Keywords: functionalized silica, chitosan-coated silica, adsorption isotherms, Cu(II), Ni(II) XVIII

This study is focused on the preparation of three types of silica-based composites for the capture of Cu(II) and Ni(II) ions. The first strategy consists in coating chitosan on colloidal fumed silica after acidic treatment yielding the composite SiO 2 +CS. The second strategy can be separated into two routes: the first one involves surface grafting of silica with aminopropyltriethoxysilane to obtaining silica particles covered by amino groups (SiO 2 (NH 2 )). The second one involves in surface condensation of triethoxysilylbutyronitrile, followed by acidic hydrolysis of the surface-bound nitrile groups affording silica particles covered by carboxyl groups (SiO 2 (CO 2 H)). In the last step, chitosan has been grafted on the surface bound NH 2 or -CO 2 H groups yielding the composites SiO 2 (NH 2 )+CS or SiO 2 (CO 2 H)+CS. The third strategy involves in the modified CS surface with -CO 2 H groups, followed by coating onto the non-modified silica nanoparticles to obtain the composite SiO 2 +CS(CO 2 H). The novel hybrid materials were characterized by IR spectroscopy, scanning electron microscopy, atomic force microscopy, and zeta potential measurements.

Batch experiments were conducted to study the sorption performance of these composites for Cu(II) and Ni(II) removal from aqueous solution at optimum pH at 298 K. The kinetics were evaluated utilizing pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The adsorption kinetics followed the mechanism of the pseudo-second-order equation for all types of adsorbents. The adsorption isotherms were evaluated utilizing Langmuir, Freundlich, and Temkin models. The best interpretation for equilibrium data was given by Langmuir isotherm model. This study demonstrates that the adsorption capacities for Cu(II) ion is more efficient for the SiO 2 +CS (256 mg g -1 ) compared to SiO 2 (NH 2 ) (75 mg g -1 ).

However, the carboxyl grafted CS-coated silica (SiO 2 +CS(CO 2 H) exhibited an excellent adsorption capacity (333 mg g -1 ). In case of Ni(II), based on Langmuir isotherm the maximum adsorption capacity found to be 182 mg g -1 for SiO 2 +CS, and 210 mg g -1 for SiO 2 (CO 2 H) + CS. Using single-metal solutions, these adsorbents were found to have an affinity for metal ions in order as Cu(II) > Ni(II). The adsorption of Cu(II) ion by SiO 2 +CS was affected by the nature of the respective anion. Application of these composite materials to remove Cu(II) and Ni(II) from aqueous solution was shown to be more efficient than the adsorption capacities of many sorbents probed by other research groups.

1.4.4

Surface of silica (37) Interface of silica in aqueous medium (37) Hydrophilicity and hydrophobicity (38) Surface charging (38) 1.4.5 Chemistry and reactivity of silica (40) The solubility of silica Solubility of amorphous silica (41) Polycondensation of silicic acid (42)

1.4.6

Colloidal silica (44) Aggregation of particles (44) List of Tables

Table 2. 3 The BET specific surface value of grafted silica with amine groups (76) Table 2. 4 Effective specific surface value of silica and amine-grafted silica materials (77) Table 2. 5 The elemental percentage of SiO 2 (CO 2 H) ( 82)

Table 2. 6 The BET specific surface value of carboxyl grafted silica particle (83) Table 2. 7 Effective specific surface of native silica and carboxyl-grafted silica particles (84) Table 2. 8 The mean hydrodynamic size of carboxyl grafted silica particle (87) Table 3.1 Structures of some reagents functionalized with CS to uptake the adsorption of copper or nickel ions, and to enhance the solubility in aqueous solutions (99) The microporous molecular structure of the zeolite ZSM-5 (12) Fig. 7 Overview of the structuration of this PhD thesis (17) Fig. 8 The particle nano-scale (23) Fig. 9 Dimensionality of nanomaterial (24) Fig. 10 Example of a molecular nanowire, the separation between the two ferrocenyl extremities being about 4 nm (25) Fig. 11 Illustration of the shape of some nanomaterial in form of dots, rods, tubes, cages and layers (26) Fig. 12 The transition of a bulk metal via a nanocluster to a molecule (26) Fig. 13 The size quantization effect of optical property of nanoparticles (27) Fig. 14 The tetrahedron coordination skeleton of Si atom surrounded by 4 oxygen atoms, found for example in amorphous silica (30) Fig. 15 Morphology of colloidal polymer-silica nanocomposites (31) XIII Fig. 16 Silica nanoparticles prepared in two different medium 1) ethanol (EtOH) and 2) diglyme

List of Figures

showing skeleton (a) and hollow structures (b) (31) Fig. 17 Structure of mesoporous silicas (32) Fig. 18 The two main types of silica and their corresponding sub-classes (33) Fig. 19 Two preparative routes for the synthetic of amorphous silica (34) Fig. 20 Sol-gel process (35) Fig. 21 Different types of silanols and siloxane-bridges on a SiO 2 surface (37)

Fig. 22 The surface charge of silica as function of pH in presence of different NaNO 3 (mol/L) concentration (39) Fig. 23 Molecular structure of the octahedral Si(cat) 3 2-anion (41)

Fig. 24 Solubility of amorphous silica at various pH (42) Fig. 25 Polycondenzation of silicic acid to form of chain (i), ribbon (ii), layers (iii), and network (iv) (43) Fig. 26 Three different types of particles aggregation (44) Fig. 27 The FT-IR spectra of native Aerosil 200 (a), and Activated Aerosil 200 (b), respectively

Fig. 28 Principle of SEM (47) Fig. 29 Characterization of native Aerosil (a), and Activated Aerosil (b), by scanning electron microscopy (SEM)

Fig. 30 The principle of AFM technique (48) Fig. 31 The AFM image of native silica (48) Fig. 32 The TEM image of the single aggregated (original magnification x 40,

Final magnification x 80,000)

Fig. 33 Evolution of the pH of 0.5% colloidal untreated SiO 2 and activated SiO 2 (50)

Fig.34a

Evolution of the conductivity of 0.5% colloidal SiO 2 over 3 days (51)

Fig.34b

Evolution of the conductivity of 0.5% colloidal Activated SiO 2 over 3 days (51) Fig. 35 Schematic representation of zeta potential measurement using LDE (52) Fig. 36 The zeta potential value of untreated SiO 2 , and activated SiO 2 , respectively

Fig. 37 The Dynamic Light Scattering technique (54) Fig. 38 Overview of chemical modification of silica surface (64) Fig. 39 Immobilization of Ru(0) nanoparticles and a Ru(II) catalyst on mesoporous silica (65) Fig. 40 Silica-immobilized La(III) as catalysts for organic transformations (66) Fig. 41 Surface modification by aluminum alkyl impregnation on SBA-15 silica (66) Fig. 49 Synthesis of amino-modified ordered mesoporous silica (73) Fig. 50 Preparation of composites by reaction of APTES-grafted silica with various organic and organometallic reagents (74) Fig. 51 The FT-IR spectrum of SiO 2 (NH 2 ) (75)

Fig.52

The surface morphology of grafted silica with amine groups: SiO 2 (NH 2 ) (77) Fig. 53 The AFM images (2-D (a) and 3-D (b)) of the SiO 2 (NH 2 ) material ( 78)

Fig. 54 The zeta potential values of SiO 2 , and SiO 2 (NH 2 ) in function of the pH (79) Fig. 55 Size distribution in volume of SiO 2 (NH 2 ) (80) Fig. 56 Two-step surface modification of silica via grafting with cyano groups (81) Fig. 57 Selected applications of carboxyl-functionalized silica (82) Fig. 58 The FT-IR of cyanide grafted silica (a), and carboxyl grafted silica (b) (83)

Fig.59

Surface morphology of SiO 2 (a), and the SiO 2 (CO 2 H) (b), respectively

Fig. 60 The AFM image of the SiO 2 (CO 2 H) particle ( 85)

Fig. 61 The zeta potential of grafted silica with carboxyl groups (86) Fig. 62 Structures motives of three natural biopolymers: cellulose (a), chitin (b), and chitosan (c)

Fig. 63 Production process of CS (96) Fig. 64 Protonation of fully deacetylated CS in acidic aqueous solution (97) Fig. 65 Synthesis of carboxyalkyl CS (100) The surface morphology of SiO 2 (a), and SiO 2 +CS LMW (b), respectively

Fig. 77 The zeta potential values of SiO 2 compared with CS-coated silica

Environnemental Pollution

Pollution is the introduction of contaminants into a natural environment, which causes harmful effects to the ecosystem, including living organism and last but not least to humans.

Pollution can take a physical form, such as noise, heat (thermal pollution), or light and other electromagnetic radiations (electrical pollution from cell phone antennas etc.). However, in the majority of cases, the term "pollution" is associated with chemical pollution. Pollutants can be released into the environment by several ways. There are different types of pollutions of our biosphere* like air pollution, land pollution and water pollution. As a consequence of the steady growth of the world population, unfortunately combined with a non-ecological, even irresponsible consummation of energy and raw materials, the most important and still increasing contribution to pollution stems from anthropogenic sources. Moreover, naturally sources may cause pollution such as volcanic eruptions, forest fires, etc. Air pollution is the accumulation of any hazardous substances into the atmosphere, that danger to human life and other living organism [1,2]. Air pollution is caused by the release of air pollutants (SO 2 , NO x , CO, NH 3 , O 3 , volatile organic compounds, etc.) at a rate faster as than they can be accommodated by the environment (removed from the atmosphere) [3]. Some sources of air pollution are power plants, aerosol sprays, combustion of plants, nuclear weapons, smoke from engines and factories, etc. An important urban compound is sulfur dioxide combined with from the use of fossil fuels in heat and power production. Moreover, the growing traffic gave the nitrogen oxides and volatile organic compounds [4][5][6]. Land pollution is the pollution of earth's surface, releases from household, industries, and agriculture activities. The sources of land pollutions are human sewages, oil refineries, construction debris, mining, etc. Thermal pollution and the global temperature rise are considered as principal sources causing the climate change [7]. The warming of the Earth's surface and lower atmosphere is to be the result of the greenhouse effect, mostly due to human-produced increases in atmospheric greenhouse gases.

*The biosphere is the part of the earth where life is; it includes the surface, the oceans, rivers and the lower atmosphere.

Agricultural soils can constitute a source of the principal greenhouse gases: nitrous oxide (N 2 O), methane, and carbon dioxide [8,9]. In the context of this PhD work, we focus exclusively on water pollution and decontamination, what will be discussed in detail below.

Water pollution

The water pollution constitutes one of the most important environmental problems of our blue planet, the origin of this pollution being due to anthropic activity. Inorganic nitrogen pollution seems to be one of water problem found around the world [10]. Three major environmental harmful consequences due to inorganic nitrogen are water acidification, cultural eutrophication (toxic algae), and direct toxicity of inorganic nitrogen compounds [11]. Ammonium (NH 4 + ), nitrite (NO 2 -

), and nitrate (NO 3 -) are the most common ionic reactive forms of dissolved inorganic nitrogen in aquatic ecosystems [12]. The distribution of nitrate indicates that serious nitrate concentration (10-50 mg NO 3 -/L) occur in areas with important agricultural activities. Elevated nitrate concentrations are found in Northern China, Northern India, and the North-western regions of the U.S.A., and contribute also to the overproduction of toxic algae in the "Bretagne" region of France [13][14][15][16]. Apart from the elevated level of nitrogen-based salts, there are also reports describing the problems caused by high phosphate concentration in rivers in China and Thailand, which drains from agricultural activities [17,18]. Another global problem is the widespread geographic contamination of water sources by heavy metal ions. In the case of Thailand, the water pollution due to high level of metal ions and other contaminants combined with the low quality of water sources for drinking water production is widespread in many regions of Thailand. This problem will be explained more in detail in the following section.

The particular case of water pollution in Thailand

The water pollution in Thailand will certainly tend to get worse in the future unless proper measures will be undertaken. The contaminated and often untreated wastewater stemming from household, agricultural, and industrial activities are the main cause of the water pollution. Thailand exports agricultural products for example rice, cassava, etc [19]. To increase the productivity, large quantities of pesticides and fertilizers are used, that are distributed in both ground and surface water. The contaminated wastewater released from the industry is one of factors contributing to the water pollution problem in Thailand. For example, large amounts of waste products are drained from rubber industry [20], and palm oil mills [21].

Thailand consists of 25 river basins. Major rivers, which are particularly exposed to pollutants discharged from household, industrial, and agricultural activities are the Lower Chao Praya, Tha Chin, Mae Klong and Songkhla Lake [22][23][24]. The largest source of groundwater is located in the Lower Central Plain, particularly in the Bangkok Metropolitan Region and surrounding provinces [25]. In the middle part of Thailand, numerous researches have been done to investigate the water quality. A high level of toxic metals is found in the Chao Praya river estuary [24]. Furthermore, the Gulf of Thailand (Inner Gulf), a major marine resource in term of fisheries, aquaculture, coral, oil and mineral resource, also suffers problems due to water pollution [26][27][28]. In the North of Thailand, nitrate and fluoride ions originating from fertilizers are found in groundwater as well as in the Mae Ping river which is the most important alternative surface water source for the Chiang Mai province [29] . In the south of Thailand, there is a serious long time problem due to high concentration of heavy metal ions as well as arsenic. These toxic compounds are found in both contaminated surface water and ground water in the Ron Phibun district of the Nakhon Si Thammarat province.

People in this mining industry area had and still have problems because of a long-time consumption of surface and ground water [30][31][32].

Unfortunately, water pollution in Thailand is continuing, a serious environmental problem caused by the funding limitations in cities, operation costs, lack of operational experience, the absence of a high-efficient waste-water treatment station before releasing to environment and the growing population. In my opinion, for example recycling of waste products from seafood industry could be considered as promising ecological and also economically interesting solution to reduce the consequences of this national problem. For this reason, the University of Phayao, where I'm employed in the teaching stuff, is financially supporting this research project concerning the water pollution in my home country.

Pollutants in water and wastewater

There are many types of pollutants in water, which can be divided as radioactive materials, sediments, pathogens and parasites (viruses, bacteria, etc.), organic compounds (gasoline, oil, solvents, pesticides, phenols, phthalates, polychlorinated biphenyls (PCBs), detergents, organic matter, etc.), and inorganic compounds (nitrate, fluoride, chromium, copper, lead, etc.). These pollutants reduce the quality of life and may eventually cause even death. Focusing on inorganic pollutants, especially problematic metallic ions from discharge of industrial wastewater, let's cite cadmium, mercury, copper, nickel, lead, chromium, and arsenic, etc. Many metallic ions are known to be toxic and carcinogenic to humans because they can accumulate in living organisms, causing several disorders and diseases. One welldocumented example is lead poisoning, also called saturnism [33]. Lead interferes with a variety of body processes and is toxic to many organs and tissues, including the reproductive systems. It interferes also with the development of the nervous system and is therefore particularly toxic to children, causing potentially permanent learning and behavior disorders.

Another example is natural arsenic contamination (Arsenicosis), which is a cause for concern in many countries of the world including Argentina, Bangladesh, Chile, China, India, Mexico, Taiwan and the south of Thailand [34][35][36][37]. People who have drunken arsenic-contaminated water (The WHO's guideline value for arsenic in drinking water is 0.01 mg / l) over a long period are suffering a skin problem called "black foot disease".* Furthermore, skin cancer, cancers of the bladder, kidney and lung may be the consequence of an Arsenicosis [38].

* Blackfoot disease (BFD) is a peripheral vascular disease. The symptoms of BFD start with spotted discoloration on the skin of the extremities, especially the feet.

The chemical equilibrium of metallic ions in water, their toxicity and specification

In an aquatic environment, a given metal exist in a chemical equilibrium between free metal ions, dissolved inorganic coordination complexes and metals associated with colloidal particles, etc. For example, copper ions can be complexed with dissolved organic matter [39],

clay mineral, iron, and manganese in aqueous media.

The toxicity of metal ions depends first of all the concentration (see table 1) and the chemical form of a given metal. Chemical speciation of metal ions form can exist in different oxidation states depending on type of metal ions and environmental conditions. Speciation of metallic ions is very important for example nickel. Nickel is found in several oxidation states (from 0 to + 4), the Ni(II) ion being the most prevalent form of nickel in aquatic sources.

Ni(II) ions found in aqueous solution (neutral pH) are hydrated as hexa-aquo complex :

[Ni(H 2 O) 6 ] 2+ [40]. Chronic nickel poisoning can affect several organs including the respiratory systems, skin, and kidney. In neutral aqueous solution, Cu(II) salts such as copper sulfate dissolve to give the paramagnetic aqua complex [Cu(H 2 O) 6 ] 2+ , which has octahedral molecular geometry. For Cu(NO 3 ) 2 (stoichiometric amounts in solution : 10 mg/L of copper and 19.5 mg/L of nitrate), the distribution at basic pH, but less than 10, the dominant species is [Cu 2 (OH) 2 ] 2-. When the pH is above 10, anionic forms of copper hydroxides are [Cu(OH) 3 ] - and [Cu(OH) 4 ] 2-. The precipitation of copper hydroxide [Cu(OH) 2 ] starts at around pH 8 [41].

Bioaccumulation is the process by which organisms (including humans) can take up contaminants more rapidly than their bodies can eliminate them. Some metal ions, such as copper, which are an essential nutrient in low concentration (copper proteins have diverse roles in biological electron transport and oxygen transportation), become toxic at an elevated level. Some harmful effects to human due to a sustained high copper level are cardiac enlargement, and arteries with smooth muscle degeneration. It has also been reported that copper can cause cell damage [42a]. The maximum tolerated values for some selected metal ions in drinking water in France are listed in Table 1 [42b].

Metal-containing Wastewater Treatment Techniques

In contrast to the most part of organic matter, metal ions are not biodegradable.

Therefore, they accumulate their amounts along the food chain. For this reason, it is very important to remove metals from contaminated water. Various methods are used for removal of metals from water and wastewater. Some conventional techniques that have been used to treat industrial wastewater include ion exchange, filtration, chemical precipitation, flotation, and adsorption (see Fig. 1).

Ion exchange

The ion exchange technique is currently used for the removal of heavy metal from contaminated water [43,44]. Ion exchange resins consist of insoluble polymeric matrices containing a functional group. The principle of this technique is the exchanging a counter ion of the exchanger resin with an ion in aqueous solution. Ion exchange can be classified in two types of resin; cationic exchange resin, and anionic exchange resin (Fig. 2). The exchange reaction of a strong cationic exchange resin (R-SO Advantages of this technique are metal selectivity, and high regeneration of the adsorbent materials. However, a serious drawback of ion-exchange resins is the production a large volumes of strong acids as consequence of the release of H + , causing environmental problems due to acidification.

Filtration techniques

Alternatively, filtration techniques including reverse osmosis (RO) [46], nanofiltration [47], and ultrafiltration are used to remove pollutants from water.

Reverse osmosis (RO)

RO is a widespread membrane separation process used to remove metals from wastewater (Fig. 3) [48]. A common membrane material employed in RO is for example cellulose acetate, which is used for the separation of metallic ions. The RO process in the presence of the disodium salt of ethylene diamine tetraacetic acid as chelating agent for Cu(II) and Ni(II) removal have been carried out by Madoarres et al. Using the membrane RE2012-100, the authors of this study succeeded to increase the metal ion removal up to 99%. This high rejection percentage was explained by an increase of the coordination sphere around Cu(II) and Ni(II) by chelatation of the added agent EDTA around the metal ions and thereby blocking the solute to pass through the pores of the membrane used [49].

Nanofiltration (NF)

NF is one another membrane filtration process, which is considered to be intermediate between ultrafiltration and reverse osmosis in terms of operating conditions. The nominal pore size of the membrane is typically about 1 nanometer. NF processes operate at pressures between 50 and 150 psi*, which is much lower than in RO (200 to 1000 psi), but higher than in UF (10 to 70 psi). For example, H. Saitua et al developed a Nanofiltration pilot plan to evaluate the efficiency and mechanism of arsenic removal from naturally contaminated groundwater [47]. A commercial NF-300 membrane (TFC polyamide), enclosed in an OSMO 19E-HR 500-ECN membrane housing, was used for this study. This negatively charged membrane with 1.5 m 2 active surface shows high rejection over 95% for arsenate (HAsO

Ultrafiltration (UF)

The UF technique represents a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. This separation technique allows the passage of water and low molecular weight molecules through the membrane, while macromolecules and suspended solids (basis of pore size 5-20 nm, and molecular weight of the compounds in the range of 1000-100,000 Da) cannot be permeated. The traditional UF shows therefore low efficiency for the removal of small contaminants from water.

However, the use of water-soluble metal-binding polymers in combination with UF is a hybrid approach to combine selectively and to recover also heavy metals. In the complexation-UF process cationic forms of heavy metals are first complexed by a macromolecular ligand in order to increase their molecular weight with a size larger than the pores of the selected membrane that can be retained whereas permeate water is then purified from the heavy metals (Fig. 4). In a recent study of Barakat and Schmidt, the polymerenhanced ultrafiltration process has been investigated for removal of heavy metals such as Cu(II), Ni(II), and Cr(III) from synthetic wastewater solutions, using carboxy methyl cellulose as a water-soluble polymer and a polyethersulfone membrane with a 10000 Da cutoff. Maximum capacity of metals removal was obtained at pH  7 [50]. This MEUF technique gave a high removal percentage (90%) for Cd(II), Cu(II), Pb(II), and Zn(II), except for Ni(II) [51].

Another
In general, filtration technology may attain high efficiency for removing heavy metal ions, but limitations in its use are (i) the high operating cost, (ii) the problem of membrane fouling [52],

and (iii) low permeate flux [53].

* The hydrodynamic size (or radius) is the effective radius of an ion in a solution measured by assuming that it is a body moving through the solution and resisted by the solution's viscosity. If the solvent is water, the hydrodynamic radius includes all the H 2 O molecules attracted to the ion. As a result, it is possible for a small ion to have a larger hydrodynamic radius than a large ionif it is surrounded by more solvent molecules.

Fig. 4 Schematic representation of the principle of polymer-enhanced filtration

Chemical precipitation

The chemical precipitation [54] is widely used in the industrial sector to remove heavy metals because it is easy to operate. Metal precipitation from water consists in the conversion of soluble metal to an insoluble solid phase that can be precipitated. The precipitate then can be removed by filtration or clarification processes. Typically, the metal precipitate is in hydroxide form. Lime (CaO) is known to act as collector for the precipitation of Cu(II) in form of copper(II) hydroxides from aqueous solution [55]. The most important factor for precipitation is the control of the pH of the solution. The combination between Electro-Fenton* treatment and chemical precipitation was used to remove Zn(II) from rayon industry wastewater [56]. About 99% of Zn(II) was removed in the range of pH 9-10 using lime as precipitant. Chemical precipitation of heavy metals from acid mine drainage was also achieved using the 1,3-benzenediamidoethanethiol dianion (BDET, known commercially as MetX), which selectively and irreversibly binds soft heavy metals from aqueous solution. In a study of Atwood et al., BDET was found to remove >90% of several problematic metals from acid mine drainage samples taken from abandoned coal mines [57]. The concentrations of metals such as iron may be reduced at pH 4.5 from 194 ppm to below 0.009 ppm. The major advantage of utilizing the low-cost organosulphur complexation ligand within the mines lays in the reduction of metal waste formed during current treatment methods. However, chemical precipitation requires in general a large amount of precipitants for removing metals. The water treatment often requires pH adjustment, and toxic sludge is generated from the process.

* The classical Fenton's reagent involves using ferrous sulfate and hydrogen peroxide as reagents to oxidize organic compounds, where the Fe(II) ion plays an active role in generating the strongly oxidizing radicals •OH radicals actually responsible for attacking the organic molecules. A modification of the classical Fenton process by electrochemical dissolution of Fe, supplying the Fe(II) ion to the reactions, is called electro-Fenton method.

Flotation

Flotation is employed to separate a liquid or a solid from a liquid phase using bubble attachment [58]. The colloid flotation process used for the precipitation of metal ions is based on imparting ionic metal species present in wastewater a hydrophobic character. The initially hydrophilic metal ions are converted to hydrophobic by interaction with a surfactant (surface active agent, also called "collector" in this context). Both stages, sorption and flotation, can operate in the same treatment device. When air bubbles are introduced in the second stage into a flotation cell, metal ion -surfactant assemblies are removed by the action of air bubbles (foaming phase) because of the favored interactions between the exposed hydrocarbon chains and the air bubbles (see Fig. 5). Anionic sodium dodecyl sulfate (SDS) and cationic hexadecyltrimethyl ammonium bromide (HTAB) were used as surfactants for precipitation of Cu(II), Zn(II), Cr(III), and Ag(I) from waste waters at low pH (74% removal). Under basic pH condition, even a 90% removal could be achieved [41].

Adsorption

Adsorption, a physico-chemical treatment process, is found to be an effective and economic method for removing heavy metals from aquatic system [59]. The adsorption process offers both ease in design and operation. Metal ions can be removed by adsorption on adsorbent materials. The principal materials used for adsorption of pollutants are activated carbon, zeolites, clay, etc.

Activated carbon (AC)

AC is an efficient adsorbent widely used for removal organic and inorganic pollutants from water because its high porosity and the enormous surface area per unit mass (300-1500 m 2 .g -1 ) [60][61][62]. AC is an efficient sorbent for Cr(III) (98%), Cr(VI), Pb(II) (98%), Cd(II) (92%), Hg(II) (80%) retention from wastewater [63][64][65][66][67]. AC can be divided in three types based on size and shapes: Powder Activated Carbon (PAC), Granular Activated Carbon (GAC), and Activated Carbon Fibers (ACF). For example, GAC was used as a packed bed in column for removal of Pb(II) [68]. ACF was prepared from the fibers of jute and coconut. Chemical activation by phosphoric acid is the most suitable process to produce fibrous activated carbon. The thus obtained materials have high porosity (S BET up to 1500 m 2 g -1 ). This ACF exhibits maximum adsorption capacities for phenol (181 mg/g) [69]. Modified activated coconut shell carbon was used to adsorb heavy metal from water. Coconut shell carbon was modified with chitosan and/or phosphoric acid. The chitosan-coated and acidtreated coconut shell carbon shows good results for Zn(II) ion removal from synthetic industrial wastewater. Particle sizes of 1.0 nm and 1.25 nm exhibit the highest removal percentage [70]. However, drawbacks in the use of AC are the high costs (commercial activated carbon is more expensive compared to other adsorbents such as zeolites and plant products), and the fact that AC can in general not be regenerated and recycled after utilization (note that some types can be regenerated using acidic aqueous solution) [68].

Zeolites

These microporous materials consist of a mineral framework of tetrahedral aluminosilicates linked together by oxygen atoms. Zeolites have large pores (Fig. 6) and channels within their three-dimensional network and are commonly used as commercial adsorbents [71a]. They show high effectiveness for the capture of metal ions such as Cu(II), Ni(II), Zn(II), Cd(II), Pb(II), and Cr(II). This is due to the fact that the Al(III) ion is small enough to occupy the position in the center of the tetrahedron of four oxygen atoms, and the isomorphous replacement of Si 4+ by Al 3+ produces a negative charge in the lattice. The net negative charge is balanced by the exchangeable monovalent cations Na + and K + . These cations are in solutions exchangeable with the above-cited heavy metal cations [72]. The naturally occurring mineral clinoptilolite is the most abundant zeolite in nature. It has been shown to have a high selectivity for certain heavy metals (Pb(II), Cd(II), Zn(II), and Cu(II)).

Various types of zeolites are prepared from coal fly ash (CFA) as well as from a SiO 2 -Al 2 O 3 system. For example, Na-X, Na-A, zeolite-13X (Na-13X), zeolite Na-P1 (Na-P1), zeolite Y (Na-Y), sodalite, hydroxy-sodalite, phillipsite, K-chabazite, and Analcime were prepared using the sodium hydroxide fusion process and using different NaOH concentration. For example, zeolite X (38.7% SiO 2 and 20.9% Al 2 O 3 ) was used to remove Ni(II), Cd(II), Cu(II) and Pb(II) in bi-, tri-and tetra-metallic component systems using a batch method [73]. The zeolites A and X, prepared from kaolin, were used to remove divalent metal ions (Cd, Cu, Pb, Zn and Ni) from synthetic solutions. The optimum adsorption, using 0.8 g of each zeolite, reached to equilibrium after 30 min for Cd, Cu, Zn and Ni. But, the contact time reached to equilibrium up to 60 min for nickel adsorption [74]. A natural Bulgarian zeolite with a SiO 2 content of 68.7 wt% is reported to exhibit a copper removal of 89.3% at pH 5. Another study has demonstrated that also Y zeolite ion-exchangers containing the series Cs + , Rb + , Na + and Li + as alkali metal ions may retain copper and nickel ions, the exchange efficiency for copper being superior than the retention of Ni(II) under competitive conditions [75][76][77][78]. An important advantage of zeolites compared to activated carbon is the relative low price, but a limitation is their reactivity and slow degradation upon contact with an acidic aqueous environment.

Clay

Clay is another naturally occurring aluminosilicates. Basic types of clay are montmorillonite, kaolinite, and micas. Acid-modified clay presents porous structures, which offer the possibility to use them as sorbent to capture metal ions [79]. In a study of Vengris et al., a clay stemming from a deposit in North Lithuania was activated by treatment with 20% HCl to increase porosity. This activated clay was used for removal of Cu(II), Ni(II), and Zn(II) from wastewater [80]. In another study, the surface of a montmorillonite-illite-type clay has been activated by an aqueous solution of H 2 SO 4 to create new pores by replacing some cations (Al 3+ , Ca 2+ ) by H + [81]. Although clay can like zeolites be considered as low-cost adsorbent, an inconvenient is the need of high temperature in the calcination process some times applied to improve its mechanical resistance and to eliminate some impurities [82].

Emerging new techniques

Despite the established techniques presented above, there is a need for new innovative and low-cost effective techniques for removing of metals from wastewater, in particular for countries in Asia and Africa. Biosorption is one of the promising technologies (emerging technique) for removing metals from water. Using biosorbents, their various advantages are low cost as biomaterials (considered as low-cost if it is requires little processing), their environment friendly features such as biodegradability and biocompatibility, easy availability (biomass), and minimization of chemical volume usage. Biosorbents can be obtained from industrial waste materials (activated sludge biomass). In the past, our laboratory has participated in a study to use low-cost chemically modified starch-enriched flour stemming from paper industry to adsorb organic dyes [83]. Some biosorbents, like chitosan, alginate, plant products, etc., have also been used to capture metals in aqueous solution. Moreover, it appears very promising to develop further the use of biomaterials as biosorbent because of their effectiness and versatility to bind metal ions (the functional group on their structure can act as active site). The adsorption capacities of a given adsorbents depends on the characteristics of the adsorbent for each water pollutants and its concentration in water. Biosorbents bear functional groups on their backbone structure, which confers them the propensity to bind readily various metal ions [84]. Rice straw was used for removal of Cu(II), Zn(II), Cd(II) and Hg(II) ions from industrial wastewater [85]. Adsorption process reached the equilibrium before 1.5 h. Adsorption of metal ions on rice straw decreases in the order Cd(II) > Cu(II) > Zn(II) > Hg(II). Maximum adsorption values, at pH 5.0 at room temperature, were 0.128, 0.132, 0.133 and 0.110 mmol.g -1 for Cu(II), Zn(II), Cd(II) and Hg(II), respectively. The adsorption capacity for removal of Pb(II) and Cd(II) on Carica papaya seeds, were 1667 mg/g and 1000 mg/g, respectively [86]. Cr(II) was removed using modified Phanerochaete chrysosporium (a fungal biomass). Modified macrofungus Pleurotus platypus (fruit bodies of Oyster mushrooms) was used for Cd(II) removal from industrial wastewater. The removal percentage was up to 68%, when the flow rate system was 5 mL/min. This packed bed column can be regenerate for three times recycles [87]. Numerous articles demonstrate that chitosan is a good biosorbent combining low-cost and high efficiency for the removal of heavy metal ions [88,89]. Chitosan offers also other advantages such as being a high abundant and renewable resource, friendly to environment (biocompatibility, biodegradability) and last but not least ease of chemical modification. In this context, the use of this polysaccharide-biosorbent for the removal of Cu(II) and Ni(II) ions constitutes an import part of this PhD thesis and will be discussed in the following chapters.

In parallel to the use of biopolymers as adsorbents, another novel generation of adsorbents is emerging. The progress in the preparation and characterization of nanoparticles has also stimulated the utilization of these materials (magnetic nanoparticles such MnFe 2 O 4 , ferrite nanoparticles, carbon nanotube supported ceria, alumina-silica nanoparticles, TiO 2 , ZrO 2 , Fe 3 O 4 ,..) for the removal of both inorganic an organic pollutants. The state of the art concerning water treatment by adsorption on nanoparticles has been reviewed in 2012 [90].

Even hybrid materials combining biosorbent and inorganic nanoparticles are known. One example are monodisperse chitosan-bound magnetic Fe 3 O 4 nanoparticles (13.5 nm), which have been probed for the removal of Cu(II) [91].

Part 2 Aims of this Research Study

To realize my research project, this work has been supported by funding of a PhD grant allowing to start a cooperation between the University of Phayao and the group "Matériaux et Surfaces Structurés" within the Université de Franche-Comté on the preparation of novel low-cost and environmentally friendly composite materials. One goal was to prospective the possibility to use an industrial waste products for water treatment.

The objective of this research project focuses on the preparation of novel materials using silica-based particles as supporting polyelectrolyte. Silica was chosen as a supporting material because of its large surface area, low costs, high mass exchange characteristics, nonswelling and excellent mechanical resistance. Its surface can easily be modified by many reagents to improve the adsorption properties. In this research project, there are two strategies developed for the preparation of silica-based nano-composites to enhance the adsorption capacity for metal ions removal. The first one consists in coating silica particles with the biopolymer CS, the second one is functionalization of silica with surface-bound amine and/or carboxyl groups. Prior functionalization, silica was activated with acid solution to obtain more reactive silanol groups on its surface and subsequently the activated silica surface was then coated with chitosan and/or silanized with amino and/or carboxyl groups.

The manuscript is divided into four sections:

The first chapter focuses on silica particles. After a general information about nanoparticles and SiO 2 , the surface chemistry of silica and its properties as colloidal silica particles in aqueous solution are represented. The silica surface was activated by treatment with acid solution in order to augment the number of surface silanol groups. The activated silica powders were characterized by various techniques such as FT-IR to detect the silanol groups on the silica surface; scanning electron microscope and atomic force microscopy were used to study the image of the composites, and the specific surface was determined using the BET method. The behavior of silica in aqueous solution was studied by pH and conductivity measurements, zeta potential and hydrodynamic size measurements.

The second chapter describes the functionalization of silica-particles by amine and/or carboxyl groups under reflux conditions. These composites bear amine and/or carboxyl groups that can bind divalent metal ions. The functionalized composites were characterized by FT-IR to characterize the functional groups grafted on the silica surface, as well by the other techniques mentioned for chapter 1.

The third chapter deals on the biopolymer chitosan (CS) including the production process of CS, its intrinsic properties and advantages, the physicochemical properties, and the deacetylation degree (%DD) of CS. In order to obtain the adsorption capacity, functionalized CS with various organic reagents was evaluated. In this work, also the synthesis of carboxylic modified CS was undertaken. As in the precedent chapter, the prepared composite was characterized by FT-IR spectroscopy. To functionalize the silica surface by organic molecules, biopolymer used for coating the silica nano-materials to enhance its adsorption properties for metal ions removal. CS was chosen to functionalize the silica surface because the principal amino groups on its structure, which makes CS a promising biosorbent with excellent capacity to entrap heavy metal ions. The obtained materials were characterized by different techniques. Their properties in aqueous solution was measurement such as the stability in aqueous solution using pH, conductivity, zeta potential, and hydrodynamic particles size.

