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Available online at www.sciencedirect.com

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The selective binding character of a molecular imprinted particle for Bisphenol A from water Yue-Ming Ren a,*, Jing Yang a, Wei-Qing Ma c, Jun Ma b,**, Jing Feng a, Xiao-Li Liu a a

College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China c Qingdao Municipal Engineering Design Research Institute, Qingdao 266001, PR China b

article info

abstract

Article history:

A molecular imprinted particle for Bisphenol A (BPA-MIP) was successfully used for se-

Received 22 August 2013

lective recognition of BPA in the water. The contaminants such as 3, 30 , 5, 50 -Tetra-

Received in revised form

bromobisphenol A (TBBPA), phenol and phenol red (PSP) were selected as the latent

24 November 2013

interferon to investigate the selectivity. The binding efficiencies of BPA-MIP for different

Accepted 27 November 2013

phenols were explored at various initial concentrations in the single and mixed water.

Available online 7 December 2013

Various selective parameters such as Kd, K and K’ of BPA-MIP for BPA were calculated. The influences of humic acid (HA) and common ions on the BPA binding were investigated. A

Keywords:

physical model was proposed to illustrate the selective binding performance. The results

Molecular imprinted

showed that BPA-MIP possessed strong selectivity for BPA in competitive water, while the

BPA

other

Selective binding

TBBPA > phenol > PSP. The HA and common ions indicated little effect on the BPA binding

Selectivity coefficient

process onto BPA-MIP. It was found that the molecular geometry and the hydrogen

Water environment

bonding interactions between the hydroxyl and carboxyl played an important role in

similar

phenols

had

the

influence

for

BPA

binding

at

the

order

of

recognizing the target molecular in the binding process. ª 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

BPA is one of a well-known endocrine disrupting chemical that has been widely used as an intermediate in the production of epoxy resins, polysulfones and polycarbonates plastics. In general, BPA can dissolve from plastics into water or food (Krishnan et al., 1993; Brotons et al., 1995). It has been reportedly observed in rivers, seas and soils (Staples et al., 1998), and it can interfere with natural hormone systems

(Hunt et al., 2003; Maffini et al., 2006). Therefore, the rapid enrich and recognition of BPA is both important and requisite in the water environment. Adsorption is a common technique for the binding of some contaminants such as phenols and heavy metals, therefore,  various adsorbents are widely researched (Sæiban et al., 2008; Al-Sarawy et al., 2005). The activated carbon adsorption is very efficient for estrogens, but it is easily impacted by the interfering substances such as humic acids in the environmental water (Fukuhara et al., 2006). Therefore, the search for cost-

* Corresponding author. Tel./fax: þ86 451 82569890, þ86 451 86282292, þ86 451 82368074. ** Corresponding author. E-mail addresses: [email protected] (Y.-M. Ren), [email protected] (J. Ma). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.11.042

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effective and selective binding methods is still warranted (Krupadam et al., 2010). Molecular imprinting is a state-of-theart technique what has become a powerful method for the preparation of selective materials that have the ability to recognize a specific chemical material (Wulff, 1995; Masque´ et al., 2001; Chassaing et al., 2004). Molecular imprinting technique not only can be used in the selective removal of BPA from water, but also used in the pre-enrichment process of detection such as flow injection fluorimetric determination (Bravo et al., 2005), LC-MS (Watabe et al., 2005) and HPLC (Alexiadou et al., 2008). These reports showed that the interference from the water samples effectively decreased after packed the molecularly-imprinted polymer particles in the pretreatment column, and it resulted in a significant increase in sensitivity and more reliable results. Most MIPs are prepared by the co-polymerization and precipitation polymerization reaction in the reports of molecularly imprinted technology. Researchers (Takeda and Kobayashi, 2005; Baggiani et al., 2010) prepared an imprinted polymer with molecular recognition properties for BPA by the co-polymerization and thermal polymerization, respectively. Hiratsuka et al. (2013) reported a magnetic molecularly imprinted polymer for BPA by a multi-step swelling and polymerization method. Whereas, these reactions often need harsh conditions of nitrogen, heating, acid or alkali, and the MIPs exhibit a low binding efficiency in the water. Recently, more and more reports are focused on the surface molecular imprinting by a solegel process, which is a new-style type, low cost and easy manipulation. The materials prepared by this mean possess many advantages including more accessible binding sites and high selectivity for targets, and they can be applied in the water (Fang et al., 2005). Zhu et al. (2010) synthesized the BPA imprinted particles by the solegel process. The selective solid-phase extraction of BPA from chemical cleansing and cosmetics samples were compared with its analogues, and it revealed high selectivity and molecular recognition. According to the literature, the selectivity in the mixed solution can be denoted by the selectivity coefficient. The molecular imprinted polymers of phenol, 2, 4dichlorophenol and 2, 4-dichlorophenoxyacetic acid were also reported (An et al., 2008; Pan et al., 2010; Han et al., 2010). At the same time, the selective adsorption coefficient was studied visually to display the selectivity of the imprinted polymers. Up to date, the possible imprinting and selective binding mechanism have not been very clear yet. A few people had put forward the possible binding models to illustrate the imprinting process. Piletsky et al. (1996) proposed a “gate effect” model. Li et al. (2009a, b) further confirmed this model, and the template is analogized to the “key”, corresponding with the binding site of molecular imprinted polymer which is analogized to the “lock”. However, the reports about selective recognition performance of imprinted polymers for template in the complicated water were few. In our previous work, a molecular imprinted particle for BPA (BPA-MIP) was prepared using the surface molecular imprinting technique with a solegel process on the surface of silica nanoparticles firstly (Ren et al., 2012). The optimal preparation conditions and the characters of the BPA-MIP were explored. The preliminary selective recognition was evaluated in its single system. The objective of this study was

