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Environmental Pollution 158 (2010) 2317e2323

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Environmental Pollution

journal homepage: www.elsevier .com/locate/envpol

Synthesis of core-shell magnetic molecular imprinted polymer by the surfaceRAFT polymerization for the fast and selective removal of endocrine disruptingchemicals from aqueous solutions

Ying Li a, Xin Li a,*, Jia Chu a, Cunku Dong a, Jingyao Qi b, Yixing Yuan b

aDepartment of Chemistry, Harbin Institute of Technology, Harbin 150090, Chinab School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China

Magnetic molecular imprinted polymers have potential as adsorptive

materials in water treatment.

a r t i c l e i n f o

Article history:Received 23 October 2009Received in revised form3 February 2010Accepted 4 February 2010

Keywords:Endocrine disrupting chemicalsMolecularly imprinted polymerIron oxideRemoval

* Corresponding author. Tel.: þ86 451 86282153; faE-mail address: lixin@hit.edu.cn (X. Li).

0269-7491/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.envpol.2010.02.007

a b s t r a c t

In this study, we present a general protocol for the making of surface-imprinted core-shell magneticbeads via reversible addition-fragmentation chain transfer (RAFT) polymerization using RAFT agentfunctionalized iron oxide nanoparticles as the chain transfer agent. The resulting composites werecharacterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) analysis, thermogravi-metric analysis (TGA), vibrating sample magnetometer (VSM), and scanning electron microscopy (SEM)and transmission electron microscopy (TEM). The surface-imprinted magnetic beads were demonstratedwith a homogeneous polymer films (thickness of about 22 nm), spherical shape, and exhibited magneticproperty (Ms ¼ 0.41 mA m2 g�1) and thermal stability. Rebinding experiments were carried out todetermine the specific binding capacity and selective recognition. The as-synthesized surface-imprintedcore-shell magnetic beads showed outstanding affinity and selectivity towards bisphenol A over struc-turally related compounds, and easily reach the magnetic separation under an external magnetic field. Inaddition, the resulting composites reusability without obviously deterioration in performance wasdemonstrated at least five repeated cycles.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Endocrine disrupting chemicals (EDCs) are chemicals with thepotential to elicit negative effects on the endocrine systems ofhumans and wildlife. Over the past few decades, the EDCs pre-sented in the environment have received considerable attention inthe scientific and public community (Martin-Skilton et al., 2006;Matthiessen and Johnson, 2007). It has been shown that EDCscannot be completely eliminated by conventional treatmentmethods in complex environmental matrices, resulting in theformation of by-products with even a higher endocrine disruptingaction (Korshin et al., 2006; Gogate and Pandit, 2004; Stackelberget al., 2004). For the removal and perhaps recovery of the EDCsfrom the environment, it is desirable to use adsorptive materials,which have high recognition and selectivity towards the EDCs,together with rapid sorption kinetics.

x: þ86 451 86418750.

All rights reserved.

Molecularly imprinted polymers (MIPs) are a new kind of smartmaterials that display remarkable recognition properties (Piletskyet al., 2001). Nowadays, molecular imprinting is a widely usedmethod for the preparation of tailor-made materials with a highrecognition towards a targetmolecule, normally the template usingwhich the material was prepared (Bolisay et al., 2006; Haupt andMosbach, 2000; Henry et al., 2005; Janiak and Kofinas, 2007;Karim et al., 2005; Mahony et al., 2005; Spivak, 2005).

Selective recognition and removal of EDCs from aqueous solu-tion using MIPs have been recently demonstrated (Alexiadou et al.,2008; Mathieu et al., 2007; Meng et al., 2005). Conventionally, MIPsare synthesized by using the free-radical polymerization technique.However, the rate of chain propagation in the free-radical poly-merization process is difficult to control, and the resultant polymergenerally has a broad size distribution due to side reactions. Whilesuch MIPs can exhibit high affinity and selectivity, their adsorptivecapacity and accessibility to the adsorptive sites for a targetmolecule are in general poor (Chen et al., 1999). To solve theseproblems, different living/control radical polymerization tech-niques have been developed. Amongst, the reversible addition-fragmentation chain transfer (RAFT) polymerization, first reported