In the last chapter, the potential of the novel prepared composites for wastewater treatment is investigated. There are many pollutants found in water, but for this thesis we focused on divalent transition metal cations as model pollutant and Cu(II) and Ni(II) cations have been chosen. The composites evaluated as adsorbent for the capture of Cu(II) and Ni(II) are amine-functionalized silica, CS-coated silica, carboxyl-grafted silica, and carboxyl-grafted silica coated with CS. The most suitable pH for adsorption of these metal ions was determined and the effect of contact time for adsorption was probed. To study the adsorption isotherms, Langmuir, Freundlich, Temkin, BET, and Dubinin-Radushkevich models were applied. The adsorption kinetics were also studied by pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The plan of this research project is summarized in Fig. 7 to provide an outlook concerning the organization and structuration of this work. The use of nanoparticles as adsorbents has been mentioned in section 1.2.2 of the preceding chapter. A brief general definition of the term "nanomaterials" seems therefore appropriate. Nanomaterials can be defined as functional objects where one or more components are seized in the nanoscale (1-100 nm), as shown in Fig. 8. Their shape and morphology can be made visible using techniques like Scanning Electron Microscopy or Atomic Force Microscopy, etc. They are fabricated from metals, polymers, ceramics, glasses or molecular precursors using the so-called top-down or bottom-up strategies.* -The top-down approach often uses the traditional workshop or microfabrication methods where externally-controlled tools are used to cut, mill, and shape materials into the desired shape and order.

-Bottom-up approaches, in contrast, use the chemical properties of single molecules to cause single-molecule components to (a) self-organize or self-assemble into some useful conformation, or (b) rely on positional assembly. These approaches utilize the concepts of molecular self-assembly and/or molecular recognition, which also used in our laboratory to self-assemble for example luminescent 1D molecular wires (see below). According to a targeted application, different shapes of nanomaterials can be tailor-made fabricated. For example, cobalt-iron cyanide hollow cubes with interesting magnetic properties have been assembled by hydrothermal synthesis in the presence of poly(vinyl)pyrrolidone [2]. Amine-terminated functionalized silica nanospheres have been developed as biosensors for glucose detection [3] or gold nanoparticle embedded silicon nanowires were used as biosensor to enhance the sensitivity for label-free DNA detection [4].

In the domain of heterogeneous catalysis, cage-like nanostructured NiO-silica composite particle have been designed and probed as catalysts for the oxidation of organic pollutants [5].

Since the discovery of fullerenes and related carbon-based nanomaterials [6], the chemistry and physics of carbon nanotubes (CNT), which can be considered as a monodimensional variation, has known a tremendous growth. Some selected applications include their use in devices for fuel cell (hydrogen storage) or solar cell [7][8][9][10]. The concept to use single-walled carbon nanotubes coated with MgH 2 for hydrogen storage has been investigated both from a theoretical point of view and experimentally [11]. The use of CNT as sensors and actuators has recently been reviewed [12]. Also at the Université of Franche-Comté, some research groups work on applications of CNT in pharmaceutical sciences or on their experimental development and theoretical modeling as chemosensors [13]. In this context, single-walled carbon nanotubes and the interaction energy in the presence of amino acids inside and outside the CNT have been studied. It was found that outside adsorption provides a better detection performance than inside adsorption. Theoretical calculations suggest the possibility to use carbon nanotubes as nanocapsules to transport molecule without chemical modification [14,15]. Graphene, a 2-D form of carbon-based nanomaterial is currently a very hot topic after awarding the physics Nobel Prize in 2010 to two Russian researchers (K. Novoselov and A.

Geim) for their import contribution on the preparation and understanding of this promising carbon allotrope. For example, graphene has proven is usefulness as biosensor for the detection of glucose, hemoglobin, or cholesterol, etc [16]. Bilayer graphene sheets (BLGS) are currently receiving increasing attention because of their unique vibrational properties and large stiffness. These properties of BLGS make them suitable candidates for manufacturing nanosensors; electromechanical resonators, and nanodevices [17].

Inorganic nano-multilayers of composition CrAlN/AlON have been obtained by sputtering techniques using under N 2 atmosphere Cr-Al alloys and Al 2 O 3 as precursors. The mixed metal-nitride-oxide layers were probed as hard protective coatings because of their high hardness, good wear and high corrosion/oxidation resistance at elevated temperatures (900 • C) [18]. Organometallic molecular rods like the depicted nano-sized ferrocenyl compound have been engineered using classical organic synthesis with the aim to use them as conducting materials for molecular electronics (Fig. 10). The famous spherical fullerenes obtained by soot extraction with toluene can now be prepared even in multi-gram scale. This opened the possibility for molecular chemists to further functionalize these fascinating objects by attaching covalently organic groups or metal complexes on the surface of these C 60 -C 84 objects [20,21]. Other nano-sized spherical objects obtained using the toolbox of molecular chemistry is the so-called dendrimers. These starshaped molecules formed by successive chemical assembly of several layers (called generations) found applications for drug delivery, catalysis and as sensors [22]. Finally, also the synthesis of metallic nanoparticles can be achieved by controlled decomposition of molecular precursor complexes. Several techniques were used for the characterization of the obtained nanoparticles (approximately 2.5 nm) that organize into spherical or elongated superstructures of 45 nm made of small individual nanoparticles [23]. Some typical structural motifs and shapes of current nanomaterials are represented in Fig. 11.

Fig. 11 Illustration of the shape of some nanomaterial in form of dots, rods, tubes, cages and layers.

Quantum effect

The special nature of nanoclusters (particles of any kind with a size too small to exhibit characteristic bulk properties), is due to a quantum confinement of electrons which leads to a change of relevant properties of a nano-sized material compared to a bulk material.

G. Schmid and co-workers propose an explication of the transition from a bulk material to a nano-sized material as depicted in Fig. 12 [24]. The electronic situation of metal atoms represented in Fig. 12a can be described a quasi-delocalized electronic state of overlapping valence (VB) and conductivity bands (CB).

The situation of particles where the diameter (d) corresponds to = de Broglie wavelength) in the ground state is shown in Fig. 12b. Fig. 12c shows bonding (MO b ) and antibonding (MO ab ) molecular orbitals of localized bonds between few atoms in a molecular cluster [24].

As already stated, nanoscale dimensions confer nano-objcets special properties compared to bulk objects of the same materials due to quantum confinement effects. For example, the size of the particles has an influence on the wavelength of their absorbance and light emission. The size quantization effect in a series of a given material of various nanosizes may give cause different colours. For example, ZnS-capped CdSe semiconductor nanoparticles (quantum dots) with a diameter of 2 nm give rise to a blue-shifted emission, but having a diameter of 6 nm, they emit with a red-shift (see Fig. 13). The size is inversely related to the band gap energy (dictating the fluorescence emission) of the nanoparticles [25,26]. The same quantum confinement effect is observed for the photoluminescence (PL) of silicon quantum dots. The PL maximum is shifted to higher energy when decreasing the size of silicon quantum dots (Si QD). The average diameter of Si QD embedded in silicon nitride films showing PL peak energies of 1.7, 2.25, 2.5, and 2.77 eV was 4.9, 3.7, 3.2, and 2.9 nm, respectively. This phenomenon can be rationalized by an increase of the band gap energy when decreasing the size of Si QD [27]. The PL appearance in porous silicon is also attributed to quantum confinement effects [28]. part" of this acronym includes research on silica nanostructures and silica nanoparticles, carbon nanotubes, gold nanoparticles, molecular rods etc. For example, deposition of thin silica films on silicon wafer (100 nm diameter) using the sol-gel method was studied. To investigate the initial stages of sol-gel thin film formation, effects of sol parameters on the deposition and aggregation of silica nanoparticles were analysed [29]. The behaviour of a single ammonia molecule encapsulated in C 60 fullerene nano-cage has also been studied to understand how the fullerene modifies the spectral signature of a NH 3 molecule in vibrationinversion mode called umbrella. This work shows that simulation of the spectra of ammonia can be used to probe the temperature of the surrounding media in which fullerene is observed [30]. Our team has investigated the electrodeposition of latex particles (50 nm diameter) on functionalized polythiophene films obtained by electropolymerisation. Three-dimensional structures of the polythiophene derivatives could be evidenced by AFM. This deposition on the film offers promising possibilities for surface functionalization and selective latex particles adhesion [31]. The retention of Cu(II) and Ni(II) complexes chelated with polyaminocarboxylates in the presence of polyethyleneimine (PEI) or chitosan, assisted by ultrafiltration, was also investigated in our laboratory. Metal retention with PEI of 98% in the pH range of 4-9 was obtained [32]. Adsorption of the polymer poly(vinylimidazole) onto an Aerosil 200 silica surface at pH 3, 5, and 7 was studied in our group. It has been shown that the adsorption decreases with increasing the pH of the solution and with increasing in NaCl concentration, demonstrating that the amount of the adsorbed polymer strongly depends on pH and ionic strength. The uptake of basic imidazole by acidic silica is strongly due to electrostatic interaction [33]. Moreover, the coagulation of monodispersed silica (around 15 nm diameter) in the presence of Ca(II) ions was studied. The adsorption is pH dependent. It was found that, one Ca(II) ion adsorbed releases one H + from the silica surface. During the coagulation, shrinkage of the silica nanoparticles to 6 nm occurred giving an aggregation of strongly linked particles, as evidenced by X-ray scattering technique [34]. Another paper of our group deals on the behaviour of silica nanoparticles suspended in aqueous solution, which aggregates after addition of Al 13 . Then osmotic stress was applied to the system in order to extract water and to obtain a smaller particles volume. It was found that the system shows different aggregation behaviours under acidic (pH 5) and basic (pH 9) conditions. In the latter case, the reaction was much faster than under acidic condition and the dispersion contained both aggregated and non-aggregated particles, depending on the concentration of Al 13 [35].

These are just some few selected examples to give an overview about the properties and applications of various nanomaterials, which may help to overcome some intrinsic limitations of bulk materials. Since our research group has acquired some experience in the past on the preparation and physicochemical properties of silica, we have chosen to use silicabased nanomaterials and composites in this present PhD work for the retention of pollutants from model wastewater solution. To facilitate the understanding for this choice, some information on the physical and chemical properties of SiO 2 are presented. Furthermore, synthesis and characterization techniques for this versatile inorganic compound are detailed in the following section.

Silicon (Si) and silicon oxide (SiO)

In Earth's crust, silicon is the second most abundant element with oxygen, and constitutes 27.7% of mass of the crust. Elemental silicon plays an import role in solid-state physics as well as constituent in inorganic and organic silicon chemistry [36,37]. The reactivity of silicon depends on the particle size. Silicon nanoparticles (SiNPs) are silicon nanomaterials with crystalline structure with an average size around 1 to 10 nm. They are also referred to silicon nanodots, silicon quantum dots, nanocrystalline silicon, and silicon nanoclusters. Silicon nanowires (SiNWs) are 1D silicon crystalline nanomaterials with crosssectional diameter of less than about 100 nm and are used in electronic devices. Silicon nanoribbons or silicon nanomembranes are 2D silicon crystalline nanomaterials with a thickness < 100 nm. Porous silicon is generated by etching crystalline silicon wafers in aqueous solution containing HF; electrochemical etching, anodization under galvanostatic conditions. Porous silicon is also produced by light-simulated etching of crystalline silicon in HF without electrodes or electrochemical etching [37]. Silicon in form of silicon thin film is fabricated by Vapor Deposition and thermal processing and employed in photocopy systems, X-ray detectors and photovoltaic solar cells. Silicon nanocrystallites embedded in a silicon oxide matrix shows electrical interesting and optoelectronic properties [36][37][38].

In the universe, the reaction of silicon with O 2 mainly leads to gaseous SiO and small amounts of SiO 2 . Gaseous SiO is obtained from the condensation of Si and O 2 at hightemperature between 1250-1400 o C as shown in equation (1) [36].

However, solid SiO does not exist naturally on Earth. Solid SiO was first prepared in 1995 by Potter by reduction of SiO 2 with C, Si or SiC [36]. SiO is always present whenever silica or silicates are reduced at high temperatures such as in carbothermal reduction of SiO 2 to produce silicon carbide SiC. In this context, silicon or SiO x particles is also related to solid SiO. It is used as a vapour-deposition material in the fabrication of SiO x thin film for optical or electronic applications [38,39].

Silica (SiO 2 )

Silica, also known as silicon dioxide, is the second, thermodynamically more stable oxide, commonly found in nature. Sand and quartz are the two most common forms of SiO 2 .

The tetrahedral [SiO 4 ] 4-building blocks can be joined together by two or four oxygen corners.

The tetrahedral coordination of the oxygen ions around the silicon atom is shown in Fig. 14. , where M is an extraframework alkali or alkaline earth cations (Li + , Na + , K + , Ca 2+ , Sr 2+ ,…) [41,42]. Natural zeolite, clinoptilolite (Fig. 6) is one of the most common found in sedimentary rocks, the formula is (Na, K) 6 Si 3 0A l6 O 72 • nH 2 O [43]. There are about 800 different zeolites, which can be classified by 119 different zeolite structure types [42]. The zeolite ZSM-5 has been used as heterogeneous catalyst for direct NOx decomposition [44], the utility of these porous compounds as adsorbents has already been outlined in the General Introduction.

Synthetic silica is known to exist in both in form of microparticles and nanoparticles.

After chemical modification, the latter may be subdivided as shown in Fig. 15. For example, colloidal nanocomposite particles of polymer-silica composites with different silica content represent different morphologies determined by transmission electron microscopy (TEM), or small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), etc.

Polypyrrole-silica nanocomposite can adopt a raspberry morphology (Fig. 15a) [45], but exist also in a variation possessing a SiO 2 core coated with a polypyrrole-shell allowing to use them as capsules (Fig. 15c) [46]. Another core-shell structure in which the silica core is surrounded by a layer of alkylammonium ions together with solvent was elaborated by synthesis of SiO 2 nanoparticles from hydrolysis and condensation of Si(OEt) 4 in aqueous tetrahydrofuran solution in the presence of different long-chained primary amines [46,47]. Poly(4-vinylpyridine)-silica nanocomposites (150-170 nm) containing 34-38 wt% of silica content have a so-called « currant-bun » morphology (Fig. 15b). Poly(methylmethacrylate) (PMMA)-silica nanocomposites (180 nm & 600 nm containing 15 to 60 wt% of SiO 2 ), have a silica core coated by a PMMA-shell (Fig. 15c), and polystyrene-silica nanoparticles with a mean diameter between 330-460 nm containing 13 to 26 wt% SiO 2 exhibit a polymer core coated by silica-shell (see Fig. 15d). Coating of Rhodium nanoparticles (obtained by reduction of Rh(III) with hydrazine) with silica is also represented as shown Fig. 15e. This metallic core-silica shell architecture was obtained by subsequent hydrolysis of tetraethyl orthosilicate [48]. A report of cubic-shaped skeletal silica nanoparticles using a cubic template (calcium carbonate with calcite crystal structure) whose surface is coated by dehydroabietic acid (DAA). The DAA adsorbed calcite were suspended in two different media (EtOH and diglyme), subsequently sol-gel process in the presence of TEOS lead to core-shell nanoparticles formation. The thin silica shell is obtained after removal of the calcite core by dilute acid etching. Two different types were identified by SEM (Fig. 16): The cubic skeletal one prepared in EtOH, and the hollow one prepared in dyglyme [49]. Applications of silica include the use as solid phase extraction cartridges (for extracting polar analytes such as epinephrine, and metanephrine from urine samples) [50], drug delivery devices (porous silica) [51], chemosensors [1,8- [54], etc. Nanoporous silica was used as adsorbent for the removal of anionic dyes [55].

Silica-coated gold nanoparticles have been used as a radiotracer in industrial process [56]. For

Types of silica

Silica can be divided into two forms: crystalline silica and amorphous silica (see Fig. 18). Crystalline silica exists in forms such as quartz, cristobalite, and tridymite, which are an important constituent of a many minerals and gemstones, both in pure form or mixed with related oxides. Amorphous silica can be classed into three types: vitreous silica [58], silica M (obtained from amorphous or crystalline silica irradiated with fast neutrons) [59] and microamorphous silica. Vitreous silica (fumed silica or pyrogenic silica) is obtained by reduction of high purity quartz with coke in electric arc furnaces in production of silicon and ferrosilicon alloy. Microamorphous silicas can be divided into three classes: (i) microscopic sheets (ribbon or fiber), (ii) common amorphous forms (spherical particles of SiO 2 < 1000 Å in diameter, the surface consist of anhydrous SiO 2 and SiOH groups), and (iii) hydrated amorphous silica.

Advantages of amorphous silica

This research work is focused on amorphous silica nanomaterials, because they offer several important properties making them a unique support for introducing functional components [60][61][62][63][64].

1. Amorphous silica nanoparticles exhibit high porosity, providing space required for attaching functional groups or entities.

2. The effective porosity of silica nanomaterials allow grafted molecules inside their matrix to keep original their inherent properties of original molecules and may even add novel ones (e. g. optical and magnetic properties).

3. Silica nanomaterials are non-toxic and biocompatible, a very important requirement for any development in medical and pharmaceutical sciences. Colloidal silica is also used as a wine, beer, and juice-fining agent. Note however that inhaling finely divided crystalline silica dust in very small quantities over time can lead to silicosis, bronchitis, or cancer, as the dust becomes lodged in the lungs and continuously irritates them, reducing lung capacities.

4. Silica exhibits a high surface area and is a non-swelling material. The surface chemistry of silica facilitates the chemical modification of silica-based nanomaterials.

5. Silica possesses an excellent high thermal resistance. 6. Last but not least, silica can be considered as a low-cost material, which is accessible in large quantities. In amorphous silica, the bulk structure is determined by random packing of [SiO 4 ] 4- units. The density  of amorphous silica (2.20 g/cm -3 ) is lower than that of crystalline silica, which ranges between 2.21 to 3.01 g/cm -3 [59]. The bond length of the slightly polarized covalent Si-O bond between the electropositive element Si and the electronegative Oxygen atom is 155 pm, as established by X-ray, electron-, and neutron diffraction [65].

Synthesis of silica

Several strategies have been developed for the synthesis of silica materials in the form of sols, gels, and powders in function of the desired applications. Synthetic amorphous silica can be obtained by wet preparation in a solvent or via a thermal synthetic route (Fig. 19). Both wet and thermal ways will be described in the next topic. For this work, we have chosen pyrogenic silica (Aerosil 200) as silica-based nanomaterial for surface modification because of its advantageous properties just described before.

Sol-gel silica

Sol or gel silica results from the sol-gel process; a versatile method used for preparation of silica gels and precipitated silica (see Fig. 20). This process produces silica in an aqueous solution (wet process), its surface chemical properties are different from those of high temperature silica.

Fig. 20 The sol-gel process Silica nanoparticles are formed by various methods. In the Stöber method [66], silica nanoparticles are produced by a sequence of hydrolysis and condensation processes of alkoxysilanes. The overall equations can be written as:

The Stöber process is a variation of the sol-gel method for the generation of colloidal monodispersed silica nanoparticles. The particle size can be tuned by changing the process parameters. A key parameter affecting the size of primary silanol species [(RO) 3 Si-OH] derived from the TEOS monomer when fixing the concentration of tetraethylorthosilicate (TEOS or Si(OEt) 4 ) is the nature of solvent. For example, when fixing the concentration of TEOS (0.5 M) and using 0.01-0.1 M NH 3 and 1.1-4.4 M H 2 O, it was found that in ethanol as solvent the size of the primary particle is larger (around 8 nm) than for those generated in methanol (around 4 nm). The difference in primary size is probably caused by a higher supersaturation ratio of [(OR) 3 Si-OH] in methanol than in ethanol [67]. Also bioencapsulated forms have been prepared using the sol-gel process and been used in the medical sector [68].

The sol-gel covalent crosslinking process was also used for the preparation of cellulose/silica hybrid biomaterials [69].

Aerogel silica

Sol-gel technology is also used to preparation of aerogel silica [70]. An aerogel is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage, which would be happen if the gel had been directly from a liquid. Note that SiO 2aerogels are manufactured materials with the lowest bulk density of any known porous solids of only 0.1 g.cm -3 .

Xerogel silica

This porous form is also prepared by the sol-gel method based on hydrolysis and polycondensation of TEOS but in an acid medium. The ratio of water / TEOS typically used is 4:1. The formation of the species ≡Si-OOCCH 3 and CH 3 COOC 2 H 5 in TEOS / ethanol systems was proposed (in the presence of acetic acid), several reaction steps are shown as follows

The highest surface area value attained was 850 m 2 g -1 for samples submitted to a gelling temperature of 20 °C [71].

Pyrogenic silica

Fumed silica is produced in a flame process (thermal process) using hydrogen, oxygen (air) and silicon tetrachloride (SiCl 4 ) as raw materials according to equations 8 and 9 [65a].

The by-product HCl is recycled to produce silicontetrachloride and hydrogen from silicon metal according eq.10:

Fumed silica is also produced by oxidization of metallic silicon according eq. 11:

All synthetic amorphous silicas are white powders. Their densities vary around 50 g/L to 600 g/L. First are produced primary particles (2-20 nm), which can grow together to form agglomerates, which are connected by van der Waals and hydrogen bonding. Agglomerates exhibit particles sizes around several hundred m in powder or up to 3000 m in microgranular form [65b]. Characterization of the specific surface, often determined by the BET method, is one of the most important parameter in reinforcement, rheological behaviour, and adsorption applications. Fumed silica exhibits specific surfaces in the range between 10 m 2 /g to 400 m 2 /g.

Surface of silica

The surface of silica is mainly composed of siloxane groups (Si-O-Si) in the inward region and silanol groups (Si-OH) distributed on the surface. The latter can be classified in three types as isolated (single) silanol, geminal (double) silanol, and vicinal silanol (see Fig. 21). The silanol groups on silica surface readily react with many reagents. The chemically modification via attachment of functional groups will be described in Chapter 2. Many of the adsorption, adhesion, chemical and catalytic properties of silica depend on chemistry and geometry of their surface, therefore the surface chemistry is a topic of intensive studies and has been reviewed by Zhuravlev [72]. The content of residual SiOH groups was measured at high temperature. Above 400-450 o C, more hydroxyl groups are removed, retaining large siloxane areas. The physical adsorbed water was removed at 115 o C and the bound water was present as hydroxyl groups layer on the surface. According to literature, silanol groups are not only found on the surface, but may also occur inside of the silica skeleton and very fine ultramicropores (diameter < 1 nm) [72].

Interface of silica in aqueous medium

At the silica-water interface, two types of surface groups can be found as singly coordinated SiOH groups and doubly coordinated Si 2 O surface groups. In solution, only singly coordinated SiOH groups undergo protonation and deprotonation, while doubly coordinated Si-O-Si surface groups are unreactive vis-à-vis protons because of the high electropositive repulsion of two Si 4+ ions.

Using the Brönsted acid/base properties, two protonation reactions can occur on the silica surface:

where K 1.1 and K 1.2 are the first protonation and second protonation constants for singly coordinated SiOH groups [73].

Hydrophilicity and hydrophobicity

The silica surface contains both hydrophilic and hydrophobic patches. Silanol groups impart hydrophilicity, while siloxane groups confer hydrophobicity [74]. The silanols can be protonated or deprotonated depending on the pH environment. At pH 7, silanols are negatively charges because they are deprotonated (the silica surface becomes negative due to the formation of siloxides Si-O -). Laser Doppler electrophoresis (LDE) is a technique used to measure the zeta potential value of dispersion particles in aqueous solution. In the case of silica, the zeta potential is a function of surface charge of the silica particles in aqueous solution. Therefore, zeta potential values can predict the stability of the material in the liquid phase. This technique will be discussed in the experimental part.

Surface charging

Silica particles in aqueous solution exhibit a surface charge due to dissolution of surface species and interfacial ion-exchange. The surface charge of silica is assumed to originate from the deprotonation of silanol groups (eq. 14) [75].

The site densities of both surface groups ( SiOH and  SiO ) respect to the mass-action law:

 SiO -)  H  SiO   exp ( e  0 ) ( 15
)
where K is the intrinsic equilibrium constant (with pK= -logK),  H is the activity of the protons (with pH = -log  H ),  0 the surface potential, e the elementary charge, and  the inverse thermal energy [76]. The main cause for the surface charge generation is the dissociation of the silanol groups. In aqueous solution, the silica surface forms -Si(OH), -Si(OH) 2 , -Si(OH) 3 at the interface [77].

The charge plays a crucial role in the adsorption of matter and in particle and/or cell adhesion. The surface charge of silica can be determined using X-ray photoelectron spectroscopy [78]. Surface charging behavior of silica in water has been studied. From literature, amorphous silica shows point of zero charge (pzc) at around pH 3 depending on source of silica [73,77]. Moreover, the surface charge in function of pH is due to the ionic strength (I)*. A survey of the literature reveals that the silanol number per nm 2 is quite variable and can vary from 2-8 depending on the type of silica, temperature and characterization technique ( 29 Si NMR, BET, TGA,..). For fumed silica it is about 2 nm -2 , whereas for fully hydroxylated surface of sols, gel, or precipitated silica (each surface Si atom has -OH groups), the silanol number is around 5 or larger. The silanol number is also affected by the age of a sample. For example, after one year of storing an Aerosil 200 sample, its silanol number had changed

Chemistry and reactivity of silica

The solution chemistry of silica will be discussed in following sections dealing on the solubility and polymerization of silica. The surface chemistry of silica involving the chemical reactivity and transformation of surface-bound silanol groups will be treated in Chapter 2.

The solubility of silica a) In HF solution

The Si-O bond is from a thermodynamic point of view a very stable bond. The bond dissociation energy necessary to cleave a Si-O-Si bond is about 136 kcal mol -1 . Nevertheless, the Si-F bond is still stronger. For that reason gaseous hydrofluoric acid (HF) converts silica to silicon tetrafluoride according eq. ( 16). As explained, this transformation is not due to the acidity of hydrofluoric acid, but to formation of a Si-F bond, whose bond dissociation energy has been determined to be 167 kcal mol -1 .

In aqueous solution, reaction with HF yields hexafluorosilicic acid, in which the silicon atom of the octahedral hexafluorosilicate anion is coordinated by six fluoride ligands (17) [79].

This reactivity has been exploited by Xu and Asher to prepare hollow particles [80]. In a first stage, monodisperse silica particles with a diameter of 300 nm were prepared by the Stöber process, and modified by dispersion polymerization to envelop a polystyrene shell around the silica core particles. In a second stage, the silica core was removed by action of HF to give capsules.

b) Dissolution in hot concentrated alkaline

Silica is also soluble in hot concentrated alkaline or in fused hydroxide to give sodium metasilicate Na 2 SiO 3 as shown in eq. ( 18). This water-soluble salt is also formed upon reaction with Na 2 O [81].

c) Dissolution in presence of aromatic vicinal diols

It is also literate-known that silica can be dissolved by complexation by aromatic vicinal diols Ar(OH) 2 in basic aqueous solution. For instance, upon addition of catechol to colloidal silica in 8-15 M aqueous NH 4 OH solution colourless crystals of an octahedral hypervalent silicon complex are formed, whose molecular structure is depicted in Fig. 23. The driving force for this complexation is the thermodynamically favourable chelate effect due to formation of five-membered cycles. The solubility of silica is dependent on many factors such as the state of silica (crystalline or amorphous form), pH of the solution, temperature, and presence of other ions.

Solubility of amorphous silica

Amorphous silica has a uniform solubility in neutral pH region. The dissolved species is almost monomer (pH range from 3 to 6). The rate of dissolution increases with increasing pH. From pH 9 to pH 10.7, there is an increase in the solubility of amorphous silica as shown in Fig. 24. In this range, amorphous silica is in solubility equilibrium with neutral monomer as well as silicate ions (equation 21 and 22) [59].

At pH above 10.7, all the solid phase of amorphous silica is dissolved to form soluble silicate.

Note that porous vitreous silica can also be soluble in nitric acid solution at pH 3. The effect of impurities (Al 3+ , Ca 2+ , Mg 2+ , SO 4

, and F -) on a silica surface reduces the solubility. For example, the dissolution rate of silica decreases when only one aluminium atom adsorbed on silica surface as anionic aluminosilicate per 2 nm 2 [77]. Impurities present on the surface of pyrogenic silica (Aerosil) are Al, Fe, Ti, and Cl [73].

Polycondensation of silicic acid

The polycondensation of silicic acid is a reaction leading to an increase in molecular weight of silica. It involves the successive polycondensation of monosilicic acid as shown in reaction 23 [83].

Three steps of polycondensation are recognized as: i) Polycondensation of molecular species to form particles.

ii)

The growth of particles.

iii)

The particles are linked together into branched chains, then become networks, and finally extending throughout the liquid medium to generate a gel.

The isoelectric point (pzc) of silica is around pH 2. Therefore, below pH 2 the catalyst is a proton, which forms an active cationic complex by protonation of a silanol. Above pH 2, the OH -ion is the catalyst generating active anionic silica species. The self-condensation of the monomer to form various structures like chain (1D), ribbon (1D), layers (2D), and finally, the condensation of the layers generates a 3D network is shown in Fig. 25.

Fig. 25 Polycondensation of silicic acid to form of chains (i), ribbons (ii), layers (iii), and networks (iv) [83].

The polymerization of silicic acid [Si(OH) 4 ] is strongly pH dependent. In a strongly alkaline solution, monosilicic acid does not polymerize. Polymerization of silica in H 2 O/K 2 O solution was studied [84]. When silica concentration increased, it promotes silica polymerization, and that increasing in the K 2 O/SiO 2 ratio. An increasing pressure may also cause depolymerization of aqueous silica.

Colloidal silica

Colloidal silica is a suspension of SiO 2 particles with diameters in the 1-1.000 nm range [85]. The colloidal silica particles are used in many applications such as binder, nanoparticle-based fluorescent bioassays, sensors, etc. The stability of colloidal silica is an important parameter to understand the aggregation properties in aqueous suspensions.

Aggregation of particles

Aggregation occurs when colloidal particles are bonded together in liquid phase. The colloidal particles are linked together in many ways, hence the aggregation includes gelling (i), coagulation (ii), and flocculation (iii) as shown in Fig. 26.

Silica gels: the particles are linked together via branched chains that fill the whole volume of thesol. Factors controlling gel characteristics are the size of original silica particles, amount of silica particles in solution, the pH, salt, temperature, etc.

Coagulation: the silica particles come together into relatively closed-packed clumps (more concentrated than in original sol). It is proposed that coagulation of silica in the presence of alkaline cations is caused by interparticle hydroxo-bridging [86].

Flocculation: the silica particles are linked together by bridges of flocculating agent. FT-IR is among the most useful techniques to identify either organic or inorganic species [88]. It can be used to quantify some components of an unknown mixture in form of solids, liquids, and gasses. In FT-IR, the data are collected and then converted from an interference pattern to a spectrum. By interpreting the infrared absorption spectrum, the chemical bonds in a molecule can be determined. FT-IR spectra of pure compounds are generally unique and they are like a molecular "fingerprint". For most common materials, the spectrum of an unknown substance can be identified by comparing it with a library of a known compounds. Characteristic absorptions of silica in the 4000 cm -1 to 1300 cm -1 region are gathered in Table 1.1 [89]. In the present work, both native SiO 2 and activated Aerosil 200 were characterized by FT-IR. The silicas were mixed with dry KBr and spectra were recorded with a resolution of 4 cm -1 . The FT-IR spectrum of the neat SiO 2 displays a number of characteristic bands at 3411 cm -1 , 1093 cm -1 (vs) and 804 cm -1 (m) (Fig. 27). These peaks are assigned to silanol groups (-SiOH) with H 2 O adsorbed, stretching vibration of -Si-O-Si-(siloxane backbone), and Si-O bond rocking, respectively [90]. The spectrum of Activated silica displays a sharp band at 3504 cm -1 is assigned to the -OH stretching vibration band due to physisorbed water and hydrogen-bonded silanol groups [91]. The -Si-O-Si-absorption at 1093 cm -1 and the Si-O stretching of silanol groups at 804 cm -1 have been shifted 1089 cm -1 and 817 cm -1 in the case of Activated silica. The vibration occurring at 453 cm -1 in SiO 2 confirms the -Si-O-Sifunctional group also appearing in Activated silica, but it has been shifted to 486 cm -1 .

(

Fig. 27 The FT-IR spectra of native Aerosil 200 (a), and Activated Aerosil 200

Energy Dispersive Spectroscopy (EDS)

EDS is based on the detection of X-rays produced by a sample placed in an electron beam [92]. The sample is excited by the electron beam subsequently to produce X-rays to discharge the excess energy. The X-rays energy is a characteristic for each element. The element percentage presents in our samples was analyzed by the EDS technique and the results for each element found are given in Table 1.2. The EDS analysis gave the percentage by number of atoms (wt %) of each of the elements identified. The analysis of Aerosil 200

shows a large peak of silicon (64.6 % atomic). Another strong one is the oxygen peak (34.68% atomic). Small amounts of hydrogen are also detected (0.33%). Other small peaks (residual impurity elements) are due to chlorine (0.2% atomic) and potassium (0.2%).

Scanning Electron Microscopy (SEM)

SEM is used to visualize a surface texture of materials with high resolution (less than 2 nm) [START_REF] Lubda | Comprehensive pore structure characterization of silica monoliths with controlled mesopore size and macropore size by nitrogen sorption, mercury porosimetry, transmission electron microscopy and inverse size exclusion chromatography[END_REF]. It can be used for determining particle size, shape as well as chemical composition and distribution from the characteristic X-ray signal. In SEM, the electron beam scans across the specimen surface point by point (Fig. 28). The signal collected from each point is used to construct an image on the display, with the cathode ray tube beam and the column beam following a synchronized scanning pattern. This means the displayed image is the variation in detected signal intensity as the column beam is scanned across the sample. The morphology of our materials was studied by SEM. The SEM used is a typical SEM tungsten cathode with a resolution of 3.5 nm (at a voltage of 30 KV). All samples were metallized by sputtering with gold (about 5 nm) before examination. The SEM of the outer surface morphology of native silica is shown in Fig. 29a the image represents a rough and irregular geometry as well as the SEM image of Activated silica treated with acidic aqueous solution (Fig. 29b).

Atomic Force Microscopy (AFM)

The AFM technique is a powerful technique to characterize the residual scratches, indentations of a given surface. AFM is an accurate and versatile technique for measuring surface morphology of a material at the nano-scale. A very fine sensor tip, mounted at the end of a small deflecting cantilever, is in contact with the sample surface to be investigated (see Fig. 30). The deflection of a cantilever holding the tip is monitored by a detector. The sensor tip is moved across the surface of sample in numerous line scans, which produces a threedimensional image of the surface with ultra-high resolution.

A droplet of the sample (colloidal SiO 2 suspension) was deposited onto a thin glass substrate. To dry the sample before scanning, the sample was kept in a desiccator for a week.

The surface morphology of native Aerosil 200 was characterized by AFM in the contact mode using an aluminum coated silicon tip. The cantilever was 450 m long and its stiffness was about 0.27 N.m -1 . The AFM image was obtained by scanning a 4 µm x 4 µm surface area, revealing the presence of colloidal nodules on the glass substrates, having an average height of 50 nm. The surface of the particles is quite rough and irregular (Fig. 31). Fig. 31 The AFM image of native silica Aggregation of silica particles occurred when native Aerosil 200 particles were dispersed in DI water, the interaction between the silanol groups on the surface of the silica particles with water happened via hydrogen bonding [72]. This result corresponds to the report of J.

Eisenlauer and E. Killmann [87b]. This report confirmed our work that the primary silica particles are fused together to form aggregates as shown in Fig. 32.

Different types of activated silica

We chose a 0.5% silica content to prepare colloidal silica for pH and conductivity studies. The pH value of native SiO 2 and activated silica were measured during 0.5 to 72 h interval. As expected, the pH of Activated SiO 2 is inferior to that of untreated SiO 2 (Fig. 33).

This can be ascribed by partial protonation of the silica surface after acid treatment and small amounts of residual acid traces (see eq. 26). All pH values of colloidal silica remained quite stable during 3 days of monitoring.

Conductivity

To determine the conductivity data of all colloidal silica suspension, 0.5% solutions were prepared in the same manner as used for pH measurement. The conductivity of both two types of colloidal silica was measured during a 0.5 to 75 h interval. Within the first 4 h, the conductivity of native Aerosil particles (suspended in aqueous solution) strongly increases because of the formation of protonated and deprotonated silanol groups at the interface of silica with the aqueous medium (see eq. 12 and 13) [74]. The conductivity continuously increases until 75 h with the value of about 7.4 ± 0.8 S/cm (Fig. 34a). For activated SiO 2, the conductivity is quite stable and the average value is around 100 S/cm (Fig. 34b). The activated silica suspension exhibits a ten times stronger conductivity than native Aerosil. This may be due to some residual hydrochloric acid molecules disturbing the surface as shown in equation ( 26) [77].