91

to evaluate the selective binding behavior of BPA-MIP for BPA in the single and multicomponent mixtures such as TBBPA, phenol and PSP with the similar molecular structure. The influences of the humic acid (HA) and common ions on the BPA binding were also investigated. Furthermore, a specific binding mold about hydrogen bonding structures of various phenols was proposed to explain the selective rebinding mechanism according to the binding effect of BPA, TBBPA, phenol and PSP in aqueous solution.

2.

Experimental

2.1.

Materials

Bisphenol A (BPA), 3, 30 , 5, 50 -tetrabromobisphenol A (TBBPA), phenol red (PSP), phenol, Humic acid (HA) and all other chemicals were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd (Taijin, China). Distilled water used throughout the experiments was obtained from laboratory purification system.

2.2.

Preparation of samples

The preparation method of samples has been confirmed in our early work (Ren et al., 2012). The uniform silica nanoparticles were synthesized by TEOS hydrolysis with ammonium hydroxide according to the report by Sto¨ber et al. (1968). The nanoparticles were finally washed with anhydrous ethanol and dried. BPA-MIP was prepared by the surface molecular imprinting technique with a solegel method. As a reference, BPA-NIP was prepared by the same protocol without the template molecule.

2.3.

Experimental procedure

All binding experiments were carried out in 100 mL flasks, each solution contained 30 mL adsorbate and 0.015 g of the prepared samples. The flasks were shaken at the speed of 200 rpm at 25  C for 4 h (having achieved the binding equilibrium). Solution pH remained at about 6.5 in all tests. The single binding experiments were conducted by preparing solution of BPA, TBBPA, phenol or PSP with each initial concentration of 10 mmol L1. For the binary test, one was that the initial concentration of TBBPA, phenol or PSP distributed from 1 mmol L1e50 mmol L1, while BPA concentration remained at 20 mmol L1, the other was that the initial concentration of BPA changed from 1 mmol L1e50 mmol L1, while the concentration of TBBPA, phenol or PSP were 15 mmol L1, 15 mmol L1, 10 mmol L1, respectively. The triple mixed water system of BPA/TBBPA/phenol and BPA/phenol/PSP were prepared with the initial concentration of TBBPA, phenol or PSP at 15 mmol L1, 15 mmol L1 and 10 mmol L1, respectively, while the concentration of BPA changed from 1 mmol$L1 e50 mmol L1. One test of HA effect was that the initial concentration of HA changed from 1 mg L1e5 mg L1 with the BPA of 20 mmol L1, and the other was that the initial concentration of BPA differentiated from 10 mmol L1e100 mmol L1 with HA of 3 mg L1. The experiment of ions effect was that the concentration of BPA varied from 10 mmol L1e100 mmol L1, and the

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initial concentration of Naþ, Kþ, Ca2þ, Mg2þ, SO2 4 and Cl were 10.58 mg L1, 0.39 mg L1, 0.40 mg L1, 1.15 mg L1, 1.44 mg L1 and 19.88 mg L1, respectively. In order to test the reproducibility, the experiments were carried in triplicate and the error of reproducibility was found to be within 2%.

2.4.

Analysis

After binding equilibrium, the saturated samples were separated by centrifugation, and the residual concentrations of BPA, TBBPA, phenol and PSP were determined by high performance liquid chromatography (HPLC) 1525 equipped with a Waters 717 autosampler and a Waters 2487 dual l detector. Waters symmetry C18 column (4.6 mm  150 mm, 5 mm particle sizes) was used as separate column. In the mixed system, BPA/phenol, BPA/PSP, or BPA/phenol/PSP were quantified at l of 228 nm and 280 nm with an eluent of methanol and water ratio of 70:30 at a flow rate of 0.5 mL/min at 30  C. In addition, BPA/TBBPA, or BPA/TBBPA/phenol were quantified at l of 228 nm and 280 nm with an eluent of methanol and water ratio of 80:20 at a flow rate of 0.3 mL/min at 30  C.