Y. Li et al. / Environmental Pollution 158 (2010) 2317e23232318

by Rizzardo and co-workers (Chiefari et al., 1998), is a mostpromising method. The RAFT polymerization method allows one tocontrol the polymerization of a wide range of monomers withoutusing metal catalysts under mild polymerization conditions. Inaddition, the resultant polymer obtained by RAFT polymerization isend-capped by the moieties derived from the RAFT agent. Asa result, the functional groups can be easily introduced into thechain ends of the polymer by adjusting the structure of the RAFTagent used in the RAFT process (Lowe and McCormick, 2007).Recently, the RAFT polymerization technique has been extensivelyused to prepare MIPs (Lu et al., 2007; Southard et al., 2007). Thinfilms of molecularly imprinted polymers that combine covalentimmobilization of azo initiators with RAFT-mediated living radicalpolymerization on mesoporous silica beads have been reported(Titirici and Sellergren, 2006).

Separation of a solid from an aqueous solution represents a hugeissue in engineering aspect. A great deal of recent research efforthas been made at magnetic solids, which can be easily separatedfrom the medium simply by applying an external magnetic field.Recently, the preparation of magnetic MIP nanowires has beendescribed (Li et al., 2006). Nanopore alumina was used as template.The magnetic MIP nanowires displayed excellent properties fortheophylline. Bovine serum albumin surface-imprinted sub-micrometer particles (500e600 nm) with magnetic susceptibilitiesprepared by usingminiemulsion polymerizationmethod have beenreported (Tan et al., 2008). More recently, we have developeda general approach to rational design of MIPs for the determinationof toxic pollutants from extreme environments (Li et al., 2009a,b).

In this work, superparamagnetic core-shell beads withmagneticiron oxide covered with silica as the core and MIP as the shell wereprepared using the suspension polymerization method. Therecognition and removal efficiency of the beads for EDCs fromaqueous environment was investigated. Bisphenol A (BPA) [2,2-bis(4-hydroxyphenyl) propane] was chosen as a model contaminantfor it is considered as a highly active estrogen frequently present inthe environment (Chen et al., 2007; Hu et al., 2002; Suzuki et al.,2004).The preparation of the surface-imprinted core-shellmagnetic beads is schematically illustrated in Scheme 1. It involvesa couple of steps, including the synthesis of Fe3O4 microspheres,deposition of a thin layer of silica on the sphere surfaces, MIP-functionalized onto the silica surface, and final extraction of BPAand generation of the recognition site. The as-synthesized surface-imprinted core-shell magnetic beads showed a high affinity,selectivity, and easy separation behavior.

2. Experiment section

2.1. Materials

Ultra-dried tetrahydrofuran (THF), phenylmagnesium bromide and 4-(chlor-omethyl)phenyltrichlorosilane were obtained from the J&K Chemical Ltd. Acryl-amide (AAm) was provided by the Shanpu Chemical Co., Ltd. (Shanghai, China).Tetraethyl Orthosilicate (TEOS) was purchased from the Bodi Chemical Reagent Co.,Ltd. (Tianjin, China). 4,40-Sulfonyldiphenol, BPA, phenol, azobisisobutyronitrile(AIBN) and divinylbenzene (DVB) were purchased from the Guangfu ChemicalIndustry (Tianjin, China). Chromatographically pure toluene was obtained from theKermel Chemical Reagent Co., Ltd. All other chemicals were of analytical grade andused as received. For practical water samples, tap water was taken from the watersupply system on our campus. The characterization of the drinking water samplewas expressed as follows: pH ¼ 6.80, turbidity ¼ 0.35 (NTU), and total organiccarbon (TOC) ¼ 2.01 mg L�1.

