Highly selective cloud point extraction of thorium(iv) ions in environmental samples for their ultra-trace level determination by total reflection X-ray fluorescence spectrometry

Abstract

The toxicological effects of thorium on human health are twofold due to its chemical as well as radiological toxicity. The mining of natural thorium resources and anthropogenic uses of thorium contribute to its increased human exposure. Thorium compounds are known as potential occupational carcinogens. The low solubility of thorium compounds in natural water systems requires ultra-trace level detection of Th(IV) ions. This calls for the selective preconcentration of Th(IV) in the presence of ubiquitously present uranium and lanthanides along with naturally abundant alkali, alkaline earth, and transition metals. In this work, the bis(phosphoramidate) ligand was examined for selective cloud point extraction (CPE) of the Th(IV) ion. The CPE parameters were optimized for quantitative recovery of Th(IV). It was found that at pH 4 the ligand and sodium dodecyl sulfate in the micelle medium made a supramolecular complex with Th(IV) via a cation exchange mechanism. The CPE procedure was found to have an extraction efficiency and recovery of (98.9 ± 0.2)% and 99% respectively. The preconcentration factor of the method was 91. All these parameters resulted in quantitative preconcentration of Th(IV) in the surfactant rich phase (SRP) which was then directly analyzed by total reflection X-ray fluorescence (TXRF) spectrometry. The small sample volume requirement of the TXRF technique makes it an excellent candidate for microlitre volume SRP analysis without compromising on the preconcentration factor. Oxidative pyrolysis of the SRP before TXRF analysis improved the thorium detection limit from 0.9 to 0.1 μg L⁻¹. The proposed analytical methodology was validated by analyzing certified reference materials (CRMs) and real samples via spike addition. The overall process had a maximum precision of 2% on the reported results.