenylacetylene: (1) without any catalyst, (2) GO-BPED-PS (20 mg), (3) Ni(OCOCH3)2 (20 mg), (4) NiBr2 (20 mg), (5) NiCl2 ( 20 mg), (6) bulk Ni (20 mg), and (7) GO-BPED-PS@NiNPs (20 mg; 1 mol%). Experimental conditions: benzaldehyde (1.0 mmol), morpholine (1.2 mmol), and phenylacetylene (1.5 mmol), and DMSO (3.0 mL) at 90 °C under air, the error bars correspond to one standard deviation n = 2.

Table 2 .b

 2 Effect of the temperature on the time/yield plot for the A3 coupling reaction of benzaldehyde, morpholine, and phenylacetylene in presence GO-BPED-Experimental conditions: benzaldehyde (1.0 mmol), morpholine (1.2 mmol), and phenylacetylene (1.5 mmol), GO-BPED-PS@NiNPs (20 mg, 1mol%) and γ-Valerolactone (3.0 mL) for 3h; the error bars correspond to one standard deviation n = 2.

Figure 7 .

 7 Figure 7. GO-BPED-PS@NiNPs catalyzed three-component coupling of benzaldehyde, morpholine, and terminal alkynes; Experimental conditions: benzaldehyde (1.0 mmol), morpholine (1.2 mmol), and terminal alkynes (1.5 mmol), GO-BPED-PS@NiNPs (20 mg, 1mol%) and γ-Valerolactone (3.0 mL) at 100 °C for 3h.

  First, graphene oxide (GO) was chemically modified by N,N-bis(2-pyridylmethyl)amino groups (BPED) through a multistep procedure. The influence of metal ion concentrations, pH, contact time, and temperature on their adsorption onto the GObased adsorbents was investigated and the GO-EDA-CAC-BPED adsorbent showed the highest ability to adsorb Cu(II), Ni(II), and Co(II) ions. In addition, it was shown that the adsorption behavior processes of metal ions onto both the GO-based adsorbents correspond to a pseudo-second-order kinetic model and Jossens isotherm model. The thermodynamic adsorption parameters were also calculated and the obtained negative values for both enthalpy change (ΔH°) and entropy change (ΔS°) demonstrated the exothermic features for the GO-EDA and GO-EDA-CAC-BPED adsorbents. Finally, it was shown that all the GO-based adsorbents exhibit good recyclability and reusability for Cu(II), Ni(II), and Co(II) ions.

Publication 1 :Publication 2 :Publication 3 :Publication 4 :Publication 6 :

 12346 "Comparative studies on the adsorption of metal ions from aqueous solutions using various functionalized graphene oxide sheets as supported adsorbents" L. Chaabane, E. Beyou, A. El ghali, M.H.V. Baouab, Journal of Hazardous Materials, (IF: 9.03) 389 (2019) 121839. "Functionalization of graphene oxide sheets with magnetite nanoparticles for the adsorption of copper ions and investigation of its potential catalytic activity toward the homocoupling of alkynes under green conditions" L. Chaabane, E. Beyou, D. Luneau, M.H.V. Baouab, Journal of Catalysis, (IF: 7.88) 388, (2020) 91-103. "Comprehensive study of the adsorption of heavy metal ions from aqueous solutions by zwitterionic graphene oxide sheets as adsorbents" L. Chaabane, E. Beyou, M.H.V. Baouab, Carbon (IF: 4.30) (2020) (under review). "Ni nanoparticles onto zwitterionic graphenen oxide sheets supports as catalyst for A3 coupling reaction with δValerolactone as renewable dipolar aprotic solvent". L. Chaabane, E. Beyou, M.H.V. Baouab, Green chemistry (IF: 9.42) (2020) (Preparation). Publication 5:"Functionalization of developed bacterial cellulose with magnetite nanoparticles for nanobiotechnology and nanomedicine applications" L. Chaabane, H. Chahdoura, R. Mehdaoui, E. Beyou, MLahcini,M.H.V. Baouab, Carbohydrate Polymers (IF: 7.18), 18 (2020) 139-143. "Synthesis and characterization of Ni (II), Cu (II), Fe (II) and Fe3O4 nanoparticle complexes with tetraaza macrocyclic Schiff base ligand for antimicrobial activity and cytotoxic activity against cancer and normal cells" L. Chaabane, H. Chahdoura, S. Mejdi, E. Beyou, M. Lahcini,M.H.V. Baouab, Applied Organometallic Chemistry (IF: 3.12) (2019), https://doi.org/10.1002/aoc.4860.

Table 1 .

 1 Comparison of the maximum adsorption capacities, pH, and exploited kinetic and isotherm models to various adsorbnets for the removal metal ions from aqueous solutions.

	Adsorbents	Heavy metal ions	Qmax (mmol.g -1 )	pH	Kinetic model	Isotherm model	Ref.
	Acrtivated carbon	Cr(VI)	0.846	2.0	Pseudo-Second-order	Langmuir	[214]
	Coconut tree sawdust	Cr(VI)	0.025	3.0	Pseudo-Second-order	Langmuir	[218]
	Coconut shell carbon	Cr(VI)	0.258	5.0	-	Langmuir	[219]
		Pb(II)	0.594	5.5	Pseudo-Second-order	Langmuir-Freundlich
	SWCNTs oxide	Cu(II)	0.375	5.5	Pseudo-Second-order	Langmuir-Freundlich	[227]
		Cd(II)	0.097	5.5	Pseudo-Second-order	Langmuir-Freundlich
		Cd(II)	0.259	6.0	Pseudo-Second-order	Langmuir
	MWCNTs-EDA-CD	Cu(II)	0.586	5.0	Pseudo-Second-order	Langmuir	[229]
		Zn(II)	0.165	6.0	Pseudo-Second-order	Langmuir
	GO	Cr(VI)	1.782	5.0	Pseudo-Second-order	Langmuir	[236]
		Cd(II)	2.541	8.3	Pseudo-Second-order	Langmuir
	GO-APL	Hg(II)	1.133	8.3	Pseudo-Second-order	Langmuir	[240]
		As(II)	1.756	7.6	Pseudo-Second-order	Langmuir
	GO-PEI	U(VI)	2.664	5.0	-	-	[245]
	GO-CS-EDT	Cr(VI)	1.657	2.0	-	-	[246]

Table 1 .

 1 Parameters obtained from UV absorbance analysis for different pH and concentration solutions of Cu(II), Ni(II) and Co(II) ions .In case of the presence of impurities (formation of metal hydroxides solid particles at pH>7) in the aqueous suspension, the surnageant was filtered through a 0.45 µm membrane filter. Morover, the adsorption kinetics were carried out using different adsorption time intervals. Additionally, the isothermal adsorption

	Metal ions	pH	λmax	Line of regression	R 2
		4	766	Y=0.151X	0.999
		5	736	Y=0.261X	0.998
	Cu(II)	6	722	Y=0.226X	0.999
		7	720	Y=0.372X	0.997
		8	716	Y=0.216X	0.998
		4	394	Y=0.105X	0.998
		5	392	Y=0.137X	0.999
	Ni(II)	6	390	Y=0.154X	0.996
		7	388	Y=0.297X	0.994
		8	384	Y=0.114X	0.998
		4	514	Y=0.091X	0.999
		5	512	Y=0.143X	0.996
	Co(II)	6	508	Y=0.209X	0.998
		7	506	Y=0.367X	0.999
		8	504	Y=0.144X	0.997

Table 1 .

 1 (AC) ratios of carbon-containing bonds to total area (AT) according to the XPS results.

Table 2 .

 2 (AN) ratios of nitrogen-containing bonds to total area (AT) according to the XPS results.

Table . 4

 . . Kinetic parameters calculated from both pseudo-first-order and pseudosecond-order equations for adsorption of Cu(II), Ni(II) and Co(II) onto adsorbents (a)The Elovich equation is widely used to describe the second-order kinetic and considers that the solid surface of adsorbent is heterogeneous. The values of kinetic constants of the Elovich model for all heavy metal ions are listed in table 5.

