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Surface Modification of Hydrophobic Resin with Tricaprylmethylammonium Chloride for the Removal of Trace Hexavalent Chromium Jyh-Herng Chen,*,† Kai-Chung Hsu,‡ and Yu-Min Chang§ †

Department of Materials and Mineral Resources Engineering, ‡College of Engineering, and §Department of Environmental Engineering, National Taipei University of Technology, 1, Section 3, Chung-Hsiao East Road, Taipei, 10608, Taiwan, R.O.C. S Supporting Information *

ABSTRACT: Chromium(VI) is one of the most toxic contaminants in the environment, even at low concentration. The conventional chemical precipitation process for Cr(VI) wastewater treatment becomes less effective at low Cr(VI) concentration. By a two-step solvent−nonsolvent method, tricaprylmethylammonium chloride (Aliquat 336) can be immobilized onto the surface of porous Amberlite XAD resin with good stability, giving Aliquat 336-modified resins (AMRs). Furthermore, the exposure of the cationic ammonium functional group facilitates the adsorption and stripping efficiency of Cr(VI). The amount of immobilized tricaprylmethylammonium chloride was 2.0 mmol/g of resin. A kinetic study showed that the adsorption of Cr(VI) was under film-diffusion control followed by intraparticle-diffusion control. The Cr(VI) adsorption capacity was as high as 1.37 mmol/g. More than 99% of the adsorbed Cr(VI) could be stripped during regeneration. For stability and reusability, the AMRs maintaine da high level of Cr(VI) adsorption even after four cycles of adsorption/stripping. The experimental results for actual wastewater demonstrated that the AMRs can be used effectively for the treatment of trace Cr(VI)-containing wastewater.

1. INTRODUCTION Chromium compounds are extensively used in electroplating, leather tanning, chromite beneficiation, fertilizers, and several other industries.1 In industrial effluents, Cr(VI) is one of the most toxic contaminants.2 Even at low concentrations, Cr(VI) is considered to have mutagenic and carcinogenic effects on biological systems.3 It is important to reduce Cr(VI) to a very low level before chromium-containing effluents are discharged into the environment. Conventionally, Cr(VI)-containing wastewater is treated by a chemical precipitation method. This process requires the reduction of Cr(VI) to Cr(III), followed by pH adjustment to form the insoluble chromium(III) hydroxide as a precipitate.4 Chemical precipitation is a favorable method for the treatment of wastewater containing high concentrations of Cr(VI). Using NaOH, although removal efficiencies up to 99.9% have been achieved, residual Cr is always >8.7 mg/L, and a low-density sludge is formed, leading to the production of high sludge volumes.5 For the removal of trace Cr(VI), chemical precipitation is less effective because of a low reduction efficiency.6,7 Taking these environmental and efficiency concerns into account, efficient separation techniques are needed to reduce the concentration of Cr(VI) to the recommended values. Various methods for the removal of Cr(VI) from wastewater have been developed, including electrochemical precipitation,8 reverse osmosis,9 solvent extraction,10,11 and membrane separation.12 Attempts have also been made to develop lowcost adsorbents to remove Cr(VI) from industrial effluents, with fireclay,13 polysaccharides,14,15 wood pulp,16 and chitosancoated biosorbent17 all being potential alternatives. Synthetic polymer-based materials have also been studied intensively because of their characteristic properties, such as renewable origin and well-defined physical and chemical properties. Such © 2013 American Chemical Society

materials include commercially available anion-exchange resins,18 polymeric hydrogels,19,20 polymeric ligand-based exchanger,21 and modified poly(4-vinylpyridine) silica gel.22 Solvent-impregnated resins (SIRs), in which a liquid extractant is contained within the pores of particles by physical sorption of the extractant on the particle surface, also show potential applications.23 Recently, it was demonstrated that the surface of a hydrophobic resin, such as Amberlite XAD-4 resin, can be modified with a surface-active extractant by a two-step solvent− nonsolvent method for Cu and Zn recovery.24,25 The two-step solvent−nonsolvent method is based on the concept of immobilization of the surface-active extractant by phase segregation.26 In the solvent treatment step, the surface-active extractant adsorbs onto the swelled polymer matrix. In the nonsolvent treatment step, the phase segregation caused by the nonsolvent ensures the immobilization of extractant by entanglement of the chains of the extractant with the surface molecular chains of the resin, in addition to physical adsorption. In this study, we focused on the removal of trace Cr(VI) anions from aqueous solution using Aliquat 336- (tricaprylmethylammonium chloride-) modified resins (AMRs) prepared by the solvent−nonsolvent method. Aliquat 336 (tricaprylmethylammonium chloride) was chosen for the following two reasons: Aliquat 336 has been used to recover and purify various ionic compounds, such as those containing Cd, Co, Ni, Pa, V, Zn, and Cr.27−31 Because of its relatively long alkyl chains, Aliquat 336 is a surface-active compound and will preferentially adsorb Received: Revised: Accepted: Published: 11685

April 17, 2013 July 25, 2013 July 31, 2013 July 31, 2013 dx.doi.org/10.1021/ie401233r | Ind. Eng. Chem. Res. 2013, 52, 11685−11694

Industrial & Engineering Chemistry Research

Article

onto a polymeric surface when in contact.32 Therefore, by the solvent−nonsolvent method, Aliquat 336 can be immobilized onto the surface of a hydrophobic resin. In this study, experiments on adsorption kinetics and isotherms were performed to investigate the diffusion coefficient of Cr(VI) species, the adsorption capacity, and the adsorption mechanism of Cr(VI) on AMRs. The performance of the AMRs was demonstrated by adsorption/stripping studies of Cr(VI).