2.2. Preparation of Fe3O4@SiO2 microspheres

Iron oxide nanoparticles were first synthesized by solvothermal reductionmethod (Deng et al., 2005). Fe3O4@SiO2 microspheres were prepared according tothe method reported with minor modification (Fang et al., 2008). Typically, 2 mLFe3O4 water-basedmagnetic fluid (40 mgmL�1) was dispersed in 60 mL ethanol and10 mL of highly purified water by sonication for 15 min, followed by the addition of

1.0 mL ammonium hydroxide (25 wt%) and 2.0 mL TEOS sequentially. The mixturewas reacted for 24 h at the room temperature under a continuous stirring. Theresultant product was collected by an external magnetic field, and rinsed six timeswith ethanol and highly purified water, respectively. Finally, the Fe3O4@SiO2

microspheres obtained were dried under vacuum at 60 �C for 24 h.

2.3. Synthesis of 4-(chloromethyl)-phenyltrichlorosilane functionalized Fe3O4@SiO2

microspheres (Fe3O4@SiO2-Cl)

To prepare the Fe3O4@SiO2-Cl, 2.0 g of Fe3O4@SiO2 was dispersed in a solution of4.0 mL 4-(chloromethyl) phenyltrichlorosilane in 55 mL of anhydrous toluene. Afterultrasonic treatment for 20 min, a solution of triethylamine (2.0 mL) in anhydroustoluene (5.0 mL) was slowly added to the former mixture. After the addition wascomplete, the resulting mixture was heated at reflux for 24 h under nitrogenprotection. The product was washed with methyl alcohol for five times. Finally, theFe3O4@SiO2-Cl obtained was dried under vacuum at 60 �C for 24 h.

2.4. Synthesis of Fe3O4@SiO2-RAFT agent

14.5mLphenylmagnesiumbromidewasdispersed in130mLultra-dried THF, andreacted with 3.5 mL carbon disulfide at 45 �C for 2 h. The reaction mixture was thenadded 0.5 g Fe3O4@SiO2-Cl and kept at 65 �C for 72 h under nitrogen protection. Afterpolymerizationwas complete, the resulting products (Fe3O4@SiO2-RAFTagent) werewashed with methanol and acetone, separately, for 5 times, and the nanoparticleswere dried under vacuum at 60 �C for 24 h.

2.5. Preparation of MIP nano-film on the surface of Fe3O4@SiO2-RAFT (Fe3O4@SiO2-MIP)

6mmol AAm, 20mmol DVB and 220mg Fe3O4@SiO2-RAFT agent were dispersedinto a 10 mL chloroform solution of BPA (1 mmol). After sealing, shaking, andpurging the mixture with nitrogen, a 5 mL acetonitrile solution of 50 mg AIBN(initiator) were added into this suspensionwith glass syringe. The resultant mixturewas stirred at 60 �C for 24 h under nitrogen protection, and then separately washedseveral times with chloroform, methanol-acetic acid (9:1, v/v) solution and acetoneuntil the template was not detected. Finally, the resultant polymer was dried undervacuum at 60 �C for 24 h. As a reference, non-imprinted polymer coated magneticnanoparticles (Fe3O4@SiO2-NIP) was prepared without the template, was alsoprepared in parallel with the Fe3O4@SiO2-MIP by using the same synthetic protocol.

2.6. Characterization

X-ray diffraction (XRD) analysis was carried out on an XRD-6000 X-raydiffractometer (Shimadzu). Fourier transform infrared spectra (FT-IR) were recordedon an Avatar 360 (Nicolet) instrument. Transmission electron microscopy (TEM),scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS)images were obtained on a Tecnai G20 (Philip) and S4800HSD (Hitachi) micro-scopes, respectively. Magnetic properties were measured with a LakeShore 7307(Lakeshore Cryotronic) VSM at 300 K. Thermogravimetric analysis (TGA) was per-formed by a ZRY-2P thermal analyser (Shanghai Balance Instrument Company,China). The TOC content of the aqueous solution was measured on a TOV-Vcpn

analyzer (Shimadzu).

