Removal of Reactive Blue 13 from Dyeing Wastewater by Self

Article pubs.acs.org/jced

Removal of Reactive Blue 13 from Dyeing Wastewater by SelfAssembled Organobentonite in a One-Step Process Min Yao, Xingwang Zhang, and Lecheng Lei* Industrial Ecology and Environment Research Institute, Department of Chemical and Biological Engineering, Yuquan Campus, Zhejiang University, Hangzhou, Zhejiang 310027, China ABSTRACT: A bentonite/surfactant one-step process including the self-assembly of organobentonite and removal of pollutants simultaneously has been proposed as a potential superadsorption process for organic pollutant removal, due to its low operation cost and simplified process compared with a traditional organobentonite process. Here, the selfassembly of organobentonite was optimized, and then the optimized process was applied to remove anionic dye (reactive blue 13) from dyeing wastewater. Results showed high removal efficiencies (more than 93 %) of reactive blue 13 in this one-step process. The functions of cationic surfactant were: (i) interaction with dye anions during the initial reaction; and (ii) self-assembly of organobentonite and then applications to remove anionic dyes as adsorbents. The adsorption data of RB13 could be described by Langmuir isotherm model very well, and the adsorption capacity of selfassembled organobentonite was 50 % higher than that of traditional organobentonite. X-ray diffraction (XRD) and Brunauer− Emmett−Teller (BET) analysis were used to elucidate the adsorption mechanism. Results indicated that self-assembled organobentonite had a larger basal spacing than traditional organobentonite, and CTMA+ and dye were adsorbed on the external surface or intercalated into the interlayer of bentonite.

1. INTRODUCTION Organic dyes are important products in the fine chemical industry, widely used in textile, leather, papermaking, paint, plastic, and even the food and drink industry. During the process of dye producing and application, a large amount of wastewater produced inevitably is highly colored, of low BOD (biochemical oxygen demand) and high COD (chemical oxygen demand). Some azo dyes, especially many reactive dyes and reaction products, are reported to cause allergies, irritation, dermatitis, cancer, and mutation to humans.1,2 Additionally, due to their high water solubility, it is predicted that there are about (10 to 20) % of reactive dyes lost in wastewater during the production process and nearly 50 % entering the effluents during dyeing or other application processes.3 Thus, it is very important to remove these dyes from wastewater before they are discharged into the unpolluted natural water bodies. The most commonly used processes for dye removal are adsorption, flocculation, oxidation, microbial degradation, and so forth. The adsorption process is considered as a relatively efficient method for the removal of dyes and other organic pollutants. Natural clay minerals such as bentonite have been chosen as low-cost and high-efficiency adsorbents for the removal of various dyes and organic and inorganic pollutants.4−6 Bentonite is a predominantly montmorillonite clay, which has been increasingly gaining attention as a promising adsorbent because it is inexpensive and readily available and its sheet-like structures also provide high specific surface area and cation exchange performance. Although bentonite does well in the adsorption of cationic dyes, it fails in the adsorption of anionic dyes due to the repulsive interactions between the negative charges on bentonite surface and anionic dye.7,8 © 2012 American Chemical Society

Therefore, organobentonite, synthesized by bentonite and cationic surfactants with cationic exchange, is used to adsorb anionic dyes and displays a good adsorption capability for most anionic dyes.9−11 The traditional organobentonite process in removing pollutants from wastewater consists of two separate steps: the first is the preparation of organobentonite, followed by the application of the organobentonite for pollutant removal from wastewater by adsorption. The mostly used synthesis method for organobentonite includes pulverizing bentonite, mixing bentonite−cationic surfactant solution, and then separating, drying, and grinding products.12 However, large quantities of power and water are consumed, and the organobentonite is difficult to settle, which could be the limiting factors for application in wastewater treatment.13 Therefore, to simplify the traditional organobentonite process, a simple one-step process involving both the self-assembly of organobentonite and the removal of pollutants simultaneously in a system was proposed by adding raw bentonite and appropriate cationic surfactant into the wastewater. In a one-step process, cationic surfactant in wastewater could be utilized to assemble organobentonite with raw bentonite through the regulation of surfactant structure and load capacity, so as to remove anionic dyes and some nonionic organic pollutants better in a one-step process.15−17 It is well-known that surfactants always exist as an additive in the wastewater produced by the dyeing or textile industry,14 which gives a possibility that it could realize the adsorption removal of Received: December 13, 2011 Accepted: June 8, 2012 Published: June 14, 2012 1915

dx.doi.org/10.1021/je300216e | J. Chem. Eng. Data 2012, 57, 1915−1922

Journal of Chemical & Engineering Data

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Figure 1. Wastewater treatment procedures designed for one-step and traditional organobentonite processes. −·−, preparation of organobentonite; −−, bentonite/surfactant one-step process; the whole figure, traditional organobentonite process.