2.4. Adsorption of Chromium(VI). The adsorption kinetics of Cr(VI) by AMRs was studied by immersing 0.1 g of resin (2 mmol of Aliquat 336/g of resin) in 1 L of Cr(VI) solution (10−4 M) under continuous stirring. The Cr(VI) solution was prepared by dissolving a suitable amount of potassium dichromate (K2Cr2O7) in distilled and deionized water. The removal rate of Cr(VI) was determined by analyzing the Cr(VI) concentration in the solution at different contact times. The adsorption isotherm was determined by adding different amounts of AMR to 500 mL of Cr(VI) solution (10−4 M) at 25 °C under continuous stirring. In this study, the concentration of Cr(VI) solution was fixed at 10−4 M to avoid any effects of the Cr(VI) concentration on the distribution of Cr(VI) species in solution. After the predetermined equilibrium time, the concentration of Cr(VI) in the solution was measured. For all samples, the concentration of Cr(VI) was measured on an atomic absorption spectrometer (Perkin-Elmer AAnalyst 100) with a chromium cathode lamp (Perkin-Elmer) at a wavelength of 357.9 nm.33 All data are averages of three replicable determinations. 2.5. Stripping of Cr(VI) from AMRs. The stripping behavior of Cr(VI) from the AMRs was studied in batch mode. Before the stripping experiments, each AMR was first equilibrated with Cr(VI) (10−4 M) at pH 3 for 24 h to obtain the maximum loading of Cr(VI). The Cr(VI)-loaded resin (0.1 g) was then stripped with 1 L of NaOH solution (pH 12) for 24 h. During the stripping process, an aliquot of sample solution was withdrawn and assayed for Cr concentration with an atomic adsorption spectrometer. Then, the resins were transferred to fresh alkaline water solution, and the stripping procedure was repeated twice. 2.6. Removal of Cr(VI) from Actual Wastewater. In this study, the AMRs were also tested with actual wastewater. The wastewater was obtained from a local chromium plating factory. The Cr(VI) concentration and pH value of the wastewater were 44.43 mg/L and 1.2, respectively. The wastewater was used as collected without further treatment. The adsorption ability of Cr(VI) by the AMRs was assessed by immersing 10 g of AMR in 1 L of wastewater under continuous stirring at room temperature for 24 h.

2. EXPERIMENTAL SECTION 2.1. Materials. Tricaprylmethylammonium chloride {Aliquat 336, CH3N[(CH2)7CH3]3Cl} was obtained from Alfa Aesar. Potassium dichromate (K2Cr2O7) was purchased from Kanto Chemical (Tokyo, Japan). The chromium standard for atom adsorption analysis was obtained from J.T. Baker. For pH adjustment, sodium hydroxide (NaOH) and hydrochloric acid (HCl) were obtained from Showa (Saitama, Japan) and Aldrich (Milwaukee, WI), respectively. All chemicals were of reagent grade and were used without further purification. Amberlite XAD-4 resin, which was purchased from Fluka, is a polystyrene resin with a specific surface area of 750 m2/g and a pore volume of 1.04 m3/g. Before modification with Aliquat 336, the Amberlite XAD-4 resin was purified by immersing 10 g of the resin in 30 mL of 50 wt % methanol solution containing 4 M HCl for 12 h to remove impurities and monomeric materials. After being filtered, the resin was kept in contact with distilled and deionized water at 70 °C for 1 h to remove the residual hydrochloric acid and methanol. This step was repeated several times until the pH was neutral. Finally, the resin was air-dried at 75 °C. 2.2. Surface Modification of Hydrophobic Resin. For the two-step solvent−nonsolvent method, the hydrophobic resin was immersed first in a solvent containing surface-active extractant and then in a nonsolvent.25 In principle, various types of hydrophobic resins would be suitable for the purposes of this study. However, considering the relatively high surface area and percent swelling of the polymeric adsorbent in the solvent, Amberlite XAD-4 resin was selected. For surface modification, 1 g of the pretreated dry Amberlite XAD-4 resin was immersed in 50 mL of ethanol solution (solvent) for 30 min. After addition of a specified amount of Aliquat 336 (which was varied), the system was equilibrated for 1 h under continuous stirring at 25 °C. Then, a suitable amount of distilled and deionized water (nonsolvent) was added so that the final total water content was equal to 70 wt %. After 1 h, the Aliquat 336-modified resins (AMRs) were separated by vacuum filtration and washed with distilled and deionized water. The AMRs were air-dried at 45 °C. The amount of immobilized Aliquat 336 was determined by the weight difference of the resins before and after immobilization. 2.3. Stability of Immobilized Aliquat 336. The stability of the AMRs was investigated by immersing 1 g of AMR in 10 mL of water under continuous stirring to simulate the conditions of adsorption and stripping experiments. The stability of immobilized Aliquat 336 was determined by measuring the difference in dry weight before and after the immersion treatment. To further demonstrate the stability, the AMRs were also subjected to ultrasonic treatment in water. In this case, 1 g of the modified resin was immersed in 10 mL of distilled and deionized water in a beaker. The beaker was subjected to an ultrasonic bath (Cole-Parmer 8891, 42 kHz), maintained at 25 °C, for different periods of time. All data reported are averages of three samples.

3. RESULTS AND DISCUSSION 3.1. Surface Modification by the Solvent−Nonsolvent Method. In this study, ethanol was a suitable solvent because ethanol can dissolve Aliquat 336 and swell the polymeric matrix of Amberlite XAD-4 resin. Figure 1 shows the volume expansion rate of Amberlite XAD-4 resin as a function of the concentration of ethanol. The packed volume of the Amberlite XAD-4 resin first increased almost linearly with increasing ethanol concentration until 50 wt %. For ethanol concentrations between 50 and 70 wt %, the volume expansion rate of Amberlite XAD-4 resin reached a maximum of about 7%. However, as the ethanol concentration increased to 80 wt %, the expansion of the packed volume decreased slightly to 6%. Scanning electron microscopy (SEM) images of Amberlite XAD-4 resin treated with 50 and 80 wt % ethanol indicate that the decrease of the packed volume was due to the fracture of the resin structure at high ethanol concentration. Therefore, a 70 wt % ethanol/water mixture was considered to be appropriate for use as the solvent for the solvent−nonsolvent method. 11686

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nonsolvent treatment step. This was accomplished by adjusting the composition of the Aliquat 336/ethanol/water solution to be in the lower region of the phase diagram, to ensure phase segregation and immobilization of Aliquat 336. Figure 3 shows the amount of immobilized Aliquat 336 as a function of initial Aliquat 336 concentration in ethanol

Figure 1. Volume expansion rate of Amberlite XAD-4 resin with respect to the concentration of ethanol. Insets: SEM images of Amberlite XAD-4 resin treated with 50 and 80 wt % ethanol (scale bars = 100 μm).

