L R Bonetto
email: lrbonetto@gmail.com

F Ferrarini

C D Marco

J S Crespo

R Guégan

M Giovanela

Removal of methyl violet 2B dye from aqueous solution using a magnetic composite as an adsorbent

Keywords: halloysite-magnetite-based composite, methyl violet 2B, adsorption, kinetics, thermodynamics

Despite the important role of the textile industry in the global economy, its effluents generate deep concern with regard to treatment and disposal. Adsorption is a promising technique for the removal of textile color effluent at relatively lowcost and with satisfactory efficiency. This study aimed to evaluate the adsorption capacity of a halloysite-magnetite-based compositein the removal of methyl violet 2B cationic dye. Afterpreparation, the obtained composite was characterized by applying several instrumental techniques, includingX-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and determination of pH at the point of zero charge. The influences ofstirring rate, adsorbent mass, pH,initial concentration ofdyeand contact time on the adsorption process werealso studied. The adsorption capacityof the composite was then investigated in a temperature-controlled batch system. The experimental results showed that the adsorption kinetics were better described using a pseudosecond-order model. Regarding the adsorption equilibrium, the experimental results suggest that both the Langmuir and Freundlich models were applicable. The thermodynamic data showed that dye adsorption onto the composite was spontaneous and endothermic and occurred byphysisorption.The employed composite could also be regeneratedat least four times using a 1.0 mol L -1 solution of NaOH as an eluent and was shown to be a promisingadsorbentfor the removal ofcationic dyes.

Introduction

Thefuture of Earthandits biodiversityisa common concern foreveryone.Rapid technologicaland industrial developmentin recent decadeshas led to anumber of environmentalproblems, particularly with regardto the pollutionof water resources [START_REF] Gupta | Application of low-cost adsorbents for dye removal -A review[END_REF].Synthetic dyes areone of themajor classes ofpollutants responsiblefor imbalancesin aquatic ecosystems; their complexmolecularstructureskeeps themrelatively stable, making them difficultto remove [START_REF] Forgacs | Removal of synthetic dyes from wastewaters: a review[END_REF].

Color is usually the first contaminant to be recognized in wastewater because a very small amount of synthetic dye in water (< 1.0 mg L -1 ) is typically highly visible, affecting the aesthetic merit, transparency and gas solubility of water bodies. These dyes absorb and reflect sunlight, thereby interfering with the growth of aquatic species and hindering photosynthesis.

Additionally, they can have acute and/or chronic effects on organisms, depending on their concentration and exposure time [START_REF] Pereira | Dyes-Environmental Impact and Remediation[END_REF].

Methyl violet (MV) is particularly important because of its broad applications in textiles, paints and print inks [START_REF] Mittal | Removal and recovery of hazardous triphenylmethane dye, Methyl Violet through adsorption over granulated waste materials[END_REF][START_REF] Pal | Microwave-assisted synthesis of platinum nanoparticles and their catalytic degradation of methyl violet in aqueous solution[END_REF].MV isoften usedin the dyeingofcotton, silk, paper, bamboo, strawand leather [START_REF] Ofomaja | Biosorption of Methyl violet onto palm kernel fiber: Diffusion studies and multistage process design to minimize biosorbent mass and contact time[END_REF]. In biomedical fields, MV is the active ingredient in Gram's biological stain for bacteria classification [START_REF] Esteva | Trypanosoma cruzi: methoprene is a potent agent to sterilize blood infected with trypomastigotes[END_REF][START_REF] Laube | Skin infections and ageing[END_REF][START_REF] Saji | Efficacy of gentian violet in the eradication of methicillin-resistant Staphylococcus aureus from skin lesions[END_REF][START_REF] Wainwright | Dyes for the medical industry[END_REF]. It can sometimes also be used as a moderateclass disinfectant but has beenfound to be poisonous to most animals. The inhalation of MV may cause irritation to the respiratory tract, whereas ingestion typically causes irritation to the gastrointestinal tract [START_REF] Li | Bioadsorption of methyl violet from aqueous solution onto Pu-erh tea powder[END_REF].

Dyes can be classified according to their chemical structure or method of application.

MV belongs to a class of intensely colored organic compounds called triphenylmethane dyes due to the presenceofthreearylgroups, each of which is bonded toa nitrogen atomthat interactswith one or twomethyl groups.Moreover, MV is a basic dyebecause of thepresence ofa positive chargeonthe aminogroup [START_REF] Zollinger | Color chemistry: syntheses, properties and applications of organic dyes and pigments[END_REF].

MVis a mixture of tetramethyl, pentamethyl and hexamethyl pararosaniline chlorides [START_REF] Sabnis | Handbook of Biological Dyes and Stains[END_REF]. This last chemical compound is also known as crystal violet or MV 10B and is the primary representativeof these three dyesreportedin the literature [START_REF] Chakraborty | Adsorption of Crystal Violet from aqueous solution onto NaOH-modified rice husk[END_REF][START_REF] Chen | Effective removal of high-chroma crystal violet over TiO 2 -based nanosheet by adsorption-photocatalytic degradation[END_REF][START_REF] Chen | Multifunctional kaolinitesupported nanoscale zero-valent iron used for the adsorption and degradation of crystal violet in aqueous solution[END_REF][START_REF] Gabal | MWCNTs decorated with Mn 0.8 Zn 0.2 Fe 2 O 4 nanoparticles for removal of crystal-violet dye from aqueous solutions[END_REF][START_REF] Pal | Synergistically improved adsorption of anionic surfactant and crystal violet on chitosan hydrogel beads[END_REF][START_REF] Singh | Optimizing adsorption of crystal violet dye from water by magnetic nanocomposite using response surface modeling approach[END_REF]. Conversely, in MV 2B,there is a predominance of the pentamethylated compound [START_REF] Sabnis | Handbook of Biological Dyes and Stains[END_REF]. Due to this difference, the particular shade of MV differs, depending on the amount of each component present.

