Preparation and Evaluation of Water-Compatible Surface Molecularly

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Preparation and Evaluation of Water-Compatible Surface Molecularly Imprinted Polymers for Selective Adsorption of Bisphenol A from Aqueous Solution Feifei Duan,†,‡,§ Chaoqiu Chen,‡ Lin Chen,†,§ Yongjiao Sun,⊥ Yunwei Wang,‡ Yongzhen Yang,†,∥ Xuguang Liu,*,†,§ and Yong Qin*,‡ †

Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan, Shanxi 030024, China ‡ State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030024, China § College of Chemistry and Chemical Engineering, ∥Research Center on Advanced Materials Science and Technology, and ⊥College of Information Engineering, Taiyuan University of Technology, Taiyuan, Shanxi 030024, China S Supporting Information *

ABSTRACT: Water-compatible molecularly imprinted polymers (MIPs) for adsorbing bisphenol A (BPA) in aqueous solutions are synthesized using water-soluble monomer as surface hydrophilicity-increasing agent via surface addition−fragmentation chain transfer polymerization. The formation and structure of these hybrid materials are verified by Fourier transform infrared spectroscopy, contact angle studies, thermogravimetric analysis, and scanning electron microscopy. The characterization and adsorption results indicate that the molecularly imprinted polymers prepared with 2-acrylamido-2-methylpropanesulfonic acid (AMPS/MIPs) are water-compatible (the contact angle is 14°). The excellent dispersion of AMPS/MIPs in water provides more opportunity for BPA molecules to access the imprinted cavities and improves their recognition characteristics. The kinetics and isotherm data of AMPS/MIPs can be well described by the pseudo-second-order kinetic model and the Langmuir isotherm, respectively. The thermodynamic studies indicate that the adsorption process is a spontaneous exothermic process.

1. INTRODUCTION In recent years, endocrine disrupting chemicals (EDCs) have attracted extensive attention because they can lead to the disturbance of central regulatory functions in humans and wildlife, resulting in a risk to humans and animals.1 Bisphenol A (BPA) is regarded as a representative material among EDCs because it is mainly used as a monomer in the production of polycarbonate, epoxy resins, and other plastics, which are widely used in industry and households and hence result in the release of BPA into the surrounding environment.2 Moreover, because of strong polarity and low volatility, aquatic environments are its primary reservoirs. Therefore, the analysis and removal of trace BPA contamination from municipal wastewater, surface water, and groundwater is very necessary. Various methods have been used in removal of BPA such as adsorption,1a,2b,3 photo degradation2a,4 and biological treatment.1b,2c Among these methods, adsorption is a superior and widely used method in terms of mild operation conditions and low impact on the environment. Molecularly imprinted polymers (MIPs) are promising and facile adsorption materials for separation procedures and chemical analyses because they exhibit high affinity and selectivity toward target molecule. Nowadays, MIPs have been widely used in solid-phase extraction,5 chemical sensors,6 and catalysis.7 Traditionally, MIPs are synthesized by copolymerization of functional monomer and cross-linkers assembled around the target molecule (template) in an aprotic and nonpolar solvent. After removal of the template, the © 2014 American Chemical Society

recognition sites identical to the shape, size and chemical structure of the template molecule are formed in the polymeric matrix; thus, these formed recognition cavities can selectively rebind template molecules. Recent research shows that MIPs have promising potential for BPA analysis and removal because of their high affinity and selectivity.8 However, MIPs synthesized in organic solvents show excellent selective adsorption ability toward BPA in organic solvents but poor in aqueous environments because the presence of great excess of water can disturb the hydrogen bond formed between template and functional monomer.1c,9 Therefore, there is a need for preparing water-compatible MIPs for analysis and removal of trace BPA in aqueous environments. However, it is difficult to synthesize MIPs directly in aqueous environment because the hydrogen bonds between the functional monomer and template can be easily disturbed1c and sometimes the template molecules have poor solubility in aqueous phase (such as organic environmental pollutes BPA,1a 4,4′-(1-phenylethylidene) bisphenol,10 pesticides11). To improve the watercompatibility of MIPs, two main strategies have been developed. One representative approach is a postmodification of MIPs by surface grafting hydrophilic polymer layers or by chemical modification of the MIPs particles.12 However, the Received: Revised: Accepted: Published: 14291