The solvent−nonsolvent method involved mixing solvent, extractant, and nonsolvent in different stages of preparation. Therefore, the solution behavior of the solvent/extractant/ nonsolvent system was investigated. Figure 2 shows the phase

Figure 3. Amount of immobilized Aliquat 336 and water uptake as functions of initial Aliquat 336 concentration in ethanol solution.

solution. The amount of immobilized Aliquat 336 first increased with increasing Aliquat 336 concentration until 17 vol %. Then, the amount of immobilized Aliquat 336 reached a saturation amount of about 2.0 mmol/g of resin. Based on the density of Aliquat 336 (0.884 g/cm3) and the specific surface area of the Ambrelite XAD-4 resin (750 m2/g), it was estimated that the most compact arrangement of immobilized Aliquat 336 was about 2.3 mmol/g of resin. Therefore, the surface of Amberlite XAD-4 resin loaded with 2.0 mmol of Aliquat 336 was approximately covered with a monolayer of immobilized Aliquat 336 molecules. The immobilization of Aliquat 336 changed the surface characteristics of the Amberlite XAD-4 resin, which became hydrophilic because of the presence of the cationic moiety of the immobilized Aliquat 336. The hydrophilic nature of the AMRs can be demonstrated by the amount of water uptake. Figure 3 also shows the amount of water uptake with respect to the amount of immobilized Aliquat 336. Only a small amount of water uptake (0.03 g/g of resin) occurred in the nonmodified Amberlite XAD-4 resin because of the hydrophobic nature of the resin. With increasing amount of immobilized Aliquat 336, the hydrophilicity of the resin increased. The amount of water uptake increased slowly at first and then more rapidly. For resin immobilized with a monolayer of Aliquat 336, the water uptake reached about 0.51 g/g of resin. The preceding experimental results reveal the advantageous characteristics of AMRs prepared by the solvent−nonsolvent method. In the solvent treatment step, the surface molecular chains of the polymer matrix of the resin can be partially swelled (or dissolved) by a solvent. The swelling of the resin by the solvent promotes the incorporation of the extractant into the swelled polymer matrix. The surface-active nature of Aliquat 336 ensures monolayer adsorption on the surface of the resin. In the nonsolvent treatment step, a polar nonsolvent that is miscible with the solvent but immiscible with the extractant is added to elute the solvent from the swelled interfacial region.

Figure 2. Phase diagram of the ethanol/Aliquat 336/water system at 25 °C and 1 atm. The arrows represent changes in composition during solvent−nonsolvent modification of Amberlite XAD-4 resin.

diagram of the ethanol/Aliquat 336/water system at 25 °C and 1 atm. Because Aliquat 336 is soluble in ethanol and only marginally soluble in water, the phase diagram of the ethanol/ Aliquat 336/water system shows two distinct regions. In the upper ethanol-rich region, the mixture of ethanol, Aliquat 336, and water forms a clear homogeneous solution. In the lower water-rich region, the system forms a mixture of two immiscible phases because of the low solubility of Aliquat 336 in water. Figure 2 also shows the changes in solution composition during the solvent−nonsolvent modification of Amberlite XAD-4 resin. In the solvent treatment step, the composition of the Aliquat 336/ethanol/water mixture was in the clear homogeneous region to ensure good incorporation of Aliquat 336 molecules into the resin matrix. The deposition of Aliquat 336 on the surface of the resin occurred in the subsequent 11687



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The elution of the solvent by the nonsolvent causes phase segregation of the extractant from the solvent. The immobilization of the extractant, therefore, is realized by the entanglement of the chains of the extractant with the surface molecular chains of the resins. The polar nature of water (nonsolvent) also promotes the exposure of the polar functional groups of Aliquat 336, which, in turn, increases the hydrophilicity of the AMRs and the amount of accessible functional groups for Cr(VI) removal from wastewater. 3.2. Stability of AMRs. One major concern with using extractant-modified resins is the stability of immobilized extractant. Because Aliquat 336 is slightly soluble in water, some immobilized Aliquat 336 could be leached out during the process if the Aliquat 336 molecules are not strongly attached to the resin. It is therefore necessary to study the stability of the immobilized Aliquat 336 in the AMRs. The stability of the AMRs was first investigated by immersing the AMRs in water under magnetic stirring. Figure 4 shows that Figure 5. Batch adsorption kinetics of Cr(VI) on AMRs and relative errors of the model fitted to experimental values at pH 3, 7, and 12. The solid lines in the lower panel represent the pseudo-second-order kinetic model.

