Preparation of Molybdate Anion Surface-Imprinted Material for

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Preparation of Molybdate Anion Surface-Imprinted Material for Selective Removal of Molybdate Anion from Water Medium Baojiao Gao,*,† Xiaojing Li, Tao Chen, and Li Fang*,‡ †

Department of Chemical Engineering, North University of China, Taiyuan 030051, People’s Republic of China School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, People’s Republic of China

‡

ABSTRACT: A MnO4− anion surface-imprinted material was prepared with the new surface-imprinting technique of “pre-graft polymerizing and post-crosslinking/imprinting” for the selective removal of molybdate anions from a water medium. Monomer dimethylaminoethyl methacrylate (DMAEMA) was first graft-polymerized on micron-sized silica gel particles by using a surfaceinitiated graft-polymerization method, resulting in the grafted particles PDMAEMA/SiO2. Subsequently, the grafted macromolecule PDMAEMA was quaternized with epichlorohydrin as a reagent, and the quaternization transform of the grafted particles was realized, obtaining the functional grafted particles QPDMAEMA/SiO2, on which original tertiary amine groups had been transformed into quaternary ammonium groups. The adsorption property of the functional grafted particles for molybdate (MoO42− anion) was examined. On this basis, the MoO42− anion imprinting toward the functional macromolecules QPDMAEMA was conducted with 1,6-hexanediamine as a cross-linker, and MoO42− anion surface-imprinted material IIPQPDMAEMA/SiO2 was obtained, namely, MoO42− anion surface-imprinted material IIP-QPDMAEMA/SiO2 was prepared with the “pre-graft polymerizing and post-crosslinking/imprinting” method. The ion recognition and combination characters of IIPQPDMAEMA/SiO2 particles were researched in depth. The experimental results show that this MoO42− anion surface-imprinted material has specific recognition selectivity and combination affinity for the template ion, the MoO42− anion. Relative to the two contrast anions, WO42− and MnO4−, the selectivity coefficients of IIP-QPDMAEMA/SiO2 for the MoO42− anion were 6.41 and 5.39, respectively, displaying high ion recognition ability of this imprinted material.

1. INTRODUCTION Molybdenum (Mo) is an essential trace element for the human body, and the recommended dietary intake is 75−250 g/day for adults and older children.1 However, if its uptaken is in excess, it will become toxic to humans and will cause some diseases such as anemia, bone and joint deformities, liver and kidney abnormalities, and so on.2,3 The World Health Organization (WHO) recommends a maximum level of 0.07 mg/L Mo in drinking water.4 Molybdenum can exist in various oxidation states ranging from −2 to +6, and among Mo compounds, the dominant soluble species found in oxygenated environments is the tetrahedrally coordinated oxoanion molybdate (MoO42−). Anomalously high concentrations of Mo in water may associate with metallurgical and mining activities, and so discharges containing high Mo from industrial processes could pollute the environment if not treated properly.5,6 Besides, Mo (existing as molybdate) is the most concentrated trace element in the seawater due to its stability and weak adsorption behavior. The removal of molybdenum ions from wastewater as well as from groundwater is significant from the environmental point of view. The removal of molybdate has been achieved by various methods such as coprecipitation, the flotation technique and the adsorption method.7−9 Among them, adsorption is a simple and facile technique used for molybdate treatment, and various adsorbents have been reported in the literature including mineral substances and natural materials (like pyrite, goethite, kaolinite, siliceous materials and zeolites), resins and bioadsorbents (chitin, chitosan, and biomass).10−14 However, the general solid adsorbents have a common and serious drawback, and it is the absence of adsorption selectivity for the © 2014 American Chemical Society

goal substance. Up to now, molecule (or ion)-imprinted polymers (MIPs) are known as the solid adsorbents with the best adsorption selectivity. We think that for the selective removal of molybdate anions from water, the molybdate-anionimprinted materials should be considered as preferable solid sorbents. Molecularly imprinted polymers (MIPs) are a class of synthesized smart polymeric materials, in which a great deal of imprinted cavities designed for the target template molecule is distributed, and these cavities are complementary to the template molecule in shape, size, and functionality. Therefore, MIPs have specific molecular recognition ability and high binding affinity for the template molecule,15−18 so that MIPs are often called 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), and they specially recognize the template ions.19−21 Now, MIPs as well as IIPs have been extensively employed in many fields, in which the fine selectivity for the target substances is required. Especially, in the environmental pollution treatment, MIPs and IIPs have been used as effective solid sorbents for removing harmful and toxic substances from aqueous medium.22−25 The conventional method of preparing molecular imprinted polymers (MIPs) is the entrapment way, and the prepared MIPs have many disadvantages, such as taking time, having a Received: Revised: Accepted: Published: 4469

December 22, 2013 February 22, 2014 February 23, 2014 February 24, 2014 dx.doi.org/10.1021/ie404321v | Ind. Eng. Chem. Res. 2014, 53, 4469−4479

