Adsorption Behavior of Cationic Dye on Bentonite Functionalized With 3-aminopropyltriethoxysilane

The current study focused on the modification of Algerian bentonite clay (Bent) with the product of hydrolysis of 3-aminopropyltriethoxysilane (APTES) using the intercalation process. The modified clay (Bent-APTES) was investigated as an adsorbent solid for methylene blue dye (MB) removal from wastewater.The Bent-APTES was characterized by X-ray diffraction (XRD), chemical analyses, Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The cationic exchange capacity was also determined. Several adsorption parameters were evaluated on the adsorption performance. The results showed that the equilibrium adsorption data was found to fit better to the Langmuir adsorption model, and the adsorption capacity for the removal of MB on Bent-APTES was 217.39 mg g -1 . The kinetic process of adsorption could be described by the pseudo-second order model. Consequently, the modified clay could be served as an efficient adsorbent for cationic dyes in wastewater treatment. the successful intercalation process. These modified clay exhibited great ability for removal of methylene blue (MB) from wastewater. The effects of contact time, initial dye concentration, pH value, and adsorbent mass on the adsorption process were discussed. Sorption of MB onto Bent-APTES was spontaneous monolayer adsorption, which could be well described by pseudo-second order and Langmuir isotherm models. With these findings, this work may promote the development of new absorbents for the removal of dye from wastewater.


Introduction
With the rapid development of human society, harmful dyes are getting one among the most sources of pollution and represent a major environmental concern (mith et al. 2016). The presence of dyes in water, even at low concentrations, is very harmful to living beings (Weng et al. 2007 et al.;Borhade et al. 2017).
Methylene blue, a cationic dye, is one sort of the water-soluble dyes with high chromaticity which is extensively utilized in chemical indicators, dyes, biological stains and medecines. It is quite visible and stable in water at room temperature (Russo et al. 2016). As an synthetic dye, methylene blue is largely widely used in many fields, including paper, food processing, plastics and printing. All of them are closely linked to our life. Therefore, dyes removal from wastewater is a matter of utmost urgency. there are several methods to remove dyes from effluents, such as flocculation, membrane, electrochemical methods, coagulation and adsorption (Mondal et al. 2013;Vergili et al. 2012;Verma et al. 2012). Among different ways, adsorption is an equilibrium separation process that removes dyes from effluents effectively (Elmoubarki et al. 2015;Yagub et al. 2014).
Clay minerals are widespread, low cost and environmentally friendly materials which demonstrated their efficiency in the remediation of the environmental pollution (Belhadri et al. 2019). Current studies have established that the adsorption capability can be improved by modification of adsorbents (Kausar et al. 2018).
Chemical modification of silica surface is an area of intense interest from both fundamental and practical points of view. Bentonite (Bent) is a clay mineral of group having silica tetrahedral sheets layered between alumina octahedral sheets. Due to isomorphic substitution within the layers (for example, Al 3+ replaced by Mg 2+ or Fe 2+ in the octahedral sheet; Si 4+ replaced by Al 3+ in the tetrahedral sheet), the clay layer is negatively charged which is counterbalanced by the cations located in the interlayer space. The hydration of inorganic cations on the exchange sites causes its surface to be hydrophilic (Shen et al. 2007). It is frequently exploited to get nanocomposites and environmental materials due to its aspect ratio (Zhang et al. 2006;Ianchis et al. 2012;Huskić et al. 2013), capacity for swelling (He et al. 2005) and ion exchange, softness, high solvent resistance, good mechanical strength and high capacity for delamination (Bertuoli et al. 2014).
Silylation was proven an efficient method to modify the surfaces of clay minerals (Su et al 2012;Zhang et al. 2006, Ruiz-Hitzky andFripiat 1976;Tao et al. 2011).
The linkage between organic components and clay via covalent bonds enables a durable immobilization of the reactive organic groups, avoiding their leaching in the surrounding medium when the silane grafted clays are used in solutions [ Su et al 2012;Waddell et al. 1981].
This paper reports changes in the structure of an Algerian bentonite clay which has been intercalated with a 3-aminopropyltriethoxysilane, to our knowledge this clay (Algerian bentonite) has never been modified by this molecule.
X-ray diffraction (XRD), chemical analysis, thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy were employed to characterize the modified clay minerals. The objective of the research is to see the behavior of this clay by the modification by silylation procedure and its application in the adsorption of MB from aqueous solutions. The effect of various operational parameters such as the initial MB dye concentration, pH, temperature and the adsorbent dosage on adsorption was studied. Different isotherm models were investigated for fitting the adsorption equilibrium data, and also the kinetics of adsorption process were also studied.