Fig. 1showsthe chemical structuresof MV 2B and 10B. The small structural difference observed(the presence of an additionalmethyl group in MV 10B) is responsible for thedifferent physical and chemicalproperties of the two dyes, which include theabsorption maximumin the visible region, solubility, staining intensityandmelting temperature [START_REF] Sabnis | Handbook of Biological Dyes and Stains[END_REF]. A wide range of technologies has been developed for the removal of synthetic dyes from waters and wastewaters to decrease their environmental impacts. These include physical methods, such as membrane filtration processes and sorption techniques;chemical methods, such as coagulation, electroflotation, advanced oxidation processes and irradiation; and biological methods, such as aerobic and anaerobic microbial degradation and the use of enzymes [START_REF] Pereira | Dyes-Environmental Impact and Remediation[END_REF]. Although chemical and biological methods are effective at removing dyes, they require specialized equipment and are usually energy intensive; in addition, large amounts of by-products are often generated. Conversely, the physical methodsare generally effective at removing dyes without producing unwanted by-products [START_REF] Al-Degs | Effect of solution pH, ionic strength, and temperature on adsorption behavior of reactive dyes on activated carbon[END_REF][START_REF] Vandevivere | Treatment and reuse of wastewater from the textile wet-processing industry: Review of emerging technologies[END_REF].

With regard to the physical methods, sorption of synthetic dyes on inexpensive and efficient solid supports has been considered a simple and economical process for the removal of dyes from water and wastewater, producing high quality water; this process is an attractive alternative for the treatment of contaminated waters, particularly where the sorbent is inexpensive and does not require a pre-treatment step before its application. Sorption is superior to other techniques for water reuse in terms of initial cost, flexibility and simplicity of design, and ease of operation [START_REF] Pereira | Dyes-Environmental Impact and Remediation[END_REF].

Activated carbon is the most widely used sorbent, and it has excellent sorption properties for a considerable number of synthetic dyes. However, the preparation of carbon sorbents is generally energy intensive, making commercially available products relatively expensive. Because a large amount of carbon sorbent is needed to remove the dye from a large volume of effluent, high cost can hinder its application [START_REF] Yagub | Dye and its removal from aqueous solution by adsorption: A review[END_REF]. In addition, the technology for manufacturing good-quality activated carbon is not fully availablein developing countries.

This limitation has prompted increasing research interest in the production of low-cost alternatives to activated carbon from a range of carbonaceous and mineral precursors [START_REF] Pereira | Dyes-Environmental Impact and Remediation[END_REF].

According to Bujdák [START_REF] Bujdák | Effect of the layer charge of clay minerals on optical properties of organic dyes. A review[END_REF], future investigations could be directed towards the optimization of wastewater treatment processes using suitable clay templatesand nanomaterials. In this way, adsorbents with magnetic properties have been extensively studied by many researchers. Zhao et al. [START_REF] Zhao | Adsorption kinetics, isotherms and mechanisms of Cd(II), Pb(II), Co(II) and Ni(II) by a modified magnetic polyacrylamide microcomposite adsorbent[END_REF] have reported, for example,a novel magnetic hydroxamic acid modified polyacrylamide/Fe 3 O 4 adsorbent (M-PAM-HA) for the removal of Cd(II), Pb(II), Co(II) and Ni(II) ions from aqueous solutions. Ai et al. [START_REF] Ai | Removal of methylene blue from aqueous solution by montmorillonite/CoFe 2 O 4 composite with magnetic separation performance[END_REF] and Sun et al. [START_REF] Sun | Preparation and adsorption property of xylan/poly(acrylic acid) magnetic nanocomposite hydrogel adsorbent[END_REF] have developeda xylan/poly(acrylic acid) magnetic nanocomposite hydrogel adsorbent and montmorillonite/CoFe 2 O 4 magnetic composite, respectively, for removalof methylene blue from wastewater.

The novelty brought by these adsorbentsis theimprovement in accelerating separation speed, thereby enhancing water treatment efficiency. As a result, solid phase can be easily recovered by the application of an external magnetic field [START_REF] Gupta | Chromium removal by combining the magnetic properties of iron oxide with adsorption properties of carbon nanotubes[END_REF][START_REF] Rocher | Removal of organic dyes by magnetic alginate beads[END_REF][START_REF] Sivashankar | Magnetic composite an environmental super adsorbent for dye sequestration -A review[END_REF]. Moreover, adsorbents with magnetic properties produce no contaminants during wastewater treatment. As main disadvantage, most commercially magnetic particles are expensive and cannot be applied to large-scale processes, but magnetic modification of low cost adsorbents could lead to materials suitable for biotechnology and environmental applications [START_REF] Sivashankar | Magnetic composite an environmental super adsorbent for dye sequestration -A review[END_REF]. Research hasshown that it ispossible to producepromisingadsorbentsin this field, creating new options for textile industry effluent mitigation [START_REF] Chang | Preparation and characterization of Fe 3 O 4 /graphene nanocomposite and investigation of its adsorption performance for aniline and p-chloroaniline[END_REF][START_REF] Duan | Halloysite nanotube-Fe 3 O 4 composite for removal of methyl violet from aqueous solutions[END_REF][START_REF] Pan | Selective Recognition of 2,4,6-Trichlorophenol by Molecularly Imprinted Polymers Based on Magnetic Halloysite Nanotubes Composites[END_REF][START_REF] Xie | Magnetic halloysite nanotubes/iron oxide composites for the adsorption of dyes[END_REF][START_REF] Yang | Fabrication of β-cyclodextrin conjugated magnetic HNT/iron oxide composite for high-efficient decontamination of U(VI)[END_REF][START_REF] Zhang | Halloysite nanotubes coated with magnetic nanoparticles[END_REF][START_REF] Zhang | Novel magnetic Fe 3 O 4 @C nanoparticles as adsorbents for removal of organic dyes from aqueous solution[END_REF].

Therefore, the aim of this work was to prepare, characterize and evaluate the removal of MV 2B from aqueous solution using a halloysite-magnetite-based composite (HNT-Fe 3 O 4 )

as an adsorbent. The adsorption kinetics, equilibrium and thermodynamics of MV 2B onto this composite were also investigated. Extran solution (15% v/v) for 24 h. After this period, they were washed thoroughly with tap water and then soaked in a solution of HNO 3 (5% v/v) for 24 h. Finally, the material was rinsed with deionized water and left to dry at room temperature.