2.5.

Expressions

The binding capacity (q) and efficiency (E) were calculated by the following equations (Cayllahua et al., 2009): q¼

ðC0  Ce ÞV W

(1)

E¼

C0  Ce  100% C0

(2)

where q (mmol g1) and E are the binding capacity and efficiency of adsorbate, respectively, C0 and Ce (mmol L1) are the initial and equilibrium adsorbate concentrations in the solution, respectively, V (L) is the volume of the solution, W (g) is the amount of the samples added to the solution. Langmuir isotherm models are expressed as (Bhattacharyya and Gupta, 2008): Ce Ce 1 ¼ þ qe qm KL qm

K¼

Kd ðBPAÞ Kd ðMÞ

(5)

where K is the selectivity coefficient and M represents TBBPA, phenol or PSP. A comparison of the K values of BPA-MIP with the competitors allows an estimation of the effect of imprinting on selectivity. A relative selectivity coefficient of K’ can be defined as expressed in Eq. (6) (Gao et al., 2008): K0 ¼

KMIP KNIP

(6)

where KMIP and KNIP are the selectivity coefficient of BPA-MIP and BPA-NIP for BPA with respect to the competition species of M, respectively.

3.

Results and discussion

3.1. Binding efficiency of different phenols in the single water system TBBPA, phenol and PSP are selected as the interferences since their chemical molecular structures are similar to BPA to some extent. Binding efficiency of various phenols onto the BPA-MIP and BPA-NIP in a single water system was displayed in Fig. 1. It could be seen that BPA-MIP had the highest removal rate of 88% for BPA, which was higher at the percentage of 65%, 56% and 91% than for TBBPA, phenol, and PSP, respectively, which showed a great binding affinity for BPA (Ren et al., 2012). However, the combine of all the phenols onto BPA-NIP was lower than BPA-MIP did, and the removal of BPA by the BPANIP decreased greatly. The difference in the BPA removal between BPA-MIP and BPA-NIP reached about 50%. This is larger than that obtained by Noir et al. (2007), which at approximately 20% difference in BPA removal were observed between BPA-MIP and BPA-NIP synthesis. This could be due to different functional monomer used in the two studies. For other adsorbates, the BPA-MIP and BPA-NIP nearly had the same removal rates for BPA, and the difference of their removal was

(3)

where qe (mmol g1) is the binding capacity at equilibrium time, qm (mmol g1) is the maximum binding capacity, Ce (mmol L1) is the same as Eq. (1), KL (L mmol1) is Langmuir binding coefficient. The distribution coefficient (Kd) is defined as the ratio of the adsorbate concentration in the solid phase to that in the equilibrium solution (Usman, 2008), and it was calculated for each adsorbate according to Eq. (4) (Gao et al., 2008): Kd ¼

Cp Ce

(4)

where Kd (L g1) represents the distribution coefficient, Cp (mmol L1) is the equilibrium binding concentration onto BPAMIP and BPA-NIP, Ce (mmol L1) is the same as Eq. (1). The selectivity coefficient of BPA-MIP for the binding BPA with respect to the competition species (assigned as M) can be obtained from the equilibrium binding data according to Eq. (5) (Gao et al., 2008):

Fig. 1 e Binding efficiency of BPA, TBBPA, phenol and PSP onto BPA-MIP and BPA-NIP in the single water system. Sorbent: 0.5 g LL1, Cadsorbate: 10 mmol LL1, pH: 7, T: 25  C, t: 4.0 h.

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Fig. 2 e Binding capacity of BPA, TBBPA, phenol, and PSP onto BPA-MIP and BPA-NIP in the binary mixed water system with various concentrations of competitive molecules: (a) BPA/TBBPA, (b) BPA/phenol and (c) BPA/PSP. C0BPA: 20 mmol LL1, C0competitive molecules: 1e50 mmol LL1, Sorbent: 0.5 g LL1, pH: 7, T: 25  C, t: 4.0 h.