3, 7, and 12. In general, the kinetic study at different pH values showed a rapid sorption within the first few hours, followed by a slower uptake until a final equilibrium was reached in about 24 h. At pH 3 and 7, the kinetic data showed that the initial adsorption rates were almost the same within the first 2 h. In the subsequent slow uptake region, the adsorption rate at pH 7 was lower than that at pH 3. The adsorption at pH 12 was relatively slower than that at both pH 3 and 7 throughout the entire kinetic region. Figure 5 also shows that the equilibrium amount of Cr(VI) adsorbed decreased with increasing pH. Because the ammonium structure of Aliquat 336 has a permanent positive charge due to the quaternary ammonium functionality, the immobilized Aliquat 336 can form salts with Cr(VI) anions over the pH range considered in this study. The adsorption kinetic behavior of Cr(VI) anions at different pH values, therefore, must originate from changes in the chemical composition of the solution phase. The concentration of anionic species and the relative abundance in solution with respect to the total Cr(VI) concentration ([Cr(VI)]total) and the total anion concentration ([anions]total) were determined by MINEQL+ calculations (Table S1, Supporting Information). At pH 3, the dominant Cr(VI) species is HCrO4−, with a relative abundance of greater than 99%. The adsorption of Cr(VI) is mainly in the form of HCrO4−. At pH 7, the relative abundances of CrO42− and HCrO4− are 75.3% and 24.6%, respectively. Both forms of Cr(VI) can be adsorbed by AMRs. For pH 12, the adsorption of Cr(VI) species is mainly for CrO42− ([CrO42−]/[Cr(VI)]total = 99.5%). Therefore, the decrease of the adsorption rate with increasing pH is due to the increasing amount of CrO42−. For practical applications of adsorption processes, the identification of adsorption mechanism is an essential issue. The mechanisms of solute sorption onto an adsorbent can be subdivided into reaction-based and diffusion-based models.34 In this study, both types of kinetic models were tested to elucidate the mechanism of Cr(VI) adsorption onto AMRs. 3.3.1. Reaction Kinetic Models. The adsorption kinetics of Cr(VI) onto the AMRs was fitted with a pseudo-first-order rate

Figure 4. Stability of immobilized Aliquat 336 under ultrasonic and stirring conditions.

the amount of immobilized Aliquat 336 decreased slightly during the first 10-min period of immersion. This was due to the removal of loosely attached Aliquat 336 molecules. Then, the amount of immobilized Aliquat 336 remained constant. More than 93% of the Aliquat 336 remained on the surface of the Amberlite XAD-4 resin. To further demonstrate stability, the AMRs were subjected to ultrasonic treatment (42 kHz) in water. The removal rate of Aliquat 336 in the first 5 min was slightly higher than that under stirring conditions. Then, the amount of immobilized Aliquat 336 remained constant. More than 92% of the Aliquat 336 remained on the Amberlite XAD-4 resin even after 30 min of ultrasonic treatment. The high stability of the AMRs is mainly due to the entanglement of the chains of the Aliquat 336 molecules with the polymeric matrix during the solvent−nonsolvent process. For practical applications, the stability of immobilized Aliquat 336 after the adsorption (pH 3) and stripping (pH 12) steps was also monitored for a complete cycle of adsorption and stripping processes. The experimental results showed that 93% of the originally immobilized Aliquat 336 remained on the resin during the adsorption process at pH 3 for 24 h, consistent with the results in Figure 4. After 24 h of stripping at pH 12, the amount of immobilized Aliquat 336 remained almost the same (>93%). 3.3. Adsorption Kinetics of Cr(VI) by AMRs. Figure 5 shows the batch adsorption kinetics of Cr(VI) on AMRs at pH 11688



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expression. This model has been applied to the sorption of Cr(VI) onto several adsorbents.35 The linear form of the pseudo-first-order rate equation is36 ln(qe − qt ) = ln qe − k1t

(1)

where qt and qe are the amounts of Cr(VI) adsorbed at time t and at equilibrium, respectively, and k1 (s−1) is the pseudo-firstorder rate constant. If the pseudo-first-order kinetic model is applicable, a plot of ln(qe − qt) versus t should give a straight line, from which k1 and ln qe can be determined as the slope and intercept, respectively. The values of the correlation coefficient (R2) for such a fit were relatively low (Table S2, Supporting Information). In addition, the values of qe obtained from the linear plots did not agree with the experimental values. Therefore, the pseudo-first-order rate equation is not applicable to the adsorption of Cr(VI) on the AMRs. The linear form of the pseudo-second-order equation can be expressed as37 t 1 1 = + t 2 qt q k 2qe e −1

Figure 6. Plots of qt versus t0.5 at pH 3, 7, and 12.

obtained at pH 3, 7, and 12. All of the plots show a common feature of a three-stage adsorption mechanism. The initial linear portion can be attributed to film diffusion, the second linear portion to intraparticle diffusion, and the last linear portion to adsorption−desorption equilibrium. Similar results were observed for the adsorption of Cr(VI) on rice straw.39 This feature of this diffusion plot indicates that the adsorption kinetics of Cr(VI) by the AMRs might be jointly controlled by film diffusion (outer diffusion) and intraparticle diffusion (inner diffusion). The values of the rate constant (k), intercept (Ci), and correlation coefficient (R2) obtained for the plots in Figure 6 are reported in Table S3 (Supporting Information). The values of k in both the film-diffusion- and intraparticle-diffusioncontrolled regions generally decreased with increasing pH. As pointed out previously, the decrease of the diffusion rate with increasing pH might be due to the increasing abundance of CrO42− at higher pH (Table S1, Supporting Information). Because the hydrated CrO42− ion is larger than the hydrated HCrO4− ion,40 the migration of CrO42− is slower than that of HCrO4−. The linear fits of the second stage (intraparticle region) did not pass through the origin, and the intercept (Ci) decreased with increasing pH. The deviation of the intercept from the origin might be due to the difference in the masstransfer rates of Cr(VI) species in the initial film-diffusion stage of adsorption. 3.3.3. Diffusion Coefficient. The determination of diffusion coefficients is of importance in the design of mass-transfer processes. Because the adsorption process was found to be jointly controlled by film diffusion and intraparticle diffusion, the diffusion coefficient of Cr(VI) species at different stages of adsorption can be estimated with suitable models. Because the film-diffusion-controlled region appeared in the initial stage of the adsorption process, a modified shrinking-core model for the film-diffusion-controlled region can be described by the equation41

