Removal of Aniline from Aqueous Solution using Pine Sawdust

Pine sawdust (PS) was modified with citric acid (CA) and β-cyclodextrin (CD) for aniline removal, and its adsorption properties were investigated usi...
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Removal of Aniline from Aqueous Solution using Pine Sawdust Modified with Citric Acid and β‑Cyclodextrin Yanbo Zhou,* Xiaochen Gu, Ruzhuang Zhang, and Jun Lu Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, No. 130, Meilong Road, Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: Pine sawdust (PS) was modified with citric acid (CA) and β-cyclodextrin (CD) for aniline removal, and its adsorption properties were investigated using a batch and column experiment. On the basis of the Langmuir analysis, the maximum adsorption capacities were determined to be 16.3, 117.0, and 233.6 μmol g−1 of the base-treated PS, CA-modified PS, and CA+CD-modified PS, respectively, which showed that CA+CD-modified PS possesses the strongest adsorption ability for aniline because of the hydrogen bond interaction, hydrophobic interaction, and cyclodextrin inclusion. The adsorption capacities of aniline on modified PS increased with an increase in the ionic strength and changed little with an increase in pH from 5.0 to 10.0. The column experiment results showed that the space velocity strongly influenced the aniline uptake capacity. As modified PS is easily obtained and exhibits high adsorption capacity, it is a promising candidate for use in the removal of aniline from wastewater.

1. INTRODUCTION

utilization of sawdust will benefit both the environment and the field of wood agriculture and is therefore urgently needed. Many effective biosorbent modification methods have been developed, introducing special functional groups onto natural biosorbents in order to obtain relatively large adsorption capacity and relatively high adsorption selectivity.15−17 βCyclodextrin (β-CD) is one of the most commonly used cyclodextrin in many fields including foods, pharmaceuticals, cosmetics, and chemical products. Through host−guest interactions, it can remarkably increase capacity to form inclusion complexes with organic molecules. If β-CD is used for modifying sawdust, a better efficiency of the adsorption of organic pollutants can be expected.18,19 Usually, the chemical modification of β-CD is carried out using cross-linking agents such as epichlorohydrin, diisocyanates, polycarboxylic acids, and anhydrides.19 Citric acid (CA) can be used as a cross-linking agent to modify cellulosecontaining materials by forming an ester linkage. The condensation between β-CD and CA can progress at a temperature lower than 473 K without the use of harmful organic solvents and additives.20,21 Moreover, a β-CD polymer cross-linked by CA contains both β-CD and carboxyl groups (−COOH), which may cooperate with the adsorption of metal ions and organic compounds. In this study, base-treated pine sawdust (B-PS), CA-modified PS (CA-PS), and β-CD+CA-modified PS (CD-CA-PS) were prepared. The adsorption of aniline by these three modified PSs was studied, and the effects of contact time, initial concentration, pH, ion strength, and adsorbent dose are discussed. Furthermore, fixed-bed adsorption studies were

Aniline is frequently used as an organic intermediate in the process of producing dyes, rubbers, medicine, and paint. Aniline is very harmful to aquatic life because of its high toxicity and accumulation in the environment.1 It is estimated that 30,000 tons of aniline are discharged into the environment annually.2 Labeled as a priority pollutant by the Ministry of Environmental Protection of China, aniline has a maximal discharge concentration of 5.0 mg L−1 (0.054 mmol L−1) according to the wastewater discharge standard.3 Because of its dramatically negative environmental impact, the development of efficient aniline removal technologies has increasingly become a significant environmental concern. Traditionally, aniline-containing wastewater is treated using photocatalytic oxidation, electrolysis, adsorption, oxidation, biodegradation, membrane, etc.4−6 From the technical and economic points of view, adsorption has proved to be an effective method for aniline removal. Although activated carbon is used widely in various cleaning procedures, it remains expensive. Therefore, over the past 20 years, an increasing number of environmental scientists and engineers have considered low-cost natural biosorbents as alternatives to activated carbon because these materials are biodegradable, renewable, and nontoxic.7 Another important reason is that a natural biosorbent contains a considerable amount of floristic fiber, protein, and some functional groups such as carboxyl, hydroxyl, and amidogen, which can form hydrogen bonds with organic compounds or bind organic compounds through the ion-exchange effect.8−10 Studies have shown that sawdust is a promising adsorbent for removing heavy metals and dyes.11,12 However, most current studies on sawdust have focused on heavy metals and dyes; there is limited information about the removal of aniline.13,14 In addition, the capacities of natural sawdust are comparable to but slightly lower than those of activated carbon. The highly efficient © 2013 American Chemical Society

