Preparation of Arsenate Anion Surface-Imprinted Material IIP-PDMC

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Preparation of Arsenate Anion Surface-imprinted Material IIP-PDMC/SiO2 and Study on its Ion Recognition Property Baojiao Gao, Junmei Du, and Yanyan Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400440k • Publication Date (Web): 17 May 2013 Downloaded from http://pubs.acs.org on May 18, 2013

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Industrial & Engineering Chemistry Research

Preparation

of

Arsenate

Anion

Surface-imprinted

IIP-PDMC/SiO2 and Study on its Ion Recognition Property

Baojiao Gao*, Junmei Du, Yanyan Zhang Department of Chemical engineering, North University of China, Taiyuan 030051, People' s Republic of China

*To whom correspondence should be addressed Tel: 86-0351-3924795 Fax: 86-0351-3922118 E-mail: [email protected]

ACS Paragon Plus Environment

Material


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Abstract In this work, by molecular design, a new ion surface-imprinting technique was set up based on surface-initiated graft-polymerization, and arsenate anion surface-imprinted material with high performance was prepared for the first time. A redox initiating system was constituted by the amino groups on the surface-modified particles and the ammonium persulphate in the solution. The cationic monomer 2-methacryloyloxyethyl-trimethyl ammonium chloride (DMC) was used as functional monomer and N,N'-Methylenebisacrylamide (MBA) was used as crosslinker. In the solution, the monomer molecules were arranged around the template ion, arsenate anion, via ion exchange action, and then the arsenate anion surface-imprinting was carried out along with the surface-initiated graft-polymerization of DMC and MBA, forming arsenate anion surface-imprinted material IIP-PDMC/SiO2. The experimental results show IIP-PDMC/SiO2 possesses specific recognition selectivity and excellent binding affinity for arsenate anion. The selectivity coefficients of IIP-PDMC/SiO2 for arsenate anion are 8.814 and 7.898 relative to chromate and nitrate ions, respectively.

Keywords Arsenate anion; Surface imprinting technique; Ion recognition; Graft polymerization; Ion exchange

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1. Introduction Water pollution by arsenic has attracted increasing attention all over the world due to its mobility and toxicity. The arsenic pollution occurs from both natural and anthropogenic sources. The arsenic in the earth's crust causes the ground water contamination regionally owing to the natural processes such as erosion of arsenic-containing rocks and sediments, whereas some anthropogenic activities cause major arsenic pollution, for example, mineral-processing solutions and effluents containing arsenic are the important source of arsenic pollution 1, 2. In the aquatic environments, arsenic is present mostly in inorganic species, arsenate and arsenite. Arsenate, As (V), is the predominant arsenic form in oxidizing conditions. At the present, arsenic pollution has brought great threat to the health of human beings and on ecological environment

3, 4

. In view of the high toxicity of arsenic, now arsenic in drinking water standard has been

regulated at 10 ppb by the US Environmental Protection Agency (USEPA), and the World Health Organization (WHO) recommends the same standard

5, 6

. Therefore, it has become one of important

research subjects in environmental science field to try to remove the arsenic pollution from water, and such study has been received much attention worldwide. A number of methods have been studied for arsenic removal from water, such as coagulation–precipitation, ion exchange, membrane separation, adsorption method and so on

7-10

. Among

theses methods, Adsorption process is proved to be cost-effective and simple to perform 11, 12, so various solid adsorbents are developed. The results of many studies reveal that iron oxide based sorbents are promising sorbent materials for arsenic removal from high-arsenic water because they have strong affinities towards inorganic arsenic species and have relatively high adsorption capacities towards arsenate

13,14

.

However, those iron oxide-based sorbents have some limitations, for example, they have not been shown to deeply eliminate arsenic from water to produce cleaned water with a very low arsenic concentration, say 10

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µg/L, and there is still a issue of handling the contaminated waste residues 15, 16. Therefore, it is necessary to develop new adsorbents with high performance so as to remove arsenate more efficiently. Perhaps, ion-imprinted polymers are such a kind of solid adsorbents. Molecular imprinting is a technology to create recognition sites in a macromolecular matrix using a template molecule. Within molecularly imprinted polymers (MIPs), a great deal of imprinted cavities designed for the template molecule is distributed, and these cavities are complementary to the template molecule in shape, size and functional groups. Therefore, MIPs have specific molecular recognition ability and high binding affinity for the template molecule, and are described as artificial antibodies or receptors. As the template is an ion in the preparation process of MIPs, the resultant products are called ion-imprinted polymers (IIPs). IIPs retain all the virtues of MIPs, but they can specially recognize the template ion. In recent years, IIPs as selective sorbents for template ions have received much attention, and their applications in selective pre-concentration, separation and enrichment of ions as well as in the removal of toxicant ions from aqueous medium for protecting the environment have been reported 17-20. But then, most of the reported IIPs are metal ion-imprinted polymers, whereas anion-imprinted polymers are seldom recorded in the literature

21, 22

. In metal ion-imprinting process, the selectivity of a polymeric adsorbent is

based on the coordination geometry and coordination number of the ions as well as on ionic charges and sizes

23, 24

. The aim of this work is to prepare arsenate anion surface-imprinted material and to evaluate its

ion recognition ability so as to supplied new solid adsorbent for high effectively removing the arsenic pollution in water. Ion exchange will play a key role in the present designed anion-imprinting process. Entrapment way is the conventional method to prepare MIPs and IIPs , and it has some disadvantages, such as time-consuming and complicated preparation process, less recognition sites inside matrices particles obtained via crushing and grinding the imprinted polymeric monolith, and greater diffuse

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barrier for the template molecules coming from thick matrices, leading to the poor recognition property. In order to overcome these drawbacks, many researchers including our group are devoting to the development of molecule surface-imprinting techniques