(2) −1

where k2 (g mmol s ) is the rate constant of second-order adsorption. The kinetic parameters of the pseudo-second-order equation were obtained by plotting t/qt versus t, where the parameters qe and k2 can be determined from the slope and intercept, respectively. Table S2 (Supporting Information) shows that the values of the correlation coefficient (R2) were very high (>0.99). However, the equilibrium amounts of adsorbed Cr(VI) predicted by the pseudo-second-order model were only marginally close to the experimental values (within 12%). The fitted results for the pseudo-second-order model are also shown in Figure 5. Similar trends can be observed for all three pH values in that the pseudo-second-order model overestimated the qt value at the initial stage of adsorption but underestimated the value at the stage before equilibrium. The relative error of the fitted model to the experimental values, defined as (qt,cal − qt,exp), is also shown in Figure 5. Therefore, although the fit with the pseudo-second order showed very high R2 values, the systematic deviation between the model and the experimental data indicate that the adsorption kinetics of Cr(VI) on the AMRs cannot be appropriately described by the pseudo-second-order model either. 3.3.2. Diffusion Kinetics Model. The adsorption process in a porous material can be described by three consecutive steps as follows: (1) mass transfer of the solute from the bulk solution through a diffusion layer to the particle surface (film diffusion), (2) diffusion of the solute within the porous adsorbent particle to the adsorption sites (intraparticle diffusion), and (3) adsorption of the solute at the adsorption sites. As a very general guideline, if equilibrium is achieved within 3 h, the process is usually kinetically controlled, whereas if more than 24 h is require for equilibrium to be attained, it is diffusioncontrolled.34 The diffusion mechanism of the adsorption process was determined from the diffusion model.38 According to this theory

qt = kt

0.5

+ Ci

X = (total amount of metal adsorbed on AMR at time t ) t 3Df /(AMR capacity) = C dt rδC̅ Bo 0 (4)

∫

(3)

where X is the fractional attainment of equilibrium, Df is the film diffusion coefficient, C is the Cr(VI) concentration in the liquid phase, r is the radius of the resin, δ is the thickness of the film, and C̅ Bo is the adsorption capacity of the sorbed species in

−1 −0.5

where k is the diffusion rate constant (mmol g s ), obtained from the slope of the straight line in a plot of qt versus t0.5. Figure 6 shows the qt versus t0.5 plots of the kinetic data 11689



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the resin. The integral ∫ C dt was evaluated graphically by the trapezoid rule. By plotting X versus ∫ C dt, the film diffusion coefficient Df was determined from the slope. For the intraparticle-diffusion-controlled region, the modified shrinking-core model can be written as42 2/3

1 − 3(1 − X )

+ 2(1 − X ) =

6De̅ Ce̅ r 2

∫ C dt

The effective intraparticle diffusion coefficient is related to the aqueous molecular diffusion coefficient (D̅ aq) in the pore space by the expression45 Daq ̅ =

De̅ ε τ

(6)

where ε is the porosity and τ is the tortuosity factor. In this study, the porosity of Amberlite XAD-4 resin was taken to be 0.51, and the tortuosity was estimated as τ = 1/ε1/2.46 Table S4 (Supporting Information) also includes the values of D̅ aq determined by eq 6. The implicit assumption of eq 6 is that the true mobility of a species in the pores is the same as that in the absence of the resin matrix. Therefore, the value of D̅ aq estimated from D̅ e might indicate the physical nature of the diffusion of Cr(VI) in the pores.45 Comparing Df and D̅ aq, several interesting points can be noted. The mobility of HCrO4− in the pores, D̅ aq,pH 3, is close to the value of Df,pH 3, indicating that the pores are filled with water because of the hydrophilicity of the AMRs. D̅ aq,pH 7 is smaller than Df,pH 7, but it is close to Df,pH 12, indicating that the intraparticle diffusion at pH 7 is mainly controlled by the diffusion of CrO42− ions in the pores of the AMRs. D̅ aq,pH 12 is much smaller than Df,pH 12. This might be because, at pH 12, the high concentration and high diffusion coefficient of OH− impede the diffusion of CrO42− ions in the pores of the AMRs. The reported diffusion coefficient of OH− is 5.23 × 10−5 cm2/s.43 3.4. Adsorption Equilibrium of Cr(VI) on AMRs. 3.4.1. Langmuir Isotherm. To understand the nature of Cr(VI) adsorption on the AMRs, the equilibrium adsorption isotherms were investigated. Figure 8 shows the adsorption

(5)

where D̅ e is the effective intraparticle diffusion coefficient for sorption (assumed to be constant), X is the fractional conversion of resin at equilibrium, and C̅ e is the equilibrium concentration of the sorbed species in the resin (mmol/L). By plotting 1 − 3(1 − X)2/3 + 2(1 − X) versus ∫ C dt in the intraparticle-diffusion-controlled region, the effective intraparticle diffusion coefficient D̅ e can be determined from the slope. Figure 7 shows a plot of the observed data and the applied models. The estimated film diffusion coefficients at different pH

Figure 7. Plot of the experimental data and the applied models.

values are listed in Table S4 (Supporting Information). At pH 3, the dominant Cr(VI) species is the HCrO4− ion. The film diffusion coefficient at pH 3, Df,pH 3, was found to be 1.43 × 10−5 cm2/s. At pH 12, the dominant Cr(VI) species is CrO42−. The film diffusion coefficient of CrO42− (Df,pH 12) was found to be 0.98 × 10−5 cm2/s, which is close to the reported CrO42− molecular diffusion coefficient in water (i.e., 1.07 × 10−5 cm2/ s).43 These results confirm that the diffusion coefficient of CrO42− is smaller than that of HCrO4−. At pH 7, the diffusion coefficient of Cr(VI) species, Df,pH 7, was found to be equal to 1.29 × 10−5 cm2/s. Because the Cr(VI) species at pH 7 is a mixture of HCrO4− (24.6%) and CrO42− ions (75.3%), Df,pH 7 is an average diffusion coefficient of mixed Cr(VI) species, which can also be estimated from the weighted average of Df,pH 3 and Df,pH 12.44 The value of Df,pH 7, estimated as 0.246 DHCrO4− + 0.753DCrO42−, is 1.16 × 10−5 cm2/s, which is close to the Df,pH 7 value obtained from the film-diffusion-controlled region. The estimated effective intraparticle diffusion coefficients of Cr(VI) in the AMRs are 5.11 × 10−6, 3.28 × 10−6, and 2.03 × 10−6 cm2/s for pH 3, 7, and 12, respectively (Table S4, Supporting Information). The decreases of Df and D̅ e with increasing pH value are mainly due to the increasing content of CrO42− species.