Received: Revised: Accepted: Published: 887

November 12, 2013 December 18, 2013 December 27, 2013 December 27, 2013 dx.doi.org/10.1021/ie403829s | Ind. Eng. Chem. Res. 2014, 53, 887−894

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Figure 1. Thermochemical reactions of pine sawdust with citric acid and β-cyclodextrin.

performed at different flow rates for the purpose of possible industrial application.

mL of deionized water to avoid any excess CD, CA, and sodium dihydrogen phosphate. The samples were suction-filtered and dried at 333 K until a constant weight was obtained and then preserved in a silica-gel desiccator and labeled CD-CA-PS. CA Modification. CA-PS was prepared using the same steps as described for the modification of CD-CA-PS, except for the use of β-CD. This synthesis mechanism is illustrated in Figure 1. 2.3. Characterization Methods. Scanning electron microscopy was carried out using a Phillips SEM 501 electron microscope, and the elemental analysis of the biosorbent was carried out using an elemental analyzer (Elementar Analysensysteme GmbH varioMICRO CHNS). X-ray photoelectron spectroscopy (XPS) was performed using a Shimadzu ESCA3400 spectrometer. The amount of acidic groups was determined using the titration method.22 The dry sample (0.25 g) was slurried in water, and 0.1 M NaOH was added to it and stirred for 24 h. The mixture was back-titrated with 0.1 M HCl until the phenolphthalein end point. 2.4. Batch Adsorption Study. The kinetic adsorption experiment was conducted in an orbital shake at 298 K. Approximately 0.1 g of B-PS, CA-PS, and CD-CA-PS was introduced into an adsorption bottle containing 50 mL of an aqueous aniline solution with an initial concentration of 0.4 mmol L−1 and 1.0 mmol L−1 (pH 6.2). The adsorption bottle was placed in a shaker at a constant temperature (293 K). The concentration (ct, in millimoles per liter) of the aniline solution was determined at different times. The concentration of aniline was determined using a UV−vis spectrophotometer (Shimadzu Brand UV-3000) by monitoring the absorbance changes with N-(1-naphthyl) ethylenediamine (545 nm). All samples were analyzed in triplicate. Analytical precision, determined as the percentage relative standard deviation (RSD) for the triplicate test, was on the order of 2.0%. The adsorption amount (qt) was

2. EXPERIMENTS 2.1. Materials. The PS used in the present investigation was obtained from the local countryside in Liaocheng City Shandong Province, China. Citric acid, β-CD, aniline, and the other reagents used were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All working solutions were prepared by diluting the stock solution with distilled water to the required concentration. The initial pH of the working solution was adjusted by the addition of an HCl or NaOH solution (0.5 mol L−1). 2.2. Preparation of Modified Pine Sawdust. The collected PS was washed with fresh water several times in order to remove all the dirt particles and then dried in an oven at 333 K for a period of 12 h. Dried PS was milled and sieved to retain the 0.15−0.25 mm (60−100 mesh) fractions for further pretreatment and modification. First, 50 g of the sieved PS straw was boiled in 1 L of 0.5 M NaOH for 1 h at 373 K. Then, the slurries were poured onto a 100 mesh sieve and rinsed with deionized water three times to ensure the removal of the base or color factors retained in the materials. Third, the completely washed PS was dried overnight at 333 K and labeled B-PS. The purpose of the pretreatment process was to increase the proportion of active surfaces and to prevent the elution of tannin compounds that could stain the treated water. CA+CD Modification. B-PS (1 g), citric acid (0.3 g), sodium dihydrogen phosphate (0.125 g), β-cyclodextrin (0.15 g), and deionized water (10 mL) were mixed in a 100 mL flask and vibrated with ultrasound for 15 min. Then, the flask was placed in a boiling water bath. After 1 h, the mixture was heated in an electric thermostatic oven at 418 K for 4 h. After being allowed to cool, the primary product was purified by washing with 500 888

