Study on adsorption of Th(IV) using surface modified dibenzoylmethane molecular imprinted polymer
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- Ji, X.Z., Liu, H.J., Wang, L.L. et al. J Radioanal Nucl Chem (2013) 295: 265. doi:10.1007/s10967-012-1979-4
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The adsorption of Th(IV) was studied using a novel dibenzoylmethane molecular imprinted polymers, which was prepared using acryloyl-β-cyclodextrin as a monomer on surface modified functional silica gel. X-ray photoelectron spectroscopy and FTIR were employed to confirm the reliability of the synthetic polymer. Scanning electron microscope was used to analyze the surface properties of the experimental materials. UV-spectrophotometer was employed to investigate the adsorption property and the concentration of Th(IV). Adsorption kinetics and adsorption isotherm were carried out. In pH 3.5, the adsorption equilibrium could reach a balance in 45 min, the resultant adsorbing capacity was 30.8 mg g−1, and the remove ratio of Th(IV) was 88.1 %.
KeywordsTh(IV)DibenzoylmethaneMolecular imprinting polymerAdsorption
Thorium was considered as one kind of strategic elements. With the quick development of global economic, the thorium was massive mining in recent years and some pollution by traces of thorium also acceded and need be solved urgently. Shtangeeva  had studied the uptake of uranium and thorium by native and cultivated plants. Zoriy et al.  had studied the biomonitoring of environmental pollution by thorium and uranium in selected regions of the Republic of Kazakhstan. Höllriegl et al.  had studied the solubility of uranium and thorium from a healing earth in synthetic gut fluids. Sato and Kirishima  had studied the separation of thorium and uranium by sulfide method. Harrison et al.  had studied separation and measurement of thorium, plutonium, americium, uranium and strontium in environmental matrices. Masa et al.  had studied the determination of trace element concentrations and stable lead, uranium and thorium isotope ratios by quadrupole-ICP-MS in NORM and NORM-polluted sample leachates. Carretas et al.  had studied the uranium(III, IV) and thorium(IV) pyrazolylmethane complexes. Kutahyal and Eral  had studied the Sorption studies of uranium and thorium on activated carbon prepared from olive stones. Amaral and Morais  had studied the thorium and uranium extraction from rare earth elements in monazite sulfuric acid liquor through solvent extraction. Lindley and Parks  had studied the near-complete transuranic waste incineration in a thorium fuelled pressurised water reactor. Permana et al.  had studied the breeding and void reactivity analysis on heavy metal closed-cycle water cooled thorium reactor. Ozay et al.  had studied the P(4-vinyl pyridine) hydrogel use for the removal of UO2+ and Th4+ from aqueous environments. However, how to get a quick and resultful material for the determination and adsorption of Th(IV) is still a work badly in need for us to do.
Molecular imprinting technology (MIT) is one of synthesis and preparation technology designed to obtain spatial structure and binding sites exactly match with the template. The molecular imprinting polymers have specific selective recognition of the template for the cavity when template molecular was removed, which was exactly match with the template on the spatial structure and binding sites . With these advantages, more and more researches have focused on this aspect. MIT has been considered as a worthy application in many areas, such as separations [14, 15], catalysis , sensors , biomimetic materials [18, 19] and so forth. In recent years, MIT has greatly developed due to the use of excellent materials such as β-cyclodextrin (β-CD) and silica gel [20–22]. Recently, Li’S group has made a great progress in MIT, and they take β-CD and silica gel into MIT and carry the experiments to selectively recognize biomacromolecular protein in aqueous phase for the first time [23–25].
Dibenzoylmethane (DBM) is a minor β-diketone, which is a kind of outstanding metal-chelating agent . To the best of our knowledge, there was still no report of study on the adsorption of Th(IV) using DBM molecular imprinting polymers to date. In our present study, DBM molecular imprinting polymers were prepared by surface MIT using functional silica gel as a solid phase carrier and acryloyl-β-CD as a monomer. In order to prepare a material can selectively recognize the Th(IV). The experiment results showed that molecular imprinted polymers (MIPs) could selectively recognize the Th(IV), even in a high concentration of effective ions.
