Selective Recognition of 2,4,6-Trichlorophenol by Molecularly

ARTICLE pubs.acs.org/JPCC

Selective Recognition of 2,4,6-Trichlorophenol by Molecularly Imprinted Polymers Based on Magnetic Halloysite Nanotubes Composites Jianming Pan,†,‡ Hang Yao,§ Longcheng Xu,† Hongxiang Ou,‡ Pengwei Huo,† XiuXiu Li,† and Yongsheng Yan*,† †

School of Chemistry and Chemical Engineering, and ‡School of the Environment, Jiangsu University, Zhenjiang 212013, China § School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China

bS Supporting Information ABSTRACT: Magnetic nanoparticles were attached to carboxylic acid functionalized halloysite nanotubes (HNTsCOOH) by high-temperature reaction of ferric triacetylacetonate in 1-methyl-2-pyrrolidone. Then, based on magnetic halloysite nanotubes particles (MHNTs), the magnetic molecularly imprinted polymers (MMIPs) were synthesized for the selective recognition of 2,4,6-trichlorophenol (TCP). MMIPs were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) analysis, thermogravimetric analysis (TGA), vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), elemental analysis, and Raman spectroscopy. MMIPs were demonstrated with an imprinted polymer film (5.0-15.0 nm) and exhibited magnetic property (Ms = 2.74 emu g-1) and thermal stability. Batch mode adsorption studies were carried out to investigate the specific adsorption equilibrium, kinetics, and selective recognition. The Langmuir isotherm model was fitted to the equilibrium data better than the Freundlich model, and the monolayer adsorption capacity of MMIPs was 246.73 mg g-1 at 298 K. The kinetic properties of MMIPs were well-described by the pseudo-second-order equation, initial adsorption rate, and half-adsorption time. The selective recognition experiments demonstrated high affinity and selectivity toward TCP over structurally related phenolic compounds, and hydrogen bonds between TCP and methacrylic acid (MAA) were mainly responsible for the recognition mechanism. In addition, MMIPs could be regenerated, and their adsorption capacity in the fifth use was about 11.0% loss in pure TCP solution, about 16.1% loss in coexisting phenolic compound solution. The MMIPs prepared were successfully applied to the separation of TCP from environmental samples.

’ INTRODUCTION 2,4,6-Trichlorophenol (TCP) is very common and has been listed in the priority pollutants into the aquatic environment by the United States Environmental Protection Agency (U.S. EPA) and China due to its toxicity, stability, and bioaccumulation.1-3 The sources of TCP are fossil fuel combustion, municipal waste incineration, and chlorination of water containing phenol or certain aromatic acids with hypochlorite or during disinfection of water.4 Discharge of TCP-contaminated wastewater into aquatic environment without adequate treatment can lead to negative effects on the water quality and pose adverse effects on the human nervous system and many health disorders.5 In regular monitoring, selective recognition and removal of target chlorophenols from complex matrixes in aquatic environment is frequently required. Thus, the great priority has been given to the development of novel molecular recognition and selective separation techniques.6 Molecular imprinting is a well-established and simple technique for synthesizing molecularly imprinted polymers (MIPs) r 2011 American Chemical Society

with specific molecular recognition properties.7 Owing to the chemical, mechanical, and thermal stability together with high selectivity for the template molecules, MIPs have been utilized for a wide variety of applications, including in chromatography,8 solidphase extraction,9 drug-controlled release, and sensor devices,10 where the MIPs act as a “artificial antibodies”. By preparing the MIPs film on a solid-support substrate, the surface-imprinting technique provides an alternative way to improve mass transfer and reduce permanent entrapment of the template.11 In previous investigations, SiO2,12 TiO2,13 and Fe3O414 have been widely used in the surface-imprinting process. When compared with conventional solid-support substrates, magnetic nanoparticles (MNPs) of MO (where M is Mn, Co, Ni, or Fe) have been proved to be efficient and promising materials. When MNPs are encapsulated inside of MIPs, the resulting Received: November 22, 2010 Revised: January 23, 2011 Published: March 17, 2011 5440

dx.doi.org/10.1021/jp111120x | J. Phys. Chem. C 2011, 115, 5440–5449

The Journal of Physical Chemistry C

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Scheme 1. Process of Preparing the Carboxylic Acid Functionalized Halloysite Nanotubes