CTMAB was (10, 20, 40, 60, 80, 100, 120, 150, 200, 300, 400, and 500) % CEC of the clay saturated) as a 1 mass fraction aqueous suspension at room temperature for 24 h under vigorous stirring. The treated bentonites were separated from water by vacuum filtration and washed several times with deionized water until free bromide anions are present (tested by AgNO 3 ). The organobentonite (CTMAB modified bentonite) was dried at 333 K and ground to 0.074 mm size, the same as the raw bentonite, which would be used in the experiments later. The detailed batch adsorption procedures for the traditional organobentonite process and one-step process are shown in Figure 1. 2.3. Adsorption Experiments. The batch adsorption experiments were carried out in the laboratory by contacting a certain volumn of RB13 aqueous solution (adsorbate) with the bentonite/CTMAB or organobentonite as an adsorbent at 200 rpm and 298 K for 4 h to ensure apparent equilibrium. A preliminary kinetic investigation revealed that equilibrium could be reached in less than 1 h. When the equilibrium was obtained, the mixture was separated by centrifugation (8000 rpm for 5 min and 278 K), and the supernatant solution was analyzed for residual dye concentrations. Then the adsorbed amount was calculated. The effect of adsorbent dosage on the adsorption was studied by adding different doses of bentonite [(1 to 50) mg] and 100 % CEC (bentonite) of CTMAB into 50 g of RB13 solution (RB13 concentration: 0.1 mmol·kg−1). Similarly, the effect of CTMAB amount [(0.4 to 20) mg] on RB13 adsorption was studied at the fixed RB13 concentration (0.1 mmol·kg−1) and bentonite dose (10 mg) in 50 g of aqueous solution. In the determination of the equilibrium adsorption isotherm, 10 mg of bentonite and 100 % CEC (bentonite) of CTMAB were added into 50 g of different concentrations [(0.05 to 0.3) mmol·kg−1] of RB13 solution. To evaluate the kinetic characteristics, 200 mg of bentonite and 100 % CEC (bentonite) of CTMAB (added as solid or liquid form, respectively) were added into 1 kg of 0.1 mmol·kg−1 RB13 solution with a mechanical stirrer. Small portions of the samples (4 g) were withdrawn at regular time intervals and filtered immediately to determine the concentration of the RB13 dye and CTMAB. In addition, the solution conductivity was measured simultaneously during the kinetic process.

pollutants accompanied with the self-assembly of organobentonite directly while the bentonite solely adds into those wastewaters. The one-step process makes large-scale application of organobentonite in dyeing wastewater treatment possible. In previous studies, it was mainly concerned with the removal efficiency; however, the rule of surfactant assembling bentonite on-demand was ignored, as well as the interaction among surfactant, dyes, and bentonite in the one-step process. In this study the self-assembly of organobentonite and the removal mechanism of bentonite in bentonite/surfactant onestep process was both tested. The objectives of this study were: (i) to determine the optical conditions for the self-assembly of organobentonite by regulating the effect of bentonite and surfactant dosage on the removal of anionic dye (reactive blue 13) and cationic surfactant (cetyltrimethylammonium bromide, CTMAB) from simulated dyeing wastewater in a bentonite/ surfactant one-step process; (ii) to reveal the removal mechanism of reactive blue 13 by self-assembled organobentonite in a one-step process by the analysis of the adsorption isotherm and kinetics process; (iii) to further explain the adsorption mechanism by X-ray diffraction (XRD) and Brunauer−Emmett−Teller (BET) analysis of the adsorbents in a one-step process.

2. MATERIALS AND METHODS 2.1. Materials. The raw bentonite (Ca-bentonite) used in this study was obtained from Hangzhou Huate Co., Ltd. (Inner Mongolia, China). Its cation exchange capacity (CEC) was 1.084 mmol·g−1, and the organic carbon content was 0.14 mass fraction, respectively. The bentonite sample was gently ground to 0.074 mm size. Cationic surfactant cetyltrimethylammonium bromide (CTMAB, 0.99 mass fraction) and C.I. reactive blue 13 (tetrasodium, [3-[[8-[(4-amino-6-chloro-1,3,5-triazin-2-yl)amino]-1-hydroxy-3,6-disulfo-2-naphthyl]azo]-4-hydroxynaphthalene-1,5-disulfonato(6-)] cuprate(4-); CAS No. 14692-76-3; abbreviated as RB13; 0.90 mass fraction) were supplied by Tianyu Co., Ltd. (Hangzhou, China). All of the reagents were used in the experiments without further purification. The chemical structure of RB13 was given in the literature by Lin et al.18 2.2. Preparation of Organobentonite. As a comparison, organobentonite was prepared.12,13 The raw bentonite was treated with aqueous solutions of CTMAB (the amount of 1916



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better than 4 % on the same three samples. The detection range is (3 to 100) mass fraction of carbon.