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with graphitic carbon as the reference at a binding energy of 284.8 eV. Figure S-1 of the Supporting Informationshows the C1s spectra of the PS prior to and after the CD+CA modification. The deconvolution of the C1s spectra reveals four peaks, as listed in Table S-3 of the Supporting Information. Taking the content of C−O as a reference, the change in R values of the C in other chemical states reflects their contributions to the surface complexation. The percentage values of single-bonded C in C−O and double-bonded C in CO on the B-PS surface decrease after the modification, suggesting the introduction of carboxyl groups. Figure 2 shows the scanning electron microscope (SEM) images that reveal the rough and microporous structure of the

calculated according to eq 1. Then, the adsorption isotherm experiment was performed with different initial concentrations: 0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 1.5, 2.0, and 3.0 mmol L−1. After the adsorption reached equilibrium, the equilibrium concentration (ce, in millimoles per liter) of the aniline solution was determined. The equilibrium adsorption amount (qe, millimoles per gram) was calculated according to eq 2: qt = V (c0 − ct )/m

(1)

qe = V (c0 − ce)/m

(2)

where qe and qt denote the amount of solute adsorbed (mmol g−1) at equilibrium and time t (min), respectively; V (L) represents the volume of the aniline solution; and m (g) represents the mass of the adsorbent. We examined the influences of various factors on the adsorption amount of B-PS, CA-PS, and CD-CA-PS. Further, by varying the adsorbent dose, the influence of the adsorbent amount on the adsorption amount of B-PS, CA-PS, and CDCA-PS was examined. The adsorption behaviors of aniline onto B-PS, CA-PS, and CD-CA-PS were also observed at different solution pH values (pH 3.0−10.0) by adding a NaOH or HCl solution and different ionic strengths ranging from 0.1% to 10% by adding a NaCl solution. 2.5. Column Study. Considering that in practical applications, a packing column is used the most, we studied the dynamic adsorption by using a column system. A fixed-bed column (length, 200 mm; diameter, 14 mm) was used in the column adsorption tests. The mass of CD-CA-PS in the column was 2.0 g (bed height, 7.0 cm) and the influent aniline concentration was 0.4 mmol L−1. The pH value of the influent aniline solution was 6.5. The adsorption experiments were carried out in the up-flow direction at flow rates of 3.5 mL/min and 20 mL/min, and the effluent solutions were collected. The interval of 10 mL was selected for the sampling in order to determine the residual concentrations in the effluent solutions.

3. RESULTS AND DISCUSSION 3.1. Characterization. The main chemical compositions of the native PS are lignin, cellulose, and hemicelluloses, as shown in Table S-1 of the Supporting Information.23 In the pine sawdust, cellulose and lignin account for up to 70.8% of the dry weight. The cellulosic hydroxyl groups could combine with citric acid anhydride to form an ester linkage and introduce carboxyl groups into the straw fiber.24 We found that all the modified samples easily swell in water because of the abundant hydrophilic acidic groups (−OH, −COOH). The elemental analysis and acidic group content results for B-PS, CA-PS, and CD-CA-PS were obtained (listed in Table S-2 of the Supporting Information). The acidic group contents of B-PS, CA-PS, and CD-CA-PS are 0.22, 2.10, and 1.92 mmol g−1, respectively. The C% of CD-CA-PS is smaller than that of B-PS and CAPS; the O% increases after the CD and CA modification. The O % and molar ratio of O/C of CD-CA-PS are the highest among all the observed values. As the formula of CA is C6H8O7 and the molar ratio of O/C is greater than 1, we can conclude that the increase in the oxygen content is due to the increase in the carboxyl content. A nonlinear least-squares curve-fitting software (XPSPEAK 4.1) is used for deconvolving the XPS data. In order to compensate for the charging effects, all spectra are calibrated