magnetic molecularly imprinted polymers (MMIPs) can be easily collected and separation by an external magnetic field without additional centrifugation or filtration, which makes separation easier and faster.15 Moreover, MMIPs can not only selectively recognize the template molecules in the complex matrix but also possess more imprinted cavities within the polymer network because of the high surface-to-volume ratio of MNPs.16,17 Recently, in order to avoid leakages of MNPs and fragility of the resultant magnetic composites, special attentions have been directed to the combination of iron oxide nanoparticles and nanosized materials such as carbon nanotubes (CNTs). Vasilios’s group attached magnetite nanoparticles on CNTs by means of an interlinker molecule.18 Tannenbaum’s group described a facile method for the decoration of CNTs with nearly monodisperse γ-Fe2O3 magnetic (maghemite) nanoparticles by a modified sol-gel process.19 Tan et al. published a work that the magnetic iron oxide particle deposition could be controlled selectively on the outer, inner, or both surfaces of CNTs.20 Qu et al. published a study that the Fe2O3 nanoparticles were introduced into the CNTs via a wet chemical method.21 But CNTs were expensive and not easy to be obtained. Thus, the great priority may be given to the development of a substitute for CNTs. Halloysite nanotube (HNTs) is a two-layered aluminosilicate clay mineral, possessing hollow nanotubular structure in the submicrometer range and large specific surface area.22 Moreover, HNTs with molecular formula of Al2Si2O5(OH)4 3 nH2O are a naturally occurring unique tubular structure in nanoscale which have a similar geometry of carbon nanotubes. In contrast with other nanosized materials such as CNTs, SiO2, and TiO2, HNTs are readily obtainable, much cheaper, and possessing of special structure and large reserves in China. Therefore, when combining MNPs with HNTs, the resultant magnetic composites could be the promising multifunctional candidate for the surfaceimprinting process. Herein, we report an effective method to achieve magnetic halloysite nanotubes (MHNTs) via robust linkages by thermal decomposition of organic iron precursors such as ferric triacetylacetonate [Fe(acac)3] in the presence of carboxylated HNTs. And then MHNTs were coated with a thin MIPs film. This film was obtained using TCP as template, methacrylic acid (MAA) as functional monomer, 2,20 -azobisisobutyronitrile (AIBN) as initiator, and ethyl glycol dimethacrylate (EGDMA) as crosslinker. The characterization, magnetite leakage, adsorption capacity, kinetics, selectivity, and regeneration of these MMIPs were investigated. Finally, the MMIPs were used as sorbent for solid-phase extraction (SPE) and separation of TCP from environmental water samples.

’ EXPERIMENTAL SECTION Materials. HNTs were collected from by Zhengzhou Jinyangguang Chinaware Co. Ltd., Henan, China. Prior to use, they were

milled and sieved followed by being oven-dried at 373 K for 24 h. MAA, oleic acid, toluene, acetone, dimethyl sulfoxide (DMSO), and HPLC-grade methanol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). EGDMA (Shanghai Xingtu Chemical Co. Ltd., Shanghai, China) was washed consecutively with 10% aqueous NaOH, water, and brine, dried over MgSO4, filtered, and then distilled under reduced pressure. AIBN (Shanghai No. 4 Reagent & H.V. Chemical Co. Ltd., Shanghai, China) was recrystallized from methanol prior to use. 2,4-Dichlorophenol (DCP), TCP, 4-chlorophenol (CP), poly(vinylpyrrolidone) (PVP), Fe(acac)3, 3-aminopropyltriethoxysilane (APTES), triethylamine, N, N-dimethylformamide (DMF), succinic anhydride, and 1-methyl-2pyrrolidone were all purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Except that AIBN is chemical grade, all the reagents are analytical grade. Deionized ultrapure water was purified with a Purelab ultra (Organo, Tokyo, Japan). Instruments. Infrared spectra (4000-400 cm-1) were recorded on a Nicolet NEXUS-470 FT-IR apparatus (U.S.A.). Raman spectra were recorded in the range of 200-800 cm-1 at ambient temperature using a WITEC Spectra Pro 2300I spectrometer equipped with an Ar-ion laser, which provided a laser beam of 514 nm wavelength. The identification of crystalline phase was performed using a Rigaku D/max-γB X-ray diffractometer (XRD) with monochromatized Cu KR radiation over the 2θ range of 20-70° at a scanning rate of 0.02 deg s-1. The morphology of MMIPs was observed by a transmission electron microscope (TEM, JEOL IEM-200CX). Magnetic measurements were carried out using a VSM (7300, Lakeshore) under a magnetic field up to 10 kOe. A Vario EL elemental analyzer (Elementar, Hanau, Germany) was employed to investigate the surface elemental composition of the prepared composites. Thermogravimetric analysis (TGA) was performed for powder samples (about 10 mg) using a Diamond TG/DTA Instruments (Perkin-Elmer, U.S.A.) under a nitrogen atmosphere up to 800 °C with a heating rate of 5.0 °C min-1. The 1H NMR spectrum was collected on a DXT-300 MHz Bruker NMR spectrometer. A TBS-990 atomic absorption spectrophotometer (Beijing Purkinge General Instrument Co. Ltd., Beijing, China) with a deuterium background correction and a GF990 graphite furnace atomizer system was used. Synthesis of Amino and Carboxylic Acid Functionalized HNTs. The synthesis routes of amino-functionalized (HNTsNH2) and carboxylic acid functionalized HNTs (HNTs-COOH) are shown in Scheme 1.24 Briefly, activated HNTs were dispersed into the 100 mL of toluene with stirring for 1.0 h. In the next step, 3.0 mL of triethylamine and 3.0 mL of APTES were added dropwise into the mixture mentioned above under vigorous stirring. The reaction was continued 24 h at 80 °C under a nitrogen atmosphere. Then the HNTs-NH2 were collected and washed several times by deionized ultrapure water and ethanol. When the HNTs-NH2 were dried at 50 °C under vacuum, dry HNTs-NH2 were added into 25 mL of 0.1 M succinic 5441