under 20%. These binding comparison of BPA-MIP and BPANIP for each phenols suggested that the BPA-MIP was specific to BPA but non-specific to other phenols, and the BPA-NIP showed non-specific for any substances. In addition, the order of removal rate of these four substances onto BPA-MIP was BPA > TBBPA > phenol > PSP. However, there was a special case that the removal rate of phenol onto BPA-NIP was a little higher than that onto BPA-MIP. The same results also occurred in the mixed water system (Fig. 2b, Fig. 3b). This might be due to the main binding occurred at the specific sites existing on the BPA-MIP, while the non-specific adsorption dominated the binding process onto the BPA-NIP. The molecular structures of chemicals used in this study are shown in Fig. 1. It is well known that TBBPA (543.87) is a kind of brominated derivative of BPA, and its relative molecular weight is almost twice than BPA (228.29), it has the same structure as BPA except four extra bromine atoms, which has been led to the serious impact on BPA adsorption onto BPAMIP. The phenol (94.11) has a single benzene ring with a hydroxyl group. PSP (354) contains three phenol rings and heterocyclic substances, and the two phenol rings with two hydroxyl groups are similar to BPA. All the above results implied that the binding ability of BPA-MIP was influenced by both the spacial molecular structure and the hydroxyl

functional groups, while it had no relation on the relative molecular weight of each phenol.

3.2. Binding capacity of different phenols in the binary water system The binding ability of BPA-MIP and BPA-NIP for above estrogens was investigated in different binary mixed water systems, such as BPA/TBBPA, BPA/phenol and BPA/PSP solution with various initial concentrations of adsorbates. As shown in Fig. 2, along with the concentration of the competitive molecular increased in different static binary adsorption, BPAMIP and BPA-NIP had almost stable affinity for all the adsorbates. Furthermore, it was demonstrated that the average binding capacity of BPA-MIP for BPA almost remained fixed, and it could attain about 21.0 mmol g1, 27.4 mmol g1 and 33.5 mmol g1 in their respective binary solution, respectively. The average binding capacity of BPA-MIP for TBBPA, phenol and PSP were 12.8 mmol g1, 8.8 mmol g1 and 5.1 mmol g1, respectively. Our previous results suggested that the average binding capacity of BPA onto BPA-MIP was about 25.3 mmol g1 at the initial BPA concentration of 20 mmol L1. The average binding capacity would decrease within the limits with the competing targets existing, and such reduce was different in

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Fig. 3 e The effect of various phenols on the isotherms of BPA binding onto BPA-MIP and BPA-NIP in the binary water systems: (a) BPA/TBBPA, (b) BPA/phenol and (c) BPA/PSP. C0BPA: 1e50 mmol LL1, C0TBBPA: 15 mmol LL1, C0phenol: 15 mmol LL1, C0PSP: 10 mmol LL1, Sorbent: 0.5 g LL1, pH: 7, T: 25  C, t: 4.0 h.

the order of TBBPA > phenol > PSP. Obviously, TBBPA had the maximum interference for BPA recognition. It was supposed that the excellent binding selectivity of the adsorbate onto BPA-MIP was dependent on the identification of the imprinted double benzene rings in the water. This might arise from the fact that the molecular structure of TBBPA is more similar to BPA than phenol and PSP showed. In addition, it could be seen that BPA-NIP had close average capacity of about 7 mmol g1 for BPA and TBBPA. In fact, the distinction of the binding capacity for the two phenols was not obvious, which suggested that there were no excellent recognition sites onto the BPANIP, and the imprinting sites onto BPA-MIP exploited the advantages to the full during the rebinding process of the targets. The result of a slight lower capacity occurred for BPA (9.5 mmol g1) binding than that of phenol (11.3 mmol g1) in the BPA/phenol mixture, which was consistent with the result in their own single system. This might be due to the simple benzene ring of the phenol, which was easy to be combined by non-specific site onto BPA-NIP in the water phase, but the recognition by BPA-MIP was relied on the shape of the imprinted sites and the coordinate chemical bonds onto the adsorbent. However, BPA-NIP possessed the higher rebinding

capacity of 12.0 mmol g1 for BPA in the BPA/PSP system, but it was very poor for the PSP binding (0.4 mmol g1), also, it could be seen that the binding ability of BPA-MIP and BPA-NIP for PSP was both very low. It was obvious that the imprinting process had no effect on the rebinding for PSP. Moreover, the spatial structure of PSP was so different from BPA that it had three benzene rings, which resulted in the lowest binding capacity. All the above results indicated that the spatial structure of the phenols brought out the fine binding capacity in the water, and another key factor was related with the hydroxyl complexation recognition of the adsorbents with the imprinting sites. In the binary mixture solution, the high and special selectivity of BPA-MIP for BPA were displayed well.

3.3.

