Effect of the surfactant loaded amount into the bentonite on the elimination of two anionic dyes: Adsorption, Kinetics, and Isotherm studies

The present work focused on the preparation of hybrid-bentonite adsorbents by treating sodium bentonite (Na-B) with different amounts of hexadecyltrimethylammonium bromide (HDTMA + Br - ), corresponding to 50, 100 and 200% the value of the cation exchange capacity (CEC). The samples prepared, namely 50HDTMA-B, 100HDTMA and 200HDTMA-B, were characterized by different techniques including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA ). The removal efficiency of two anionic dyes, namely. Congo Red (CR) and Indigo Carmine (IC), from aqueous solutions, was studied as a function of the amount of HDTMA, contact time, hybrid adsorbent and initial dye concentration. In addition, a kinetic study was conducted to fit the experimental data in order to compare the adsorption rate constants and the equilibrium sorption capacities of anionic dyes on the prepared materials. The results obtained indicated that the adsorption kinetics of clay samples could be best described by the pseudo-second-order model, and the adsorption capacity of the hybrid-Bentonite is affected by the amount of modifier used. Based on Weber-Morris intra-particle diffusion equation, it was found that the CR and IC dyes diffused faster as the cationic surfactant loading increased from 0 to 100% of the CEC value. The Langmuir isotherm exhibited the best fit, with an adsorption capacity equal to 292 and 289 mg g -1 , for CR and IC dyes, respectively.


Introduction
Textile dyeing is a process that has been practiced since the early days of mankind. At the beginning, only natural dyes were used, but afterwards, they were progressively replaced by synthetic dyes [1] The presence of dyestuffs in effluents discharged from factories does pose serious threats to the environment when they are released in the biosphere without prior treatment. Dyestuffs are dangerous pollutants that have serious effects on the aquatic flora and fauna, but also on the human and animal populations that consume untreated water as well. [2] Many synthetic coloring dyes can be seriously harmful to public health and may engender allergies in humans; they can also affect the aquatic life and even the food chain. [3]. Congo red (CR) and indigo carmine (IC) have extensively been applied in textile dyeing processes due to their technical simplicity, low purchase and production costs, ease of application, durability and also because they are available in various colors. [4,5] These two dyes (CR and IC) are classified as azo dyes and vat dyes, respectively; they both carry negative charges on their molecules. Furthermore, it has widely been reported that the discharge of CR and IC into the environment may lead to skin and eye irritation, injuries to the cornea and conjunctiva, dermatitis and cancer for humans. [6,7] For this reason, an effective treatment is urgently required for the purpose of eliminating these harmful substances from the environment. Indeed, several techniques, such as coagulation and flocculation, reverse osmosis, oxidation or chemical reduction, adsorption, photocatalytic degradation, etc., have been employed in the removal of dyes from aqueous systems. [8,9] The adsorption technique proved to be effective and relatively cheap for the elimination of dyes [10] because this process is highly effective in purifying contaminated water. The use of clays as adsorbents is actually of great interest in the treatment of industrial wastewater because they present high surface area, with a negative charge, high cation exchange capacity and wide availability in nature.
Nowadays, it is widely admitted that natural clays are poor adsorbents of anionic dyes. For this reason, it was decided to modify these natural clays by means of surfactants, by introducing an organic phase on the clay interlayer surface that acts as a partitioning medium for the anionic contaminants. Recent works have suggested that chemically modified clays represent a novel and promising class of adsorbent materials. Such modification may consist of replacing the compensating ions by alkyl ammonium ions. The clay surface is thus converted from an initially hydrophilic state to a hydrophobic state, which makes it possible to give rise to strong interactions with organic molecules either in the vapor state or dissolved in water. The adsorption of anionic dyes on organo-clays has been extensively investigated. [11,12] The adsorption capacity and adsorption process depend on the molecular structure, amount of modifier or ammonium cations, chemical properties of organic compounds, and surface area of clay mineral. [13][14][15]. Few studies are available on the removal of CR and IC dyes by hybridclays. In this context, Dazhong Shen et al. [16] were able to remove 95% IC from water using polydiallydimethylammonium modified Bentonite (PDADMA-Bentonite) at a dosage of 0.75 g L −1 . As for Chenglong Xia et al. [17], they reported that the adsorption capacities of hectorites modified with cetyltrimethyl ammonium bromide (CTAB) and octadecylamine, for CR dye, were greatly improved compared to natural hectorite. In addition, Li Wang et al. [18] indicated that the synergistic effects of several factors, such as the changes in the crystalline structure, specific surface area and average pore size, may help to increase the adsorption capacity of cethyltrimethylammonium-modified montmorillonite (CTAB-MMT) when appropriate amounts of CTAB are used (2.0 CEC) in the case of CR dye elimination. The present study aimed at preparing hybrid-clays through the modification of Bentonite clay using cationic surfactants. Its main purpose consisted in highlighting the remarkable potential of hexadecyltrimethylammonium-exchanged Bentonite for the removal of anionic dyes from aqueous solutions. Indeed, two dyes, i.e. an azo dye CR and a vat dye IC, were chosen as target pollutants for the purpose of investigating and understanding the adsorption behavior in terms of adsorption kinetics and equilibrium isotherms. The effects of various parameters, like the amount of surfactant, contact time, adsorbent dosage and initial dyes concentration, were investigated for the removal of dyes from aqueous solutions.