Si-OH + H Cl

Zeta potential measurement

The particles in suspension display a zeta potential. In solution, the presence of a net charge on a particle affects the distribution of ions surrounding the particle, resulting in an increase in the concentration of counter-ions. The region over which this influence extends is called "electrical double layer". This double layer is thought to comprise two separate regions (Fig. 35): The inner region (Stern layer) consists of strongly bound ions, whilst the outer region (diffuse layer) consists of loosely associated ions. The zeta potential is the electrical potential at the slipping plane. Zeta potential measurement by Laser Doppler electrophoresis (LDE) is used to assess the charge stability of a disperse system. A voltage is applied across a pair of electrodes at either end of a cell containing the particle dispersion. Charged particles are attracted to the oppositely charged electrode. Their velocity is measured and expressed in unit field strength as their electrophoretic mobility (U E ). LDE measures small frequency shifts (Δf) in the scattered light that arise owing to the movement of particles in an applied electric field [98]. The frequency shift is equal to where υ is the particle velocity, λ is the laser wavelength, and θ is the scattering angle. The measured U E is converted into zeta potential (ζ) through the Henry's equation [99]. There is no significant difference between the two recorded curves (three repeated measurements), but nevertheless the conductivity was somewhat higher for activated silica.

This may imply that just few siloxanes units on the silica surface have been activated. Note that the content of silanol groups after acid treatment has not been determined.

Hydrodynamic size measurements

Dynamic light scattering (DLS) is used to determine the size and size distribution of DSL technique (Fig. 37), called "hydrodynamic diameter", refers how a particle diffuses within a fluid. The hydrodynamic diameter is the diameter of a sphere that has the same translational diffusion coefficient as the particle. The DLS is performed by measuring the rate at which the intensity of the scattered light fluctuates when detected using a suitable optical arrangement.

Fig. 37 The Dynamic Light Scattering technique

As already mentioned, Aerosil 200 is composed of spherical primary particles which a diameter of around 12 nm, which are highly aggregated in the dry state. These particles remain highly aggregated even when dispersed under sonication ( explained by repulsion between some residual H + ions on the surface of the Activated SiO 2 particles.

Conclusion

The surface of Aerosil 200 nanoparticles was chemical activated with hydrochloric acid solution to obtain more silanol groups on their surface. The untreated silica and the activated silica particles were characterized by FT-IR spectroscopy. The spectrum of activated silica displays the characteristic sharp bands at 3458 cm -1 of the -OH groups due to physisorbed water and hydrogen-bonded silanol groups. There is no significant difference of BET specific surface value between untreated silica and activated silica. Stability of the composites in aqueous solution was studied by measuring values of pH, and conductivity. The obtained result displays that the pH values are quite stable at the period time studied (72 h).

The effect of time to measure did not significant on the pH studied. These composites show stability properties in aqueous solution. The conductivity value of untreated silica suspension after 75 h is about 7.4 ± 0.8 S/cm. After silica was activated by acid aqueous solution, the conductivity is quite stable and the average value is around 969 ± 8 S/cm. Zeta potential value of untreated silica and activated silica are nearly zero at pH 2. When pH increase more than 2, zeta potential values decrease accordingly, resulting from the increase of the concentration of the negative charges on the silica surface. For that result, at pH > 2, the negative charges silica could capture metal cations in aqueous solution. The basic information of untreated silica and activated silica in nano-size are obtained. In order to develop the adsorption properties, silica nanoparticles will be chemically surface modified by introduction of organic functional groups (Chapter 2).

Experimental part

Materials and Apparatus

Hydrophilic

Preparation of activated silica

Activated silica (Activated SiO 2 ) was prepared in the following manner: 5 g of Aerosil 200 silica particles were suspended in 100 ml of 1.1 M of hydrochloric acid aqueous solution to obtain activated silica. After agitation during 24 hours (50 rpm/min), the activated silica was washed with DI water. Then the sample was centrifuged and the supernatant removed; then 200 mL of DI was added to the activated silica and the suspension was again centrifuged. This operation was repeated several times (10 times) until the pH of the supernatant remained stable ( pH 3, measured by a pH meter) and the conductivity was found to be around 914 S/cm. Then the activated silica was dried at 100 o C and kept in desiccator until used.

Chapter 2

Surface-Functionalization of Silica Introduction

As outlined in the preceding chapter, Silica is known as a good supporting material because of its large surface area, high mass exchange characteristics, non-swelling, and excellent mechanical resistance. Moreover, the presence of surface-bound silanol groups permits easily a chemical modification of this inorganic polymer by introduction of new functional groups. This kind of surface modification provides a unique opportunity to modulate the interface properties of a solid support, retaining in many cases its physical properties like porosity and mesoscopic order. For example, it was shown mesoporous silica SBA-15 can be grown in the presence of Pt nanoparticles (2.9 nm diameter) preserving the mesoscopic order, the final pore lattice parameter in SBA-15 rising around 10% [1]. A functionalized silica material should have a sufficient chemical stability under experimental conditions and when used as sorbent, a high adsorption capacity combined with a sufficient selectivity. Silica surfaces can be modified by physical or chemical treatment as summarized in Fig. 38.

Chemical modification of silica surface by organofunctionalization

Hydrogen bonding, electrostatic interaction, physical interaction, etc.

Covalent grafting of silane agent

Alkoxysilane Chlorosilane

Fig. 38 Overview of chemical modifications of silica surfaces (i) The physical treatment includes thermal and hydrothermal processes leading to changes of the density of silanol and siloxane concentration on the silica surface. For example, Zhuravlev and Potapov reported a diminution of silanol groups on the silica surface (4.9 Si-OH /nm 2 to 2 Si-OH/nm 2 ) when the temperature was raised from 190 o C to 400 o C [2].

(ii) Chemical treatment leads to a change in the chemical characteristic of the surface of silica. This chemical modification can be achieved by attachment of functional groups, which can be introduced using two main approaches:

-The first one implies interaction of organic molecules to silica via hydrogen bonding, electrostatic interaction, etc. For example, the surface-grafted organometallic catalyst

Results and Discussion

Grafting of Organosilane Agents on a Silica Surface -State of the Art

The most convenient way to alter a silica surface through chemical modification via covalent grafting is the immobilization of the organic functional group onto the SiO 2 surface.

This derivatization of the silica surface involves covalent coupling of the silane agent with silanol groups of the silica surface. The general formula of a silane agent can be represented as F n SiX (4-n) (n = 1, 2, 3), where F contains organic functional groups, and X represents a hydrolysable group such as alkoxy -OR or halide [11]. An example of condensation of a chlorosilane in order to use it as a coupling agent in the preparation of a silica-supported carbene-Cu(II) composite, which can be used as a catalyst. First, the chlorosilane agent was grafted onto the surface of silica as represented in Fig. 42. In a second step, the SiO 2 -grafted silane was further N-alkylated with various imidazoles [12]. However, one inconvenient of this method is the extrusion of hydrogen chloride liberated during the initial condensation reaction. In the case of alkoxysilanes, the most common alkoxy groups are -OMe and -OEt, which produce less harmful methanol and ethanol as elimination-products. Therefore, the alkoxysilane route is generally preferred over the chlorosilane condensation. Some representative alkoxysilane coupling agents, which have been used to incorporate onto silica surfaces, are summarized in Table 2.1. These organosilanes play the role of coupling agent or precursor [13]. Advantages of organosilanes-grafted silica are apart from the strong binding on the silica surface are the possibility to post-functionalize the amino, halide, thiol and nitrile endgroups F. The typical schematic elaboration of alkoxysilane-functionalized silica particles is represented in Fig. 43. To resume, alkoxysilane agents are coupling agents allowing a facile modification of inorganic supports under relative mild conditions. Functional groups at the end-chain of the grafted silica have strong affinity to bind inorganic or organic molecules [14][15][16][17][18][19][20][21][22]. The covalent bonding of the grafted silica materials can also enhances the long-term stability of the silica in acidic medium [23]. A wide range of alkoxysilane reagents is nowadays commercially available by various suppliers. The sum of all these advantages explains the "success story" of these agents giving rise to an import number of articles dealing on their use for surface-silanization and sol-gel chemistry.

For this work, both aminopropyltriethoxysilane and cyanopropyltriethoxysilane have been chosen to prepare SiO 2 particle bearing amino-and/or carboxyl groups on the surface. It is well-established that due to the presence of a electron pair (non-bonding doublet) on the nitrogen atom of silica surface bound amino group and the negative charges of carboxyl groups these groups may act as donor sites allowing complexation of metal cation ions i.e.

Cu(II), Ni(II), Hg(II), V(I), etc. in aqueous solution [24][25][26]. Silanization of the surface of the SiO 2 particles used in this work with APTES and/or CPTES was therefore expected to enhance the selectivity and capacity for metal adsorption [27]. Although a lot of work has been published concerning the silanization of various silica surfaces, we are just aware of one work on the functionalization of Aerosil 200 microparticles with APTES or CPTES [28].

Therefore, these two strategies are presented herein in detail to evaluate the impact of the intrinsic characteristics of Aerosil 200 (size effect, specific surface area and morphology, surface charge and z-potential …). First is presented the preparation and characterization of APTES-functionalized Aerosil 200. Then the chemical modification of our silica particles with CPTES is described.

Functionalization of Silica with APTES and APTMS

The covalent coupling between silica and alkoxysilanes often employs APTES (3), which possesses three -O-CH 2 -CH 3 groups and a propyl chain terminated with an amino group. Silanization of silica with APTES is accomplished by refluxing silica with APTES in dry toluene to allow a homogenous diffusion of the silane and coverage of the silica surface [29]. The condensation process of surface silanol groups with the ethoxy groups of APTES liberates ethanol (Fig. 48). shown Fig. 49. This material has then been evaluated as nano-sorbent for the removal of chlorophenols from aqueous media [30]. Figure 50 summarizes some other reactions. The preparation of the dioctadecyl Lglutamide-derived lipid-grafted porous silica particles (5) includes coupling of N, N"didodecyl-L-glutamide with APTMS-silica particle in THF using diethylphosphorocyanide as a condensing agent [31]. The histidine-grafted composite (6) was obtained in a two-step sequence: Condensation of APTES-SiO 2 with glutaraldehyde yielded first an imine, which was than reacted with histidine [32]. Ammonium-functionalized silica was obtained by acidification of amine-grafted silica with hydrochloric acid solution. This protonation converts the amino groups to positively charged ammonium moieties (7) [33]. NH 2 -grafted silica has also been reacted with succinic anhydride in toluene to provide the carboxylfunctionalized silica (8) linked through an amide group [34]. An organometallic surface modification has been achieved by reaction of ferrocene aldehyde with APTES-grafted silica (imine condensation) to give the interesting hybrid material (9). Survey of internal surface of a pellet of this material by electron microscopy coupled to Fe-EDX back-scatter techniques revealed that the ferrocenyl sites were on average evenly distributed across the entire sample [14]. In this work, the APTES-grafted Aerosil 200 was prepared according to the procedure represented in Fig. 48. The amine grafted silica material SiO 2 (NH 2 ) was then characterized by FT-IR spectroscopy in order to confirm the presence of amino groups on the silica surface.

The quantity of amino-groups contained on the surface was also measured by the Kjeldahl method and Elemental analysis. Characterization of the SiO 2 (NH 2 ) composites was also done by BET surface area measurements, SEM, and AFM. Furthermore, the physicochemical characteristics in aqueous solution such as pH, conductivity, zeta potential, and hydrodynamic particles size have been investigated and are discussed.

The IR spectrum of functionalized Aerosil particles

Three major bands, namely the -OH stretching vibration band due to physisorbed water (hydrogen-bonded to silanol groups) [35], stretching vibration of -Si-O-Si-(siloxane backbone), and bulk mode of SiO 2 at 3411 cm -1 , 1093 cm -1 (vs) and 804 cm -1 (m), respectively are found in amine-functionalized silica as shown in Fig. 51. The appearance of a weak band at 2972 cm -1 and 1654 cm -1 is assigned to the stretching vibration of CH 2 groups and primary amine (N-H) bending from APTES molecules, respectively [22,29,36]. This gives an indication of the success of modification of silica particle. In comparison with literature, for APTES-grafted Aerosil 200 the new absorption band at 2932 cm -1 and 1640 cm - 1 are assigned to C-H stretching (correspond with the absorption characteristics of APTES spectrum) and H-O-H bending vibration (corresponding to adsorbed water) [24b]. Moreover, in our SiO 2 (NH 2 ) spectrum the band at around 1597 is not found this may be due to low concentration of APS molecules. The band at 3411 cm -1 is shifted to higher wave number (3479 cm -1 ) due to the interaction between the hydroxyl groups on silica with the amine groups of the APTES reagent [28b].

Fig. 51 The FT-IR spectrum of SiO 2 (NH 2 )

Determination of the N-content by Elemental analysis and by the Kjeldahl method

The amino content of two types of grafted silica was determined by Elemental analysis and the Kjeldahl method (Table 2.2) [37]. The carbon, hydrogen, and nitrogen content obtained from Elemental analysis was found to be 2.41%, 0.51%, and 0.76%, respectively.

The Kjeldahl method consists in three analysis steps : digestion, distillation, and titration. The nitrogen content obtained from the Kjeldahl method is about 0.82 % for SiO 2 (NH 2 ).

Elemental analysis allows the calculation of the covering of our APTES-grafted silica.

A nitrogen percentage of 0.76% was calculated for SiO

BET specific surface

The BET specific surface area of the SiO 2 (NH 2 ) was investigated and determined to about 131± 4 m2 /g. This BET value of the SiO 2 (NH 2 ) composite is reduced compared to that of native silica (182 ± 4 m 2 /g) as shown in Table 2.3. This can be explained by an aggregation of the functionalized silica particles. Therefore, the BET value of the amine-grafted silica is lower than neat silica [38]. Our result is in agreement with BET data from literature, the BET value decreases with an increase of the surface density of amino groups [39]. To obtain more information about the grafting density (loading) [13], the BET value was used to calculate the amine grafting density or surface coverage,  ( mol m -2 ), as follows (equation 1):

 = fc 10 6 / [(M c nc -fc M group ) S BET ]
fc is the mass fraction of carbon in the sample, M c is the molecular weight of carbon, nc is the number of carbon atoms within the grafted ligand, M group is the molecular weight of the grafted ligands; it means the difference between the molecular weight of the ungrafted reagent and that of the leaving group and S is the specific surface of the silica (m 2 .g -1 ).

The grafting density of SiO 2 (NH 2 ) calculated from the carbon content is 3.86 mol m - to 1.79 APTS groups/nm 2 [13]. It can be concluded that the chosen procedure for the preparation of amine-grafted silica seems to be a suitable one for chemically modification of a silica surface containing both geminated and isolated silanol functions [40].

Determination of the effective specific surface

Exact knowledge of the specific surface area of silica is essential to express concentration of reactive surface species. We have attempted to obtain information concening the specific surface of activated silica and amine-grafted silica employing the CTAB To conclude, both SEM and AFM present a more regular, smoother, and homogeneous surface texture of the amine-grafted silica compared to that of neat silica (Fig. 31).

adsorption technique [41a].

pH and conductivity

From pH and conductivity measurements, the average pH value of the amino-grafted silica suspension composite was determined to be about pH 9.6 when adding 0.5% of the composite to DI water. SiO 2 (NH 2 ) suspension shows a stable pH at least for 3 days. The chemical stability of silica in aqueous solution is pH-dependent and decreases significantly in alkaline solutions. The solubilization of APTES-grafted silica is very low at pH 1 and much higher at pH 5.7 [23]. Our composite intended for the adsorption of metal ions conserves its stability in acidic medium and it will therefore be utilized as an adsorbent for the removal of metallic ion at pH 3-5 (Chapter 4). The conductivity of the SiO 2 (NH 2 ) suspension was quite stable and determined to be about 50 ±10 S/cm. As deduced from pH and conductivity measurements, the amino-functionalized silica sols exhibits stability in aqueous solution for at least three days of study.

Zeta potential

The chemical modification of Aerosil 200 by grafting with APTES changes the surface properties. Since in DI water (pH 5.6), the silanol groups on the surface of native silica exhibit a negative zeta potential of -19.6 mV due to the ionization of the surface hydroxyl groups. After silanization with APTES, the surface was expected to show a basic character. It is also documented in the literature that after surface modification the zeta potential of the amine-grafted silica nanoparticles (139.4 nm) changes to a less negative zeta potential (-10.1 mV) compared with unmodified silica (-35.3 mV) [20].

The effect of pH on the zeta potential at 25 o C was studied. It was found that the effect of pH causes the zeta potential of SiO 2 (NH 2 ) to be positive between pH 2 and pH 8. However, SiO 2 (NH 2 ) shows a low zeta potential value of about 3.8 mV at pH  9. After functionalization, the zeta potential of the grafted silica was changed compared to native SiO 2 as shown in Fig. 54. From the curve, the point of zero charge (pzc) of the amine-grafted silica was around pH 9. Therefore, at pH 2 -8, the zeta potential was positive with a value between 41.7 mV to 20.2 mV for SiO 2 (NH 2 ). At those pH ranges, the functional amine groups on the silica surface are probably protonated (at least partially), so positively charged (NH 3 + ) species occur [23]. And under more alkaline conditions, an increase of the OH -concentration in the solution will reduce the density of positive charges. In consequence, the zeta potential diminishes further, reaching the point of zero charge at ~ pH 9. As a reminder, the conductivity is an electrical current in a solution, which depends on the ionic strength of the solution. The more the conductivity (high ionic strength) is high, the more the double layer becomes compressed. Indirectly, the conductivity has an influence on the zeta potential value. The amine-grafted silica suspension exhibits a higher conductivity than the unmodified SiO 2 . From zeta potential measurement, at pH 2-5 the amine-grafted silica exhibits a higher zeta potential value than SiO 2 protonation of the amino groups to give positively charged NH 3 + species. Our results are similar to those of an earlier study of Foissy and Persello [42]. In this study, a graph is represented showing a plot between surface charge density ( o ) versus pH for ultra-pure silica precipitated from pyrogenic Aerosil solutions (NaCl electrolyte). The curve shows that at pH > 2 the surface charge density of silica raises due to an increase of the ionic strength [42].

Hydrodynamic size measurement

During the preparation, the amine-grafted silica was dried at 100 o C before being used.

We suppose when dispersing the dried particles in aqueous solution by ultrasonication for 1 hour, hydrogen bonding between the amino groups on the surface of the modified silica and water occurs. The hydrodynamic size of the hydrated amine-grafted silica was found to be more than 1 m (Fig. 55). Their sizes are in the micrometer range, caused by the apparition of an aggregation of the amine-grafted silica particles in aqueous solution ( pH 8), which are more aggregated than native silica (mean size ≈ 270 nm). In consequence, the particles

Functionalization of Silica with Carboxyl Groups

To introduce a carboxyl group on a silica surface, the alkoxysilane agent CPTES has been used for silanization of the silica surface [43]. CPTES consists of three ethoxy groups and a cyanide group at the end-chain as shown in Fig. 57. Silanization of silica with CPTES can be accomplished by reflux in dry toluene allowing the diffusion of the CPTES precursor to be condensed onto the surface of silica. The CN-grafted silica (intermediate composite) can be afterward hydrolysed to -CO 2 H groups by strong acids like sulphuric acid. The procedure of the synthesis of the carboxyl-grafted silica is represented in Fig. 56. After aging and drying, the final product was obtained by hydrolysis with sulphuric acid [45].

Carboxyl-grafted silica was further reacted with hexamethyldisilazane to obtain bifunctional groups of trimethylsilyl-carboxyl modified SBA-15 (10), shown in Fig. 57 [46].

Recently, Y. Fang et al. have condensed the CPTES agent onto an Aerosil 300 surface and then hydrolysed the CN-grafted silica in sulphuric acid. In order to utilize this material as electrolyte, this carboxyl-grafted composite was further mixed with the two ionic liquids 1hexyl-1-3-methylimidazolium iodide and 1-allyl-3-methylimidazolium iodide and two additives of N-methylbenzoimidazole and guanidinium thiocyanate. This mixture was then spread on a TiO 2 electrode. The carboxyl-grafted silica emerged in the ionic electrolyte was found to exhibit an improved charge transfer reaction at the TiO 2 electrode (Fig. 57) [47]. In this present work, Aerosil 200 nanoparticles were functionalized with CPTES to obtain particles loaded with carboxyl groups on the silica surface. The aforementioned twostep sequence was carried out according to a procedure described in the literature (Fig. 56) [44]. The first step implies immobilization the silica surface with CPTES by reflux in dry toluene. The resulting -CN grafted silica was the acid-hydrolysed to yield the -CO 2 H groups by treatment with H 2 SO 4 . The silica composite SiO 2 (CO 2 H) covered with carboxyl groups was characterized in the same manner as SiO 2 (NH 2 ) including Elemental analysis, FT-IR, BET specific surface, SEM, and AFM, respectively. The particle suspension in DI water was analysed by measuring the pH, conductivity, the zeta potential, and the hydrodynamic size.

Elemental analysis

After chemical modification, the elemental content of carbon, hydrogen and nitrogen has been determined. Results of the elemental analysis are shown in Table 2.5. The carbon percentage of SiO 2 (CO 2 H) was 0.41%, corresponding to a grafting density of 0.47 mol.m 2 .

The carbon content of the carboxyl-modified silica was used to calculate the grafting density utilizing equation (1).

From literature, for a silica gel having a BET surface area of 320 m 2 /g calculated from TGA, the organic loading (-CO 2 H) was found to be 0.95 ± 0.05 mmol/g [48].

The FT-IR spectra of silica particles grafted with nitrile and carboxyl groups

The nitrile-containing material exhibits a band at 2266 cm -1 assigned to the C≡N stretching vibration as shown in Fig. 58a. After hydrolysis of the -CN groups to -CO 2 H groups, three characteristic bands from silica particle such as the -OH stretching vibration band corresponding to physisorbed water and hydrogen-bonded silanol groups, stretching vibration of -Si-O-Si-(siloxane backbone), and bulk mode of SiO 2 at 3490 cm -1 , 1014 cm -1

(vs) and 825 cm -1 (m), were detected in the IR spectrum of SiO 2 (CO 2 H) (Fig. 58b). The occurrence of a weak band at 1711 cm -1 due to the C=O stretching vibration and disappearance of the absorption band of the nitrile group at 2266 cm -1 confirms the successfully conversion to silica particles loaded with carboxylic groups [44]. Fig. 58 The FT-IR spectra of cyanide-grafted silica (a), and carboxyl-grafted silica (b)

BET specific surface

The BET specific surface area of the carboxyl-grafted silica composite was also investigated. This BET value of SiO 2 (CO 2 H) is lowered from 182 m 2 /g to about 118 m 2 /g compared to native SiO 2 as shown in Table 2.6 [49, 24b]. This confirms a successful surface modification of the silica nanoparticles. Our result is consistent with the data reported by K.Y.

Ho et al. where the BET specific surface area of carboxyl-grafted ordered mesoporous silica decreases to 757 m 2 /g compared to unmodified silica (1071 m 2 /g) [49]. Elemental analysis and BET value of native silica and grafted silica allow the calculation of the carboxyl groups grafting density. It was found that the grafting density of carboxyl groups containing on the silica surface calculated from equation ( 1) is 0.47 mol.m -2 corresponding to the 0.28 groups/nm 2 .

Note that grafting density of carboxyl groups on SB15 silica (obtained by cocondensation of TEOS and CTES, followed by hydrolysis of nitrile groups) was reported to be in the range between 0.60 groups/nm 2 and 2.10 groups/nm 2 (depending on the concentration of TEOS) [50]. However, the grafting density remains constant with further increases in the silane reagent concentration because of the limited amount of silanol (SiOH) groups available on the surface of silica [24d].

Determination of the effective specific surface of SiO 2 (CO 2 H)

We have also attempted to determine the specific surface of SiO 2 (CO 2 H) using the CTAB adsorption technique.

Surface morphology of SiO 2 (CO 2 H) by SEM, and AFM

The surface of the carboxyl grafted silica composite was characterized by SEM and AFM. The SEM images of SiO 2 (CO 2 H) silica as well as that of SiO 2 are shown in Fig. 59.

The surface morphology of SiO 2 (CO 2 H) was more uniform and more regular (Fig. 59b) when compared with that of SiO 2 (Fig. 59a).

pH and conductivity

The pH value of suspended colloidal carboxyl-grafted silica is quite stable over a time period with the average pH being about 4.5 [49]. According to conductivity measurements, the carboxyl-grafted silica suspension has a conductivity of 4 S/cm, which remained stable at least for 48 h. Note that the conductivity value of carboxyl-grafted silica (~ 4 S/cm), depending on the ionic strength of the solution, was quite similar with that of native silica (~7 S/cm).

Zeta potential

A study on the influence of pH to zeta potential of the in aqueous solution (pH of the solution was not reported in ref. [20]). This modified silica has more negative zeta potential compared with untreated silica (-35.3 mV) due to the change of surface charge of the silica particles after modification [20].

The effect of pH on the zeta potential was studied in the pH range from 2-9. At pH  2, the carboxyl-functionalized silica shows a positive zeta potential of 7.0 mV. This can be explained by protonation of carboxyl groups to positively charged -CO 2 H + species [51,52].

At higher pH, the carboxyl-functionalized silica represents negative zeta potential of -58.8

mV, -56.9 mV, and -61.9 mV at pH 5, 7, and 9, respectively. The negative charge zeta potential of the carboxyl-modified silica was higher than the SiO 2 due to deprotonation of the carboxyl groups grafted on the silica surface:

SiO 2 -CO 2 H + H 2 O SiO 2 -CO 2 -+ H 3 O +
Under more alkaline condition, the OH -concentration increases, thus the negative charges on the grafted silica surface due to formation of anionic carboxylate groups increases further.

Fig. 61 The zeta potential of grafted silica with carboxyl groups

Hydrodynamic size measurement

The carboxyl-grafted material was dried at 100 o C before use. After dispersion of the dried particles in aqueous solution under sonication for 1 hour, the hydrogen bonds between water and surface charge of the modified silica takes place and so the particle size becomes more important. The hydrodynamic size of the carboxyl-functionalized silica composite is shown in Table . 2.8. Compared to native silica, the hydrodynamic size of the carboxyl-grafted Aerosil was lower (172 ± 20 nm) than that of SiO 2 (328 ± 25 nm). This might be caused by the fact that repulsion between negative charges on each carboxyl-functionalized silica particle occurs. Note, that in contrast to our findings, no noticeable change between the diameter of non-functionalized silica particles (139 nm) obtained by the Stöber method and that of derived SiO 2 (CO 2 H) nanoparticles (135 nm) could be evidenced by TEM micrographs [20]. regular, smoother, and homogeneous surface texture than that of neat silica. The BET specific surfaces of all grafted silica composites decreased compared to untreated silica (328 ± 25 nm) according to functional groups on the surface of silica. The nitrogen content on the silica surface were measured utilizing the Kjeldalh method and found to be 0.82 ± 0.02 % for SiO 2 (NH 2 ). Elemental analysis is also used to determine C, H and N content of both amineand carboxyl-functionalized silica. For SiO 2 (NH 2 ), the N-content was found to be 0.76%.

This value corresponds to a concentration of amine groups containing on silica surfaces of 4.75×10 -4 mol/g. For SiO 2 (CO 2 H), the C-content from Elemental analysis was 0.41%, corresponding to a carboxyl groups concentration on the silica surfaces of 0.85×10 -4 mol/g.

The BET value of all grafted silica materials decreases compared to unmodified silica, this is due to an increase of aggregation. Both Elemental analysis and BET led to calculation of grafting density utilizing the Berendsen equation. The grafting densities, calculated from carbon-content, are 4.03 mol m -2 and 0.47 mol m -2 for SiO 2 (NH 2 ) and SiO 2 (CO 2 H),

respectively. The grafting density, calculated from the N-content (%), was 3.86 mol m -2 or 1.94 APTES groups/nm 2 . In this work, the grafting density was higher than reported in the literature: 1.7 mol m -2 or 1.79 APTES groups/nm 2 [12]. In case of the SiO 2 (CO 2 H) composite, the grafting density was found to be 1.20 CO 2 H groups/nm 2 . The silanization processes used in this present work afford a good grafting density for both anime-and /or carboxyl-modified silica. Measuring the zeta potential and hydrodynamic particles size provided information on the characteristics of these composites in aqueous solution. The stability of the composite was studied by pH and conductivity measurements. These modified silica particles exhibit sufficient stability in aqueous solution during a 3 days period. The potential of these two types of grafted silica composites for the capture of divalent metal cations in aqueous solution will be evaluated and discussed in Chapter 4.

Experimental part

Materials and apparatus The nitrogen content of APTES-grafted silica particle was determined by the SERAC (UFC) using the Kjeldahl method by means of a TurboTherm (Gerhardt, Germany) apparatus. The experimental procedures for pH and conductivity measurements were similar to those described in chapter I.

2.3

Preparation of the grafted silica

Functionalization of silica surface with APTES

Synthesis of SiO 2 (NH 2 )] was carried out according to the procedure represented in Fig. 2.11 [25]. 10 g of silica particles were suspended in 300 ml solution of APTES in dry toluene (10% v/v). The mixture was refluxed with stirring for 12 hours. The slurry was filtered and the remaining APTES was washed out successively with toluene, ethanol and diethyl ether, respectively. The product was then dried at 70 o C for 8 hours.

Functionalization of silica surface with carboxyl groups

The synthesis of SiO 2 (CO 2 H) was carried out according ref. [49]. There are two steps for the preparation of SiO 2 (CO 2 H). A CN-containing organic layer was first grafted onto Aerosil 200 silica by reflux of 2.5 g silica in 250 ml of dry toluene containing 0.1 mol of CPTES at 108 o C for 18 hours. After evaporation of the solvent, the cyanide-functionalized silica SiO 2 (CN) was dried at 100 o C for 24 hours. The CN group was then hydrolyzed to carboxylic acid by refluxing with 50% sulfuric acid at 135 o C for 3 hours. The grafted silica was collected, then washed and dried to obtain SiO 2 (CO 2 H) material. The schematic synthesis of the hydrophobic fumed silica nanoparticles with carboxyl groups is shown in Fig. 56.

Determination of the effective specific surface by adsorption of CTAB

A sample was prepared by suspending 0.5 g of silica particles in 100 ml of DI water and 20 ml of CTAB (3 g/L) was added to the sample solution, which was then allowed to stand for 24 h. After adsorption of the CTAB solution on the surface of the sample, the initial CTAB solution and supernatant of mixture of sample and CTAB were quantified for total organic carbon concentration by TOC analysis using a carbon analyzer (Shimadzu, TOC-5050).

Chapter 3 Chitosan, Modified Chitosan and Chitosan-Coated Silica Surfaces Introduction

A silica surface modified with a biopolymer is interesting since suitable functional groups on a biopolymer backbone confer them the propensity to capture pollutants like phenol, dyes, metal ions in aqueous solution [1]. Biopolymers such as gelatin, alginate, cellulose, collagen, and chitosan (CS) have been deposited by several groups onto the surface of silica nanoparticles. Some examples of biopolymer-functionalized silica surfaces for targeted applications are represented. For example, a hydrophobic silica surface obtained by silanization with methyltrimethoxysilane has been reacted with gelatin to give a hybrid material providing high ductility, low elastic modulus, and high mechanical strength [2].

Silica (precipitated type) was also modified with alginic acid to prepare hydrogel films. These films were tested in the presence of glycerol as membrane coating, affording an improved protection vis-à-vis hydrophobic loss of osmotic distillation PTFE membrane (pore diameter: 0.1 m) compared to a non-treated PTFE membrane [3]. Fumed silica interacting with epoxidized natural rubber via hydrogen bonds was produced by a mechanical mixing and analyzed by FT-IR. It was shown that the fumed silica content affects the mechanical properties of the composite, which is of importance for the resistance of tyres in rubber industry [4]. The bayberry tannin (BT) -grafted silica beads was used as support for the synthesis of Pd(0) catalysts. The BT acts as stabilizer to prevent the aggregation of Pd(0) particles and as an antioxidant for keeping the Pd(0) particles from oxidation. The Pd(0) nanoparticles stabilized by BT-grafted silica beads exhibit a highly active and reusable in liquid-hydrogenation of unsaturated organic compounds [5]. In order to study its infrared emissivity, a hybrid collagen/silica composite was prepared in aqueous suspension by deposition of collagen onto the surfaces of SiO 2 spheres at pH 5. Structural features and thermal properties were measured with FTIR, SEM, TEM, and TGA-DTA. The results showed that self-aggregation of collagen macromolecules takes place during the adsorption of collagen on the SiO 2 surface and that the biopolymer is attached through electrostatic forces and hydrogen bonding [6].

Another natural biopolymer becoming increasingly important is CS thanks to its advantageous properties especially being a low-cost material, a renewable resource, widely abundant, environmental friendly (biocompatibility, biodegradability), and last but not least its ease of chemical modification. Moreover, CS is commonly used as a sorbent for removal of metal ions from water because of the presence of potentially complexing amino groups and hydroxyl groups. These aspects have been the topic of several review articles [7].

The chemical modification of CS constitutes another promising method for the preparation of new materials. It enables one to introduce special properties thus enlarging the scope of potential applications for this biopolymer. For example, one inconvenience and limitation of CS for some applications is its poor solubility in neutral and alkaline solutions. This limited solubility in aqueous solution can be overcome by carboxylmethylation rendering the derivative N-carboxylmethylchitosan soluble in a wide range of pH [8]. Some examples of the modification of CS to obtain the carboxymethylchitosan derivatives are discussed in the topic of Chemical modification of CS.

With the objective to combine the intrinsic propensity of the natural biopolymer for pollutant uptake with those of silica particles as support, we attempted to elaborate new adsorbent materials consisting of a chitosan shell coated on a silica nanoparticle core. The chitosan shell was chemically modified to improve adsorption capacity for complexation of divalent metal ions. As support, Aerosil 200 particles (activated by acidic treatment or chemically modified as described in Chapters 1 and 2), have been chosen. After giving some helpful background information on the constitution, chemistry and properties of CS, the elaboration of three new organo-mineral silica based-composites (CS-coated silica, CS-coated carboxyl grafted silica, and carboxyl modified CS-coated silica) is described in this chapter.

Constitution, Chemistry and Properties of Chitosan Origin, Structure and Production of Chitosan

Chitin is the second most abundant natural amino polysaccharide after cellulose.

Chitin is found in the skeleton of crustacean, insects, and some fungi, etc. As shown in Fig. 62, CS is structurally very related to the polysaccharide cellulose, an OH function being by a acetylated amino-group. CS itself is the N-deacetylated derivative of chitin. CS constitutes a major component of arthropod and crustacean shells of lobsters, crabs, shrimps, and cuttlefish [9,10]. Crustacean shells consist of 30-40% proteins, 30-50% calcium carbonate, and 20-30% chitin and pigments such as carotenoids [11]. These proportions vary with species and seasons. CS is obtained from a chemical purification process involving 4 steps (Fig. 63):

-(i) Demineralization step: the sample shell is demineralized with HCl and then washed to neutral pH, rinsed and filtered.

-(ii) Deproteinization step: this is accomplished by reacting the sample with NaOH.

-(iii) Deacetylation step: the solid product is added to sodium hydroxide leading to the deacetylation of chitin to CS and involves the removal of acetyl groups from the molecular chain of chitin, leaving behind a complete amino group [12].

-(iv) Decoloration step: the product is washed and dried under UV light from the sun (oxidative degradation of carbon-carbon double bonds present in astaxanthin) [13].

Fig. 63 Production process of CS [13] CS powder (70-80% degree of deacetylation (DD)) exhibits a degree of crystallinity of 30-50% as determined by X-ray diffraction and IR spectroscopy [14], affected by the N-acetyl glucosamine unit. The percentage of this residual unit depends on the original source and the treatment during production and can be determined by a variety of spectroscopic methods for structural analysis presented in a recent review article [15].

