Selective solid-phase extraction of trace thorium(IV) using surface-grafted Th(IV)-imprinted polymers with pyrazole derivative
Introduction
Thorium, which is an important radioactive element widely distributed over the earth's crust, not only has extensive application in industry, e.g., optics, radio, gas mantle, aeronautics and aerospace, metallurgy and chemical industry, and material, but is also used for energy, for example, as nuclear energy for electricity production in power plants [1]. However, thorium is a toxic heavy metal, which is extremely mobile and once entered living bodies will provoke inner irradiation (especially due to the γ-active decay products), having as a final result the appearance of cancer [2]. Also, thorium and its compounds are hazardous causing environmental problems. In view of the extensive application, toxicity, and hazard, the development of reliable methods for the separation, monitoring, and recovery of thorium in environmental and geological samples is of a particular significance [3]. Direct determination of thorium is still difficult, owing to thorium's trace concentration in nature and presence of complex matrix [4]. As a result, a preconcentration or sample cleanup step, to facilitate selective separation of analytes prior to its detection, is required [5].
Various separation techniques have been developed in the past for this purpose, including liquid–liquid extraction (LLE) [6], [7], solid-phase extraction (SPE) [8], [9], extraction chromatography [10], ion exchange [11], etc. Of these, although the use of LLE has remained as the preferred technique for several years due to its high selectivity behavior [12], it has several technical problems like the generation of amounts of organic wastes that are difficult to dispose of. In contrast, the use of SPE has turned out to be a more eminent and promising technique, owing to its many advantages, such as higher enrichment factors, lower consumption of reagents, flexibility, and more importantly environmental friendliness [13]. So far, many different types of adsorbents for SPE have been reported, for instance, XAD resins, ion-exchange resin, silica gel, cellulosic derivatives, and porous glass beads [14]. Unfortunately, these solid adsorbents have poor ion selectivity, which leads to high interference of other existing species with the target metal ions. By reason of this, the use of ion-imprinted polymers (IIPs) as adsorbents for SPE has increased substantially in recent years [15], [16], [17], [18].
Ion imprinting is a versatile technique for preparing polymeric materials that are capable of high ionic recognition. In general, polymerization is carried out in the presence of a print ion or template, which forms a complex with the constituent monomers. The subsequent removal of the template leads to the formation of cavities within the polymeric structure that function as specific recognition sites [18]. Such an imprinted polymeric material shows an affinity for the template ion over other structurally related compounds. Surface imprinting is one of the important types of imprinting methods. Surface imprinting polymers not only can avoid grinding and sieving, but also possess high selectivity, good mass transfer, and fast binding kinetics. More importantly, the template ions can be removed completely from IIPs [19], [20]. More recently, several studies have reported ion-imprinted polymers based on surface imprinting [21], [22], [23].
The pyrazole unit is one of the core structures in a number of natural products. It is very important in coordination chemistry because nitrogen-containing heterocycles have good coordination capability with metal ions [24], [25]. Pyrazole derivatives also have strong chelation capability and large extraction capacity for Th(IV) ions [26], [27], [28]. But there is little information available in literature about applying pyrazole derivative to the Th(IV) ion-imprinted polymers as a monomer. Based on these aspects, in this study, 1-phenyl-3-methylthio-4-cyano-5-acrylicacidcarbamoyl-pyrazole (PMTCAAC P) was synthesized and chosen as a functional monomer, whose cyano and carbonyl groups were responsible for the thorium(IV) complexation. The silica gel was modified by amidation reaction between amino and maleic anhydride. The Th(IV)-imprinted polymers based on the modified silica gel were prepared via a surface-grafted approach in the presence of Th(IV) ion template. After removal of Th(IV) ions, the adsorption behavior of analytes on the imprinted polymers and the experimental conditions for the preconcentration process were investigated in detail. In addition, a method using Th(IV)-imprinted adsorbent for selective solid-phase extraction coupled with UV–vis spectrophotometry for determining Th(IV) was developed, and applied to the analysis of biological and water samples.
Section snippets
Apparatus
A Perkin-Elmer Lambda 45 UV–vis spectrometer (USA) and 10-mm quartz cells were used for the determination of metal ions concentrations. IR Spectra (4000–400 cm−1) were recorded on IRPrestige-21 (Shimadzu, Japan) using KBr pellets. LC–MS was performed on Agilent 1100 Series LC/MSD (USA). 1H NMR was taken on Varian INOVA-300 (USA) in DMSO-d6 with TMS as the internal standard. A pHs-l0C digital pH meter, Pengshun Scientific instruments research (Shanghai, China), was used for the pH adjustments. A
Characteristic of the FT-IR spectra
IR spectra were obtained from Silica-COOH, imprinted and non-imprinted polymers. The IR spectra of the Silica-COOH showed vinyl C–H band at 3079 and methyl C–H band at 2955 cm−1, respectively. Bands around 1707 cm−1 was assigned to CO of COOH stretching vibrations. Around 1101, 798 and 471 cm−1 resulted from stretching and bending vibrations of Si–O–Si, respectively [14]. Compared with the Silica-COOH, the IR spectra of the ion-imprinted polymers showed some new peaks as follows: at 1387 cm−1 (–CH3
Conclusion
A new surface-grafted Th(IV)-imprinted material with PMTCAACP as the functional monomer and the surface-modified silica gel as the support was prepared successfully for selective solid-phase extraction of thorium(IV). The imprinted polymers showed good characteristics, such as high affinity, selectivity and adsorption capacity, good reusability, and fast kinetics process for Th(IV). The kinetics and mechanism for the adsorption of Th(IV) on the imprinted polymers followed the Lagergren first
Acknowledgement
This project was supported by the Scientific Research Fund of the Hunan Provincial Education Department (no. 06B081).
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