Materials
All the chemicals and reagents used were of analytical grade. Algerian clay sample (Bent) used in this study was taken from a mine in Magnia region, northeastern Algeria. The silane used in this research is 3aminopropyltriethoxysilane (APTES). The chemical formula of APTES is NH2 -CH2 -CH2 -CH2 -Si (OCH3)3.

Characterization methods
Powder X-ray diffraction (XRD) patterns of the clays were obtained using a Bruker AXS D-8 diffractometer with Cu Kα radiation in the 2θ range from 5° to 70°.The morphological features of clays were investigated using a scanning electron microscope (PHILIPS XL30) with an accelerating voltage 20kV. The Fourier transform infrared (FTIR) spectra were obtained by the KBr method using a FT/IR-4200 type FTIR spectrometer. The chemical composition of the samples was performed by X-ray fluorescence using a Philips PW2540 Magix spectrophotometer yielding the percentage of each constituting element in weight. The thermal analysis (TG-DTA) was performed in air using a start Pyris DTA-TGA analyzer. All the samples were heated to 1000°C at heating rate of 5°C min -1 . For aqueous solution analysis, a Hanna 210 microprocessor pH meter was used for pH value measurements.

Preparation of the pillared clay
For preparing pillared clay (Bent-APTES), clay was sedimented to eliminate the fraction superior to 2µm.
The obtained material is named Bent.
The preparation of pillaring solutions is as follows: First, 20 g of 3-aminopropyltrimethoxysilane and 60 ml of methanol are mixed with stirring at room temperature. Next, 6 ml of HCl (16%) are added dropwise. After this, the solution is stirred for 27 h. 300 ml of acetone are added and the solution heated to reflux for 1 h. Finally, cooled to room temperature while stirring for 3 h. The final pH was 9.7. This solution is stored in dark for 2 hours. In fact, this treatment leads to a condensation of the silanes into polysiloxanes.
The intercalated clay was obtained by adding dropwise the amount of pillaring solution (4mL / min) to the sedimented clay in suspension (5 g in 1670 mL of deionized water) (Fetter et al. 1994). The modified clay was centrifuged and washed with deionised water (2L/g) and dried in thin layers in air. The sample was reference Bent-APTES (scheme 1).

Determination of cation-exchange capacity (CEC)
The cation exchange capacity (CEC) was determined with the conductimetric method described in previous work (Peech et al. 1961;Chiu et al. 1990).

Adsorbate and other chemicals
Analytical grade of MB dye was selected as the model organic cationic dye in this study. The molecular formula of MB dye is C16H18ClN3S with the molecular weight of 319.851 g/mol.
A stock solution of dye having a concentration of 1000 mg/L was prepared by dissolving appropriate amount of MB dye powder in 1 L deionized water, and then it was diluted to the desired concentration of each experiment. The UV/VIS (V-670) spectrophotometer was used to measure the concentration of MB dye in solution at wave length of λmax = 665 nm.
The pH solution was adjusted using reagent grade HCl and NaOH.

Adsorption experiments
All the adsorption experiments were done in triplicate under similar conditions. The experimental variables examined were contact time, bentonite concentration in suspension, pH of suspension and MB concentration. The experiments were carried out as follows: a 0.5 g of dry adsorbent Bent-APTES was added to 500 ml of aqueous MB solution of specific concentration and then stirred at room temperature at different time intervals (0 -160 min). The resultant solution was centrifuged and the residual MB concentration was measured by a UV spectrophotometer to determine the amount of MB uptake. In this study, the adsorption experiments were performed in the initial pH range between 2 and 9, in these cases, pH of the solution was adjusted to the desired value by adding HCl or NaOH as required. The effect of adsorbent dosage on MB adsorption was studied by increasing the amounts of adsorbent from 0.25 g to 4.0 g. To establish the adsorption isotherms, this study was carried out with different initial MB concentrations varying from 5 mg/L to 340 mg/L.
The adsorbent MB uptake, per unit mass of clay at time t, qt (mg/g) was evaluated by the following equations: where qt and (mg/g) are adsorption amount of the dye at time t and equilibrium, C0, Ct and (mg L -1 ) are the concentration of MB solution at initial, time t and equilibrium, respectively, V (L) is the volume of MB solution, m (g) corresponds to dosage of adsorbent.