2.7. Adsorption measurement

To investigate the adsorption dynamics of the Fe3O4@SiO2-MIP (i.e. Fe3O4@SiO2-NIP, Fe3O4@SiO2, and bulk Fe3O4), 40 mg imprinted polymer was dispersed inmethanol solution of 0.1 mmol L�1 BPA (20 mL). The mixture was shaken continu-ously at 25 �C. The specimens were sampled at different time intervals: 10,15, 20, 30,40, 60, 80, 120, 150 and 180 min. The adsorption efficiency was calculated based onthe difference of BPA concentrations before and after adsorption. To investigate theadsorption equilibrium of the Fe3O4@SiO2-MIP (i.e. Fe3O4@SiO2-NIP, Fe3O4@SiO2,and bulk Fe3O4), 10 mg imprinted polymer was equilibrated with varied initialconcentrations (0.02e0.5 mmol L�1) of adsorbate in each tube. After 12 h, thesaturated polymer was separated by a strong magnet. The concentration of the BPAwas measured by UVevis spectrophotometer (Shimadzu UV3100) at 279 nm. Inorder to estimate the selectivity of Fe3O4@SiO2-MIP for BPA, 40 mg Fe3O4@SiO2-MIPwas dispersed in 20 mL of aqueous solutions containing 0.1 mmol L�1 of BPA,bisphenol S (BPS) and phenol respectively. The mixture was shaken in HY verticalmulti-purpose vibrator at 25 �C for 4 h. The concentrations of BPA, BPS and phenolwere determined by UVeVis spectrophotometer at the conditions given above. Inaddition, the Fe3O4@SiO2-MIP solid with adsorbed species was regenerated witha methanol/acetic acid (9/1, v/v) mixture. The solid was filtered off and washed withdistilled water, dried in vacuum, and reused for the subsequent adsorption of BPA.

The adsorption equilibrium of BPA on Fe3O4@SiO2-MIP was fitted to Langmuirequation:

1Qeq

¼ 1Qm

þ 1bQmCeq

(1)

Scheme 1. Synthesis route of surface-imprinted core-shell magnetic beads and their application for removal of BPA with the help of an applied magnetic field.

Y. Li et al. / Environmental Pollution 158 (2010) 2317e2323 2319

Where Qeq (mg g�1) is the equilibrium adsorbed amount of BPA, Qm (mg g�1) is thesaturation capacity, Ce (mg mL�1) is the equilibrium concentration, and b (mLmg�1)is the adsorption equilibrium constant.

To evaluate the adsorption kinetics of BPA, two kinetic models were used in thiscase, assuming that the measured concentrations are equal to adsorbent surfaceconcentrations. The pseudo-first-order rate expression of Lagergren model isgenerally expressed as follows:

log�Qeq � Qt

� ¼ logQeq � k1t2:303

(2)

Where t is the rebinding time (min), Qt (mg g�1) is the adsorption capacity ofdifferent time, Qeq (mg g�1) is the equilibrium rebinding capacity, k1 (min�1) is thefirst-order rate constant.

The pseudo-second-order kinetic rate equation is expressed as:

tQt

¼ 1k2Q2

eqþ tQeq

(3)

Where k2 is the rate constant of second-order adsorption (g(mg min)�1).

3. Results and discussion

3.1. Characterization of the Fe3O4@SiO2-MIP

Fig. 1 compares the XRD patterns of magnetite, Fe3O4@SiO2 andFe3O4@SiO2-MIP which display several relatively strong reflectionpeaks in the2q regionof10e80�. Thepeakpositionswereunchangedupon coating of SiO2 and polymerization, indicating that the crys-talline structure of the magnetite was essentially maintained.

The SEM and TEM images of samples magnetite, Fe3O4@SiO2

and Fe3O4@SiO2-MIP are shown in Fig. 2. All simples displayedspherical shape before and after being encapsulated by silica andMIP. Fig. 2(a) shows the SEM image of the uncoated Fe3O4 micro-sphere. This image reveals a relatively uniform size distributionwith a mean diameter of about 330 nm. It is also seen that eachsynthesized magnetite microsphere consists of a large number ofmagnetite nanoparticles. Fig. 2(b) shows the SEM and TEM (inset)

images of sample Fe3O4@SiO2. It can be seen that the thickness ofthe silica layer is about 18 nm, clearly showing that the magnetitemicrospheres were fully coated by silica. Actually, in this process,the thickness of the silica layer can be controlled by varying theTEOS concentration in the reaction mixture. Fig. 2(c) clearly showsthe core-shell structure of sample Fe3O4@SiO2-MIP. The polymershell had a thickness of about 22 nm and appeared to be uniform(inset), which could be attributable to the intrinsic characteristicsof the controlled/living polymerization mechanism of the RAFTpolymerization process.