Preparation of HNT-Fe 3 O 4 composite

The HNT-Fe 3 O 4 composite was prepared using the chemical precipitation method [START_REF] Duan | Halloysite nanotube-Fe 3 O 4 composite for removal of methyl violet from aqueous solutions[END_REF]. Initially, 6.25 g of HNT was added into a 500 mL solution of 7.275 g of FeCl 3 . 6 H 2 O and 3.75 g of FeSO 4 . 7 H 2 O. The suspension was thenrefluxed for 3hin aglycerin bathat 120°C undernitrogen gas flow. When the mixed suspension was cooledto 50°C, NaOH solution (4.0 molL -1 ) was addeddropwise with vigorousmagnetic stirring. ThepH of the final suspension was controlled to be in the range of 9.0-10.0. The mixture was aged at 50°C for 2.5 h and then filtered and washed with deionized water repeatedly. At the end of this procedure, the obtained composite was dried at 60°C for 24 h.

Characterization of HNT-Fe 3 O 4 composite

The crystalline phases in the HNT and HNT-Fe 3 O 4 composite were characterized by X-ray powder diffraction using a Shimadzu XRD-6000 diffractometer (Tokyo, Japan). The samples were scanned at room temperature in reflection mode using incident CuK α radiation (λ = 1.5405 Å) at a step width of 0.05° min -1 from 2θ = 2° to 80°.

The morphology of the HNT and HNT-Fe 3 O 4 composite was examined by scanning electron microscopy (SEM) using a ShimadzuSSX-550 (Tokyo, Japan) operating at an accelerating voltage of 10 kV. Before analysis, the samples were sputter-coated with a thin gold layer for 2.5 min. The qualitative analysis of iron in the HNT-Fe 3 O 4 compositewas performed by energy dispersive spectroscopy (EDS) of the same sample used in the SEM analysis and in the same microscope, operating at an accelerating voltage of 15 kV.

Nitrogen adsorption-desorption experiments were performed in order to obtain information on the specific surface area of the HNT-Fe 3 O 4 composite. The measurements were carried out at 77 K using a Quantachrome Instruments Nova 2200e Surface Area Analyzer (Boynton Beach, FL, United States of America). About 100 mg of the sample was outgassed at 383 K for 24 h under a residual pressure of 0.01 Pa. Data were recorded for relative vapour pressures from 0.05 to 0.99. The specific surface area was determined using the Brunauer-Emmet-Teller (BET) equation based upon the cross-sectional area of nitrogen (0.163 nm 2 ) at 77 K.

The point of zero charge (pH PZC ) of the HNT-Fe 3 O 4 composite was determined by adding 50 mL of 0.01 mol L -1 KNO 3 with a previously adjusted initial pH (i.e., pH i values of the solutions were adjusted from 2.0 to 11.0 with 0.10 mol L -1 of KOH or HNO 3 ) to several 150 mL Erlenmeyer flasks. Each Erlenmeyer contained 0.10 g of the magnetic compositeand was securely capped immediately with a sheet of aluminum foil. Then,the Erlenmeyers were shaken at a stirring rate of 400 rpm using a thermostated water bath shakerat a constant temperature of 25°C; the samples were then allowed to equilibrate for 24 h. At the end of this experimental procedure, the suspensions were filtered, and the final pH (pH f ) values of the solutions were recorded using a Digimed DM-20 pHmeter (São Paulo, SP, Brazil). The value of pH PZC is the point where the curve of ΔpH (pH i -pH f ) versus pH i crosses the line equal to zero [START_REF] Mall | Characterization and utilization of mesoporous fertilizer plant waste carbon for adsorptive removal of dyes from aqueous solution[END_REF].

Adsorption experiments

The adsorption experiments were performed in a Dist multiple spindle stirrer (Florianópolis, SC, Brazil) using 25 mL of dye solution at 25°C for 6 h. Subsequently, the Erlenmeyers were exposed to a magnetic field using a 3000 G magnet for 5 min to separate the magnetic composite from the aqueous solutions.

The initial and final concentrations of the MV 2B remaining in the solutions were determined by visible spectrophotometry using a Thermo Scientific Evolution 60 spectrophotometer (Waltham, MA, United States of America) fitted with a quartz cellwith a path length of1.0 cm. Absorbance measurements were made at 582 nm (i.e., the maximum wavelength of MV 2B dye). A calibration curve was constructed withdye concentrations ranging from 1.0 to 15.0 mg L -1 .

The removal percentage (%R, %), the amount of MV 2B adsorbed at time t (q t , mg g - 1 ), and the adsorption capacity at equilibrium (q e , mg g -1 ) were calculated by applying Equations ( 1), ( 2) and( 3), respectively [START_REF] Duan | Halloysite nanotube-Fe 3 O 4 composite for removal of methyl violet from aqueous solutions[END_REF][START_REF] Xie | Magnetic halloysite nanotubes/iron oxide composites for the adsorption of dyes[END_REF]:

%R = (C 0 -C t ) C 0 × 100 ( 1
)
q t = C 0 -C t m × V(2) q e = C 0 -C e m × V (3)
where C 0 , C t and C e are the concentrations of MV 2B initially, at time t and at equilibrium, respectively (mg L -1 );m is the weight of the HNT-Fe 3 O 4 composite (g); andV is the volume of the MV 2B solution (L).

Results and discussion

Preparation and characterization of HNT-Fe 3 O 4 composite

X-ray diffraction (XRD)

The XRD patterns of the HNT and HNT-Fe 99° (0 0 2) can be indexed to HNT [START_REF] Duan | Halloysite nanotube-Fe 3 O 4 composite for removal of methyl violet from aqueous solutions[END_REF][START_REF] Xie | Magnetic halloysite nanotubes/iron oxide composites for the adsorption of dyes[END_REF][START_REF] Lvov | Functional polymer-clay nanotube composites with sustained release of chemical agents[END_REF]. These results indicated that magnetic particles were successfully anchored onto the surface of the clay.