Industrial & Engineering Chemistry Research

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(Unic Company, Shanghai, China), Zetasizer Nano-Zeta potential analyzer (Malvern Instrument Company, UK), and THZ-92C constant temperature shaker equipped with gas bath (Shanghai Boxun Medical Treatment Equipment Factory, Shanghai, China), STA449 thermogravimetric analyzer (TGA, Netzsch Company, German), air atmosphere, and a heating rate of 10 °C/min. 2.2. Preparing Functional Grafted Particles QPDMAEMA/SiO2 and Examining Their Adsorption Property for Molybdate Anion. 2.2.1. Preparation of Grafted Particles PDMAEMA/SiO2. (1) By using the surface-initiated graftpolymerization method,37 the grafted particles PDMAEMA/ SiO2 were prepared, and the main experimental procedures were as follows. The micron-sized silica gel particles were first surface-modified with coupling agent 3-aminopropyltrimethoxysilane (AMPS), and amino groups were introduced onto the surfaces of silica gel particles, obtaining the modified particles AMPS-SiO2. (2) In an aqueous solution, the surface-initiated graft-polymerization of DMAEMA was conducted by initiation of the amino group/persulfate redox initiating system. The modified particles AMPS-SiO2 (1.5 g) were added into a fournecked flask equipped with a mechanical agitator, a reflux condenser, a thermometer and a N2 inlet, followed by adding 80 mL of distilled water and 16 mL of monomer DMAEMA. N2 was bubbled for 30 min to remove air. The content in the flask was heated to 35 °C, and 20 mL of aqueous solution, in which 0.16 g of initiator ammonium persulfate (APS) was dissolved, was added. The graft polymerization of DMAEMA was performed under a N2 atmosphere at 35 °C for 6 h. (3) The resultant particles were extracted with methanol in a Soxhlet Apparatus for 24 h to remove the polymer physically attaching to the particles and then dried under a vacuum, finally obtaining the grafted particles PDMAEMA/SiO2. The FTIR spectrum of the grafted particles PDMAEMA/SiO2 was determined to characterize their structure. The grafting degree of PDMAEMA on PDMAEMA/SiO2 particles was determined by TGA, and the grafted particles PDMAEMA/SiO2 used in this work have a grafting degree of 24 g/100g. 2.2.2. Realizing Quaternization Transformation of Grafted Particles PDMAEMA/SiO2. The grafted particles PDMAEMA/ SiO2 (1 g) were added into a four-necked flask equipped with a mechanical agitator, a reflux condenser and a thermometer, followed by adding 50 mL of epichlorohydrin, which was used as a reagent and also was used as a solvent. The quaterization reaction between the ternary amine groups of the grafted macromolecule PDMAEMA and epichlorohydrin was allowed to be carried out at 60 °C for 3 h. The collected product particles were washed with acetone repeatedly and dried under a vacuum, obtaining the functional grafted particles QPDMAEMA/SiO2. The quaternization transformation degree of QPDMAEMA/SiO2 was determined with the silver nitrate titration method; i.e. the quaternization transformation degree was obtained through determining the content of Cl− ions of QPDMAEMA/SiO2 particles via the Volhard method, and it was 65%. The determination procedure is described briefly as follows. The functional grafted particles QPDMAEMA/SiO2 were soaked in distilled water, and the grafted polymer layer was allowed to swell. Excessive AgNO3 standard solution with a given concentration was added into the mixture, and all of the Cl− ions of QPDMAEMA were transformed into AgCl as white precipitate. The residual Ag+ ion was determined with the backtitration method with NH4SCN solution as a titrant. The amount of Cl− ions of the soaked QPDMAEMA/SiO2 particles

complicated preparation process, having fewer recognition sites inside the matrix particle obtained by crushing and grinding the imprinted polymeric monolith, and having greater diffusion resistance for the template molecules because of the thick matrix.26,27 These shortcomings seriously limited the applications of MIPs and IIPs. In order to overcome these drawbacks, recently, the various surface-imprinting methods have been actively developed,28,29 and the researchers try to build molecular recognition systems on the surfaces of solid particles or microspheres. It needs to be pointed out that among various surface-imprinting methods, the use of double-imprinting technology by the sol−gel process has some of the noteworthy features.30,31 We also devote ourselves to develop the new surface-imprinting methods. In recent years, we put forward two novel molecule surface-imprinting techniques, “pre-graft polymerizing and post-crosslinking/imprinting”32−34 and “synchronously graft-polymerizing and imprinting” methods,35,36 and both of them are successful. The aim of the present study is to prepare a molybdate anion surface-imprinted material by using the new surface imprinting technique of “pre-grafting and post-crosslinking/imprinting” mentioned above and is to investigate the recognizing and binding performance of the imprinted material toward the molybdate anion so as to supply a valuable reference for preparing effective solid sorbents that can selectively remove the molybdate anion from water. The MoO42− anion surfaceimprinted material prepared in this research work possesses specific recognition selectivity and high combining ability for the molybdate anion. As far as we know, the molybdate anion surface-imprinted material is reported for the first time. It is significant and valuable to introduce molybdate-anion-imprinted materials into the removal of the molybdate anion from a water medium in the field of water environment protection. Further, the cost of the molybdate anion-imprinted material prepared in this work is not high, and so to prepare and apply such materials is feasible.

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. Silica (about 125 μm in diameter) was purchased from Ocean Chemical Limited Company (Qingdao City, China). γ-Aminopropyltrimethoxysilane (AMPS) was obtained from Nanking Chuangshi Chemical Aux Ltd. (Province Jiangsu, China). N,N-Dimethylaminoethyl methacrylate (DMAEMA) was supplied by Feixiang Chemical Limited Company (Province Jiangsu, China) and was purified by vacuum distillation before use. Epichlorohydrin was from Beijing Zhonglian Chemical Reagent Factory (Beijing, China). Ammonium molybdate ((NH4)6Mo7O24·4H2O) was obtained from Tianjin Ruijinte Chemicals Co. Ltd. (Tianjin City, China), and the pH of the prepared aqueous solution was adjusted to 8, implying that in this system MoO42− is the existing form of Mo(VI) species (see below). 1,6-Hexanediamine was obtained from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). Sodium tungstate and potassium permanganate (Na2WO4·2H2O and KMnO4) were purchased from Shanghai Shuntai Chemicals Co. Ltd. (Shanghai, China). Other reagents were all commercial chemicals of analytical pure grade and purchased mainly from the Beijing Chemical Reagent Company of China National Medicine Group (Beijing, China). The instruments used in this study were as follows: PerkinElmer 1700 infrared spectrometer (Perkine-Elmer Company, USA), Unic-2602 UV/vis spectrophotometer 4470