Binding isotherms in the mixed water system

The binding isotherms fitting to Langmuir model of BPA and the binding affinity of TBBPA, phenol and PSP onto BPA-MIP and BPA-NIP in the binary and triple water system are shown in Figs. 3 and 4, respectively, and the parameters of Langmuir isotherm model are given in Table 1. Apparently, BPA binding onto BPA-MIP and BPA-NIP ideally matched with

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Fig. 4 e The effect of various phenols on the isotherms of BPA binding onto BPA-MIP and BPA-NIP in the triple mixed water system: (a) BPA/TBBPA/phenol and (b) BPA/PSP/phenol. C0BPA: 1e50 mmol LL1, C0TBBPA: 15 mmol LL1, C0phenol: 15 mmol LL1, C0PSP : 10 mmol LL1, Sorbent: 0.5 g LL1, pH: 7, T: 25  C, t: 4.0 h.

the Langmuir model (R2>0.99). The combination still occurred on the surface of adsorbents by monomolecular layer sorption although the presence of the other interfering substances. It seemed that the competitive targets had little effect on the fit to Langmuir model for BPA onto the adsorbents. In each binary mixture (Fig. 3), the Langmuir constant qm, which is a measure of the maximum equilibrium binding capacity of monolayer, was obtained as 21.4 mmol g1, 26.8 mmol g1 and 31.1 mmol g1 of BPA binding onto BPA-MIP, respectively. Our previous experiment had proved that the value of this qm was 30.26 mmol g1 in BPA single water system (Ren et al., 2012). The decrease of the capacity in the interface solution seemed unconspicuous, which hardly any decrement occurred especially in the BPA/PSP system. The sequence of this abatement was TBBPA > phenol > PSP, that is to say, the influence of the phenols on BPA binding onto BPA-MIP was the same in different case. It also indicated that the TBBPA disturbed the adsorption greatly. Meanwhile, it could be seen that the average binding capacities of BPA-MIP for TBBPA, phenol and PSP could only hold a part for BPA at the percentage about 70%, 29% and 16%. Considerable, it could also conclude that BPA binding difference between BPA-MIP and BPA-NIP in the binary water system was still about three times despite of the existence of the competitors, which was close to the result in the BPA single solution. BPA-NIP had little distinction on the BPA and the competitive molecular binding capacity in the binary solution. The imprinting process

improved the force of the chemical key between the targets and the imprinted sites although the similar substances existing in the water. Therefore, the good fit for Langmuir equation was probably a proof for the predominant status of the chemical adsorption (Yu et al., 2008), that is to say, the fit for Langmuir also proved that the chemisorption was the ratelimiting step controlling BPA binding process. The same conclusions could be drawn from the triple mixed water system (Fig. 4). In the BPA/TBBPA/phenol mixture, the qm value of BPA-MIP for BPA reached 20.7 mmol g1 which was almost double than that of TBBPA and phenol, and in the BPA/PSP/phenol mixture. This value showed 29.0 mmol g1 which was about 3.6 and 5.8 times than that of phenol and PSP, respectively. The maximum binding capacity of BPA-NIP for BPA, TBBPA, phenol and PSP kept low at the range of 0.5e15 mmol g1. The capacity difference between BPA-MIP and BPA-NIP were about more than two times, which was lower than in the binary solution. In the mixed water system, the competitors had little impact on the binding between BPA and the imprinted sites regardless of the similar molecules structure, and highly selective recognition of BPAMIP possessed in our study.

3.4.

Binding selectivity

A further analysis of the binding selectivity based on the obtained data in relation to Kd, K and K0 in the binary and

Table 1 e The parameters of the Langmuir isotherm model in the binary and mixed water system of BPA binding onto the BPA-MIP and BPA-NIP. BPA-MIP

BPA-NIP

qe(exp) mmol g1 qm(cal) mmol g1 KL L mmol1 BPA/TBBPA BPA/phenol BPA/PSP BPA/TBBPA/phenol BPA/PSP/phenol

21.18 26.11 30.01 20.19 26.17

21.37 26.80 31.09 21.04 26.89

2.242 1.252 1.551 1.607 2.590

R2 0.999 0.999 0.999 0.999 0.999

qe(exp) mmol g1 qm(cal) mmol g1 KL L mmol1 7.02 9.01 9.65 6.65 8.71

7.21 9.22 10.01 7.05 9.63

0.138 0.131 0.179 0.906 0.368

R2 0.999 0.994 0.992 0.999 0.993

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Table 2 e Kd, K and K0 values of competitors with respective to BPA in various binary water system. BPA/TBBPA

BPA/phenol

BPA/PSP

KdBPA KdTBBPA KBPA/TBBPA 0 K KdBPA Kdphenol KBPA/phenol 0 K KdBPA KdPSP KBPA/PSP 0 K