3.1.2Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS)

BET analysis

The adsorption and desorption isotherms of nitrogen gas revealed classical features for both non swelling clay mineral and iron oxides. The BET calculation applied to the nitrogen desorption isotherm gave a total specific surface area for the HNT-Fe 3 O 4 composite of 46 m 2 g -1 , an intermediate value between the starting materials for which their specific surface area was reported to be in the range of 50-60 and almost 9m 2 g -1 for kaolinitehalloysite clay minerals [START_REF] Joussein | Halloysite clay mineralsa review[END_REF]and magnetite particles [START_REF] Salazar-Camacho | Characterization and surface reactivity of natural and synthetic magnetites[END_REF], respectively.

3.1.4pH PZC determination

The pH PZC curve of the HNT-Fe 3 O 4 composite is shown in Fig. 4. The pH PZC is the pH value where positive and negative charges are equal on the surface of a material, which makes it possible to describe the properties of the resulting electrical double layer interfaces [START_REF] Raij | Determinação do ponto de carga zero em solos[END_REF].

For pH values lower than pH PZC , the adsorbent presents a positive surface charge that favors the adsorption of negatively charged compounds, such as anionic dyes. In the situation when the pH is greater than pH PZC , the adsorbent presents a negative surface charge that favors the adsorption of positively charged compounds, such as MV 2B [START_REF] Mall | Characterization and utilization of mesoporous fertilizer plant waste carbon for adsorptive removal of dyes from aqueous solution[END_REF]. Thus, it can be concluded that the adsorption of this dye by HNT-Fe 3 O 4 composite should be favored in media with a pH greater than 5.02.

Effect of experimental conditions on the adsorption process

To evaluatethe effect of the experimental conditions on theadsorptionprocess(e.g., stirring rate,adsorbent dosage, initial pH, initial dye concentrationand contact time), the results were comparedin termsof %R and q t , which were calculatedusing Equations( 1)and (2), respectively. For this calculation, it wasnecessary to know thevalue of the finaldye concentrationat the endof each experiment as obtainedusing acalibration curve(r=0.9997).

Stirring rate

The experiments used to evaluatethe influence of thestirring rateonthe adsorption processwere conductedin aDist multiplestirrerspindle at 25°Cwithan initialweightof 0.15 gof compositein 25mL of 90 mgL -1 of MV2B(pH~4.2)for 6 h.The effect of the stirring rate on the dye removal percentage is shown in Fig. 5. As shown, %R increases as the stirring rate of the system increases, indicating the occurrence of a stronger interaction between the composite and the dye. Furthermore, a higher removal percentage occurs when the stirring rate of the system is equal to 700 rpm. For higher values, the %R begins to decrease; the associated high rate of turbulence in this case might promote the entrainment of the MV 2B particles before they can interact with the composite.

The decrease in percentage removal may also be explained by the fluid dragging HNT-Fe 3 O 4

particles due to the high rate of the system, which also affects the interaction of the composite with the dye [START_REF] Suzuki | Adsorption engineering[END_REF].

Adsorbent dosage

The experiments performedto evaluatethe influence of theadsorbent massonthe adsorption processwere also conductedin aDist multiplestirrerspindle at 25°C with0.025, 0.05, 0.10, 0.15, 0.20 and 0.25 g of composite. A volume of25mL ofMV2Bat a concentration of90 mgL -1 was used,with the stirring rate previously optimizedat apH value of approximately4.2for 6 h.The adsorbent dosage effect on the MV 2B removal percentage is shown in Fig. 6. The results show that %R increases as the adsorbent dosage increases. It was found that the MV 2B removal percentage increased from 18.3% to 94.7% when the adsorbent dosage was increased from 0.025 to 0.25 g. This increase likely occurreddue the increment of the number of adsorption sites available for adsorption [START_REF] Cardoso | Removal of remazol black B textile dye from aqueous solution by adsorption[END_REF]. However, q t shows the opposite behavior of the removal percentage, likely due to the increase in the adsorbent mass at a determined dye concentration;fixed volume also leads to unsaturated adsorption sites during the adsorption process. Additionally,a high adsorbent mass may be favorable to particle aggregation, which would lead a decrease in total surface area of the adsorbent and an increase in diffusional path length [START_REF] Cardoso | Removal of remazol black B textile dye from aqueous solution by adsorption[END_REF].In addition, this reduction of q t can be mathematically explained by combining Equations ( 1) and ( 2), as shown in Equation ( 4):

q t = %RC 0 V 100 m ( 4
)
where %R is the removal percentage (%), C 0 is initial concentration of MV 2B (mg L -1 ), m is the weight of HNT-Fe 3 O 4 composite (g), V is the volume of MV 2B solution (L), and q t is the amount of MV 2B adsorbed at time t (mg g -1 ).

According toEquation (4), the amount of dye adsorbed at a time t (q t ) and the mass of adsorbent (m) are inversely proportional. For fixed values of the dye percentage removal (%R), the treated volume of dye solution (V) and the initial dye concentration (C 0 ), the increase of adsorbent mass leads to a decrease in q t values [START_REF] Calvete | Application of carbon adsorbents prepared from the Brazilian pine-fruit-shell for the removal of Procion Red MX 3B from aqueous solution-Kinetic, equilibrium, and thermodynamic studies[END_REF].

Based on these results, the value of adsorbent mass should be chosen based on the intersection of the curves %R and q t with the justification that this location will produce the most cost-effective system [START_REF] Duan | Halloysite nanotube-Fe 3 O 4 composite for removal of methyl violet from aqueous solutions[END_REF]. However, for the adsorption experiments, an adsorbent amount of 0.15 gwas chosen because this dosage results in a satisfactory removal percentage (i.e., above 85%) and q t values (e.g., 12.18 mg g -1 ), particularly when compared to 0.10 g, which typically yields a removal percentage ofapproximately 60% and a q t of 12.61 mg g -1 .

The use of an amount greater than 0.15 g does not show significant advantages in terms of percent removal with an increase of approximately 5%.