Materials and methods
DBM, β-CD (biochemical reagent), ethyleneglycol dimethacrylate (EGDMA), (3-aminopropyl)trimethoxysilane (APS), azodiisobutyronitrile (AIBN), Th(NO3)4·4H2O (SP), and acryloyl chloride, Silica gel (200–300 mesh) were purchased from Tianjing Kermel Chemical Technology Co., Ltd. Toluene was removed water by sodium metal. Methanol, dimethyl formamide, acetone, etc. were analytical reagent and used without further purification. All the solutions were used in this work was distilled water.
Simultaneous Thermal Analysis STA449C Jupiter (NETZSCH GmbH), XPS (Perkin Elmer PHI 550 ESCA/SAM, USA), UV-spectrophotometer (Shanghai, China), FTIR (Shimadzu, Japan), JSM-6610 Scanning Electron Microscope (JEOL), etc.
Synthesis of functional silica gel
A mixture of 5 g activated silica gel and 100 mL toluene was stirred and refluxed with APS (4 mL) dropping into the flask, and kept reacting for 24 h. The mixture solution was filtrate and the solid product was washed by toluene, ethyl ether, acetone and methanol, respectively. The amino-functionalized silica gel (Si–NH2) was prepared under drying at 80 °C for 8 h.
A mixture of 5 g of Si–NH2, 75 mL of toluene and 3 mL of acryloyl chloride was stirred and kept reacting for 5 h. The mixture solution was filtrate and the solid product was washed by toluene and acetone for three times and was dried at 80 °C for 10 h, then the functional silica gel (F–Si) was prepared.
Preparation of imprinted polymers and non-imprinted polymers
100 mL DMF was added to 0.6 g of acryloyl-β-CD (synthesized as ) and 0.0224 g of DBM. The solid materials was dissolved and stirred for 24 h at room temperature. Then, 1.00 g functional silica gel, 4.00 mL EGDMA and 0.50 g AIBN were added to the solution. After the mixture was purged with nitrogen for 10 min to remove the oxygen, it was stirred and heated by water bath for 24 h. The abstained polymer precipitation was filtered and dried in vacuum oven at 70 °C for 10 h to abstain imprinted polymers (MIPs).
Element analysis of each material
Atomic content ( %)
MIPs + Th
Effect of pH
A series of 50 mg L−1 Th(IV) working solutions were transferred into a 10 mL flask, and the pH values were adjusted to 1.0–6.0 with HNO3 and NH3·H2O, and then the volume was adjusted to 10 mL with distilled water. 10 mg of MIPs power was added, and then the mixture was shaken vigorously for 60 min. Centrifuged and the concentrations of thorium ions in the solutions were determined by UV-spectrophotometer.
To 10 mL 50 mg L−1 Th(IV) solution, pH 3.5, 10 mg MIPs (or 10 mg NIPs) power was added. The flask was shaked at room temperature and the Th(IV) was adsorbed by the MIPs (or NIPs) for 5–60 min correct order. Centrifuged and UV-spectrophotometer was employed to analyze the Th(IV) concentration in the adsorbed solution.
A group of 10 mg MIPs power was added into 10 mL different concentration of Th(IV) solution, pH 3.5, shaking to adsorb for 45 min. Centrifuged and UV-spectrophotometer was employed to analyze the Th(IV) concentration in the adsorbed solution.
Effect of interfering ions
A group of different concentration of interfering ions such as Fe(II), Cu(II), Ca(II), Pb(II), Al(III), Fe(III) and even UO22+(VI) were added to 50 mg/L Th(IV) solution, pH 3.5, shaking to adsorb for 45 min. Centrifuged and UV-spectrophotometer was employed to analyze the Th(IV) concentration in the adsorbed solution.