dx.doi.org/10.1021/jp111120x |J. Phys. Chem. C 2011, 115, 5440–5449


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Scheme 2. Synthesis Route of MMIPs and Their Application for Removal of 2,4,6-TCP with the Help of an Applied Magnetic Field

anhydride in DMF. The mixture was stirred for 24 h; the resulting HNTs with carboxylic-function groups at their surface (HNTs-COOH) were washed by DMF. Finally, the obtained HNTs-COOH were dried at 50 °C under vacuum. Synthesis of Magnetic HNTs (MHNTs). The synthetic process of MHNTs followed a modified method by thermal decomposition of organic iron precursors (Scheme 2).25 As follows, 1.0 g of carboxylated HNTs was dispersed in 30 mL of 1-methyl-2-pyrrolidone by ultrasonication at room temperature, and then the mixture was heated to 195 °C under a nitrogen atmosphere. An amount of 0.883 g of Fe(acac)3 (2.5 mmol) was dissolved in 20 mL of 1-methyl-2-pyrrolidone and added quickly into the 1-methyl-2-pyrrolidone/HNTs-COOH solution under vigorous stirring. The reaction was continued for another 1.0 h with stirring. After being cooled to room temperature, the mixture was dispersed in ethanol, and the resulting black precipitate was collected by a Nd-Fe-B permanent magnet. After washing several times by acetone, the MHNTs were dried in a vacuum oven at 50 °C overnight. Moreover, black magnetite nanoparticles were also synthesized by a similar way without any HNTs-COOH. Synthesis of Magnetic Molecularly Imprinted Polymers (MMIPs). The MMIPs were prepared following the literature26 with the optimal synthesis conditions. The synthesis route of MMIPs and their application for removal of TCP with the help of an applied magnetic field are also shown in Scheme 2. The TCP (1.0 mmol) and MAA (4.0 mmol) were dispersed into the 10 mL of DMSO in an ultrasonic bath for 1.0 h. This step was to preparation of the preassembly solution. The MHNTs (1.5 g) were mixed with 1.5 mL of oleic acid and stirred for 10 min. Then 20 mmol of EGDMA and the preassembly solution were added into the mixture of MHNTs and oleic acid. This mixture was stirred (300 rpm) continually for 30 min to obtain the prepolymerization solution. Moreover, the PVP (0.4 g) used as dispersant was dissolved into 100 mL of DMSO/water (9:1, v/v) in a three-necked round-bottomed flask under stirring. Then the

prepolymerization solution was added into the three-necked flask, and then 0.3 g of AIBN was also added into it. The mixture was stirred at 300 rpm and purged with nitrogen gas to displace oxygen while the temperature increased to 70 °C. The reaction was allowed to proceed at 70 °C for 24 h. After the polymerization, the mixture was dispersed in ethanol and the MMIPs were collected by a Nd-Fe-B permanent magnet. Then the obtained MMIPs were washed with the mixture solution of methanol/ acetic acid (95:5, v/v) using Soxhlet extraction to remove the template molecules. Finally, the obtained MMIPs were dried at 50 °C under vacuum. In comparison, the magnetic nonimprinted polymers (MNIPs) were also prepared as a blank in parallel but without the addition of TCP. Moreover, the magnetic molecularly imprinted polymers based on magnetite nanoparticles (m-MMIPs) were also prepared by a similar way. Batch Mode Binding Studies. The experimental parameters such as pH, contact time, initial concentration of TCP, and temperature on the adsorption of TCP were studied in a batch mode of operations. For this purpose, a certain amount of sorbent (MMIPs or MNIPs) was dispersed in testing solution of TCP (10 mL). After the desired time, the MMIPs and MNIPs were isolated by an external magnetic field, and the concentration of TCP in the solvent phase was determined with high-performance liquid chromatography (HPLC). HPLC analysis was performed on a Shimadzu LC-20A system (Shimadzu, Kyoto, Japan) equipped with a UV-vis detector (set at 288 nm for all phenolic compounds). The injection loop volume was 20 μL, and the mobile phase consisted of deionized ultrapure water and methanol with a volume ratio of 30:70. The flow rate of the mobile phase was 1.0 mL min-1. The oven temperature was set at 25 °C. Magnetite Leakage Studies. In order to estimate the amount of magnetite that is likely to leach from the MMIPs, 100 mg of the MMIPs and m-MMIPs were placed in test tubes containing 10 mL of 50% (v/v) acetic acid solution and shaken by a rotary 5442