Initial pH

To optimize pH, experiments were performed with a range of values between 3.0 and 11.0 in increments of unity, adjusted by adding HCl or NaOH solutions. The concentration of the MV 2B solution was set to 90 mg L -1 by applying the stirring rate and adsorbent dosage previously determined to be optimal (i.e., 700 rpm and 0.15 g, respectively)for 6 h at 25°C.The effect of the initial pH on the MV 2B removal percentage is shown in Fig. 7. As shown, %R increases as pH increases. In an acidic medium, little interaction occurs between the adsorbent and adsorbate. This behavior is expected because the pH PZC value of the adsorbent is lower than 7.0. Under these conditions, the HNT-Fe 3 O 4 composite is positively charged (i.e., with the same net charge as the dye), resulting in electrostatic repulsion between the composite and the dye. For pH values lower than 3.2, MV 2B begins to change color towards yellow, which ultimately changes the maximum absorption in the visible region. All of these factors result in a non-optimal scenario for adsorption.

In systems with higher pH(e.g., 9.0 to 11.0), there is a large increasein %R,includingvalues near100% at pH11.0. According toDuynsteeandGrunwald [START_REF] Duynstee | Organic Reactions Occurring in or on Micelles. II. Kinetic and Thermodynamic Analysis of the Alkaline Fading of Triphenylmethane Dyes in the Presence of Detergent Salts1[END_REF], in the pH rangebetween9.0and 13.0,a reactionbetweenthe dyeandOH -ions occurs. These ions attack the central carbon atom of the MV 2B molecule, forming a base carbinol with a different molecular geometry.Theresulting compoundhas nocolor,which explainsthe high %Rvalues in thispH range.

Despite thenearly complete removal ofcolor, as shownby the high%R value,the dye still remains in solution. Whenthesecarbinolbases form, the absorption peakshifts tothe UVregion, making it difficult to study theadsorption processofthecomposite because the calibration curveused to quantifythe dyewas constructedusing themaximum absorptionof the dyeat582nm. Finally, theOH -ionscompetewith the adsorbent in the interactionwithMV2B molecules.

Based on these observations, the best conditionsfor adsorption are in the range ofpH valuesbetween 5.0and 9.0; this is the range whereremoval is improvedwithout a loss of colordue tochemical reactions. Because of the differencesin percentageremovalvarybyonly 5%, aneutral pH value was adopted for this study.Furthermore, at this pH value(which is higher than the pH PZC of the HNT-Fe 3 O 4 composite) the adsorbent surface is now completely negatively charged and, as a result, itcan better interact with the MV 2B molecules (positively charged) by electrostatic interactions favoring the adsorption process (Fig. 8).

Effect of initial dye concentration and contact time

To optimizethe initial concentration of dye and contact time on the adsorption process, experiments wereconductedwith 25 mLofMV2Bat concentrations of60, 90,120, 150 and180 mgL -1 over time intervalsof 10, 20, 40, 60, 90, 120, 150, 180, 240 and360min. The stirring rate, adsorbent dosage and initial pH used were those previously optimized at 25°C.

As shown in Fig. 9, q t increases as the initial concentration of MV 2B in solution increases; however, these values tend to be constant after a certain period of time. The increase in q t in the experiments that contained higher initial concentrations of dye can be explained by the existence of a higher concentration gradient of MV 2B, which increases the diffusive contribution of the mass transfer process [START_REF] Duan | Halloysite nanotube-Fe 3 O 4 composite for removal of methyl violet from aqueous solutions[END_REF][START_REF] Liu | Adsorption of methyl violet from aqueous solution by halloysite nanotubes[END_REF].This increasein the values ofq t at equilibriumdoes not follow aproportionality,presentingshorter intervalsas theinitial concentrationof MV2Bincreases, particularlyat higherconcentrations. According toGusmãoet al. [START_REF] Guimarães Gusmão | Application of succinylated sugarcane bagasse as adsorbent to remove methylene blue and gentian violet from aqueous solutions -Kinetic and equilibrium studies[END_REF], this is an indicationthat theremoval percentagedecreases asthe initial concentrationof MV2Bincreasesdue to the greateramount ofdye presentin the bulk solution. The mass of HNT-Fe 3 O 4 composite used (i.e., 0.15 g)islikely not sufficient to promotea highpercentageremovalforhigher concentrations, as discussed in section 3.2.2; this is alsoin accordance withHameed [START_REF] Hameed | Equilibrium and kinetic studies of methyl violet sorption by agricultural waste[END_REF].Theadsorbeddye moleculeslikely exert anelectrostatic repulsiononthose stillin solution, increasing resistance to mass transfer. Regarding the influenceof thecontact time,q t is shown to increase as theprocess evolvesuntil equilibriumis reacheddueto the longer period,so thatthe interaction betweenthe soluteandcompositeoccurs, increasing the amount ofadsorbedMV2B [START_REF] Duan | Halloysite nanotube-Fe 3 O 4 composite for removal of methyl violet from aqueous solutions[END_REF].However, thecurves shown inFig. 9 show thatthe removal isfasterin the initial minutesofthe processbecausemore adsorption sites are available during this period. Asthese sites become progressivelyoccupied, adsorptionbecomesslowerand less efficient [START_REF] Mall | Characterization and utilization of mesoporous fertilizer plant waste carbon for adsorptive removal of dyes from aqueous solution[END_REF]. Another important aspectto note isthat as the initial concentration ofMV2B increases, moretime is required forequilibrium to be reached.

The reasonfor this phenomenon is thata higher concentration ofadsorbatein the solutionincreases theelectrostatic repulsionbetween moleculespresent in the medium, increasing the diffusive resistancetomass transferwithinthesolution but notin the interfaceas previouslydiscussed; this results in a slower process [START_REF] Hameed | Equilibrium and kinetic studies of methyl violet sorption by agricultural waste[END_REF].

22

Adsorption kinetics

To identifythe mechanism ofadsorption, the potentialremoval rateand the controlof the steps involvedin the process,two kineticmodels(i.e., thepseudo-first-and pseudo-secondorder)can be adopted [START_REF] Liu | Adsorption of methyl violet from aqueous solution by halloysite nanotubes[END_REF]. Kinetics experiments were performed at the same conditions as described in the section 3.2.4. The pseudo-first-and pseudo-second-order kinetic models are shown in Fig. 10. Thepseudo-first order kinetic modelcanbe expressedbyEquation(5) [START_REF] Plazinski | Theoretical models of sorption kinetics including a surface reaction mechanism: A review[END_REF]:

ln q e -q t = ln q e -k 1 t( 5)

where k 1 is the pseudo-first-order rate constant (min -1 ). The k 1 and q e parameters can be determined from the slopeandintersection of the linesoflog (q e -q t ) versus t. These values are summarized in Table 1.