March 12, 2014 August 19, 2014 August 21, 2014 August 21, 2014 dx.doi.org/10.1021/ie5028099 | Ind. Eng. Chem. Res. 2014, 53, 14291−14300

Industrial & Engineering Chemistry Research

Article

reacting with RAFT agent. One milliliter of deionized water and 1 mL of γ-chloropropyl trimethoxysilane were added to 30 mL of ethanol with 0.3 g of oxidized CMSs and the mixture was stirred at 65 °C for 12 h. After the reaction, the silanized CMSs were obtained by filtration, washing with ethanol, and drying at 50 °C overnight. RAFT agent was immobilized onto silanized CMSs as follows: 10 mL of bromobenzene and 1.2 g of magnesium were added into 30 mL of tetrahydrofuran under stirring at 35 °C for 2 h. Then 10 mL of carbon disulfide was slowly added over 0.5 h, and the reaction system was maintained for 2 h. Next, 0.8 g of silanized CMSs was added into the resulting mixture, and the reaction temperature was kept at 35 °C for 48 h. The RAFT/ CMSs were obtained by filtration with ethanol until the filter liquid became colorless and drying overnight under vacuum. 2.2. Synthesis of MIPs. Molecularly imprinted polymers (MIPs) on CMSs were prepared as follows: 0.228 g of BPA (1 mmol), 0.832 g of AMPS (4 mmol) or 430 μL of 4-VP (4 mmol), 3.76 mL of EGDMA (20 mmol), and 0.1 g of RAFT/ CMSs were dispersed in 25 mL of DMF at room temperature for 1 h. Then 0.065 g of 2,2′-azobis(isobutyronitrile) (AIBN) was added into the mixed system. The suspension was deoxygenized with nitrogen for 1 h and then sealed. The polymerization was performed at 50 °C for 24 h under nitrogen. The product (denoted as AMPS-BPA/MIPs and 4VP-BPA/MIPs, respectively) was centrifuged and washed with ethanol three times to remove unreacted reagents. Then the template molecule was removed by washing the product with 30 mL of mixture solution (ethanol and acetic acid, v/v = 9/1) under ultrasound five times. Finally, the product was washed with ethanol to remove the remaining acetic acid and dried under vacuum. The molecularly imprinted polymers prepared using AMPS and 4-VP as hydrophilic monomer were denoted as AMPS/MIPs and 4-VP/MIPs, respectively. For comparison, the surface nonimprinted polymers (AMPS/NIPs) using AMPS as monomer were also prepared in the same way without adding template molecules (BPA). 2.3. Adsorption Experiment. Adsorption of BPA was carried out in a stirred batch system. In brief, BPA was dissolved in ethanol as a stock solution (2000 mg/L) because of its low solubility in water and further diluted with a large amount of water to the required concentration before being used. All adsorption experiments were performed in sealed 100 mL glass conical flasks that contained 50 mg of adsorbent and 50 mL of BPA solution in the appropriate concentration. The suspension was placed in a water bath at 20 °C. The adsorption kinetic study under different temperature was carried out with an initial BPA concentration of 50 mg/L to determine the minimum time required for adsorption to reach equilibrium and the temperature for adsorption to reach the maximum adsorption capacity. The concentration of BPA was measured at different time intervals ranging from 20 min to 2.5 h. The adsorption isothermal experiments were similar to the adsorption kinetic study. The initial BPA concentration ranged from 2 to 50 mg/L. The competitive adsorption test was performed in solution of BPA, bisphenol F (BPF), and resorcinol (R), separately, and all the initial concentrations of BPA, BPF, and R were 50 mg/L. The effect of pH on the adsorption capacity of AMPS/MIPs toward BPA was studied with an initial BPA concentration of 50 mg/L in a pH range of 2.0−11.0 at 293 K. The solution pH was adjusted with a 0.1 M HCl or NaOH solution.