Results and discussion
The FTIR spectra of MIPs shows that the template does not change the adsorption peak of each chemical group very much, suggesting that the template only combines with the monomer acryloyl-β-CD by self-assemble with hydrophobic interaction and hydrogen bonding interaction, but not forming chemical bonds. The peaks around 1,102 and 472 cm−1 resulted from the stretching and bending vibration of Si–O–Si. The adsorption peaks around 2,958 and 2,989 cm−1 resulted from the methyl C–H of acryloyl-β-CD, respectively. In the FTIR spectrum of MIPs, the peaks at 1,110 cm−1 and 1,602 cm−1 were ascribed to the δC–H on benzene ring and νC=C of the template DBM. The FTIR spectra of MIPs + Th shows not only the peaks which also accedes in the MIPs, but also some more peaks at 420 cm−1 (Th–O stretching vibrations) and 1,592 cm−1 (C=O–Th stretching vibrations).
To further verify the findings from the FTIR results, X-ray photoelectron spectroscopy (XPS) studies of MIPs, NIPs and MIPs + Th were conducted the C1s, O1s, N1s, Si2p and Th4f7 regions. Deconvolution of each atom peak produces a peak of binding energy at 286.4, 532.3, 399.5, 102.7 and 334.9 eV, respectively. The elements percentage of each atom was analyzed (Table 1). As the synthesis process went on, the element percentage of N and Si reduced, but the C and O increased. It is likely due to the reason that the crosslink reagent EGDMA could not be grafted on the functional silica gel so much in MIPs compared to NIPs due to the resist of template DBM molecular. Therefore, the element percentage of C and O were lower but the N and Si were higher in MIPs than the NIPs. So the surface of MIPs is very crude while that of NIPs is much smooth (Fig. 3). In MIPs + Th, because of the Th(IV) was adsorbed by the metal-chelating agent DBM on the surface of MIPs, the atomics content of C, O, N and Si reduced. These results indicate that MIPs were successfully prepared by the cross-linking reactions grafted onto the silica gel surface, in good agreement with the results of FTIR analysis.
Effect of pH
The study of adsorption dynamics shows that the adsorption increases as the time increases in MIPs, which is markedly higher than that for NIPs in 45 min. This observation is due to the fact that the molecular imprinting polymer of MIPs has many metal-chelating agent DBM molecules on the surface of the polymers which exactly match with the Th(IV), and the surface of the polymer is much cruder than that of NIPs. Therefore, the Th(IV) can be easier adsorbed on MIPs than NIPs.
Interfering ions analysis
The impact on adsorption of Th(IV) by interfering ions were carried out to explore the selection of Th(IV) by the molecular imprinting polymer we had prepared. The adsorption of Th(IV) could have the similar capacity when the concentration of Fe(II), Cu(II), Ca(II) and Pb(II) were lower than 40 mg L−1, and the maximum allowed concentration were found to be 35 mg L−1 for Al(III), Fe(III) and even UO22+(VI).
In this work, we reported a novel DBM molecular imprinting polymer prepared by surface imprinting method using functional silica gel as the solid carrier and acryloyl-β-CD as the monomer, which was successfully used for the adsorption of Th(IV). The polymers were confirmed by FTIR, scanning electron microscope (SEM) and XPS characterizations. The MIPs in this study had good adsorption capacity and remove ratio of Th(IV) at pH 3.5, and the adsorption equilibrium could reach a balance in 45 min. The adsorption capacity of Th(IV) was 30.8 mg g−1, and the remove ratio was high to 88.1 %, the maximum allowed concentration were found to be 40 mg L−1 for Fe(II), Cu(II), Ca(II) or Pb(II) and 35 mg L−1 for Al(III), Fe(III) or even UO22+(VI).
This study was supported by National Natural Science Foundation of China (No. 20707008), Natural Science Foundation of Hunan Province (No. 07JJ5004) and Technology Department Foundation of Hunan Province (No. 2011GK3191).