Result and discussion
Characterization of the adsorbent

Composition of clays
The chemical analysis (Table 1) shows an increase in the silicon and oxygen content and the complete disappearance of calcium, sodium and potassium for the Bent-APTES intercalated clay. These results confirm the operation of the exchange of cations (Na + , Ca 2 +, K + ) by the organosilicon oligomer species.

X-ray diffraction (XRD)
The X-ray diffraction pattern of the clay and the modified clay are reported in Fig. 1. The basal spacing for natural clay Bent is 1.56 nm. After silylation, the value increases to 1.95 nm, indicating that silane has entered the Bent galleries (Fetter et al. 1994, Jlassi et al. 2017Su et al. 2012) However, the new reflection around 8° (2θ) is attributed to the (002) reflection of the silylated product, similar to those of the cationic surfactant modified montmorillonite (Su et al. 2012;He et al. 2004). According to previous works (Chen et al. 2013), the height of the interlayer space can be estimated with the basal spacing and the thickness of the sheet of phyllosilicate with value of 0.96 nm. The gallery height of the bentonite was estimated, from the observed basal spacing, to be 0.96 nm and by calculating the interlayer space height for modified clay (0.99 nm). The value is large enough to accommodate the 3aminopropyltrimethoxysilane molecule. In addition, the full width at half maximum (FWHM) of the (d001) peak has declined by a slight way after silylation for modified bentonite Bent-APTMS (FWHM= 0.503) compared to natural bentonite (FWHM= 0.349). This could be interpreted a slight decrease in the ordered. However, despite decrease in the ordered nature the structure of bentonite was preserved.
As indicated by the XRD analysis of modified bentonite, APTMS pillaring on the bentonite surface was successfully achieved.

Cation-exchange capacity (CEC)
The material Bent-APTES has a CEC of the order of 31.03 meq/100g of clay; it is significantly lower than that of the starting clay (89.86 meq/100g). This decrease can be explained by the fact that the organosilicon oligomer initially fixed as a compensating cation occupies the exchange sites of the interlamellar space of the clay. As indicated by the XRD, FTIR, TGA and SEM analysis of modified bentonite, APTES pillaring on the bentonite surface was successfully achieved.

Adsorption Kinetic study of the adsorption
Kinetic process gives fundamental knowledge about the interaction adsorbent-adsorbate. The kinetic adsorption evaluation of BM onto Bent-APTES (Fig. 4) was performed to establish its optimal time for adsorption. For MB, the adsorption is very rapid in the first 5 min and afterward, a very slow increase in the MB uptake is observed up to 20 min. The rapid initial increase in the amount of adsorbed dye may be due to an increase in the number of vacant sites available at the beginning Fig. 4 The kinetic of MB removal onto Bent-APTES adsorbent The first-order and the pseudo-second-order kinetic models were used to elucidate the adsorption mechanism.
where and (mg/g) are the amounts of adsorbate adsorbed at equilibrium and at time t, respectively, and 1 (min −1 ) is the rate constant of the first-order model.

Bent-APTES
The experimental data were fitted to pseudo-first-order (Fig. 5a) and pseudo second-order kinetic models ( Fig. 5b) in order to investigate the mechanism of adsorption for the adsorptive removal of MB.
The kinetic parameters for the adsorption of dye on the clay minerals were calculated from the corresponding plots (Table 2). Correlation coefficients of the pseudo second-order kinetic model was relatively greater than that of the pseudo first order kinetic model, The calculated qe (69.93 mg g −1 ) was close to the experimental qe (69.05 mg g −1 ), implying that the MB adsorption can be described more appropriately by the pseudo-second-order model. This result is in accordance with (Al-Futaisi et al. 2007), evidencing that the adsorption process occurred by a chemisorption mechanism. The fast removal of the dyes from solution, reaching the equilibrium in a rather short time, gave evidence that functionalized clay was efficient adsorbent.
of adsorbate adsorbed at equilibrium and at time t, respectively.