dx.doi.org/10.1021/ie404321v | Ind. Eng. Chem. Res. 2014, 53, 4469−4479


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shaken in a constant temperature shaker. The supernatant was taken at a certain time interval, the concentrations of these supernatants were determined by spectrophotometry, respectively, and the corresponding adsorption amounts were calculated. The adsorption amount as a function of time was figured, and the time of equilibrium adsorption was determined from the flatting part of the curve. On that basis, the isothermal binding experinent of IIPQPDMAEMA/SiO2 particles for MoO42− anion was carried out. MoO42− anion solutions of 20 mL (pH = 8) with different concentrations (1−7 mmol/L) were taken and transferred into a number of conical flasks, respectively. IIP-QPDMAEMA/ SiO2 particles of 0.05 g weighted accurately were added into these solutions, respectively. These mixtures were shaken in a constant temperature shaker for 4 h, and the binding process was allowed to reach equilibrium. After standing, the equilibrium concentrations of the MoO42− anion in the supernatants were determined by spectrophotometry. The equilibrium binding amounts Qe (mmol/g) of IIP-QPDMAEMA/SiO2 toward the MoO42− anion were calculated still according to eq 1, and the binding isotherm was plotted. The tungstate anion (WO42−) and permanganate anion (MnO4−) are also oxyanions like the MoO42− anion, and their structures are similar to the MoO42− anion to a certain extent, for example, containing four oxygen atoms and having the spatial structure of a tetrahedron. Thus, in this investigation, both WO42− and MnO4− anions were selected as the contrast anions to examine the recognition selectivity of IIPQPDMAEMA/SiO2 toward the MoO42− anion. The solutions of WO42− and MnO4− anions in a concentration range of 1−7 mmol/L were prepared with Na2WO4·2H2O and KMnO4, respectively. The pH values of these solutions were adjusted to pH = 8 with diluted HCl solution. According to the same method as for the MoO42− anion, the isothermal binding experiments of IIP-QPDMAEMA/SiO2 toward WO42− and MnO4− anions were conducted, and the binding isotherms were also plotted. In the binding experiments, the equilibrium concentration of the WO42− anion in the supernatants was determined with the thiocyanate-spectrophotometric method at 400 nm,39 and that of the MnO4− anion was determined also with the spectrophotometric method at 525 nm. The determination method of the concentration of the WO42− anion is described as follows. The supernatant (0.5 mL) containing the WO42− anion was transferred into a conical flask of 50 mL, followed by adding 5 mL of a mixed acid of sulfuric acid and phosphoric acid and 1−2 drops of concentrated nitric acid. The mixed solution was heated until white smoke rose. Then, it was cooled and was transferred into a volumetric flask of 50 mL, followed by adding 5 mL of stannous chloride, 6 mL of potassium rhodanide solution and 2 mL of titanium dichloride solution. After shaking well, the solution in the volumetric flask was diluted to a constant volume, and then its absorbency was determined with the blank solution as a reference solution. The WO42− concentration in the original supernatant was determined by using the standard curve. 2.4.2. Evaluating Binding Property of IIP-QPDMAEMA/SiO2 with Dynamic Method. At room temperature, IIP-QPDMAEMA/SiO2 particles (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 MoO42− solution of 5 mmol/L was allowed to flow gradually through the packed column at a rate of 5 BV/h, and the effluents with one volume (1 BV) interval were collected. The MoO 4 2− anion

was calculated with the data of the volume and concentration of the added AgNO3 standard solution and the data of the volume and concentration of the titrant NH4SCN solution. The FTIR spectrum of QPDMAEMA/SiO2 particles was also determined to verify their structure change. Their zeta potentials at different pH values were determined to examine their surface electrical property. 2.2.3. Examining Adsorption Property of QPDMAEMA/ SiO2 Particles for Molybdate Anion. Ammonium molybdate solutions in a concentration range of 1−7 mmol/L were prepared, and the pH values of these solutions were adjusted to pH = 8 with a diluted NaOH solution. On the basis of the dynamic adsorption test (the time of equilibrium adsorption was 4 h), the isothermal adsorption experiments of QPDMAEMA/SiO 2 particles for the molybdate anion (MoO42−) were carried out with these solutions in a constant temperature shaker. The equilibrium concentrations of the MoO42− anion in the supernatants were determined by spectrophotometry at 460 nm.38 The equilibrium adsorption amounts (mmol/g) were calculated according to eq 1, and the adsorption isotherms were plotted.