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Table 1. Elemental Composition of the Particles from Elemental Analysis particle type

C (%)

H (%)

N (%)

HNTs

0.221

0.566

0

HNTs-NH2 HNTs-COOH

1.68 2.11

1.78 1.81

0.393 0.358

MHNTs

2.07

1.64

0.277

MMIPs

45.52

5.88

0.634

MNIPs

44.41

5.69

0.599

Figure 1. (a) FT-IR spectrum of the MHNTs (a) and MMIPs (b). (b) Raman spectrum of the MHNTs (a) and MMIPs (b).

shaker for 24 h. The amount of the magnetite leached into leach media was determined by a graphite furnace atomic absorption spectrophotometer. Sample Preparation and SPE Procedure. Water samples were obtained from Yangtse Rive, the Grand Canal, and tap water, Zhenjiang, China. Freshly collected water samples were immediately filtered through a Millipore cellulose nitrate membrane (pore size was 0.45 mm) to remove suspended particles. Then the pH of the water samples was adjusted to 4.0 with 0.1 mol L-1 NaOH or 0.1 mol L-1 HCl before SPE. An amount of 50 mg of MMIPs was put into a beaker, and then 250 mL of water sample with spiked concentration ranging from 50 to 200 μg L-1 was added into the beaker and the mixture was stirred for 3.0 h at 25 °C. Then the MMIPs with adsorbed TCP were separated rapidly from the solutions by a Nd-Fe-B permanent magnet. Subsequently, the supernatant solutions were discarded and the MMIPs were transferred into a testing bottle and were washed with 2 2.0 mL of acetonitrile. Finally, the TCP were desorbed from the MMIPs with 2 2.5 mL of methanol solution containing 5.0% acetic under the action of ultrasound. The extracts were combined and evaporated to dryness under nitrogen gas at 40 °C, and the residues were dissolved with 2.0 mL of 20% aqueous methanol for further HPLC analysis.

’ RESULTS AND DISCUSSION Characterization of MMIPs and MNIPs. The infrared spectra of the MHNTs (a) and MMIPs (b) were measured and are shown in Figure 1a, respectively. The main functional groups of the predicted structure can be observed with corresponding infrared absorption peaks. The peaks at 3707 and 3622 cm-1 of MMIPs and MHNTs were attributed to the stretching vibrations of inner-surface hydroxyl groups.22 Characteristic bands of Si-O-Si for MHNTs and MMIPs were observed around 1040 cm-1. The absorption bands at 527 and 463 cm-1 of MMIPs corresponded to the Fe-O bond for spinel Fe3O4 particles, which was also obtained for MHNTs.19 For MHNTs and MMIPs, two additional vibrational bands at around 1460 and 1389 cm-1 could be assigned to the covalently bound between the iron oxide nanoparticles and HNTs-COOH. Moreover, the absorption band at 1637 cm-1 in the spectrum of MHNTs and MMIPs could be assigned to the carbonyl group of 1-methyl-2pyrrolidone coordinating with Fe in Fe3O4 particles.25 The results indicated that magnetic nanoparticles were facilely attached to HNTs-COOH by high-temperature reaction of ferric triacetylacetonate in 1-methyl-2-pyrrolidone. Furthermore, MMIPs showed the strong absorption bands around 1729, 1262, and 1150 cm-1,

Figure 2. Micrographs from a transmission electron microscope of HNTs (a), HNTs-COOH (b), MHNTs (c), and MMIPs (d).

which were assigned to the CdO stretching vibration of carboxyl (MAA) and C-O symmetric and asymmetric stretching vibration of ester (EGDMA), respectively.27 The characteristic peak of C-Cl bond stretching vibration was observed at 662 cm-1.28 The peaks at 2965 and 2869 cm-1 of MMIPs, which were absent in MHNTs, indicated the presence of C-H stretching bands of both -CH3 and -CH2 groups. The absorption band at 3445 cm-1 of the MMIPs could be attributed to the stretching vibration of O-H bonds from MAA molecules. All the results confirmed that the copolymerization of MAA and EGDMA on the surface MHNTs in the presence of AIBN has been initialled. In order to effectively distinguish the maghemite or magnetite nanoparticles on the surface of MHNTs, the Raman spectroscopy of MHNTs and MMIPs in the wavelength range of 200-800 cm-1 is shown in Figure 1b. Several broad peaks around 304, 552, and 672 cm-1 were observed in MHNTs and MMIPs. Those peaks were typical Raman peaks of Fe3O4, which can be assigned to Eg, T2g, and A1g modes of Fe3O4.29 In addition, few miscellaneous peaks could be assigned to the trace amounts of maghemite resulting from oxidation of Fe3O4 to Fe2O3 during the imprinting process. Elemental analysis was employed to ascertain each modification. The results are shown in Table 1. Compared with HNTs, the nitrogen composition in HNTs-NH2 increased from 0 to 0.393%, suggesting that 0.281 mmol of amido groups was immobilized on every gram of the HNTs-NH2 particles. After the modification of HNTs-NH2 by carboxylic acid, the compositions of carbon and hydrogen were increased from 1.68% to 2.11% and from 1.78% to 1.81%, respectively. The results indicated that carboxylic groups 5443



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Figure 3. Thermogravimetric analysis of the HNTs-COOH, MHNTs, MMIPs and MNIPs.