Figure 2. SEM micrographs of pine sawdust before and after modification: (a) B-PS, (b) CA-PS, (c) CD-CA-PS. 889

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trations, qe(B‑PS) = 11.03 μmol g−1, qe(CA‑PS) = 62 μmol g−1, and q e ( C D ‑ C A ‑ P S ) = 132.07 μmol g − 1 , and the ratio of qe(PS):qe(CA‑PS):qe(CD‑CA‑PS) = 1.0:5.6:12.0, indicating that −COOH in CA-PS and CD-CA-PS significantly contributes to the adsorption of aniline. Further, CD-CA-PS adsorbs aniline more, demonstrating that aniline can be covered by the hydrophobic cavity of CD-CA-PS. The maximum adsorption capacity of CD-CA-PS is approximately twice that of CA-PS, showing that β-CD and −COOH in CD-CA-PS probably cooperates with the adsorption of aniline; hence, aniline is in a more stable situation when adsorbed by CD-CA-PS than when absorbed by CA-PS or B-PS. Adsorption Isotherms. Adsorption isotherms describe how pollutants interact with adsorbents, and it is critical for adsorbent design optimizing. Thus, experimental data are fitted to the isotherm models (by Langmuir and Freundlich) which are well-known and widely applied.19 The nonlinear forms of these models are expressed by eqs 5 and 6:

surfaces of B-PS (Figure 2a) and CA-PS (Figure 2b) and CA− CD−PS (Figure 2c). The SEM observations of the three samples reveal their complex and porous surface texture and porosity. The samples are clearly porous, and there are pores of varying sizes within the particles. This may increase the external surface area and porosity of the materials, which facilitates the adsorption of aniline. A comparison of the micrographs reveals that the modification does not change the PS surface structure significantly. Therefore, according to the characterization results, the structures of CA-PS and CD-CA-PS are cellulose-based, and the surfaces of CA-PS and CD-CA-PS are covered with carboxyl groups. Esterification appears to have resulted only in changes in the surface chemistry and does not clearly change the porosity of the sample. 3.2. Adsorption Kinetics and Isotherm Studies. Adsorption Kinetics. B-PS, CA-PS, and CD-CA-PS are applied to the adsorption of aniline, whose initial concentration was 1.0 and 0.4 mmol L−1. In less than 20 min, all adsorption reached saturation. Pseudo-first-order and pseudo-second-order models are used for investigating the adsorption kinetics of aniline. These equations are expressed as eqs 3 and 4:25

qt = qe(1 − e

−k1t

)

qt = k 2qe 2t /(1 + k 2qet )

Langmuir isotherm: qe = qmKLce/(1 + KLce)

(5)

Freundlich isotherm: qe = KFce1/ n

(6)

where qe denotes the amount of solute adsorbed per unit mass of the adsorbent (μmol g−1), ce the equilibrium concentration of the solute in the bulk solution (mmol L−1), qm the maximum monolayer adsorption capacity (μmol g−1), and KL the constant related to the energy of adsorption (L/mmol). KF and 1/n denote the Freundlich constants related to the adsorption capacity and adsorption intensity of the adsorbent, respectively. The adsorption isotherm of B-PS, CA-PS, and CD-CA-PS for aniline is shown in Figure 4, and the parameters of the two

(3) (4)

where qe and qt denote the amount of solute adsorbed (μmol g−1) at equilibrium and time t (min), respectively; k1 represents the rate constant of the pseudo-first-order adsorption (min−1); and k2 represents the rate constant of the pseudo-second-order adsorption (g (μmol min)−1). A nonlinear regressive analysis is conducted, and a comparison of fitted curves from the two kinetic equations is shown in Figure 3. The kinetic parameters are given in Table S-

Figure 4. Adsorption isotherms of aniline on B-PS, CA-PS, and CDCA-PS (c0 = 0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0 mM; T = 293 K; adsorbent = 0.1 g; V = 50 mL; pH 6.2; t = 240 min).