				First-order Kinetic model		Second-order Kinetic model
	Adsorbents	Metal Ions	Qe(mmol.g -1 )	K1×10² (min -1 )	Qe(mmol.g -1 )	R²	K2×10² (g.mmol -1 .min -1 )	Qe(mmol.g -1 )	R²
		Cu(II)	1.844±0.097	0.230±0.0001	2.152±0.0266 0.989	1.193±0.0214	1.972±0.0072 0.997
	a	Ni(II)	1.789±0.075	0.690±0.0001	1.651±0.0250 0.990	0.947±0.0532	1.945±0.0083 0.996
		Co(II)	1.510±0.105	0.701±0.0006	1.352±0.0147 0.994	1.176±0.0482	1.644±0.0081 0.997
		Cu(II)	2.080±0.095	0.921±0.0007	1.735±0.0146	0.993	1.362±0.0836	2.202±0.0060 0.997
	b	Ni(II)	1.874±0.100	1.151±0.0007	1.663±0.0143	0.995	1.121±0.0952	2.019±0.0064 0.998
		Co(II)	1.612±0.095	1.197±0.0007	1.339±0.0152	0.994	1.872±0.0197	1.703±0.0072 0.999
		Cu(II)	2.521±0.098	1.266±0.01	2.463±0.0254	0.989	0.790±0.0618	2.762±0.0053 0.997
	c	Ni(II)	2.044±0.067	1.358±0.001	1.774±0.0189	0.994	1.440±0.0545	2.183±0.0034 0.999
		Co(II)	1.696±0.092	1.220±0.0001	1.636±0.0197	0.993	1.024±0.0937	1.883±0.0096 0.995
		Cu(II)	3.464±0.100	1.750±0.0005	3.063±0.0429	0.962	0.579±0.0275	3.824±0.0068 0.996
	d	Ni(II)	3.254±0.098	2.275±0.0005	2.792±0.0417	0.978	1.187±0.0102	3.438±0.0039 0.997
		Co(II)	3.054±0.093	2.326±0.0052	2.541±0.0421	0.979	1.310±0.0507	3.227±0.0050 0.998

GO, (b) GO-EDA, (c) GO-EDA-CAC and (d) GO-EDA-CAC-BPED.

Table 5 .

 5 Kinetic parameters of Elovich equation for the adsorption of Cu(II), Ni(II) and

872 Ki2 (mmol.g -1 .min -1/2 )

	Adsorbents	Parameters	Cu(II)	Metal ions Ni(II)	Co(II)
		R 2	0.992	0.997	0.993
		Ki1 (mmol.g -1 .min -1/2 )	0.124±0.0043	0.114±0.0025	0.100±0.0032
	a	C1 (mmol.g -1 .min -1/2 ) R 2	0.019±0.0240 0.969	-0.010±0.0137 0.958	-0.025±0.0176 0.933
		Ki2 (mmol.g -1 .min -1/2 )	0.037±0.0033	0.039±0.0042	0.031±0.0041
		C2 (mmol.g -1 .min -1/2 )	1.029±0.0604	0.928±0.0755	0.841±0.075
		R 2	0.991	0.989	0.976
		Ki1 (mmol.g -1 .min -1/2 )	0.123±0.0058	0.114±0.0042	0.095±0.0052
	b	C1 (mmol.g -1 .min -1/2 ) R 2	0.142±0.0477 0.969	0.039±0.0341 0.994	0.124±0.0426 0.976
		Ki2 (mmol.g -1 .min -1/2 )	0.019±0.0024	0.025±0.0013	0.014±0.0015
		C2 (mmol.g -1 .min -1/2 )	1.650±0.0483	1.313±0.0268	1.297±0.0314
		R 2	0.998	0.987	0.972
		Ki1 (mmol.g -1 .min -1/2 )	0.176±0.0027	0.170±0.0077	0.129±0.0089
	c	C1 (mmol.g -1 .min -1/2 ) R 2	-0.049±0.0148 0.945	-0.040±0.0427 0.960	-0.089±0.0489 0.971
		Ki2 (mmol.g -1 .min -1/2 )	0.059±0.0101	0.027±0.0038	0.037±0.0044
		C2 (mmol.g -1 .min -1/2 )	1.421±0.1661	1.541±0.0637	1.008±0.0734
		R 2	0.953	0.955	0.964
		Ki1 (mmol.g -1 .min -1/2 )	0.489±0.0762	0.410±0.0623	0.356±0.0487
		C1 (mmol.g -1 .min -1/2 )	-0.863±0.4081	-0.332±0.3345	-0.064±0.2613
	d	R 2	0.934 0.0.100±0.0264 0.947 1.545±0.0345 0.217±0.0831
		C2 (

mmol.g -1 .min -1/2 )

		-0.018±0.0373	-0.0246±0.0489	-0.059±0.1175
	R 2	0.995	0.947	0.

963 Ki3 (mmol.g -1 .min -1/2 )

		0.083±0.0039	0.045±0.0107	0.026±0.0518
	C3 (mmol.g -1 .min -1/2 )	2.175±0.0524	2.563±0.1445	2.638±0.0694

Table 7a .

 7a Langmuir adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	Qref(mmol.g -1 )	Qcalc. (mmol.g -1 )	b (L.mmol -1 )	KL (L.g -1 )	RL	R 2
			293	2.203±0.097	2.770±0.080	0.812±0.043	2.257±0.043	0.231±0.010	0.992
		Cu(II)	313 333	2.518±0.095 2.864±0.090	3.085±0.091 3.115±0.007	1.009±0.028 2.547±0.018	3.125±0.0268 0.207±0.009 7.936±0.018 0.088±0.007	0.993 0.996
			353	3.147±0.032	3.289±0.020	5.846±0.050	19.230±0.005	0.041±.020	0.999
			293	1.909±0.075	2.087±0.008	2.147±0.031	4.484±0.031	0.055±0.008	0.997
	GO	Ni(II)	313 333	2.232±0.105 2.419±0.110	2.403±0.003 2.551±0.004	2.476±0.010 3.920±0.014	5.952±0.010 10.0±0.014	0.030±0.003 0.027±0.004	0.999 0.999
			353	2.606±0.085	2.762±0.001	4.582±0.005	12.658±0.005	0.022±0.001	0.999
			293	1.713±0.105	1.908±0.007	1.559±0.028	2.976±0.028	0.097±0.007	0.998
		Co(II)	313 333	1.798±0.111 1.968±0.078	1.901±0.004 2.035±0.001	3.437±0.016 5.857±0.006	6.535±0.016 11.904±0.006	0.065±0.004 0.034±0.001	0.999 0.999
			353	2.121±0.075	2.183±0.002	6.361±0.009	13.888 ±0.009 0.032±0.002	0.999

Table 7b .

 7b Langmuir adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	Qref(mmol.g -1 )	Qcalc. (mmol.g -1 )	b (L.mmol -1 )	KL(L.g -1 )	RL	R 2
			293	2.219±0.095	2.590±0.014	1.393±0.044	3.610±0.044	0.149±0.014	0.990
		Cu(II)	313 333	2.0774±0.110 1.573±0.094	2.544±0.012 1.897±0.015	0.963±0.039 0.963±0.057	2.450±0.039 1.828±0.057	0.197±0.012 0.180±0.015	0.992 0.992
			353	1.532±0.125	1.322±0.010	0.590±0.040	0.905±0.040	0.227±0.106	0.998
			293	2.044±0.100	2.352±0.010	1.261±0.037	2.967±0.037	0.056±0.010	0.992
	GO-EDA	Ni(II)	313 333	1.703±0.099 1.482±0.125	2.020±0.014 1.763±0.016	0.990±0.056 0.915±0.068	2.00±0.056 1.615±0.068	0.089±0.014 0.090±0.016	0.993 0.993
			353	1.022±0.097	1.282±0.024	0.636±0.109	0.816±0.109	0.174±0.024	0.992
			293	1.696±0.098	1.908±0.014	1.117±0.055	2.212 ±0.055	0.014±0.014	0.993
		Co(II)	313 333	1.459 ±0.120 1.272 ±0.098	1.692 ±0.016 1.464±0.014	1.094±0.069 1.144±0.059	1.851±0.069 1.675±0.059	0.128±0.016 0.136±0.014	0.993 0.996
			353	1.130±0.096	1.302±0.013	0.789±0.057	1.267±0.057	0.153±0.013	0.997

Table 7c .