BPA-MIP

BPA-NIP

0.480 0.315 1.524 2.73 0.400 0.167 2.395 3.50 0.478 0.0046 103.9 49.38

0.087 0.154 0.565 0.144 0.210 0.685 0.162 0.077 2.104

mixed solution was done. The results were shown in Table 2 and Table 3. The distribution coefficient (Kd) is a useful index for comparing the sportive binding capacities of different materials for a particular ion under the same experimental conditions. A high Kd value indicates a high adsorbate retention by the solid phase through sorption and chemical reactions, while a low represents a high amount of the adsorbate remains in the solution (Alumaa et al., 2002). Obviously, it could be observed that the Kd values of BPA-MIP for BPA remained stable nearby 0.47e0.48 in the binary or triple competitive solution. Except for a little lower value of 0.4 in the BPA/phenol solution, which meant that the binding ability of BPA-MIP for the imprinted BPA remained high and constant in the mixture. However, the Kd values of BPA binding onto BPA-NIP presented very low at the range of 0.085e0.162, which illustrated that BPA-NIP had a poor affinity for BPA. That is to say, more BPA was retained onto the BPA-MIP through the binding reactions of the imprinted sites although the similar competitors existed, and the distribution of BPA had not been effected by the interferences in the mixture solution. In addition, the Kd values of BPA-MIP for TBBPA revealed just a little less than that for BPA in their mixture systems. On the other hand, obviously, the Kd values of BPA-MIP for phenol and PSP were less than for BPA far

Table 3 e Kd, K and K0 values of competitors with respective to BPA in various mixed water system. BPA/TBBPA/phenol

BPA/PSP/phenol

KdBPA KdTBBPA Kdphenol KBPA/TBBPA KBPA/phenol 0 K BPA/TBBPA 0 K BPA/phenol KdBPA Kdphenol KdPSP KBPA/phenol KBPA/PSP 0 K BPA/phenol 0 K BPA/PSP

BPA-MIP

BPA-NIP

0.468 0.402 0.234 1.164 2.0 2.592 6.579 0.477 0.263 0.007 1.814 68.14 6.343 63.68

0.092 0.205 0.303 0.449 0.304

0.085 0.297 0.079 0.286 1.07

away. This could be explained by the imprinting effect. The abundant binding sites with template molecule in a predetermined special size were available for the selective recognition of BPA onto BPA-MIP. These results were accord with the above conclusions. The selectivity coefficient (K) is possible to estimate the degree of binding ability of BPA-MIP for BPA when other interferences existed in the water. High values of K indicate that BPA-MIP possess a strong selectivity and identify for BPA. In the binary mixture of BPA/TBBPA, BPA/phenol and BPA/ PSP, the K values of BPA-MIP was found to be 1.524, 2.395 and 103.9, respectively, which was out and away higher than the values of 0.565, 0.658 and 2.104 for BPA-NIP binding. Similarly, in the triple mixture of BPA/TBBPA/phenol and BPA/PSP/ phenol, also, the K values of BPA-MIP displayed variant enormously while unconspicuous of BPA-NIP. And they were higher than that of BPA-NIP greatly. BPA-MIP had obvious selectivity in the mixed water. Noticeable, the K values of BPA-MIP in the BPA/PSP mixture was the highest among the experimental system, which illustrated that BPA-MIP had the weakest affinity for PSP. And a binding sequence was BPA > TBBPA > phenol > PSP comparing with these coefficients. Meanwhile, similar binding patterns were observed both in the binary and triple water mixture by analyzing of relative selectivity coefficient (K0 ). In general, K0 is an indicator to express a binding affinity of recognition sites to the imprinted template molecules (Ren et al., 2008). And the values of K0 can indicate the enhanced extent of recognition and selectivity of the BPA-MIP for BPA comparing to the BPANIP, and high values of K0 represent preferable imprinting effect of BPA-MIP. The K0 results indicated that the selective binding of BPA-MIP for BPA was 2.73, 3.50 and 49.38 times greater than that of BPA-NIP in the binary water system (Table 2). Meanwhile, the K0 values of BPA-MIP for BPA/TBBPA and BPA/phenol in their ternary system were 2.592 and 6.579, respectively (Table 3), and K0 values of BPA-MIP for BPA/ phenol and BPA/PSP in their ternary system suggested 6.343 and 63.68, respectively (Table 3). It was worth mention that the K0 values of BPA-MIP for phenol in the two different ternary systems showed very closely, that is to say, they were not really relevant to the mixture where the phenol participated. In the BPA/PSP and BPA/PSP/phenol mixture, the K0 values of BPA-MIP showed the highest, respectively. Comparing to the results of different K and K0 values of BPAMIP in the binary and ternary water system, the imprinting effect and selectivity sequence of various phenols could be deduced as BPA > TBBPA > phenol > PSP. This sequence was equal to the favorable combination of the phenols by the BPA-MIP. The most similar spacial structure of TBBPA to BPA resulted in the most powerful effect on the BPA binding while very distinct spacial structure of PSP resulted in little effect on the BPA binding in the mixture. The above results clearly revealed that BPA-MIP possessed high selective recognition for BPA in the competitive water.