The coefficientof correlation (r) for thepseudo-first-ordermodelrangedfrom 0.8734 to 0.9566forinitial concentrationsof MV2Bof60, 90, 120, 150 and 180 mg L -1 .The experimental valuesof q e, (q eexp , mg g -1 ) diverged fromthe calculated values(q e cal , mg g -1 ), as shownin Table 1. This result indicates thatthe adsorption ofthe MV 2B by the compositedoes not follow thepseudo-first-orderkinetic model(i.e., the process is nota phenomenon controlled bydiffusion;othersteps areresponsible for controlling thekinetics observed).Therate constantk 1 can be interpretedas atemporalscalefactor, which is intended toindicate therate at whichthe systemreaches equilibrium.Higher values ofk 1 indicate lesstime to reachthat condition and vice versa. This parametermay or may notbe dependent onthe experimental conditionsof the process [START_REF] Plazinski | Theoretical models of sorption kinetics including a surface reaction mechanism: A review[END_REF].As shown in Table1, the adsorption process does not reachequilibriumin a short periodaccording to the pseudo-first-order model becausethe values ofk 1 are on the orderof10 -2 ,withthe greatest differenceoccurringin the experimentsin which the initialdye concentrationsare 90 and 120mg L -1 .This findingalso indicates that k 1 does not depend on the initial concentration of MV 2B parameter.

The pseudo-second-order kinetic model can be expressed by Equation ( 6) [START_REF] Ho | Pseudo-second order model for sorption processes[END_REF]:

t q t = 1 k 2 q e ² + 1 q e t(6)
where k 2 is the pseudo-second-order rate constant (g mg -1 min -1 ). Analogous to thepseudofirst-orderkinetic model, the k 2 andq e parameters weredetermined from thelinearand angularcoefficients of the equationsformedby regressingt/q t versus t, respectively. Thervalueat different concentrationsof the linearportionsis greater than0.9997, which suggests that the adsorptionoftheMV 2Bfollows thepseudo-second-orderkinetic modelwherein therateof chemicalinteractions between thedye andthe sites ofadsorbent isthe step thatcontrols theoverallkinetics of the process. Furthermore, the calculated data (q e,cal ) are consistent with the experimental data (q e,exp ), as shown in Table 1. Thekinetic constantk 2 has the same physicalmeaningas k 1 and mayor may not alsodepend onthe experimental conditionsof the adsorption process [START_REF] Plazinski | Theoretical models of sorption kinetics including a surface reaction mechanism: A review[END_REF]. In this case, the results shown in Table 1 demonstrate that k 2 does not depend on the initial concentration of MV 2B parameter either. The results ofk 2 are also on theorder of10 -2 , which indicatesthat adsorptiondid notreachequilibriumwithin a short time period, according to the pseudo-second-order model.Unlike whatwaspreviouslyobserved, the value ofk 2 differs slightlyin experiments with lowerMV2B concentrations (C 0 = 60 mg L -1 ) than for the other experiments, which show a valueapproximately 4times higher, indicating thatequilibrium is attained more rapidlyunder these conditions.

Adsorption equilibrium

Adsorption isotherm modelsare fundamentalto describing the interactive behavior between theadsorbateandadsorbent and are also important for investigating mechanisms of adsorption. In this study, equilibrium data were analyzed using the Freundlich and Langmuir isotherms. It is important to emphasize that the equilibrium experiments were performed at the same conditions of adsorption kinetics.

To evaluate which model was best suited to describe these processes, r and the error function(F error ) were analyzed; a lower result from the error function indicated a smaller difference between adsorption capacity calculated by the model (q cal ) and the experimental (q exp ). F error canbe expressedaccording toEquation (7) [START_REF] Lima | Adsorption of Cu(II) on Araucaria angustifolia wastes: Determination of the optimal conditions by statistic design of experiments[END_REF]:

F error = q i cal -q i exp q i exp 2 ( 7
) P i
where q i cal is a value of q predicted by the fitted model;q i exp is a value of q measured experimentally;i indicatesthe values of the initial dye concentration of the experiments (60, 90, 120, 150 and 180 mg L -1 ); and Pis the number of experiments performed.

The Freundlich and Langmuir isotherm linearized models are shown in Fig. 11. The empirical Freundlich equation is applicable to adsorption on heterogeneous surfaces, where the interaction between the adsorbed molecules is not limited to the formation of a monolayer. The Freundlich constant (k F ) is related to the adsorption capacity of the adsorbent: the higher the value, the greater the affinity for the adsorbate. The empirical parameter 1/nis related to the strength of adsorption, which varies with the heterogeneity of the material. When the values of 1/n are between 0.1 and 1.0, the adsorption process is considered favorable [START_REF] Liu | Adsorption of methyl violet from aqueous solution by halloysite nanotubes[END_REF]. The linearized form of the Freundlich equation is expressed according to Equation (8) [START_REF] Foo | Insights into the modeling of adsorption isotherm systems[END_REF]:

ln q e = ln k F + 1 n ln C e (8)
where k F is the Freundlich constant (L g -1 ) and 1/n is a dimensionless empirical parameter.

The k F and 1/n values were determined from the linear and angular coefficients of the equations formed by regressing ln q e as a function of ln C e , respectively. These values, along with r and F error , are shown in Table 2. In general, the experimental data are shown to be well adjusted to the Freundlich model because the r value is greater than 0.98, and F error is near 0.0022.The value of 0.2696 for the parameter 1/n is lower than 1.0, suggesting that the adsorption is favorable under the conditions proposed by the Freundlich model.

The Langmuir model is based on the assumption of monolayer adsorption on a structurally homogeneous adsorbent, where all sorption sites are identical and energetically equivalent [START_REF] Suzuki | Adsorption engineering[END_REF]. The linearform of theLangmuirequation canbe expressedaccording toEquation (9) [START_REF] Foo | Insights into the modeling of adsorption isotherm systems[END_REF]:

C e q e = C e q m + 1 q m k L (9)
where q m is the theoretical maximum adsorption capacity corresponding to monolayer coverage (mg g -1 ), and k L is the Langmuir constant (L mg -1 ). The k L and q m weredetermined from thelinearand angularcoefficients of the equationsformedby regressingC e /q e as a function of C e , respectively.