procedures are complicated and time-consuming. Another representative approach is the use of hydrophilic monomers, such as β-cyclodextrin (β-CDs)11,13 and 2-acrylamido-2methylpropanesulfonic acid (AMPS),14 in the synthesis process of MIPs. This approach is simple and can improve the surface hydrophilicity of MIPs. For example, Guo et al. synthesized MIPs with β-CD as functional monomer through bulk polymerization for recognition of pyrethroids in aqueous media.11 Chang et al. prepared creatinine imprinted organic− inorganic hybrid polymers using tetraethoxysilane and AMPS.14 It is worth noting that AMPS is a water-soluble functional monomer which can be polymerized both in water and organic solution (N,N-dimethylformamide, DMF).15 Therefore, the imprinting process with AMPS as functional monomer in organic solution avoids disturbing the interaction between template and monomer, and the obtained water-compatible MIPs can be applied in the removal and enrichment of ultralow concentration pollutants in water. In recent years, surface initiated controlled/living free radical polymerization technology has been extensively studied and explored to synthesize MIPs for the production of well-defined polymers with narrow polydispersity and controlled structure and composition. Addition−fragmentation chain transfer (RAFT) polymerization is a versatile controlled/living freeradical polymerization which can be employed with a wide range of monomers and reaction conditions, allowing the controlled molecular weight polymers with very narrow distribution (usually 8). The monomer, AMPS, dissociates completely over the entire pH range because of its strongly ionizable sulfonate group.15 Thus, the decrease of the adsorption capacity of AMPS/MIPs when pH > 9 might be due to the repulsive electrostatic interaction between the negatively charged surface of AMPS/MIPs and the bisphenolate anion. While the decrease of the adsorption capacity of AMPS/MIPs

4. CONCLUSION In summary, surface molecularly imprinted polymers for adsorbing BPA in aqueous phase are fabricated using 4-VP and water-soluble AMPS as functional monomers via surface RAFT polymerization. Static water contact angle measurements and water dispersion stability results reveal the improved surface hydrophilicity for AMPS/MIPs in comparison with 14298

dx.doi.org/10.1021/ie5028099 | Ind. Eng. Chem. Res. 2014, 53, 14291−14300

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CMCs and 4-VP/MIPs, leading to excellent adsorption capacity and high selectivity toward BPA in water. The kinetics and isotherm data can be well fitted with the pseudo-second-order kinetic model and the Langmuir isotherm, respectively. The adsorption process was a spontaneous exothermic process. In addition, the presence of strong acid (pH ≤ 3) or strong alkali (pH > 7) conditions and higher temperature of the solution were unfavorable. Furthermore, the AMPS/MIPs show excellent reusability. The experimental results obtained in this work show that the water-soluble AMPS is a promising monomer for preparing water-compatible MIPs for BPA adsorption in aqueous phase.

ASSOCIATED CONTENT

S Supporting Information *

FTIR spectrum of 4-VP/MIPs, contact angle of silanized CMSs and AMPS/CMSs, dispersion of CMCs and functionalized CMCs in pure water, SEM image of AMPS/NIPs, XPS spectrum of AMPS/MIPs, binding capacity of oxidized carbon microspheres to BPA, and selective adsorption of oxidized CMSs. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Regeneration cycles for AMPS/MIPs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from Program for Changjiang Scholar and Innovative Research Team in University (IRT0972), National Natural Science Foundation of China (20971094, 21176169, 51152001), Ph.D. Programs Foundation of Ministry of E ducation of China (20101402110007), International S&T Co-operation Program of Shanxi Province (2010081017), Research Project Supported by Shanxi Scholarship Council of China (2012-038), the Hundred Talent Program of the Chinese Academy of Sciences, and the Hundred Talent Program of Shanxi Province. 14299

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