Behavior of CS in Solution

CS is almost insoluble in water at neutral pH and in organic solvents. The solubility of CS depends on the % DD, pH and the percentage of protonation of the free amino groups. CS can be solubilized in diluted acidic solution (acetic acid, formic acid, etc.) [10,16] and exhibits in acidic solution cationic polyelectrolyte properties because of the protonation of -NH 2 groups as shown in Fig.

64 [17]. The degree of protonation of CS depends on the pK a (acid dissociation constant) of the acid used to dissolve CS [18]. In case of strong mineral acids (i.e. sulfuric acid, hydrochloric acid, nitric acid) dissociating completely in water, the protonation of the amino group is shown in Fig. 65. Studies by A. Domard have evidenced that interactions between CS and carboxylic acids (weak acids) are electrostatic ones between -NH 3 + and -COO -functions [19].

Chemical Modification of CS

A lot of research deals with the chemical modification of CS in order to improve its adsorption properties and selectivity. This chemical modification can be realized without change/degradation of the fundamental skeleton of CS. In order to widen the application of CS, many attempts have been made to optimize its properties by crosslinking [20,21], layer coating [22][23][24], depositing [25][26][27], grafting, and functionalizing [28][29][30][31][32][33][34]. Note however that chemical modification utilizing crosslinking agents may decrease the metal uptake efficiency and sorption capacities [35].

An important amount of CS derivatives has been obtained by grafting new functional groups onto the CS backbone. Some reasons for chemical modification by grafting new functional groups to CS are listed as follows:

(1) to increase the density of sorption sites,

(2) to change the pH range for metal sorption, and

(3) to change the sorption sites and/or the sorption mechanism in order to increase the sorption selectivity.

State of the Art

The active sites of CS (-NH 2 and -OH groups) show an effectiveness to interact with metal ions like Cu(II), Co(II), Ni(II), Cr(VI), and Au(III), etc [36][37][38][39][40][41][42], but are also able to bind anionic pollutants such as As(III) arsenite and As(V) arsenate [43]. Due to its non-toxicity, biodegradability, and antibacterial and antiviral properties, CS is employed as base material in medical and pharmaceutical applications [44]. In addition, CS can be formulated in various forms such as powder, gels, and films, etc depending on the desired use. For example, CS has been used as bone tissue, since it promote cell growth and mineral rich matrix deposition by osteoblasts* cells in culture [45,46]. CS can be moulded into porous structures to allow osteoconduction* [47]. CS has been known, with high potential, as a carrier for various active agents [48][49][50][51][52]. In the case of gene delivery, CS as natural cationic polysaccharide can interact with the negatively charged DNA (introduced into the target cells to get transcribed and genetic information) and form polyelectrolyte complexes. This provides a better-protected DNA from nuclease degradation leading to better transfection efficiency [53].

*Osteoblasts are large cells which are responsible for the synthesis and mineralization of bone during both initial bone formation and later bone remodeling. Osteoconduction means that bone grows on a surface.

A selection of some reagents which have linked with CS such as acrylamide, N,N"biisomaleimide, N,N"-biisophthalimide, N,N"-phthalimidomaleimide, and polyamidoamine (PAMAM) is shown in Table 3.1. N,N"-biisomaleimide

(2)
N,N"-biisophthalimide

(3)
N,N"-phthalimidomaleimide

(4)
Mercaptoacetic acid (5) Ethylenediaminetetraacetic acid (EDTA)

Diethylenetriaminepentaacetic acid (DTPA)

Carboxymethylchitosan

A further chemical modification of CS, which has received much attention in recent decades, is the so-called carboxymethylation. Apart from low toxicity, antibacterial activities, etc. [54][55][56] functionalization via carboxymethylation renders CS more hydrophilic thus dramatically increasing the solubility of CS at neutral and alkaline pH value, whereas CS itself is only soluble at pH < 6. This solubility of carboxyl-modified CS involves H-bonding between water and the polar COO -or carboxyl groups of the modified CS [57]. Moreover, these carboxymethylated CS derivatives own the ability for capturing metal ions, which can be even superior to that of CS [37].

Depending on the reaction conditions, the site for carboxymethylation can be controlled allowing the targeted synthesis of several derivatives including N-carboxymethyl For this work, a N,O-carboxymethyl-modified CS composite was synthesized using monochloroacetic acid according to a modified procedure described in the literature and shown in Fig. 66 [61]. This carboxyl-grafted CS was then further reacted with the silica particles as described below.

Fig. 66 Schematic modification of CS by grafting with carboxyl groups

The CS-coated Silica Surface

The study of organic-inorganic hybrid materials is an interesting subject because the latter combine both organic and inorganic properties. Biopolymers would enhance adsorption capacities, the selectivity of metal ions, and the flexibility of hybrid materials. On the other hand, silica could be used to balance some of the disadvantage of biopolymers such as lower chemical resistance, thermal resistance, stability, and mechanical performance. Therefore Silica-CS hybrid materials can be employed to combine the advantages. The hybrid materials can be fabricated as beads, membranes, films, etc. However, in this work, the silica particles used were fumed silica (Aerosil 200, Degussa) whose surface was modified by the introduction of the CS biopolymer. The hybrid material was fabricated as colloidal particles in order to be handled easily and to get more surface sites for capturing metal ions. Herein, some different forms of materials and preparation procedures of the hybrid materials were represented. The hybrid composites, based on the silica particles, were first treated with acid solution or modified before grafting the CS macromolecules. In contrast, CS could be modified with reagents like polyethylene glycol (PEG) to obtain a porous material. Then, the modified CS was attached to the silica particles.

To give an idea and overview about silica-CS hybrid composite materials including their preparation, some selected examples used for water treatment are presented. Q.H. Shi et al. activated silica beads (97 nm) with hydrochloric acid and interacted them with CS dissolved in 1% acetic acid. Finally, the CS-coated silica beads were treated with glutaraldehyde. In this cross-linking process, glutaraldehyde reacts with the amino groups of CS molecules to improve the stability of the CS film (Fig. 67) [62]. Another procedure for the preparation of glutaraldehyde-modified CS coated silica particles involves treatment of SiO 2 particles (150 nm) with dodecyl sulphonic acid sodium salt in NaCl solution. A CS solution is then added into the former solution to adsorb CS onto the silica particles. Finally, the CScoated silica is cross-linked with glutaraldehyde (Glut) [63]. To study the adsorption of anionic dyes on a silica/chitosan hybrid material, the group of A. R. Cestari first silanized silica gel (70-230 mesh) with 3-aminopropyltrimethoxysilane (APTMOS) and than reacted it with glutaraldehyde to give Glut-SiO 2 . The latter was finally cross-linked with CS at room temperature (Fig. 67) [64]. A macroporous CS-coated silica gel (100-200 mesh size) has been evaluated for the immobilization of trypsin. The surface pores of the bead were generated by PEG molecular imprinting and cross-linking steps. The modified CS was then reacted with silica gel to give a macroporous CS-coated silica, which was finally cross-linked with epichlorhydrin to increase the stability of the coated chitosan layer. In this case, the poor resistance of silica at basic conditions is enhanced since the silica surface is coated with CS [66].

In the following biomedical example, an elegant electrophoretic deposition (EPD) method has been developed for the fabrication of nanocomposite silica-chitosan coatings.

The EPD has been performed from suspensions of silica and hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 with a chemical composition similar to that of bone tissue in ethanol-water mixture containing CS. The suspensions were ultrasonicated to obtain a homogeneous dispersion of the nanoparticles. CS provides an effectively electrostatic stabilization of the ceramic particles and its positive charge allows the cathodic electrodeposition of the particles.

Moreover, CS acts as an effective binder that provides adhesion of the particles to the substrate surface and prevents cracking [67].

In this work, several CS-coated silica hybrid composites were prepared by mixing CS solution and colloidal silica at room temperature to elaborate novel performing sorbents for the retention of divalent cations form aqueous solution.

Degree of Deacetylation (%DD) of CS

The %DD influences numerous characteristics of CS including crystallinity, degradation, solubility, charge density, compactness of matrices and hydrophobic properties, etc [11,67]. Of course, also the nitrogen content is correlated with the %DD and constitutes an important parameter especially for metal ion uptake applications, since a high %DD implies a higher number of free amine groups available for binding with metal ions.

Therefore is seemed to be important to obtain an accurate value %DD of our CS samples and two independent techniques have been used:

(i) Infrared spectroscopy offers several advantages such as being nondestructive and fast, since there is no special need for the preparation of the samples before recording the ATR-IR spectra. Furthermore, the high precision of the determination of the %DD is consistent with elemental analysis as shown below. The degree of deacetylation was determined by recording the absorbance at 1655 cm -1 and at 3450 cm -1 (Sabin and Block, 1997) [69].

where A 1655 is the absorbance at 1655 cm -1 of amide I band as a measure of N-acetyl group content, (C=O band in amine group), A 3450 is the absorbance at 3450 cm -1 of the hydroxyl band of CS. The %DD of two types of CS was calculated using equation (1).

According to the report of Sabnis and co-workers, the values of 97.67 and 24.486 are slope and c values, respectively. They were determined from standard curve plotted between DD% (using chitosans with known degree of deacetylation determined by titrimetry) and absorbance ratio (A 1655 /A 3450 ) (see more detail in ref. [69]).

The degree of deacetylation thus determined via the FT-IR method is shown in Table 3.3 and was found to be around 73, 75, and 77 for CS Marine, CS Yuhuan, and CS LMW, respectively. The maximum value of % DD was ranked in order as follows: CS LMW (77%DD) > CS Yuhuan (75%) > CS Marine (73%). (ii) As second independent method, elemental analysis has been chosen for the determination of %DD of CS. The determination of the amount of carbon and nitrogen of CS allows to calculation the %DD using equation 2 [70].

where C/N is the percent ratio of carbon or nitrogen in a CS macromolecule. They may vary from 5.145 for completely N-deacetylated chitosan (C 6 H 11 O 4 N as repeating unit) to 6.861 for the fully N-acetylated polymer (C 8 H 13 O 5 N as repeating unit). The amount of carbon and nitrogen is shown in Table 3.4, the degree of deacetylation was then calculated utilizing equation 2. The %DD of CS MW was found to be 77 %. Note that fully deacetylated CS has an amino concentration of about 6.21 mmol/g [71]. This amino concentration helps to calculate the amino amount contained in CS. From IR spectroscopy data, the amino amount of CS LMW was found to be 4.22 mmol/g. From elemental analysis, it was found to be 4.78 mmol/g. In comparison, the %DD of CS LMW determined from the two independent methods (77 %DD by IR spectroscopy and 77%DD by elemental analysis) was close to each other. In other words, the %DD value of CS determined by IR matches well with that determined by elemental analysis.

In comparison with the literature, the nitrogen amount of CS is found to be 7.325%

(%DD  76) [72]. This value was close to that of our chitosan (CS LMW) where the nitrogen amount was 7.31% (77%DD determined by elemental analysis).

Characterization

Preparation of Carboxymethyl-modified CS

The synthesis of a

Preparation of CS-coated Silica Hybrid Composites

With the goal to prepare novel adsorbent for metal cation removal, three types of modified CS-coated silica particles have been prepared as summarized in Fig. -NH 2 and -COOH groups can be taken in consideration. However, this hypothesis is less likely, since formation of an amide requires in general more severe reaction conditions.

Furthermore, no evidence for the formation of amide groups was obtained by FT-IR spectroscopy.

To obtain the third type, namely SiO 2 +CS(CO 2 H), Aerosil 200 particles were coated with carboxymethylated CS. Again, the interaction between this CS derivative and the silica surface occurs most probably through hydrogen bonding between carboxylate groups and Si-OH groups according to the FT-IR spectra.

Fig. 69 Preparation of CS-coated silica particles

Color code: blue = non-modified CS, green = carboxymethyl-modified CS

Characterization of the Composites by Elemental Analysis

The C, H, N-contents of composites In the literature, the CS amount containing glutaraldehyde crosslinked with silica gel was found to be 8.5 mg/g of wet bead [62].

Determination of the BET Specific Surface

The BET specific surface (S BET ) of the new silica composites was also determined. As one may expect, the specific surface of the particles decreased significantly after the modification of silica nanoparticles (see Table 3.6). The S BET value of the CS-coated silicas was less than 10 m².g -1 . An obstruction and blocking of the adsorption sites might occur after surface-coating of the silica nanoparticles with a CS layer to explain this important lowering of the S BET . This finding suggested that the surface of silica nanoparticles be functionalized with the carboxyl group or they could be covered entirely by a CS layer. The specific surface decreased according to the physical or chemical modification.

We are aware of a recent study where the S BET value of the silica xerogel hybrid materials containing CS was measured in function of the CS content [74]. This study revealed that the specific surface area is considerably reduced when increasing the amount of CS. The neat xerogel exhibits a S BET of 620 m².g -1 , whereas the hybrid containing 30% of CS has a S BET value of 150 m².g -1 . When increasing the CS percentage to 50%. The S BET value drops to less than 20 m².g -1 . So our findings are in line with the latter study.

FT-IR Spectroscopy

As can be seen in Fig. 70a-c, the FT-IR spectra of CS LMW, CS Marine, and CS Yuhuan are similar and displayed a characteristic -OH stretching vibration at 3450 cm -1 , which overlaps the -NH stretching vibration for amines resonating. The C-H stretching vibration is observable at 2923 cm -1 . At 1650 cm -1 are found the C=O stretching vibration for amide I. Other partially overlapping absorptions at 1560 cm -1 are due to the -N-H bending of the amine group. Other absorptions can be assigned to amide II at 1379 cm -1 for -CH symmetric bending vibration in -CHOH-, at 1022 cm -1 for the -CO stretching vibration in -COH [9]. Based on literature data, the most frequently identified bonds which correspond to a given wave number (cm -1 ) in IR spectra are presented in table 3.7 [7]. These data are close to those of our CS spectra (Table 3.8). The characteristic absorption bands of CS(CO 2 H) are shown in Fig. 71 (curve d) and in Table 3.9. The two bands at 1627 cm -1 and 1429 cm -1 are the characteristic absorption bands of the COO -carboxylate group due to an asymmetric vibration and symmetric vibration according to the literature [61]. Note that after deprotonation of a -CO 2 H group which resonates at about 1730 cm -1 , the resulting -COONa group absorption shift to lower wavenumber at around 1627 cm -1 . We got no evidence for the presence of -CO 2 H groups in our sample. The amide II band present in CS LMW at 1595 cm -1 disappeared after carboxymethylation indicating the binding of the carboxyl group to the amino group of CS. [75,76]. The band at 3450 cm -1 was shifted to 3471 cm -1 after carboxymethylation [38,77]. In the case of CS-coated silica, intense siloxane stretching bands around 1080-1100 cm -1 dominate the spectra (Fig. 7172). The bands related to alkyl chains are detected at 2879 cm -1 in SiO 2 +CS LMW (Fig. 71a), at 2922 cm -1 in SiO 2 +CS Yuhuan (Fig. 71b), at 2929 cm - 1 in SiO 2 +CS Marine (Fig. 71c), at 2918 cm -1 in SiO 2 (NH 2 )+CS LMW (Fig. 72a), at 2877 cm -1 in SiO 2 (CO 2 H)+CS LMW (Fig. 72b), and at 2933 cm -1 in SiO 2 +CS(CO 2 H) (Fig. 72c).

The residual amide bonds of the deposited CS give rise to the C=O stretching bands in the range between 1650-1679 cm -1 . Finally, the large band observed around 3450 cm -1 is attributed to both silanol groups on the surface of silica and hydroxyl groups presented on CS In the case of SiO 2 (NH 2 )+CS LMW (Fig. 72a) the band at around 3448 cm -1 (SiO 2 +CS LMW) is shifted to 3485 cm -1 . This shift may be due to the -NH 2 group grafted onto the surface of silica.

Due to the carboxylic acid moieties present in SiO 2 (CO 2 H) (Fig. 60b), an absorption was observed at 1711 cm -1 . However, this band did not appear in SiO 2 (CO 2 H)+CS LMW (Fig. 72b). This absence could be attributed to a relatively small number of CO 2 H groups compared to amide moieties in CS and the above discussed interactions between the CS and the CO 2 H groups.

For SiO 2 +CS(CO 2 H) (Fig. 72c), the band at 1429 cm -1 and 1627 cm -1 due to COO - symmetric and asymmetric vibration from CS(CO 2 H) were shift to 1440 cm -1 and 1650 cm -1 after the coating of CS(CO 2 H) onto the silica surface.

From FT-IR data, a proposed model of the possible interaction between CS and silica is represented in Fig. 73. The CS chains were adsorbed on silica particles via different types of interactions such as an electrostatic attraction [82] and H-bonding [83] as describe on pages 112-113.

Morphology Analysis of the Composite by SEM

The surface morphology of the CS LMW polymer was investigated by SEM as shown in Fig. 74a. The CS LMW image shows a dense and smooth surface texture which was similar to the observation of the surface morphology of CS by E.-J. Lee et al. [74]. In contrast, the morphology of the carboxymethyl-CS material (CS(CO 2 H)) was rough. There were some porous points on the surface of the CS(CO 2 H) (see Fig. 74b). This could favour the accessibility for metal adsorption. 75a-d) [84]. In contrast, the morphology of the carboxymethyl CS-coated silica (SiO 2 +CS(CO 2 H)) showed a heterogeneous texture of the carboxymethyl CS molecules (Fig. 75e). found to be around 0.02 M. After intensively rinsing with DI water, the conductivity value of SiO 2 (CO 2 H)+CS LMW was found to be around 3-6 S/cm. The pH and conductivity measurements conducted on these hybrid composites imply that these composites are quite stable in aqueous solution at least for 72 hours.

Zeta Potential

In order to study the interfacial properties of the colloidal silica material, the zeta potential of the hybrid composites was determined at 25 o C. The results are shown in Fig. 77a-b. For comparison, the zeta potential values of the unmodified particles were also represented. The point of zero charge (pzc) of the Aerosil 200 silica particles was around pH 3. At pH > 3, the zeta potentials decreased. This induced an increase of the concentration of the negative charges on the silica surface. An unequivocal reversal of the sign of the zeta potential was noticed after coating the two types of silica by CS at an acidic pH. The pzc of the coated silica composite was reached at a pH 6.5. Therefore, the zeta potentials of the hybrid composites were positive at pH < pzc. This could be explained by the protonation of amino groups to give positively charged -NH 3 + species at acidic pH (in the absence of a salt, the pKa value of CS is 6.5) [86]. The zeta potential turned to negative values at pH higher than pzc. Under alkaline conditions, the amino-groups were deprotonated. Therefore, the zeta potential was negative. Note that the zeta potential values of the five types of CS-coated silica (SiO 2 +CS LMW, SiO 2 +CS Marine, SiO 2 (CO 2 H)+CS LMW, SiO 2 (NH 2 )+CS LMW, and SiO 2 +CS(CO 2 H)) were quite close. This result could be the consequence of a large degree of coating by CS on our silica particles. The results of zeta potentials were consistent with the results reported by T. Witoon and his team. The zeta potentials of the CS-silica hybrid material prepared at low pH (2-3) were positive. At this low pH, amino groups of CS were highly protonated and the zero charge of silica surface was formed. So, the presence of CS macromolecules in the mixtures prevents the aggregation of silica nanoparticles [87].

Hydrodynamic Size Measurement

The hydrodynamic size of the six modified silica materials was also investigated. These values were in accordance with the deposition of CS supported on the silica surface.

These results were similar to the results reported by J.-S. Chang and co-workers. The average size of the hybrid material which was measured by light-scattering grew to 9.7 µm due to CS deposition [73].

Conclusion

In this chapter, the biosorbent CS LMW, CS Yuhuan, and CS Marine have been characterized by several methods. The %DD was found to be 73, 75, and 77% for CS Marine, CS Yuhuan, and CS LMW, respectively. The degree of deacetylation of the CS LMW was obtained by infrared spectroscopy and elemental analysis. Independently of the method, the %DD of CS LMW was found to be 77%. The results showed that there was no significant difference for the %DD values determined from the two methods. In order to improve the adsorption properties and solubility, CS LMW was chemically modified by synthesis of N, O-carboxymethylchitosan the derivative CS(CO 2 H). The composite was characterized using FT-IR spectroscopy confirming the successful introduction of carboxyl groups on the CS scaffold. The surface morphology of a functionalized CS composite was characterized by SEM technique. The morphology of the modified composite of CS(CO 2 H) showed a rough and porous texture which would enhance its adsorption properties. In the literature, the N-carboxymethylchitosan derivatives exhibited an promising efficiency for Cu(II) adsorption [78]. This N-carboxymethylchitosan derivatives was also used to bind with Au(III) [38]. In order to overcome the disadvantages of CS (swelling), the CS was deposited on Aerosil 200 as supporting material.

These CS-coated silica (activated and/or functionalized) composites were successfully prepared using reaction mild conditions. The composites were characterized using FT-IR, SEM, AFM, zeta potential, hydrodynamic particles size, and BET specific surface the same as neat CS and modified CS. The behavior of the composites in aqueous solutions was studied by measuring the pH and conductivity values. The hybrid composite showed a stability property in aqueous solution for at least 72 hours. The carbon amount of CS coated on the silica surface was measured by the Total Organic Carbon (TOC) method. It was found to be 1.67 x 10 -3 mol/g for CS LMW. The amine content was also determined by elemental analysis. It was found to be 4.51 x 10 -3 mol/g for SiO 2 +CS LMW. The CS amount determined from the latter method was higher than that determined by the former method. In the former method, the solid sample required the chemical procedure to convert the solid sample to solution (dissolving with acetic acid). Therefore some residue would be lost.

Experimental Part

Materials and Apparatus

Chloroacetic acid (99.3%) was purchased from Aldrich Chemical Company. Inc., Switzerland. Sodium bicarbonate was purchased from Prolabo, Switzerland. Glacial acetic acid (99.9%) was purchased from Sigma-Aldrich, Germany. All reagents were analytical grade. The amount of CS adsorbed on silica surface was quantified by means of a TOC analyzer instrument (TOC-5050, Shimadzu, Japan).

Preparation of

Preparation of Organic-inorganic CS/silica Hybrid Composites

A 0.5% of each colloidal (non-grafted and/or grafted) silica was added to 100 ml of a 2% CS solution (2 g of CS LMW or CS Yuhuan, or CS Marine dissolved with 100 ml of 1% CH 3 COOH) while being stirred with a magnetic stirrer for 1 night. The above process led to the formation of silica-CS suspension (see Fig. 78a). This suspension was poured into 500 ml of 0.1 M NaOH solution while being stirred to neutralize excess acid. Then the particles were successively washed with DI water. The material was dried in an oven at 100 o C for 24 hours, crushed and kept in desiccator until it was used. In the case of carboxymethyl CS, it was first dissolved in 100 ml of DI water for 24 hours before adding colloidal silica into the CS solution (see Fig. 78b) [61].

Determination of Concentration of CS Adsorbed on Silica by Total Organic Carbon (TOC)

The carbon content of CS-grafted untreated silica particle was quantified by TOC analysis (Shimadzu, TOC-5050, Japan). The sample was prepared as follows: an 0. The tests to determine the adsorption efficiency for the binding these metal cations were carried out at room temperature. We used various adsorption isotherms to study the interaction between adsorbates and adsorbents, kinetic isotherms such as pseudo-first-order, pseudo-second-order, intraparticle diffusion, and the Elovich equation (which assumes that the active sites of the sorbent are heterogeneous and exhibit different activation energies for chemisorption) were also investigated to study the adsorption rate. The impact of other adsorption parameters such as initial solution pH, contact time, and initial solution concentration was also investigated.

Adsorption

Since adsorption is an effective method to remove heavy metals from water, this technique has stimulated the interest of many research groups [1,2]. The term "Adsorption" For the consideration of the adsorption process, there are two aspects that must be addressed:

(1) Kinetics: the rate at which the adsorption process occurs.

(2) Adsorption isotherms: these are important parameters for describing how the adsorbate interacts with the adsorbent.

On the basis of the types of attraction forces existing between the adsorbate and the adsorbent, adsorption can be classified into two categories: physical adsorption (physisorption), and chemical adsorption (chemisorption).

-The physisorption: the adsorbate molecules are attracted by van der Waals forces with the adsorbent molecules. The physical adsorption can be a monolayer or multilayer adsorption and is an exothermic process characterized by low enthalpy values (around 5-40 kJ/mol).

-In contrast, the chemisorption involves specific interactions between the adsorbent surface and the adsorbate; i.e. a chemical reaction between the surface and the adsorbate takes place. Chemisorption is an exothermic process implying high enthalpy values (energy involved can reach several hundreds of kJ/mol) due to chemical adsorption. Characteristic is a much higher heat of adsorption with specific interaction (chemical bonds). Chemisorption by nature is limited to a monolayer process. The chemisorption process necessitates some activation energy, so chemisorption may be much slower than physisorption [3b].

The adsorption process on a porous material takes place in four definable steps shown in Fig. 79. It could be more or less definable in steps, which depends on the characteristic of the adsorbent components such as surface area, porosity, size, etc [4]. -The L-type (L refers to Langmuir) isotherm is a common type isotherm. It is identified by its initial region concave to the concentration axis. As the adsorbate concentration increases, the isotherm may reach a plateau (Fig. 80a).

-The S-type (S refers to Sigmoid) exhibits a low adsorption affinity at low adsorbate concentrations and enhanced adsorption after inflection (Fig. 80b).

-The H-type (H refers to High affinity) isotherm has a very high initial slope indicating strong adsorption at low adsorbate concentrations. The adsorption at higher concentrations leads to similar changes to those found in both L and S-types. (Fig. 80c).

-The last one, C-type (C refers to Constant partition) isotherm refers to a system in that the initial concentration ratio between adsorbate and adsorbent is the same. (Fig. 81d). This isotherm type provides information about the adsorption mechanism. Ni(II) ions on colloidal hybrid organic-inorganic silica composites" [7]. In addition, the adsorption kinetics such as pseudo-first-order, pseudo-second-order, and intraparticle diffusion and adsorption parameters at high concentration are discussed in this chapter.

Part A Adsorption of Copper(II) ions on the Composites

In this first part, various silica materials, namely the amino-functionalized silica

(

A1 Effect of Initial Solution pH

The mass of an adsorbent is one of the key parameters influencing the adsorption process. The adsorption capacity for Cu(II) of a polymer-based adsorbent raises when increasing the polymer concentration, since the number of reactive sites responsible for the adsorption process is increased [8]. The amount of copper adsorbed onto the adsorbents (mg/g) was calculated based on the difference of the Cu(II) concentration in aqueous solutions before and after adsorption, the volume of aqueous solution, and the weight of the modified silica materials according to equation 1 where C o is the initial metal concentration, mg/L, C e is the final metal concentration, mg/L, M is the weight (g) of the modified silica material, and V is the volume (L) of the metal solutions.

In this present work, the adsorbent dose was constant at 0.4% (w/w) for all adsorption studies. As previously explained in Chapter 3, the affinity of CS to capture metal ions depends on the pH of the solution. Therefore, the effect of solution pH was investigated. The adsorption at an initial concentration of 500 mg/L Cu(II) was studied on two selected materials, namely SiO 2 +CS LMW and SiO 2 (NH 2 ). Considering that at lower pH (≤ 2) the SiO 2 +CS LMW is dissolved, this low pH range was not investigated. The adsorption at pH > 6 was not because the precipitation of Cu(OH) 2 from solution. Therefore, the experiments were carried out in a pH range between 3 and 6. CS in an acidic medium becomes a polyelectrolyte because of the protonation of amino groups. The following equilibrium reaction describes the ionization of CS:

In the absence of a salt, the pK a value of CS is found to be equal to 6.5. At pH  6.5, the amino groups on CS are protonated, this reduces the number of binding sites available for the adsorption of Cu(II) ions [9,10]. In addition, at pH lower than 3 the CS can be dissolved.

A2 Effect of the Initial Cu(II) Concentration

The initial concentration of metal ions in solution has a strong effect on the adsorption capacity of pollutants. In general, the adsorption capacity increases when increasing the initial concentration of metals [8].

The effect of the initial concentration of Cu(II) was investigated using various Cu(II) concentrations in the range between 100 and 1.000 mg/L. The adsorption parameters: pH 5, adsorbent dose (0.4%), and the contact time (24 h) were fixed [11]. The adsorbents used in this part were I (SiO 2 (NH 2 )), group II (SiO 2 +CS LMW), and group IV (SiO 2 +CS(CO 2 H)).

A comparison of the influence of the initial Cu(II) concentration of solution for the adsorption between group I and II is shown in Fig. 82. It was found that an increase of the initial Cu(II) concentration augments the adsorption capacity for all three types of modified silica materials. Fig. 82 shows that SiO 2 +CS LMW possesses a higher efficiency as a good adsorbent for adsorption of Cu(II) than SiO 2 (NH 2 )). This is due to the fact that the former adsorbent (nitrogen content of SiO 2 +CS LMW = 4.29 mmol/g) owns considerably more -NH 2 binding sites than SiO 2 (NH 2 ) (0.47 mmol/g). In consequence, the adsorption onto the CS-coated silica materials was found to be higher than the adsorption onto SiO 2 (NH 2 ).

Fig. 82 The effect of initial concentration of Cu(II) ions for adsorption onto

SiO 2 +CS LMW material in comparison with SiO 2 (NH 2 )
The comparison between group I (SiO 2 (NH 2 )), and group IV (SiO 2 +CS(CO 2 H)),

shows in Fig. 83 that an increase of the initial Cu(II) concentration gives rise to the increase of the adsorption capacity for two types of modified silica materials. At the initial Cu(II) concentration of 500 mg/L, the adsorption onto SiO 2 +CS(CO 2 H) was found to be significantly higher than that of SiO 2 (NH 2 ). This difference is due to the amount of available active sites (-CO 2 H) on CS for the binding of Cu(II) ions. Therefore, the adsorption isotherms were further studied to get a better understanding on the adsorption of Cu(II) onto each selected modified silica materials. respectively. These values of adsorbed Cu(II) were determined by Atomic Absorption Spectroscopy (AAS) and calculated using equation 1.

For these types of CS-coated silica materials: SiO 2 +CS LMW, and SiO 2 +CS Marine, the adsorption capacity was strongly related to the binding sites, which are only amino groups or both amino and hydroxyl groups cooperating with each other to provide an appropriate special conformation for binding of the Cu(II) ion. The content of adsorbed copper on SiO 2 (NH 2 )+CS LMW is strongly related to amino binding sites present both on the silica core and the CS shell. The copper concentration adsorbed on SiO 2 (CO 2 H)+CS LMW involved both the amino and the carboxyl groups cooperating with each other to provide an appropriate space conformation for binding the Cu(II) ion. In case of SiO 2 +CS(CO 2 H), the amount of copper adsorbed depends both on carboxyl groups or both carboxyl and hydroxyl groups and as well as residual amino groups.

To understand the adsorption system, the adsorption isotherms were further investigated. The FT-IR spectra of the metal loaded adsorbent were also recorded.

The two-parameters Adsorption Isotherms

Equilibrium studies were performed to evaluate various isotherms models to explain the effectiveness of the adsorption process and thereby the adsorption capacity and energy of adsorption. Table 4.2 shows some adsorption isotherms in non-linear form.

Langmuir isotherm

The Langmuir adsorption isotherm (Langmuir, 1918) is widely used for the description the adsorption of a pollutant from an aqueous solution and is based on the following hypotheses:

-the Langmuir isotherm describes an equilibrium adsorption assuming a mono-layer adsorption onto the surface of the sorbent;

-the adsorption takes place at specific homogeneous sites with a finite number of identical sites, -the adsorption takes place without any interaction between the adsorbed molecules and,

-the energy of the adsorption is constant and there is no transmigration of adsorbate on the surface plane [11,12].

The Langmuir isotherm can be represented by the following relationship:

where q e and C e are amount adsorbed (mg/g), and adsorbate concentration in solution (mg/L) at equilibrium. The plot of 1/C e versus 1/q e should give a straight line. The intercept has the value of 1/K L q m , where K L (L/mg) is the Langmuir constant and q m (mg/g) is the maximum adsorption capacity [13]. ]. From the highest initial metal concentration C o (mg/L) and the Langmuir constant K L (L/mg), can be expressed a dimensionless equilibrium parameter R L , defined by

The value of R L indicates the type of the isotherm to be either unfavorable (R L > 1), linear (R L = 1), favorable (0 < R L <1) or irreversible (R L = 0). For example, CS-coated sand (5% chitosan content) has been used to adsorb Cu(II) and the Langmuir isotherms best fits to describe the adsorption of Cu(II) onto the surface [14]. The Langmuir isotherm model also fits well to explain the adsorption of the dyes Acid Red 37 and Acid Blue 25 onto cross-linked Chitosan-ethylene glycol diglycidyl ether beads and showed a better conformity compared to

Freundlich and BET isotherms [15].

Freundlich isotherm

The Freundlich constant (Freundlich, 1909) predicts the quantity of metal ion per gram of sorbent at the unit equilibrium concentration [16]. The empirical Freundlich model considers mono-layer adsorption of solute by the adsorbent and assumes that the adsorbent has an energetically heterogeneous surface and different affinity for adsorption and the existence of an interaction between the molecules that adsorbed onto that heterogeneous surface. The Freundlich equation suggests that the adsorption energy exponentially decreases on completion of the adsorptional centres of an adsorbent. The Freundlich adsorption isotherm is given by where q e , and C e are amount adsorbed (mg/g) and the adsorbate concentration at equilibrium in solution (mg/L). Adsorption capacity (K F ) and intensity of adsorption (n) are Freundlich constants, which affect the adsorption process. The constants K F and n are calculated from the previous equation by plotting log C e versus log q e (intercept = log K F and slope = 1/n). The n value is a measure of the nature and strength of the adsorption process and of the distribution of active sites. If the intensity of adsorption is less than one (n < 1), bond energies increase with the surface density. In contrast, if the intensity of adsorption is more than one (n > 1), bond energies decrease with the surface. CS has been used to capture Pb(II) and the adsorption is best explained in this case by the Freundlich isotherm [17]. Freundlich isotherm model provided the best fit for the adsorption of Cu (II) onto chitosan (CS)/sisal fiber (SF)/banana fiber (BF) hybrid composite [18].

Temkin isotherm

The Temkin model (Temkin and Pyzhev, 1940) is used to represent the equilibrium adsorptive behavior between two phases composing the adsorption system considering the effects of some indirect adsorbate/adsorbent interactions on adsorption isotherm. The heat of adsorption of all molecules in the layer would decrease linearly with coverage. The Temkin isotherm is expressed as where q e is the amount of metal ions adsorbed per gram of composite (mg/g), C e is the equilibrium concentration of metal ions in solution (mg/L), B T = RT/b T , and b T is a Temkin isotherm constant related to heat of sorption (J/mol), K T is the equilibrium binding constant (L/mg) corresponding to the maximum binding energy, R is gas constant (8.314 J/mol K), and T is the absolute temperature (K) [14]. The isotherm constant can be obtained from the plot of q e versus ln C e . B T is the adsorption heat linked to the Temkin isotherm constant b T as equation 7.

The adsorption of Cr(VI) ions from aqueous solution by ethylenediamine-modified crosslinked magnetic chitosan resin have been explained by the Temkin isotherm [19].

Dubinin-Radushkevich isotherm

Dubinin and Radushkevich (Dubinin, 1960) have reported the characteristic sorption curve which is related to the porous structure of the sorbent [14,15]. The Dubinin-Radushkevich (D-R) isotherm is postulated within a sorption space close to the sorbent surface to evaluate the sorption free energy. The D-R isotherm, in non-linear form, is generally expressed as follows:

(8.1) q e = exp (- 2 q e q m exp where is the constant related to the adsorption free energy (mol 2 /kJ 2 ), and q e , q m , R, and T are explained above, is the Polanyi potential which can be calculated by using the following equation:

 = RTln (1+1/C e ) ( 8.2)
E (eq. 9) is the mean free energy of adsorption (kJ/mol) per molecule of metal ion when it is transferred to the surface of the sorbent from infinity in the solution.

E = (9) 1 -2

E is in the range of 8-16 kJ/mol and refers to an ion-exchange reaction. Dubinin-Radushkevich isotherm has shown good fitting at higher concentration areas for adsorption of Cu(II), Zn(II), and Pb(II) onto Croatian zeolite clinoptilolite [20].