3.5.

Influence of humic acid and ions on BPA binding

Humic acid (HA), a kind of macromolecular organic matter with a large number of hydroxyl-reactive groups in the surface, having a strong complexing capacity for cations and

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Fig. 5 e The effect of HA concentration on BPA binding onto BPA-MIP and BPA-NIP in the mixed water system: (a) C0BPA: 20 mmol LL1, C0HA: 1e5 mg LL1 and (b) C0BPA: 10e100 mmol LL1, C0HA: 3 mg LL1. Sorbent: 0.5 g LL1, pH: 7, T: 25  C, t: 4.0 h.

mineral surfaces, has been recognized as an important factor controlling the behavior of various elements in the natural aquatic environment (Takahashi et al., 1997; Wan and Liu, 2006). The effect of HA on BPA binding onto the adsorbents was investigated. Fig. 5a showed the results of binding experiments in the presence of HA at the initial BPA concentration of 20 mmol L1, with HA concentrations at a range of 1e5 mg L1, over a pH of about 7.0. Evidently, HA concentration affects slimly the on the binding of BPA onto the BPA-MIP, with the increasing of HA concentration, the affinity of BPAMIP for BPA kept almost constant (the relative error of the maximum and minimum is less than 5%), which was about more than two times than that of BPA-NIP. Fig. 5b revealed the Langmuir isotherms of BPA-MIP and BPA-NIP in the presence of 3 mg L1 HA, with the initial concentration of BPA at the range of 10e100 mmol L1. It was found that the binding data fit Langmuir isotherms with the correlation coefficient (R2) were 0.99. Moreover, the maximum binding capacity (qm) from the Langmuir isotherm of BPA-MIP was 29.2 mmol g1, which was close to in the BPA single water system (qm ¼ 30.26 mmol g1). However, the binding affinity of BPA-NIP was different when the HA existing in the water. This illustrated the imprinted bond played an important role in recognizing the target molecular in the binding process. It was pleased that the presence of HA almost had no effect on the BPA binding onto BPA-MIP. This was consistent with the results obtained by Dickert and Tortschanoff (1999, 2001). Molecularly imprinted polyurethanes were used as sensitive coating for detection of PAHs with hardly any effect by humic acid. In absence of other ions in the water, the binding efficiency and selective ability of BPA-MIP and BPA-NIP for BPA have been discussed. Nevertheless, the composition of actual water body is very complicated. There are a lot of anions and cations, such as Naþ, Kþ, Ca2þ, Mg2þ, SO2 4 , Cl and so on. The influence of various ions in the simulated seawater was investigated. Seen from Fig. 6, the binding of BPA-MIP and BPA-NIP for BPA was still fit well to the Langmuir equation with high correlation coefficient (R2>0.99) in the presence of

various ions. The binding capacity of BPA-MIP for BPA calculated from the Langmuir isotherm were 31.3 mmol g1, which was slightly higher than that in BPA single water system (qm ¼ 30.26 mmol g1). It indicated that the binding was not influence by the common ions in the water. In general, a multi-component adsorbates-adsorbents generally exhibit three possible types of behavior: synergism (the effect of the mixture is greater than that the single components in the mixture), antagonism (the effect of the mixture is less than that of each of the components in the mixture) and noninteraction (the mixture has no effect on the adsorption of each of the adsorbates in the mixture) (Srivastava et al., 2006, 2008). The combined effect of the HA and other ions in the mixture solution seemed to be non-interaction. This indicated that BPA-MIP synthesized in our work was in favor of detecting or binding BPA in the water.

Fig. 6 e The influence of various ions on the BPA binding onto BPA-MIP and BPA-NIP in the mixed water system. C0BPA: 10e100 mmol LL1, C0NaD: 10.58 g LL1, C0KD: 0.39 g LL1, C0Ca2D: 0.40 g LL1, C0Mg2D: 1.15 g LL1; C0SO42L: 1.44 g LL1, C0Cl-: 19.88 g LL1. Sorbent: 0.5 g LL1, pH: 7, T: 25  C, t: 4.0 h.

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Fig. 7 e Possible binding structures of various phenols in a specific site onto the BPA-MIP surface in the aqueous solution. The a1, a2, b1, b2, c1, c2, and d1 in the dotted circle are hydrogen-bonding between the hydrogen atom of hydroxy groups and the oxygen atom of carboxyl groups.

3.6.