 7c Langmuir adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA-CAC adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	Qref(mmol.g -1)	Qcalc. (mmol.g -1 )	b (L.mmol -1 )	KL (L.g -1 )	RL	R 2
			293	2.864±0.098	3.311±0.009	0.175±0.023	5.714±0.023	0.179±0.009	0.992
		Cu(II)	313 333	3.068±0.120 3.336±0.092	3.401±0.005 3.546±0.002	2.470±0.014 4.622±0.006	8.403±0.014 16.393±0.006	0.114±0.005 0.095±0.026	0.997 0.999
			353	3.446±0.097	3.546±0.001	10.846±0.002	38.461±0.002	0.024±0.001	0.999
			293	2.129±0.067	2.352±0.010	1.985±0.037	4.6728±0.037	0.043±0.010	0.995
	GO-EDA-CAC	Ni(II)	313 333	2.249±0.102 2.385±0.075	2.409±0.004 2.512±0.004	2.712±0.016 4.103±0.015	6.535±0.004 10.309±0.015	0.027±0.017 0.026±0.004	0.998 0.998
			353	2.555±0.085	2.624±0.002	8.286±0.008	21.739±0.008	0.013±0.026	0.999
			293	1.815±0.092	1.923±0.003	3.250±0.014	6.250 ±0.014	0.069±0.003	0.999
		Co(II)	313 333	1.951 ±0.098 2.104±0.049	2.000±0.001 2.159±0.022	6.225±0.006 6.521±0.007	12.500±0.006 14.084±0.007	0.032±0.005 0.038±0.022	0.999 0.999
			353	2.206±0.057	2.247±0.002	8.240±0.010	18.518±0.010	0.021±0.002	0.998

Table 7d .

 7d Langmuir adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA-CAC-BPED adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	Qref(mmol.g -1)	Qcalc. (mmol.g -1 )	b (L.mmol -1 )	KL (L.g -1 )	RL	R 2
			293	3.556±0.100	3.891±0.009	0.074±0.020	13.513±0.020 0.128±0.009	0.988
		Cu(II)	313 333	3.352±0.118 3.100±0.120	3.731±0.011 3.496±0.013	0.697±0.026 2.288±0.031	10.309±0.026 0.143±0.002 8.000±0.031 0.212±0.013	0.984 0.983
			353	2.691±0.095	3.105±0.018	1.740±0.051	5.405±0.051	0.169±0.018	0.973
			293	3.254±0.098	3.508±0.009	3.392±0.026	11.904±0.026 0.226±0.009	0.990
	GO-EDA-CAC-BPED	Ni(II)	313 333	3.015±0.122 2.675±0.098	3.401±0.013 3.086±0.014	1.973±0.038 1.521±0.045	6.711±0.038 4.694±0.045	0.146±0.013 0.060±0.014	0.983 0.984
			353	2.112±0.065	2.403±0.019	1.569±0.069	3.773±0.069	0.081±0.019	0.982
			293	3.156±0.093	3.401±0.008	0.086±0.024	11.627±0.024 0.029±0.008	0.992
		Co(II)	313 333	2.901±0.098 2.647±0.082	3.236±0.015 2.761±0.013	2.145±0.044 1.877±0.042	6.944±0.004 5.586±0.042	0.045±0.015 0.051±0.013	0.981 0.987
			353	2.206±0.103	2.733±0.022	1.201±0.075	3.278±0.075	0.186±0.022	0.971

Table 8a .

 8a Freundlich adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO adsorbent at different temperature.

	Support	Metal ions	Temperature (K)	KF	n	R 2
			293	1.112±0.048	2.008±.0429	0.943
		Cu(II)	313 333	1.352±0.048 1.915±0.025	2.070±0.044 3.048±0.019	0.937 0.973
			353	2.396±0.043	3.546±0.028	0.926
			293	1.216±0.058	2.808±0.047	0.901
	GO	Ni(II)	313 333	1.462±0.058 1.702±0.065	3.267±0.036 3.558±0.050	0.897 0.793
			353	1.901±0.047	3.717±0.038	0.882
			293	1.017±0.049	2.906±0.039	0.907
		Co(II)	313 333	1.262±0.047 1.490±0.035	3.984±0.038 4.854±0.024	0.854 0.894
			353	1.630±0.029	5.102±0.019	0.927

Table 8b .

 8b Freundlich adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	KF	n	R 2
			293	0.782±0.067	2.262±0.060	0.869
		Cu(II)	313 333	0.916±0.052 1.217±0.061	2.118±0.046 2.237±0.050	0.927 0.906
			353	1.526±0.045	2.247±0.036	0.948
			293	0.884±0.063	2.439±0.0527	0.883
	GO-EDA	Ni(II)	313 333	0.886±0.062 0.752±0.065	2.331±0.049 2.320±0.050	0.902 0.899
			353	0.469±0.058	2.169±0.043	0.934
			293	0.912±0.671	2.415±0.057	0.881
		Co(II)	313 333	0.778±0.073 0.701±0.056	2.460±0.057 2.680±0.0431	0.863 0.903
			353	0.590±0.056	2.597±0.042	0.912

Table 8c .

 8c Freundlich adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA-CAC adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	KF	n	R 2
			293	1.549±0.054	1.739±0.050	0.904
		Cu(II)	313 333	2.048±0.063 2.471±0.057	2.544±0.055 3.012±0.041	0.864 0.888
			353	2.817±0.069	3.921±0.040	0.831
			293	1.172±0.070	2.314±0.058	0.858
	GO-EDA-CAC	Ni(II)	313 333	1.481±0.053 1.685±0.064	3.225±0.042 3.584±0.052	0.867 0.795
			353	1.991±0.036	4.769±0.023	0.912
			293	1.264±0.046	3.937±0.035	0.862
		Co(II)	313 333	1.485±0.036 1.620±0.029	4.926±0.025 5.181±0.019	0.887 0.923
			353	1.755±0.035	5.847±0.012	0.885

Table 8d .

 8d Freundlich adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA-CAC-BPED adsorbent at different temperatures.

	As it can be seen, Langmuir isotherm model shows better correlation values (R 2 )
	and better fit with the experimental data than the Freundlich isotherm model,
	indicating that the adsorptions of metal ions onto the different GO-based adsorbents
	Support are based on a monolayer onto homogenous surfaces. Moreover, the Langmuir Metal ions Temperature (K) KF	n	R 2
	293 monolayer saturation capacity (Q) for Cu(II), Ni(II) and Co(II) ions follows the order 2.653±0.108	2.626±0.091	0.684
	313 GO-EDA-CAC-BPED>GO-EDA-CAC>GO-EDA>GO. The higher Q values at 293 K 2.372±0.102 Cu(II) 333 2.081±0.094 obtained for the GO-EDA-CAC-BPED adsorbent (QCu(II)=3.891±0.009 mmol.g -1 , QNi(II)=	2.481±0.092 2.364±0.088	0.704 0.742
			353	1.848±0.101	2.147±0.092	0.870
			293	2.302±0.102	2.898±0.086	0.638
	GO-EDA-CAC-BPED	Ni(II)	313 333	1.909±0.096 1.566±0.088	2.481±0.086 2.304±0.0788	0.730 0.791
			353	1.105±0.097	1.706±0.083	0.847
			293	2.278±0.094	2.941±0.078	0.732
		Co(II)	313 333	2.064±0.010 1.786±0.090	3.225±0.092 3.058±0.080	0.682 0.759
			353	1.377±0.103	2.531±0.091	0.768

Table 9a .