Harkin-Jura isotherm

The Harkin-Jura (Harkin and Jura, 1944) isotherm describes a multilayer adsorption and it can be explained by existence of a heterogeneous pore distribution. The Harkin-Jura can be expressed as shown in equation 10. Where A and B are the Harkin-Jura constants, q e , and C e have already been mentioned above [21]. The Harkin-Jura isotherm has been used to describe the adsorption of Dyes (Reactive Yellow 2, RY2) onto Cocoa (Theobroma Cacao) Shell [22].

Table 4.2 Overlook of some two-parameters adsorption isotherms models

Adsorption isotherm models " Two parameters"

Isotherms Plot Description Reference Langmuir + = q e = q m K L C e 1 K L C e q e vs C e 1 1
This model assumes a monolayer adsorption onto the surface of the sorbent [11][12][13][14][15] Freundlich

q e = C e K F 1/n

Log qe vs Log Ce

This empirical model considering a mono-layer adsorption of solute by the adsorbent onto heterogeneous surface [16][17][18] Temkin This model assumes the presence of multilayer adsorption with the existence of heterogeneous pore distribution [21,22] The three-parameters adsorption isotherms

Herein, the adsorption isotherm models represented by three parameters are given in Table 4.3 and some of the adsorption isotherms with three parameters were briefly discussed.

Redlich-Peterson isotherm

The isotherm first proposed by Redlich and Peterson (1959) was modified by Jossens and his co-workers (Jossens, 1978) [13,23]. It incorporated the features of the Langmuir and

Freundlich isotherms into a single equation as follows:

where K L is the Redlich-Peterson isotherm constant (L/mg), and n is the Redlich-Peterson isotherm exponent. The q e , q m , and C e have already been mentioned above. The exponent, n, lies between 0 and 1. There are two limiting behaviours: Langmuir form for n = 1 and Henry's law form for n = 0. Adsorption of copper from wastewater by chestnut shell is well described by the Redlich-Peterson isotherm [24].

Sips isotherm

The Sips (Sips, 1948) model an empirical model, which is a hybrid of the Langmuir and the Freundlich isotherms.

The Sips model takes the following form for single solute equilibrium data:

where K L is the Sips isotherm constant (L/mg) and n is the Sips model exponent. The q e , q m , and C e have already been mentioned above [23]. At low sorbate concentrations it is effectively reduced to the Freundlich isotherm and thus it does not follow Henry's law. At high sorbate concentrations, it predicts a monolayer adsorption capacity characteristic of the Langmuir isotherm. Chitosan-glutaraldehyde beads have been used to capture Cu(II) and the adsorption was best explained in this case by the Sips isotherm [25].

Toth isotherm

The Toth isotherm (Toth, 1962) is an empirical equation developed to improve Langmuir isotherm fittings (experimental data), and used to describe heterogeneous adsorption systems, which satisfying both low and high-end boundary of the concentration. Its correlation presupposes an asymmetrical quasi-Gaussian energy distribution, with most of its sites having an adsorption energy lower than the peak (maximum) or mean value. The Toth isotherm can be represented by equation 13:

where K L is the Toth isotherm constant and n is the Toth isotherm exponent [13,23]. The q e , q m , and C e have already been mentioned above. For the value of n = 1, this isotherm is

C e

This model involved properties of both Freundlich and Langmuir isotherm. It may be used to represent adsorption equilibrium over a wide concentration range [23,24] Sips (Kobble-

Corrigan) q m + 1 K L (C e ) = = + 1 K L (C e ) = = + 1 K L (C e ) 1/n = = K L (C e ) 1/n + 1 K L (C e )
q e = = (C e ) 1/n 1 vs q e q m An empirical model, at low ion concentration, this model reduces a Freundlich isotherm and at high ion concentration, it predicts Langmuir isotherm [23,25] Toth q e = q e q m (1+(K L C e ) n ) This model predicts the adsorption on heterogeneous surface. It assumes a quasi-Gaussian energy distribution [23,26] Fritz-Schluender q e = q m + K L C e 1 q m (C e ) n vs q e

C e (C e ) n

This model describes the adsorption on heterogeneous surfaces [27,28] Radke-Prausnitz q e = q m q e = q m 1+K L (C e ) n K L C e vs q e

C e (C e ) n

This model is suitable for weakly adsorbed materials [23,29] Error Analysis

To evaluate the fit of the different isotherm models to the experimental data, the linear coefficient of determination was examined. The linear coefficient (R 2 ) represents the percentage of variability in the dependent variable. It has been explained by the regression line. Apart from R 2 , other error function (chi-square (X 2 ) test, it is basically the sum of the squares of the differences between the experimental data and the data which calculate from models, was also calculated to measure the data. The X 2 can be calculated from equation 16

as follows:

where q e and q e,m are the amount of metal ions adsorbed per gram of composite (mg/g) from experiment and from models, respectively [30a].

Adsorption isotherms: The Case of CS-coated Silica and Amine-Grafted Silica Materials

To investigate the adsorption isotherms at room temperature, a series of Cu(II) solutions at pH 5 in the concentration range between 100 and 1.000 mg/L was prepared, the mass of adsorbent being 0.4%. The contact time was 24 h. The adsorption isotherm of the adsorption of Cu(II) ions by the three types of the CS-coated silica materials (SiO 2 +CS LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine) were compared with that of SiO 2 (NH 2 ).

The experimental equilibrium data for Cu(II) adsorption onto our materials was calculated from eq. 1. There are various adsorption isotherm models that could be used to interpret our experiment data. In our case, the plot of the solute concentration adsorbed on the solid surface (mg/g) as a function of the solute concentration in the solution at equilibrium (mg/L) could be described by several adsorption models. Langmuir, Freundlich, Temkin, and Sips isotherm models were employed for the experimental data. The representations of the experimental data by four model equations result in a linear curve with the correlation coefficient (R 2 ) values as tabulated in Table 4.4. Apart from R 2 , the chi-square (X 2 ) was also used to measure the goodness-of-fit. The X 2 values are represented in Table 4.4.

The Temkin isotherm was used for representing the equilibrium adsorptive behavior between two phases composing the adsorption system considering the chemisorption of Cu(II) as adsorbate onto the adsorbent. The R In the case of the Langmuir isotherm, the plots yield a straighter line in the range between 0.97-0.99. The maximum adsorption capacity was calculated utilizing Langmuir isotherm given by equation 2. The maximum adsorption capacity for Cu(II) adsorbed on the adsorbents based on Langmuir isotherm model was found to be 256.4, 140.8, and 109.9 mg/g for SiO 2 +CS LMW, SiO 2 +CS Yuhuan, and SiO 2 +CS Marine, respectively. In contrast, the maximum adsorption capacity of the SiO 2 (NH 2 ) was found to be much lower (75.8 mg/g). In comparison with literature, the adsorption capacity for Cu(II) uptake by amino-functionalized silica gel was found to be 66.7 mg/g [31]. In the case of amine-grafted activated carbon, the adsorption capacity was found to be 54.6 mg/g at pH 5.8 [32]. The maximum adsorption capacities for Cu(II) adsorbed on the adsorbents were ranked in the following order as shown in Table 4.4:

SiO 2 +CS LMW > SiO 2 +CS Yuhuan > SiO 2 +CS Marine > SiO 2 (NH 2 )
From these findings, it can be deduced that the %DD value of CS influences the adsorption of Cu(II). Fig. 86 shows the comparison of the experimental Cu(II) adsorption curves with theory models of Langmuir, Freundlich, and Sips isotherm plots (plotting between C e versus q e ). When comparing the values of the error functions, it was found that Langmuir model best fits the Cu(II) adsorption onto three types of CS-coated silica materials as well as amine grafted silica material. The models show high R 2 and low X 2 values. Fig. 86a-d confirms that the adsorption of Cu(II) onto CS-coated silica materials as well as amine grafted silica material matches well with Langmuir isotherm.

Adsorption isotherms: The case of CS-coated Amine and/or Carboxyl

Grafted Silica Materials and Carboxymethyl CS-coated Silica

The adsorption experiments of Cu(II) using the chemically modified silica materials (SiO 2 (CO 2 H)+CS LMW and SiO 2 (NH 2 )+CS LMW) as well as the carboxymethyl CScoated silica SiO 2 +CS(CO 2 H) were also investigated at room temperature. To determine the equilibrium isotherm, a series of Cu(II) solutions at pH 5 in the range between 50 and 1.000 mg/L was prepared. The calculated constants of the four isotherm equations (Eq.

) along with R 2 and RMSE values are presented in Table 4.6.

The Temkin isotherm was used for representing the equilibrium adsorptive behavior between two phases composing the adsorption system considering the chemisorptions of the adsorbate (in our case Cu(II)) onto the adsorbent). The R In the case of SiO 2 +CS(CO 2 H), according the R 2 values, the adsorption of Cu(II) was close to prediction in the following decreasing order:

Langmuir > Sips > Freundlich > Temkin isotherms (Table 4.6).

For the Temkin isotherm, the R 2 was found to be 0. Where  is related to the chemisorption rate (mg/g min),  is related to the surface coverage (mg/g min) during any experiment. A slope (1/) and an intercept (1/ ln ) are obtained from a plot of q t versus ln t [38].

External mass transfer

The adsorbate can be transferred from the solution phase to the surface of the adsorbent through several steps. The steps may include external diffusion, pore diffusion, surface diffusion and adsorption on the pore surface. The external mass transfer can be written in a mathematical form as follows:

where K L is the liquid-solid external mass transfer coefficient (m/s), A S is the specific surface area of adsorbent (m 2 /Kg), C t is the liquid phase concentration of sorbate at time (t) in the bulk solution (mM), C S is the concentration of sorbate at sorbent interface (mM). The K L can be estimated using the limit theorem: at t = 0, C approaches to C 0 and C S approaches to zero as where  b is the bulk density of the adsorbent (kg m -3 ), d p is the mean particle diameter (m), and h is the initial sorption rate (mmol/g/min) [39].

Intraparticle diffusion (Vermeulen)

The intraparticle diffusion has been used in the analysis of a mass transfer mechanism of metal adsorption. The Vermeulen's intraparticle diffusion is expressed as where F t 2 is equal to q t / q e (fractional at time t), D e is the effective diffusion coefficient (m 2 /s) and t is time (second), R a (m) is the radius of spherical adsorbent particles. The slope of the plot of ln [1/(1)F t 2 )] versus t gives D e [40].

Intraparticle diffusion (Weber and Morris)

The intraparticle diffusion model based on the theory proposed by Weber and Morris is expressed as

where k int is the intraparticle diffusion rate constant (mg/g min Pseudo-first order ln( (q e -q t )/ q e ) = -k 1 /t ln(q e -q t ) vs t

This model fits to experimental adsorption kinetic results when physical adsorption is dominant [34] Pseudo-second order This model describes chemical adsorption [37] Elovich equation

It assumes that the active sites of the sorbent are heterogenous and therefore exhibit different activation energies for chemisorption [38] SiO 2 +CS Marine > SiO 2 +CS LMW > SiO 2 +CS Yuhuan > SiO 2 (NH 2 ). The Elovich equation which assumes that the active sites of the sorbent are heterogeneous was also used to interpret the experimental data. The R 2 values for the Elovich equation plots were in the range between 0.80-0.97, and lower than those of the pseudosecond-order (see Table 4.9). Therefore, in contrast to the other three models, the Elovich model is not appropriate to describe the data.

Adsorption Kinetics: CS-coated Amine and/or Carboxyl-Grafted Silica

Materials and Carboxymethyl CS-coated Silica

The adsorption kinetics were furthermore investigated for the adsorption of Cu(II) (500 mg/L) onto the two group III CS-coated silica materials SiO 2 (NH 2 )+CS LMW, and SiO 2 (CO 2 H)+CS LMW) as well as the group IV adsorbent SiO 2 +CS(CO 2 H). The amount of adsorbent was constant (0.4%). The adsorption took place at pH 5, and the contact time was in the range between 0 and 1440 min. As adsorption kinetics, pseudo-first-order, pseudo-secondorder, intraparticle diffusion, and Elovich were employed to interpret the experimental data.

The adsorption rate of Cu(II) ions at the solid-solution interface onto three CS-coated silica materials with an equilibrium established between the two phase was studied using the pseudo-first-order which is commonly used to the adsorption of liquid-solid system based on adsorbent capacity) and the pseudo-second-order which derived by assuming that two surface sites could be occupied by one divalent adsorbate ion). Moreover, the plots showed a curvature which was attributed to boundary layer diffusion effect or external mass transfer effect. The rate constants, k int, were 2.03 mg/g min 1/2 , 0.55 mg/g min 1/2 , and 0.60 mg/g min From these observations, it can be deduced that the -NH 2 , -OH, and -CO 2 H groups are involved as binding sites to form complexes with the Cu(II) ion, thus causing the spectral changes.

A6.2 Energy Dispersive Spectroscopy (EDS)

The EDS spectrometer was utilized for elemental analysis of the copper adsorbed on the CS-silica hybrid materials. Five points on the surface of the copper-loaded modified silica materials were analyzed. Fig. 98a-b shows three peaks of sulphur, silica and copper, which were the three major constituents of the composites (SiO 2 +CS LMW and SiO 2 +CS Marine).

The amount of the copper in percentage is shown in Table 4.11. The average copper amount for SiO 2 +CS LMW was 20.52 ± 3.4 %, and for SiO 2 +CS Marine it was 16.53 ± 1.3 %.

The finding indicated that the amount of the copper of both hybrid composites observed from each of the five points, represented as a percentage was close to each other.

From the results, it can be assumed that the copper ions were quite homogeneously adsorbed on the CS-coated silica hybrid composites.

A7 Interaction of Copper(II) with CS

The IR results provide the information that the complexation between CS-coated silica and the metal cation may be through the free amino groups on CS structure. Fig. 99a shows a metal ion bounded with one amine group as a donor [42]. In a report of Domard and coworkers, the Density Functional Theory (DFT) method was used to study the interaction between Cu(II) and CS. The computing showed as most stable form the pending complex (Fig. 99a). This complex consists of only one Cu(II), one amino group and oxygen atoms of the three other ligands involved in the highest occupied molecular orbital (HOMO). The interaction involves the d orbitals of the Cu(II) ion and the p orbitals of the N atoms [42,43].

The study of the interaction between Cu(II) and CS by Desbrières 99a). In contrast, when the pH is above 5.8, the complex {[Cu(-NH 2 )] 2+ , 2OH -, II} ligated by two amino groups is formed (Fig. 99b) [44].

mg/g [70a]

, CS/clay/magnetite composite was found to be 17.2 mg/g [70b], maghnite was found to be 21.78 mg/g [70c], and zeolite was found to be 20.9 -132.2 mg/g [71][72][73][74][75].

However, under similar adsorption conditions (pH 5), significantly lower q max values in the range of 20.9 -97.2 mg/g are reported for modified silica, and q max values in the range of 35 salts onto CS in swollen CS beads [81]. According to the Freundlich isotherms, it was found that the adsorption of Cu(II) from sulphate solution is higher than that from CuCl 2 solution.

The sulphate ion had the ability to enhance the adsorption of copper ion onto chitosan beads. The result of these three independent studies confirm that the Cu(II) adsorption depends on the nature and charge (monovalent vs divalent) of the counter ions in solution. Of course this preliminary investigation needs to be explored more in depth in future studies. Note, that in contrast to the increasing number theoretical and experimental studies on the interaction of Cu(II) with CS on a molecular level, the coordination and bonding mode (supramolecular, hydrogen bridges, …) of the anion has been neglected and deserves to be taken more in consideration.

Part B Adsorption of Nickel(II) on the Composites

After our thorough study on the various parameters affecting the capture Cu(II) salts on CS-coated silica hybrid materials from aqueous solution, we were intrigued to extend our project on other divalent transition metals. Since the three SiO 2 +CS LMW, SiO 2 (CO 2 H)+CS LMW), and SiO 2 +CS(CO 2 H) appeared to be promising adsorbents for the uptake of Cu(II) ions, these three materials were selected to probe their propensities for the uptake of Ni(II)

ions. The batch adsorption of Ni(II) on the silica composites were also carried out at room temperature. Again, first the effect of contact time and of the initial solution pH for Ni(II) ions adsorption were investigated, then the equilibrium isotherms and adsorption kinetics were studied. The Ni(II)-loaded silicas (functionalized and/or hybrid) were also characterized using FT-IR, furthermore EDS was used to investigate the adsorption behaviour of NiSO 4 on these composites.

B1 Effect of the Solution pH

To

B3 Effect of Contact Time

The effect of contact time on the extent of adsorption was also investigated at room temperature. The adsorption parameters (pH 7, adsorbent dose = 0.4%, and initial concentration of Ni(II), 1.000 mg/L) were fixed. The adsorption of Ni(II) ions by three types of composites was studied in the interval between 0 -1.440 min.

Fig. 103

B4 Adsorption Isotherm

In order to get a better understanding on the Ni(II) adsorption process, a study of the

B5 Adsorption Kinetics

Experiments were carried out for Ni(II) different concentrations and contact times (min). Each experiment was repeated twice under identical conditions. The pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich constants values are given in Tables 4. 15-4.16. The kinetic curve of Ni(II) ions reveals that the adsorption was initially rapid. In order to evaluate the kinetic mechanism controlling the adsorption process, pseudo-secondorder kinetics was employed to interpret the experimental data. Fig. 108-110 and Table 4.15

indicate that based on the obtained correlation coefficients (R 2 ), the pseudo-second-order equation was the model that provided the best fit for the experimental kinetic data. The R 2 values for the pseudo-second-order plot were found to be 0.99 for all the three types of modified silica materials suggesting chemical sorption as the rate-limiting step of the adsorption mechanism and excluding involvement of a mass transfer in solution. Similarly, K.A. Krishnan and co-workers reported that the adsorption of Ni(II) onto activated carbon (sugarcane bagasse pith) best fits for the experimental kinetic data with a k 2 value in the range between 1.6457 x10 -3 min -1 to 9.1386 x10 -3 min -1 [82].

For kinetics, the lower case "k 1 and k 2 " are used to study the rate constant and they give definite equilibrium established between the two phases (the adsorption rate of Ni(II) ions at the solid solution interface onto four CS-coated silica materials). Both the pseudo-first and pseudo-second order models could not explain the diffusion mechanism; therefor the kinetic results were interpreted using the intraparticle diffusion equation. The k int is the intraparticle diffusion rate constant (mg/g min

B6.2 Energy Dispersive Spectroscopy

The Ni(II)-loaded modified silica materials were furthermore analyzed by EDS.

B7 Interaction of Nickel(II) with CS

From the FT-IR data, an interaction between the biopolymer-coated silica hybrid composites (CS-coated silica) and Ni(II) ions via both -NH 2 and -OH groups was evidenced.

The structure of the supposed complex is shown in Fig. 115. The CS chelates the nickel ions by electron-dononation from both oxygen and nitrogen atoms [86]. Note that from a molecular point of view, the interaction between divalent metal ions and the amino residues of 4.19 [89]. The concentration of free metal ion [M n+ ], free carboxylate anion [L -] and 1:1 metal-ligand complex [ML + ], were calculated from equations for the total concentration of ligand-containing species, electroneutrality and the dissociation constant of the carboxylic acid. The value of stability constant for a 1:1 complex can be calculated using equation 35.

From the log K values, the stabilities of the metal complexes follow the order Cu 2+ > Ni 2+ , consistent with the Irving-Williams order. The order of log K values for a particular metal ion with the carboxylate anions is the same as the order for the basicities of these anions [89].

Conclusion

In this chapter, the capacity and aptitude of functionalized silica particles of type SiO 2 (NH 2 ) and in particular those of the chitosan-coated composites SiO 2 +CS LMW, SiO 2 +CS Marine, and SiO 2 +CS Yuhuan for the adsorption of Cu(II) from aqueous solution has been probed. The adsorption kinetics were evaluated using pseudo-first-order and pseudosecond-order models; it was found that pseudo-second-order kinetics describe the Cu(II) adsorption in a very satisfying manner. Another important finding is the fact that the Langmuir isotherm model fits well with the adsorption studies, suggesting that the adsorbed Cu(II) ions form a monolayer on the adsorbent surface. Based on the Langmuir isotherms, the maximum adsorption capacities were found to be 256. To determine the equilibrium isotherm, a series of Ni(II) initial concentration solutions ranging between 100 -1.000 mg/L were prepared. The maximum total time of an experiment was 1440 minutes. The maximum adsorption capacity was calculated using both Langmuir isotherm, Freundlich, Temkin, and BET isotherm models

The adsorption kinetic of Ni(II) on the composites were evaluated at both pH 5, and 7.

The concentration of residual Ni(II) in solution at different time intervals in the experiment was analyzed during the experiment. A 1-2 ml of sample solution was taken at 30, 60, 180, 300 and 1440 minutes, respectively. Samples were analysed by AAS.

Calibration curves

To determine the residual concentration of Cu(II) or Ni(II) in the supernatant by AAS using the parameter shown in Table 4. The intraparticle diffusion model (Weber and Morris) was also investigated. The plot for the obtained data showed a straight line plot. Therefore, the adsorption fitted to intraparticle diffusion.

The adsorption kinetics for Cu(II) or Ni(II) adsorption onto the modified silica materials are well described by the pseudo-second-order model, assuming that chemisorption is the rate-controlling mechanism.

There are numerous of advantages of using these novel materials as adsorbents for the treatment of water as follows:

1. The surface of SiO 2 -particles can be easily chemically modified to afford novel hybrid materials.

2. They were highly effective for the capture of both Cu(II) and Ni(II) in aqueous solution at room temperature.

3. There is an improved chelation capacity of the metal ions for modified silica particles coated with LMW chitosan.

4. A colloidal system also avoids problems brought about by the viscosity of CS solution, ensuring easy handling.

5. The adsorbents are of low toxicity and friendly to the environment due to the polymeric CS shell coating on the silica nanoparticles (core), since CS is biodegradable.

6.

As biosorbent, our materials act as a good candidates for an utilization as adsorbents for water treatment with adsorption capacities superior to numerous other literature-know adsorbents.

To conclude, this thesis, which was the first one started in our research group on the elaboration and use of silica-based composite particle for water treatment, has demonstrated that this strategy is promising and competitive in terms of efficiency and costs compared with other adsorbents studied by other laboratories for this purpose. In this work, the recycling and desorption processes of our composites have been completely neglected due to lack of time.

Of course we are aware that the recovery/recycling of an adsorbent is of primordial importance for a potential large-scale use. One possibility for decomplexation of surfacecoordinated metal ions could be electrochemical techniques or by decomplexation using a chelating agent having a stronger affinity to bind the metals ions than CS.

Nonetheless, a lot of things concerning synthesis of the composites, their characterisation and their adsorbption capacities could and should certainly be improved:

In order to augment the number of reactive silanol groups on the surface, our Aerosil (300 ± 30 m 2 /g) is desirable to see whether the absorption capacities can be modulated in function of these parameters; e.g. this study will give us more informaton on the effect of silica particles sizes on the adsorption capacity. This project could also be extended to other inorganic oxides such as Al 2 O 3 or TiO 2 particles.

In this work, the degree of deacetylation DA of the different CS types was inferior to a DA of 80 %. However, the use of completely deacetylated CS should be investigated to increase the available amino contents on the CS framework to raise the number of binding site for metal ions, thus optimizing the adsorption capacity. Also the functionalization of CS to carboxymethyl CS could thus be optimized. Also other chemical modifications of CS using the know-how of our group in organic chemistry could constitute an interesting projet for future work.

Morever, to improve the adsorption properties of the sorbents, the coupling of metal ion adsorption with other purification techniques deserves attention. Indeed, some preliminary studies performed in our laboratory indicated that combining the adsorption propensity of the grafted particles with ultrafiltation gives quite encouraging results concerning the retention of Cu(II) ions.

A lot of complementary techniques have been used to characterize the composition, surface structure, particle sizes and adsorption properties of our materials. However, a collaborative work with other laboratories having other facilities absent in our Institut could provide further information on our composites. For example, 13 C and 29 Si solid-state NMR techniques are more and more employed and could complete the characterization of our composites. Furthermore, the use of X-ray powder diffraction may be an additional tool for the characterisation of our composites.

The present study was limited to the use of Ni(II) and Cu(II) as pollutant. To expand the scale of adsorption by our materials, the adsorption of other metal ions such as Cd(II), Hg(II), Pb(II) and Cr(VI) deserves investigation. An interesting challenge for future studies will also be the exploration of the retention behaviour and selectivity of our materials in the presence of mixtures of different metal ions or couterions. This would represent a next step towards the evaluation and suitableness of our CS-coated silica materials for the removal of pollutants from aqueous solution under real world wastewater conditions.

RESUME DE THESE Fonctionnalisation de silices colloïdales pour l'adsorption de cations métalliques Cu(II) and Ni(II) Elaboration de composites pour le traitement des eaux

Ce doctorat porte sur la fonctionnalisation de silices colloïdales en vue de la rétention de micropolluants métalliques dans des effluents. Les nanoparticules et microparticules ouvrent des potentialités d'application dans de nombreux secteurs industriels (chimie, environnement, pharmacie...). Ainsi, ces travaux de recherche portent sur la synthèse et la caractérisation de matériaux composites submicroniques : il s'agit de silices colloïdales sur lesquelles sont greffés des silanes ou supportés des polysaccharides. Une des applications de ces travaux de recherche porte sur l'adsorption de métaux de transition sur ces composites en solution aqueuse. Dans le cadre de ce doctorat, les caractéristiques des composites sont définies par leur morphologie de surface, par l'étude des groupements fonctionnels présents, par détermination de leurs surfaces spécifiques ainsi qu'en solution aqueuse par détermination de leurs diamètres hydrodynamiques et de leurs potentiels zéta. Dans un premier temps, la fonctionnalisation de la silice a permis le greffage de groupements carboxyliques et amines dont tes taux de greffage obtenus ont été respectivement de 0, 47

Abstract

This study is focused on the preparation of three types of silica-based composites for the capture of Cu(II) and Ni(II) ions.

The first strategy consists in coating chitosan on colloidal fumed silica after acidic treatment yielding the composite SiO 2 +CS. The second strategy can be separated into two routes: the first one involves surface grafting of silica with aminopropyltriethoxysilane to obtaining silica particles covered by amino groups (SiO 2 (NH 2 )). The second one involves in surface condensation of triethoxysilylbutyronitrile, followed by acidic hydrolysis of the surface-bound nitrile groups affording silica particles covered by carboxyl groups (SiO 2 (CO 2 H)). In the last step, chitosan has been grafted on the surface bound NH 2 or -CO 2 H groups yielding the composites SiO 2 (NH 2 )+CS or SiO 2 (CO 2 H)+CS. The third strategy involves in the modified CS surface with -CO 2 H groups, followed by coating onto the non-modified silica nanoparticles to obtain the composite SiO 2 +CS(CO 2 H). The novel hybrid materials were characterized by IR spectroscopy, scanning electron microscopy, atomic force microscopy, and zeta potential measurements. Batch experiments were conducted to study the sorption performance of these composites for Cu(II) and Ni(II) removal from aqueous solution at optimum pH at 298 K. The kinetics were evaluated utilizing pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. The adsorption kinetics followed the mechanism of the pseudo-second-order equation for all types of adsorbents. The adsorption isotherms were evaluated utilizing Langmuir, Freundlich, and Temkin models. The best interpretation for equilibrium data was given by Langmuir isotherm model. This study demonstrates that the adsorption capacities for Cu(II) ion is more efficient for the SiO 2 +CS (256 mg g -1 ) compared to SiO 2 (NH 2 ) (75 mg g -1 ). However, the carboxyl grafted CS-coated silica (SiO 2 +CS(CO 2 H) exhibited an excellent adsorption capacity (333 mg g -1 ). In case of Ni(II), based on Langmuir isotherm the maximum adsorption capacity found to be 182 mg g -1 for SiO 2 +CS, and 210 mg g -1 for SiO 2 (CO 2 H) + CS. Using singlemetal solutions, these adsorbents were found to have an affinity for metal ions in order as Cu(II) > Ni(II). The adsorption of Cu(II) ion by SiO 2 +CS was affected by the nature of the respective anion. Application of these composite materials to remove Cu(II) and Ni(II) from aqueous solution was shown to be more efficient than the adsorption capacities of many sorbents probed by other research groups.

Keywords: functionalized silica, chitosan-coated silica, adsorption isotherms, Cu(II), Ni(II)

Fig. 1 5 )Fig. 2 6 )Fig. 3 7 )Fig. 4 9 )Fig. 5 11 )Fig. 6

 152637495116 Fig.1 Summarization of techniques used in industrial wastewater treatment (5) Fig.2 Polystyrene cation and anion exchanger resins (6) Fig.3 Illustration of the reverse osmosis membrane process (7) Fig.4 Schematic representation of principle of polymer-enhanced filtration technique (9) Fig.5 Schematic representation of the sorptive flotation technique (11) Fig.6The microporous molecular structure of the zeolite ZSM-5(12)

Fig. 42 68 )Fig. 43 69 )Fig. 44 69 )Fig. 45 70 )Fig. 46

 426843694469457046 Fig.42 Treatment of a SiO 2 surface with trichloro(4-(chloromethyl)phenyl)silane and postfunctionalization by N-alkylation. The subsequent Cu-complexation sequence is not shown

Fig. 66 Fig. 70 FTFig. 72 FT-IR of SiO 2 (Fig. 73

 667072273 Fig.66 Schematic modification of CS by grafting with carboxyl groups (101) Fig.67 Schematic preparation of silica-CS hybrid materials (102) Fig.68 Three routes for preparation of CS-SiO 2 microspheres (103) Fig.69 Preparation of CS coated silica particles (108) Fig.70 FT-IR of CS LMW (a), CS Marine (b), CS Yuhuan (c), and CS(CO 2 H) (d), respectively

(SiO 2 Fig. 78 Fig. 81 Fig. 82 Fig. 83 Fig. 84 Fig. 85 Fig. 87 SiO 2 (Fig. 92 Fig. 93 Fig. 94 Fig. 96 FTFig. 97 FTSiO 2 (

 278818283848587292939496972 Fig. 78Schematic preparation of CS-coated Silica hybrid materials:CS-coated silica (modified or unmodified) (a), Carboxymethyl CS-coated silica(b)

Fig. 1 1 . 2 . 1

 1121 Fig. 1 Summarization of techniques used in industrial wastewater treatment

3 -HFig. 2

 32 Fig. 2 Polystyrene cation and anion exchange resins

Fig. 3

 3 Fig. 3 Illustration of the reverse osmosis membrane process

  naturally contaminated groundwater. This NF device was also used to reject divalent cations such as Ca(II) and Mg(II). * 1 psi approximately equals 6,894.757 Pa

  emerging modification is the Micellar-enhanced ultrafiltration (MEUF). Its advantage is to combine the high selectivity of RO and the high flux and the low-energy requirements involved in UF processes. The high removal efficiency can be rationalized by the effective trapping of solutes by the micelles. This separation technique is based on the addition of surfactants to the solution to be filtered. When surfactants are present in aqueous solutions above a certain concentration, i.e., the critical micelle concentration, aggregates are formed that may entrap a solute existing in the solution. The increased hydrodynamic size* of the solutes enables their rejection by polymeric UF membranes. The micellar solution is then passed through a UF membrane having pore sizes small enough to reject the micelles containing the attracted metal ions. The permeate side will consequently contain very low concentrations of unbound metal ions. MEUF has for example been used by De la Rubia et al.to eliminate of Cd(II), Cu(II), Ni(II), Pb(II), and Zn(II) from synthetic waste water. Sodium dodecyl sulfate (SDS) and linear alkylbenzene sulfonate (LAS) were use as anionic surfactant.

Fig. 5

 5 Fig. 5 Schematic representation of the sorptive flotation technique

Fig. 6

 6 Fig. 6 The microporous molecular structure of the zeolite ZSM-5 [71b]

Fig. 7

 7 Fig. 7 Overview of the structuration of this PhD thesis

Fig. 8

 8 Fig. 8 The particle nano-scale

Fig. 9

 9 Fig. 9 Dimensionality of nanomaterial

Fig. 10

 10 Fig. 10 Example of a molecular nanowire, the separation between the two ferrocenyl extremities being about 4 nm [19].

Fig. 12

 12 Fig.12 The transition of a bulk metal via a nanocluster to a molecule

Fig. 13

 13 Fig.13 The size quantization effect of optical property of nanoparticles

Fig. 14

 14 Fig. 14 The tetrahedral coordination skeleton of the Si atom surrounded by 4 oxygen atoms, found for example in amorphous silica (Adaped from A. M.Coclite, 2010[40])

Fig. 15

 15 Fig. 15 Morphologies of colloidal polymer-silica nanocomposites

Fig. 16

 16 Fig. 16 Silica nanoparticles prepared in two different media 1) ethanol and 2) diglyme showing skeletal (a) and hollow structures (b).

  the removal of anions (F -, Cl -, Br -, I -, HSO 4 -), mesoporous silica was functionalized by a covalently bound naphthalene-based receptor using the sol-gel process [57a]. In addition, mesoporous silica nanoparticles have been used in the field of cancer therapy [57b].

Fig. 17

 17 Fig. 17Structures of

  Fig. 17Structures of mesoporous silicas (adapted from F. Hoffmann, 2006)

Fig. 18

 18 Fig.18 The two main types of silica and their corresponding sub-classes

Fig. 19

 19 Fig. 19 Two preparative routes for the synthesis of amorphous silica

Fig. 21

 21 Fig. 21 Different types of silanols and siloxane-bridges on a SiO 2 surface (modified from ref. [72])

  Fig. 22 shows the correlation of the surface charge ( o ) density and the pH determined by potentiometric titration with various concentration of NaNO 3 . Ionic strength is affected to the surface charge of the sol suspended in that solution. In high ionic strength (I = 1 M/I) solution represents the higher surface charge density than low ionic strength (I = 0.002 M/I) [77]. * The ionic strength of a solution is a function of the concentration and of the charge of all ions present in that solution.

Fig. 22

 22 Fig. 22 The surface charge of silica as function of pH in presence of different NaNO 3 (Mol/L) concentrations (source: E. Papirer, 2000)

from 1 .

 1 8 to 2.6 [77a]. The silanol number of Aerosil 200 has also been determined by adsorption of CO molecules and found to be between 1.2 and 2.1 [77a]. The silanol number of Aerosil 200 determined by thermogravimetric analysis (TGA) was found to be 4.4 nm 2 (at 100 o C) and 1.0 nm 2 at higher temperature (550 o C) [78a]. According to Cabot and Foissy, the total surface SiOH density of Aerosil 200 is about 5 nm -2 [33]. A similar silanol number (4.55 SiOH nm -2 ) has been calculated theoretically [78b]. This topic has been treated more in detail in a review article from Mishra et al. [77b].

Fig. 23

 23 Fig. 23 Molecular structure of the octahedral Si(cat) 3 2-anion [82]

Fig. 24

 24 Fig.24 Solubility of amorphous silica at various pH (source: R.K.[START_REF] Iler | The chemistry of silica. Solubility, polymerization, colloidal and surface properties, and biochemistry[END_REF]

Flocculation

  of dispersion silica by polyvinyl alcohol (PVA) depends on type of silica and pH of the solution [87a]. Maximum adsorption of PVA by flocculation occurs at point of zero charge of the oxide. When increasing the pH, the adsorption and flocculation decreases.

Fig. 26 1 . 5

 2615 Fig. 26 Three different types of particles aggregation

Fig. 28

 28 Fig.28 Principle of SEM[94]

Fig. 29

 29 Fig. 29 Characterization of native Aerosil (a) and Activated Aerosil (b) by SEM

Fig. 30

 30 Fig. 30 The principle of AFM technique (Modified from Y. L. Lyubchenko, 2011[95a])

Fig. 32

 32 Fig. 32 The TEM image of aggregated Aerosil 200 particles (original magnification x 40,000; final magnification x 80,000) [87b].

  1.3.).

