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Hyper-cross-linked Polystyrene-co-divinylbenzene Resin Modified with Acetanilide: Synthesis, Structure, and Adsorptive Removal of Salicylic Acid from ...
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Hyper-cross-linked Polystyrene-co-divinylbenzene Resin Modified with Acetanilide: Synthesis, Structure, and Adsorptive Removal of Salicylic Acid from Aqueous Solution Jianhan Huang,*,† Xialie Wang,‡ Xiaomei Wang,‡ and Kelong Huang† † ‡

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Department of Bioengineering and Environmental Science, Changsha University, Changsha 410003, China

bS Supporting Information ABSTRACT: A series of novel hyper-cross-linked resins were prepared from macroporous cross-linked chloromethylated polystyrene-co-divinylbenzene (PS) by adding a different quantity of acetanilide in the Friedel-Crafts reaction. The results indicated that the synthesized resins possessed predominant mesopores (2-5 nm) and medium-values specific surface area with a few acetanilide groups on the surface. Among these resins, HJ-W02 had the largest adsorption capacity toward salicylic acid and hence selected as the model resin for adsorptive removal of salicylic acid from aqueous solution. The kinetic and isotherm data could be well-fitted by pseudo-second-order rate equation and Freundlich model, respectively. Increase of the solution pH had a negative effect while Cd2þ posed a positive effect on the adsorption. Hydrogen bonding between the carbonyl groups of HJ-W02 and carboxyl (hydroxyl) groups of salicylic acid played an important role. The breakthrough point of HJ-W02 toward salicylic acid was 122.5 BV and 5% of sodium hydroxide aqueous solution regenerated it completely.

’ INTRODUCTION Salicylic acid is often employed as a medicine intermediate to produce aspirin, lopirin, fenamifuril, diflunisal, salicylamide, and benorylatum.1 It can get rid of horniness, shrink pores, and wipe off trivial wrinkles, so it is frequently used as a cosmetic at a low concentration (0.2-1.5%). However, salicylic acid is bad for the health at a high concentration; it can induce headaches and nausea and it even affects the liver and kidneys.2,3 As a result, the efficient removal and recycling of salicylic acid from aqueous solution has received considerable attention in recent years. Adsorption is a usual approach for removal of organic pollutants from aqueous solution.4-6 Activated carbon has an outstanding capability in adsorption of organic pollutants due to its high specific surface area and dominant micropores.7 However, how to regenerate the used activated carbon for repeated use is a big problem. Synthetic polymeric adsorbents, as excellent alternatives for activated carbon recently, have attracted increasing attention due to their possible diverse chemical structures and feasible regeneration property.8,9 In the 1970s, Davankov synthesized a kind of hyper-crosslinked polystyrene-co-divinylbenzene (PS) using bifunctional cross-linking agents and Friedel-Crafts catalysts from linear PS or low cross-linked PS.10 By this method, a large number of rigid methylene cross-linked bridges were formed between the polymeric chains, and the polymeric skeleton was strengthened accordingly.11 The specific surface area of the synthesized resin was shown to be relatively high with the pore diameter distribution being dominated by mesopores. Adsorption experiments indicated that the synthesized resin displayed excellent adsorption behaviors toward aromatic compounds, especially nonpolar or weakly polar aromatic compounds. r 2011 American Chemical Society

If macroporous low cross-linked chloromethylated PS is used as the reactant, some other reagent like acetanilide is also added in the reaction; there are at least two possible considered reactions. One is the Friedel-Crafts reaction of the chloromethylated PS itself, and the other is the Friedel-Crafts reaction between the chloromethylated PS and acetanilide.10 Moreover, because of the addition of a different quantity of acetanilide in the reaction, the respective extent of the two reactions should be different and which will determine the structure and the adsorption selectivity of the synthesized resin. In this work, a series of novel hyper-cross-linked resins were prepared from macroporous cross-linked chloromethylated PS by adding different quantities of acetanilide. Their pore structures, surface chemical structures, and adsorption capacities toward salicylic acid were compared. Subsequently, the adsorption behaviors of HJ-W02 toward salicylic acid from aqueous solution were investigated and the possible interaction between HJ-W02 and salicylic acid was elucidated by Fourier transform infrared spectroscopy (FTIR).