 9a Jossens adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	i	j	m	R 2
			293	1.626±0.08	0.329±0.055	1.300±0.063	0.997
		Cu(II)	313 333	2.204±0.112 22.652±3.602	0.435±0.062 7.357±1.452	1.265±0.053 0.798±0.041	0.996 0.986
			353	26.204±9.210	11.685±7.207	0.952±0.027	0.994
			293	2.815±0.0828	0.989±0.055	1.154±0.015	0.998
	GO	Ni(II)	313 333	4.837±0.335 7.047±0.661	0.026±0.005 0.027±0.007	1.045±0.023 1.089±0.032	0.997 0.995
			353	11.715±0.760	0.0716±0.010	1.00±0145	0.918
			293	2.152±0.156	0.889±0.125	1.112±0.036	0.997
		Co(II)	313 333	4.841±0.220 14.544±1.527	2.296±0.155 7.583±0.951	1.050±0.013 0.965±0.014	0.999 0.996
			353	29.601±4.893	15.426±2.847	0.924±0.015	0.994

Table 9b .

 9b Jossens adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA adsorbent at different temperatures

	Support	Metal ions	Temperature (K)	i	j	m	R 2
			293	2.083±0.100	0.379±0.053	1.415±0.055	0.997
		Cu(II)	313 333	1.628±0.072 1.203±0.039	0.343±0.046 0.338±0.032	1.325±0.050 1.318±0.034	0.998 0.999
			353	1.044±0.063	0.509±0.076	1.109±0.044	0.998
			293	1.943±0.059	0.554±0.041	1.251±0.024	0.999
	GO-EDA	Ni(II)	313 333	1.273±0.053 1.031±0.038	0.368±0.044 0.3291±0.036	1.311±0.042 1.324±0.039	0.998 0.999
			353	0.550±0.027	0.227±0.039	1.334±0.061	0.998
			293	1.411±0.029	0.401±0.022	1.282±0.018	0.997
		Co(II)	313	1.181±0.044	0.383±0.038	1.291±0.033	0.999
			333	1.137±0.044	0.522±0.048	1.183±0.026	0.999

Table 9c .

 9c Jossens adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA-CAC adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	i	j	m	R 2
			293	2.115±0.965	0.308±0.289	1.417±0.017	0.998
		Cu(II)	313 333	5.115±1.061 14.475±1.102	1.351±0.360 3.961±0.407	1.053±0.018 1.014±0.021	0.995 0.995
			353	38.521±3.122	10.991±1.053	0.997±0.014	0.997
			293	2.710±0.169	0.716±0.097	1.251±0.024	0.999
	GO-EDA-CAC	Ni(II)	313 333	4.347±0.334 6.884±0.615	1.453±0.189 2.315±0.314	1.117±0.031 1.095±0.030	0.996 0.994
			353	37.272±7.280	15.728±3.441	0.933±0.019	0.998
			293	4.922±0.230	2.358±0.162	1.041±0.013	0.996
		Co(II)	313 333	14.302±1.534 29.352±4.528	7.442±0.957 15.354±2.647	0.972±0.015 0.929±0.014	0.999 0.996
			353	46.480±1.740	22.792±0.942	0.949±0.003	0.997

Table 9d .

 9d Jossens adsorption isotherm parameters for Cu(II), Ni(II) and Co(II) ions adsorption onto the GO-EDA-CAC-BPED adsorbent at different temperatures.

	Support	Metal ions	Temperature (K)	i	j	m	R 2
			293	8.547±0.965	1.417±0.406	1.329±0.118	0.992
		Cu(II)	313 333	6.557±0.872 4.766±0.585	1.050±0.306 0.708±0.219	1.368±0.127 1.438±0.1359	0.994 0.992
			353	3.446±0.425	0.546±0.182	1.451±0.142	0.997
			293	7.025±1.057	1.234±0.376	1.315±0.112	0.996
	GO-EDA-CAC-BPED	Ni(II)	313 333	4.495±0.573 2.926±0.227	0.801±0.229 0.487±0.101	1.307±0.104 1.379±0.079	0.996 0.993
			353	2.171±0.950	0.419±0.109	1.417±0.097	0.998
			293	7.221±0.950	1.455±0.354	1.241±0.080	0.996
		Co(II)	313 333	4.619±0.690 3.380±0.334	0.860±0.282 0.619±0.149	1.312±0.118 1.349±0.088	0.999 0.996
			353	2.082±0.214	0.323±0.105	1.478±0.130	0.999

Table 10a .

 10a Thermodynamic parameters for adsorption of Cu(II), Ni(II) and Co(II) ions by the GO adsorbent under optimal conditions.

Table 10b .

 10b Thermodynamic

	Support	Metal ions	Temperature (K)	∆G° (kJ.mol -1 )	∆S° (J.mol -1 .K -1 )	∆H° (kJ.mol -1 )
			293	-1.983
		Cu(II)	313 333	-2.965 -5.735	114	32.06
			353	-8.676
			293	-3.655
	GO	Ni(II)	313 333	-4.641 -6.374	73	18.25
			353	-7.449
			293	-2.656
		Co(II)	313 333	-4.885 -6.857	85	22.19
			353	-7.721
	Support	Metal ions	Temperature (K)

parameters for adsorption of Cu(II), Ni(II) and Co(II) by GO-EDA under optimum conditions.

∆G° (kJ.mol -1 ) ∆S° (J.mol -1 .K -1 ) ∆H° (kJ.mol -1 ) GO-EDA Cu(II)

		293	-2.620
		313 333	-1.964 -0.673	-59	-20.14
		353	0.880
		293	-1.386
	Ni(II)	313 333	-0.256 0.561	-48	-15.64
		353	1.172
		293	-1.256
	Co(II)	313 333	-0.616 1.060	-55	-17.74
		353	1.902

Table 10c .

 10c Thermodynamic parameters for adsorption of Cu(II), Ni(II) and Co(II) ions by the GO-EDA-CAC adsorbent under optimal conditions.

	Support	Metal ions	Temperature (K)	∆G° (kJ.mol -1 )	∆S° (J.mol -1 .K -1 )	∆H° (kJ.mol -1 )
			293	-4.245
		Cu(II)	313 333	-5.539 -7.743	108	27.82
			353	-10.711
			293	-3.755
	GO-EDA-CAC	Ni(II)	313 333	-4.885 -6.459	87	22.09
			353	-9.036
			293	-4.464
		Co(II)	313 333	-6.572 -7.323	65	14.35
			353	-8.566
	Support	Metal ions	Temperature (K)

Table 10 d. Thermodynamic parameters for adsorption of Cu(II), Ni(II) and Co(II) ions by the GO-EDA-CAC-BPED adsorbent under optimal conditions.

∆G° (kJ.mol -1 ) ∆S° (J.mol -1 .K -1 ) ∆H° (kJ.mol -1 ) GO-EDA-CAC- BPED

		293	-6.342
	Cu(II)	313 333	-6.071 -5.757	-22	-13.02
		353	-4.952
		293	-6.033
	Ni(II)	313 333	-4.954 -4.281	-35	-16.22
		353	-3.891
		293	-5.976
	Co(II)	313 333	-5.043 -4.762	-38	-17.34
		353	-3.484

Table 11 .

 11 Comparison of the adsorption capacities of various adsorbents for the removal of Cu(II), Ni(II) and Co(II) ions from aqueous solutions.

Table 11

 11

	highlights that our GO-based adsorbents exhibit the highest
	adsorption capacities. In particular, the adsorption capacities of GO-EDA-CAC-BPED

for removing Cu(II), Ni(II) and Co(II) ions are 23 times higher than the GO-Cellulose adsorbent one

[74]

.

Table 1 .

 1 (AC) ratios of carbon-containing bonds to total area (AT) according to the

	XPS results.