Fig. 33

 33 Fig. 33 Evolution of the pH of 0.5% colloidal untreated SiO 2 and activated SiO 2

Fig. 34a

 34a Fig. 34a Evolution of the conductivity of 0.5% colloidal SiO 2 over 3 days

Fig. 35

 35 Fig.35 Schematic representation of the zeta potential measurement using LDE

Fig. 36

 36 Fig.36 The zeta potential value of untreated SiO 2 and activated SiO 2 , respectively

  particles emulsions and molecules dispersed or dissolved in a liquid [102]. It measures the Brownian motion corresponding to the size of particles. The intensity of the scattered light fluctuates at a rate that is dependent on the size of the particles (smaller particles move more rapidly). Analysis of these intensity fluctuations yields the velocity of the Brownian motion and hence the particle size using the Stokes-Einstein relationship as Where d(H) is hydrodynamic diameter, D is translational diffusion coefficient, k is Boltzmann's constant, T is absolute temperature, and  is viscosity. The diameter obtained by

  Ru(NCMe) 3 (sulphos)](OSO 2 CF 3 )/SiO 2 and/or (Ru(II)/SiO 2 was obtained by immobilization of [Ru(NCMe) 3 (sulphos)](OSO 2 CF 3 ) and/or (Ru(II)-sulphos) on silica[3,4]. The binding modes of both complexes involve a hydrogen bonding between silanol groups of the silica surface and SO 3 -groups from both the sulphos ligand [ -O 3 S(C 6 H 4 )CH 2 C(CH 2 PPh 2 ) 3 and the CF 3 SO 3 -anion (trifluoromethanesulfonate) as depicted in Fig. 39. A comparative study of the hydrogenation of various heterocycles by either these silica-immobilized Ru(II) complexes or Ru(0) nanoparticles deposited on the same support has been performed.

Fig. 39

 39 Fig. 39 Immobilization of Ru(0) nanoparticles and a Ru(II) catalyst on mesoporous silica

Fig. 40

 40 Fig. 40 Silica-immobilized La(III) as catalysts for organic transformations

Fig. 41

 41 Fig. 41 Surface modification by aluminum alkyl impregnation on SBA-15 silica

Fig. 42

 42 Fig. 42 Treatment of a SiO 2 surface with trichloro(4-(chloromethyl)phenyl)silane and postfunctionalization by N-alkylation. The subsequent Cu-complexation sequence is not shown.

Fig. 43

 43 Fig. 43 Schematic representation of covalent grafting of an alkoxysilane onto a silica surface

Fig. 44

 44 Fig. 44 Synthesis of CPMS-grafted silica gel with sulfasalazine (SSZ)

Fig. 45

 45 Fig. 45 Synthesis of MPS-grafted silica with N-didodecylglutamidemaleimide and octadecyl acrylate

Fig. 46

 46 Fig. 46 Formation of charged and neutral Hg(II) complexes resulting from adsorption of Hg(II) on MPS-functionalized silica at various pH

Fig. 47

 47 Fig.47 Synthesis of MPTMS-functionalized silica (Aerosil A200 V)

Fig. 48

 48 Fig. 48 Surface modification of silica by grafting with APTES

Fig. 49

 49 Fig. 49 Synthesis of amino-modified ordered mesoporous silica

Fig. 50

 50 Fig. 50 Preparation of composites by reaction of APTES-grafted silica with various organic and organometallic reagents

2 (NH 2 )

 22 corresponding to 4.75× 10 -4 mol of amine functions (-NH 2 ) per gram of silica (after a reaction time of 12 h). For comparison, the amino content of APTES-grafted Aerosil 200 was found to be 5.46 x 10 -3 mol per gram of silica after a reaction time of 72 hours using a methanol to water 2 : 1 ratio according the protocol described in ref. 28a. This important discrepancy between our protocol and that of ref. 28a requires undoubtedly a re-examination.

2. 1 . 5 Fig. 52 Fig. 53

 155253 Fig. 52 The surface morphology of grafted silica with amine groups: SiO 2 (NH 2 )

Fig. 54

 54 Fig. 54 The zeta potentials of SiO 2 , and SiO 2 (NH 2 ) in function of the pH

  become larger in aqueous solution. The hydrodynamic size value of the APTES-grafted silica (>1 m) is consistent with a study of P. Punyacharoennon et al. [28]. They found that the APTES-grafted Aerosil 200 forms micron-sized nanoclusters in suspension. Our finding is also consistent with the results of Y. An et al. who noticed an aggregation of SiO 2 -NH 2 nanoparticles at pH 7-8 due to the change of surface charge after surface modification [20].

Fig. 55

 55 Fig. 55 Size distribution in volume of SiO 2 (NH 2 )

Fig. 56

 56 Fig. 56 Two-step surface modification of silica via grafting with cyano groups

Fig. 57

 57 Fig.57 Selected applications of carboxyl-functionalized silica

Fig. 59 Fig. 60

 5960 Fig. 59 Surface morphology of SiO 2 (a), and SiO 2 (CO 2 H) (b), respectively

Fig. 62

 62 Fig. 62 Structural motives of three natural biopolymers: cellulose (a), chitin (b), and chitosan (c)

Fig. 64

 64 Fig. 64 Protonation of fully deacetylated CS in acidic aqueous solution

(

  iii) The third one is the preparation of N-carboxymethylchitosan. It consists of the reacting the free amino groups of CS with glyoxylic acid to produce a soluble aldimine and then reduction of the aldimine product with sodium cyanoborohydride (NaBH 4) . The formation of N-carboxylmethyl CS mechanism is shown in Fig.65[60].

Fig. 65

 65 Fig. 65 Synthesis of carboxymethyl CS (according to Ref. [56-58])

Fig. 67

 67 Fig. 67 Schematic preparation of silica-CS hybrid materials

Fig. 68

 68 Fig. 68 Three routes for preparation of CS-SiO 2 microspheres (according to ref. [65])

  69. Preparative details concerning the synthesis of the six new silica-based composites are described in the To prepare the first family, Activated silica particles were directly coated with CS (CS LMW, CS Yuhuan, and CS Marine) providing the three CS-coated silica materials SiO 2 +CS LMW, SiO 2 +CS Yuhuan, and SiO 2 +CS Marine. The interaction between CS and the surface occurs probably through hydrogen-bonding between silanols groups and the primary amine groups of CS. But partial deprotonation of the acidic Si-OH groups by the basic NH 2 -functions cannot be ruled out. In this case, electrostatic interactions of the type -NH 3 + ••• -O-Si are expected. Note that recently supramolecular structures of CS on the surfaces of fumed silica have been evidenced by mass spectrometry, combined with thermorgravimetric and UV-Vis and IR studies [73]. The second family comprising the SiO 2 (NH 2 )+CS LMW composite was prepared by functionalizing Aerosil 200 particles via covalent bonding using APTES (see Chapter 2). The amine-grafted silica was then coated with CS at room temperature to obtain SiO 2 (NH 2 )+CS LMW. The other variant SiO 2 (CO 2 H)+CS LMW was obtained in a two-step sequence. In the first step, 4-(triethoxysilyl)-butyronitrile (CPTES) was condensed on the silica surface, followed by acidic hydrolysis of the surface bound nitrile groups to obtain the carboxyl groups terminated silica particles. In the second step, CS was deposited on the carboxylgrafted silica core. The interaction between CS and the carboxyl group could involve CS-NH 2 •••HOOC-SiO 2 hydrogen bridges or be electrostatic in nature through -NH 3 + ••• -O 2 Cinteractions. Alternatively, a covalent linking through amide groups by condensation between

Fig. 70 FT

 70 Fig. 70 FT-IR of CS LMW (a), CS Marine (b), CS Yuhuan (c), and CS(CO 2 H) (d), respectively

[ 73 ,Fig. 72 FT

 7372 Fig. 71 FT-IR of SiO 2 +CS LMW (a), SiO 2 +CS Yuhuan (b), and SiO 2 +CS Marine (c)

Fig. 73

 73 Fig. 73 Schematic possibility of interaction between active sites of CS and silanol groups of silica

Fig. 74

 74 Fig. 74 SEM images of unmodified CS (a) compared to carboxymethyl CS (b)

Fig. 75

 75 Fig. 75 SEM images of CS-coated silica materials

Fig. 77

 77 Fig. 77 The zeta potential values of SiO 2 compared with CS-coated silica (SiO 2 +CS LMW, and SiO 2 +CS Marine) (a), and the SiO 2 compared with CS LMW-coated grafted silica SiO 2 materials (SiO 2 (CO 2 H)+CS LMW, SiO 2 (NH 2 )+CS LMW, and with carboxymethyl CS-coated silica SiO 2 +CS(CO 2 H) (b)

  Carboxymethyl-modified CS CS LMW was dispersed in 100 ml isopropyl alcohol and stirred for 30 min at ambient temperature. An amount of 10 mol/L sodium hydroxide (15 ml) was added to the CS solution (in six equal portions) and stirred for 45 min. Then, chloroacetic acid (4 g)/ isopropyl alcohol solution was added to the mixture solution in four equal portions. The solution was stirred for 5 h at 50 o C. The solid product was filtered off, washed with ethanol (80%, v/v) until a filtrate reached a neutral pH. The product was dried at 50 o C to obtained N,O-Carboxymethyl chitosan. The schematic preparation is shown in Fig. 66 [61].

Fig. 78

 78 Fig. 78 Schematic preparation of CS-coated Silica hybrid materials: CS-coated silica (modified or unmodified) (a), Carboxymethyl CS-coated silica (b)

  1 g amount of the composite was suspended in 10 ml of 1% CH 3 COOH solution leading to dissolution of CS from the composite. The amount of carbon of CS of the initial concentration of CH 3 COOH (1%) and the mixed solution between CH 3 COOH and CS adsorbed on silica was determined. The concentration of CS was then calculated. the application of CS-coated silica hybrid materials as well as amine-modified silica material presented in the preceding chapter for water treatment via adsorption of divalent cations from aqueous solution in a batch experiment. These modified silica-based materials were utilized as adsorbents for removal of Cu(II) and Ni(II) from water.

  was coined in 1881 by German physicist Heinrich Kayser and can be defined as the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to a surface [3a]. Another definition describes this term as preferential concentration of one component of a system at an interface where the local (i.e., interface) concentration of one or more components of one or both phases is different from those on the bulk phase [3b]. Adsorption involves two components including adsorbate and adsorbent. The adsorbate is the substance (i.e., metal ion) being adsorbed on the surface of the adsorbent. The adsorbent is a substance (i.e. CScoated silica), on which the adsorption takes place.

  Bulk solution transport: the movement of the material (metal ions) from the bulk liquid to the boundary layer of a fixed film of liquid surrounding the adsorbent, (II) Film diffusion transport: transport, by diffusion, of the metal ions through the stagnant liquid film, (III) Pore transport: the transport of the metal ions to be adsorbed through the pores by a combination of molecular diffusion through the pore and/or by diffusion along the surface of the adsorbent, and(IV) Adsorption: the attachment of metal ions to adsorbent at an available site.

Fig. 79

 79 Fig. 79 Schematic illustration of adsorption steps

Fig. 80

 80 Fig. 80 Shapes of adsorption isotherms in solid-liquid systems

Fig. 81 shows

 81 Fig. 81 shows the effect of pH for the adsorption of Cu(II) on both SiO 2 (NH 2 ) and SiO 2 +CS LMW. The pH does not significantly affect the adsorption of Cu(II) on SiO 2 +CS LMW, while it shows a strong influence for the adsorption of Cu(II) on SiO 2 (NH 2 ).

However, at pH > 6

 6 the precipitation of Cu(II) ions with hydroxide ions to form of Cu(OH) 2 occurs. By changing the pH of the metal ions solution, the functional group of adsorbents such as amino groups also changed. Consequently, the surface of the SiO 2 +CS LMW behaves as a positive charge, reducing the uptake of Cu(II) ion. The adsorption of Cu(II) depends therefore on the solution pH and the binding sites available for adsorption. The optimum pH for the adsorption of Cu(II) onto the modified silica materials was found around pH 5. For this reason, the adsorption isotherms and adsorption kinetics for the adsorption of Cu(II) onto the modified silica materials were studied at pH 5.

Fig. 81

 81 Fig. 81 Effect of the initial pH on the adsorption of Cu(II) ions on amine-grafted silica, and CS LMW-coated silica composites (500 mg/L Cu(II), sorbent = 0.4%)

Fig. 83 Fig. 84

 8384 Fig.83 The effect of initial concentration of Cu(II) ions for adsorption onto SiO 2 +CS(CO 2 H) compared to SiO 2 (NH 2 )

  In the case of the silica materials SiO 2 (NH 2 )+CS LMW and SiO 2 (CO 2 H)+CS LMW as well as for carboxymethyl CS-coated silica SiO 2 +CS(CO 2 H), the adsorption occured within the first 10 min with an adsorption value of about 80 mg/g. Fig.85shows that the content of adsorbed Cu(II) increased only slowly after 10 min. The content of Cu(II) adsorbed onto SiO 2 (CO 2 H)+CS LMW remained stable after 960 min with an adsorbed amount of 103 mg/g, while the adsorption of Cu(II) onto SiO 2 +CS(CO 2 H) was quite stable after 1.080 min with an adsorbed amount of 100 mg/g. In contrast, for the SiO 2 (NH 2 )+CS LMW) after 1.440 min, no further increase was noticed for the adsorption capacity of Cu(II). The adsorbed amount of Cu(II) onto the SiO 2 (NH 2 )+CS LMW) was 79 mg/g. Based on these results, the maximum contact time for the adsorption isotherms study was 1.440 min (24 h).

Fig. 85

 85 Fig. 85 Effect of contact time for the adsorption of 500 mg/L Cu(II) ions onto the SiO 2 (NH 2 )+CS LMW, SiO 2 (CO 2 H)+CS LMW), and SiO 2 +CS(CO 2 H) composites.

  The b T (a Temkin isotherm constant) related to heat of sorption. The heat of adsorption of all molecules in the layer would decrease linearly with coverage.

Fig. 86

 86 Fig. 86 Comparison of experimental adsorption curves of Cu(II) adsorbed on the modified silica materials with Langmuir isotherm plots: SiO 2 +CS LMW (a), SiO 2 +CS Yuhuan (b), SiO 2 +CS Marine (c), and SiO 2 (NH 2 ) (d)

Fig. 87

 87 Fig. 87 Comparison of experimental data of adsorption curve of Cu(II) adsorbed on the hybrid materials with Langmuir isotherm plots: SiO 2 (NH 2 )+CS LMW (a), SiO 2 (CO 2 H)+CS LMW (b), and SiO 2 +CS(CO 2 H)(c)

Fig 88a .Fig. 88a Elovich

 88a88a Fig 88a. For comparision, the maximum adsorption capacities of three types of CS-coated

Fig. 89

 89 Fig. 89 Pseudo-second-order curve of the Cu(II) adsorption on SiO 2 +CS LMW

Fig. 93 Fig. 94 Fig. 95

 939495 Fig. 93 Pseudo-second-order curve of the Cu(II) adsorption onto SiO 2 (NH 2 )+CS LMW

Fig. 96 FT

 96 Fig. 96 FT-IR of Cu loaded SiO 2 +CS LMW (a), and Cu loaded SiO 2 (NH 2(b)

Fig. 97 FT

 97 Fig. 97 FT-IR of Cu loaded SiO 2 (NH 2 )+CS LMW (a), and Cu loaded SiO 2 +CS(CO 2 H) (b)

Fig. 98

 98 Fig. 98 SEM images and corresponding EDS spectra of Cu-loaded SiO 2 +CS LMW (a), and Cu loaded SiO 2 +CS Marine (b), respectively

  et al. determined by both potentiometric study and spectroscopic method confirms that at pH  5.3, Cu(II) ions are complexed with one -NH 2 group of CS under the most stable form {[Cu(-NH 2 )] 2+ , 2OH -, H 2 O, I} (Fig.

( a )

 a Complex I, pH 5.3 (b) Complex II, pH > 5.8

Fig. 99

 99 Fig. 99 Complexation between Cu(II) ions and amino groups of CS

. 5 -

 5 139.3 mg/g are reported for modified CS. In addition, M. Hébrant et al. report that the extraction of copper(II) by colloidal dispersions of porous fumed silicas (FS200 and/or FS390) grafted with 5-phenyl-azo-8-hydroxyquinoline found to be 97% for an initial copper concentration of 5 x 10 -5 M) [70d]. Considering, the content of 5-phenyl-azo-8hydroxyquinoline attached to the silica is limited (0.15 mm/g at the maximum).

NO 3 -

 3 solution. The same finding was also briefly communicated by T. Mitani and co-workers, who investigated the effect of counter ions such as SO 42-and Cl -on the adsorption of Cu(II)

Fig. 100

 100 Fig. 100 Comparision of the Cu(II) adsorption onto SiO 2 +CS LMW from CuSO 4 x 5H 2 O and CuCl 2 x 2H 2 O solutions at pH 5

  study the effect of pH on the effective of adsorption of Ni(II), experiments were conducted in the pH range 3-7, using of adsorbent concentration of 0.4% and a Ni(II) concentration 500 mg/L. Fig. 101 displays the effect of pH on the adsorption of Ni(II) onto 0.4% SiO 2 (CO 2 H)+CS LMW. The adsorption capacity increases with increasing the pH of the solution going from pH 3 to 7, the maximum uptake of Ni(II) occurring for SiO 2 (CO 2 H)+CS LMW at pH 7. A similar result was obtained for adsorption of Ni(II) on SiO 2 +CS LMW. Remind that at pH 7, amino groups binding sites are not protonated; therefore the non-bonding electron doublets are fully available to interact with metal cations (see eq. 31-33). An example for the adsorption of various heavy metal ions, among them Ni(II) and Cu(II), by resacetophenone loaded 3-aminopropyltriethoxysilane grafted on silica gel at pH 6-7 is reported by A. Goswami [55]. At pH > 8, the adsorption capacity decreases due to the precipitation of Ni(II) with hydroxide forming [Ni(OH) 2 ] [82-84a]. At an acidic pH, the surface charge on the particles became positive due to protonation, thus inhibiting the binding of the metal cation due to a deactivation and an electrostatic repulsion with the protonated amino group.

Fig. 101

 101 Fig. 101 Effect of initial solution pH for adsorption of Ni(II) (500 mg/L) on 0.4% SiO 2 +CS LMW and SiO 2 (CO 2 H)+CS LMW

Fig. 102

 102 Fig. 102 Effect of the initial Ni(II) concentration on the adsorption onto 0.4% SiO 2 +CS(CO 2 H) (contact time = 24 h, pH 7)

Fig. 103

 103 Fig. 103 Effect of contact time for adsorption of Ni(II) (1.000 mg/L) onto SiO 2 +CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 (CO 2 H) at pH 7

Fig. 104 Fig. 105

 104105 Fig. 104 Comparison between experimental adsorption data and Langmuir isotherms for Ni(II) onto 0.4% SiO 2 +CS LMW at pH 5 and pH 7

Fig. 107a 7 Fig. 107b

 107a7107b Fig. 107a Maximum Ni(II) adsorption capacities of six types of CS-coated silica materials at pH 5 and pH 7

Fig. 108

 108 Fig. 108 Pseudo-second-order plot of Ni(II) adsorption onto SiO 2 +CS LMW

  Fig. 114 SEM image and corresponding EDS spectra for the nickel loaded CSsilica hybrid particles : Ni(II) loaded SiO 2 (CO 2 H)+CS LMW

  CS has been the object of several studies supported by IR, UV-vis, ESR, and XPS techniques and modeling[55,[87][88]. Therefore, it is reasonable to assume that formation of classical surface-bound "Werner-type" coordination complexes, with the high-spin Nickel d 8 ion ligated in an octahedral environment consisting of chelating NH 2 and H 2 O molecules, occurs also on the surface of our composites.

Fig. 115

 115 Fig. 115 Complexation between Ni(II) ions andCS-coated silica material[86]

However, the electronegativity

  scale developed by Linus Pauling attributes to nickel a value of 1.91 and 1.90 for copper (on a scale running from about 0.7 (an estimate for francium) to 2.20 (for hydrogen) to 3.98 for fluorine). Electronegativity has no units but "Pauling units" are often used when indicating values mapped on to the Pauling scale Therefore, the fixation trend of metal ions on CS-coated silica materials Cu(II) > Ni (II) is not readily explained using solely the electronegativity as argument. However, the wellestablished Irwing-Williams Series (Mn(II) < Fe(II) < Co(II) < Ni(II) < Cu(II) > Zn(II), [47]) supports the higer affinity for the binding of Cu(II) with respect to Ni(II) ions on our novel materials. Note also that the ionic radii for Cu(II) and Ni(II) ions are 0.71 and 0.72 Å, respectively. Of course a more detailed analysis on the different binding abilities of divalent transition metal ions with hard nitrogen and oxygen donors would require a more profound discussion of the crystal field theory (the crystal field of Cu(II) is weaker than that of Ni(II), however octahedral Cu(II) complexes are subjected to the Jahn-Teller effect affording an extra stability), Pearson's Hard and Soft Acids and Bases (HSAB) concept (http://en.wikipedia.org/wiki/HSAB_theory)* and a Frontier orbital analysis of the interaction between the HOMO of the ligand and the LUMO of the metal center. However, these approaches are beyond the scope of this PhD manuscript and are therefore not further developed. *According the HSAB concept, Cu(II) and Ni(II) ions are considered as border line cases between hard and soft lewis acids; H 2 O, carboylate, ROH and RNH 2 as hard Lewis bases.

50 mg/L and 1 .

 1 000 mg/L of Cu(II). Batch adsorption experimental studies were carried out at room temperature with a known initial concentration of Cu(II) ions and an amount of the silica based composites (SiO 2 (NH 2 ), SiO 2 +CS Marine, SiO 2 +CS LMW, and SiO 2 +CS Yuhuan) was added. The pH of the suspended aqueous solutions were adjusted with 0.1 M HCl or 0.1 M NaOH. After attaining equilibrium, the adsorbents were centrifuged and the residual aqueous-phase concentration of metal was analyzed by AAS.To determine the equilibrium isotherm, a series of Cu(II) solutions at pH 5 with initial concentration ranges between 50 -1.000 mg/L were prepared. The maximum total time of an experiment was 1.440 minutes. The maximum adsorption capacity was calculated from Langmuir isotherm and Freundlich isotherm plots. The sorption kinetic studies of Cu(II) at pH 5 on the sorbent were evaluated. The concentration of residual Cu(II) in solution at different time intervals in the experiment was analyzed. During the experiment, 1-2 ml of sample solution was taken between 0 to 1.440 minutes, respectively. Samples were analysed by AAS.The results were plotted as adsorbed Cu(II) versus time (min). Each experiment was conducted twice under identical conditions.Adsorption Procedure of Ni(II)Batch adsorption experiments were conducted using three types of carboxylfunctionalized silica (SiO 2 (CO 2 H), silica coated with CS LMW (SiO 2 +CS LMW), and carboxyl-functionalized silica coated with CS LMW (SiO 2 (CO 2 H)+CS LMW)), as adsorbent for the uptake of Ni(II) at room temperature. The Ni(II) stock solution was prepared in DI water using NiCl 2 .6H 2 O and then added to 0.4% of the suspended adsorbents. The pH of the suspending aqueous solution was adjusted with 0.1 M HCl or 0.1 M NaOH.

Fig. 116 Fig. 117

 116117 Fig. 116 Calibration curve for Cu(II)

pH 8 ,SiO 2 (

 82 the zeta potential values of SiO 2 (NH 2 ) show a positive zeta potential between 41.7 mV to 20.2 mV. In case of SiO 2 (CO 2 H), the zeta potential value of this material changed to a negative zeta potential with the value in the range of -58.8 mV to -61.9 mV (at pH[5][6][7][8][9]. The pH value of colloidal amino-, carboxyl grafted silica are quite stable at the period time study with an average pH value of about 9.6 and 4.5 for SiO 2 (NH 2 ) and SiO 2 (CO 2 H), respectively.Chapter three deals on the chemical modification of CS. The synthesis of a N,Ocarboxymethylchitosan polymer by reaction of CS with acid in isopropyl alcohol via alkylation reaction provides the carboxymethylchitosan material CS(CO 2 H). With the aim to prepare novel adsorbent for metal cation removal, the six new silica-based composites (SiO 2 +CS LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine, SiO 2 (NH 2 )+CS LMW, SiO 2 (CO 2 H)+CS LMW and SiO 2 +CS(CO 2 H)) have been prepared and the preparative details concerning the synthetic procedure are presented. These hybrid materials were characterized using FT-IR spectroscopy, elemental analysis, and BET specific surface analysis. The elemental analysis allowed the calculation of the number of amine functions (-NH 2 ). Consequently, the amine function density of SiO 2 +CS LMW was 4.51×10 -3 mol per gram. For the other hybrid composite, SiO 2 (CO 2 H)+CS LMW, the amine function density was 4.21×10 -3 mol per gram. In the case of SiO 2 (NH 2 )+CS LMW, a somewhat higher nitrogen percentage of 6.05% was obtained consistent with the presence of amino groups both on SiO 2 (NH 2 ) and CS LMW. Consequently, the number of amine functions in CS LMW could not be directly calculated. The S BET value of the silica composites containing CS was measured in function of the CS content. The S BET value drops to less than 10 m².g -1 for the CS-coated silica materials, whereas the neat SiO 2 exhibits a S BET of 1824 m².g -1 . An increasing of CS molecule affect to a reducing of S BET value. This agrees with the report of E.-J. Lee and co-workers (2010). The neat xerogel exhibits a S BET of 620 m².g -1 , whereas the hybrid containing 30% of CS has a S BET value of 150 m².g -1 . The morphology of five types of the hybrid materials (SiO 2 +CS LMW, SiO 2 +CS Marine, SiO 2 (NH 2 )+CS LMW, CO 2 H)+CS LMW and SiO 2 +CS(CO 2 H)) was also investigated by SEM. The surface of the silica particles, before coating with CS, was quite rough and irregular. However, after biopolymer coating, the CS-coated silica hybrid composites (SiO 2 +CS LMW, SiO 2 +CS Marine, SiO 2 (NH 2 )+CS LMW, and SiO 2 (CO 2 H)+CS LMW showed a rigid texture with small gains of the silica nanoparticles distributed homogeneously in the CS matrix as current bun (Fig.1.8, chapter 1) which was due to the high coverage of CS. In contrast, the morphology of the carboxymethyl CS-coated silica (SiO 2 +CS(CO 2 H)) showed a heterogeneous texture of the carboxymethyl CS layer. The pH of the hybrid materials (7.6-diffusion model (Weber and Morris). From these results, the straight line did not pass the origin for all the three adsorbents; intraparticle diffusion can be excluded as only ratecontrolling step. Moreover, the plots showed a curvature, which was attributed to boundary layer diffusion effect or external mass transfer effect. A comparative study of solutions containing both CuSO 4 and Cu(NO 3 ) 2 on the adsorption behavior of Cu(II) by CS-coated silica particles shows that the adsorption of Cu(II) from sulphate solution is higher than that from Cu(NO 3 ) 2 solution in the case of SiO 2 +CS LMW as sorbent. However, the influence of the counterion on the adsorption process needs to be explored more in depth in future studies and should be extended to other counterions.In the case of Ni(II) adsorption onto SiO 2 +CS LMW and SiO 2 (CO 2 H)+CS LMW), the isotherm of Langmuir and Freundlich were both fit for the Ni(II) adsorption studied.However, Langmuir best fitted for the adsorption studied. Based on Langmuir isotherm, the maximum adsorption obtained at pH 7 was found to be 189 mg/g for SiO 2 +CS LMW, and found to be 210 mg/g for SiO 2 (CO 2 H)+CS LMW. The effectiveness for adsorption of Cu(II) or Ni(II) by our materials could be due to the carboxyl (-CO 2 H) or the amine (-NH 2 ) groups on the materials made them possible to offer chelate groups for increasing adsorption capacity. The adsorption kinetics was studied. It was found that the pseudo-second-order was the model that provided the better fit for adsorption kinetics of Ni(II) onto three types of CScoated silica materials (SiO 2 +CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS(CO 2 H)).

  200 particles have been activated by acidic treatment with HCl. The conductivy measurements have indicated the presence of some residual HCl despite washing with DI water. Perhaps a better more careful rinsing followed by a prolonged drying at a temperature above 100°C could eliminate residual HCl and chemisorbed water to optimize the degree of silanization. This study was limited to Aerosil 200 particles with a specific BET surface area of around 200 ± 25 m 2 /g. The comparison with other silica particles having smaller and larger particles sizes and specific surface areas such as Aerosil 150 (150 ± 15 m 2 /g) and Aerosil 300

Table 1

 1 Some maximum values for drinking water in EU(5)

Table 1 . 1

 11 The IR spectrum bands of OH groups on silica surface(45)

	Table 1.2	Elemental percentage of native Aerosil 200	(46)
	Table 1.3	BET specific surface values of native SiO 2 and activated SiO 2	(49)

Table 1 . 4

 14 The hydrodynamic sizes of native SiO 2 and activated SiO 2(54)

	Table 2.1	Some alkoxysilane coupling agents used for modification of silica	(68)
		surface

Table 2 . 2

 22 Elemental analysis of SiO 2 (NH 2 )

Table 3 .

 3

	2	Three types of commercial CS used in this work	(105)
	Table 3.3	Absorbance of CS from IR spectroscopy	(106)
	Table 3.4	Percent content of C and N of CS by Elemental analysis	(106)
	Table 3.5	Elemental analysis of the silica particles	(109)
	Table 3.6	The BET specific surface area (S BET ) of neat silica and biopolymer-	(110)
		coated silica
	Table 3.7	IR spectra of CS and their position assignment	(110)
	Table 3.8	IR analysis of pure CS	(111)
	Table 3.9	IR analysis of the different types of CS-coated silica hybrid materials	(112)
	Table 3.10	Hydrodynamic size of untreated silica, and six hybrid composites	(119)
	Table 4.1	Classification of the modified silica materials	(131-132)
	Table 4.2	Overlook of some two parameters adsorption isotherm models	(142)
	Table 4.3	Some three parameters adsorption isotherms	(145)
	Table 4.4	Isotherm constants	(147)
	Table 4.5	R L values based on Langmuir isotherms	(147)
	Table 4.6	Isotherm constants	(151)
	Table 4.7	R L values based on Langmuir isotherms	(152)
	Table 4.8	Some adsorption kinetics	(157-158)

Table 4 .

 4

9

Pseudo-first-order, pseudo-second-order, Intraparticle diffusion, and

(159)

XII

Elovich constants values

Table 4 .

 4 10 Pseudo-first-order, pseudo-second-order, Intraparticle diffusion, and

	(162)
	Elovich constants values

Table 4 .

 4 11 The EDS value of five-point measurement on sample surface (167)

	Table 4.12	Some maximum adsorption capacities of Cu(II) by various adsorbents	(170-171)
	Table 4.13	Langmuir, Freundlich, and Temkin isotherms for adsorption of Ni(II)	(178)
		onto SiO 2 +CS LMW and SiO 2 (CO 2 H)+CS LMW at pH 5 and pH 7
	Table 4.14	R L values based on Langmuir isotherms only at 1000 and 3000 mg/L	(178)
	Table 4.15	Pseudo-first-order, and pseudo-second-order constant for adsorption of	(183)
		Ni(II)
	Table 4.16	Intraparticle diffusion and Elovich parameters for the Ni(II) adsorption	(183)
	Table 4.17	IR analysis of the CS-coated silica hybrid-metal complex	(186)
	Table 4.18	The percentage of nickel content of three points of the hybrid composite	(187)
	Table 4.19	The log K values for 1: 1 metal-carboxylate complexes	(188)
	Table 4.20	Some maximum adsorption capacities of Ni(II) by various adsorbents	(188-189)
	Table 4.21	Different models for electronegativity values of copper and nickel	(190-191)
	Table 4.22	Operating parameters for metal analysis using	(195)

Table 1

 1 Some maximum values for drinking water in EU

	Metals	Maximum concentration	Unit
	Arsenic	10	g/L
	Cadmium	5	g/L
	Copper	2	mg/L
	Nickel	20	g/L
	Lead	10	g/L
	Mercury (total)	1	g/L

Table 1 .1 The

 1

IR spectrum bands of OH groups on a silica surface

Type of OH group Peak (cm -1 )

	Free hydroxyl groups	3745 -3750
	Vicinal	3650 -3660
	Adjacent pairs of SiOH groups with H-bonded to each other	3540 -3550
	H 2 O adsorbed on the above	3400 -3500

Table 1 . 2

 12 Elemental percentage of native Aerosil 200

	Technique	Elemental Analysis, (Atomic %)
	Si	64.6
	O	34.7
	Cl	0.2
	K	0.2

Table 1 .

 1 3 BET specific surfaces of native SiO 2 and activated SiO 2

	Sample	BET, m 2 /g
	Untreated silica, SiO 2	182 ± 4
	Activated SiO 2	184 ± 4

Behavior and Characterization of Aerosil 200 in Aqueous Solution 1.6.1 pH measurements

Table 1 .4 The hydrodynamic sizes of native SiO 2 and Activated SiO 2

 1

	No	Sample	Z-Ave	Peak 1	Volume percentage Peak 2	Peak 3
	1		305 nm	89.9	8.9	1.3
	2	Aerosil 200 silica	323 nm	100	-	-
	3		356 nm	94	6	-
	Average		328 25 nm
	1		147 nm	100	-	-
	2	Activated silica	133 nm	100	-	-
	Average		140  10 nm

1 h) in water with average hydrodynamic particle sizes of 300 nm

[103]

. The hydrodynamic sizes (in volume percentage) of neat SiO 2 and activated SiO 2 were measured for this work using a Zetasizer instrument. As shown in Table

1

.4, the average hydrodynamic size of neat SiO 2 was 328 ± 25 nm and found to be 140 ± 10 nm for activated SiO 2 Moreover, Table

1

.4 shows that the mean hydrodynamic sizes of the particles after acidic treatment were more homogeneous and smaller than that of untreated silica. This can be

  fumed Aerosil 200 silica (neat silica SiO 2 ) was purchased from EVONIK industries, Japan. All reagents are analytical grade. The water used in all experiments was obtained from Millipore filtration units (18.2 MΩ cm). Fourier Transform IR spectra were recorded using a VERTEX 70 Spectrometer equipped with a DTGS detector (BRUKER). The morphology of the composites was studied by Scanning Electron Microscopy (JSM 5600, JEOL, Germany) and Atomic Force Microscopy (PicoSPM, Molecular Imaging, USA). BET analyses have been performed the by the Welience service at the University of Burgundy.

Conductivity data were measured using a conductivity meter from Mettler-Toledo, Switzerland. Zeta potentials were obtained by means of a Zetasizer NanoZ (Malvern Instruments). The suspensions were prepared by suspending the particles in DI water, and adjusting the pH using 0.1 M HCl or NaOH. After sonication of the samples during 30 minutes, the zeta potentials were determined and pH was measured. The device was calibrated with a standard suspension, whose  value was 686.8 mV, so the deviation is about  10%.

During the experiments, three  measurements were performed, then the average and standard deviation were calculated. Elemental analyses have been carried out by UMR 7565 CNRS Université Henri Poincaré Faculté des Sciences, Boulevard des Aiguillettes Case 79, BP 70279, VANDOEUVRE les NANCY.

Table 2 . 1

 21 Some alkoxysilane coupling agents used for modification of silica surfaces

	Name	Abbreviation	Structure
	3-Chloropropyltrimethoxysilane	CPMS
	3-Mercaptopropyltrimethoxysilane	MPS
	3-Methacryloxylpropyltrimethoxysilane	MPTMS
	Aminopropyltrimethoxysilane	APTMS
	Aminopropyltriethoxysilane	APTES
	Cyanopropyltriethoxysilane	CPTES

Table 2 .2 Elemental analysis of SiO 2 (NH 2 )

 2

	Sample	Elemental analysis (%, w/w)	Kjeldahl method (%, w/w)
	SiO 2 (NH 2 )	C 2.41	H 0.51	N 0.76	N 0.82 ± 0.02
			Elemental Analysis, (Atomic %)*
	SiO 2	Si	O	Cl	K
		64.6	34.7	0.2	0.2

*EDS analysis gave the percentage by number of atoms (wt %) of each of the elements identified

Table 2 . 3

 23 The BET specific surface value of grafted silica with amine groups

	No.	Sample	BET, m 2 /g
	1	SiO 2	182 ± 4
	2	SiO 2 (NH 2 )	131 ± 4

Table 2

 2 .4 shows that the specific surface of SiO 2 (NH 2 ) decreased to just one third compared to that of non-grafted silica. The same trend has been noticed for the mesoporous silica cited in ref.24b and 24c. In the first case, the BET surface area drops from 910 to 336 m 2 /g after silanization with APTES, in the latter case, the BET surface area drops from 672 to 409 m 2 /g after silanization with APTMS. However the values obtained by

the CTAB adsorption technique appear much too low, since the BET surface of Aerosil 200 should be about 200 m 2 /g as shown in Table

1

.3. It therefore seems that the CTAB adsorption technique is not appropriate to give reliable results in the case of Aerosil 200. Perhaps the experimental procedure needs to be modified to afford more realistic values. Note that the surfactant CTAB is also known to adsorb as bi-or multilayer on silica surfaces [41b].