’ MATERIALS AND METHODS Chemicals and Reagents. The chloromethylated PS was purchased from Langfang Chemical Co. Ltd. (Hebei province, China). A.R. salicylic acid was applied as the adsorbate (molecular formula: C6H4(o-OH)COOH, molecular weight: 138.1, Wuhan Fengzhulin Chemical Co. Ltd., China), being Received: September 16, 2010 Revised: December 26, 2010 Accepted: December 27, 2010 Published: January 27, 2011 2891

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Figure 1. The preparation method for the hyper-cross-linked resin modified with acetanilide.

used without further purification. Acetanilide (Jiangsu Yonghua Fine Chemical Co. Ltd., China), anhydrous zinc chloride (Shangdong Jinxin Chemical Co. Ltd., China), and anhydrous ethanol (Changsha Mingrui Chemical Co. Ltd., China) were also analytical reagents and used in this study. Preparation of the Acetanilide-Modified Hyper-crosslinked Resins. A schematic representation of the preparation of the hyper-cross-linked resins modified with acetanilide is depicted in Figure 1. Forty grams of chloromethylated PS was swollen by the addition of 120 mL of nitrobenzene at room temperature over a period of 12 h and then a different quantity of acetanilide (0%, 2%, 7%, 15%, and 22% relative to chloromethylated PS, w/w) dissolved in nitrobenzene was added into the reaction flask. At a moderate stirring speed, 2.0 g of anhydrous zinc chloride applied as the catalysts was added into the reaction flask at 323 K. After the added zinc chloride was dissolved completely, the reaction mixture was heated to 388 K within 1 h. After the reaction was held for ca. 10 h at 388 K, the resulting hyper-cross-linked resins named HJ-W00, HJW02, HJ-W07, HJ-W15, and HJ-W22 were obtained. To remove the residual nitrobenzene and zinc chloride in the pores of the solid particles, the solid particles were filtrated from the solution and rinsed with 1% of hydrochloric acid and anhydrous ethanol until the effluent from hydrochloric acid was transparent. Finally, the solid particles were washed with deionized water until neutral pH, rinsed with anhydrous ethanol on a Soxhlet extractor for ca. 10 h, and then dried in vacuum at 323 K for 8 h. Characterization of the Resin. The specific surface area of the resin was determined by nitrogen adsorption/desorption curves measured at 77 K using a Micromeritics Tristar 3000 surface area and porosity analyzer (Micromeritics Corp., Norcross, GA). Before such experiment was performed, the resin was pretreated by exposure to nitrogen gas for 24 h so that any water and other impurities present in the material were displaced. The FTIR spectrum of the resin was collected on a Nicolet 510P Fourier transform infrared instrument (Thermo Nicolet Corporation, USA) via KBr disk method.

Kinetic Experiment. With regard to the kinetic curves for the adsorption of the resin toward salicylic acid, about 1.0000 g of the resin was mixed with 500 mL of salicylic acid solution with an initial concentration of 253.0, 401.5, or 499.8 mg/L, respectively. The mixture was continuously shaken in a thermostatic oscillator (agitation speed: 150 rpm) at 298 K and 0.5 mL of salicylic acid solution was sampled as fast as possible at a different time interval. The concentration of the salicylic acid was measured by UV spectrometry (UV-2450 spectrophotometer, Japan) at a wavelength of 296.5 nm until equilibrium was reached; the adsorption capacity at contact time t was calculated as

qt ¼

ðC0 -Ct ÞV W

ð1Þ

Here qt is the adsorption capacity at contact time t (mg/g), C0 and Ct are the initial concentration of salicylic acid and that at contact time t (mg/L), V is the volume of the solution (L), and W is the weight of the resin (g). Isotherm Experiment. In a cone-shaped flask with a stopper, about 0.1000 g of the resin was accurately weighed and mixed with 50 mL of salicylic acid solution. The initial concentration of salicylic acid was 100-500 mg/L with 100 mg/L interval. Hydrochloric acid and sodium hydroxide were applied to adjust the solution pH; Na2SO4 and CdCl2 were employed to consider the effect of inorganic salt and heavy metal ion on the adsorption. The flasks were then shaken for ca. 24 h at a desired temperature (298, 303, 308, or 313 K) until equilibrium was reached, equilibrium concentration of salicylic acid was measured, and the equilibrium adsorption capacity was calculated as qe ¼