Qe =

V (C0 − Ce) m

(1)

where Qe (mmol/g) is the equilibrium adsorption amount, C0 and Ce (mmol/L) are the initial and equilibrium concentrations of MoO42− anions, respectively, V (mL) is the volume of the adsorption solution, and m (g) is the mass of the functional particles QPDMAEMA/SiO2. 2.3. Preparation and characterization of molybdate anion surface-imprinted material IIP- QPDMAEMA/SiO2. QPDMAEMA/SiO2 particles of 1 g were added into 400 mL of MoO42− anion solution of 7 mmol/L with pH 8. This mixture was placed into a constant temperature shaker, and was shaken for 4 h until the adsorption reached equilibrium, followed by collecting the particles by filtrating with a Buchner funnel, on whose bottom a moistened filter paper was placed, and drying them under vacuum. Such particles (0.5 g) were placed into an aqueous solution of MoO42−anion with pH 8 and with a concentration of 8 mmol/L to prevent desorption. A DMF solution (30 mL) of 1,6-hexanediamine with a concentration of 7.5 mmol/L was added into this system. The cross-linking reaction and imprinting process were carried out at 45 °C for 24 h. After the reaction finished, the product particles were washed first with ethanol and then with a 2 mol/L NaCl solution, in which a little NaOH (its concentration was 0.01 mol/L) was contained, repeatedly, so as to remove the residual cross-linking agent 1,6-dibromohexane and the template MoO42− anions. After drying under a vacuum, the resultant particles were namely the MoO42− anion surface-imprinted material IIP-QPDMAEMA/SiO2. The infrared spectrum of the particles IIP-QPDMAEMA/SiO2 was determined with the KBr pellet method to confirm the structure change. 2.4. Examining the Binding Characteristic of IIPQPDMAEMA/SiO2 for the Molybdate Anion. 2.4.1. Evaluating the Binding Property of IIP-QPDMAEMA/SiO2 with Static Method. The binding kinetics experiment was first conducted to determine the time of equilibrium adsorption, and it was also about 4 h. The procedure of the kinetics experiment is described briefly as follows. A MoO42− anion solution of 40 mL with a given concentration was taken and transferred into a conical flask, and about 0.1 g of IIPQPDMAEMA/SiO2 particles was added. The mixture was 4471



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Scheme 1. Schematic Expressions of Preparation Process of QPDMAEMA/SiO2 Particles

The two mixed systems were allowed to be shaken for 4 h in a constant temperature oscillator, and the completing adsorption was allowed to reach equilibrium. After standing, the concentration of each anion in the supernatants was determined, and the distribution coefficient of each anion was calculated according to eq 2, and this equation was originated from refs 40 and 41.

concentrations in these effluents were determined by spectrophotometry, and the dynamic binding curve of the MoO42− anion was plotted. By using the data of the concentrations and bed numbers of these effluents, the break binding amount and saturated binding amount for MoO42− anion on the packed column were calculated. The dynamic binding curves of WO42− and MnO4− anions were also figured, and their break binding amounts and saturated binding amounts were also calculated in the same way as the MoO42− anion. 2.4.3. Experiments of Binding Selectivity. In order to further examine the recognition specificity of IIP-QPDMAEMA/SiO2 particles for the MoO42− anion relative to WO42− and MnO4− anions, the competitive adsorption experiment was conducted. Two binary mixed solutions, MoO42−/WO42− and MoO42−/MnO4−, were prepared. In the two solutions, the concentration of each anion was the same and was 4 mmol/L. The two mixed solutions (20 mL) were placed into two conical flasks with a cover, respectively, and the imprinted particles IIPQPDMAEMA/SiO2 (0.5 g) weighted accurately were added.

Kd =

Qe Ce

(2)

where Kd represents the distribution coefficient (L/g) of an anion, Qe (mmol/g) is its equilibrium binding amount, and Ce (mmol/L) is its equilibrium concentration. The selectivity coefficients k of the imprinted particles IIPQPDMAEMA/SiO2 for the MoO42− anion relative to the two competition anions were obtained according to eq 3 with the distribution coefficient data, respectively. 4472



k=

Kd Kd′

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three particles, SiO2, the grafted particles PDMAEMA/SiO2 and the functional grafted particles QPDMAEMA/SiO2.

(3)

where Kd is the distribution coefficient of the MoO42− anion, whereas K′d represents the distribution coefficient of a certain competition anion. The value of k represents and marks the recognition selectivity of IIP-QPDMAEMA/SiO2 for the MoO42− anion relative to the competing anion.40,41 A relative selectivity coefficient k′ is also defined as expressed in eq 4,40,41 and the value of k′ can reveal the enhanced extent of the adsorption affinity and selectivity of the imprinted material IIP-QPDMAEMA/SiO2 toward the template aion with respect to the nonimprinted material QPDMAEMA/SiO2.

k′ =

k impr k non ‐ impr

(4)

where kimpr is the selectivity coefficient of IIP-QPDMAEMA/ SiO2 for MoO42− anion with respect to the competition species, and knon‑impr is the selectivity coefficient of QPDMAEMA/SiO2 for the MoO42− anion also with respect to the same competition species. 2.6. Examining Desorption Performance. IIPQPDMAEMA/SiO2 particles (1 g) that had adsorbed the MoO42− anion in a saturation state were packed into a piece of glass pipe. A NaCl aqueous solution of 2 mol/L containing a little of NaOH (its concentration was 0.01 mol/L) was used as eluent. The eluent was allowed to flow gradually through the column at a rate of 2 BV/h in the countercurrent manner. The effluents with one volume (1 BV) interval were collected, and the concentrations of the MoO42− anion in these effluents were determined by spectrophotometry. The dynamic desorption curve was plotted. The amounts of the MoO42− anions eluted out were calculated with its concentration and bed number, and the elution percents for the effluents with different bed numbers were further calculated to estimate the desorption performance of IIP-QPDMAEMA/SiO2 particles.

Figure 1. Infrared spectra of three kinds of particles.