Preparation and characteristics of Bentonite
The natural Bentonite (B) used in this study was acquired from the mining site of Maghnia, a small town in the extreme northwestern region of Algeria. Afterwards, it was purified and then converted to its sodic form, namely sodium Bentonite (Na-B), according to the method published in a previous study. [19] The chemical compositions of B and Na-B are shown in Table 1. Table 1.

Insert
Furthermore, the specific surface area was determined from the N2 gas adsorption isotherm at 77 K using a Volumetric Analyzer (Nova-1000). Moreover, the cation exchange capacity (CEC) of clay samples was calculated by means of the methylene blue adsorption method [20] and was found to be 37 meq/100 g for dry natural Bentonite and 93 meq/100 g for dry sodic Bentonite. The results obtained indicated that the BET specific surface area increased from 45.04 m 2 g -1 in natural Bentonite (B) to 66.21 m 2 g -1 in sodium Bentonite (Na-B).

Reagents
The surfactant hexadecyltrimenthylammonium bromide, denoted as HDTMA + Br -(C19 H42 Br N), and the anionic dyes used in this study, with purity > 99%, were purchased from Biochem.
The chemical structures and properties of these dyes are clearly depicted in Table 2. Table 2.

Preparation and characterization of hybrid-Bentonites
HDTMA-intercalated Bentonite samples were prepared by intercalation of the ammonium cation into the Na-B suspension (5% clay in water (w/w)), based on a previously described procedure [21] which consists of using amounts of HDTMA corresponding to 50, 100 and 200% of the cation exchange capacity of Na-B; these amounts were next dissolved in 500 mL of distilled water and the resulting mixtures were then added to 25g Na-B. The admixture was first agitated for 24 h at room temperature and then centrifuged at 3000 rpm for 15 min.
Afterwards, the suspension obtained was filtered, washed several times with distilled water Thermogravimetric analyses (TGA). The infrared spectra of clay samples were recorded within the wavenumber range extending from 500 to 4000 cm -1 using a Perkin Elmer FT-IR Spectrophotometer. Moreover, the X-ray diffraction measurements were performed on a Philips X-Pert diffractometer using Ni-filtered Cu Ka radiation.

Adsorption experiments
The adsorption study was conducted on synthetic wastewater containing Congo red or indigo carmine dyes. An aqueous solution was prepared by dissolving dyes in deionizer water to the desired concentration. With regard to the kinetic study, each experiment was carried out in a 100 mL Erlenmeyer flask containing 25 mL of anionic dye solution (50 mg L -1 ) and 25 mg of adsorbent at natural pH 6.6; the mixture was then subjected to stirring at the speed of 250 rpm.
Afterwards, the samples were withdrawn from time to time, up to 180 min, and aliquots were taken from theses samples and centrifuged at 3000 rpm. The concentration of dye in the supernatant was quantified by the Analytik Jena (SPECORD210) UV-vis spectrophotometer at λmax and was subtracted from the initial concentration in order to determine the amounts of dye adsorbed in each case. The amount adsorbed per mass unit of adsorbent at time t (qt), the amount adsorbed per mass unit of adsorbent at equilibrium (qe) and the adsorption yield (Y) were calculated using the following expressions: Note that C0 and Ce are the initial and equilibrium dye concentrations (mg L -1 ), respectively; Ct is the concentration of dye within the aqueous solutions at time t (mg L -1 ), qt is the amount of dye adsorbed per mass unit of adsorbent at time t (mg g -1 ), V is the volume of the aqueous solution (L) and m is the amount of adsorbent (g). Furthermore, the adsorption isotherms were

Characterization of adsorbents
The FTIR spectra of the adsorbent samples are illustrated in Figure 1  Insert Figure 2.
The XRD patterns of sodic Bentonite and HDTMA-modified Bentonite are clearly illustrated in Figure 3. The interlayer distance (d001) was calculated from the measured diffraction angle (2θ) value using Bragg's equation (inset in Figure 3). Insert Figure 3.