25-28

, and try to make the imprinted caves to lie on the surfaces

of solid particles. As a result, the surface-imprinted materials are more effective for recognizing and combining the template molecules or ions. In the present study, a new strategy to prepared arsenate anion surface-imprinted material IIP-PDMC/SiO2 was design with 2-methacryloyloxyethyl-trimethyl ammonium chloride (DMC) as functional monomer and micron sized SiO2 as matrix. For this new strategy, there were two main technical points: (1) The combination of the monomer DMC and the template ion, arsenate anion, was based on ion exchange; (2) The surface-initiated graft-polymerization of DMC and the imprinting process of arsenate anion were synchronously carried out. In this investigation, the recognition and binding character of the arsenate anion surface-imprinted material were examined in depth. The result showed that the arsenate anion surface-imprinting is successful. As far as we know, the arsenate anion surface-imprinted material is reported for the first time, and especially the used method of imprinting anion on silica gel particles is specific. It is significant to introduce arsenate anion-imprinted material to the removal of arsenate with high toxicity from water, and such investigation is very valuable in the field of environment protection. 2. Experimental Section 2.1. Material and equipment Silica (about 125 µm of diameter) was purchased from Ocean Chemical Limited Company (Qingdao City, China), and was of reagent grade. γ-Aminopropyltrimethoxysilane (AMPS) was obtained from Nanking Chuangshi Chemical Aux Ltd. (Province Jiangsu, China), and is of analytical grade. 2-Methacryloyloxyethyl-trimethyl ammonium chloride (DMC, an aqueous solution of 80%) was purchased

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from Shanghai Banchen chemical Co, Ltd. (Shanghai, China), and was of analytical grade. Ammonium persulphate (APS) was supplied by Shanghai Fushu Chemical Engineering Co, Ltd. (Shanghai, China), and was of analytical grade. N,N'-Methylenebisacrylamide (MBA) was

supplied by Shanghai Yuanye

Bio-Technique Co, Ltd. (Shanghai, China), and was of analytical grade. Sodium arsenate (Na3AsO4), potassium chromate (K2CrO4) and other reagents were analytical pure grade and all them are purchased from Chinese companies. The instruments used in this study were as follows: Perkin-Elmer 1700 infrared spectrometer (Perkine-Elmer Company, USA), LEO-438VP scanning electronic microscope (SEM, LEO Company, UK), STA449

thermogravimetry

analyzer(TGA,

Netzsch

Company,

German),

Unic-2602

UV/Vis

spectrophotometer (Unic Company, Shanghai, China), THZ-92C constant temperature shaker equipped with gas bath (Shanghai Boxun Medical Treatment Equipment Factory, Shanghai, China), HK-8100 Inductively coupled plasma emission spectrometer (ICP-AES, Beijing Huake Yitong Analytical Instruments Co., Ltd., Beijing, China). 2.2. Preparation and characterization of arsenate anion surface-imprinted material IIP-PDMC/SiO2 2.2.1. Surface modification of silica gel particle with APMS According to the procedure described in Ref. (29), silica gel particles were first surface-modified with coupling agent APMS, and amino groups were chemically introduced to the surfaces of silica gel particles, forming the modified particles AMPS-SiO2 that had a APMS bonding amount of 1.50 mmol/g, which was determined by both TGA and acid base titration method (back drop method ) 29. 2.2.2. Surface-imprinting of arsenate anion The arsenate anion surface-imprinting was conducted in an aqueous solution system. 100 mL of distilled water (100 mL), 3.0 g of Na3AsO4 as template, 5 mL of monomer DMC and 0.47g of crosslinker

5


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MBA were added in turn into a four-necked flask equipped with a mechanical agitator, a reflux condenser, a thermometer and a N2 inlet, and The pH value of the solution was adjusted to pH=5. The solution was fully stirred so as to make the ion exchange process between Cl- ion of the monomer and arsenate anion to fully take place. The modified particles AMPS-SiO2 (1.0 g) were placed into the flaks, and N2 was bubbled for 30 min to exclude air. The content was heated to 35℃, and 0.058g of initiator APS was added. The graft/cross-linking polymerization reaction was performed under N2 atmosphere at 35℃for 10 h. The resultant particles were collected by filtering. To remove the template, arsenate anion, the product particles were fully soaped and washed with diluted NaOH solution, in which 2 mol/L of NaCl was contained. Through vacuum drying, arsenate anion surface-imprinted material, IIP-P(DMC-co-MBA)/SiO2 (simplified as IIP-PDMC/SiO2) was obtained. For comparison, non-imprinting material P(DMC-co-MBA)/SiO2 was also prepared in absence of template Na3AsO4. 2.2.3. Characterization of surface-imprinted material IIP-PDMC/SiO2 The arsenate anion surface-imprinted material IIP-PDMC/SiO2 was characterized by using several methods. (1) The infrared spectrum of IIP-PDMC/SiO2 particles was determined with KBr pellet method to confirm their structure. (2) The morphology of the particles was examined with SEM via comparing the morphology change of silica gel particles before and after imprinting. (3) The degree of weigh loss of IIP-PDMC/SiO2 particles as well as that of the modified particles AMPS-SiO2was determined by thermal gravimetric analysis (TGA) so that for the IIP-PDMC/SiO2 particles, the amount of grafted/crisslinked copolymer P(DMC-co-MBA) on the surfaces of silica gel particles was determined and it was 45.8 g/100g. For the non-imprinting material P(DMC-co-MBA)/SiO2, the amount of grafted/crisslinked copolymer P(DMC-co-MBA) was also determined, and it was 46.5 g/100g. It was obvious that both results were nearly identical.