Results of control experiments indicated that no evaporation and adsorption on glass surface occurred. The test of the effect of solution pH (adjusted by HCl or NaOH, Figure 2) showed

3. RESULTS AND DISCUSSION 3.1. Effect of Bentonite Amount on the Removal of RB13. The removal of RB13 at different bentonite dosages is shown in Figure 3 in traditional and one-step processes. It was

Figure 2. Effect of initial solution pH on the removal of RB13 (bentonite and CTMAB addition: 10 mg and 100 % CEC in 50 g of 0.1 mmol·kg−1 of RB13 solution). ■, raw bent→RB13; ●, CTMAB modified bent→RB13; ▲, raw bent + CTMAB→RB13. Figure 3. Effect of bentonite dosage on the removal of RB13 (solution mass: 50 g). ■, raw bent→RB13; ●, CTMAB modified bent→RB13; ▲, raw bent + CTMAB→RB13.

that pH had a negligible influence on the dye removal in two processes (here no detailed discussion), which probably attributed to that chemisorption may be the operative mechanism,9 so the experiment was conducted under nature pH of the dye wastewater without any adjustment. Triplicate parallel samples were tested under identical conditions for all experiments. 2.4. Analyses. The concentration of RB13 was estimated by complete UV−vis spectra scan using a UV−vis spectrophotometer (GBC Cintra 303, Australia), to avoid the error of the fixed-point determination. The conductivity of the solution was determined by a conductivity meter (DDSJ-308A, Leici, China). The total organic carbon (TOC) was measured using a total organic-carbon analyzer (TOC-VCPH, SHIMADZU). The CTMAB concentration was calculated by the simple mass balance, which was equal to the TOC minus the carbon amount of RB13. The bromide ion concentration was determined by an ion chromatograph (ICS-1100, DIONEX). X-ray diffraction (XRD) patterns of the raw and adsorbed bentonite were acquired by an X-ray powder diffractometer (X’Pert Pro, PANalytical) with Cu Kα radiation (λ = 0.154 nm, 40 kV, 300 mA). All of the XRD patterns were obtained from 2.5° to 80° with the scan speed of 4 deg·min−1. The samples of raw and modified bentonite were dry, and the adsorbed samples were the centrifugal solid without any drying [(60 to 70) % of the moisture content]. The specific surface areas and pore volumes of the raw and adsorbed bentonite were measured from N2 (0.36 nm) adsorption/desorption isotherms with a sorptiometer (ASIC2, Quantachrome Co., USA) after vacuum degassing for 16 h at 407 K. The surface areas and micropore volumes were calculated based on the BET equation and t-plot method, respectively. The total pore volume was expressed as the adsorption amount of N2 at the maximal relative pressure. The organic carbon content of bentonite was measured by an Aurora 1030 TOC analyzer (UAS) at 1173 K. Average RSDs (relative standard deviations) for intersample testing were

evident that there was little of the dye molecules adsorbed onto the raw bentonite because the negatively charged surface sites on clay did not favor the adsorption of dye anions due to electrostatic repulsion.9 As the adsorbent dosage increased in the one-step process, the removal of RB13 increased accompanied with the appearance of the hydrophobic sites on the bentonite surface by cation exchanges of CTAB+ cations. Meanwhile, self-assembled organobentonite in the one-step process revealed the advantage of better removal efficiency than traditional organobentonite at lower adsorbent doses. For example, when adsorbent dose was 10 mg at the 50 g solution (0.2 g per kg solution), the removal of RB13 in one-step process was more than 95 %, while that of CTMAB modified bentonite was less than 80 %. 3.2. Effect of CTMAB Amount on RB13 Removal. As seen from Figure 4a, the presence of cationic surfactant showed a rather prominent effect on the removal of RB13. The adsorption capacity of RB13 increased nearly linearly when the initial CTMAB concentration was less than 100 % CEC of bentonite, reached a peak and then decreased when the initial CTMAB concentration was more than 200 % CEC. The presence of RB13 had little effect on the adsorption of CTMAB onto bentonite when the initial concentration of CTMAB was less than 200 % CEC of the clay. When the CTMAB initial concentration further increased, the residue CTMAB concentration also increased rapidly, and there were a certain amount of surfactants releasing to the liquid phase from traditional organobentonite (Figure 4b). Some researchers considered different types of adsorption based on the density and extent of the adsorbed surfactant layer: at low concentrations of cationic surfactant (below the CEC), the surfactant coverage and density were decisive for the enhancement of the pollutant adsorption; at high surfactant concentrations (above the CMC), the solubilization of pollutant in the bulk phase 1917



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the continuous decrease of the removal rate of RB13.22 Moreover, to sum up, the best self-assembly conditions of organobentonite were as follows: the appropriate amount of bentonite was 0.2 g per kg solution, and CTMAB was 100 % CEC of the clay. 3.3. Adsorption Isotherms. Equilibrium isotherms are often used to describe the experimental data of equilibrium adsorption. Figure 5 shows the adsorption isotherms of RB13

Figure 5. Adsorption isotherms of RB13 onto bentonite in the traditional and one-step processes. ●, CTMAB modified bent→RB13; ▲, raw bent + CTMAB→RB13.