Figure 4. X-ray diffraction (XRD) patterns of MMIPs (a), MNIPs (b), and MHNTs (c).

were successfully introduced onto the surface of the HNTs. After polymerization, it could be observed that carbon and hydrogen composition were significantly increased. The increase of nitrogen composition may be attributed to the AIBN. It could also be found that the elemental compositions of the MMIPs were different from that of the MNIPs, indicating the TCP molecules were not able to completely leach from the MMIPs. According to the results, 6.69% of the template TCP (13.22 mg of TCP on every gram of the MMIPs) was remained on the packing after washing of the MMIPs. The morphology of HNTs, MHNTs, and MMIPs was observed by TEM. As can be observed in Figure 2a, the HNTs have a diameter in the range of 80-120 nm, and the nanotubes were open within the ends. The inner diameter of HNTs was more or less 30 nm, while the average thickness of the wall was about 25 nm. For the HNTs-COOH (Figure 2b), the inner diameter significantly decreased due to the modification with amino and carboxylic acid functional groups, indicating the preferential decomposition of ferric triacetylacetonate solution may initial on the external surface of HNTs-COOH. In Figure 2c, black particles (Fe3O4 particles) with the small size of 10 nm on average adhered stably on the tube surface, which could be attributed to the interaction between carboxy groups from HNTs-COOH and iron nanoparticles. From the morphology of MMIPs (Figure 2d), the rough surface with black particles (Fe3O4 particles) is shown. According to the thickness of MMIPs (140 (10 nm), the thickness of the imprinted film for MMIPs was almost 5.0-15.0 nm, which may be benefit for the fast extraction equilibrium within a short time. Figure 3 shows the thermogravimetric analysis (TGA) of the HNTs-COOH, MHNTs, MMIPs, and MNIPs. As shown in Figure 3, the HNTs-COOH displayed little weight loss even at 300 °C, indicating the carboxylic functional group from HNTs-COOH could still be stable at 195 °C without decomposition. Moreover, the first weight loss stage can be ascribed to the evaporation of water molecules for each particle, which was 4.25%, 9.19%, and 10.33% for MHNTs, MMIPs, and MNIPs, respectively.30 The second weight loss stage started at 550 °C for MHNTs and 450 °C for MNIPs and MMIPs, respectively. In this stage, there were no significant differences of the mass loss of the MMIPs and MNIPs, which were 84.05% and 86.23% for MMIPs and MNIPs, respectively. Compared with MMIPs and MNIPs,

MHNTs cannot be easily decomposed at high temperatures, which showed little weight loss (13.17% below 800 °C). Thus, the remaining mass for MMIPs and MNIPs was attributed to the thermal resistance of MHNTs particles, and the quantity of MHNTs particles in the MMIPs and MNIPs was 5.62% and 4.58%, respectively. Figure 4 shows the XRD patterns of MMIPs (a), MNIPs (b), and MHNTs (c). In the 2θ range of 20°-70°, six characteristic peaks that corresponded to Fe3O4 (2θ = 30.15°, 35.54°, 43.21°, 53.41°, 57.13°, and 62.34°) were observed in the MHNTs, MMIPs, and MNIPs, and the peak positions could be indexed to (220), (311), (400), (422), (511), and (440) (JCPDS card 19-0629 for Fe3O4). Moreover, it also could be seen that the XRD pattern of MMIPs was similar to that of MNIPs, indicating they had the same cylinder wall structure and interplanar spacing. As shown in Figure 4c, characteristic peaks around 24.75° and 38.48° were both assigned to the raw HNTs particles, and a minor amount of quartz (2θ = 37.61°) also existed in the MHNTs.22 In Figure 4, parts a and b, the three characteristic peaks mentioned above declined owing to the encapsulation by the polymer layer. Figure 5a shows the magnetic hysteresis loop of MHNTs, MMIPs, and MNIPs, respectively. It was obvious that there was a similar general shape and trend of the three curves, indicating three particles were superparamagnetic.31 The saturation magnetization (Ms) values obtained at room temperature were 2.74, 0.829, and 0.759 emu g-1 for MHNTs, MMIPs, and MNIPs, respectively. Because of the small particle surface effect, such as a magnetically inactive layer containing spins that were not collinear with the magnetic field, the Ms values for three particles were lower than that of the theoretical value for magnetite, which was 92 emu g-1.32,33 MMIPs and MNIPs possessed similar saturation magnetization, indicating the they had similar morphological structure and size distribution. Moreover, the results from Figure 5b strongly suggested that the remained magnetic force in MMIPs could be attracted by an external magnetic field effectively. The results also illustrated that MMIPs was a feasible magnetic separation carrier. Moreover, the amount of magnetite that was likely to leach from the MMIPs was estimated. The photographs of MMIPs (left image) and m-MMIPs (right image) suspended in water are shown in the Supporting Information. 5444