Figure 8. Adsorption isotherms of Cr(VI) at pH 3, 7, and 12. The dashed and solid lines represent the simulated Langmuir and two-step adsorption isotherm models.

isotherms of Cr(VI) at pH 3, 7, and 12. Because the previous results indicated that a monolayer of Aliquat 336 was immobilized on the surface of the AMRs prepared by solvent−nonsolvent method, the Langmuir adsorption isotherm model was first tested. The Langmuir model assumes monolayer adsorption onto a surface containing a finite number of adsorption sites with a uniform energy of adsorption and no transmigration of adsorbate on the surface. The linear form of the Langmuir isotherm equation is47 Ce 1 1 = + Ce qe qmax KL qmax 11690

(7)



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where Ce (mmol/L) is the equilibrium concentration of Cr(VI); qe (mmol/g) is the amount of Cr(VI) adsorbed per gram of AMR; and qmax (mmol/g) and KL (L/mmol) are constants related to the maximum adsorption capacity and the equilibrium adsorption constant, respectively. When Ce/qe is plotted against Ce (figure not shown), the values of qmax and KL can be obtained from the intercept and slope, respectively, of the resulting straight line. Table S5 (Supporting Information) lists the fitted parameters. Based on the correlation coefficients, the Langmuir adsorption model seems to describe the adsorption isotherms reasonably well for pH 3 and 7 (R2 > 0.98). The fitted curves are also shown in Figure 8 (dashed lines). However, it was observed that the equilibrium data deviated from the fitted Langmuir models at higher equilibrium concentrations, indicating that an additional adsorption mechanism might be involved at higher Cr(VI) surface loading on the resin. For pH 12, the isotherm cannot be described by the Langmuir model (R2 = 0.716). 3.4.2. Multistep Isotherm. By visual inspection of the isotherms (Figure 8), it was observed that the adsorption isotherms at pH 3 and 12 reflected a characteristic multistep type of adsorption. To represent the multistep-shaped isotherms measured in this study, a nonlinear mathematical model can be obtained from the sum of Langmuir-type isotherms with the additional assumption that there are potential interactions among the adsorbed molecules at high surface loading.47 Each step on the curve represents different existing specific types of adsorption mechanisms that can be described by the Langmuir equation. A two-step isotherm is given by47 qe =

a1k1Ce a 2k 2[(Ce − b2) + |Ce − b2|] + 1 + k1Ce 2 + k 2[(Ce − b2) + |Ce − b2|]

regarding the energetic background of the adsorption processes.47 For comparison, the adsorption of Cr(VI) by the AMRs was compared with those of other systems related to the adsorption of Cr(VI). Some of the results are presented in Table S6 (Supporting Information). It is clear that the AMR system provides a relatively higher extractant loading and qmax value than most of the other adsorption systems, particularly for the Aliquat 336/XAD system. This might be due to because, during the nonsolvent treatment step, phase segregation promotes the immobilization of the monolayer of extractant, as well as the exposure of the polar functional groups of Aliquat 336, which, in turn, increases the hydrophilicity of the AMRs and the number of accessible functional groups for Cr(VI) adsorption. This also explain the higher adsorption capacity of the AMRs per mole of extractant. 3.4.3. Adsorption Mechanism of Cr(VI) on AMRs. Based on the adsorption kinetics and isotherm results, the adsorption mechanism of Cr(VI) on AMRs at different pH values is postulated as follows. At pH 3, for which the dominant Cr(VI) species is HCrO4−, the first-step equilibrium is mainly due to the adsorption of HCrO4−. The second-step equilibrium appeared as the Cr(VI) loading was about 1.1 mmol/g. Considering the ion-exchange resin as a high-capacity strongly ionized homogeneous electrolyte, this adsorption loading is equivalent to a Cr(VI) concentration inside the AMRs of approximately 2.0 M, that is, qe divided by water content (0.51 mL/g of resin). Therefore, the second-step equilibrium might be due to the formation of dichromate (Cr2O72−) at high adsorption loading of HCrO4−.48,49 Consequently, the adsorption mechanism is proposed as follows First step R3NH4 + + HCrO4 − → R3NH4 +(HCrO4 −)

(8)

(9)

Second step

where ai represents the convergence limit of the adsorption capacity (mmol/g) for the corresponding step, ki is the corresponding adsorption equilibrium constant (L/mmol), and b2 is the critical concentration limit of the associated adsorption mechanism (mmol/L). The fitness of the models for the data was assessed from the correlation coefficient (R2) and five error functions, namely, the sum of squares of errors (ERRSQ), the hybrid fractional error function (HYBRID), Marquart’s percentage standard deviation (MPSD), the average relative error (ARE), and the sum of the absolute errors (EABS). The statistical analysis showed that the two-step isotherm model describes the adsorption isotherms significantly better than the single-step Langmuir isotherm model at pH 3 and 12 (Table S5, Supporting Information). However, at pH 7, both the single- and two-step isotherm models describe the isotherm equally well. The fitted two-step adsorption isotherm models are also shown in Figure 8 (solid lines). Table S5 (Supporting Information) also includes the parameters calculated from the two-step model. The cumulative adsorption capacities, defined as Smax = ∑ai, were found to be 1.37, 1.14, and 0.69 mmol/g for pH 3, 7, and 12, respectively. For pH 3, 7, and 12, the fitted values of b2 were found to be 0.04, 0.02, and 0.08 mmol/L, corresponding to surface loadings, qe, equal to 1.1, 0.68, and 0.22 mmol/g, respectively. The ki constant, which depends on temperature, characterizes the adsorption equilibrium. Interpretation of individual ki values, however, is rather difficult without information