As compared with the spectrum of SiO2, in the spectrum of the grafted particles PDMAEMA/SiO2, two new bands have appeared at 1732 and 1395 cm−1, respectively. The former is attributed to the stretching vibration absorption of carbonyl group CO of the ester group, and the latter is ascribed to the vibration absorption of the C−N bond of tertiary amine groups. Meanwhile, the stretching vibration absorption bands of methyl group −CH3 and methylene −CH2− appear at 2925 and 2850 cm−1, respectively. The above spectrum data change demonstrates that the monomer DMAEMA has produced surfaceinitiated graft-polymerization by the initiating of the −NH2/ S2O8 2− system, and the grafted particles PDMAEMA/SiO2 have been formed. In the spectrum of QPDMAEMA/SiO2 particles, the characteristic vibration absorption of the epoxy group has appeared at 908 cm−1, indicating that the quaterization reaction between the tertiary amine groups of the grafted macromolecule PDMAEMA and epichlorohydrin has been produced and the functional grafted particles QPDMAEMA/SiO2 bearing a lot of quaternary ammonium groups as well as epoxy groups have been formed. 3.2.2. Zeta Potential Curve. Figure 2 gives the zeta potential curves of three particles, SiO2, PDMAEMA/SiO2 and QPDMAEMA/SiO2, and the following facts can be observed. (1) As compared with the zeta potential of SiO2 particles, in a certain range of pH, the zeta potential of PDMAEMA/SiO2

3. RESULTS AND DISCUSSIONS 3.1. Preparation Process and Chemical Structure of Functional Grafted Particles QPDMAEMA/SiO2. In this work, the functional grafted particles QPDMAEMA/SiO2 were prepared via two reaction stages, surface-initiated graftpolymerization and polymer reaction 1. Micron-sized silica gel particles were surface-modified with coupling agent AMPS, obtaining modified particles AMPS-SiO2, on which amino groups were introduced onto the surfaces of silica gel particles. (2) A surface-initiating system was constituted by the amino group on AMPS-SiO2 particles and persulfate in the solution. (3) By the initiating of this surface-initiating system, monomer DMAEMA was graft-polymerized on silica gel particles, getting the grafted particles PDMAEMA/SiO2. (4) The quaterization transform of the tertiary amine groups of the grafted macromolecule PDMAEMA was carried out with epichlorohydrin as a reagent, resulting in the functional grafted particles QPDMAEMA/SiO2, on which quaternary ammonium groups with a high density were contained, and at the same time, epoxy groups were also contained (it is a preparation of the following imprinting). The preparing process and chemical structure of QPDMAEMA/SiO2 particles are schematically expressed in Scheme 1. 3.2. Characterization of QPDMAEMA/sio2 Particles. 3.2.1. FTIR Spectrum. Figure 1 gives the FTIR spectra of the

Figure 2. Zeta potential curves of three kinds of particles. 4473



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particles is positive, and it is caused by the protonation of N atoms of the tertiary amine groups of the grafted macromolecule PDMAEMA. However, after pH > 5, the zeta potential of PDMAEMA/SiO 2 particles decreases with increasing pH due to the weakening of the protonation of the tertiary amine groups. Therefore, the zeta potential of PDMAEMA/SiO2 particles is highly sensitive to pH value. (2) The value of the positive zeta potential of QPDMAEMA/SiO2 particles is higher than that of PDMAEMA/SiO2 particles, and furthermore, in a greater range of pH (3−9) its value remains always high. This is associated with that there are quaternary ammonium cations with a high density on the surfaces of QPDMAEMA/SiO2 particles. 3.3. Character of High Adsorption Capacity of QPDMAEMA/SiO2 Particles for MoO42− Anion. The isothermal adsorption experiments for the MoO42− anion in near neutral solutions (pH = 8) were conducted with the three particles, SiO2 particles, the grafted particles PDMAEMA/SiO2 and the functional particles QPDMAEMA/SiO2, respectively. The adsorption isotherms are presented in Figure 3.

Figure 4. Varying diagrams of the form of Mo(VI) species with pH and concentration.

effectively performing the surface-imprinting of MoO42− anions on the surfaces of QPDMAEMA/SiO2 particles. 3.4. Chemical Processes of Preparing MoO42− Ion Surface-Imprinted Material IIP-QPDMAEMA/SiO2. As mentioned above, under the condition of pH = 8, Mo(VI) exists as MoO42−. Under such pH condition, the adsorption of QPDMAEMA/SiO2 particles for MoO42− anions is first allowed to reach saturation, and then the cross-linking agent 1,6hexanediamine is added. The ring-opening reaction between the two terminal primary amine groups of 1,6-hexanediamine and the epoxy groups on the side chains of the grafted macromolecule QPDMAEMA will be carried out favorably, leading to the cross-linking of the grafted macromolecules QPDMAEMA. As a result, MoO42− anions are enveloped in the cross-linking networks, resulting in the imprinting of MoO42− anions. As the template anions are washed away, large numbers of MoO42− anion-imprinted caves will remain within the thin polymer layer on the surfaces of silica gel particles. Consequently, the MoO42− anion surface-imprinted material IIP-QPDMAEMA/SiO2 is formed. The chemical processes of “pre-graft polymerizing and post-crosslinking/imprinting” described above can be schematically expressed in Scheme 2. In order to confirm the structure of the imprinted particles IIP-QPDMAEMA/SiO2, their infrared spectrum was compared with that of the functional particles QPDMAEMA/SiO2 as shown in Figure 5. In comparison with the spectrum of QPDMAEMA/SiO2, in the spectrum of IIP-QPDMAEMA/ SiO2, the characteristic absorption band of the epoxy group at 908 cm−1 has disappeared, whereas the absorption band of methylene −CH2− at 2850 cm−1 has been strengthened greatly, and it comes from a large number of the methylene groups in the cross-linking-bridge of 1,6-hexanediamine. The spectrum data changes above-mentioned demonstrate that the cross-linking and imprinting processes have occurred, and the MoO42− anion surface-imprinted material IIP-QPDMAEMA/ SiO2 has been prepared. 3.5. Binding Characteristic of IIP-QPDMAEMA/SiO2 for MoO42− Anion. 3.5.1. Binding Isotherm. The isothermal binding experiments of the imprinted particles IIP-QPDMAEMA/SiO2 were first conducted for MoO42−, WO42− and MnO4− anions with the static method (batch method), respectively, and at the same time the isothermal adsorption experiments of the nonimprinted particles QPDMAEMA/SiO2 for the three anions were also carried out. Figure 6 gives the adsorption isotherms of QPDMAEMA/SiO2 particles for the