Selective binding mechanism

According to the results obtained in this study, it can conclude that BPA-MIP possess the significant selective binding capacity for the imprinted BPA in the mixed water, while the other similar phenols have the order of influence ability for the BPA binding was TBBPA > phenol > PSP. This result proved indirectly that the specific binding sites existed and played an important role in the selective binding process. Hydrogen bonding is an important intramolecular/intermolecular interaction, and it is also a useful adsorption mechanism for adsorptive separation, whereas its directionality and short range confer specific selectivity (Xu et al., 2008). Water can also form hydrogen bond to influence on the binding in our experimental mixture, but the results showed that the surface of the imprinted adsorbent was hydrophobic interaction. Among interactions, hydrophobic interactions, van der Waals forces and long range electrostatic interactions are not directional, therefore, they are not considered specific (Bikadi et al., 2007). Zhang and Hu (2008) put forward that the nonspecific adsorption would occur for MIP based on noncovalent synthesis approach as more functional monomers are needed during the imprinting synthesis. Similarly, in our

binding mixture, non-specific adsorption for the four phenols might occur during the binding process, but it was secondary. It was assumed that hydrogen bonding played an important role in the selective adsorption process for the specific binding. Zhang and Hu (2008) reported that specific adsorption would occur selectively for the imprinted molecular by MIP, which could form hydrogen bonds with the carboxyl and carbonyl groups on the binding sites. According to the selective binding results and the surface character of the MIP, the ChemOffice 2004 software package (American Cambridge Company) was employed to model binding pattern of target molecule with monomer. Fig. 7 showed the possible hydrogen bonding structures of four phenols binding into a specific site on the surface of BPA-MIP in the water. The related bond lengths and angles were also shown. We supposed that double intermolecular hydrogen bonds had been formed between the hydroxyl on the benzene ring of BPA and carbonyl from the functional monomer (DTPA). The direction (about 180 ) and length of the two OeHeO bonds had been determined during the imprinting process. The memory cavity with a certain shape retained after removing the template of BPA. During the recognition process, the double hydrogen bonding (Fig. 7a, labeled as a1 and a2) rendered a stable

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surface specific site. It was obviously that the three phenols comprised two phenolic benzene rings except for phenol with a single in their molecular structure, which is essential for the high-affinity binding of phenols compounds into the specific binding sites on BPA-MIP. But the benzene rings and the hydroxyl groups located at different angle in the space (Fig. 7aed), which resulted in various binding forms by the hydrogen bonds. Seen from Fig. 7a, the perfect binding occurred for BPA in a shaped specific site, two hydrogen bonds could be formed between the hydrogen atom of hydroxy groups and the oxygen atom of carboxyl groups. The binding sites contained two specific functional groups which could quickly recognize BPA molecules through the force of hydrogen. Importantly, the dihedral angles of OeHeO were both about 180 resulting in the most stable affinity for the imprinted BPA binding. However, for the possible binding of TBBPA and PSP in the specific site (Figs. 7b, c), it could be seen that the spacial angle and the bond distance of the two OeHeO (labeled as b1, b2, c1 and c2) changed greatly. An unstable binding for TBBPA and PSP could be formed due to different site distributing of the two benzene rings in the space. Especially, although the hydrogen bond might be formed, PSP molecular could not be contained completely in a specific binding site owing to the dimensional structure. This might be the reason that there was some binding for TBBPA while hardly affinity for PSP in any competitive binding solution. Similarly, seen from Fig. 7d, the linear OeHeO bond (d1) could be formed only by one side in a specific site, which resulted in the formation of an unsteady H-bonded combination although it showed appropriate for phenol binding. Such single bond could be distorted by the other weak forces in the water. From the above binding mode of a specific site, the sequence of binding affinity for various phenols (BPA > TBBPA > phenol > PSP) in the water could be interpreted exhaustively. Thus, it was reasonable to assume that representation of the double hydrogen bonding network would offer advantages for BPA binding. The distance plot and contact angle of the hydrogen bonding determined the binding stability. And the binding results indicated that the volume and dimensional structure of the molecular acted as another important role in the BPA binding onto BPA-MIP.

4.

Conclusions

The results obtained from the single and competitive binding experiment of the phenols suggested that the synthesized BPA-MIP exhibited high selectivity in the water. The selectivity of BPA-MIP was found to be influenced by the similar phenols at the order of TBBPA > phenol > PSP. The BPA-MIP exhibited excellent binding affinity even in the presence of the HA (1e5 mg L1) and the common ions in the water. The physical binding structures of various phenols in a specific site onto the BPA-MIP surface was proposed to interpret the selective binding specialty. In conclusion, the dimensional molecular structure and the double hydrogen bonding interaction between carboxyl and hydroxyl played an important role in the selective recognizing of BPA in the binding process.

99

Acknowledgment We appreciate the financial support of the National Natural Science Foundation of China (No.51108111, No.51178134, and No.51378141), Fundamental Research Funds for the Central Universities (HEUCF 20130010) and Heilongjiang Natural Science Foundation (E201125).

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