Elsevier

Talanta

Volume 81, Issues 1–2, 15 April 2010, Pages 30-36
Talanta

Selective solid-phase extraction of trace thorium(IV) using surface-grafted Th(IV)-imprinted polymers with pyrazole derivative

https://doi.org/10.1016/j.talanta.2009.11.032Get rights and content

Abstract

A new pyrazole derivative 1-phenyl-3-methylthio-4-cyano-5-acrylicacidcarbamoyl-pyrazole (PMTCAACP) was synthesized and chosen as a complexing monomer for the preparation of surface-grafted ion-imprinted polymers for selective solid-phase extraction of thorium(IV). The silica gel, modified with maleic anhydride, was prepared as a carrier material. In the ion-imprinting process, Th(IV) was complexed with PMTCAACP, and then imprinted in the polymers grafted to the surface of modified silica gel. Subsequently, the template Th(IV) ions were removed with 6 mol/L HCl solution. The obtained ion-imprinted particles for Th(IV) showed specific recognition, and rapid adsorption and desorption kinetics process. The maximum static and total dynamic adsorption capacity of the ion-imprinted polymers (IIPs) for Th(IV) was 64.8 and 37.4 mg/g, respectively. The relative selectivity coefficient values of the imprinted adsorbent for Th(IV)/U(VI), Th(IV)/Ce(III), Th(IV)/La(III), and Th(IV)/Zr(IV) were 72.9, 89.6, 93.8, and 137.2 times greater than non-imprinted matrix, respectively. The interference effect of common cations tested did not interfere with the recovery of Th(IV). The enhancement factor of 20.2, the detection limit of 0.43 μg/L, and the precision of 2.47% (n = 7) of the method under the optimized conditions were obtained. Additionally, the calibration curve (r = 0.9993) was linear in the range of 1.43–103 μg/L of thorium(IV). The prepared IIPs were shown to be promising for solid-phase extraction coupled with UV–vis spectrophotometry for determination of trace Th(IV) in real samples.

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 Cdouble bondO 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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