Figure 3. Adsorption isotherms of three kinds of particles for molybdate anion. Temperature: 25 °C; pH = 8.

It is well-known that in aqueous solutions, chemical species of Mo(VI) change with the aqueous pH value as well as with the concentrations of the metal species (by the way, so do the species of W(VI)), appreciably. The diagram of Mo(VI) species with different concentrations and pH values is shown in Figure 4.40 It is obvious that in alkaline solutions like pH = 8 and at lower concentrations, Mo(VI) exists as a MoO42− anion (incidentally, at pH 8, W(VI) also exists as a WO42− anion). It can be seen from Figure 3 that the adsorption ability of SiO2 particles for the MoO42− anion is very poor, or it can be considered that silica gel particles nearly do not adsorb MoO42− anions. However, after grafting the macromolecule PDMAEMA, the grafted particles PDMAEMA/SiO2 have produced some adsorption ability for MoO42− anions, but the adsorption capacity is only 0.23 mmol/g. The functional grafted particles QPDMAEMA/SiO2 display very strong adsorption ability for MoO42− anions with an adsorption capacity of 0.40 mmol/g by right of the strong electrostatic interaction between the quaternary ammonium cations with a high density on QPDMAEMA/SiO2 particles and MoO42− anions. The difference in the adsorption ability of PDMAEMA/SiO2 and QPDMAEMA/SiO2 particles is originated from the difference of their surface electrical performances as shown in Figure 2. The strong adsorption ability of the functional grafted particles QPDMAEMA/SiO2 for MoO42− anions lays a solid base for 4474



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Scheme 2. Schematic Expressions of Preparing Surface-Imprinted Particles IIP-QPDMAEMA/SiO2

Figure 5. Infrared spectrum comparison of QPDMAEMA/SiO2 and IIP-QPDMAEMA/SiO2 particles.

Figure 7. Binding isotherms of IIP-QPDMAEMA/SiO2 particles for three anions. Temperature: 25 °C; pH = 8.

three anions, leading to their high adsorption capacities. For the small adsorption capacity differences between the three anions displayed in Figure 6, it probably arises from their ionic structure differences as well as from the differences of the negative charges carried by them. In a word, the functional particles QPDMAEMA/SiO2 nearly have no adsorption selectivity for the three anions. By the way, it was confirmed by data processing that the above adsorption isotherms basically fit to the Langmuir model. However, Figure 7 shows that the binding property of the imprinted particles IIP-QPDMAEMA/SiO2 for the template MnO4− anion is obviously different from that for the two contrast anions, WO42− and MnO4−. The binding capacities of WO42− and MnO4− anions greatly decrease, and they have reduced from 0.49 mmol/g to 0.11 mmol/g for the WO42− anion and from 0.64 mmol/g to 0.15 mmol/g for the MnO4− anion. It fully reveals that the imprinted particles IIPQPDMAEMA/SiO2 nearly do not recognize and bind the two contrast anions. Whereas for the template ion, MoO4−, the imprinted particles IIP-QPDMAEMA/SiO2 still maintain a high binding capacity, and it even increases from 0.40 mmol/g to 0.46 mmol/g, showing specific recognition selectivity and excellent binding affinity. The specific ionic recognition selectivity of IIP-QPDMAEMA/SiO2 particles for the MoO4− anion is explained as follows.

Figure 6. Adsorption isotherms of QPDMAEMA/SiO2 particles for three anions. Temperature: 25 °C; pH = 8.

three anions, whereas Figure 7 gives the binding isotherms of IIP-QPDMAEMA/SiO2 for the three anions, respectively. It is displayed in Figure 6 that the functional particles QPDMAEMA/SiO 2 (nonimprinted material) have high adsorption ability for all three of the anions, and the adsorption capacity is in a range of 0.40−0.64 mmol/g. The reason for this can be explained as follows. There are quaternary ammonium cations with a high density on the surfaces of the functional particles QPDMAEMA/SiO2, and so the functional particles can produce a strong electrostatic interaction for each of the 4475