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2.3. Study on binding character of IIP-PDMC/SiO2 for arsenate anion 2.3.1. Evaluating binding property of IIP-PDMC/SiO2 with static method The binding property of IIP-PDMC/SiO2 for arsenate anion was examined with batch method (static method). Aqueous solutions of Na3AsO4 were prepared in a concentration range of 0.1-1.4 mmol/L, and the pH values of these solutions were adjusted to pH 5 with diluted HCl solution. The adsorption kinetics experiments were first conducted to determine the time of equilibrium adsorption, and it was about 5 h. On that basis, the isothermal binding property of IIP-PDMC/SiO2 particles for arsenate anion was investigated. At the constant temperature of 25℃, Na3AsO4 solutions of 25 mL with different concentrations were placed into a number of conical flasks with cover, and about 0.05 g of IIP-PDMC/SiO2 particles weighted accurately was added into these solutions, respectively. These mixtures were shaken in a constant temperature oscillator for 5 h, and the binding process was allowed to reach equilibrium. After standing for layering, the arsenate concentrations of those supernatants were determined by ICP-AES. The equilibrium binding amount of arsenate anion, Qe (mmol/g), was calculated according to Eq. (1).

Qe =

V (C0 − Ce ) m

(1)

where C0（mmol/L）and Ce（mmol/L）are the initial and equilibrium concentration of arsenate, respectively, V (mL) is the volume of arsenate solution, and m (g) is the mass of the used adsorbent IIP-PDMC/SiO2 particles. Both CrO42- anion and AsO43- anion are the isoelectronic ions, and they have similar chemical structure and a tetrahedral configuration. NO3- anion is one of common oxyanions in water, but its chemical structure is entirely different with AsO43- anion. In this investigation, CrO42- and NO3- anions were selected as two contrast anions to examine the recognition character of IIP-PDMC/SiO2 for arsenate anion.

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According to the same method, the equilibrium binding amounts of CrO42- and NO3- anions on IIP-PDMC/SiO2 particles were determined. The concentration of CrO42- ion in the solution was determined by spectrophotometry at 540 nm (after the complexation of CrO42- ion with 1, 5-diphenyl carbazide 30). The concentration of NO3- ion in the solution was determined also by spectrophotometry at 220 nm. 2.3.2. Evaluating binding property of IIP-PDMC/SiO2 with dynamic method The binding property of IIP-PDMC/SiO2 for arsenate anion was further examined with column method (dynamic method). At room temperature, IIP-PDMC/SiO2 particles (about 1 g) were packed into a piece of glass pipe with an internal diameter of 1.0 cm, and the bed volume (BV) of the packed column was 2 mL. The arsenate solution (pH=5) with a concentration of 1.2 mmol/L was allowed to flow gradually through the packed column at a rate of five bed volumes per hour (5 BV/h) in the countercurrent manner. The effluents with one volume (1 BV) interval were collected, and the concentrations of AsO43- ion in these effluents were determined by ICP-AES. The dynamic binding curve was plotted, and the break binding amount and saturated binding amount were calculated with the data of the concentration and bed number of these effluents, respectively. Using the same method, the dynamic binding curves of IIP-PDMC/SiO2 particles towards two contrast anions were also determined. 2.3.3. Experiments of binding selectivity In order to further examine the recognition specificity of IIP-PDMC/SiO2 particles for arsenate anion, the competitive adsorption experiments were carried out. Two binary mixed solutions, AsO43-/CrO42and AsO43-/ NO3-, were prepared, and the concentration of each anion in the two mixed solutions was the same, 0.8 mmol/L. Furthermore, the pH values of the two solutions were adjusted to pH 5. The two binary mixed solutions (25 mL) were placed into two conical flasks with cover, respectively, and 0.1 g of IIP-PDMC/SiO2 particles weighted accurately was added into the two solutions, respectively. The static

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adsorption experiments were conducted. After reaching binding equilibriums (5 h), the concentration of each anion in the two supernatants was determined, and the distribution coefficient for each anion was calculated according to Eq. (2) 31, 32.

Kd =

Qe Ce

（2）

where Kd represents the distribution coefficient (L/g) of a certain anion; Qe （mmol/g）is its equilibrium combinding quantity; Ce（mmol/L）is its equilibrium concentration. The selectivity coefficient k of IIP-PDMC/SiO2 particles for AsO43- anion relative to a certain competition anion, CrO42- or NO3- ion, can be obtained from the distribution coefficient data according to Eq. (3), and the value of k marks the recognition selectivity of IIP-PVI/SiO2 for AsO43- anion. K d ( Arsennate species) K d′

where K d ( Arsennate

(3)

species) is the distribution coefficient of arsenate species under the given pH

condition (pH=5), whereas K d′ represents the distribution coefficient of a certain competition anion. 2.6. Desorption experiment IIP-PDMC/SiO2 particles with a certain amount, which had adsorbed arsenate anion in a saturation state, were packed into a piece of glass pipe with an internal diameter of 1.0 cm, and the bed volume (BV) of the packed column was allowed to be about 2 mL. An aqueous solution of NaCl with a concentration of 3 mol/L (pH=10) was used as eluent. The eluent was allowed to flow gradually through the column at a rate of two bed volumes per hour (2 BV/h) upstream. The effluents with one volume (1 BV) interval were collected, and the concentrations of arsenate anion in these effluents were determined. The dynamic desorption curve was plotted, and the elution property of IIP-PDMC/SiO2 particles was estimated.