Table 2 . 4

 24 Effective specific surface value of silica and amine-grafted silica materials

	Material	Specific surface by CTAB, m 2 /g
	SiO 2	36.1 ± 2
	SiO 2 (NH 2 )	10.4 ± 1

Table 2 . 5

 25 The elemental percentage of SiO 2 (CO 2 H)

	Sample	C	Elemental analysis (%, w/w) H N
	SiO 2 (CO 2 H)	0.41	0.45	< detection limit

Table 2 . 6

 26 The BET specific surface value of carboxyl-grafted silica particles

	Sample	BET, m 2 /g
	SiO 2	182 ± 4
	SiO 2 (CO 2 H)	118 ± 2

Table 2

 2

.

7

shows that the specific surface of SiO 2 (CO 2 H) is reduced to only one third compared to that of non-grafted silica. It was found that the specific surface value of SiO 2 (CO 2 H) is close to the specific surface of the SiO 2 (NH 2 ) (10.4 ± 1 m 2 /g). However, again the values obtained by this method appear much too low, as already discussed in section 2.1.4.

Table 2 . 7

 27 Effective specific surface of native silica and carboxyl-grafted silica particles

	Material	Specific surface by CTAB, m 2 /g
	SiO 2	36.1 ± 2
	SiO 2 (CO 2 H)	10.0 ± 6

  SiO 2 (CO 2 H) material at 25 o C was performed, and the zeta potential values are presented in Fig. 61. Previously, the SiO 2 has a negative zeta potential of -19.6 mV (pH 5.6) due to the ionization of the surface hydroxyl groups. The results show that the SiO 2 (CO 2 H) displays a negative zeta potential of about -38.6 mV (pH 4.5) superior to that of neat silica. Since the pK a of carboxylic acid is around 4.8, it follows that there are more negative charges on the grafted silica than that of untreated silica [51]. Y. An et al. reported that SiO 2 -CO 2 H exhibits a negative zeta potential (-42.2 mV)

Table 2 . 8

 28 The mean hydrodynamic size of carboxyl grafted silica particle Three types of grafted silica composites were characterized by FT-IR to confirm the presence of the functional groups of amine and/or carboxyl groups after functionalization process. Surface morphology of the materials was characterized using SEM and AFM techniques. Both SEM and AFM images of SiO

	No.	Materials	Average particle size, nm
	1	SiO 2	328 ± 25
	2	SiO 2 (CO 2 H)	172 ± 20
			Conclusion
	The amine-functionalized silica composite SiO 2 (NH 2 ) and carboxyl-functionalized silica
	composite SiO 2 (CO 2 H) were successfully prepared under mild conditions via covalent
	bonding.

2 (NH 2 ) and SiO 2 (CO 2

H) display a more

Table 3 . 1

 31 Structures of some reagents for functionalization of CS to uptake the adsorption of copper or nickel ions, and to enhance the solubility in aqueous solutions

	Reagents	Structure
	Acrylamide
		(1)

  CS, O-carboxymethyl CS, N, N-carboxymethyl CS, and N, O-carboxymethyl CS, shown in

	Fig. 65. Some methods for preparation of carboxymethylchitosan derivatives are described
	below.
	(i)	The first method is the preparation of the O-carboxymethylchitosan sodium
	salt. CS is first activated in alkaline solution (42 wt% NaOH) at 0 o C for 48 h. Then the pH is
	adjusted to 8 with HCl. The solid product is then collected, washed with methanol and dried
	at 40 o C [58].
	(ii)	The second method is the preparation of the N, O-carboxymethylchitosan. CS
	is suspended in a solvent (1:4 (v/v) water/isopropanol) and then alkalization is carried out
	using 50% NaOH at 40-70 o C for 1 h. Chloroacetic acid (ClCH 2 CO 2 H) in isopropanol is then
	added to the reaction mixture. The reaction is stopped by adding 70% ethanol. The pH is
	adjusted to neutrality with acetic acid. The solid product is rinsed with 70-90% ethanol (to
	desalt) and dried at 60

o C. At high alkaline concentrations (NaOH 50%), alkylation with ClCH 2 CO 2 gives both N-and O-alkyl CS derivatives at the C2 (NH 2 groups) and C6 (OH groups)

[59]

.

Table 3 .2 Three types of commercial CS used in this work

 3

	Name		CS LMW	CS Yuhuan	CS Marine
	Company	Aldrich, U.S.A	Yuhuan Ocean Biochemical, China Marine chemical, India
	DD, %		75-85	85	70
	M W , kDa		150	52	390
	Viscosity, mPa.s*		20-300	50-800**	-
	Appearance	White powder	Yellow light powder	White powder
	*mPa s = Millipascal Second.	**The viscosity is determined at 20.0℃ with 1%chitosan in 1% acetic acid.

Table 3 . 3

 33 Absorbance of CS from IR spectroscopy

		Absorbance
	Type of CS	1655, cm -1	3450, cm -1	%DD
	CS Marine	0.09	0.10	73
	CS Yuhuan	0.15	0.17	75
	CS LMW	0.14	0.18	77

Table 3 .4 Percent content of C and N of CS by Elemental analysis

 3

		Element
	Type of CS			%DD
		%C	%N
	CS LMW	40.39	7.31	77

Table 3 . 5

 35 Elemental analysis of the silica particles

	Material	C	Percent (%wt) H	N	N, mmol/g
	SiO 2	< detection limit	0.33	< detection limit	-
	SiO 2 +CS LMW	34.88	5.71	6.31	4.51
	SiO 2 (CO 2 H)+CS LMW	33.49	5.64	5.89	4.21
	SiO 2 (NH 2 )+CS LMW	34.75	5.54	6.05	-

Table 3 . 6

 36 The BET specific surface area (S BET ) of neat silica and biopolymer-coated silica

	S BET , m 2 /g	SiO 2 SiO 2 182 ± 4	SiO 2 +CS LMW 5 ± 1	SiO 2 +CS Marine 1.6 ± 1	SiO 2 (CO 2 H)+CS LMW 5 ± 1

Table 3 .

 3 7 IR spectra of CS and their position assignment

	Wave number, cm -1	Position assignment
	3450, 3500	OH group (at C3 and C6)
	3360	broad band derived from OH and NH groups overlay each other
	2920, 2880	CH 2 groups related to the symmetric and asymmetric bonds in a saccharide ring
	1660	oscillation of C=O in amino groups
	1630	protonated amino groups
	1590, 1560	NH 2 amino groups and oscillation of NH group in amide group, respectively
	1420	oscillations of OH and CH groups
	1380	-CH bond, C-CH 3 deformations in amide groups
	1150, 1040	-C-O-C in glycosidic bonds
	850, 840	CH 3 COH groups

Table 3 .

 3

	Position assignment		Frequency, cm -1
		CS LMW	CS Marine	CS Yuhuan
	C-OH stretching vibration	1078	1022	1024
	N-H bending for amine II	1595	1560	1598
	C=O stretching vibration for amide I	1658	1650	1664
	C-H stretching vibration	2896	2871	2873
	O-H, and N-H stretching vibration	3450	3450	3450

8 IR analysis of pure CS

Table 3 .

 3 9 IR analysis of the different types of CS-coated silica hybrid materials

	Frequency, cm -1
	Functional groups

SiO 2 +CS LMW SiO 2 (CO 2 H) +CS LMW SiO 2 (NH 2 ) +CS LMW SiO 2 +CS(CO 2 H)

	Siloxane (from silica)	1101	1081	1107	1109
	-COO -symmetric vibration (carboxylic acid)	-	-	-	1440
	-COO -asymmetric vibration (carboxylic acid)	-	-	-	1650
	Carbonyl (residual amide in CS)	1656	1658	1679	-
	Alkyl stretching vibration	2879	2877	2918	2933
	-OH (silanols from silica or hydroxyl groups onto CS)	3448	3448	3485	3471

The dried composites SiO 2 +CS LMW, SiO 2 +CS Marine, SiO 2 +CS Yuhuan, SiO 2 (CO 2 H)+CS LMW, SiO 2 (NH 2 )+CS LMW, and SiO 2 +CS(CO 2 H) were

  ultrasonicated for 30 min before measuring their size. The hydrodynamic size of the six hybrid composites is given in Table.3.10. The hydrodynamic size of the six hybrid composites was higher than 1 m.

Table 3 .

 3 10 Hydrodynamic size of untreated silica, and six hybrid composites

	Materials	Average hydrodynamic size
	SiO 2	328 ± 25 nm
	SiO 2 +CS LMW	> 1 µm
	SiO 2 +CS Marine	> 1 µm
	SiO 2 +CS Yuhuan	> 1 µm
	SiO 2 (CO 2 H)+CS LMW	> 1 µm
	SiO 2 (NH 2 )+CS LMW	> 1 µm
	SiO 2 +CS(CO 2 H)	> 1 µm

2 +CS LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine, SiO 2 (NH 2 )+CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS(CO 2 H) were

	tested as
	adsorbent for Cu(II), and Ni(II) ions removal form aqueous solution at optimum pH
	describing in Chapter 4.

The hybrids composites of SiO

+CS LMW, SiO 2 +CS Marine, and

  SiO 2 (NH 2 )), the CS-coated activated silica (SiO 2

	SiO 2 +CS Yuhuan), the CS-coated amine-grafted silica (SiO 2 (NH 2 )+CS LMW), CS-coated
	carboxyl grafted silica (SiO 2 (CO 2 H)+CS LMW), and the carboxymethyl CS-coated silica
	(SiO 2 +CS(CO 2 H)) were tested at room temperature as adsorbent for the capture of Cu(II)
	ions from aqueous solution. The equilibrium isotherms, adsorption kinetics, and adsorption
	thermodynamics were investigated. The Cu(II)-loaded adsorbents were characterized using
	FT-IR spectroscopy and Energy Dispersive Spectroscopy (EDS), respectively.

Note:

In this present work, the first adsorbent tested as adsorbent for the adsorption of Cu(II) ion was the hybrid silica material "SiO 2 +CS LMW" (the silica core was coated by a CS shell. To probe whether the adsorption capacity could be further improved, various other types of materials were also investigated. The adsorbents used in this study can be classified into four groups as shown in Table

4

.1.

Table 4 . 1

 41 Classification of the modified silica materials

	Group	Material
	(I)

SiO 2 (NH 2 )

		The first group (I): the silica
		surface was chemically
	Amine content per gram of silica
	0.47 mmol/g	modified by grafting with
		amine-and/or carboxyl group.
	SiO 2 (CO 2 H)
	Carbon content on SiO2(CO2H) =
	0.085 mmol/g

Table 4 . 1

 41 Classification of the modified silica materials

	Group	Material
			For the second group (II): the
		SiO 2 +CS LMW,	silica surface was coated by
	(II)	SiO 2 +CS Yuhuan, and	different types of CS (CS LMW
		SiO 2 +CS Marine	or CS Yuhuan, and/or CS
			Marine).
		Nitrogen content on SiO2+CS =
		4.29 mmol/g
		SiO 2 (NH 2 )+CS LMW
			Group (III): the silica surface was
			chemically modified by grafting
			with amine groups and/or
	(III)		carboxyl groups on its surface.
			Then it was coated by CS LMW.
		SiO 2 (CO 2 H)+CS LMW
			Group (IV): the CS was first
			chemically modified by
	(IV)	SiO 2 +CS(CO 2 H)	functionalization with carboxymethyl groups. Then the
			carboxymethyl CS was coated on
			the silica surface.

Table 4 . 3

 43 Some three-parameters adsorption isotherms

			Adsorption isotherm models "Three parameters"
		Isotherm			Plot	Description	Reference
	Redlich-Peterson	q e = =	+ q mK L C e 1 K L (C e ) n	q e	vs	(C e ) n

2 +CS LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine, and SiO 2 (NH 2 ) was

  in the range between 0.90 and 0.95. The B T is the Temkin constant, which is related to the heat of adsorption of all the molecules in the layer. It decreases linearly with coverage due to adsorbent-adsorbate interactions. Morever, the adsorption is characterized by a uniform distribution of the binding energies. According to the results of our experiment, the B T values were 55.37 J/mol, 46.52 J/mol, 31.99 J/mol, and 14.67 J/mol for SiO

2

of the Temkin isotherm for the SiO

2 +CS LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine, and SiO 2 (NH 2 ), respectively

  and these results suggest a chemical adsorption.

	One of the Freundlich constants is K F indicating the relative sorption capacity of the
	system, and the values of K F were found to be 0.717, 0.914, 1.829, and 0.105 mg/g for
	SiO 2 +CS

LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine, and SiO 2 (NH 2

), respectively. The other Freundlich constant n values for the adsorption of Cu(II) indicated the favorability of the adsorption. They were found to be 1.747, 1.545, 1.407, and 1.263 for SiO

2 +CS Marine, SiO 2 +CS Yuhuan, SiO 2 +CS LMW, and SiO 2 (NH 2 ), respectively

  . These n values were > than 1, indicating that the adsorption of Cu(II) onto the three types of CS-coated silica materials as well as the amine grafted silica material was favorable at high concentration but less at lower concentration [30b].

	Table 4.4 Isotherm constants
	Isotherm models	Materials	SiO 2 +CS LMW SiO 2 +CS Yuhuan	SiO 2 +CS Marine	SiO 2 (NH 2 )
		K T (x10 -2 , L/mg)	0.74	8.13	2.37	1.01
	Temkin	B T	55.37	46.52	31.99	14.67
		R 2	0.95	0.92	0.91	0.90
		X 2	6.00	6.80	8.15	6.94
		K F (mg/g)	0.717	0.914	1.829	0.105
		n	1.407	1.545	1.747	1.263
	Freundlich	R 2	0.98	0.97	0.98	0.99
		X 2	2.88	3.12	3.37	1.49
		K L ( L/g )	1.68	0.53	0.30	1.79
		q m (mg/g)	256.4	140.8	109.9	75.8
	Langmuir	R 2	0.99	0.97	0.99	0.99
		X 2	2.73	2.30	2.64	1.36
		K S	1.45	2.28	0.43	3.58
	Sips	n R 2	1.093 0.99	0.863 0.95	0.929 098	0.890 0.97
		X 2	3.05	2.35	3.37	1.49

Table 4 .

 4 5 R L values based on Langmuir isotherms

[Cu(II)] Initial , mg/L SiO

2 +CS LMW SiO 2 +CS Yuhuan SiO 2 +CS Marine SiO 2 (NH 2 )

	1000	0.373	0.654	0.769	0.358

SiO 2 (NH 2 )+CS LMW, and SiO 2 (CO 2 H)+CS LMW was found between

  2 of the Temkin isotherm for the adsorption of Cu(II) onto the 0.93-0.98 (see Table4.6). The B T values were 36.27, and 41.93 for the

SiO 2 (NH 2 )+CS LMW, and SiO 2 (CO 2 H)+CS LMW, respectively

  . These parameters of heat of adsorption indicate again chemisorption.

	The Freundlich parameter "n" indicates the favourableness of the adsorption. The two
	adsorbents SiO 2

(NH 2 )+CS LMW and SiO 2 (CO 2 H)+CS LMW showed

  n values > than 1 as shown in Table4.6, indicating that the adsorption intensity is favorable at high concentration.

	Fig 87(a-b) shows a comparison of the experimental data of the Cu(II) adsorption
	curve on the hybrid materials with theory models of Langmuir isotherm plots (plotting
	between C e versus q e ). When comparing the experimental data with theoretical data, it was
	found that the adsorption of Cu(II) onto SiO

2 (NH 2 )+CS LMW and SiO 2 (CO 2 H)+CS LMW is

  close to the prediction of the adsorption by the Langmuir isotherm (Fig 4.8a-b). Based on the Langmuir isotherm, the maximum adsorption capacities for adsorbed Cu(II) was found to be 270.3 mg/g, and 232.6 mg/g for SiO

2 (NH 2 )+CS LMW and SiO 2 (CO 2 H)+CS LMW, respectively

  (seeTable 4.6). The maximum adsorption capacities for copper adsorbed can be ranked in the following order:

SiO 2 (NH 2 )+CS LMW > SiO 2 (CO 2 H)+CS LMW.

  The group II adsorbents are better adsorbents than the group I adsorbents. It is assumed that Cu(II) was adsorbed onto the CS-coated modified silica materials by monolayer adsorption.

	The Langmuir isotherm can be expressed in terms of a dimensionless constant
	separation factor or equilibrium parameter (R L ). The values of R L for initial Cu(II)
	concentration (1.000 mg/L) were shown in Table 4.7. The R L values of the SiO 2 (NH 2 )+CS
	LMW was 0.441. For SiO 2 (CO 2 H)+CS LMW, the value of R L was 0.613. This indicated
	that adsorption of Cu(II) was favorable at high initial concentration of Cu(II).
	In the case of the Sips isotherm, the n values are 1.134, 0.873 and 1.156 for
	SiO 2 (

NH 2 )+CS LMW, SiO 2 (CO 2 H)+CS LMW and SiO 2 +CS(CO 2 H), respectively

  . The n values as shown above are close to one. This could suggest a homogeneous adsorption.According to R 2 and X 2 values (Table4.6), the adsorption of Cu(II) onto

SiO 2 (NH 2 )+CS LMW and SiO 2 (CO 2 H)+CS LMW matches

  well with Langmuir isotherms as can be seen in Fig.87a-b.

	Table 4.6 Isotherm constants
	Isotherm models	Materials	SiO 2 (

NH 2 )+CS LMW SiO 2 (CO 2 H)+CS LMW SiO 2 +CS(CO 2 H)

		K T (x10 -2 , L/mg)	19.4	2.84	2.68
	Temkin	B T R 2	36.27 0.93	41.93 0.98	30.51 0.90
		X 2	6.29	13.69	11.01
		K F (mg/g)	0.884	1.548	0.521
		n	1.469	1.544	1.341
	Freundlich	R 2	0.95	0.88	0.96
		X 2	5.99	6.80	7.92
		K L (L/g)	1.27	0.63	2.08
	Langmuir	q m (mg/g) R 2	270.3 0.99	232.6 0.98	333.3 0.99
		X 2	5.62	5.83	1.85
		K S	0.71	2.88	0.99
		n	1.134	0.873	1.156
	Sips	R 2	0.98	0.99	0.98
		X 2	9.99	25.83	3.03

Table 4 .

 4 7 R L values based on Langmuir isotherms [Cu(II)] Initial , mg/L SiO

2 (NH 2 )+CS LMW SiO 2 (CO 2 H)+CS LMW SiO 2 +CS(CO 2 H)

	1000	0.441	0.613	0.325

2 (NH 2 )+CS LMW and SiO 2 (CO 2 H)+CS LMW). The q m value of SiO 2 +CS(CO 2 H) was higher than that of SiO 2 +CS LMW.

  90. The B T was 30.51 for The n values suggest a favorableness for the adsorption of Cu(II) onto the carboxymethyl CS-coated silica materials at high adsorbate concentration.According to R 2 and X 2 , the adsorption of Cu(II) onto SiO 2 +CS(CO 2 H) matched well with Langmuir isotherms (see Table4.4). The maximum adsorption capacities for Cu(II) adsorbed on the adsorbents was found to be 333.3 mg/g. This let assume that Cu(II) was This could be due to both the functional groups attached on modified CS (-CO 2 H) and residual -NH 2 acting as active sites to capture Cu(II) ions.In the case of Sips isotherm, the n values of all types of our materials are close to the unity as shown in Tables 4.4 and 4.6. Therefore, homogeneous adsorption occurred for the adsorption of Cu(II) onto seven types of materials (SiO 2

	SiO 2 +CS(CO 2 H) in accordance with a chemisorption. However, the model was not suitable
	to describe the data compared to the other three models, since R 2 is low and RMSE quite high
	(see table 4.6).
	In case of the Freundlich isotherm, according to the R 2 of 0.96, the value of K F was
	calculated and found to be 0.521 mg/g for SiO 2 +CS(CO 2 H). The other Freundlich constant
	(n) value for the adsorption of Cu(II) indicated the favorableness for adsorption. It was found
	to be 1.341 for SiO 2 +CS(CO 2 H). adsorbed onto the modified CS-coated silica material (SiO 2 +CS(CO 2 H)) by monolayer
	adsorption. From the literature, the adsorption of Cu(II) was found to be 318 mg/g for
	carboxyl-grafted CS derivative at pH 6 [35]. The R L values of the SiO 2 +CS(CO 2 H) was in
	the range between 0.325-0.906. This indicates that the adsorption of Cu(II) is favorable at
	high initial concentration of Cu(II) solution, similar to CS-coated grafted silica materials
	(SiO 2 +CS) and non-modified CS-coated grafted silica materials (SiO

+CS LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine, SiO 2 (NH 2 ), SiO 2 (NH 2 )+CS LMW, SiO 2 (CO 2 H)+CS LMW and SiO 2 +CS(CO 2 H)).

	The results reveal that the adsorption equilibrium data best fit with the Langmuir
	isotherm. Therefore, the maximum adsorption capacity of Cu(II) onto our novel materials,
	obtained from the adsorption isotherm study (based on Langmuir isotherm) is represented in

Table 4 . 8

 48 1/2 ), t is the time (min), and C is a intraparticle diffusion constant (or the intercept of the line, mg/g). It is directly proportional to the boundary layer thickness. A value of C close to zero indicates that diffusion is the only controlling step of the adsorption process. The k int can be determined from the straight line plots of q t against t 1/2 . According to equation(24), if the plots give a straight line and pass through the origin, then the adsorption is controlled solely by the intraparticle diffusion. In contrast, if the plots show multi-linear plots, such plots indicate that two or more steps take place. The first portion is the external surface adsorption or boundary layer diffusion of solute molecule (ions). The second portion is the gradual adsorption stage, where intraparticle diffusion is rate-controlling. The final portion is the equilibrium adsorption stage where intraparticle diffusion starts to slow down due to low adsorbate concentrations[12, 38]. Some adsorption kinetics are shown in Table4.8 Some adsorption kinetics

		Adsorption kinetics
	Kinetic	Plot	Description	Reference

Table 4 . 9

 49 Pseudo-first-order, pseudo-second-order, Intraparticle diffusion, and Elovich constants values

	Kinetic isotherms	SiO 2 +CS LMW SiO 2 +CS Yuhuan	SiO 2 +CS Marine	SiO 2 (NH 2 )
	Pseudo-first-order	k 1 (x 10 -3 min) q e (mg/g) R 2	2. 30 76.40 0.97	2.07 24.62 0.96	3.22 54.60 0.98	2.30 32.24 0.97
	Pseudo-second-order	k 2 (x10 -3 g/mg/ min) q e (mg/g) R 2	0.04 98.04 0.97	0.29 37.74 0.97	0.06 70.42 0.99	0.01 99.01 0.97
	Intraparticle diffusion	k int (mg/g min 1/2 ) C (mg/g) R 2	2.42 1.97 0.96	0.63 9.56 0.99	1.73 8.48 0.89	1.24 5.88 0.80
		 (g./mg min)	0.06	0.24	0.07	0.11
	Elovich	R 2	0.94	0.80	0.97	0.91

Table 4 .

 4  show the adsorption kinetics study for copper adsorption onto these group III and IV adsorbents. It was found that both pseudo-first-order and pseudo-second-order models were the model that provided a good fit for adsorption kinetics of Cu(II) onto SiO

2 (NH 2 )+CS LMW and

	SiO 2 (CO 2 H)+CS LMW). When comparing the k 2 values of the group III (SiO 2 (CO 2 H)+CS
	LMW) adsorbent with the group I material (SiO 2 (NH 2 ), it can be seen that Cu(II) is adsorbed
	more rapidly onto the modified group III silica material than onto group I. This is probably
	due to the fact that SiO 2 (CO 2 H)+CS LMW own two functional groups (-NH 2 and -CO 2 H) in
	the CS shell, whereas SiO 2 (NH 2 ) bears only a -NH 2 monolayer on its surface.
	In case of SiO

2 (NH 2 )+CS LMW, it

  was found that both the pseudo-first-order and pseudo-second-order were fit to describe the adsorption mechanism with the correlation coefficient (R 2  0.98). However, the pseudo-first-order was better fit with the R 2 value of 0.99 than the pseudo-second-order model. According to the pseudo-first-order model, it could be assumed that one adsorbate species (Cu(II) ion) reacts with one active site (-NH 2 ) on the surface of SiO 2 (NH 2 )+CS LMW. This result was similarly to the adsorption of Cu(II) onto sodium hydroxide modified waste mycelium of A. awamori where the adsorption mechanism followed the pseudo-first-order odel [41a].In order to gain insight into the mechanisms and rate controlling steps affecting the kinetics of adsorption, the kinetic experimental results were analyzed by the intraparticle diffusion model (Weber and Morris) to elucidate the diffusion mechanism. The k int is the intraparticle diffusion rate constant, mg/g min 1/2 .

Table 4 .

 4 10 Pseudo-first-order, pseudo-second-order, Intraparticle diffusion, and Elovich

	constants values
	Kinetic isotherms	SiO 2 (

NH 2 )+CS LMW SiO 2 (CO 2 H)+CS LMW SiO 2 +CS(CO 2 H)

	Pseudo-first-order	k 1 (x 10 -3 min -1 ) q e (mg/g) R 2	2.07 65.01 0.99	2.30 21.16 0.97	0.46 23.49 0.82
	Pseudo-second-order	k 2 (x 10 -3 g/mg/ min) q e (mg/g) R 2	0.06 85.47 0.98	0.59 104.17 0.99	0.44 99.01 0.96
	Intraparticle diffusion	k int (mg/g min 1/2 ) C (mg/g) R 2	2.03 7.09 0.98	0.55 76.62 0.89	0.60 81.35 0.95
		 (g./mg min)	0.47	18.52	19.95
	Elovich	 (g./mg min)	0.07	0.24	0.28
		R 2	0.95	0.83	0.78

2 (NH 2 )+CS LMW, SiO 2 (CO 2 H)+CS LMW, and

  1/2 , for SiO

	SiO 2 +CS(CO 2 H), respectively. In addition, the k int value for Cu(II) adsorption onto
	SiO 2 (CO 2 H)+CS LMW and SiO 2 +CS(CO 2 H) is close to the k int value (0.2491 mg/g min 1/2 )
	for the adsorption of Cu(II) on CS-bentonite material [41b]. Moreover, the C value given in

Table 4 .

 4 10 is showing that SiO 2 +CS(CO 2 H) has a higher C value (C = 81.35 mg/g) than

Table 4 .

 4 11 The EDS value of five-point measurement on sample surface

	Materials	1	2	3	%Atomic 4	5	Average
	SiO 2 +CS LMW	20.1	18.2	21.8	18.9	17.6	20.5±3.4
	SiO 2 +CS Marine	16.7	16.1	14.6	17.1	18.2	16.5±1.3

Table 4 .

 4 12 Some maximum adsorption capacities of Cu(II) by various adsorbents.The maximum adsorption capacity was obtained in the case of aChitosan-coated sand with a value of 228 mg/g.[14]. The adsorption of Cu(II) on CS-coated silica (present work), 109.9 -333.3 mg/g, was higher than the adsorption of Cu(II) on a CS-coated alumina hybrid with the work reporting by F. Kopecky and co-workers who found that the maximum adsorption capacities for Cu(II) on CS suspended in aqueous solution were 190 mg/g for CuSO 4, 170 mg/g for Cu(ClO 4 ) 2 and 140 mg/g for Cu(NO 3 ) 2[80d]. When SO 4 2-anions are present in the solution, the adsorption of Cu(II) onto SiO 2 +CS LMW was higher than that in

	Materials	Cu(II))	pH	Reference
	Cinnamomum camphora (Camphor tree)	17.9	4	[50a]
	Agricultural waste (Sugar beet pulp)	28.5	4	[50b]
	Aspergillus niger	28.7	5	[50c]
	Activated carbon (Tunisian date stones)	31.25	5	[51a]
	Sawdust (Meranti wood)	37.17	6.6	[51b]
	Sodium hydroxide treated Orange peel	50.3	5.3	[52a]
	Coffee wastes	70	5	[52b]
	Fly ash	69.9	6	[53]
	Sissoo sawdust (Dalbergia)	263.2	6	[53]
	Activated carbon number E4 34011	125.0	6	[53]
	Activated carbon (rice husk)	112.43	-	[54]
	Natural sorbent	187	6.5	[20]
	Resacetophenone loaded 3-aminopropyltriethoxysilane-silica gel	11.80	6.0-7.5	[55a]
	N-[3-(trimethoxysilyl)-ethylenediamine grafted silica	16.59	5.5-6	[55b]
	Cyanex 272 [bis(2,4,4-trimethylpentyl)phosphinic acid] immobilized silica	20.9	5.3	[56]
	4-amino-2-mercaptopyrimidine immobilized silica gel	28.41	5-6	[57]
	Modified SBA-15 mesoporous silica with 3-aminopropyl-triethoxysilane or with salicylaldehyde	44	4.8	[58]
	Amine-functionalized silica SBA-15	52.7	-	[59]
	Aminopropyl functionalized silica gels	97.2	5	[60]
	Amino-functionalized Fe 3 O 4 nanopartiles	25.77	6	[61]
	Melamine-based dendrimers amine-functionalized SBA-15 silica	126.2	4	[62]
	CS-GLA	8.7	6	[63a]
	Crosslinked CS with epichlorohydrin	35.5	6	[63b]
	Modified magnetic CS	35.5	5	[63c]
	CS beads	64.6	4.5	[25]

2 +CS LMW, SiO 2 (CO 2

  shows the influence of the contact time for the Ni(II) adsorption on SiO

H)+CS LMW as well as on SiO 2 (CO 2 H). It is evident that adsorption of Ni(II) ions rapidly occurs onto both SiO 2 +CS LMW (83.7 mg/g) and SiO

2 (CO 2 H)+CS LMW

	This result is in accordance with the report of K.A. Krishnan and co-workers where the
	adsorption of Ni(II) onto activated carbon (sugarcane bagasse pith) attains equilibrium at
	around 240 min [82]. It was found that the adsorption capacities of Ni(II) onto SiO 2 +CS
	LMW and SiO 2 (CO 2 H)+CS LMW materials are quite comparable. However, the content of
	nickel adsorbed onto SiO 2 +CS LMW and SiO 2 (CO 2 H)+CS LMW was higher than for
	SiO 2 (CO 2 H). The adsorption capacity is strongly related to the binding sites, composed of
	functional groups such as amino-, hydroxyl-and carboxyl groups cooperating with each other

(87.

0 mg/g) as well as on SiO 2 (CO 2 H) (74.5 mg/g) during the first 30 min, then gradually approaches a maximum value and finally attains equilibrium at around 180 min.

providing an appropriate conformation for chelation of the Ni(II) ion.

2 +CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS(CO 2 H), respectively

  . According to the linear regression (R 2 ), the plots yield a straighter line in the range between 0.94 -0.99. Moreover, the chi-square (X 2 ) values confirm the Langmuir isotherms fitted better to the experimental data than those of the Freundlich isotherms as shown in Table4.13. The maximum adsorption capacity was calculated utilizing the

	Langmuir isotherm given in equation 2. The maximum adsorption capacity for Ni(II), based
	on the Langmuir isotherm model at pH 5, is found to be 104 mg/g, 166 mg/g, and 227 mg/g
	for SiO 2

adsorption equilibrium was undertaken using SiO 2 +CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS(CO 2 H) as adsorbent, at pH 5 and pH 7 and choosing as 1.440 min as optimum contact time. Four isotherm models (Langmuir, Freundlich, Temkin, and BET) were used in the present study to describe the adsorption data.

It was found that the BET model was not suitable to describe correctly the adsorption of Ni(II) by our materials because all linear regression were lower than 0.8. However, the Langmuir, Freundlich, and Temkin isotherm models provided good fits (see Table

4

.16). Fig.

104

-106 show the isotherm plots between Langmuir isotherms and experimental data for adsorption of Ni(II) onto SiO

+CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS(CO 2

  SiO 2 +CS LMW, 210 mg/g for SiO 2 (CO 2 H)+CS LMW, and 263 mg/g for SiO 2 +CS(CO 2 H). dimensionless separation factor R L values for the adsorption of Ni(II) onto our adsorbents at pH 5 and pH 7 are represented in Table4.14. These R L values implied that the adsorption of Ni(II) onto the adsorbents was favorable at higher pH.

	The

H), respectively. However, at pH 7 the adsorption of Ni(II) was superior than at pH 5, where there is a competition between the adsorption of Ni(II) and protonation of amine sites of CS (pKa 6.3). At pH 7, the maximum adsorption capacity (q m ) for the adsorption of Ni(II) reaches 182 mg/g for

Table 4 .

 4 13 Langmuir, Freundlich, and Temkin isotherms for adsorption of Ni(II) onto

	SiO 2 +CS LMW and SiO 2 (CO 2 H)+CS LMW at pH 5 and pH 7
	Isotherm models	Materials	K L	q m , mg/g	R 2	X 2
		SiO 2 +CS LMW pH 5	0.4 x 10 -3	104	0.99	1.12
		SiO 2 +CS LMW pH 7	3.0 x 10 -3	182	0.96	2.51
	Langmuir	SiO 2 (CO 2 H)+CS LMW pH 5 SiO 2 (CO 2 H)+CS LMW pH 7	0.8 x 10 -3 1.1 x 10 -3	166 210	0.98 0.96	3.07 5.14
		SiO 2 +CS(CO 2 H) pH 5	0.7 x 10 -3	227	0.94	2.12
		SiO 2 +CS(CO 2 H) pH 7	0.5 x 10 -3	263	0.98	2.93
		Materials	K F , mg/g	n	R 2	X 2
	Freundlich	SiO 2 +CS LMW pH 5	0.230	1.85	0.99	3.26
		SiO 2 +CS LMW pH 7	0.609	3.89	0.99	3.80
		Materials	K F , mg/g	n	R 2	X 2
		SiO 2 (CO 2 H)+CS LMW pH 5	0.493	3.33	0.99	6.99
	Freundlich	SiO 2 (CO 2 H)+CS LMW pH 7	0.452	3.10	0.99	9.32
		SiO 2 +CS(CO 2 H) pH 5	0.887	1.83	0.96	35.29
		SiO 2 +CS(CO 2 H) pH 7	0.812	1.80	0.98	35.17
		Materials	K T	B T	R 2	X 2
		SiO 2 +CS LMW pH 5	2.57 x 10 -3	243.3	0.99	1.11
		SiO 2 +CS LMW pH 7	5.96 x 10 -3	82.9	0.97	1.05
	Temkin	SiO 2 (CO 2 H)+CS LMW pH 5 2.45 x 10 -3	59.0	0.98	5.22
		SiO 2 (CO 2 H)+CS LMW pH 7	2.79 x 10 -3	91.2	0.99	6.49
		SiO 2 +CS(CO 2 H) pH 5	3.47 x 10 -2	12.9	0.81	44.64
		SiO 2 +CS(CO 2 H) pH 7	2.67 x 10 -2	14.1	0.95	42.93

Table 4 .