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There are a good deal of MoO4− anion-imprinted caves within the thin polymer layer on the surfaces of particles IIPQPDMAEMA/SiO2. These caves are highly matched with MoO4− anions in spatial structure, acting site, and size, whereas they are unmatched with the two contrast anions, resulting in the specific recognition selectivity for MoO4− anions and very poor binding ability for WO42− and MnO4− anions. Although the three oxyanions have some spatial structure similarities, for example, containing four oxygen atoms and having the spatial structure of a tetrahedron, there are some subtle structure differences between them. For example, the shapes of the tetrahedron are probably different, the bond lengths are different (Mo−O, 1.67−1.81 Å;43 W−O, 1.71−1.85 Å;44 Mn−O, 1.63 Å45), and the corresponding bond angles are also diverse. In addition, the negative charges carried by them are different. It is these subtle differences in structures that lead to that the combining sites in the imprinted cave of the MoO4− anion are inappropriate for WO42− and MnO4− anions. These subtle differences result in the mismatching of these imprinted caves with WO42− and MnO4− anions. Therefore, it is inevitable that IIP-QPDMAEMA/SiO2 particles do not recognize and bind the two contrast anions, and whereas for MoO4− anions, they possess special binding selectivity. 3.5.2. Dynamic Binding Curve. The isothermal binding experiments of the imprinted particles IIP-QPDMAEMA/SiO2 were also carried out for MoO42−, WO42− and MnO4− anions with the dynamic method (column method), respectively, and at the same time, the dynamic isothermal adsorption experiments of the nonimprinted particles QPDMAEMA/SiO2 for the three anions were also performed. Figure 8 gives the

Figure 9. Dynamic binding curves of IIP-QPDMAEMA/SiO2 particles for three anions. Temperature: 25 °C; pH = 8; 5 mmol/L; flow rate: 5 BV/h.

WO42− solutions. The leaking volumes of MnO4− and WO42− solutions significantly reduced. The leaking volume of MnO4− solution decreases from 45 BV to 11 BV and that of WO42− solution decreases from 31 BV to 7 BV. In contrast, for MoO4− solution, the leaking volume still remains high, and even it has been improved from 23 BV to 29 BV. Apparently, the result of dynamic binding experiments again indicates that the imprinted particles IIP-QPDMAEMA/SiO2 hardly recognize and bind WO42− and MnO4− anions, whereas for the template ion, MoO 4 −, the imprinted particles again display specific recognition selectivity and excellent binding affinity. By calculation, for the WO42− anion, the leaking and saturated adsorption amounts are only about 0.07 mmol/g and 0.14 mmol/g, respectively, and for MnO4−, they are also only about 0.11 mmol/g and 0.18 mmol/g, respectively. However, for the MoO42− anion, they actually reach 0.29 mmol/g and 0.47 mmol/g, respectively. The above experimental results still arise from the mismatching of the imprinted caves with WO42− and MnO4− anions and from the great matching of the imprinted caves with MoO4− anions. 3.5.3. Selectivity Coefficients of IIP-QPDMAEMA/SiO2 for MoO4− Anion. Two binary ionic mixed solutions with pH 8, MoO42−/WO42− and MoO42−/MnO4−, in which the concentration of each anion was 4 mmol/L, were prepared, respectively, and the competing adsorption experiments of IIP-QPDMAEMA/SiO2 particles were carried out. Tables 1 Table 1

Figure 8. Dynamic adsorption curves of QPDMAEMA/SiO2 particles for three anions. Temperature: 25 °C; pH = 8; 5 mmol/L; flow rate: 5 BV/h.

adsorb material

dynamic adsorption isotherms of QPDMAEMA/SiO2 particles for the three anions, and Figure 9 gives the dynamic binding isotherms of IIP-QPDMAEMA/SiO2 for the three anions, respectively. It can be observed from Figure 8 that as the solutions of the three anions with the same concentration (5 mmol/L) flow upstream through the column packed with QPDMAEMA/SiO2 particles, there is relatively little difference in their leaking volumes, and they are 45 BV for the MnO4− anion, 31 BV for the WO42− anion, and 23 BV for the MoO4− anion, respectively. This result also reveals that the packed column of QPDMAEMA/SiO2 particles nearly has no adsorption selectivity. However, Figure 9 obviously displays that the leaking curve of MoO4− solution is clearly different from that of MnO4− and

QPDMAEMA/SiO2

adsorbate

MoO42−

Kd/(L/g) k k′

0.062

WO4

IIP-QPDMAEMA/SiO2

2−

MoO42−

0.085

WO42−

0.173

0.73

0.027 6.41

8.78

and 2 list the data of the distribution coefficient of each anion in the two binary ionic mixed solutions and the data of the Table 2 adsorb material