Figure 3. Kinetic adsorption of aniline on B-PS, CA-PS, and CD-CAPS (c0 = 0.4 and 1.0 mM; T = 293 K; adsorbent = 0.1 g; V = 50 mL; pH 6.2).

isotherms are presented in Table S-5 of the Supporting Information. It can be seen that the equilibrium adsorption amount of aniline increases rapidly with an increase in the equilibrium concentrations. From Table S-5 of the Supporting Information, we observe that the R2 values obtained from the Langmuir isotherms are higher than those obtained from the Freundlich isotherm, whereas the relative value of X2 is lower, which suggests that the Langmuir model yields a considerably better fit than the Freundlich model for aniline adsorption.

4 of the Supporting Information. The results with the values of R2 and X2 show that the pseudo-second-order model can predict the kinetic process better than the pseudo-first-order model.26 According to the pseudo-second-order fit results, for lower concentrations, the maximum adsorption capacities of the three materials with respect to aniline with a ratio of qe(B‑PS):qe(CA‑PS):qe(CD‑CA‑PS) = 1.0:3.8:9.7. For higher concen890

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the presence of the CD sites, and hydrogen bonding in the presence of carboxylic groups. CD molecules have a remarkable capability to form a stable encapsulation without any covalent bonds being formed.33 These complexes are chemical species consisting of two or more associated molecules, where one of the host molecules can provide access for a guest component to its cavity. Furthermore, the character of guest molecules, such as size, shape, and polarity, also influenced the inclusion process. Aniline has a chemical structure that may not only enable a deeper penetration into the adsorbent’s network but also favor the formation of an inclusion complex between aniline and the CD sites. Moreover, it can become a cation through the protonation effect, which is good for ion-exchange interactions. Of course, on the basis of the electron-donating nature of the −OH- and −COOH-containing groups in sawdust materials and the electron-accepting nature of some organic compound ions at a low pH, the ion-exchange mechanism can be considered preferentially. The differences in the functional groups between the adsorbents may explain the results shown in Figure 4. B-PS exhibits weak adsorption ability because it has relatively few functional groups. Compared to B-PS, CA-PS shows higher adsorption ability because of CA modification, which introduces many carboxylic groups onto B-PS. Carboxylic groups can promote the formation of hydrogen bonds and be used for the ion-exchange process, both of which can contribute to the adsorption of aniline. When the material changes to CDCA-PS, it displays the best adsorption for aniline, not only because of the inclusion complex formation in the presence of the CD sites but also because of the hydrogen bonding or ionexchange interactions in the presence of carboxylic groups. The hydrogen bonding occurring among CA, CD, and aniline can help explain this phenomenon. The −NH2 of aniline can form a hydrogen bond (N−H−O hydrogen bond) with the O atom of the hydroxyl and carboxyl groups in the adsorbents. Zhang et al.34 studied the inclusion mechanism between cationic dye and the charged CD molecule and found that the additional electrostatic interaction would result in a stronger binding property. 3.4. Influences of Operational Parameters on Adsorption Amount of B-PS, CA-PS, and CD-CA-PS. Influence of pH. The pH of the solution plays an important role in the entire adsorption process, influencing not only the aniline solution chemistry but also the surface charge of the adsorbents. As shown in Figure 5a, the pH value has a considerable influence on the aniline adsorption process. The three adsorbents have a similar trend with respect to the adsorption amount and pH change. Under an acidic condition, the adsorption amount increases with an increase in the pH value, whereas under alkaline conditions, the adsorption amount decreases slightly with an increase in the pH value. It is well-known that aniline exhibits a weak ionic character under certain conditions and the degree of ionization also changes with pH. When the pH value is very low, the aniline can easily form C6H5NH3+ with H+ in solution (pKa of aniline = 4.63). The electrostatic repulsion will reduce the binding affinity between C6H5NH3+ and the less negatively charged adsorbent. On the other hand, at a pH value greater than pKa, aniline is typically in a nonionic molecular form. The quantity of H+ decreases with an increase in the pH value, which results in the strengthening and increase of the hydrogen bond. The hydrophobic interaction plays a key role in the entire