Material

AC-C/AT (%) AC-N/AT (%) AO-C-O/AT (%) AC=O/AT (%)

Table 2 .

 2 (AN) ratios of nitrogen-containing bonds to total area (AT) according to the

	XPS results.

Table 4 .

 4 Langmuir adsorption isotherm parameters for Cu(II) ions adsorption onto (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED at different temperatures.

	Adsorbents	Temperature (K)	Qref (mmol.g -1 )	Qcalc. (mmol.g -1 )	b (L. mmol -1 )	KL (L.g -1 )	RL	R 2
			3.180±0.060	3.155±0.004	7.512±0.010	25.641±0.010	0.056±0.027	0.996
	(a)		3.415±0.095	3.546±0.003	8.545±0.008	30.303±0.008	0.039±0.015	0.998
			3.619±0.082	3.703±0.002	15.882±0.004	58.823±0.004	0.022±0.006	0.997
			3.777±0.038	3.802±0.001	17.533±0.001	66.666±0.001	0.016±0.017	0.999
			3.808±0.062	3.958±0.246	8.233±0.001	33.333±0.001	0.050±0.022	0.997
	(b)		3.462±0.071 3.068±0.092	3.563±0.004 3.115±0.003	5.571±0.009 3.445±0.008	20.408±0.009 11.111±0.008	0.080±0.014 0.148±0.091	0.998 0.999
			2.854±0.057	2.965±0.007	2.775±0.019	8.333±0.019	0.115±0.065	0.998

Table 5 .

 5 Freundlich adsorption isotherm parameters for Cu(II) ions adsorption onto (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED at different temperatures. Even if the regression coefficient values (R 2 ) are higher for the Langmuir model, it is believed that adsorption of Co(II) and Ni(II) ions onto GO-BPED and GO-BPED-PS involve a GO-based surface that is not completely homogeneous so the Jossens model represents a combination between Langmuir and Freundlich isotherms and it can be applied to both homogenous and heterogeneous systems. The calculated parameter values as well as the correlation coefficients at different temperatures are tabulated in

	Adsorbents	Temperature (K)	KF	n	R²
			2.596±0.093	3.496±0.064	0.809
	(a)		2.770±0.096	3.597±0.064	0.801
			3.083±0.103	4.081±0.056	0.799
			2.935±0.065	10.309±0.032	0.721
			3.052±0.069	2.624±0.036	0.946
	(b)		2.466±0.108 2.071±0.087	2.439±0.985 2.421±0.080	0.897 0.868
			1.700±0.099	2.222±0.095	0.834

Table 5 .Table 5 .

 55 Jossens

	Adsorbents	Temperature (K)	i	j	m	R²
		293	6.117±2.107	0.035±0.051	1.134±0.051	0.999
	(a)	313 333	9.070±2.458 10.570±3.451	2.504±0.820 4.256±1.675	1.137±0.041 1.106±0.046	0.999 0.999
		353	17.270±6.270	6.415±2.452	1.029±0.054	0.999
		293	11.480±6.321	0.215±0.052	1.080±0.052	0.999
	(b)	313 333	9.233±1.425 7.940±1.652	0.200±0.034 0.185±0.062	1.067±0.032 1.054±0.031	0.999 0.999
		353	5.213±0.211	0.073±0.011	1.226±0.023	0.999

adsorption isotherm parameters for Cu(II) ions adsorption onto (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-BPED at different temperatures.

Table 6 .

 6 Thermodynamic parameters for adsorption of Cu(II) ions onto the

	adsorbents (a) (GO-EDA-CAC)@Fe3O4NPs and (b) [(GO-EDA-CAC)@Fe3O4NPs]-
		BPED under optimal conditions.
	Adsorbents	Temperature (K)	∆G° (kJ.mol -1 )	∆S° (J.mol -1 .K -1 )	∆H° (kJ.mol -1 )
		293	-7.902
	(a)	313 333	-8.877 -11.280	78	15.35
		353	-12.325
		293	-8.541
	(b)	313 333	-7.848 -6.666	-93	-38.22
		353	-6.222

Table 8 .

 8 Comparison of the adsorption capacities, times of various adsorbents for the removal of Cu(II) ions from aqueous solutions.

	Adsorbents	Adsorption capacity (mmol.g -1 )	Time (min)	pH conditions	Number adsorption-desorption cycles	Ref.
	PVT-g-PS	2.65	120	5.0	2	[61]
	Ti-Mg Nanorod	1.72	30	7.0	-	[62]
	GO	1.844±0.105	540	5	10	[25]
	GO-Cellulose	0.418	180	4.5	10	[63]
	GO-EDA-CAC	2.521±0.098	360	6	10	[25]
	GO-EDA-CAC-BPED	3.464±0.112	240	7	10	[25]

Table 1 .

 1 Kinetic parameters calculated from pseudo-first-order and pseudo-secondorder models for the adsorption of Ni(II) and Co(II) ions onto a) GO-BPED and b) GO-BPED-PS.

	Adsorbent	Metal Ion	Qe(mmol.g -1 )	First-order Kinetic model K1×10² (min -1 ) Qe(mmol.g -1 )	R²	Second-order Kinetic model K2×10² (g.mmol -1 .min -1 ) Qe(mmol.g -1 )	R²
	a	Ni(II) Co(II)	2.896±0.072 2.206±0.067	2.072±0.041 1.773±0.012	2.109±0.016 1.775±0.003	0.971 0.982	2.986±0.038 2.566±0.093	2.864±0.002 2.201±0.005	0.999 0.998
	b	Ni(II) Co(II)	4.174±0.098 3.902±0.090	6.909±0.013 4.822±0.012	4.198±0.009 4.006±0.007	0.990 0.991	3.486±0.061 2.988±0.023	4.176±0.002 4.911±0.001	0.999 0.999

Table 3 .

 3 The intraparticle diffusion Kinetic for the adsorption of Ni(II) and Co(II) ions on (a) GO-BPED and GO-BPED-PS.

	Adsorbents	Parameters	Metal ion
			Ni(II)	Co(II)
		R 2	0.990	0.989
		Ki1 (mmol.g -1 .min-1/2 )	0.325±0.032	0.216±0.012
	a	C1 (mmol.g -1 .min-1/2 ) R 2	0.138±0.043 0.925	0.082±0.053 0.972
		Ki2 (mmol.g -1 .min -1/2 )	0.065±0.010	0.071±0.008
		C2 (mmol.g -1 .min-1/2 )	1.934±0.124	1.134±0.105
		R 2	0.992	0.993
		Ki1 (mmol.g -1 .min -1/2 )	0.722±0.036	0.685±0.035
	b	C1 (mmol.g -1 .min -1/2 ) R 2	-0.172±0.116 0.975	-0.087±0.111 0.972
		Ki2 (mmol.g -1 .min -1/2 )	0.052±0.037	0.061±0.040
		C2 (mmol.g -1 .min -1/2 )	3.650±0.313	3.303±0.334

Table 4 .

 4 Langmuir adsorption isotherm parameters for Ni(II) and Co(II) ions adsorption onto the adsorbents (a) GO-BPED and (b) GO-BPED-PS at different temperatures.