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Figure 5. (a) Magnetization curves at 298 K of MHNTs (a), MMIPs (b), and MNIPs (c). (b) A photograph of MMIPs suspended in water in the absence (left image) and in the presence (right) of an externally placed magnet.

Figure 6. (a) Effect of pH on adsorptive removal of TCP. (b) Effect of initial pH on equilibrium pH. Figure 7. Adsorption isotherms of MMIPs and MNIPs.

From the left image, few black particles (Fe3O4) could be found, whereas the same phenomenon was not appeared in the right image. By determination of magnetite leakage, 2.414 mg of magnetite was leached from 0.1 g of m-MMIPs and only 0.0245 mg of magnetite was leached from 0.1 g of MMIPs, suggesting that MMIPs prevented magnetite leakage successfully. Effect of pH for Adsorption Medium. Optimization of pH value for adsorption medium plays a vital role in the adsorption studies. The pH of the solution affects the degree of ionization and speciation of dichlorophenols which subsequently leads to a change in adsorption kinetics and equilibrium characteristics.34 The effect of pH on the adsorption of TCP and the effect of initial pH on final pH are shown in Figure 6, parts a and b, respectively. It was observed that adsorption capacity for MMIPs and MNIPs was approximately constant over the pH range of 2.0-5.0 and then declines in the pH range of 5.0-10 as TCP becomes ionized (Figure 6a). Moreover, adsorption of TCP at low pH 2.0-5.0 did not cause significant change in pH (Figure 6b), indicating greater adsorption at this pH range may be due to the predominant species of neutral TCP. The similar fact has also been reported previously for other adsorbents by Chen et al.3 At pH > 5.0, the

repulsion of the negatively charged TCP species and the dissociation of functional groups of adsorbents may decrease the interaction of the adsorption system. Thus, pH 4.0 for the adsorption medium was selected in the following studies. And the initial pH had the same effects for MMIPs and MNIPs, but the adsorption capacity for MMIPs was more than that of MNIPs, strongly indicating the imprinting effect. Adsorption Isotherms. The binding properties of MMIPs and MNIPs for TCP were studied by the static equilibrium adsorption, and then the equilibrium data were fitted to the Langmuir35 and Freundlich36 isotherm models. The nonlinear form of the Langmuir isotherm model is expressed by the following equation: Qe ¼

K L Q m Ce 1 þ K L Ce

ð1Þ

where Ce is the equilibrium concentration of adsorbate (mg L-1), Qe is the equilibrium adsorption capacity (mg g-1), Qm is the maximum adsorption capacity of the sorbent, and KL is the affinity constant. 5445



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Table 2. Kinetic Constants for the Pseudo-First-Order Equation and Pseudo-Second-Order Equation pseudo-first-order equation adsorbates C0 (mg L-1) T (K) Qe,exp (mg g-1) Qe,c (mg g-1) k1 (L min-1) MMIPs

MNIPs

pseudo-second-order equation Qe,c (mg g-1) k2 10-3 (g mg-1 min-1) h (mg g-1 min-1) t1/2 (min)

R2

R2

100

298

67.95

13.00

0.0380

0.9952

68.49

8.07

37.88

1.81

0.9998

100

308

70.62

9.70

0.0159

0.9459

70.42

9.25

45.87

1.54

0.9999

150

298

97.75

11.41

0.0312

0.9729

98.04

8.74

84.03

1.17

0.9999

100

298

43.54

13.67

0.0269

0.9702

44.05

5.67

11.00

4.00

0.9998

100

308

44.65

11.11

0.0300

0.9676

45.25

7.54

15.43

2.93

0.9997

150

298

73.20

8.08

0.0309

0.9642

73.53

9.53

51.55

1.43

0.9999

Figure 8. Pseudo-second-order rate equation for TCP adsorption onto MMIPs (a) and MNIPs (b) using nonlinear regression.