onto bentonite in the traditional and one-step processes. According to the system of isotherm classification by Giles et al.,23 the adsorption isotherm of RB13 onto bentonite submitted to type L (Langmuir) subgroup 2, which belonged to the Langmuir monolayer adsorption. It was clear that the presence of CTMAB in the aqueous phase made the isotherm change, which increased the adsorption capacity of RB13 markedly in a one-step process. As the initial concentration increased, the adsorption amount of RB13 onto self-assembled bentonite was about 1.5 times that of traditional organobentonite. This result indicated that cooperative removal and synergism may take place to a large extent between CTMA+ and dye anion in solution in a one-step process. On the other hand, the bentonite surface was originally hydrophilic; however, once the raw bentonite was modified with cationic surfactant, the surface became hydrophobic, which would cause the adsorbent powder to congregate and hard to disperse widely. Some pores in the organobentonite powder were permeated with air, dye molecules were difficult to permeate into the hydrophobic block, and the adsorption capacity of organobentonite obviously reduced accordingly. Furthermore, as a small amount of CTMAB on the organobentonite was released into the solution (seen from Figure 4b), the CTMAB loading became lower and thus led to the adsorption capacity decrease. To acquire the best-fit isotherm to confirm the adsorption mechanism, the adsorption data were further analyzed with respect to the Langmuir, Freundlich, and Dubinin−Radushkevich isotherms. The Langmuir equation is one of the empirical isotherms describing the adsorption equation based on the assumption that the maximum adsorption corresponds to the saturated monolayer adsorption of adsorbate on specific homogeneous

Figure 4. Effect of the CTMAB amount on (a) RB13 removal and (b) equilibrium concentration (Ce) of CTMAB (bentonite addition: 10 mg in 50 g of solution). ■, raw bent→RB13; ●, CTMAB modified bent→RB13; ▲, raw bent + CTMAB→RB13.

competed with solubilization on the surface, where the pollutant molecules migrated into the micelles, thus reducing the adsorption amount of surfactant and pollutant.19 A similar decrease was observed in the adsorption of acid dye on bentonite when the CTMAB concentration was above its critical micelle concentration (CMC),15 which may be presumably due to the solubilization of dyes in the micelles in the liquid phase.20 However, interestingly, the residue concentration (equilibrium concentration) of CTMAB in this study was low, far from its CMC when the RB13 removal efficiency began to decline. Also, the residue concentration of CTMAB sharply rose when CTMAB amount was beyond 200 % CEC of the clay (Figure 4b). The reasons for these phenomena may be as follows: (i) due to the hydrophobic interaction between the surfactant and the bentonite surface or between the surfactant molecules, CTMAB would still be further integrated in the bentonite layer, making the structure of bentonite layer packed more closely and the packing density increasing,17,21 and then the steric hindrance effects occurred, which may hinder RB13 close to the adsorption sites and led to the decline of adsorption amount of RB13; (ii) along with the initial CTMAB concentration further increasing, free micelles of the surfactant were formed, and then the RB13 molecules were incorporated by these micro reservoirs, which resulted in 1918



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sites with a constant energy.23 The saturated monolayer isotherm can be represented as the following linear form:24 Ce a 1 = + L Ce qe KL KL

Table 1. Langmuir, Freundlich, and Dubinin−Radushkevich Isotherm Constants for RB13 Adsorption adsorbent isotherm models

(1)

Meanwhile the dimensionless expression is

Langmuir

1 RL = 1 + aLC0

(2) −1

where the KL (kg·g ) is the Langmuir equilibrium constant, and aL (kg·mg−1) is the Langmuir constant related to the energy of adsorption. The KL/aL gives the theoretical monolayer saturation adsorption capacity, Q0 (mg·g−1). RL is the separation factor, also named the equilibrium parameter, which indicates the shape of the isotherms to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0). Another empirical isotherm, namely, the Freundlich equation, is often used for nonideal adsorption on heterogeneous surfaces and multilayer adsorption. It can be expressed by the following linear equation:11 log qe = log KF +

1 log Ce n

Freundlich

Dubinin− Radushkevich

where Ce (mg·kg−1) and qe (mg·g−1) are the amounts of dye unabsorbed in solution and the amount of dye adsorbed onto per unit weight of adsorbent at equilibrium, respectively. The KF (mg1−1/n·kg1/n·g−1) is the Freundlich constant, and n (g·kg−1) is the Freundlich exponent which suggests n is between 1 and 10, representing beneficial adsorption.25 The Dubinin−Radushkevich equation can be cited to describe adsorption on both heterogeneous and homogeneous surfaces and is often used to distinguish the physical or chemical adsorption of pollutants.4,26,27 Its linear form is: (4)

where qm is the Dubinin−Radushkevich monolayer capacity (mmol·g−1), β is a constant related to adsorption energy, and ε is the Polanyi potential related to the equilibrium concentration as follows:

⎛ 1⎞ ε = RT ln⎜1 + ⎟ Ce ⎠ ⎝

(5) −1

−1

where R is the gas constant (8.31 J·mol ·K ) and T is the absolute temperature. The parameter E, representing the mean free energy, gives information whether the adsorption mechanism is physical or chemical adsorption. It can be computed using the formula:27 E=