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3. Results and discussions 3.1. Preparation processes of AsO43- anion surface-imprinted material IIP-PDMC/SiO2 There are two basic processes in the preparation of AsO43- anion surface-imprinted material IIP-PDMC/SiO2, and they are “ion exchange process” and “surface-initiated graft-polymerization process”. The entire processes are described as follows. (1) In the aqueous solution of Na3AsO4 and DMC, the cationic monomer DMC are first combined around AsO43- anion by right of ion exchange action, for which AsO43- anion exchanges with Cl- anion of DMC. (2) A redox initiation system is constituted by the amino group (primary amine group) on the modified particles APMS-SiO2 and ammonium persulfate in the solution, and so primary free radicals are generated on the surfaces of APMS-SiO2 particles. (3) These free radicals on the surfaces of the particles initiate monomer DMC around AsO43- anion and crosslinker MBA to produce graft/crosslinking copolymerization, forming a thin layer of copolymer on the surfaces of silica gel particles. Meanwhile, AsO43- anions are enveloped in the crosslinking networks, namely, AsO43- anion surface-imprinting is realized. (4) After washing away the template ions, large numbers of AsO43anion-imprinted caves will remain within this thin polymer layer on the surfaces of silica gel particles, and thereupon, the AsO43- anion surface-imprinted material IIP-PDMC/SiO2 is obtained. The procedure described above can be schematically expressed in Scheme 1. It needs to be explained that arsenate exists in different forms at different pH. Therefore, as the ion-imprinting is conducted under different pH conditions, the template anion for arsenate will be different species, resulting in different imprinted materials, in which the imprinted cave will be corresponding to the template anion. For example, at pH=5, H2AsO4- ion-imprinted material will be obtained. About this problem, there will be special discussion in a later section.

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Scheme 1

3.2. Characterization of arsenate anion surface-imprinted material IIP-PDMC/SiO2 3.2.1. Infrared spectrum Figure 1 gives the infrared spectra of three kinds of particles, silica gel particles SiO2, the modified particles AMPS-SiO2 and arsenate anion surface-imprinted particles IIP-PDMC/SiO2.

Fig. 1

As compared with the spectrum of SiO2, in the spectrum of AMPS-SiO2, the band at 3435 cm-1 that is associated to silanol group has been weakened. At the same time, two new bands at 2907 cm-1 and 685 cm-1 appear. The former is the asymmetrical stretching vibration absorption of C-H bond and the latter is attributed to the bending vibration absorption of N-H bond of the primary amine group. The above spectrum data show that the coupling agent APMS has bound on the surfaces of silica gel particles and the modified particles APMS-SiO2 have been formed. In the spectrum of IIP-PDMC/SiO2, the stretching vibration absorption of the carbonyl group C=O of ester group has appeared at 1728 cm-1, and the bending vibration absorption of C-H bond of methyl group locating in the main chain of the grafted copolymer P(DMC-co-MBA) has appeared at 1398 cm-1. The two bands come from the unit of the monomer DMC in the crosslinked copolymer. For IIP-PDMC/SiO2, there are still two new absorption bands at 1681 cm-1 and 1558 cm-1. The former should be ascribed to the stretching vibration absorption of the carbonyl group of amide group, and the latter should be attributed to the in plane bending vibration absorption of N-H bond of amide group. The two

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bands come from the unit of the crosslinker MBA in the crosslinked copolymer. These spectrum changes described above fully demonstrate the following facts. It is the surface initiating system consisting of the amino group on AMPS-SiO2 particles and the persulfate in the solution that initiates the cross-linking polymerization of DMC and MBA, and it leads to the formation of the arsenate anion surface-imprinted material IIP-PDMC/SiO2. It needs to be pointed out that all of the above various absorption bands of IIP-PDMC/SiO2 as well as that of APMS-SiO2 look very weak because of the affect of the strong absorption background of SiO2. 3.2.2. Morphology Fig.2 (A) and Fig.2 (B) present the SEM images of raw silica gel particles and the imprinted particles IIP-PDMC/SiO2. It can be found that before the graft/cross-linking polymerization of DMC and MBA, the surfaces of raw silica gel particles are rough and scraggy. After the graft/cross-linking polymerization, the surfaces of the imprinted particles IIP-PDMC/SiO2 become smoother and sleeky. This is caused by the coating and filling up action of the crosslinking polymer layer, which is formed during arsenate anion surface-imprinting process.

Fig. 2 (A)

Fig. 2 (B)

3.3. Binding character of IIP-PDMC/SiO2 for arsenate species 3.3.1. Binding isotherm The adsorption experiments with batch method were first performed in the solutions with pH 5, and the non-imprinted material PDMC/SiO2 and the imprinted material IIP-PDMC/SiO2 were used, respectively. It needs to be pointed out that IIP-PDMC/SiO2 was prepared under the condition of pH 5 as described in

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Section 2.2., and so it was H2AsO4- ion surface-imprinted material. As mentioned above, arsenate in water exists in different forms under different pH conditions. According to the species distribution of arsenate at different pH

33, 34

,H2AsO4- is the dominant species in

the solution of pH 5, whereas for chromate the dominant species is HCrO4-. Fig. 3 gives adsorption isotherms of PDMC/SiO2 (non-imprinting material) for the three anions, H2AsO4-, HCrO4- and NO3-, respectively, whereas Fig. 4 gives the binding isotherms of IIP-PDMC/SiO2 (imprinting material) for the three anions, respectively.