4476

QPDMAEMA/SiO2

IIP-QPDMAEMA/SiO2

adsorbate

MoO42−

MnO4−

MoO42−

MnO4−

Kd/(L/g) k k′

0.055 0.57

0.097 5.39

0.178

0.033

9.46 dx.doi.org/10.1021/ie404321v | Ind. Eng. Chem. Res. 2014, 53, 4469−4479


Article

chain unit of macromolecule QPDMAEMA to 1,6-diaminlhexane). It can been found that the selectivity coefficient of IIPQPDMAEMA/SiO2 first increases and then decreases with the increase of the molar ratio, and there is a maximun selectivity coefficient of 6.41 as this ratio is 2:1, implying that with this ratio, the imprinting effect is the best. This result is closely correlative with the number of average action sites between the grafted functional macromolecule QPDMAEMA and MoO42− anion. It can be seen from Scheme 2 that two quaternary ammonium groups on the macromolecule chain interact with one MoO42− anion by right of electrostatic interaction, namely two chain units interact with one MoO42− anion. Besides, the cross-linking action needs two chain units for one imprinting cave. Therefore, for imprinting one MoO42− anion, four chain units of the grafted macromolecule QPDMAEMA and two 1,6diaminlhexane molecules are needed. As a result, a suitable ratio of 2:1 is inevitably led to. As the ratio is greater than 2:1, i.e. the cross-linking agent is lacking, fewer imprinting caves inside the polymer layer on the surface of silica gel particles will be led, and the poorer recognization selectivity of IIP-QPDMAEMA/ SiO2 for the MoO42− anion will result. Whereas as the ratio is smaller than 2:1, i.e. the cross-linking agent is overmuch, a greater cross-linking degree will be produced, and the adsorbed MoO42− anion on the functional particles QPDMAEMA/SiO2 will be pushed aside partially. Consequently, fewer imprinted caves inside the polymer layer also will be led to, and it also will weaken the recognition ability of IIP-QPDMAEMA/SiO2 for the MoO42− anion. The above experimental facts show that the new surface imprinting method of “pre-grafting and postimprinting” is greatly different from the conventional imprinting method, and a basic characteristic of “feed quantifying” of this new surface imprinting system is reflected. In other words, for the new surface imprinting system, the feeding can be quantified in order to obtain the imprinted materials with high performance.46 An aqueous solution of NaCl (2 M) contaning a little NaOH was used as eluent, and the elution property of the packed column of IIP-QPDMAEMA/SIO2 particles that have binded MoO42− anions in a saturated state was examined. The packed column of IIP-QPDMAEMA/SIO2 particles displayed an excellent elution property, and the elution percents can reach 99%. In a word, through the three kinds of binding experiments, static, dynamic and competing binding experiments, the specific binding selectivity of the imprinted material IIP-QPDMAEMA/SiO2 for MoO42− anion has been fully confirmed. Therefore, it can be expected that such anion surface-imprinted material can be used as an ideal solid adsorbent for selective removal of molybdate anions from water.

selectivity coefficient of the imprinted particles IIP-QPDMAEMA/SiO2 and the nonimprinted particles QPDMAEMA/SiO2 for the MoO4− anion relative to WO42− and MnO4− anions, respectively, as well as the data of the relative selectivity coefficients. It can be found from Tables 1 and 2 that the selectivity coefficients of QPDMAEMA/SiO2 (nonimprinted material) for the MoO42− anion relative to WO42− and MnO4− anions are small (0.73 and 0.57), and they are approximatively close to 1, suggesting that QPDMAEMA/SiO2 particles have no adsorption selectivity. However, IIP-QPDMAEMA/SiO2 particles have high selectivity coefficients for the MoO42− anion, and they are 6.41 relative to the WO42− anion and 5.39 relative to the MnO4− anion, respectively, implying that IIP-QPDMAEMA/SiO2 particles have special binding selectivity for the MoO42− anion. It also can be observed from Tables 1 and 2 that the relative selectivity coefficients of IIP-QPDMAEMA/ SiO2 are 8.78 and 9.46, respectively, indicating a remarkable enhancement of the adsorption affinity and selectivity of the imprinted material IIP-QPDMAEMA/SiO2 for the template molecule in relation to nonimprinted material QPDMAEMA/ SiO2. In short, all of the above three experiments, batch, column and competing binding experiments, prove that the MoO42− anion surface-imprinted material IIP-QPDMAEMA/SiO2 indeed possesses excellent recognition selectivity, and it is potential to use IIP-QPDMAEMA/SiO2 particles in the selective removal of MoO42− anions from aqueous medium. 3.6. Effect of Used Amount of Cross-Linking Agent on Binding Property of IIP-QPDMAEMA/SiO2. In this surface imprinting technique of “pre-grafting and post-imprinting,” there are two factors affacting greatly the binding property of the imprinted materials. One is the grafted degree of the functional macromolecule, and it directly affects the binding capacity of the surface-imprinted materials for the template. The other one is the used amount of cross-linking agent, and it seriously influenced the binding selectivity of the surfaceimprinted material46 for the template. The MoO42− anion surface-imprinting was performed with different used amounts of the cross-linking agent (1,6-hexanediamine) by fixing other reaction conditions, and Figure 10 gives the selectivity coefficient of IIP-QPDMAEMA/SiO2 for the MoO42− anion relative to the WO42− anion as a function of the used amounts of 1,6-hexanediamine (it is expressed as the molar ratio of the

4. CONCLUSIONS In this work, through molecular design, the MoO42− anion surface-imprinted material IIP-QPDMAEMA/SIO2 was prepared with the new surface-imprinting technique of “pre-graft polymerizing and post-crossing/imprinting.” The grafted particles PDMAEMA/SiO2 were first transformed into the functional grafted particles QPDMAEMA/SiO2 through a quaternization reaction with epichlorohydrin as a reagent. There are a mass of quaternary ammonium groups and epoxy groups in the side chains of the grafted macromolecule QPDMAEMA. By right of strong electrostatic interaction, QPDMAEMA/SiO2 particles produce strong adsorption for

Figure 10. Selectivity coefficient as a function of molar ratio of chain unit of QPDMAEMA to cross-linker. Temperature: 25 °C; pH = 8. 4477



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MoO42− anions, and then via the cross-linking reaction between epoxy groups of the grafted macromolecule QPDMAEMA and the two terminal primary amine groups of 1,6-hexanediamine, the surface-imprinting of MoO42− anions was realized, resulting in the MoO42− anion surface-imprinted material IIP-QPDMAEMA/SiO2. The imprinted particles IIP-QPDMAEMA/SiO2 possess high recognition selectivity and fine binding affinity for MoO42− anions, whereas for the two contrast oxyanions, WO42− and MnO4−, their binding property is very poor, or rather, they do not recognize WO42− and MnO4− anions basically. In addition, in this imprinting system, the suitable molar ratio of the chain unit of the grafted macromolecule to the cross-linker 1,6-hexanediamine is 2:1, and at this molar ratio, the imprinted particles IIP-QPDMAEMA/SiO2 with the best recognition selectivity can obtained.

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Corresponding Author

*Tel: 86-0351-3924795 Fax: 86-0351-3922118 E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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