1 2β

RL2 KL/kg·g−1 aL/kg·mg−1 Q0/mg·g−1 RL RF2 KF/ mg1−1/n·kg1/n·g−1 n/g·kg−1 RD−R2 qm/mmol·g−1 β/mmol2·J−2 E/kJ·mol−1

self-assembled organobentonite

0.9987 97.09 0.204 476.19 0.016 to 0.164 0.9876 198.50

0.9954 125 0.175 714.28 0.019 to 0.186 0. 9857 320.76

5.69 0.9782 0.464 2.62·10−9 13.81

6.33 0.9721 0.699 3.07·10−9 12.76

the Langmuir model exhibited better fits than the other models according to the correlation coefficients (RL2 > 0.99 > RF2 > RD−R2 > 0.95). The fact that the Langmuir equation fitted the data best confirmed the monolayer coverage of RB13 molecular onto the adsorbent, since the Langmuir isotherm assumed the surface was homogeneous. The theoretical monolayer saturation adsorption capacity Q0 was found to be (476.19 and 714.28) mg·g−1 for RB13 adsorption onto bentonite in traditional and one-step processes, respectively. The equilibrium parameter RL values in Langmuir isotherm were within the favorable limit in both bentonite processes (0 < RL < 1). The degree of favorability approached zero (which represented the completely ideal irreversible adsorption) rather than unity (which represented a completely reversible adsorption), which indicated that chemical sorption was absolutely predominant.4 The experimental data were also fitted to the Freundlich and Dubinin−Radushkevich isotherms, although the fit was not as good as the Langmuir isotherm. The magnitude of Freundlich exponent n also gave an indication of beneficial adsorption for RB13 onto bentonite when the n values were both within the scope of 1 to 10. The parameter E of Dubinin−Radushkevich was between (8 and 16) kJ·mol−1, indicating that the adsorption process was governed by an ion exchange mechanism. 3.4. Removal Mechanism by Kinetic Measurements. The removal rates of RB13 against contact time at a constant initial concentration by bentonite/CTMAB are presented in Figure 6. The kinetic equilibrium times for the bentoniteCTMAB-RB13 system required were almost 40 min, and the equilibrium removal rates of the one-step process was about 98 %, higher than that of traditional organobentonite process (80 %), regardless of the initial CTMAB form (solid or liquor), which indicated the potential application of one-step process for continuous water treatment system. Many studies have estimated that the dyes could interact strongly with the oppositely charged surfactant in the premicellar concentration range, and the interaction intensity was closely related to the molecular mass and structure of them.22,28 Figure 6 showed a significant interaction between surfactant cations CTMA+ and dye anions, which would generate a blue precipitate complex in a certain surfactant/dye molar ratio range so as to remove the dyes in the wastewater. Due to the very rapid reaction between ions, the CTMAB-

(3)

ln qe = ln qm − βε 2

constants

CTMAB modified bent

(6) −1

For the value of E < 8 kJ·mol , the adsorption process is dominated by physical effects; while between (8 and 16) kJ·mol−1, the adsorption process is governed by an ion exchange mechanism, and when E > 16 kJ·mol−1, the adsorption process may be controlled by particle diffusion. According to the Langmuir, Freundlich, and Dubinin− Radushkevich isotherm equations, a detailed analysis of the equilibrium data was done, and the adjustable parameters (KL, aL, Q0, RL, KF, n, qm, β, and E) with the correlation coefficient were listed in Table 1. All of the isotherms fitted well, in which 1919



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Figure 6. Removal of RB13 by bentonite/CTMAB against the contact time. □, raw bent→RB13; ○, CTMAB(solid)→RB13; △, CTMAB(liquor)→RB13; ▼, CTMAB modified bent→RB13; ★, raw bent + CTMAB (solid)→RB13; ⧫, raw bent + CTMAB (liquor)→RB13.

Figure 7. Concentrations of CTMAB in the solution during the kinetic process (initial CTMAB addition: 80 mg in 1 kg of solution). ○, CTMAB (solid)→RB13; △, CTMAB (liquor)→RB13; ★, raw bent + CTMAB (solid)→RB13; ⧫, raw bent + CTMAB (liquor)→RB13.