Fig. 3

Fig. 4

It is displayed in Fig. 3 that the non-imprinting material PDMC/SiO2 (P(DMC-co-MBA)/SiO2) has higher adsorption ability for all of the three anions, and the adsorption capacity is in the range of 0.17-0.25 mmol/g. The reason for this can be explained as follows. For PDMC/SiO2, the cross-linking polymer layer of P(DMC-co-MBA) on the surfaces of silica gel particles is swelled in aqueous solutions, and then anion exchange occurs, namely in the three anion solutions of H2AsO4-, HCrO4- and NO3-, the chloride ion as counter-ion of the unit of cationic monomer DMC in the crosslinked polymer layer of P(DMC-co-MBA) will exchanged with the three anions in the solutions, respectively, resulting in the strong ion-interactions. It

is

the

strong

ion-interaction

that

leads

the

strong

adsorption

action

of

PDMC/SiO2

(P(DMC-co-MBA)/SiO2) particles towards the three anions, H2AsO4-, HCrO4- and NO3-. Besides, it can be seen that by comparison, the adsorption capacity of PDMC/SiO2 particles for H2AsO4- ion is the lowest. Taking it by and large, the non-imprinted material PDMC/SiO2 has no adsorption selectivity for H2AsO413


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ion. However, Fig. 4 indicates clearly that the binding capacities of the H2AsO4- ion surface-imprinted material IIP-PDMC/SiO2 for the three anions have remarkable differences. The adsorption capacities of IIP-PDMC/SiO2 for two contrast anions, HCrO4- and NO3- ions, have been drastically decreased, and the maximum adsorption amounts decline from 0.25 mmol/g to 0.05 mmol/g for HCrO4- anion and from 0.23 mmol/g to 0.04 mmol/g for NO3- anion, respectively. However, the binding amount of IIP-PDMC/SiO2 for the template ion, H2AsO4- anion, still remains higher, and even has some improvement (the maximum adsorption amount increases from 0.17 mmol/g to 0.18 mmol/g), fully displaying the high recognition selectivity of IIP-PDMC/SiO2 for H2AsO4- anion. For the H2AsO4- anion surface-imprinted material IIP-PDMC/SiO2, a great quantity of H2AsO4- anion-imprinted caves are distributed within the thin polymer layer on the surfaces of silica gel particles. These caves are highly matched with H2AsO4- anion in size and shape, and it leads to the excellent recognition ability and strong binding action of IIP-PDMC/SiO2 particles towards H2AsO4- anion. However, these imprinted caves are unmatched with HCrO4- and NO3ions in size and shape, leading to basically non-recognizing and non-binding of IIP-PDMC/SiO2 particles towards HCrO4- and NO3- ions. The ionic radiuses of AsO43- and NO3- ions are 0.248 nm and 0.200 nm, respectively

35

. It is obvious that NO3- ion is far smaller than H2AsO4- ion. Even so, the H2AsO4-

anion-imprinted caves still do not recognize and do not bind NO3- ion owing to the mutual mismatch between the imprinted caves and NO3- ions. 3.3.2 Dynamic binding curve The adsorption experiments with column method were also performed, and the columns were packed with the non-imprinted material PDMC/SiO2 and the imprinted material IIP-PDMC/SiO2, respectively. The pH value of the tested ion solutions were also remains at pH 5. Fig. 5 and Fig. 6 display

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the dynamic adsorption curves of PDMC/SiO2 and the dynamic binding curves of IIP-PDMC/SiO2 for the three anions, H2AsO4-, HCrO4- and NO3- ions, respectively.

Fig. 5

Fig. 6 It can be observed from Fig. 5 that as the solutions of H2AsO4-, HCrO4- and NO3- ions with the same concentration (1.2 mmol/L) flow upstream through the column packed with PDMC/SiO2 particles, respectively, the leaking volumes are 52, 70 and 76 BV for H2AsO4-, HCrO4- and NO3- ions, respectively. It is apparent that there are no substantial leaking volume differences between the three anions. Furthermore, the leaking volume H2AsO4- ion is the smallest. The above data further demonstrate that the column packed with PDMC/SiO2 particles has no recognition selectivity for H2AsO4- ion. However, it can be found from Fig. 6 that for the column packed with IIP-PDMC/SiO2 particles, the breakthrough curve of H2AsO4- ion is obviously different from that of HCrO4- and NO3- ions. The leaking volumes of HCrO4- and NO3- ions have fallen substantially, and decrease from 70 to 7 BV for HCrO4- ion and from 80 to 10 BV for NO3- ion, respectively. By calculating, for HCrO4- ion, the leaking and saturated adsorption amounts are only about 0.014 mmol/g and 0.040 mmol/g, respectively, and for NO3- ion, the leaking and saturated adsorption amounts are also only about 0.022 mmol/g and 0.050 mmol/g, respectively, fully displaying that the column packed with IIP-PDMC/SiO2 particles basically does not recognize and does not bind HCrO4- and NO3- ions basically. This result still arises from that H2AsO4- anion-imprinted caves are unmatched with HCrO4- and NO3- ions in size and shape, leading to basically non-recognizing and non-binding of IIP-PDMC/SiO2 particles towards HCrO4- and NO3- ions.

15


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However, the leaking volume of H2AsO4- ion still remains higher, and is 59 BV. By calculating, for H2AsO4- ion, the leaking and saturated adsorption amounts actually reach 0.139 mmol/g and 0.185 mmol/g, respectively. Obviously, the results of dynamic binding experiments once more show that the H2AsO4anion-imprinted material IIP-PDMC/SiO2 exhibits excellent recognition selectivity and excellent binding affinity for the template ion, H2AsO4- anion, due to that the imprinted caves are highly matched with H2AsO4- anion in size and shape. It will be further discussed below. 3.3.3. Selectivity coefficients of IIP-PDMC/SiO2 for H2AsO4- anion Two binary mixed solutions of pH 5, AsO43-/CrO42- and AsO43-/NO3-, were prepared, and in fact, they are such binary mixed solutions of H2AsO4-/HCrO4- and H2AsO4-/NO3-. The competitive adsorption experiments of IIP-PDMC/SiO2 particles in the two solutions were conducted. In Table 1, the data of the distribution coefficients Kd of each ion and the selectivity coefficients k of IIP-PDMC/SiO2 for H2AsO4- ion are summarized.