	Support	Metal ion	Temperature (K)	Qmax (mmol.g -1)	Qcalc. (mmol.g -1 )	b (L.mmol -1 )	KL (L.g -1 )	RL	R 2
				2.896±0.072	3.032±0.009	2.063±0.027	6.877±0.027 0.046±0.009 0.998
		Ni(II)		3.118±0.045 3.302±0.088	3.123±0.007 3.466±0.002	2.544±0.021 3.557±0.007 12.180±0.007 0.012±0.002 0.995 8.710±0.021 0.039±0.007 0.994
				3.663±0.032	3.787±0.001	5.146±0.004 19.493±0.004 0.017±0.001 0.994
	a
				2.206±0.067	2.374±0.008	2.122±0.027	5.464±0.027 0.011±0.008 0.998
		Co(II)		2.511±0.052 2.698±0.046	2.654±0.004 2.700±0.002	3.859±0.014 10.245±0.014 0.070±0.004 0.996 5.677±0.008 15.898±0.008 0.051±0.002 0.995
				2.884±0.102	2.950±0.001	7.016±0.005 20.703±0.005 0.035±0.001 0.995
				4.174±0.098	4.200±0.001	14.515±0.003 62.901±0.003 0.064±0.001 0.995
		Ni(II)		4.191±0.032 3.407±0.025	4.198±0.002 3.565±0.006	7.266±0.004 31.740±0.004 0.072±0.002 0.994 2.358±0.015 10.347±0.015 0.038±0.006 0.994
				2.623±0.071	2.943±0.007	1.782±0.023	5.246±0.023 0.053±0.007 0.992
	b
				3.902±0.090	4.032±0.003	9.565±0.006 39.525±0.006 0.013±0.003 0.998
		Co(II)		3.444±0.091 2.884±0.027	3.585±0.003 3.902±0.008	6.501±0.009 23.310±0.009 0.017±0.003 0.997 2.420±0.024 7.674±0.024 0.040±0.008 0.996
				2.477±0.049 2.535±0.0132	1.536±0.043	4.357±0.043 0.158±0.013 0.994
			Considering these data, it may be concluded that the Langmuir model is more
		suitable than the Freundlich one to describe the adsorption behavior of Ni(II) and
		Co(II) ions onto the modified GO surfaces which exhibit homogeneous distributions
		of adsorption active sites. Therefore, the adsorption process of Ni(II) and Co(II) ions

onto the GO-based adsorbents is monolayer. It should be also noted that the calculated adsorption capacities are close to the experimental ones and follow the order GO-BPED-PS˃ GO-BPED at temperatures lower than 353K (Table

4

).

Table 5 .

 5 Freundlich adsorption isotherm parameters for Ni(II) and Co(II) ions adsorption onto the adsorbents (a) GO-BPED and (b) GO-BPED-PS at different temperatures.

Table 4

 4

	Support	Metal ions	Temperature (K)	KF	n	R 2
			293	1.873±0.070	2.463±0.061	0.845
		Ni(II)	313 333	2.052±0.066 2.396±0.067	2.806±0.057 3.556±0.046	0.851 0.822
	a		353 293	2.857±0.063 1.433±0.067	3.699±0.039 2.617±0.059	0.655 0.838
		Co(II)	313 333	1.754±0.060 2.162±0.052	3.374±0.047 5.688±0.037	0.828 0.781
			353	2.238±0.050	6.060±0.017	0.863
			293	3.886±0.086	7.886±0.042	0.757
		Ni(II)	313 333	3.439±0.046 1.968±0.054	4.436±0.036 2.082±0.050	0.848 0.917
			353	1.596±0.047	2.617±0.040	0.919
	b		293	3.273±0.106	3.522±0.063	0.713
		Co(II)	313 333	2.604±0.092 1.868±0.069	3.297±0.069 2.696±0.062	0.707 0.816
			353	1.472±0.068	2.490±0.061	0.844

).

Unlike the Langmuir model, the Freundlich model describes a multilayer adsorption on heterogeneous surfaces. The Freundlich constant KF values, correlated to the equilibrium adsorption capacities of the adsorbents, confirm that the

Table 6 .

 6 Jossens adsorption isotherm parameters for Ni(II) and Co(II) ions adsorption onto the adsorbents (a) GO-BPED and (b) GO-BPED-PS at different temperatures.

	Support	Metal ions	Temperature (K)	i (L.g -1 )	j (L.mmol -1 )	m	R 2
			293	3.717±0.259	0.675±0.113	1.343±0.061 0.998
		Ni(II)	313 333	8.653±0.488 20.254±0.501	0.894±0.202 7.945±1.693	1.301±0.081 0.999 1.090±0.026 0.997
	a		353 293	41.937±5.396 4.500±0.155	14.033±4.942 0.866±0.083	1.043±0.070 0.997 1.233±0.029 0.997
		Co(II)	313 333	9.00±0.875 17.666±1.888	2.082±0.417 7.124±2.129	1.154±0.050 0.998 1.015±0.109 0.999
			353	29.153±2.973	10.297±1.977	1.001±0.300 0.999
			293	14.527±0.856	3.170±0.360	1.099±0.096 0.999
		Ni(II)	313 333	7.321±0.782 3.754±0.187	2.542±0.274 0.407±0.069	1.101±0.080 0.999 1.387±0.078 0.999
	b		353 293	2.196±0.377 24.265±2.787	0.757±0.191 4.795±0.790	1.171±0.078 0.999 1.159±0.047 0.999
		Co(II)	313 333	11.921±1.147 6.192±0.260	2.800±0.389 0.787±0.108	1.166±0.038 0.998 1.294±0.048 0.997
			353	3.357±0.161	0.373±0.075	1.431±0.079 0.996

Table 7 .

 7 Thermodynamic parameters for adsorption of Ni(II) and Co(II) ions onto the adsorbents (a) GO-BPED and (b) GO-BPED-PS under optimal conditions.

Table 8 .

 8 Arrhenius parameters for Ni(II) and Co(II) ions adsorption onto the

	Support	Metal ions	Temperatur e (K)	A	Ea (kJ.mol -1 )	R 2
			293
			313
		Ni(II)	333	15.61	34.89	0.999
	a		353 293
			313
		Co(II)	333	12.60	26.45	0.999
			353
			293
			313
		Ni(II)	333	-8.50	-27.25	0.999
	b		353 293
			313
		Co(II)	333	-8.42	-28.31	0.999

adsorbents (a) GO-BPED and (b) GO-BPED-PS at different temperatures.

Table 9 .

 9 Comparison of the maximum adsorption capacities, adsorption equilibrium times, pH, and exploited kinetic and isotherm models to various adsorbents for the removal of Ni(II) and Co(II) ions from aqueous solutions.

	Adsorbents	Adsorption capacity (mmol.g -1 )	pH	Time (min)	Exploited Model	number of adsorption desorption cycles	Refs.
		Ni(II)	Co(II)			Kinetic	Isotherm
	Peel of Artocarpus nobilis	20.443×10 -4	-	5.9	60	PFO	Langmuir	-	[49]
	HA-Am-PAA-B	-	0.897	5	120	PSO	Langmuir	3	[50]
	GO	1.789±0.115	1.510±0.095	5	540	PSO	Jossens	10	[17]
	AERGO	0.274	0.244	5.5	20	PSO	Langmuir	5	[13]
	GO-cellulose	0.243	0.263	4.5	120	PSO	Langmuir	10	[51]
	GO-EDA-CAC-BPED	3.254±0.095	3.054±0.091	7	240	PSO	Jossens	10	[17]
	GO-BPED	2.896±0.072	2.206±0.067	7	240	PSO	Jossens	18	This
	GO-BPED-PS	4.174±0.098	3.902±0.090	7	60	PSO	Jossens	18	work

Table 3 .

 3 Evaluation of green chemistry metrics for the synthesis of a 1,3-diyne based on phenylacetylene.

		Reactant (Phenylacetylene)	Solvent (Glycerol)	Base (Na2CO3)	Auxiliary	Product
	Mass	1.02 g	12.6 g	1.27 g	----	1.01 g
	Number of moles	0.010 mol	----	0.012 mol	----	0.005 mol
	Molecular mass	102.05 g.mol -1	----	105.98 g.mol -1	----	202.07 g.mol -1
				Values
	E-factor			13.74 Kg waste/Kg product
	Process Mass Intensity			14.74 kg waste/Kg product
	Atom economy			97.1%
	Reaction mass efficiency			99.0%

Table 4 .