 4 14 R L values based on Langmuir isotherms at 1000 and 3000 mg/L

	[Cu 2+ ] Initial ,	SiO 2 +CS LMW	SiO 2 (CO 2 H)+CS LMW	SiO 2 +CS(CO 2 H)
	mg/L	pH 5	pH 7	pH 5	pH 7	pH 5	pH 7
	1000	0.714	0.250	0.555	0.476	0.546	0.628
	3000	0.454	0.100	0.294	0.232	0.286	0.360

Table 4

 4

	The adsorption of Ni(II) exhibited a good fit with the Temkin isotherm, that describes
	chemisorption of Ni(II) on the composites. The K T is the equilibrium binding constant
	corresponding to the maximum binding energy. The K T values of the adsorption of Ni(II) onto
	the adsorbents were between 2.45 x 10 -3 L/mg and 5.96 x 10 -3 L/mg. The chemisorption can
	be represented by the B T value, which indicates the heat of adsorption. The B T values for
	SiO 2 +CS LMW and SiO 2 (CO 2 H)+CS LMW were in the range between 59.0 J/mol and
	243.3 J/mol. In contrast, the B T value of SiO 2 +CS(CO 2 H) were only 12.9 J/mol at pH 5 and
	14.2 J/mol at pH 7, being inferior than that of unmodified CS-coated silica. According to the
	R 2 values of SiO 2 +CS(CO 2 H) (Table 4.13), the Temkin isotherm does not provides a better
	fit compared to the Langmuir isotherm.
	Moreover, the X 2 value was also used to interpret the adsorption data. For the adsorption of
	nickel onto SiO

.13, the n values for SiO 2 +CS LMW at pH 7 and both SiO 2 (CO 2 H)+CS LMW types at pH 5 and pH 7 were between 3.10-3.89. It can be implied that adsorption conditions of Ni(II) by SiO 2 +CS LMW at pH 7 and SiO 2 (CO 2 H) at both pH 5 and pH 7 are more favourable than for SiO 2 +CS LMW at pH 5 (n = 1.85) and for SiO 2 +CS(CO 2 H) at both pH 5 (n = 1.83) and pH 7 (n = 1.80). 2 +CS LMW, SiO 2 (CO 2 H)+CS LMW as well as on SiO 2 +CS(CO 2 H), the adsorption fitted well with the Langmuir isotherm with a low X 2 value of as seen in

Table 4 .

 4 13. Based on the Langmuir isotherm, the maximum adsorption capacity q m for Ni(II) adsorbed on the adsorbents at pH 7 was found to be 182 mg/g for SiO 2 +CS LMW, 210 mg/g for SiO 2 (CO 2 H)+CS LMW, and 263 mg/g for SiO 2 +CS (CO 2 H). The modified silica composites (SiO 2

+CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS (CO 2

2 +CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS(CO 2 H). The

). Table

4

.16 presents the intraparticle diffusion constants for Ni(II) adsorption on SiO plot for the obtained data yielded a straight-line plot. Therefore, we can conclude that the adsorption matches to intraparticle diffusion. The rate constant (

Table 4 .

 4 18), k int , was 35.7 mg/g min 1/2 and 18.1 mg/g min 1/2 for SiO 2 +CS LMW and

	SiO 2 (CO 2 H)+CS LMW, respectively. In addition, the k int of adsorption of Ni(II) on Thuja
	orientalis (cone biomass) found to be 11.55 mg/g min 1/2 (Initial Ni(II) concentration = 400
	mg/L) [85] which closed to that of our material (SiO 2 (CO 2 H)+CS LMW, k int = 18.1 mg/g
	min 1/2 , Initial Ni(II) concentration = 500 mg/L).
	Elovich equation is also used to interpret the experimental data. For SiO 2 +CS LMW
	and SiO 2 (CO 2 H)+CS LMW, the R 2 values for the Elovich equation plot were in the range

between 0.93-0.97 suggesting the sorption process fitted to Elovich equation (see

Table 4.16)

. This confirmed that the adsorption of Ni(II) onto SiO

2 +CS LMW and SiO 2 (CO 2 H)+CS LMW was

  chemical adsorption. However, for SiO 2 +CS(CO 2 H), based on the R 2 values which were lower than 0.70 suggesting that both intraparticle diffusion and Elovich were not suitable to describe the adsorption of Ni(II) onto this type of carboxymethyl CS-coated silica material.

Table 4 .

 4 15 Pseudo-first-order, and pseudo-second-order constant for adsorption of Ni(II)

	Materials	Pseudo-first-order k 1 , 1/min q e , mg/g	R 2	Pseudo-second-order k 2 , g/mg .min q e , mg/g	R 2
	SiO 2 +CS LMW	2.53 x 10 -3	14.92	0.97	1.87 x 10 -3	37.6	0.99
	SiO 2 (CO 2 H)+CS LMW	5.98 x 10 -3	20.12	0.93	0.94 x 10 -3	48.78	0.99
	SiO 2 +CS(CO 2 H)	0.23 x 10 -3	12.95	0.91	2.61 x 10 -3	34.4	0.99

Table 4 .

 4 16 Intraparticle diffusion and Elovich parameters for the Ni(II) adsorption

		Intraparticle diffusion		Elovich
	Materials	k int , mg/g min 1/2	C	R 2	, (mg/g min)	, mg/g	R 2
	SiO 2 +CS LMW	35.7	1.55	0.94	0.081	min 0.03	0.97
	SiO 2 (CO 2 H)+CS LMW	18.1	1.26	1.00	0.177	0.06	0.93
	SiO 2 +CS(CO 2 H)	22.7	1.36	0.65	0.127	0.05	0.64
							0.6
							3

Table 4 .

 4 17 IR analysis of the CS-coated silica hybrid -metal complex

				Frequency, cm -1
	Functional groups	SiO 2 +	SiO 2 +	SiO 2 +	SiO 2 +	SiO 2 (CO 2 H) +	SiO 2 (CO 2 H) +
		CS LMW	CS LMW -Ni	CS(CO 2 H)	CS(CO 2 H) -Ni	CS LMW	CS LMW -Ni
	Siloxane (from silica)	1099	1066	1083	1101	1080	1024
	-COO -symmetric
	vibration	-	-	1427	-	1423	1429
	(carboxylic acid)
	-COO -asymmetric
	vibration	-	-	1639	1639	1656	1647
	(carboxylic acid)
	Carbonyl (residual amide in CS)	1652	1660	-	-	-	-
	-OH (silanols from
	silica or hydroxyl	3448	3405	3456	3446	3444	3456
	groups onto CS)

Table 4 .

 4 18 The percentage of nickel content of three points of the hybrid composite

					%Nickel
	Material
		1	2	3	4	5	Average
	SiO 2 (CO 2 H)+CS LMW	19.6	22.4	24.5	18.8	18.4	20.8±2.6

Table 4 .

 4

	19 The log K values for 1:1 metal-carboxylate complexes*
	Metal ion Acetic acid	Benzoic acid	Formic acid	Chloroacetic acid
	Cu(II)	1.76 ± 0.06	1.51 ± 0.03	1.38 ± 0.03	1.07 ± 0.02
	Ni(II)	0.72 ± 0.05	0.55 ± 0.04	0.67 ± 0.02	0.23 ± 0.07
	*At 30 o C, ionic strength 0.4 mol/L

Table 4 .

 4 20 Some maximum adsorption capacities of Ni(II) by various adsorbents , 101.01 mg/g for activated carbon (waste apricot), 140.9 mg/g for Activated carbon[82, 84, 96a,b,c,d,e-97], between 12.61-98.01 mg/g for modified silica[55,[98][99], 5.21 mg/g for macroporous CS [79c], 15.3-163 mg/g for modified CS [84b, 100-104], 8.54 mg/g for hydroxyapatite/CS [105], 1.91 mg/g for Calcined Bofe bentonite clay [106a], 52 mg/g for Considering the metal ions electronegativity* of Sanderson as shown in Table 4.21, the adsorption selectivity of metal ions in the present study may tentatively correlated with the latter being Cu(II) (1.98) > Ni(II) (1.94).

	Materials	Maximum capacity, mg/g	pH	Reference
	Nano crystalline Hydroxyapatite	2.278	6.6	[90a]
	Modified oak sawdust	3.29	8	[90b]
	Pine bark	6.28	-	[91]
	Azadirachtaindica leaf powder	9.1	5.6	[92]
	Tamarind (Tamarindus indica L.) Bark	15.34	7	[93a]
	Askari grape dust	23.923	5	[93b]
	Moringa oleifera bark	30.38	6	[94a]
	Meranti sawdust	35.971	6	[94b]
	Papaya seeds (powder)	40.22	3	[94c]
	PEI modified biomass	55	5.5	[95a]
	Orange peel	62.89	5	[95b]
	Activated carbon (palm shell)	0.130	5	[96a]
	Activated Carbon (Algae Marine Gracilaria)	2.686	5	[96b]
	Activated Carbon (Cajanus cajan L Milsp seed shell)	29.60	5	[96c]
	Irradiation-grafted activated carbon	55.7	7.7	[96d]
	Activated carbon (coirpith)	62.5	5	[96e]
	Activated carbon (waste apricot)	101.01	5	[97]
	Activated carbon	140.85	6	[85]
	Ni(II)-imprinted amino-functionalized silica gel	12.61	8	[98]
	Resacetophenone loaded 3-aminopropyltriethoxysilane (APTES)-silica gel	14.89	6-7	[55]
	Silica-gel-immobilized P. vulgaris	98.01	6.5	[99]

[90a,b-95a,b], 0.13 mg/g for activated carbon (palm shell), 2.7 mg/g for activated carbon (Algae Marine Gracilaria), 29.60 mg/g for activated Carbon (Cajanus cajan L Milsp seed shell), 55.7 mg/g for irradiation-grafted activated carbon, 62.5 mg/g for activated carbon (coirpith)When comparing the adsorption of Cu(II) and Ni(II) (500 mg/L) onto SiO 2 +CS LMW at pH 5, it was found that SiO 2 +CS LMW binds stronger with Cu(II) (256.4 mg/g) than with Ni(II) (104 mg/g). The same trend was noticed for a cross-linked magnetic chitosan-2aminopyridine glyoxal Schiff's base resin, which adsorbed Cu(II) (1241 mg/g) better than Ni(II) (672 mg/g)

[101]

. A similarly observation was reported by A.H. Chen and coworkers who found that the affinity order of glutaraldehyde-crosslinked chitosan is Cu(II) > Ni(II)

[114]

. In order to find a correlation between the metal ion uptake and metal ion properties, several parameters have to be considered such as the electronegativity of the metal ion, electrostatic attraction due to charge to radius ratio, hydroxo complex formation abilities and preferred adsorption site on the adsorbent are responsible for the preferential adsorption of one metal ion over other

[115]

.

*Electronegativity is a chemical property which describes the tendency of an atom or a functional group to attract electrons towards itself.

Table 4 .

 4 21 Different models for electronegativity values of copper and nickel

	Electronegativity	Value in Pauling units Copper Nickel
	Sanderson electronegativity	1.98	1.94
	Pauling electronegativity	1.90	1.91

2 +CS LMW, SiO 2 +CS Yuhuan, SiO 2 +CS Marine, SiO 2 (CO 2 H)+CS LMW, SiO 2 (NH 2 )+CS LMW, and SiO 2 +CS(CO 2 H), respectively

  4 mg/g, 140.8 mg/g, 109.9 mg/g, 232.6 mg/g, 270.3 mg/g, and 333.3 mg/g for SiO . The comparison between different types of CS-coated silica materials, (CS LMW, CS Yuhuan, and CS Marine) reveals that the percentage of adsorbed Cu(II) is correlated with the %DD of CS and increases in the following order: CS LMW (DD 77 %) > CS Yuhuan (DD 75%) > Marine (DD 73%) The carboxymethyl CS-coated silica material SiO 2 +CS(CO 2 H exhibits a very high adsorption capacity for Cu(II) superior to that of the CS-coated grafted/non-grafted silica materials SiO This observation is tentatively rationalized by the presence of active functional groups (-CO 2 H / -CO 2 -) on CS, which act as binding sites with a high affinity towards Cu(II) ions [116]. Furthermore, the impact of the nature of the counter ion on the capacity for Cu(II) adsorption onto SiO 2 +CS LMW was evidenced by a comparative study using CuSO 4 and Cu(NO 3 ) 2 solutions. This work shows that under analogous conditions, the adsorption capacity of SiO 2 +CS LMW for CuSO 4 is superior to that Cu(NO 3 ) 2 . The Cu(II) adsorption from Cu(NO 3 ) 2 solution also fitted well with the Langmuir isotherm; the maximum adsorption capacity being 196.1 mg/g. The adsorption of Ni(II was investigated at room temperature using the organomineral composites

2 (NH 2 )+CS LMW, SiO 2 (CO 2 H)+CS LMW, and SiO 2 +CS LMW.

SiO 2 (CO 2 H), SiO 2 +CS LMW, and SiO 2 (CO 2 H)+CS LMW). Again

  the Langmuir isotherm provided a good fit. Other parameters such as the initial nickel salt concentration, contact time and the pH of the reaction medium have been evaluated.Concerning the pH, the maximum adsorption capacity for Ni(II) on the adsorbent was reached at pH 7 and found to be 189 mg/g for SiO 2 +CS LMW, 210 mg/g for SiO 2 (CO 2 H)+CS LMW, and 263 mg/g for SiO 2 +CS(CO 2 H), respectively.Both Cu(II) and Ni(II)-loaded materials were also characterized by IR and EDS spectroscopy as well as by SEM microscopy. A critical comparison with the adsorption capacities of numerous other sorbents described by other research groups reveals that our composites rank among the most efficient materials for this purpose. These "proof of concept" studies demonstrate the feasibility and promising potential our strategy using chemically modified SiO 2 particles in the domain of wastewater depollution.

	Experimental Part

Application of Composites by Retention of Metallic Micropollutants Chemicals and Apparatus Analytical

  grade Copper(II) nitrate and Copper(II) sulphate pentahydrate were purchased from Prolabo, Belgium, analytical grade Nickel(II) chloride was purchased from Merck, Germany. The stock solutions were prepared freshly for each experiment. The pH of the experimental solution was adjusted by adding 0.1 M NaOH or 0.1 M HCl solution. All solutions were prepared using deionized water (DI), 18.2 MΩ.cm, Millipore Milli-Q RO.After equilibrium, the residual aqueous-phase metal solution was analyzed with Atomic Absorption Spectroscopy (AAS, 50B spectra, Varian, USA). Fourier Transform Infrared Spectroscopy (FT-IR spectroscopy, VERTEX 70 Spectrometer equipped with a DTGS detector, BRUKER) was used to characterize the metal-load composites. The Energy Dispersive Spectroscopy analysis of the composites was carried out at the Centre de Microscopie Electronique, Université de Franche-Comté.

Adsorption Procedure for Cu(II)

  An adsorbate stock solution of 3.000 mg/L of CuSO 4 .5H 2 O (or Cu(NO 3 ) 2 .3H 2 O) was prepared in DI water. This stock solution was diluted to the required concentration between

Table 4 .

 4 22 Operating parameters for metal analysis using AAS

	Parameters	Cu(II)	Ni(II)
	Flame Type	Air/acetylene	Air/acetylene
	Wavelength	324.7 nm	232.0 nm
	Slit Width	0.5 nm	0.5 nm

Concentration oc Cu(II), mg/L Adsorbance 31

  . V. Manu, H.M. Mody, H.C. Bajaj and R.V. Jasra, Adsorption of Cu 2+ on amino functionalized silica gel with different loading, Ind. Eng. Chem. Res., 48 (2009) 8954-8960. 32. W. Yantasee, Y. Lin, G.E. Fryxell, K.L. Alford, B.J. Busche, and C.D. Johnson, Selective Removal of Copper(II) from Aqueous Solutions Using Fine-Grained Activated Carbon Functionalized with Amine, Ind. Eng. Chem. Res., 43 (2004) 2759-2764. 33. Z. Cheng, X. Liu, M. Han, and W. Ma, Adsorption kinetic character of copper ions onto a modified chitosan transparent thin membrane from aqueous solution, J. Hazard. Mater., 182 (2010) 408-415. 34. (a) L.T. Arenas, E.C. Lima, A.A. dos Santos Jr., J.C.P. Vaghetti, T.M.H. Costa, E.V. Benvenutti, Use of .2.2]octane/silica chloride for Cr(VI) adsorption, Colloid Surface A, 297 (2007) 240-248. (b) R. Ramya, P. Sankar, S. Anbalagan, and P.N. Sudha, Adsorption of Cu(II) and Ni(II) ions from metal using crosslinked chitosan-g-acrylonitrile copolymer, Int. J. Environ. Sci. 6 (2011) 1323-1338. 35. G.Z. Kyzas, M. Kostoglou, and N.K. Lazaridis, Copper and chromium(VI) removal by chitosan derivatives-Equilibrium and kinetic studies, Chem. Eng. J., 152 (2009) 440-448. 36. D.H.K. Reddy, D.K.V. Ramana, K. Seshaiah, and A.V.R. Reddy, Biosorption of Ni(II) from aqueous phase by Moringa oleifera bark, a low cost biosorbent, Desalination. 268 (2011) 150-157. 37. A.H. Hawari, and C.N. Mulligan, Biosorption of lead(II), cadmium(II), copper(II) and nickel(II) by anaerobic granular biomass, Bioresour.Technol., 97 (2006) 692-700.

	statistical	design	of	experiments	to	evaluate	the	sorption	capacity	of	1,4-
	diazoniabicycle[2

  µmol/m² et 3,86 µmol/m². En présence de groupements amines, le potentiel  des composites est positif jusqu' à pH 9 alors qu'il est négatif dès pH 3 pour des silices non fonctionnalisées. Dans un second temps, la silice est supportée par du chitosane dont le degré de désacétylation est de 77%. Conjointement, l'encapsulation de la silice est réalisées par du chitosane sur lequel des fonctions carboxyliques ont été greffées. La morphologie des particules est alors modifiée, leurs diamètres hydrodynamiques sont plus élevés et leurs potentiels  sont positifs jusqu' à pH basique. La rétention d'ions métalliques (Cu(II) et Ni(II)) par ces composites à différents pH est ensuite étudiée. Pour chacun des cations métalliques, les capacités d'adsorption sont déterminées ainsi que les cinétiques d'adsorption. L'application de plusieurs modèles d'isotherme d'équilibre a été réalisée. Dans le cas de Cu(II), à pH 5, les meilleures capacités d'adsorption sont obtenues pour des silices supportées par du chitosane greffé : la capacité de rétention des ions Cu(II) est de 270 mg/g à pH 5. De même, c'est ce composite qui permet la meilleurs rétention des ions Ni(II) à pH 7 avec une capacité d'adsorption de 263 m/g. Concernant la cinétique, le modèle de réaction de surface du pseudo-second ordre s'applique bien aux résultats expérimentaux.

, corresponding to 2.32 APTS groups/nm 2 . A similar calculation using the nitrogen percentage resulted in a grafting density of

3.12 mol m -2 , so the results show a slight discrepancy. M. Lazghab et al. reported that the grafting density of APTES onto silica with a higher porosity of 480 m 2 .g -1 (MP silica 63-200, Echochrom) was 2.88 mol.m -2 , equivalent

Results and Discussion

Physicochemical Properties of the CS Samples

The degree of deacetylation (%DD of CS), the molecular weight distribution, viscosity, and the behavior in aqueous solution and/or in solvents are important quality criteria, especially for an application such as wastewater treatment. Therefore the %DD and other propensities of the CS samples have been analyzed to allow a reliable interpretation of the results. Three types of commercial CS were used in this work. The molecular weight of CS is one of the important factors affecting its physicochemical properties and a key variable in the adsorption process because it influences the solubility of the polymer and the viscosity in the solution. The molecular weight presented as weight average molecular weight (M W ) of the three types of CS was 52.9 kDa for CS Yuhuan (CS from Yuhuan Ocean Biochemical, China), 150 kDa for CS LMW (CS from Aldrich, U.S.A), and 390 kDa for CS Marine (from Marine Chemical, India) (Table 3.2). The M W of CS depends on the original source and the degree of deacetylation of CS. Notabely a CS with low M W and high DD should have a high capacity for metal ion removal. The exact value of %DD was determined as described in the following topic.

To prepare CS solutions, the CS samples were dissolved in 1% acetic acid. The following observation was made thereby: the solubilization of CS Marine (Mw = 390 kDa) took more time than that of CS Yuhuan (Mw = 52 kDa). This may be due to the higher M W of the former type of CS than that of the latter type. Consequently, the latter type of CS exhibited a lower solution viscosity than CS Marine. This result is consistent with the report of H. [START_REF] Queen | Electrospinning chitosan-based nanofibers for biomedical applications[END_REF]. She reported that the highest molecular weight (600 kDa) produced more viscous solutions than that of middle molecular weight (400 kDa) and lowest molecular weight (150 kDa) [68]. Note that the viscosity of the three types of chitosan solutions has not been measured. However, Table 3.2 shows the wide range of viscosity data of CS LWM and CS Yuhuan.

The CS Yuhuan gave a yellow solution after dissolution in acetic acid. This coloration is due to the origin of this biopolymer. height of about 50 nm for SiO 2 (Fig. 76a). On the contrary, CS-coated silica displayed a homogeneous coating with the thickness between 300 and 400 nm (Fig. 76b).

pH and Conductivity Measurement

The Conductivity values of all modified silica composites, as well as non-grafted silica were also investigated. It was found that the conductivity of all composites remained quite stable after 24 hours. The conductivity of SiO 2 +CS LMW was found to be in the range 18 -22 S/cm after 24 hours. For SiO 2 +CS Marine, the conductivity was found to be in the range of 26 -31 S/cm. The conductivity value of the two bio-hybrid materials was quite close.

However, the conductivity of the CS-coated silica materials was superior to those of the carboxyl-grafted silica and SiO 2 . This could be explained by the influence of CS molecules on the silica surface. The conductivity in the presence of CS increased due to the displacement of the equilibrium dissociation of the acid and progressive dissolution of protonated CS (M.

Rinaudo et al, 1999) [85].

In the case of SiO 2 (NH 2 )+CS LMW and SiO 2 +CS(CO 2 H), their conductivity values were found around 400 S/cm and 350 S/cm, respectively. After silica was surface-coated reduced to the Langmuir isotherm. Brazilian kaolinite clay has been used to capture urea and the adsorption could be explained employing the Toth isotherm [26].

Fritz-Schluender isotherm

The Fritz-Schluender isotherm describes the adsorption on heterogeneous surfaces. It can be represented by equation 14 as follows:

where K L is the Fritz-Schluender isotherm constant and n is the Fritz-Schluender exponent.

The q e , q m , and C e have already been mentioned above [27]. Rhizopus oryzae (dead fungi)

was used as biosorbent to remove p-nitrophenol from aqueous phase and it was demonstrated that the Fritz-Schluender isotherm fits to explain the experimental data [28].

Radke-Prausnitz isotherm

The Radke-Prausnitz (Radke and Prausnitz, 1972) isotherm has been derived to make the Langmuir-Freundlich model thermodynamically consistent. It is found to be suitable for weakly adsorbed components. The Radke-Prausnitz isotherm in non-linear form can be represented as seen in equation 15 [23] where K L is the Radke-Prausnitz isotherm constant and n is the Radke-Prausnitz isotherm exponent. The q e , q m , and C e have already been mentioned above. According to the literature, the biosorption of Cd(II) onto formaldehyde-treated Cystoseira indica (brown marine alga) fits well with the Radke-Prausnitz isotherm [29].

A5 Adsorption Kinetics

The adsorption kinetics represent another important physicochemical parameter to evaluate the qualities of a good sorbent. They describe the adsorbate uptake rate and the rate controling the residence time (average amount of time that a particle spends in a particular system) of adsorbate at the solid-liquid interface.

Removal of Cu(II) by our hybrid sorbents as a function of time is shown in Fig. 90919293.

The adsorption curve for Cu(II) ions reveals that the adsorption is initially rapid. To identify the rate-controlling mechanisms during the adsorption of copper, three steps were considered:

-Mass transfer of the metallic ion from the bulk solution to the chitosan surface, -Adsorption of the metallic ion onto sites, -Internal diffusion of the metallic ion onto chitosan.

For this purpose, the pseudo-first-order, pseudo-second-order (Ho and McKay, 1998), and intraparticle diffusion were used. The better fit was estimated in terms of the coefficient of determination (R 2 ).

Pseudo-First Order

The pseudo-first order has been used for the description of a reversible equilibrium between liquid and solid phases [36]. The pseudo-first order equation is expressed as equation 17ln( (q e -q t )/ q e ) = -k 1 /t (17

where k 1 (min -1 ) is the rate constant of pseudo-first order sorption, t is time, and q e (mg/g) and q t (mg/g) are the sorption capacities at equilibrium and at time t, respectively.

Pseudo-Second Order

The pseudo-second order model assumes the rate-limiting step of a chemisorption.

The pseudo-second order chemisorption kinetic equation is expressed in equation 18 [37] where k 2 is the rate constant of the pseudo-second order sorption (g/mg/ min), t is time, and q e (mg/g) and q t (mg/g) are the sorption capacity at equilibrium and at time t, respectively.

Adsorption kinetics Kinetic

Plot Description Reference

External mass transfer

The external mass transfer has been used in the analysis of mass transfer mechanism of metal adsorption [39] Intraparticle diffusion (Vermeulen) It assumes that the adsorption process is controlled by chemical reaction [40] Intraparticle diffusion (Weber and Morris)

If the line passes through the origin then adsorption is the rate-controlling step. When the plot do not pass through the origin, it shows that adsorption is not only rate-limiting step [12,38] Adsorption Kinetics: CS-coated Silica Materials and Amine-Grafted Silica

The adsorption kinetics were investigated by adsorption of Cu(II) (500 mg/L) onto two CS-coated silica materials: group II (SiO 2 +CS LMW, SiO 2 +CS Yuhuan, and SiO 2 +CS Marine) and were compared to group IV adsorbent SiO 2 (NH 2 ). The amount of adsorbent was constant (0.4%). The adsorption took place at pH 5, and the contact time was in the range between 10 and 1.440 min. The pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Elovich adsorption kinetics were employed to interpret the experimental data.

The adsorption rate of Cu(II) ions at the solid solution interface onto four CS-coated silica materials with an equilibrium established between the two phases was studied using the pseudo-first-order and pseudo-second-order model. The k 1 and k 2 values are the rate constant of pseudo-first order sorption and the rate constant of the pseudo-second order sorption, respectively. The validity of the kinetic models is tested by the magnitude of the correlation coefficients (R 2 ) shown in Table 4.9, it was found that the both pseudo-first-order and pseudosecond-order models (Fig. 89 Both pseudo-first order and pseudo-second order kinetics did not allow clarifying the diffusion mechanism; therefore the kinetics results were analyzed using the intraparticle diffusion equation. were shifted to 3489 and 1676 cm -1 , respectively. Therefore, the copper ions interact with the -NH 2 group; however an interaction with both the -NH 2 and -OH groups cannot be ruled out.

A6 Characterization of the

The formation of the complex ML n from a metal M and a ligand L can be represented as follows M + L ML; ML + L ML 2 ; ......

.ML n-1 + L ML n

where ML n is the ligated metal complex, and n is the maximum co-ordination number.

The stability constant K of the complexation of Cu(II)-chitosan was investigated by Desbrières and co-workers by the Bjerrum method. The principle of this method is based on the determination of the different species formed in solution by potentiometric titration (see detail in reference [45]). The complex I shows a higher formation constant (log K = 5.47)

than the complex II (log K = 2.62). It can be assumed that only one amino group has a larger formation constant meaning a larger stability constant and lower dissociation [44].

The complexation of metallic ions depends on the stability constant of the metalligand complex, but also on the dissociation constant of the ligand obtained from the equilibrium reaction shown in eq. 29.

The pK a can be calculated using the Katchalsky equation as shown in equation 30.

where x is the neutralization degree of amino group. From the report of Desbrières et al., the pK a is constant and equals to 6 [45].

The interaction of CS with metal ions can be described as follows: Recently, the complex ion equilibria between N,O-carboxymethyl chitosan with metal ions such as Cu(II), Ni(II), Co(II), Mn(II) and Zn(II) have been investigated by potentiometric titration [46]. From the results, it was possible to arranged the log K values in the order Mn < Co < Ni > Zn. These thus established stability constants were consistent with the order within the Irving-William series for divalent high-spin metals [47].

According to another recent DFT computing study and geometry optimization of the possible aquo-complexes resulting from complexation between Cu(II) and CS residues, the trend of coordination with paramagnetic Cu(II) is -NH 2 > -OH at C3 > H 2 O > NHCOCH 3 [48]. In this article, the coordination number of the resulting complexes is discussed. In Therefore there is still some controversy on this subject. Since a Cu(II) ion readily coordinates H 2 O molecules, the coordination sphere around the metal centre depends surely from its environment. A sample of a CS-Cu(II) adduct obtained from aqueous solution may change the coordination number after drying by loss of water molecules and than even rehydrate partially upon exposure to humidity of air. This may also hamper the interpretation of the IR spectra. Note that gray-white anhydrous CuSO 4 becomes bluish after exposure to humid air by reformation of the well-known deep-blue CuSO 4 x 5 H 2 O complex [49b].

Numerous studies have been conducted using various types of adsorbent for Cu(II) adsorption at optimum pH. Table 4.12 presents a comparison of the adsorption capacity of the results. The adsorption of Cu(II) onto plant biomass was found to be 17.87 -263.2 mg/g [50-52a], coffee wastes was found to be 70 mg/g [52b], activated carbon (rice husk) was found to be 112.4 mg/g, activated carbon (Tunisian date stones) was found to be 31.2 mg/g, activated carbon number E4 34011 was found to be 125.0 mg/g, fly ash was found to be 69.9 mg/g [53][54], natural sorbent was found to be 187 mg/g [20], sissoo sawdust (Dalbergia) was found to be 263.2 mg/g [53], modified silica was found between 11.8 -126.2 mg/g [55][56][57][58][59][60][61][62], modified CS was found to be 8.7 -219 mg/g [63-79b], polyacrylonitrile grafted CS was found to be composite (86.2 mg/g) [77]. The adsorption of Cu(II) on our materials (SiO 2 (NH 2 )+CS LMW and SiO 2 (CO 2 H)+CS LMW) was also higher than that of CS-coated perlite beads (104 mg/g, pH 4.5), and that of the Saccharomyces cerevisiae / CS-coated magnetic nanoparticles (144.9, pH 4.5) [44,78]. Cottonseed protein-poly (acrylic acid) copolymer hydrogel composite found to be 219 mg/g (pH 4.0-5. Based on the Langmuir isotherm, the adsorption of Cu(II) onto the carboxymethyl CScoated non-grafted silica material SiO 2 +CS(CO 2 H)) showed still better adsorbent properties compared to CS-coated silica material SiO 2 +CS LMW. The former types of material exhibit probably a higher adsorption capacity due to the presence of two available groups (-NH 2 and -CO 2 H) for the uptake of Cu(II) ions.

A8 Comparision between the Adsorption of Cu(II) onto CS-coated Silica from of Copper Sulphate and Copper Nitrate Solution: Influence of the

Counter Ion

Under real world conditions contaminated water contains also several other ions (both cations and anions), affecting the adsorption of a given metal. Consequently, the effect of some commonly occurring counter ions on the adsorption behavior of Cu(II) by our materials was studied. A typical water sample containing chloride ions, and nitrate ions was doped with known quantities of adsorbate metal ions. These samples were used as the water matrix for adsorption. Adsorption of Cu(II) onto CS-coated silica material SiO 2 +CS LMW from solutions of Cu(NO 3 ) 2 was studied at room temperature. The adsorption took place at pH 5 and contact time was 24 hours. The initial concentration of Cu(II) was in the range between 50 mg/L to 1.000 mg/L. The dose of sorbent was fixed as 0.4% in solution. Similar conditions were also employed to the adsorption of Cu(II) onto SiO 2 +CS LMW from solutions of CuSO 4 to allow a comparison. Fig. 100 shows a more efficient Cu(II) adsorption from solution for CuSO 4 compared to Cu(NO 3 ) 2 . The adsorption of Cu(II) fitted well with Langmuir isotherms and the q m values were 256.4 mg/g for the adsorption from CuSO 4 solution and 196.1 mg/g for the adsorption from Cu(NO 3 ) 2 solution. This result is consistent

General Conclusion and Perspectives

The demand for new green technologies and low-cost production for realizing chemical transformations and material design is becoming a priority. A lot of attention has been devoted to the use of natural polymer for sustainable development and preservation of the environment. In context, we focused our work on the elaboration of functionalized Aerosil 200 particles as efficient absorbents to scavenge divalent metal ions from aqueous solution.

The biopolymers chitosan has been chosen to prepare novel organic-inorganic hybrid material. The present work demonstrates that the modified silica materials (SiO 2 (NH 2 ) and The pH values of colloidal silica (Activated SiO 2 ) are quite stable during 3 days of analysis (around pH 6). After activation of silica by acidid aqueous solution, the conductivity is quite stable and the average value is  969±8 S/cm. The Activated SiO 2 shows a ten times higher conductivity than neat SiO 2 . This may be due to some residual hydrochloric acid bound on thesurface and surface disturbed by acid molecules. The zeta potential values of the SiO 2 and Activated SiO 2 are nearly zero at pH 2. However, at pH > 2, zeta potential values decrease accordingly resulting from an increasing of the concentration of the negative charges on the silica surface.

In Chapter two, the chemical modification of silica is described. The SiO 2 (NH 2 ) and SiO 2 (CO 2 H) materials were synthesis via simple procedure. The surface morphology of the two materials was investigated by SEM and AFM. The surface morphology of SiO 2 (NH 2 ) and SiO 2 (CO 2 H) materials was more uniform and more regular when compared with that of SiO 2 .

The BET value of modified silica materials decreases with an increase of the surface density.

The Elemental analysis and BET data allow the calculation of the amino or carboxyl groups grafting density. Therefore, the grafting density of SiO 2 (NH 2 ) and SiO 2 (CO 2 H) are 3.94 mol m -2 and 0.48 mol m -2 , respectively. The behaviour in aqueous solution was investigated by measuring the zeta potential, pH and conductivity. At pH between pH 2 and 8.9) increased when comparing to the non-grafted silica ( pH 5.7) or carboxyl-grafted silica ( pH 4.2), due to coating of the particle surface by CS polymer. The conductivity of SiO 2 +CS was found to be in the range 18 -31 S/cm after 24 hours. In case of CS-coated modified silica materials (SiO 2 (NH 2 )+CS LMW and SiO 2 +CS(CO 2 H)) their conductivity values were found to be in the range 350 S/cm and 400 S/cm. The zeta potential values of the five types of CS-coated silica (SiO 2 +CS LMW, SiO 2 +CS Marine, SiO 2 (CO 2 H)+CS LMW, SiO 2 (NH 2 )+CS LMW, and SiO 2 +CS(CO 2 H)) were quite close with the positive zeta potential value at low pH (2-3). The hydrodynamic size of the six hybrid composites was higher than 1 m when comparing with SiO 2 (32825 nm).

In the last chapter, the novel organic-inorganic hybrid composites were revealed as promising adsorbent for efficient removal of Cu(II) and/or Ni(II) from aqueous solutions at room temperature. In case of Cu(II) adsorption onto SiO 2 (NH 2 ), SiO 2 +CS LMW, SiO 2 +CS Yuhuan, and SiO 2 +CS Marine materials, the isotherm of Langmuir fitted well for the adsorption studied. Assuming that the adsorption of Cu(II) is a mono-layer adsorption onto the surface of our sorbents at specific homogeneous sites with a finite number of identical sites and the adsorption takes place without any interaction between the adsorbed molecules.

The maximum monolayer adsorption obtained at pH 5 on the adsorbent was found to be 93.3 mg/g for SiO 2 (NH 2 ) and found to be between 97.0-158.7 mg/g for SiO 2 +CS. The adsorption capacity of Cu(II) by the CS-coated silica materials acted as a better adsorbent compared to amine grafted silica material. When the silica core was chemically modified with amine and/or carboxyl group before surface coating with CS, the maximum Cu(II) adsorption onto the modified silica materials (SiO 2 (CO 2 H)+CS LMW and SiO 2 (NH 2 )+CS LMW) was found to be 232.6 mg/g for SiO 2 (CO 2 H)+CS LMW, and 243.9 mg/g for SiO 2 (NH 2 )+CS LMW).

These two types of the modified silica materials showed the more efficiency for capturing Cu(II) than the CS-coated non-modified silica materials. This was due to the amine or carboxyl groups on the chemically modified silica core. The adsorption rate of Cu(II) ions at the solid solution interface onto four CS-coated silica materials with an equilibrium established between the two phase was studied using the pseudo-first-order, pseudo-secondorder. It was found that the pseudo-second-order was the model that provided the better fit for