Table 1

It can be found from the data in Table 1 that the recognition selectivity and binding property of the H2AsO4- imprinted material IIP-PDMC/SiO2 for three anions are essentially different. Relative to HCrO4ion, the selectivity coefficient of IIP-PDMC/SiO2 for H2AsO4- ion is 8.81, and relative to NO3- ion, it is 7.90. such high selectivity coefficients of H2AsO4- ion imply that this imprinted material IIP-PDMC/SiO2 has a high recognition specificity for H2AsO4- ion, and it comes from a great deal of the caves matching with H2AsO4- ion within the thin polymer layer on the surfaces of silica gel particles. In a word, the arsenate anion surface-imprinted material IIP-PDMC/SiO2 can effectively recognize

16



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and bind corresponding arsenate species in aqueous medium, and so this investigation result supplies the separation material with high performance for preventing and controlling the arsenate pollution of water environment. 3.3.4. Effect of pH on property of IIP-PDMC/SiO2 For arsenate anion, there are four species in aqueous solution, and they are H3AsO4, H2AsO4-, HAsO42- and AsO43-. Under different pH conditions, the dominant species is different. Here, the speciation diagram of arsenate with pH displayed in Ref. 33, 34 is cited as shown in Fig. 7. According to the speciation diagram for arsenate, as pH＜4, arsenate predominantly exists as H3AsO4+H2AsO4; In the pH range of 4-6, the dominant species is H2AsO4; In the pH range of 7-10, HAsO42- is the dominant species; As pH＞10, arsenate predominantly exists as HAsO42-+AsO43-.

Fig. 7

The arsenate anion surface-imprinting was carried out under different pH conditions, and the template ion forms are different. Therefore, the obtained imprinted materials at different pH value were various. The isothermal binding experiments of these imprinted materials for the corresponding arsenate species were conducted under different pH conditions. For this, the pH of Na3AsO4 solutions was adjusted, respectively, and the isothermal binding experiments were carried out for the various imprinted materials, obtaining different binding isotherms. The saturated binding amounts (Qm) of arsenate species on these binding isotherms were taken, and the relationship curve between Qm and pH value was figured, as shown in Fig. 8. At the same time, the corresponding template ion forms are denoted above various sections of this relationship curve.

17


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Fig. 8

Fig. 8 indicates that the binding properties of the imprinted materials prepared under different pH conditions for arsenate species are different. The binding property of the imprinted material prepared in the range of pH 4-6 is the best, and it is the reason why pH 5 was selected as the optimal pH value for the surface-imprinting process in this investigation. The binding properties of the imprinted material prepared at pH＜4 and pH＞6 are all poor. Especially, the binding property of the imprinted material prepared at pH ＞7 becomes worse, and the binding property of the imprinted material prepared at pH＞10 is the worst. By combining Fig. 7 and Fig. 8, a reasonable explanation can be given. (1) As pH＜4, the species of the template are H3AsO4+H2AsO4- as shown in Fig. 8. H3AsO4 species exists in molecule form, and it has no any ion exchange ability. In that case, the imprinting process will be negatively affected, leading to the poor binding property of the imprinted material. (2) In the pH range of 7-10, the species of the template is the divalent ion, HAsO42- anion, as shown in Fig. 8. On one hand, more monomer DMC that is monovalent cation will be consumed in the ion exchange process, leading to fewer imprinted caves within the thin polymer layer on silica gel particles and negatively affecting the binding property of the imprinted material. On the other hand, at this point, OH- anion with a certain concentration has existed in the solution. The competing of anion exchange for Cl- ion between OH- and HAsO42- anions will take place, and negatively affects the imprinting process, leading to fewer imprinted caves within the thin polymer layer and resulting in the poor binding property of the imprinted material. (3) As pH＞10, the species of the template are AsO43-+HAsO42- as shown in Fig. 8. At this point, the concentration of OH- anion has become greater, and the competing of OH- anion exchange for Cl- anion is strengthened. Furthermore, the size of AsO43- ion

18



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(ionic radius of 0.248nm) is far larger than that of OH- ion (ionic radius of 0.132nm). The two effects make the ability of ion exchange of OH- anion for Cl- anion to be stronger than that of AsO43- ion, leading to very poor imprinting result. Besides, multivalent template ion (AsO43-+HAsO42-) will consume more monomer and leads to fewer imprinted caves within the thin polymer layer, and this is also one of the reasons why the binding property of the imprinted material prepared under the condition of pH＞10 is the worst. Although regarding the relationship between the imprinting conditions of pH and the imprinting result there exist the above facts, for the practical application of treatment drinking water, whose pH is

closer to pH 7, the imprinting process can be carried out in a pH range of 6-7. 3.4. Elution property IIP-PDMC/SiO2

An aqueous solution of NaCl with a concentration of 3 mol/L and with pH 10 was used as the eluent. The eluent was made to upstream pass through the column packed with IIP-PDMC/SiO2 particles having bound H2AsO4- ion in a saturated state, and the dynamic desorption experiment was conducted. In Fig. 9 the dynamic desorption curve is presented.