The nonlinear form of the Freundlich isotherm model is given as follows: Qe ¼ KF Ce 1=n

ð2Þ

where KF (mg g-1) and n are the adsorption equilibrium constant. Moreover, comparison of Langmuir and Freundlich isotherm models for TCP adsorption onto MMIPs and MNIPs using nonlinear regression are also illustrated in Figure 7. From Figure 7, when the equilibrium concentration increased, the equilibrium adsorption capacity (Qe) for TCP first increased sharply, then increased slightly, and finally reached to the maximum point, as expected. The maximum adsorption capacity for MMIPs and MNIPs was 246.73 and 75.49 mg g-1 at 298 K, respectively. The TCP adsorbed on MMIPs was greater than that of MNIPs, indicating the significantly preferential adsorption of TCP for MMIPs. It was probably because MMIPs illustrated the good specificity for the imprinted molecule. By fitting the experimental data with Langmuir and Freundlich isotherm equations, it was also found that the Langmuir isotherm model fitted the equilibrium data significantly better than the Freundlich model, indicating monolayer molecular adsorption for MMIPs and MNIPs. Adsorption Kinetics. The kinetic data obtained were analyzed using a pseudo-first-order rate equation38 and pseudo-second-order rate equation.39 The pseudo-first-order equation can be expressed as eq 4. Qt ¼ Qe - Qe e-k1 t

ð4Þ

Figure 9. Effect on the protons of TCP molecule upon addition of 6.0 equiv of MAA in DMSO.

The pseudo-second-order equation can be expressed as eq 5 Qt ¼

k 2 Qe 2 t 1 þ k2 Qe t

ð5Þ

where Qe and Qt are the amount of adsorbate (mg g-1) onto sorbent at the equilibrium and time t (min), respectively. Values of k1 (L min-1) and k2 (g mg-1 min-1) are calculated from the plot of ln(Qe - Qt) versus t and t/qt versus t, respectively. The adsorption rate constants and linear regression values from the two rate equations are summarized in Table 2. And the pseudo-second-order rate equation for TCP adsorption onto MMIPs and MNIPs using nonlinear regression is shown in Figure 8. On the basis of the second-order model, the initial adsorption rate (h, mg g-1 min-1) and half-equilibrium time (t1/2, min) are also listed in Table 2 according to the following 5446



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Table 3. Adsorption Selectivity of MMIPs and MNIPs in a Simulated Effluent MMIPs

MNIPs

Ce (mg L-1)

Kd (L g-1)

TCP

13.21

3.29

DCP

43.24

0.657

5.01

CP

37.97

0.817

4.023

phenolic compounds

k

Ce (mg g-1)

Kd (mg L-1)

55.05

0.408

45.65 35.80

k

k0

0.595

0.686

7.30

0.897

0.455

8.84

Table 4. Intra- and Interday Precisions and Recoveries of the Assay (n = 6) intraday precision 50 μg L-1

100 μg L-1

interday precision 200 μg L-1

50 μg L-1

100 μg L-1

200 μg L-1

water samples recovery (%) RSD (%) recovery (%) RSD (%) recovery (%) RSD (%) recovery (%) RSD (%) recovery (%) RSD (%) recovery (%) RSD (%) Yangtse river Yun river

92.5 85.5

5.8 3.7

92.8 87.2

1.9 6.1

94.1 89.5

3.3 1.2

83.8 83.3

6.9 6.1

90.6 85.9

2.5 2.2

92.8 89.1

2.9 3.8

tap water

92.3

4.1

94.6

3.5

96.3

2.7

88.5

3.8

89.1

5.4

93.4

4.2

equations:40 h ¼ K2 Q e 2

ð6Þ

1 K2 Q e

ð7Þ

t1=2 ¼

The adsorption of TCP followed pseudo-second-order kinetics well because of the favorable fit between experimental and calculated values of Qe (R2 values above 0.99). And it was assumed that the chemical process could be the rate-limiting step in the adsorption process for TCP.41 According to the results of h and t1/2, with the increase of temperature and initial concentration, the initial adsorption rate and adsorption capacity increased obviously. It was possible that the initial concentration of TCP molecules provided the necessary driving force to overcome the resistances of mass transfer between the aqueous phases and the solid phase.35 Furthermore, higher temperature may provide more chances for TCP molecules to pass the external boundary layer and produced the enlargement of pore volume and surface area enabling dichlorophenol molecules to penetrate further.42 From Figure 8, pseudo-second-order kinetics lines deviated substantially from the experimental points around the first 20 min. The observed deviation from experimental data could be attributed to the sharp fall in concentration gradient after the initial rapid adsorption of TCP molecules onto the large amount of vacant binding sites.43 Within this time period, it was believed that there was a switch between mass transfer diffusion control and pore diffusion control, and a change in adsorption mechanism may has occurred after the first 20 min for TCP adsorption.44 Recognition Mechanism and Selectivity Analysis of the MMIPs. To elucidate the recognition mechanism, 1H NMR was performed to study the intermolecular interaction between the TCP and the MAA. The effect on the protons of the TCP molecule upon addition of 4.0 equiv of MAA in DMSO is shown in Figure 9. As shown in Figure 9, the chemical shift of the -OH protons of the MAA and TCP are at 12.41 and 10.47, respectively. With the addition of TCP and MAA with the molar ratio 1:4, the chemical shift of the -OH protons of the TCP increased to 11.48, indicating the hydrogen of the -OH of the TCP was