Adsorption kinetics
The use of sodic Bentonite and synthesized hybrid-Bentonite in the adsorption of anionic dyes CR and IC taken as model pollutants was investigated. Figure   98.7 and 80.5%, respectively. This is mainly due to the fact that increasing the amount of HDTMA causes them to densely pack between the layers. Therefore, it may be said that the CR and IC dyes cannot penetrate into the interlayer space with high packing density, which leads to lower removal efficiency.
[30] Comparison of CR with IC dyes suggests that the removal efficiency for CR is significantly larger than that for IC under the same modifier loadings.
In order to understand the dynamics of adsorption, the experimental data of the kinetic study were examined using the pseudo-first-order and the pseudo-second-order adsorption models.
The pseudo-first-order kinetic model, also described by the Lagergren kinetic equation, is widely employed to understand the kinetic behavior of the system.
The pseudo-second-order kinetic model is given by the equation [33]: Where qe and qt (mg g -1 ) are the amounts of dye sorbed at equilibrium and at time t, respectively; k1 and k2 are the equilibrium rate constants of the pseudo-first-order model (min -1 ) and the pseudo-second-order model (g mg -1 min -1 ), respectively.
The fitting curves of the kinetic model to the experimental data are shown in Figure 6, and the results are summarized in Table 3 (the pseudo-first-order fitting plots are not presented here).
It can clearly be seen that the correlation coefficients ( Furthermore, the amounts of CR and IC dyes removed by different types of adsorbents at equilibrium (qe), according to the pseudo-second-order model, are summarized in Table 4. Note that at 50 mg L -1 of initial dye concentration, 49.3 mg g -1 of CR and 45.5 mg g -1 of IC were removed by the adsorbent 100HDTMA-B, when the ratio (m/v) is equal to unity and pH = 6.6.
In comparison with other materials, the amounts of anionic dyes adsorbed onto the adsorbents considered in this study at equilibrium are lower than those sorbed on nanocomposites, but are higher than those removed by cethyltrimethylammonium bromide-modified kaolin, surfactant modified pumice, metal hydroxides sludge and CaCO3-loaded cellulose aerogel. This can certainly be attributed to the fact that the adsorbents used in this paper have a larger adsorption capacity than many reported adsorbents.
In this case, the CR and IC molecules were probably transported by the adsorbent particles following the intra-particle diffusion process because of the porosity of organo-clays. For the purpose of explaining the adsorption mechanism of both CR and IC dyes, it was decided to use the Weber and Morris equation given below [40]: This equation originates from Fick's second law. Note that kid is the intra-particle diffusion rate constant (mg g -1 min -0.5 ) and I is the intercept. The values recorded for I help to make an approximation of the boundary layer thickness. It is worth noting that the larger the value of I, the greater the boundary layer effect. In addition, the deviation of the straight line from the origin may be attributed to the weight transfer difference between the initial and final adsorption stages.
[41] The intra-particle diffusion model suggests that the plot of qt versus t 0.5 should be linear. In theory, these types of plots may present a multilinearity, indicating that two or more stages are taking place [42]. Figure 7 displays the amount of anionic dyes adsorbed per unit mass of adsorbent, qt versus t 0.5 for the adsorption of CR and IC dyes. Since the graph does not pass through the origin, then it may be concluded there is some degree of boundary layer control. Therefore, the intra-particle diffusion is not the only rate controlling step, but other processes can also control the rate of adsorption of anionic dyes. [43] Indeed, the curves show that two steps occur during the adsorption process. The first stage is a gradual adsorption, where the diffusion of dye molecules on the surface of the adsorbents can be rate-controlling. Final equilibrium is reached during the second stage. A linear equation was fitted to the experimental data obtained in the first 20 minutes. The values obtained for kid, I, and R 2 are given in Table   3, which indicates that the adsorption rate constant kid onto HDTMA-modified Bentonite was far larger than that onto Na-B. Therefore, the diffusion of CR or IC dyes had to be faster with the increase in cationic surfactant loading from 0 to 100% CEC, which suggests that the adsorption of anionic dyes is faster with increased adsorbent organo-philicity [44] However, dye diffusion was slower in the case of 200HDTMA-B. In addition, the diffusion rate for CR dye at the surface of the material or across the inter-lamellar space was faster than for the IC dye.