Fig. 9

It can be seen in Fig.9 that this desorption curve is cuspidal and without trailing, indicating that the template ions combined in the packed column are easy to be washed off, namely, IIP-PDMC/SiO2 particles have excellent eluting property. The reason for this is that those H2AsO4- ion-imprinted caves are distributed within the thin polymer layer on the surfaces of IIP-PDMC/SiO2 particles, and it leads the little diffusion resistance for ions. That way, the template ions can be washed off rapidly via quick ionic exchange with Cl- ions in the eluent, and the column can be recovered quickly. By calculating, the

19


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desorption ratios of H2AsO4- ion in 18 BV and 25 BV reach 98.6％ and 99.6％, respectively, suggesting that H2AsO4- ions combined in the thin polymer layer on IIP-PDMC/SiO2 particles are easy to be desorbed or eluted, This implies that the recover and reuse of the surface-imprinted material IIP-PDMC/SiO2 is feasible and very convenient.

4. Conclusions

Based on “ion exchange” and “surface-initiated graft-polymerization” and by using the cationic monomer DMC as functional monomer, arsenate anion surface-imprinting was successfully carried out on the surfaces of silica gel particles, and the arsenate anion surface-imprinted material IIP-PDMC/SiO2 with high performance was obtained. Under different pH conditions, the dominant species of arsenate are different. At pH=5, arsenate predominantly exists as H2AsO4- ion, and the imprinted material IIP-PDMC/SiO2 prepared under this pH condition has the best binding property for arsenate. Owing to that there are a great deal of arsenate species-imprinted caves within the thin polymer layer on the surfaces of silica gel particles, the imprinted material IIP-PDMC/SiO2 possesses high recognition selectivity and excellent binding affinity for arsenate anion. However, the adsorption capacities of IIP-PDMC/SiO2 for two contrast anions, chromate and nitrate anions, are very low, or IIP-PDMC/SiO2 substantially does not recognize and does not bind chromate and nitrate anions. In short, in this work a new ion surface-imprinting method based on surface-initiated graft-polymerization is set up, and arsenate anion surface-imprinted material is prepared for the first time. It can be anticipated that the new surface-imprinting method will have significant scientific value for the development of molecular imprinting techniques, and the prepared arsenate anion surface-imprinted material IIP-PDMC/SiO2 will be valuable in environmental protection area.

20



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Scheme 1 Schematic expression of chemical reaction process to prepare IIP-PDMC/SiO2 Table 1 Distribution coefficient and selectivity coefficient data Initial concentration of each anion: 0.8 mmol/L; pH=5 Fig. 1 Infrared spectra of three kinds of particles Method: KBr pellet method Fig. 2 SEM photographs of SiO2 and IIP-PDMC/SiO2 Fig. 3 Adsorption isotherms of PDMC/SiO2 for three anions Temperature: 25℃; pH=5 Fig. 4 Binding isotherms of IIP-PDMC/SiO2 for three anions Temperature: 25℃; pH=5 Fig. 5 Dynamic adsorption curves of PDMC/SiO2 for three anions BV: 2mL; Temperature: 20℃; pH=5 Fig. 6 Dynamic binding curves of PDMC/SiO2 for three anions BV: 2mL; Temperature: 20℃; pH=5 Fig. 7 Speciation diagram of arsenate with pH Fig. 8 Saturation binding amount of IIP-PDMC/SiO2 prepared at different pH values as a function of pH Fig. 9 Elution curve of arsenate ion on IIP-PDMC/SiO2 column Temperature: 25 ℃

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Technol. 2012, 92, 106. (2) Oehmen, A.; Valerio, R.; Llanos, J.; Fradinho, J.; Serra, S.; Reis, M. A.; Crespo, J. G.; Velizarov, S. Arsenic removal from drinking water through a hybrid ion exchange membrane – Coagulation process. Sep. Purif. Technol.2011, 83, 137. (3) Dong, H.-R.; Guan, X.-H.; Wang, D.-S.; Li, C.-Y.; Yang, X.; Dou, X.-M. A novel application of H2O2–Fe(II) process for arsenate removal from synthetic acid mine drainage (AMD) water.Chemosphere 2011, 85, 1115. (4) Jang, M.; Cannon, F. S.; Parette, R. B.; Yoon, S.-J.; Chen, W.-F. Combined hydrous ferric oxide and quaternary ammonium surfactant tailoring of granular activated carbon for concurrent arsenate and perchlorate removal. Water Res. 2009, 43, 3133. (5) D’Arcy, M.; Weiss, D.; Bluck, M.; Vilar, R. Adsorption kinetics, capacity and mechanism of arsenate and phosphate on a bifunctional TiO2–Fe2O3 bi-composite. J. Colloid Interface Sci. 2011,364, 205. (6) Zhang, G.-S.; Liu, H.-J.; Liu, R.-P.; Qu, J.-H.Adsorption behavior and mechanism of arsenate at Fe–Mn binary oxide/water interface. J. Hazard. Mater. 2009, 168, 820. (7) Hu, C.-Z.; Liu, H.-J.; Chen, G.-X.; Qu, J.-H. Effect of aluminum speciation on arsenic removal during coagulation process. Sep. Purif. Technol. 2012, 86, 35. (8) Klerk, R. J. D.; Jia, Y.-F.; Daenzer, R.; Gomez, M. A.; Demopoulos, G. P. Continuous circuit coprecipitation of arsenic(V) with ferric iron by lime neutralization: Process parameter effects on arsenic removal and precipitate quality. Hydrometallurgy 2012,111-112, 65. (9) Awual, M. R.; Urata, S.; Jyo, A.; Tamada, M.; Katakai, A. Arsenate removal from water by a weak-base anion exchange fibrous adsorbent.Water Res. 2008, 42, 689. (10) Ren, Z.-M.; Zhang, G.-S.; Chen, J. P. Adsorptive removal of arsenic from water by an iron–zirconium binary oxide adsorbent. J. Colloid Interface Sci. 2011, 358, 230. (11) Selvakumar, R.; Jothi, N. A.; Jayavignesh, V.; Karthikaiselvi, K.; Antony, G. I.; Sharmila, P. R.; Kavitha, S.; Swaminathan, K. As(V) removal using carbonized yeast cells containing silver nanoparticles. Water Res. 2011, 45, 583. 22