 4 Evaluation of green chemistry metrics for synthesis of 1,3-diynes.

	Reactant (Phenylacetylene)	Solvent (DMF)	Base (K2CO3)	Auxiliary	Product (1,3-dyines)
	Mass	0.928g			47.4 g	2.266 g	----	0.927 g
	Number of moles	0.0091mol			----	0.0163 mol	----	0.0045 mol
	Molecular mass	102.05g.mol -1			----	138.20 g.mol -1	----	202.07 g.mol -1
	 PMI =	0.928+47.4+2.266 0.928	= 54.51 Kg waste/1Kg product
	 E-factor =	(0.928+47.4+2.266)-0.928 0.928	= 53.51 Kg waste/1 Kg product
	 Atom economy =	202.07 (16+138.20+102.05)	× 100 = 78.85%
	 Reaction mass efficiency =	0.927 0.928	× 100 = 99%

  e.g. 120 min.) is much lower than other catalysts (a few hours).

	reaction time (
	Catalyst	Experimental conditions	Phenylacetylene (Initial weight content, mg	Cu(II) (weight content, mg)	Yields (%)	Regenerati of cycles on number	Ref.
	Cu(OAC)2 (H2O)	DMSO, 90 °C, air, 10h,	102	63.5	90	-	[20]
	SBA-15-Cu(II)	DMSO, 100 °C, air, 4h	102	2.8	97	-	[28]
	HMS-DP-Cu(II)	EtOH, 80 °C, air, 8h	102	5.2	97	3	[29]
	Ps-TEDETA-CuSO4 Piperidine, Toluene, 60°C, air, 24h	102	3.2	96	9	[30]
	Cu/Cu2ONPs@rGO Cs2CO3, EtOH, 80 °C, O2, 8h	102	2.6	99	7	[31]
	[(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II)	Na2CO3, Glycerol, 80 °C, air, 120min.	102	2.4	99	15	This work

Table 3 .

 3 Comparison of the activity of GO-BPED-PS@NiNPs catalyst with other heterogeneous catalysts in the A3-coupling reaction of benzaldehyde, morpholine, and phenylacetylene.

	Catalyst	Solvent	T (°C)	Time (h)	Yield (%)	Number of runs the catalytic	Ref.
	PMO-PLs@Cu	CHCl3	60	24	96	7	[45]
	SiO2@CuNPs	Toluene	110	5	89	-	[46]
	GNS@CuONPs	CH3CN	82	5	89	-	[47]
	Monmorillonite@AuNPs	Toluene	100	3	98	-	[48]
	Graphene@AgNPs	CH2Cl2	60	24	86	5	[49]
	GO-BPED-PS@NiNPs	GVL	100	3	99	11	This work

Acknowledgements

Abbreviations

III. EXPERIMENTAL PART

No coercivity or hysteresis can be observed in the magnetization curves, indicating that the [(GO-EDA-CAC)@Fe3O4NPs]-BPED is superparamagnetic. In addition, the saturation magnetization (Ms) of [(GO-EDA-CAC)@Fe3O4NPs]-BPED (22.45 emu.g -1 ) is lower than that of the Fe3O4NPs (74.05 emu.g -1 ) which may be due to the presence of non-magnetic GO sheets and the smaller size of the supported Fe3O4NPs [45]. On the other hand, it is observed from the inset in Fig. 8a that the [(GO-EDA-CAC)@Fe3O4NPs]-BPED material can be easily isolated by applying an external magnetic field.

In case of almost zero coercivity (Mc) and zero remanence (Mr), the average size (Dm) of the Fe3O4NPs can be calculated from the initial susceptibility (xi) using Eq.3 [46]:

Where K is Boltzmann constant, T is the temperature (300 K), xi is the susceptibility when the magnetic field is near zero (0.02611 emu.g -1 .Oe) (Fig. 8b), 𝜌 is the density of magnetite nanoparticles, Ms is the saturation magnetization. Assuming that the saturation magnetization (Ms) corresponds to that of the [GO-EDA-CAC@Fe3O4NPs]-BPED (22.45 emu.g -1 ), it was calculated a diameter of 6.01 nm which in good agreement with the results obtained from calculated the Scherrer's equation (XRD) (Eq.2) and nearly in the range of the ones evaluated from the SEM image (Fig. 6). Moreover, Fig. 9 shows the Zero field cooled (ZFC) and field cooled (FC) magnetization of [(GO-EDA-CAC)@Fe3O4NPs]-BPED as a function of temperature. It can be observed that both ZFC and FC curves of the [(GO-EDA-CAC)@Fe3O4NPs]-BPED absorbent have the same magnetization behavior for temperature higher than

Efficient removal of Ni(II) and Co(II) ions from aqueous solutions using

Zwitterionic graphene oxide sheets as adsorbents: modeling and comparative studies

 Abstract

A novel zwitterionic graphene oxide-based adsorbent was first synthesized in a two step procedure including the successive grafting of N,N-bis(2pyridylmethyl)amino groups (BPED) and 1,3-propanesultone (PS) onto the graphene oxide (GO) surface. Then, it was used as adsorbent for the removal of cobalt(II) and nickel(II) ions from aqueous solutions and the influence of solution pH, contact time, metal ion concentration, and temperature onto their adsorption behaviors by the zwitterionic GO-BPED-PS adsorbent were investigated and compared with the GO-BPED adsorbent. The maximum adsorption capacities of the GO-BPED-PS adsorbent were as high as 4.174±0.098 mmol.g -1 for the Ni(II) ions and 3.902±0.092 mmol.g -1 for the Co(II) ions. In addition, it was shown that the adsorption behavior processes of Ni(II) and Co(II) ions onto both the GO-BPED and GO-BPED-PS adsorbents correspond to a pseudo-second-order kinetic model and a Jossens isotherm model. Moreover, the calculated enthalpy (ΔH°) and entropy (ΔS°) confirmed the exothermic adsorption nature of the adsorption process onto the GO-BPED-PS adsorbent.

Furthermore, the zwitterionic GO-BPED-PS adsorbent retained good adsorption properties after recycling 18 times which is much better than the conventional adsorbents.

Langmuir model :

Freundlish model :

Jossens model :

Where Ce (mmol.L -1 ) and Qe (mmol.g -1 ) are the equilibrium concentration and the equilibrium adsorption capacity, respectively ; Q (mmol.g -1 ) = maximum monolayer adsorption capacity ; b (L.mmol -1 ) = Langmuir isotherm constant ; CRef (mmol.L -1 ) = maximum concentration of adsorbent onto the surface of the adsorbent ; RL = adsorption behavior : if 0< RL<1, the adsorption is favorable while it is unfavorable if RL>1, and if RL =1, the adsorption is linear while it is irreversible if RL=0

; KF (mmol.g-1) and n are the Freundlich isotherm constant and the adsorption intensity, respectively ; i (L.g -1 ) and j (L.mmol -1 ) are the Jossens isotherm constants (m is the range 0-1).

First, the adsorption behaviors of Co(II) and Ni(II) ions onto the modified GO surfaces was studying with both Langmuir and Freundlich isotherm models at a defined temperature and the corresponding calculated parameters are compared in Table 4 and Table 5, respectively.

Thermodynamic adsorption parameters

In order to investigate the thermodynamic properties of the adsorption process, which includes both de desorption of adsorbed solvent molecules and the adsorption of the adsorbate species, the Jossens isotherm constant "i" (Table 6) has been used to calculate the Gibb's free energy (ΔG°; kJ.mol -1 ), the enthalpy (ΔH°; kJ.mol -1 ) and the entropy (ΔS°; J.mol -1 .K -1 ) [39,40] (Eqs. 9-10).

Where, R = universal gas constant = (8.314 J.mol -1 .K -1 ); T = absolute temperature in Kelvin ; KT = thermodynamic constant related to Jossens constant "i". The values of ΔS° and ΔH° can be obtained from the slope and the intercept of the plot of ln (KT)

versus 1/T.

The thermodynamic parameters were calculated for temperatures varying from 293K to 313K (Table 7). The negative values of ΔG° obtained at temperatures varying to the adsorption sites [17] and/or a higher affinity of the adsorbate on adsorbent at high temperatures [41,42]. Moreover, the positive values of ∆S° is related to a good affinity between the GO-BPED adsorbent and the adsorbates as well as an increase of the disorder at the adsorbent/solution boundary during the adsorption process [43].