2HCrO 4 − → Cr2O7 2 − + H 2O

(10)

2R3NH4 + + Cr2O7 2 − → (R3NH4 +)2 (Cr2O7 2 −)

(11)

2R3NH4 +(HCrO4 −) → (R3NH4 +)2 (Cr2O7 2 −) + H 2O (12)

R3NH4 +(HCrO4 −) + HCrO4 − + R3NH4 + → (R3NH4 +)2 (Cr2O7 2 −) + H 2O

(13)

At pH 7, for which the Cr(VI) solution consists of 75.3% CrO 4 2− and 24.6% HCrO 4 −, the adsorption isotherm represents the mixed adsorption of CrO42− and HCrO4−. Furthermore, the adsorption capacity at pH 7 is lower than that of pH 3, possibly because of the coexistence of HCrO4− and CrO42− in solution. Upon adsorption, CrO42− requires two single-valence Aliquat 336 molecules for electroneutrality. At pH 12, for which the dominant Cr(VI) species is CrO42−, the adsorption isotherm clearly reflects two-step equilibrium. The first-step adsorption isotherm is mainly due to the adsorption of CrO42− on the AMRs. The adsorption capacity at the first step is much smaller than those at pH 3 and 7, because of the divalent nature of CrO42− and the competitive adsorption of OH− ions at high pH values. The second-step adsorption isotherm might be due to the adsorption of CrO42− 11691



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at less favorable sites, such as sites in the vicinity of anionoccupied sites. 3.5. Stripping of Cr(VI) from AMRs. Various stripping solutions can be used to strip adsorbed Cr(VI) from AMRs. In this study, a NaOH alkaline stripping solution at pH 12 was tested. Figure 9 shows the stripping kinetics of Cr(VI) from

Figure 10. Reusability of AMRs for the adsorption of Cr(VI). Conditions: AMRs, 0.027 g; Cr(VI) aqueous solution, 1 L, 10−4 M, pH 3.

(Table S6, Supporting Information). Nevertheless, there was still some slight loss of adsorption capacity because of the loss of loosely bonded extractant. One possible solution to this problem would be to combine the solvent−nonsolvent method with a protective coating.51 3.7. Treatment of Actual Chromium Plating Wastewater. For the adsorption of Cr(VI), the presence of various anion spices can affect the adsorption rate of Cr(VI) through competitive adsorption. In this study, the Aliquat 336-modified resins were used for the treatment of an actual chromium wastewater. The wastewater was obtained from a local chromium plating factory. The anionic species in the original wastewater included Cr(VI) (44.43 mg/L), Cl− (0.81 mg/L), NO3− (1.24 mg/L), SO42− (2.23 mg/L), and F− (0.61 mg/L). The trace amount of cationic species included Zn2+ and Ni2+. Figure 11 shows the results of adsorption. For 1 L of

Figure 9. Stripping kinetics of Cr(VI) from AMRs for two sequential batches.

AMRs with an initial Cr(VI) loading equal to 0.86 mmol/g during two sequential batches of stripping. In the first stripping batch (0−24 h), the stripping efficiency reached more than 80% after 7 h. The stripping efficiency then gradually leveled off at 90% after 20 h because of the establishment of an equilibrium in the batch-mode stripping process. For the second stripping batch (24−48 h), the stripping efficiency reached 99.1% after 48 h. Apparently, stripping of Cr(VI) is caused by OH− replacement. The stripping kinetic can be described with pseudo-first- and -second-order desorption kinetic models50 Pseudo-first-order qtd = qed[1 − exp(−kdt )]

(14)

Pseudo-second-order qtd =

qed 2kdt 1 + qedkdt

(15)

where kd is the rate constant and qtd and qed are the amounts of Cr(VI) desorbed at time t and at equilibrium, respectively. Table S7 (Supporting Information) lists the fitted desorption rate parameters for the first batch stripping (0−24 h). The pseudo-second-order desorption model shows a better fit to the experimental data, with R2 > 0.990. In addition, the qed value predicted by the pseudo-second-order model agrees better with the experimental value, namely, 0.79 mmol/g after 24 h. 3.6. Reusability of AMRs for the Adsorption of Cr(VI). To investigate the reusability of the AMRs, four consecutive adsorption/stripping cycles were carried out. Figure 10 shows that, over four adsorption/stripping cycles, the adsorption capacity of the AMRs decreased from 1.25 to 1.16, 0.99, and 0.97 mmol/g, whereas the stripping of Cr(VI) remained as high as 97%. It is worth noting that, even after four cycles of adsorption/stripping, the adsorption capacity of the AMRs was still higher than those of other Aliquat 336/XAD systems

Figure 11. Cr(VI) adsorption ability of AMRs from actual chromium plating wastewater.

wastewater, more than 92% of the Cr(VI) was removed by the AMRs. The amount of Cr(VI) remaining in the water phase was less than 4 mg/L. The removal rates of other anions were 6.2% for Cl−, 11.36% for NO3−, 17.08% for SO42−, and 32.73% for F−, indicating that the AMRs have a higher selectivity toward Cr(VI).