ðC0 -Ce ÞV W

ð2Þ

where qe is the equilibrium adsorption capacity toward salicylic acid (mg/g) and Ce is the equilibrium concentration (mg/L). Fixed-Bed Column Experiment. The fixed-bed column experiment for the adsorption of the resin toward salicylic acid was carried out by a glass column (16 mm of diameter and 290 mm of 2892

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Figure 2. Comparison of the specific surface area and pore volume of the acetanilide-modified hyper-cross-linked resins.

Figure 3. The pore diameter distribution of HJ-W00, HJ-W02, HJW07, HJ-W15, and HJ-W22 as well as the chloromethylated PS.

length) at 298 K. Ten millimeters of wet resin was packed in the glass column and a HL-2 pump (Shanghai Huxi Analysis Instrument Factory Co. Ltd., China) was used to ensure a constant flow rate. Then 220.0 mg/L of salicylic acid was passed through the column at a flow rate of 6 BV/h (1 BV = 10 mL) and the respective concentration of salicylic acid from the effluent was recorded until it was equal to the inlet concentration (the breakthrough point was recorded at 5% of the inlet concentration).

’ RESULTS AND DISCUSSION Pore Structure of the Resins. Figure 2 indicates that the specific surface area of the prepared hyper-cross-linked resins has the same trend as the pore volume. HJ-W00 possesses the highest specific surface area. Because of the addition of acetanilide in the Friedel-Crafts reaction, the specific surface area of the obtained resin decreases sharply. In addition, the specific surface area of HJ-W02, HJ-W07, HJ-W15, and HJW22 decrease gradually. The reason can be followed that the loaded acetanilide increases the polarity of the resin, and which may induce the pore of the obtained resin to be collapsed, decreasing the specific surface area. It can be observed from Figure 3 that Friedel-Crafts reaction results in a great transfer for the pore diameter distribution of the resin, mesopores and macropores are the main pores for chloromethylated PS, and the average pore diameter is 25.7 nm, while mesopores in the

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Figure 4. FTIR spectra of the acetanilide-modified hyper-cross-linked resins and the chloromethylated PS.

Figure 5. Comparison of adsorption capacity of HJ-W00, HJ-W02, HJW07, HJ-W15, and HJ-W22 toward salicylic acid from aqueous solution.

range of 2-5 nm play a predominant role for the hyper-crosslinked resins and the average pore diameter of HJ-W00, HJW02, HJ-W07, HJ-W15, and HJ-W22 is 2.52, 2.56, 2.64, 2.72, and 2.79 nm, respectively. Surface Chemical Structure of the Resins. A specific absorption peak in the FTIR spectrum reveals a specific chemical bond and hence the FTIR spectrum is often applied to identify the existence of the functional groups on the resin.12 As can be seen from Figure 4, the FTIR spectrum of the chloromethylated PS has two characteristic peaks with frequencies at 1265 and 669 cm-1, respectively, which can be assigned to the C-Cl vibration. After the Friedel-Crafts reaction, these two peaks appear much weakened, while the appearance of another typical peak may be noted. This latter peak with a vibrational frequency at 1703 cm-1 may be assigned to the CdO stretching of amide groups;13 meanwhile, this peak may arise from the oxidation of the CH2Cl groups.14 In addition, a strong N-H stretching peak at 3404 cm-1 is also observed for HJ-W02, HJ-W07, HJ-W15, and HJW22. These results demonstrate that acetanilide has been loaded successfully onto the surface of the resin. Adsorption Selectivity of the Resins. By comparison of the adsorption capacity of the five resins to salicylic acid (see Figure 5), it was found that the adsorption capacity of HJ-W02 toward salicylic acid was the largest and hence HJ-W02 was employed as a specific polymeric adsorbent in the present study. As the adsorption of some other adsorbents such as XAD-4, X-5, bentonite, and 2893