Compared in Fig. 3 are the FT-IR spectra of Fe3O4, Fe3O4@SiO2,Fe3O4@SiO2-Cl, Fe3O4@SiO2-RAFT, and Fe3O4@SiO2-MIP. As shown inFig. 3(a), characteristic absorption band of Fe3O4 appeared are571 cm�1. In Fig. 3(b), thepeaksat954cm�1, 799 cm�1 and1096cm�1

attributed to the stretching of SieOeH, SieO, and SieOeSi, respec-tively. The peaks at 678 cm�1 (CeCl bond stretching vibration),1635 cm�1 (C]C bond stretching vibration), 2975 cm�1 and2928 cm�1 chloromethylphenyl has been grafted onto the surface ofFe3O4@SiO2 (Fig. 3(c)). In Fig. 3(d), the band at 1635 cm�1 (C]C bondstretching vibration) enhanced and the peak at 1054 cm�1(C]S bondstretching vibration) appeared, while the CeCl stretching band dis-appeared, revealingthatFe3O4@SiO2-ClhasconvertedtoFe3O4@SiO2-RAFT. The absorption band at 1561 cm�1 is strengthened distinctly,which is the characteristic absorption of carbonyl groups of meth-acrylamide. At the same time, the characteristic peakofNeHbendingband at 1513 cm�1 verified the successful grafting of polymer shellfrom Fe3O4@SiO2 by RAFT (Fig. 3(e)).

The EDS analysis was also conducted to confirm the existence ofimprintedpolymer. In Fig. 4a, the signal of oxygenand iron appearedfor Fe3O4 nanoparticles. After encapsulated with SiO2, the signal ofsilica appeared, and the intensity of iron decreased owing to theencapsulation of Fe3O4 nanoparticles by SiO2 (Fig. 4(b)). The signalsof sulfur is present for Fe3O4@SiO2-MIP, which demonstrated thegraft of imprinted polymer onto surface of Fe3O4@SiO2 (Fig. 4(c)).

Fig. 1. XRD patterns of Fe3O4 (a), Fe3O4@SiO2 (b), and Fe3O4@SiO2-MIP(c).

Fig. 3. FT-IR spectra of Fe3O4 (a), Fe3O4@SiO2 (b), Fe3O4@SiO2-Cl (c), Fe3O4@SiO2-RAFT(d), and Fe3O4@SiO2-MIP (e).

Y. Li et al. / Environmental Pollution 158 (2010) 2317e23232320

The TGA curves of the Fe3O4@SiO2, and Fe3O4@SiO2-MIP weregiven in Fig. 5. Fig. 5(a) illustrated that in the range of 100e200 �C,the rate of weight loss for Fe3O4@SiO2 increased owing to thedehydration in the layer of SiO2. The high rate of weight loss in thetemperature ranging from 270 �C to 340 �C might be due to exis-tence of trisodium citrate. It is noted that Fe3O4@SiO2-MIP hada rapid weight loss (Fig. 5(b)) rate between 470 �C and 580 �C,which was originated from the imprinted polymer on the surface ofFe3O4@SiO2. Hence, the results fully demonstrated the existence ofimprinted polymer.

3.2. Adsorption of BPA onto Fe3O4@SiO2-MIP

3.2.1. Magnetic properties of Fe3O4@SiO2-MIPVSM was employed to study the magnetic properties of the

synthesized Fe3O4@SiO2-MIP. The magnetic hysteresis loop of thesample dried at 300 K is illustrated in Fig. 6. The saturationmagnetization of Fe3O4@SiO2-MIP was reduced to 0.41 mA m2 g�1

in comparison with that of the bulk Fe3O4 and Fe3O4@SiO2 (satu-ration magnetization are 93.49 mA m2 g�1 and 10.84 mA m2 g�1

respectively), but remained enough magnetic response to meet theneed of magnetic separation as can be seen from the photograph inFig. 6 (inset). In the absence of an external magnetic field, a yellowhomogeneous dispersion exists. When an external magnetic field(0.8 T) was applied, the yellow particles were attracted to the wallof vial in a short time (about 100 s).