4. CONCLUSIONS Aliquat 336 can be immobilized onto Amberlite XAD-4 resin by a two-step solvent−nonsolvent method. As a result, the alkyl 11692



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(11) Lanagan, M. D.; Ibana, D. C. The solvent extraction and stripping of chromium with Cyanex® 272. Miner. Eng. 2003, 16, 237. (12) Salazar, E.; Ortiz, M. I.; Urtiaga, A. M.; Irabien, J. A. Kinetics of the separation−concentration of chromium(VI) with emulsion liquid membranes. Ind. Eng. Chem. Res. 1992, 31, 1523. (13) Bajpai, S. K. Removal of hexavalent chromium by adsorption onto fireclay and impregnated fireclay. Sep. Sci. Technol. 2001, 36, 399. (14) Schmuhl, R.; Krieg, H. M.; Keizer, K. Adsorption of Cu(II) and Cr(VI) ions by chitosan: Kinetics and equilibrium studies. Water SA 2001, 27, 1. (15) Nomanbhay, M.; Palanisamy, K. Removal of heavy metal from industrial wastewater using chitosan coated oil palm shell charcoal. Electron. J. Biotechnol. 2005, 8, 43. (16) Abdel-Halim, E. A.; Abou-Okeil, A.; Hashem, A. Adsorption of Cr(VI) oxyanions onto modified wood pulp. Polym.-Plast. Technol. Eng. 2006, 45, 71. (17) Boddu, V. M.; Abburi, K.; Talbott, J. L.; Smith, E. D.; Haasch, R. Removal of arsenic(III) and arsenic(V) from aqueous medium using chitosan-coated biosorbent. Water Res. 2008, 42, 633. (18) Shi, T.; Wang, Z.; Liu, Y.; Jia, S.; Changming, D. Removal of hexavalent chromium from aqueous solutions by D301, D314 and D354 anion-exchange resins. J. Hazard. Mater. 2008, 161, 900. (19) Bajpai, S. K.; Johnson, S. Removal of Cr(VI) oxy-anions from aqueous solution by sorption into poly(acrylamide-co-maleic acid) hydrogels. Sep. Sci. Technol. 2007, 42, 1049. (20) Nastasović, A.; Jovanović, S.; Dordević, D.; Onjia, A.; Jakovljevi, D.; Novaković, T. Metal sorption of macroporous poly(GMA-coEGDMA) modified with ethylene diamine. React. Funct. Polym. 2004, 58, 139. (21) Zhao, D.; SenGupta, A. K.; Stewart, L. Selective removal of Cr(VI) oxyanions with a new anion exchanger. Ind. Eng. Chem. Res. 1998, 37, 4383. (22) Gang, D.; Hu, W.; Banerji, S. K.; Clevenger, T. E. Modified poly(4-vinylpyridine) coated silica gel. Fast kinetics of diffusioncontrolled sorption of chromium(VI). Ind. Eng. Chem. Res. 2001, 40, 1200. (23) Kabay, N.; Arda, M.; Saha, B.; Streat, M. Removal of Cr(VI) by solvent impregnated resins (SIR) containing Aliquat 336. React. Funct. Polym. 2003, 54, 103. (24) Chen, J. H.; Kao, Y. Y.; Lin, C. H.; Yang, F. R. Surface modification of Amberlite XAD-4 resin with D2EHPA by a two-step, solvent−nonsolvent procedure and the application on the selective separation of lead and copper ions. Sep. Sci. Technol. 2004, 39, 2067. (25) Chen, J. H.; Huang, C. E. Selective separation of Cu and Zn in the citric acid leachate of industrial printed wiring board sludge by D2EHPA-modified Amberlite XAD-4 resin. Ind. Eng. Chem. Res. 2007, 46, 7231. (26) Chen, J. H.; Ruckenstein, E. Generation of porous polymer surface by solvent−nonsolvent treatment. J. Appl. Polym. Sci. 1992, 45, 377. (27) Juang, R. S.; Kao, H. C.; Wu, W. H. Analysis of liquid membrane extraction of binary Zn(II) and Cd(II) from chloride media with Aliquat 336 based on thermodynamic equilibrium models. J. Membr. Sci. 2004, 228, 169. (28) Nayl, A. A. Extraction and separation of Co(II) and Ni(II) from acidic sulfate solutions using Aliquat 336. J. Hazard. Mater. 2010, 173, 223. (29) Ding, H. J.; Niu, Y. N.; Xu, Y. B.; Yang, W. F.; Yuan, S. G.; Qin, Z.; Zhou, X. H. Liquid−liquid extraction of 233 Pa(V) with Aliquat 336. J. Radioanal. Nucl. Ch. 2006, 268, 433. (30) Bal, Y.; Bal, K. E.; Cote, G. Kinetics of the alkaline stripping of vanadium(V) previously extracted by Aliquat® 336. Miner. Eng. 2002, 15, 377. (31) Saha, B.; Gill, R. J.; Bailey, D. G.; Kabay, N.; Arda, M. Sorption of Cr(VI) from aqueous solution by Amberlite XAD-7 resin impregnated with Aliquat 336. React. Funct. Polym. 2004, 60, 223. (32) Xu, J.; Paimin, R.; Shew, W.; Wang, X. An investigation of solubility of Aliquat 336 in different extracted solutions. Fiber. Polym. 2003, 4, 27.

chains of the Aliquat 336 molecules entangle with the polymer matrix of the resin to ensure good stability. The Aliquat 336modified resins (AMRs) prepared by the solvent−nonsolvent method contained a monolayer of immobilized Aliquat 336. The amount of Aliquat 336 immobilized in the AMRs reached about 2.0 mmol/g of resin. A kinetic study showed that the adsorption of Cr(VI) was film-diffusion-controlled and then intraparticle-diffusion-controlled. The intraparticle diffusion coefficient of Cr(VI) was found to be approximately equal to the diffusion coefficient in the bulk phase, indicating that the pores of the AMRs were filled with water, which facilitated the diffusion of Cr(VI) in the AMRs. The exposure of the polar functional groups of Aliquat 336 renders the AMRs hydrophilic and provides good access for Cr(VI) removal from wastewater. The AMR system was found to have a relatively higher qmax value of 1.37 mmol/g compared to other systems used for Cr(VI) removal. For stripping, the efficiency reached 99%. In terms of stability and reusability, the AMRs maintained a high level of Cr(VI) adsorption even after four cycles of adsorption/ stripping. The experimental results demonstrate the potential of practical application of this new type of resin for trace Cr(VI) treatment to reduce the environmental impact of this dangerous heavy metal.

■

ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +886-2-2771-2171 ext 2719. Fax: +886-2-2731-7185. Notes

The authors declare no competing financial interest.

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REFERENCES

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