RB13 system quickly reached equilibrium when adding CTMAB solution into the dye solution. When adding CTMAB powders to the dye solution, the RB13 removal increased to more than 95 % and then decreased to a stable level of 10 % removal rate along with contact time increasing, indicating that the dissolution of CTMAB powders and the interaction between CTMAB and RB13 occurred simultaneously, and the amount of the CTMA-RB13 complex was closely related to the solubility of CTMAB in the system. In the bentonite/CTMAB one-step process, the interaction between CTMA+ and RB13 anion still existed, which was more obvious when CTAMB was added as powders. Compared with that in the CTMAB-RB13 system, RB13 removal in the bentonite/ CTMAB one-step process mainly depended on its adsorption by bentonite self-assembled with CTMAB. Different from the traditional organobentonite process, cooperative adsorption and the synergistic effect between CTMAB and RB13 took place to a greater extent in the one-step process. To further explain the removal mechanism of the RB13 dye in the bentonite/CTMAB one-step process, the system conductivity and residual concentration of CTMA+ and bromide (Br−) were also measured continuously during the kinetics. It was showed that almost all of the bromide ions still existed in the solution without any adsorption under the current experimental conditions (it would be not considered in the subsequence), indicating that surfactant was adsorbed onto bentonite in the form of CTAB+ cations through ion exchange. The changes of CTMAB concentration shown in Figure 7 proved the interaction between CTMA+ and RB13 anion as well as CTMA+ adsorption onto bentonite in the CTMABRB13 and bentonite/CTMAB-RB13 systems. Conductivity depends on the solution properties and temperature and directly reflects the ion concentrations in solution. Corresponding to the RB13 removal kinetics, the system conductivity also showed a similar trend (Figure 8). In the CTMAB-RB13 system, the decline of conductivity at first 3.5 min illustrated the formation of a nonconductive or weakly conductive material, and then when CTMA+ exceeded a certain concentration, disaggregation of the dye−surfactant complex occurred. In the bentonite/CTMAB-RB13 system, the system conductivity decreased evidently in the traditional process due to the decrease of ion concentrations caused by RB13

Figure 8. Solution conductivity against the contact time during the kinetic process. □, raw bent→RB13; ○, CTMAB (solid)→RB13; △, CTMAB(liquor)→RB13; ▼, CTMAB modified bent→RB13; ★, raw bent + CTMAB (solid)→RB13; ⧫, raw bent + CTMAB (liquor)→ RB13.

adsorption onto bentonite; however, the system conductivity did not change significantly when the system equilibrium was reached because of the ion exchange and chemical sorption occurring simultaneously in a one-step process. 3.5. XRD and BET Analysis. Relative to the organobentonite, the organic carbon content in raw bentonite was very low and negligible. After the adsorption reactions, the organic carbon content of bentonite increased significantly (Table 2), indicating that almost all organic carbons in those bentonite originated from cationic surfactants and RB13 dye molecules. The organic carbon content showed an obvious positive correlation to the adsorption capacities of CTMAB and RB13 and also proved the fact that a higher removal efficiency was obtained in the one-step process than in the traditional one. To further explain the adsorption mechanism, XRD patterns were employed to understanding the structure of the adsorbed bentonite layer. Generally speaking, the d001 value was represented as the size of interlayer spacing. The d001 diffractions of the raw bentonite, raw bentonite after RB13 1920



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Table 2. Organic Carbon, Surface Area, and Pore Volume of Bentonite after a Series of Treatment organic carbon

specific surface area

external surface area

micropore area

total pore volume

micropore volume

types of bentonite

mass fraction

m2·g−1

m2·g−1

m2·g−1

cm3·g−1

cm3·g−1

raw bent raw bent sorbing RB13 CTMAB modified bent CTMAB modified bent sorbing RB13 self-assembled organobentonite sorbing RB13

0.14 0.61 19.94 25.77 26.09

82.99 81.00 18.60 0 0

42.05 34.86 13.46 −7.564 −5.093

22.50 36.34 4.135 7.564 5.093

0.1641 0.0599 0.0566 0 0

0.0191 0.0229 0.0023 0.0036 0.0030

adsorption, CTMAB modified bentonite, CTMAB modified bentonite after RB13 adsorption, and one-step bentonite with CTMAB after RB13 adsorption were seen at [(5.862, 5.853, 4.908, 4.707, and 4.665) 2θ] with distances of (1.5063, 1.5087, 1.7991, 1.8559, and 1.9025) nm, respectively (Figure 9). The

exchange. Therefore, it was stated clearly that the bentonite with smaller surface area had adsorbed more organic cations. The decrease of specific surface areas may also be attributed to blocking the pores of aggregates and increasing the aggregation of particles,29 which was consistent with the results of previous studies.30,31 When there were only RB13 and raw bentonite in the aqueous solution, the micropore area and micropore volume increased, and the total pore volume decreased, showing that RB13 mainly adsorbed in the mesopore or macropore location of the bentonite. Meanwhile, the decrease of specific surface area was mainly contributed by external surface area, which indicated that RB13 was adsorbed on the external surface of bentonite mostly in the absence of CTMAB. When there was surfactant present, CTMAB was adsorbed on both external surface and internal surfaces of bentonite. The cationic surfactant adsorbed on the internal surface made the interlayer space increase so as to adsorb dye molecular into the interlayer by the interaction of CTMA+ and dye anion. The cationic surfactant adsorbed on the external surface could also attract more dye molecules in the same way.

Figure 9. XRD patterns of (a) raw bentonite, (b) raw bentonite sorbing RB13, (c) CTMAB modified bentonite, (d) CTMAB modified bentonite sorbing RB13, and (e) self-assembled organobentonite sorbing RB13.