The essential features of the Langmuir isotherm can be described as a dimensionless constant; in other words, the separation factor (RL), which is defined by the following equation: RL = 1/(1 + KLc0)

(7)

where c0 denotes the initial aniline concentration (mg L−1). The value of RL indicates the shape of the isotherms to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0).27 The value of RL in the range of 0−1 confirms the favorable uptake of the aniline process. The lines in the inset of Figure 4 denote the calculated RL values of different adsorbents stacked together. Furthermore, higher RL values at lower aniline concentrations reveal that adsorption is more favorable at a lower concentration. The adsorption process described by the Langmuir equation is applicable to the homogeneous adsorption process, which means each molecule adsorbed on the adsorbent surface has equal adsorption activation energy. On the basis of the Langmuir analysis, the maximum adsorption capacities of BPS, CA-PS, and CD-CA-PS are 16.3, 117.0, and 233.6 μmol g−1, respectively, which reveal that CD-CA-PS possesses the strongest adsorption ability for aniline among these three adsorbents. For CD-CA-PS, the carboxyl sites and hydrophobic cavity generated by the grafted CD can simultaneously improve the hydrophobic property of the adsorbent and the selectivity for aniline. Compared with B-PS, the adsorption ability of CAPS and CD-CA-PS is considerably higher, approximately 7.2 times and 14.3 times greater, respectively. The results illustrate the positive effect of the modification. The adsorption capacities of CD-CA-PS are significantly higher than that of other low-cost adsorbents, such as macroporous kaolin, mesoporous molecular sieves, and polymers containing −COOH.28−30 Although the adsorption ability of CD-CA-PS is not higher than that of polymeric resins and powder activated carbons, the native PS is cheap and renewable.1,6,31 This implies that CD-CA-PS is a more favorable adsorbent for the removal of aniline from an aqueous solution among the low-cost adsorbents. 3.3. Adsorption Mechanism. In general, the process of adsorption consists of four stages: (1) bulk diffusion, (2) film diffusion, (3) intraparticle diffusion, and (4) chemical reaction. The difference in the degree of adsorption of B-PS, CA-PS, and CD-CA-PS may be attributed to the different functional groups. The cell walls of sawdust mainly consist of cellulose and lignin and many hydroxyl groups such as tannins or other phenolic compounds. CD-CA-PS is an adsorbent that retains the inclusion properties of native cyclodextrin and contains both carboxyl and cyclodextrin, whereas CA-PS contains only carboxyl groups. Several factors are significant in the process of forming an inclusion complex. Hydrophobic effect, as one of the most important factors, would induce the apolar group of a guest molecule to preferentially enter the CD cavity. In addition, hydrogen bonding, van der Waals interactions, solvent effects, and steric effects may also affect the inclusion process. Aniline contains both hydrophobic groups (phenyl group) and basic groups (amino group); the hydrophobic groups are attracted by β-CD, whereas the basic groups are chemically bonded to −COOH. This makes the adsorption more complex. Crini et al.32 reported that the adsorption mechanism is complex, probably simultaneously dominated by surface adsorption and diffusion into the adsorbent network, chemisorption via the formation of an inclusion complex in 891

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results in a higher adsorption capacity that could be obtained in high ionic strength conditions.35 Effect of Adsorbent Dose on Adsorption. The effect of adsorbent dose on removal efficiency was studied (as illustrated in Figure 6). As the adsorbent concentrations increase from 0.2

Figure 6. Effect of adsorbent dose on aniline adsorption on B-PS, CAPS, and CD-CA-PS (c0 = 1.0 mM; T = 293 K; adsorbent = 0.01, 0.02, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 g; V = 50 mL; pH 6.2; t = 240 min).