To determine whether the adsorption process is favorable, a dimensionless constant separation factor R L is defined. The adsorption process is irreversible when R L is 0.0, favorable when R L is between 0.0 and 1.0, linear when R L is equal to 1.0, and unfavorable when R L is greater than 1.0. The R L parameter can be defined based on Equation (10) [START_REF] Foo | Insights into the modeling of adsorption isotherm systems[END_REF]:

R L = 1 1 + k L C 0 ( 10
)
where C 0 is the initial dye concentration (mg L -1 ).

The isotherm parameters for MV 2B adsorption by the HNT-Fe 3 O 4 composite are shown in Table 2. As shown, the experimental data are well adjusted to the Langmuir modelas well. The R L values are between 0.130-0.048, confirming that the adsorption process is also favorable. Although the r value is greater than 0.99, its F error is higher than 0.04 and, thus, higher than the one obtained for the Freundlich model. It can be inferred that analyzing the correlation coefficient of the fitting is not the only parameter that should be account for to verify the best fitting isotherm model. Thus, thesedata suggest that both models were suitably fitted [START_REF] Rangabhashiyam | Relevance of isotherm models in biosorption of pollutants by agricultural byproducts[END_REF]. Similar results were also observed by Lima et al. [START_REF] Lima | Adsorption of Cu(II) on Araucaria angustifolia wastes: Determination of the optimal conditions by statistic design of experiments[END_REF], Namasivayam and Sureshkumar [START_REF] Namasivayam | Modelling Thiocyanate Adsorption onto Surfactant-Modified Coir Pith, an Agricultural Solid 'Waste[END_REF], and Kalavathy and Miranda [START_REF] Kalavathy | Comparison of copper adsorption from aqueous solution using modified and unmodified Hevea brasiliensis saw dust[END_REF]. According to these authors, the adsorption mechanism by both monolayer adsorption and heterogeneous energetic distribution of active sites on the adsorbent surface is possible [START_REF] Namasivayam | Modelling Thiocyanate Adsorption onto Surfactant-Modified Coir Pith, an Agricultural Solid 'Waste[END_REF].

Thermodynamics studies

Thermodynamics experiments were performed at 283, 293 and 303 K at an initial dye concentration of 90 mg L -1 . This initial concentrationwas chosenbecause it provided the best opportunity to monitorthe adsorption process.The other conditions used were the same as those used in the kinetics and equilibrium studies.

The values of the standard Gibbs free energy change (ΔG°) for each temperature were calculated from the Equation(11) [START_REF] Saikia | Malachite nanoparticle: A potent surface for the adsorption of xanthene dyes[END_REF]:

∆G o = -RT ln k c ( 11
)
where R is the universal gas constant (8.314 J mol -1 K -1 ),T is temperature (K) andk c is the equilibrium stability constant, which was calculated at each temperature by the Equation ( 12) [START_REF] Saikia | Malachite nanoparticle: A potent surface for the adsorption of xanthene dyes[END_REF]:

k c = C s C e ( 12
)
where C s and C e are the equilibrium concentrations on the adsorbent and on the aqueous phase, respectively.

The enthalpy change (ΔH°) and entropy change (ΔS°) are calculated from the slope and intercept of the plot of ln k c versus1/T, based on the Van't Hoff equation (Equation 13) [START_REF] Saikia | Malachite nanoparticle: A potent surface for the adsorption of xanthene dyes[END_REF]:

ln k c = ∆S°R - ∆H°R 1 T 13
The results of the thermodynamics experiments are shown in Table 4. The negative values of ΔG°describe the spontaneous nature of the adsorption process. As the temperature increases, ΔG°increase as well, indicating less driving force and hence slower adsorption capacity at higher temperatures [START_REF] Fujiwara | Adsorption of platinum (IV), palladium (II) and gold (III) from aqueous solutions onto l-lysine modified crosslinked chitosan resin[END_REF].

The negative value ofΔH°indicates thatthe adsorption processis exothermic. This slower adsorption capacity at higher temperatures and physisorption) wasalso observed by others researchers thatemployed magnetic composites for removal of dyes [START_REF] Konicki | Equilibrium and kinetic studies on acid dye Acid Red 88 adsorption by magnetic ZnFe 2 O 4 spinel ferrite nanoparticles[END_REF][START_REF] Mahdavinia | Study of adsorption of cationic dye on magnetic kappa-carrageenan/PVA nanocomposite hydrogels[END_REF][START_REF] Zeng | Magnetically separable Ni 0.6 Fe 2.4 O 4 nanoparticles as an effective adsorbent for dye removal: Synthesis and study on the kinetic and thermodynamic behaviors for dye adsorption[END_REF].

Reusability of HNT-Fe 3 O 4 composite

It is crucial to investigate the reusability of adsorbents for practical applications to address ecological and economic demands for sustainability [START_REF] Cao | In situ preparation of magnetic Fe 3 O 4 /chitosan nanoparticles via a novel reduction-precipitation method and their application in adsorption of reactive azo dye[END_REF].Adsorption experiments were performed at a 700 rpm stirring rate anda pH equal to 7.0 with 0.15 g of composite in25mL

ofa 90 mg L -1 solution of MV2B at25°C for 6 h. After each adsorption experiment, the adsorbent was collected by external magnet force(3000 G) andrecovered by a 1.0 mol L - contact time. Finally, it was washed several times with distilled water, dried and reused again.The effect of recycling times of HNT-Fe 3 O 4 composite on MV 2B adsorption performance was repeated four times.

Fig. 12 showed that the removal percentage of MV 2B was still near80% after four cycles, indicating that HNT-Fe 3 O 4 can be reused. The decrease in %Rmight be due to irreversible adsorption of the adsorbent during the regeneration process [START_REF] Cao | In situ preparation of magnetic Fe 3 O 4 /chitosan nanoparticles via a novel reduction-precipitation method and their application in adsorption of reactive azo dye[END_REF].