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Chem. Lett. 2009, 20, 985. (18) Wang, X.-W.; Zhang, L.; Ma, C.-L.; Song, R.-Y.; Hou, H.-B.; Li, D.-L. Enrichment and separation of silver from waste solutions by metal ion imprinted membrane. Hydrometallurgy 2009,

100, 82. (19) Jo, S.-H.; Lee, S.-Y.; Park, K.-M.; Yi, S. C.; Kim, D.; Mun, S. Continuous separation of copper ions from a mixture of heavy metal ions using a three-zone carousel process packed with metal ion-imprinted polymer. J. Chromatogr. A 2010, 1217, 7100. (20) Khajeh, M.; Heidari, Z. S.; Sanchooli, E. Synthesis, characterization and removal of lead from water samples using lead-ion imprinted polymer. Chem. Eng. J. 2011, 166,1158. (21) Bayramoglu,

G.;

Arica,

M.

Y.

Synthesis

of

Cr(VI)-imprinted

poly(4-vinyl

pyridine-co-hydroxyethyl methacrylate) particles: Its adsorption propensity to Cr(VI). J. Hazard.

Mater. 2011, 187, 213. (22) Daniel, S.; Praveen, R. S.; Rao, T. P. Ternary ion-association complex based ion imprinted polymers (IIPs) for trace determination of palladium(II) in environmental samples. Anal. Chim.

Acta 2006, 570, 79. (23) Andaç, M.; Mirel, S.; Şenel, S.; Say, R.; Ersöz, A.; Denizli, A. Ion-imprinted beads for molecular recognition based mercury removal from human serum. Int. J. Biol. Macromol. 2007, 40, 159. 23


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(24) Dakova, I.; Karadjova, I.; Georgieva, V.; Georgiev, G. Ion-imprinted polymethacrylic microbeads as new sorbent for preconcentration and speciation of mercury. Talanta 2009, 78, 523. (25) Gu, J.-Y.; Zhang, H.; Yuan, G.; Chen, L.-R.; Xu, X.-J. Surface-initiated molecularly imprinted polymeric column: In situ synthesis and application for semi-preparative separation by high performance liquid chromatography. J. Chromatogr. A 2011, 1218, 8150. (26) Kitahara, K.-I.; Yoshihama, I.; Hanada, T.; Kokuba, H.; Arai, S. Synthesis of monodispersed molecularly imprinted polymer particles for high-performance liquid chromatographic separation of cholesterol using templating polymerization in porous silica gel bound with cholesterol molecules on its surface. J. Chromatogr. A 2010, 1217, 7249. (27) Gao, B.-J.; Liu, S.-Y.; Li, Y.-B. Preparation and recognition performance of uric acid-imprinted material prepared with novel surface imprinting technique. J. Chromatogr. A 2010, 1217, 2226. (28) Gao, B.-J.; Chen, Y.-X.; Men, J.-Y. Constructing chiral caves and efficiently separating enantiomers of glutamic acid with novel surface-imprinting technique. J. Chromatogr. A 2011,

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oxyanions: A review. Water Res. 2008, 42, 1343.

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Figure legend

Scheme 1 26



4000

685

2907

T/%

1728

AMPS-SiO 2

1681 1558 1398

IIP-PDMC/SiO 2

3435

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

SiO 2

3000 2000 -1 Wavenumber/cm

Fig. 1

27


1000

Page 29 of 37

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A

Mag=300X EHT=15.00kV

100μm

Detector=SE1 Date: 15 Sep 2012

B

Mag=300X EHT=15.00kV

100μm

Detector=SE1 Date: 15 Sep 2012

Fig. 2

28



0.25 0.20 Qe/(mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

0.15 0.10

-

NO3

-

HCrO4

0.05

-

H2AsO4 0.00 0.0

0.2

0.4 0.6 Ce/(mmol/L)

Fig. 3

29


0.8

Page 31 of 37

0.16 Qe/(mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-

H2AsO4

0.12

-

NO3

-

HCrO4

0.08 0.04 0.00 0.0

0.2

0.4 0.6 Ce/(mmol/L)

Fig. 4

30


0.8


1.2

-

H2AsO4 -

1.0

HCrO4

0.8

NO3

-

C/(mmol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

0.6 0.4 0.2 0.0 0

20

40

60

80

100

120

Number of bed volumes

Fig. 5

31


140

Page 33 of 37

1.2 1.0 (C/(mmol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4

HCrO4

0.2

NO3

-

-

H2AsO4

0.0 0

20

40 60 80 Number of bed volumes

100

Fig. 6

32


-

120


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

Table 1

IIP-PDMC/SiO2

Adsorbing material Adsorbate

H2AsO4-

HCrO42-

H2AsO4-

NO3-

Kd/( L/g)

0.379

0.043

0.388

0.049

k

8.814

33


7.898

Page 35 of 37

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Fig. 7

34



0.20

pH:4-6

-

H2AsO4 0.16 Qm(mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

0.12

pH< 4 -

pH:7-10

H3AsO4+H2AsO4

2-

0.08

-

HAsO4 +OH

pH> 10

0.04

2-

3-

-

HAsO4 +AsO4 +OH

1

2

3

4

5

6

7 8 pH

9 10 11 12 13 14

Fig. 8

35


Page 37 of 37

14 12 10 C/(mmol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 4 2 0 0

5 10 15 20 Number of bed volumes

Fig. 9

36


25

Preparation of Arsenate Anion Surface-Imprinted Material IIP-PDMC

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