Effect of adsorbent dosage (solid/solution ratio)
The effect of the solid/solution ratio on the adsorption efficiency of CR and IC dyes both on Na-B and 100HDTMA-B is shown in Figure 8. It can be observed that the percentage removal of CR dye increased from 0.8 to 21.9 % and from 53.4 to 99.1% when the dosages of Na-B and 100HDTMA-B were increased from 0.1 to 2 g L -1 , respectively, as can be seen in Figure 8a.
These results clearly demonstrate the improved CR adsorption efficiency of alkyl ammonium surfactant-modified Bentonite. Therefore, it may be stated that 1gL -1 of surfactant-modified Bentonite was the optimum adsorbent dosage which represents the beginning of the saturation process, because higher amounts of adsorbent did not increase appreciably the CR adsorption. Figure 8b suggests that the percentage removal of IC dye increased from 14 to 94% when the HDTMA-B dosage augmented from 0.1 g L -1 to 0.75 g L -1 , respectively. Beyond this dosage value, the adsorption efficiency dropped to 91 % and remained unchangeable. As for Na-B, low yields were obtained; the maximum was observed for a solid/solution ratio of 0.1 g L -1 (11.7 %). Afterwards, this yield went down to 5.9 % when the adsorbent dosage increased to 2 g L -1 .
The increase in the anionic dye removal indicates that the number of sorption sites on the adsorbent surface increased; this also suggests that more active binding sites become widely available as the adsorbent dosage grows. [45] These findings are similar to those previously reported by other authors [[46]] who indicated that at a higher adsorbent dosage, a portion of the solid surface remains uncovered due to the aggregation of the adsorbent particles.
Insert Figure 8. Figure 9 indicates that any increase in the initial concentration of the organic pollutant, within the range from 20 to 500 mg L -1 , induces an increase in the adsorption capacity from 19.7 to 273.1 mg g -1 for CR dye and from 14.2 to 240 mg g -1 for IC dye, respectively. The rise in the adsorption capacity as the initial concentration goes up can be explained by the growing driving force of the anionic dye to the active sites on the adsorbent. These findings are qualitatively in good agreement with those reported by Manjot et al. [47] and Gondzalez-Pradas et al. [48]. On the other hand, some histograms suggest that increasing the initial IC concentration (Fig.9 b) leads to higher adsorption efficiency to ultimately reach a maximum of 96% at 100 mg L -1 ;

Effect of initial concentration
beyond this value, the removal efficiency gradually decreases. In addition, Figure 9a clearly shows that the yield is highest (99%) for initial CR concentrations ranging between 20 and 100 mg L -1 . Beyond this concentration, the removal efficiency gradually decreases to reach a percentage yield of 54.5% at an initial concentration of 500 mg L -1 .

Adsorption isotherm study
The adsorption isotherms were used to understand the adsorption interaction and the design where the constants KF, n are for the Freundlich model and b for the Langmuir model; qm (mg g -1 ) is the maximum adsorption capacity for monolayer surface coverage.
The experimental data for the CR and IC adsorption isotherms are presented in Figure 10.
According to Giles classification, [51] the HDTMA-hybrid Bentonite exhibits an L-type isotherm, corresponding to the formation of a dye monolayer on the surface of the adsorbent.
The fitted curves of Freundlich and Langmuir models are depicted in Figure 11. The parameters predicted by the two different models are summarized in Table 5. In general, these parameters exhibited a good fit to the linear adjustment, but the correlation coefficient was better adjusted using the Langmuir model. Figure 11.
The favorability and feasibility of the adsorption process can be determined by the dimensionless equilibrium parameter or adsorption intensity of Hall (RL) in the analysis of data using the Langmuir isotherm. This parameter is given by the following equation [52]: The quantity RL indicates whether the isotherm is irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1). Close examination of the graph obtained by plotting RL vs. C0, at 20°C (inset in Fig. 10), shows that the RL values are within the range from 0 and 1, and approached the value zero as C0 increased, suggesting the favorable adsorption of CR and IC dyes onto HDTMA-modified Bentonite.
The maximum adsorption capacities (qm) for the sorption of CR and IC onto HDTMA-modified Bentonite were found equal to 292.4 and 289 mg g −1 , respectively. Note that these values are significantly higher than those of other adsorbents. For example, this value was equal to 71.82 and 55.5 mg g -1 for IC dye for chitosan [53] and mesoporous LDH nanoparticles [54], respectively. In the case of CR, Ghorai et al. [55] and Sagar et al. [56] reported the values of 209.20 mg g -1 for Xanthan gum-graft-poly (acrylamide)/SiO2 nanocomposite and 233.24 mg g -1 for g-GG/SiO2 nanocomposite, respectively.