3.2.2. Adsorption kineticsFig. 7 shows the adsorption kinetic curves of BPA over samples

Fe3O4@SiO2-MIP, Fe3O4@SiO2-NIP, Fe3O4@SiO2, and bulk Fe3O4.Compared with Fe3O4@SiO2-NIP and bulk Fe3O4, it is evidential that

Fig. 2. The SEM and TEM images of Fe3O4 (a),

a much higher adsorption capacity was achieved on Fe3O4@SiO2-MIP. The removal efficiency of Fe3O4@SiO2-MIP, Fe3O4@SiO2-NIP,Fe3O4@SiO2, and bulk Fe3O4 were 81.22%, 29.73%, 5.34%, and 12.11%,respectively. The adsorptionprocess could bedivided into two steps,a quick step and a slowone. In the first step, the adsorption ratewasfast, and the contact time to nearly reach equilibriumwas 40min. Inthe subsequent step, the adsorption was slow to reach equilibrium.In conclusion, higher adsorption efficiency was realized in a shortertime. The pseudo-first-order and pseudo-second-order modelswere adopted to describe the sorption kinetic data. According topseudo-first-order model (eq (2)), log (Qeq�Qt) rebinding time t(min) resulting in a straight line with was plotted versus rebindingtime t (min) resulting in a straight line with coefficient of determi-nation, R2, (0.9633). k1 was calculated from the slope of the linearplot as 0.024 min�1. According to pseudo-second-order model (eq(3)), t/Qt was plotted versus coefficient of determination, R2,(0.9976). k2 was calculated from the intercept of the linear plot as0.012 g (mg min)�1. The results show that the second-order mech-anism is applicable (R2 values are the highest). These results alsosuggest that the pseudo-second-order mechanism is predominantand that chemisorptionmight be the rate-limiting step that controlsthe adsorption process (Baydemir et al., 2007).

3.2.3. Sorption isothermThe sorption isotherms of BPA on samples Fe3O4@SiO2-MIP,

Fe3O4@SiO2-NIP, Fe3O4@SiO2 and bulk Fe3O4 are presented in Fig. 8.

Fe3O4@SiO2 (b), and Fe3O4@SiO2-MIP (c).

Fig. 4. EDS spectra of Fe3O4 (a), Fe3O4@SiO2 (b), and Fe3O4@SiO2-MIP (c).

Fig. 5. TGA curves of Fe3O4@SiO2 (a), and Fe3O4@SiO2-MIP (b).

Fig. 6. The hysteresis loop of Fe3O4@SiO2-MIP beads (Saturation magnetization is0.41 mA m2 g�1). The insert shows the separation and redispersion process of a solu-tion of Fe3O4@SiO2-MIP in the absence (left) and presence (right) of an externalmagnetic field.

Y. Li et al. / Environmental Pollution 158 (2010) 2317e2323 2321

By fitting the experimental data with Langmuir equation, thesaturation capacity of samples Fe3O4@SiO2-MIP, Fe3O4@SiO2-NIP,Fe3O4@SiO2 and bulk Fe3O4 for BPA were found to be 21.30 mg g�1,5.18 mg g�1, 0.85 mg g�1 and 1.98 mg g�1, respectively. Theadsorption capacity of sample Fe3O4@SiO2-MIP is about four timeshigher that of the Fe3O4@SiO2-NIP, 25 times higher than that of theFe3O4@SiO2, and ten times higher than that of the magnetite. Thefact that the experimental data agreed well with the Langmuir

isotherm implies a homogeneous distribution of molecularlyimprinting adsorption sites on the MIP surface. It is known thatuncontrolled solution polymerization is most likely to occur,leading to thicker and inhomogeneous polymer grafts. This can behere minimized by using the RAFT polymerization technique.