Conclusion

In general, the method used to prepare the HNT-Fe 3 O 4 composite was efficient, as

shown by the characterization analysis, which verified that the magnetite nanoparticles are anchored onto the surface of the HNT.

The kinetics of the adsorption process wasshown to be better described by a pseudosecond-order model. The experimental results also suggest that both the Langmuir and Freundlich models were suitable to predict adsorption equilibrium.The experimental data concerning the Langmuir constant (k L ), maximum adsorption capacity (q m ),Freundlich constant (k F ), and 1/n were 0.112 mg L -1 , 20.04 mg g -1 , 5.835 L g -1 and 0.2696, respectively.

Moreover, the thermodynamic data have shown that the adsorption of MV 2B onto HNT-Fe 3 O 4 composite is a spontaneous and exothermal process by physisorption.

Finally, the employed composite could also be regenerated at least four times using a 1.0 mol L -1 solution of NaOH as an eluent and was shown to be a promising adsorbentfor the removal ofMV 2B cationic dye.

Fig. 1 .

 1 Fig. 1.Chemical structures of MV (a)2B and (b)10B.

3 O 4 composite are shown in Fig. 2 .

 32 The new diffraction peaks at 35.45° (3 1 1), 43.20° (4 0 0), 57.15° (5 1 1) and 62.75° (4 4 0) can be identified as Fe 3 O 4 , while the other diffraction peaks at 12.29° (0 0 1), 19.78° (0 2 0) and 24.

Fig. 2 .

 2 Fig. 2. XRD patterns of the HNT and HNT-Fe 3 O 4 composite.

Fig. 3shows

 3shows Fig. 3shows the SEM images and EDS spectra for the HNT andthe HNT-Fe 3 O 4

Fig. 3 .

 3 Fig. 3.(a) SEM image of HNT; (b) SEM image of HNT-Fe 3 O 4 composite; (c)EDS spectrum of HNT; and(d) EDS spectrum of HNT-Fe 3 O 4 composite.

Fig. 4 .

 4 Fig. 4.Curve of the point of zero charge (pH PZC ) of the HNT-Fe 3 O 4 composite. Note: the horizontal dotted line shown is used only as a visual guide.

Fig. 5 .

 5 Fig. 5. Effect of the stirring rate on the MV 2B removal percentage.

Fig. 6 .

 6 Fig. 6.Effect of the adsorbent dosage on the MV 2B removal percentage.

Fig. 7 .

 7 Fig. 7.Effect of initial pH on the MV 2B removal percentage.

Fig. 8 .

 8 Fig. 8.Electrostatic interactions between MV 2B molecules and HNT-Fe 3 O 4 composite at pH values higher than the pH PZC of the adsorbent.

Fig. 9 .

 9 Fig. 9.Effect of the initial dye concentration and contact time on the adsorption process.

Fig. 10 .

 10 Fig. 10.Kinetics experimental results: (a) pseudo-first-and (b) pseudo-second-order.

Fig. 11 .

 11 Fig. 11.(a) Freundlich and (b) Langmuir isotherm linearized models.

  magnitudeis related toadsorbateinteractionswith the adsorbentandrearrangement of thesurfaceduring theadsorption process.It is alsoobservedexperimentallyas the fractionof the surface that iscoveredbyadsorbate. Enthalpy change data is useful for distinguishing physisorptionandchemisorption.The typical value for physisorption usually lies below 84 kJ mol -1 , which agrees with the value obtained for MV 2B adsorption onto HNT-Fe 3 O 4 ; for chemisorption, bond strengths lie between 84 and 420 kJ mol -1 . Finally, the results of ΔS° indicate decreased randomness at the solid/solute interface during the adsorption of MV 2B onto the HNT-Fe 3 O 4 composite [60].This behavior (negative values of ΔG°, ΔH° and ΔS°;

Fig. 12 .

 12 Fig. 12. Reusability cycles of HNT-Fe 3 O 4 composite.

Table 1

 1 Kinetics parameters for MV 2B adsorption by HNT-Fe 3 O 4 composite.

			Initial dye concentration (mg L -1 )
		60	90	120	150	180
	q e, exp (mg g -1 )	9.12	12.18	15.05	16.72	18.23
	Pseudo-first-order
	q e, cal (mg g -1 )	1.39	3.65	5.79	4.56	3.81
	k 1 × 10 2 (min -1 )	1.57	1.02	2.09	1.34	1.46
	r	0.8734	0.9021	0.9566	0.9315	0.9079
	Pseudo-second-order
	q e, cal (mg g -1 )	9.16	12.29	15.29	16.69	18.37
	k 2 × 10 2 (g mg -1 min -1 )	5.24	1.11	1.24	1.10	1.55
	r	0.9999	0.9997	0.9999	0.9999	0.9999

Table 2

 2 Isotherm parameters for MV 2B adsorption by HNT-Fe 3 O 4 composite. comparison of the q m of a few adsorbents available in the literature for removal of MV 2B in aqueous solution is given in Table3.It is clear shown that HNT-Fe 3 O 4 composite used in this work had a moderate adsorption capacity, despite of its relatively small surface area.

	Isotherm

A

Table 3

 3 Comparison of maximum adsorption capacity(q m ) of other adsorbents for removal of MV 2B.

	Adsorbent	q m (mg g -1 )	Reference
	Modified cation exchange membrane	10.1	[56]
	Cellulose-based wastes	10.5	[57]
	Mansonia wood sawdust	16.11	[58]
	HNT-Fe 3 O 4 composite	20.04	This study
	Sunflower seed hull	92.59	[48]

Table 4

 4 Thermodynamics parameters for the adsorption of MV 2B onto HNT-Fe 3 O 4 composite.

	Temperature		Thermodynamics
	(K)	ΔG° (kJ mol -1 )	ΔH° (kJ mol -1 )	ΔS° (J mol -1 K -1 )	r
	283	-4.09
	293	-3.75	-13.23	-32.32	0.9744
	303	-3.44

solution of NaOH at

rpm for 3 h. Then, the HNT-Fe 3 O 4 composite was removed from the alkaline solution and placed in a 0.5% v/v acetic acid solution at the same stirring rate and

Acknowledgments

The authors would like to thank CAPES, CNPq and FAPERGS for their financial support.