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Figure 6. Kinetic curves of HJ-W02 toward salicylic acid from aqueous solution.

kaolin toward salicylic acid was compared with HJ-W02,1-3 it was seen that the HJ-W02 was also a superior adsorbent. The micropore specific surface areas of HJ-W00, HJ-W02, HJW07, HJ-W15, and HJ-W22 are measured to be 538.6, 446.9, 320.8, 285.7, and 261.7 m2/g, respectively; they are all a little higher than 50% of the total specific surface area. The average pore diameters of the five resins are all 2-6 times larger than the molecular size of salicylic acid (the molecular size of salicylic acid is predicted to be 0.69  0.46 nm by Gaussian 03 program). These results favor the adsorbate-adsorbate interaction via pore-filling mechanism.15 The specific surface area, polarity, and pore structure of the adsorbent are thought to be the main factors influencing the adsorption. The specific surface area of HJ-W02 (728.0 m2/g) is a little lower than that of HJ-W00 (798.3 m2/g), while the adsorption capacity of HJ-W02 toward salicylic acid (196.9 mg/g) is a little larger than that of HJ-W00 (179.7 mg/g), implying that some other factors lead to the larger adsorption capacity of HJ-W02. The surface of HJ-W02 is modified with polar amino and amide carbonyl groups, and which may enhance the adsorption of HJ-W02 toward salicylic acid according to the polarity interaction. Effect of Contact Time on the Adsorption. Figure 6 illustrates that the required time for the adsorption from the beginning to the equilibrium is ca. 480 min and a longer required time is needed at a higher initial concentration. Pseudo-secondorder rate equation is often employed to fit the kinetic data and it may be expressed by the equation16 t 1 t ¼ þ 2 qt k2 qe qe

ð3Þ

Here, k2 is the pseudo-second-order rate constant (g/(mg 3 min)). Table s1 shows that the correlation coefficients are higher than 0.99, revealing that the pseudo-second-order rate equation characterizes the adsorption well. In addition, a lower rate constant is observed at a higher initial concentration, accordant with the fact that a longer required time is needed at a higher initial concentration. On the basis of the pseudo-second-order rate equation, the initial adsorption rate (h, mg/(g 3 min)) and the half adsorption time (t1/2, min) are often used as a measure of the adsorption rate; a higher h or a shorter t1/2 implies a greater adsorption rate. They are summarized in Table s1 according to the following

Figure 7. Adsorption isotherms of HJ-W02 toward salicylic acid from aqueous solution.

equations:17 h ¼ k2 qe 2

ð4Þ

t1 = 2 ¼ 1=ðk2 qe Þ

ð5Þ

It is seen that the initial adsorption rate at a lower initial concentration is a little higher, while the required half adsorption time is much shorter, revealing that the adsorption rate at a lower concentration is greater. Generally, several steps such as external mass transfer from the liquid solution to the solid particle surface, internal mass transfer in the pore of the solid particle, and adsorption on the active sites of the solid particle are necessary for the adsorbate transferring from the solution to the active sites on the resin,18 and the intraparticle diffusion is frequently the rate-limiting step. The kinetic data are further dealt with by the intraparticle diffusion proposed by Weber and Morris:19 qt ¼ kd t 1 = 2 þC ð6Þ where kd is the intraparticle diffusion rate (mg/(g 3 min1/2)) and C is a constant. If the adsorption is controlled only by the intraparticle diffusion, the plotting of qt versus t1/2 gives a straight line and the straight line passes through the origin. If plotting of qt versus t1/2 presents a multilinear relationship or it does not pass through the origin, two or more diffusion mechanisms influence the adsorption.20 Figure s1 is the relationship of qt with t1/2 for the adsorption and it is seen that these plots show similar characteristics with two linear segments followed by a plateau. In the first stage, the linear portion passes through the origin, implying that the intraparticle diffusion is the sole rate-controlling step, and the kd evaluated from the first linear portions are 10.20, 9.706, and 9.062 mg/(g 3 min1/2), respectively. In the second stage, the regression is approximately linear but does not pass through the origin, suggesting that the intraparticle diffusion is not the sole rate-controlling step in this stage. Subsequently, the adsorption reaches equilibrium. Effect of Temperature on the Adsorption. Figure 7 indicates that the temperature is unfavorable for the adsorption and a higher temperature results in a lower adsorption capacity of the resin, indicating an exothermic process. Attempts were made to fit the experimental isotherms by Freundlich isotherm model expressed as log qe ¼ log KF þð1=nÞlog Ce 2894