3.2.4. Competitive adsorption, reuse, and analysis of environmentalwater samples

In order to evaluate the selectivity of Fe3O4@SiO2-MIP towardsBPA, BPS and phenol were selected as potential interferents due totheir similar chemical molecular structure to that of BPA. As can beseen from Fig. 9(a), the removal efficiencies for BPA, BPS and phenolby Fe3O4@SiO2-MIP were 71.38%, 32.34% and 23.17%, respectively,showing that Fe3O4@SiO2-MIP had the highest molecular recogni-tion selectivity to BPA. The adsorption selectivity over samples(Fe3O4@SiO2-NIP, Fe3O4@SiO2 and bulk Fe3O4) is low and theireffect on the removal efficiency of BPA, BPS and phenol are similar(Fe3O4@SiO2-NIP: 22.36%, 20.18% and 18.69% respectively;Fe3O4@SiO2: 4.86%, 4.12%, 4.97% respectively; bulk Fe3O4: 10.13%,9.44%, and 10.46% respectively). The results indicate that theFe3O4@SiO2-NIP, Fe3O4@SiO2 and bulk Fe3O4 have no selectivity.

Fig. 7. Adsorption kinetic curve of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-NIP, and Fe3O4@SiO2-MIP for BPA. Other conditions: 40 mg sorbent (Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-NIP, andFe3O4@SiO2-MIP), 20 mL methanol solution of 0.1 mmol L�1 BPA, temperature 25 �C.

Fig. 8. Adsorption isotherms of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-NIP, and Fe3O4@SiO2-MIP for BPA (a), fitted Langmuir equation of Fe3O4@SiO2-MIP (b). Other conditions:10 mg sorbent (Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-NIP, and Fe3O4@SiO2-MIP), 5 mLmethanol solution of 0.02e0.5 mmol L�1 BPA, shaking time 24 h, temperature 25 �C.

Fig. 9. Removal efficiency of competitive molecules. Other conditions: 40 mg sorbent(Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-NIP, and Fe3O4@SiO2-MIP), shaking time 4 h,temperature 25 �C (a). Stability and potential regeneration of the Fe3O4@SiO2-MIP(b).

Fig. 10. Adsorption of Fe3O4@SiO2-MIP for BPA in real water sample. Other conditions:40 mg sorbent Fe3O4@SiO2-MIP, 20 mL real water sample of 0.1 mmol L�1 BPA,temperature 25 �C. The characterization of the real water sample: pH ¼ 6.80,turbidity ¼ 0.35 (NTU), and TOC ¼ 2.01 mg L�1.

Y. Li et al. / Environmental Pollution 158 (2010) 2317e23232322

Y. Li et al. / Environmental Pollution 158 (2010) 2317e2323 2323

To test the stability and reusability of the Fe3O4@SiO2-MIP, fivebinding/removal (regeneration) cycleswere conductedwithBPA.Noobviousdecrease in the adsorption capacitywasobserved (Fig. 9(b)),suggesting good retention of the activity of the Fe3O4@SiO2-MIP.

In order to assess the applicability of Fe3O4@SiO2-MIP toa practical water treatment, a drinking water sample spiked withBPA in concentration of 0.1 mmol L�1 was analyzed by the proce-dure mentioned above. The sampling method described in theprevious study (Liu et al., 2009,2008) was used to prepare theworking solution of BPA. As seen from Fig. 10, the results clearlydemonstrate that the Fe3O4@SiO2-MIP can produce good recoveryand can be effectively applied in the removal of BPA from envi-ronmental water samples.

4. Conclusions

This work provides a platform to prepare surface-imprintedcore-shell magnetic beads with high affinity, selectivity and easyseparation. Although the results reported here relate only to BPA,the principles of the proposed methodology are expected to beapplicable to the removal of other EDCs from contaminated water.We believe that these surface-imprinted core-shell magnetic beadscan be one of the most promising candidates for various applica-tions, including environmental pollutants and biochemical sepa-ration, recognition elements in biosensors and biochips.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (50878061) and the State Key Lab of UrbanWater Resource and Environment (HIT, ESK200801).

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