The positive values of ∆H° confirm the endothermic nature of the adsorption process for the GO-BPED adsorbent so that the adsorption is expected to increase at higher temperature while its magnitude indicates a chemisorption process.

Effect of the catalyst nature, the solvent, the temperature and the base on the homocoupling reaction of phenylacetylene.

In this study, the homocoupling of phenylacetylene was selected as a model reaction to evaluate the catalytic activity of the Cu(II) ions adsorbed onto the magnetic GO-based adsorbents and in order to find the optimal experimental conditions. For comparison, the homocoupling of phenylacetylene was first carried out in the presence of both usual unsupported and supported copper catalysts with EtOH as solvent and

Et3N as a base in air at 80 °C. It should be noted that the weight content of copper ions located onto the [(GO-EDA-CAC)@Fe3O4NPs]-BPED and (GO-EDA-CAC)@Fe3O4NPs are 0.24 g.g -1 and 0.22 g.g -1 , respectively and corresponding to adsorption capacities of 3.8 mmol.g -1 and 3.5 mmol.g -1 , respectively.

Arbitrarily, we have used all the recovered Cu(II)-GO-based substrates from the adsorption experiment (about 10 mg per experiment) as supported catalysts for the homocoupling of alkyne and it was compared to experiments containing a copper ions amount of 4 mg that is usually used for this reaction [20]. The results are summarized in Fig. 1. As it can be seen, the homocoupling reaction of phenylacetylene is not efficient without any catalyst though in the presence of (GO-EDA-CAC)@Fe3O4NPs and [(GO-EDA-CAC)@Fe3O4NPs]-BPED substrates, as expected. For the copper-based catalysts, we observe the presence of a plateau corresponding to the higher reaction yield that can be obtained at 100% phenylacetylene conversion (determined by gas chromatography) due to the selectivity of the homocoupling reaction (presence of secondary products). The CuX2 (X: Cl, Br) unsupported catalysts are more efficient than the CuI one for reaction times higher than 150 minutes with a maximum reaction yield of 61% after 280 minutes suggesting that Cu(II) ions play a key role in the homocoupling reaction. Moreover, despite their lower Cu(II) weight content (about 2 fold lower), the supported copper catalysts such as (GO-EDA-CAC)@Fe3O4NPs-Cu(II)

and [(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) lead to higher yields in a shorter time.

Indeed, we obtained homocoupling yields in the range 80-90% for reaction times lower than 180 min (Fig. 1).

Scope and limitation of the Glaser homocoupling reaction

Considering the efficiency of the experimental conditions described above, the homocoupling of other phenylacetylene-based molecules containing electrondonating groups and electron-withdrawing groups was studied (Fig. 3).

a Experimental conditions: Phenylacetylene (1.0mmol, 1.0 eq.), Na2CO3 (1.2 mmol, 1.2 eq.), [GO-EDA-CAC@Fe3O4NPs]-BPED-Cu(II) (10mg), and glycerol (1.0 mL) at 80 °C under air. The phenylacetylenes having chloro (1b) and fluoro (1c) atoms in para-positions and trifluoromethyl groups (1h) in meta positions gave the higher yields, close to 98% (Fig. 3), while the presence of electron-donating substituents such as p-methyl (1d) and p-propyl (1e) onto phenylacetylene did not affect the homocoupling yield. Finally, we have checked the efficiency of the homocoupling reaction onto etheroxide-based alkynes and we obtained high yields (89% for (1k) and 84% for (1l), Fig. 3).

Nickel nanoparticles onto zwitterionic graphene oxide sheets supports as catalyst for A3-coupling reactions with δ-Valerolactone as renewable dipolar aprotic solvent

 Abstract

In this research, the GO-BPED-PS@NiNPs material was prepared through in-situ generations of nickel nanoparticles (NiNPs) onto the zwitterionic GO-BPED-PS sheets.

This newly prepared GO-BPED-PS@NiNPs material was structurally well-established by different analytical techniques including Fourier transform infrared (FT-IR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), X-ray diffraction (X-ray), scanning electron microscope (SEM), and vibrating sample magnetometer (VSM)

analyses. The prepared GO-BPED-PS@NiNPs was used as heterogeneous catalyst for the three-component coupling reaction (A3-coupling reaction) of aldehyde, morpholine, and terminal alkynes in γ-Valerolactone as a green solvent. Low reaction time, excellent yields of the propargylamines, benignity of the catalyst, and magnetic recyclability of the catalyst are the main advantages of the present protocol. In addition, our research indicated that the GO-BPED-PS@NiNPs catalyst could be reused up to eleven times without considerable loss of catalytic activity.

Where T is the temperature (300 K), K is Boltzmann constant, xi is the initial magnetic susceptibility (0.2314 emu.g -1 .Oe) (Fig. 5b), 𝜌 is the density of Nickel nanoparticles (8.902 g.cm -3 ), and Ms is saturation magnetization (19.48 emu.g -1 ). Thus, the magnetic particle size of Ni NPs was 11.30 nm, which is very similar to both results obtained from calculated the Scherrer's equation (XRD) and the SEM measurement.

2.2.2.

Coupling A3 study of benzaldehyde, morpholine, and terminal alkynes in the presence of the magnetic GO-BPED-PS@NiNPs material supported as catalyst Numerous complexes or salts of the transition metal ions such as Au(III), In(III), Hg(I), etc. were used as the efficient catalytic systems especially for the catalysis of organic reactions such as coupling A3 of terminal alkynes, aldehydes, and amines.

However, most of these homogeneous catalysts are expensive and have enhanced from serious limitations including difficulty in recovering the catalysts and possible contamination of the pure product with a trace of transition metal catalyst [30]. Indeed, heterogeneous magnetic nanoparticles catalysts such as nickel and nickel oxide nanoparticles are more popular and efficient catalysts for coupling A3 reactions due to the fast and cost-effective separation of the catalysts from the reaction medium. Therefore, we examined the use of GO-BPED-PS@NiNPs as a heterogeneous magnetic catalyst for the A3 coupling reaction of benzaldehyde, morpholine, and terminal alkynes.

Effect of reaction parameters including catalyst nature, solvent and temperature for the A3 coupling of benzaldehyde, morpholine, and terminal alkynes

In this study, the catalytic activity of GO-BPED-PS@NiNPs has evaluated in the model three-component coupling (A3 coupling) reaction. In order to find the optimal experimental conditions, initial experiments using benzaldehyde, morpholine, and phenylacetylene were optimized by diverse parameters such as catalyst nature, time, solvent, and temperature.

In the initial step, the GO-BPED-PS@NiNPs catalyst activate the C-H bond of the terminal alkyne to generate a NiNPs-acetylide complex on the surface of the GO-BPED-PS, according to the literature [42][43][44]. The second step includes the coupling reaction between the NiNPs-acetylide complex formed, and the iminium ion, which is produced by condensation reaction between secondary amine and aldehyde, to give the desired product. Moreover, the catalytic cycle continues until the completion of the reaction.

Regeneration and heterogeneity studies of the GO-BPED-PS@NiNPs

Stability, regeneration, and reusability of a magnetic catalyst are one of the most important parameters for commercial applications. The objective of our research is aimed to recycle the GO-BPED-PS@NiNPs catalyst for the reaction system of benzaldehyde, morpholine, and phenylacetylene under our optimal conditions. After completion of the reaction, the catalyst was easily recovered by a strong magnet and washed with hot ethanol and deionized water after each experiment, and then it was recovered up to eleven consecutive runs without a significant decrease in its catalytic activity (Fig. 8). To examine the heterogeneous nature of the catalyst, the three-component A3coupling reaction was achieved with GO-BPED-PS@NiNPs catalyst under our optimal conditions but the catalyst was isolated from the hot mixture using an external magnet after 2h (Fig 10). As expected, no increase in the activity of the product was observed when the reaction was continued for an additional 2h following the isolation of the