to 10 g L−1, the qe values of the three adsorbents decrease gradually, while the aniline removal percentage first increases and then approaches an equilibrium state. When the adsorbent concentration increases from 0.2 to 10 g L−1, the aniline removal increases from 0.36% to 2.55% for B-PS, from 1.22% to 35.15% for CA-PS, and from 4.11% to 55.17% for CD-CA-PS. Therefore, we can select a suitable adsorbent dose in the future to meet the effluent requirement. 3.5. Breakthrough Adsorption Curve. The time for breakthrough appearance is crucial for determining the operation and dynamic response of an adsorption column.36 The breakthrough curve of a fixed-bed column is expressed by ct/c0 as a function of time, as shown in Figure 7. In this study, a modified dose−response model is used for describing the column breakthrough curves: ct 1 =1− c0 1 + (vt /b)a (8)

Figure 5. Effect of solution pH (a) and NaCl concentration (b) on aniline adsorption on B-PS, CA-PS, and CD-CA-PS (c0 = 1.0 mM; T = 293 K; adsorbent = 0.1 g; V = 50 mL; t = 240 min; pH 3, 4, 5, 6, 7, 8, 9, 10; NaCl, 0.05, 0.1, 0.2, 0.5, 1.0, 1.5 mol L−1).

adsorption process. The adsorption amount for aniline is maintained at a good level at pH 6−8 probably because of the lowest surface charge and strongest interaction affinity. Furthermore, under alkaline conditions, aniline exists in a molecular form, which is favorable for adsorption. Therefore, the interacting forces between adsorbents and aniline are influenced by pH, which is associated with the adsorption abilities of the adsorbents. Hence, considering these two effects, we conclude that the optimal pH value is 6.5. The following experiments were carried out under this pH range. Effect of Ionic Strength on Adsorption. Inorganic salts such as NaCl often exist in textile dye and ink effluent with a relatively high concentration. The effect of NaCl concentration on aniline adsorption capacity of the three adsorbents was studied. From Figure 5b, we infer that when the concentration of NaCl increases from 0% to 10% with the initial aniline concentration of 1.0 mmol L−1, the aniline adsorption capacity of B-PS increases from 10 to 15 μmol g−1, whereas that of the CA-PS increases from 60 to 120 μmol g−1, and that of the CDCA-PS increases from 125 to 267 μmol g−1. It is obvious that the ionic strength influences the adsorption abilities of the three adsorbents, which can be attributed to the “salting-out” effect. The water solubility of aniline is usually opposite to the ionic strength. The principal effect of NaCl on the organic solute solubility is the formation of hydrated shells around the ions, which effectively reduces the available amount of free water for dissolving the organic solutes. The increase in ionic strength favors aniline aggregation and precipitation on adsorbents. This

Figure 7. Breakthrough curves of aniline adsorption on CD-CA-PS (c0 = 0.4 mM; pH 6.2). 892

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Notes

From the value of b, the value of q0 can be estimated using the following equation:

q0 = bc0/x

The authors declare no competing financial interest.

■

(9)

ACKNOWLEDGMENTS The authors express their sincere gratitude to the National Natural Science Foundation of China (51208201), the Fundamental Research Funds for the Central Universities (WB1114033), China Postdoctoral Science Foundation (2011M500744) and Shanghai Educational Development Foundation (11CG32) for financial support of this study.

where the value of ct/c0 denotes the ratio of the effluent and influent aniline concentrations; q0 represents the equilibrium aniline uptake per gram of the adsorbent (μmol g−1); x represents the amount of adsorbent in the column (g); v indicates the flow rate of the solution passing through the column (mL/min); and t indicates the flow time (min). The parameters of a and b (mL) are from the modified dose− response model. The parameters and values of R2 and X2 are listed in Table S-6 of the Supporting Information. In this study, the initial aniline concentration (c0) is 0.4 mM, and we choose the effluent concentration (ct) of 0.054 mmol L−1 (corresponding to the maximum effluent concentration of 5 mg L−1) from the column as the control point. Figure 7 shows that the breakthrough occurs faster with an increase in the space velocity. When the space velocity is 112 h−1, the effluent concentration is almost all over the control point, indicating that a considerably high space velocity is undesirable for engineering applications. In contrast, when the space velocity is low, a relatively long running time is observed. As the liquid space velocity decreases from 112 to 20 h−1, the running time (when the effluent concentration reaches 0.054 mmol L−1) increases from