ð7Þ

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where KF is a relative indicator for the adsorption capacity and n is a magnitude of the adsorption driving force. From the data summarized in Table s2 and since R2 > 0.99, it may be concluded that the Freundlich isotherm model gave a good fit to the adsorption data. In addition, the values of KF decrease with increasing temperature, revealing that the adsorption capacity at a higher temperature is relatively smaller and the adsorption is an exothermic process. According to the Clausius-Clapeyron equation,6,13 d ln Ce ΔH ¼ 2 RT dT

ð8Þ

Here ΔH is the adsorption enthalpy (kJ/mo1) and R is the ideal gas constant. Equation 8 can be followed by integral method below, ln Ce ¼ -ΔH=ðRTÞþC0

ð9Þ

0

Figure 8. Effect of Na2SO4 and Cd2þ on the adsorption.

where C is the integral constant. Plots of ln Ce versus 1/T can be fitted to straight lines (Figure s2), and ΔH can be calculated from the slopes of the straight lines. At a low solute concentration, the adsorption free energy ΔG (kJ/mol) can be determined as8 Zx dx ð10Þ ΔG ¼ -RT q x 0 where x is the mole fraction of the adsorbate in the solution. When the q follows the Freundlich isotherm, incorporating eq 7 into eq 10 will yield8 ΔG ¼ -nRT

ð11Þ

Here n is the constant in the Freundlich model. Adsorption entropy ΔS (J/(mol 3 K)) can be obtained via the Gibbs-Helmholtz relationship, viz, ΔS ¼ ðΔH - ΔGÞ=T

ð12Þ

The corresponding ΔH, ΔG, and ΔS of HJ-W02 toward salicylic acid are listed in Table s3. The negative values of ΔH indicate that the adsorption is an exothermic process. The absolute value of ΔH decreases with increasing salicylic acid loading, which may be attributed to the energetic heterogeneity of the surface. The value of ΔG is negative, thereby revealing that the adsorption is a spontaneous process. The same negative value of ΔS suggests a more ordered arrangement of the system after the adsorption. Effect of Na2SO4 and Cd2þon the Adsorption. Sodium sulfate often coexists with salicylic acid in industrial wastewater at a comparatively high level; hence, the effect of sodium sulfate on the adsorption ability of HJ-W02 to remove salicylic acid from aqueous solution is determined in this study and the results are shown in Figure 8. It can be observed that sodium sulfate affects the adsorption slightly. Some resins can be easily poisoned by heavy metal ions such as Cd2þ, Hg2þ, and Pb2þ; hence, Cd2þ applied as a model heavy metal ion in this study and its effect on adsorption of HJ-W02 toward salicylic acid from aqueous solution is investigated. As can be seen in Figure 8, Cd2þ poses a positive effect on the adsorption and the specific reason for this phenomenon needs further study. Effect of the Solution pH on the Adsorption. The solution pH is one of the most important factors influencing the adsorption. The solution pH affects the form of the functional groups of

Figure 9. Effect of the solution pH on the adsorption.