Effect of pH on MB Absorption
The effect of initial pH on the adsorption capacity (qt) of MB onto Bent-APTES was examined at different levels (pH 3-11) by fixing other parameters, as shown in Fig. 6a. However, as illustrated in Fig (Narine et al. 1981) have reported that the adsorption capacity of bentonite was unchanged over the pH range 4.5-8.5. On the other hand, Similar tendencies were also reported for the cationic dye adsorption by montmorillonite, palygorskite, bentonite and sepiolite (Roulia et al. 2008;Al-Futaisi et al. 2007;Bilgiç et al. 2005).

Effect of Adsorbent Dosage on MB Absorption
To obtain a cost-optimal adsorption of dyes and heavy metal species from wastewater, the minimum adsorbent dosage that ensures efficient removal of contaminants is usually determined. The influence of different Bent-APTES dosages on MB removal within 30 min of contact time is illustrated in Fig. 6b.
According to Fig. 6b, with an increase in the adsorbent dosage from 0.5 to 1 g L −1 , the elimination of MB increases. This finding may be related to active adsorption sites, which facilitate and prompt the adsorption reaction. Though higher Bent-APTES dosages provided a high efficiency of MB removal, the successive addition of the Bent-APTES dosage to above 1g L -1 did not promote the MB elimination significantly. Therefore, 1 g L −1 Bent-APTES could be selected as the optimum.

Isotherm models
The sorption behaviors of MB onto Bent-APTES adsorbent were investigated. In this study, two isotherm models as the most widely used models were tested to fit the data including Freundlich (Freundlich 1907) and Langmuir (Langmuir 1918). Langmuir model assumes that a reversible sorption process occurs on a homogenous surface forming a monolayer sorption without interactions between the adsorbed specie onto the adsorbate. The Langmuir isotherm adsorption equation is expressed as eq. 5.
where qe (mg/g) is the quantity of metal ions (mg) adsorbed per gram of solid, from the aqueous solution; (mg/L) is the equilibrium concentration of metal ions in aqueous solution; KL is the Langmuir equilibrium constant and qm (mg/g) is the maximum adsorption capacity. Both of them are calculated from the slope and the intercept of Langmuir linearized plot (Fig.7a), ⁄ vs. .
Freundlich model assumes that the surface has active sites with different affinity for the adsorbate. The sites with higher affinity would be covered firstly. The linear form of the Freundlich model is given by eq 6: log = 1 log + log (6) where (mg/g) is the quantity of metal ions (mg) adsorbed, per gram of solid, from the aqueous solutions.
(mg/l) is the equilibrium concentration of metal ions in aqueous solution. n and KF are the Freundlich constants. Both are calculated from the linear plot of log vs. log (Fig. 7b).
The calculated parameters for two isotherm models are listed in

Comparison of Bent-APTES with other adsorbents
Adsorption capacities of various adsorbents towards methylene blue dye as reported in literature were presented in Table 4.
A comparison between this work and other reported data from the literature shows that bentonite functionalized with 3-aminopropyltriethoxysilane is a better adsorbent for methylene blue compared to other adsorbents. Therefore, it could be safely concluded that the Bent-APTES adsorbent has a considerable potential for the removal of methylene blue from an aqueous solution.

Conclusion
In this study, 3-aminopropyltriethoxysilane was used to functionalize bentonite clay in order to remove methylene blue from aqueous solutions. The characterization data indicated remarkable differences between functionalized and non-functionalized clay highlighting the successful intercalation process.
These modified clay exhibited great ability for removal of methylene blue (MB) from wastewater. The effects of contact time, initial dye concentration, pH value, and adsorbent mass on the adsorption process were discussed. Sorption of MB onto Bent-APTES was spontaneous monolayer adsorption, which could be well described by pseudo-second order and Langmuir isotherm models. With these findings, this work may promote the development of new absorbents for the removal of dye from wastewater.