4. CONCLUSIONS A bentonite/surfactant one-step process realized the selfassembly of organobentonite and highly efficient removal of pollutants simultaneously. Comparing to the traditional organobentonite process, the one-step process exhibited a higher adsorption capacity of the anionic dye RB13 and cationic surfactant CTMAB. Kinetic results showed that high removal efficiencies (more than 93 %) of reactive blue 13 and CTMAB could be achieved in 40 min. Also, bentonite in one step showed a better stabilization for CTMAB and RB13 adsorption when CTMAB amount was less than 200 % CEC of the clay. In our one-step process, the cationic surfactants in the wastewater were utilized to assemble organobentonite with raw bentonite and then used for the removal of anionic dyes as adsorbents, accompanied with obvious interaction between CTMA+ and dye anions during initial reaction. The adsorption data of RB13 could be fitted to Langmuir isotherm model very well, and the equilibrium adsorption capacity of self-assembled organobentonite was 50 % higher than that of the traditional one. XRD and BET analysis showed that self-assembled organobentonite had a larger basal spacing than traditional organobentonite; parts of CTMA+ and RB13 were adsorbed on the external surface, and some were intercalated inside the interlayer of bentonite. Thus, a one-step process, including both self-assembly of organobentonite and adsorption removal of organic pollutants, would be a low-cost, simple, high-efficiency, and promising process for wastewater treatment especially for dyeing or textile wastewater treatment.

d001 value was positively correlated with the adsorption amount of adsorbent. Therefore, the basal spacing values increasing for organobentonite could be attributed to the replacement of inorganic interlayer cations and their intercalation hydration water with CTMA+ cations. Referring to the literature, the increase of basal spacing suggested that CTMA+ cations were inserted into the layers of bentonite and formed an extended structure with a certain tilt angle.15,21 Also, when RB13 was adsorbed, their basal spacing values became larger, which illuminated that RB13 was adsorbed into the interlayer of bentonite. Furthermore, one-step bentonite showed a larger basal spacing than organobentonite under the same experimental conditions, which was in accordance with that one-step bentonite with CTMAB had a higher adsorption capacity than CTMAB modified bentonite. As shown in Figure 8, the uniformity of X-ray diffraction patterns has further proven that the one-step bentonite process had similar performance with the traditional organobentonite process. As generally believed, the basal spacing of bentonite has been stretched after organic modification. However, the measured specific surface areas of the adsorbents after RB13 adsorption were all lower than that of the raw bentonite. This phenomenon could be interpreted as that the space stretched by organic cations was again filled with cations itself and the bentonite adsorbed RB13 mainly in the manner of ion 1921



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Article

(17) Ma, J. F.; Zhu, L. Z. Removal of phenols from water accompanied with synthesis of organobentonite in one-step process. Chemosphere 2007, 68, 1883−8. (18) Lin, J.; Zhang, X. W.; Li, Z. J.; Lei, L. C. Biodegradation of reactive blue 13 in a two-stage anaerobic/aerobic fluidized beds system with a Pseudomonas sp isolate. Bioresour. Technol. 2010, 101, 34−40. (19) Klumpp, E.; Heitmann, H.; Schwuger, M. J. Synergistic effects between cationic surfactants and organic pollutants on clay-minerals. Colloids Surf., A 1993, 78, 93−8. (20) Esumi, K.; Goino, M.; Koide, Y. Adsorption and adsolubilization by monomeric, dimeric, or trimeric quaternary ammonium surfactant at silica/water interface. J. Colloid Interface Sci. 1996, 183, 539−45. (21) Chen, B. L.; Zhu, L. Z.; Zhu, J. X. ConFigureurations of the bentonite-sorbed myristylpyridinium cation and their influences on the uptake of organic compounds. Environ. Sci. Technol. 2005, 39, 6093− 100. (22) Bielska, M.; Sobczynska, A.; Prochaska, K. Dye-surfactant interaction in aqueous solutions. Dyes Pigm. 2009, 80, 201−5. (23) Giles, C. H.; Macewan, T. H.; Nakhwa, S. N. Studies in adsorption II. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids. J. Chem. Soc. 1960, No. OCT, 3973−93. (24) Bulut, E.; Ö zacar, M.; Şengil, I.̇ A. Equilibrium and kinetic data and process design for adsorption of congo red onto bentonite. J. Hazard. Mater. 2008, 154, 613−22. (25) Sivaraj, R.; Namasivayam, C.; Kadirvelu, K. Orange peel as an adsorbent in the removal of acid violet 17 (acid dye) from aqueous solutions. Waste Manage. 2001, 21, 105−10. (26) Shahwan, T.; Erten, H. N. Temperature effects in barium sorption on natural kaolinite and chlorite-Illite clays. J. Radioanal. Nucl. Chem. 2004, 260, 43−8. (27) Lin, S. H.; Juang, R. S. Heavy metal removal from water by sorption using surfactant-modified montmorillonite. J. Hazard. Mater. 2002, 92, 315−26. (28) Behera, P. K.; Mohapatra, S.; Patel, S.; Mishra, B. K. Dyesurfactant interaction: solubilization of styryl pyridinium dyes of varying alkyl chain in alfa-olefinic sulfonate and linear alkyl benzene sulfonate solutions. J. Photochem. Photobiol. A 2005, 169, 253−60. (29) Atia, A. A. Adsorption of chromate and molybdate by cetylpyridinium bentonite. Appl. Clay Sci. 2008, 41, 73−84. (30) Jiang, J. Q.; Cooper, C.; Ouki, S. Comparison of modified mntmorillonite adsorbents - Part I: preparation, characterization and phenol adsorption. Chemosphere 2002, 47, 711−6. (31) Yılmaz, N.; Yapar, S. Adsorption properties of tetradecyl- and hexadecyl trimethylammonium bentonites. Appl. Clay Sci. 2004, 27, 223−8.