involved in the formation of strong hydrogen bonding with the functional monomer.45 Moreover, DMSO is an aprotic and polar solvent, confirming that hydrogen bonds between TCP and MAA were mainly responsible for the recognition mechanism. To measure the selective recognition of TCP, the recognition of competitive phenolic compounds was performed. During the experiment, 10 mL of the coexisting phenolic compound solution (CP, DCP, TCP) in which each compound contained 1.0 mg were treated according to the procedure of batch mode binding studies. And the concentration of each compound in the solvent phase was determined with HPLC. The distribution coefficients (Kd), selectivity coefficients (k), and relative selectivity coefficient (k0 ) of CP and DCP with respect to TCP can be obtained according to eqs 8-10. Kd ¼ Qe =Ce

ð8Þ

-1

In eq 8, Kd (L g ) represents the distribution coefficient. The selectivity coefficient (k) for binding of a specific phenolic compound can be obtained according to the following equation: k ¼ KdðTCPÞ =KdðXÞ

ð9Þ

X is the competitive phenolic compound. A relative selectivity coefficient k0 can be defined as eq 10. kM and kN are the selectivity coefficients of MMIPs and MNIPs, respectively. k0 ¼ kM =kN

ð10Þ

Values of Kd, k, and k0 are summarized in Table 3. From the data in Table 3, the following facts could be found: (i) The k values of MMIPs presented significant increase than those of MNIPs, showing that MMIPs had the highest molecular recognition selectivity to TCP. (ii) k0 is an indicator to express the adsorption affinity of recognition sites to the template molecules. The k0 results showed that the selectivity of MMIPs was more than 7.0 times as high as MNIPs. (iii) The values of k0 were 7.30 and 8.84 for DCP and CP, respectively, indicating the recognition for competitive phenolic compounds followed the order TCP > CP > DCP. The results suggested that the imprinting process significantly improved adsorption selectivity to the imprinted template. 5447


The Journal of Physical Chemistry C Regeneration of MMIPs and Their Practical Application. To test the regeneration of the MMIPs, five adsorption/desorption (regeneration) cycles were conducted with TCP. The mixture of methanol and acetic acid (9.0:1.0, v/v) was used as an eluant. After the supernatant solution was discarded, the MMIPs were washed with 2 2.5 mL of eluant under ultrasonic bath for 30 min. From the results in the Supporting Information, after five cycles of regeneration, the adsorption capacity of MMIPs for TCP was about 11.0% loss in pure TCP solution, about 16.1% loss in coexisting phenolic compound solution, suggesting good retention of the activity of the MMIPs. To demonstrate the applicability of the method, several water samples were analyzed. The recovery study was carried out by spiking environmental water samples. The recoveries from 83.3% ( 6.1% to 96.3% ( 2.7% were obtained for TCP (Table 4). The results indicated that the proposed method was applicable for the separation and determination of TCP in different environmental water samples.

’ CONCLUSIONS In this work, attachment of magnetic nanoparticles to HNTs via robust linkages was achieved by thermal decomposition of organic iron precursors such as Fe(acac)3 in the presence of carboxylated HNTs. Then we developed an efficient method for synthesis of magnetic molecularly imprinted polymers using magnetic HNTs particles as support. The prepared MMIPs exhibited excellent specific recognition, thermal stability, and saturation magnetization. It could be easily separated from the suspension by an external magnetic field, leading to a fast and selective recognition of TCP from aqueous solutions. After MMIPs were reused and regenerated five times, the fifth adsorption capacity was still excellent. We believe that these surfaceimprinted polymers with magnetic composites as supports can be one of the most promising candidates for environmental pollutants separation. ’ ASSOCIATED CONTENT

bS

Supporting Information. The photographs of MMIPs (left image) and m-MMIPs (right image) suspended in water and potential regeneration of the MMIPs after five cycles. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86-0511-88790683. Fax: þ86-0511-88791800. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (nos. 21077046 and 30970309) and the Ph.D. Programs Foundation of Ministry of Education of China (no. 20093227110015). ’ REFERENCES (1) Keith, L. H.; Telliard, W. A. Environ. Sci. Technol. 1979, 13, 416. (2) Zhou, W. M.; Fu, D. Q.; Sun, Z. G. Res. Environ. Sci. 1991, 4, 912. (3) Chen, G. C.; Shan, X. Q.; Wang, Y. S.; Wen, B.; Pei, Z. G.; Xie, Y. N.; Liu, T.; Pignatello, J. J. Water Res. 2009, 43, 2409.

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Selective Recognition of 2,4,6-Trichlorophenol by Molecularly

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