the resin and it also has an effect on the charge profile of the adsorbate; thereby, it induces different interaction between the resin and the adsorbate at different pH.21 The effect of the solution pH on the adsorption of HJ-W02 toward salicylic acid is illustrated in Figure 9. The pKa of salicylic acid is 2.98 and thereby the dissociation curve of salicylic acid on dependency of the solution pH is also presented in Figure 9. The adsorption of HJ-W02 toward salicylic acid on the dependency of the solution pH has the same trend as the dissociation curve of salicylic acid, revealing that the molecular form of salicylic acid is favorable for the adsorption. More definitely, the adsorption capacity is invariable until the solution pH is higher than 2.98. In acidic solution, salicylic acid is represented as a molecular form, and formation of intermolecular hydrogen bonding between the carbonyl groups of HJ-W02 and the carboxyl (phenolic hydroxyl) groups of salicylic acid stands a good chance.22 As the solution pH is higher than 2.98, salicylic acid is ionized as a negative ion and the adsorption capacity decreases rapidly. Moreover, as the solution pH is higher than 11.0, nearly no salicylic acid is adsorbed onto HJ-W02. Possible Interaction between HJ-W02 and Salicylic Acid. Figure 10 shows the typical FTIR spectra of HJ-W02 before adsorption of salicylic acid (the initial resin), after adsorption of salicylic acid, and after desorption of salicylic acid (the adsorbed salicylic acid on the surface of HJ-W02 was desorbed). After adsorption of salicylic acid, the peak of the amide carbonyl groups 2895

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Figure 10. FTIR spectrum of HJ-W02: (a) before adsorption of salicylic acid (the original HJ-W02); (b) after adsorption of salicylic acid; (c) after the adsorbed salicylic acid on HJ-W02 is desorbed.

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formaldehyde carbonyl groups. The mesopores in the range of 2-5 nm dominated the pore structure and micropore specific surface areas were a little higher than 50% of the total specific surface area. The adsorption capacity of salicylic acid was the largest as the quantity used of acetanilide employed was 2%. The pseudo-second-order rate equation characterized the kinetic curves well and the intraparticle diffusion model played a predominant role in the adsorption process. Frieundlich model depicted the isotherm well and the adsorption capacity decreased rapidly with increasing pH. Hydrogen bonding between the amide carbonyl groups of HJ-W02 and the carboxyl (hydroxyl) groups of salicylic acid was one of the main driving forces for the adsorption. The dynamic experiment showed that the breakthrough point of salicylic acid for HJ-W02 was 122.5 BV and 5% of sodium hydroxide aqueous solution regenerated it completely.

’ ASSOCIATED CONTENT Supporting Information. Additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*Fax: 86-731-88879616. E-mail: [email protected].

’ ACKNOWLEDGMENT The research was supported by the National Natural Science Foundation of China (No. 20804058), the Science and Technology Project of Changsha (No. k1003036-31), and the Science Foundation in Changsha University. Figure 11. The dynamic adsorption of HJ-W02 toward salicylic acid from aqueous solution at 298 K.

appears at 1685 cm-1, red-shifted by 18 cm-1. Moreover, after the adsorbed salicylic acid on the resin is desorbed, its frequency comes back to 1702 cm-1. Commonly, formation of hydrogen bonding leads some vibrational bands to be red-shifted.23 It is deduced that hydrogen bonding is formed between the amide carbonyl groups of HJ-W02 and the hydroxyl groups of salicylic acid, and hydrogen bonding is one of the main driving forces for the adsorption. Dynamic Adsorption. The result of mini-column dynamic adsorption of salicylic acid on HJ-W02 is shown in Figure 11, where C0 is the initial concentration of salicylic acid (mg/L) and Cv is the concentration at different bed volume of the effluent (mg/L). The breakthrough point of salicylic acid is 122.5 BV. In addition, as 5% of sodium hydroxide aqueous solution is applied to desorb the adsorbed salicylic acid from HJ-W02 resin column, nearly 100% regeneration efficiency is achieved. The continuous adsorption-regeneration run of the used HJ-W02 resin bed indicates that the eighth cycle is seen almost identical to the first cycle, implying that HJ-W02 can be regenerated completely and the consistency and stability of HJ-W02 is well-maintained after repeated use.

’ CONCLUSIONS We synthesized a series of novel hyper-cross-linked resins with their surface being modified with amino, amide carbonyl and

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