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 571-87952209. E-mail address: [email protected]. Funding

The authors would like to acknowledge financial support for this work provided by MOST project of China (No. 2008BAC32B06) and NSFC of China (No. 21076188, 20836008, 20976158, 20990221, and 21076189), the Key Innovation Team for Science and Technology of Zhejiang Province of China (2009R50047). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kadirvelu, K.; Kavipriya, M.; Karthika, C.; Radhika, M.; Vennilamani, N.; Pattabhi, S. Utilization of various agricultural wastes for activated carbon preparation and application for the removal of dyes and metal ions from aqueous solutions. Bioresour. Technol. 2003, 87, 129−32. (2) Papić, S.; Koprivanac, N.; Božić, A. L.; Meteš, A. Removal of some reactive dyes from synthetic wastewater by combined Al(III) coagulation/carbon adsorption process. Dyes Pigm. 2004, 62, 291−8. (3) Li, Q.; Yue, Q. Y.; Su, Y.; Gao, B. Y.; Li, J. Two-step kinetic study on the adsorption and desorption of reactive dyes at cationic polymer/ bentonite. J. Hazard. Mater. 2009, 165, 1170−8. (4) Bulut, E.; Ö zacar, M.; Şengil, I.̇ A. Adsorption of malachite green onto bentonite: Equilibrium and kinetic studies and process design. Microporous Mesoporous Mater. 2008, 115, 234−46. (5) Wu, Z.; Li, C. Kinetics and thermodynamics of [beta]-carotene and chlorophyll adsorption onto acid-activated bentonite from Xinjiang in xylene solution. J. Hazard. Mater. 2009, 117, 582−7. (6) Wang, Q. H.; Chang, X. Z.; Li, D. D.; Hu, Z.; Li, R. J.; He, Q. Adsorption of chromium (III), mercury (II) and lead (II) ions onto 4aminoantipyrine immobilized bentonite. J. Hazard. Mater. 2011, 186, 1076−81. (7) Tahir, S. S.; Rauf, N. Removal of a cationic dye from aqueous solutions by adsorption onto bentonite clay. Chemosphere 2006, 63, 1842−8. (8) Benguella, B.; Yacouta-Nour, A. Adsorption of bezanyl red and nylomine green from aqueous solutions by natural and acid-activated bentonite. Desalination 2009, 235, 276−92. (9) Baskaralingam, P.; Pulikesi, M.; Elango, D.; Ramamurthi, V.; Sivanesan, S. Adsorption of acid dye onto organobentonite. J. Hazard. Mater. 2006, 128, 138−44. (10) Zohra, B.; Khenifi, A.; Fatima, S.; Nourredine, B.; Zoubir, D. Adsorption of Direct Red 2 on bentonite modified by cetyltrimethylammonium bromide. Chem. Eng. J. 2008, 136, 295−305. (11) Kang, Q.; Zhou, W. Z.; Li, Q.; Gao, B. Y.; Fan, J. X.; Shen, D. Z. Adsorption of anionic dyes on poly(epicholorohydrin dimethylamine) modified bentonite in single and mixed dye solutions. Appl. Clay Sci. 2009, 45, 280−7. (12) Zhu, L. Z.; Chen, B. L.; Shen, X. Y. Sorption of phenol, pnitrophenol, and nniline to dual-cation organobentonites from water. Environ. Sci. Technol. 2000, 34, 468−75. (13) Zhu, L. Z.; Chen, B. L. Sorption behavior of p-nitrophenol on the interface between anion-cation organobentonite and water. Environ. Sci. Technol. 2000, 34, 2997−3002. (14) Saleh, M. M. On the removal of cationic surfactants from dilute streams by granular charcoal. Water Res. 2006, 40, 1052−60. (15) Zhu, L. Z.; Ma, J. F. Simultaneous removal of acid dye and cationic surfactant from water by bentonite in one-step process. Chem. Eng. J. 2008, 139, 503−9. (16) Khenifi, A.; Zohra, B.; Kahina, B.; Houari, H.; Zoubir, D. Removal of 2,4-DCP from wastewater by CTAB/bentonite using onestep and two-step methods: A comparative study. Chem. Eng. J. 2009, 146, 345−54. 1922


Removal of Reactive Blue 13 from Dyeing Wastewater by Self

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