Aqueous two-phase systems (polyethylene glycol + ammonia sulfate) for thallium extraction: Optimization of extraction efficiency, structural characterization, and mechanism exploration

Abstract

Liquid-liquid extraction using aqueous two-phase systems (ATPs) presents advantages when compared to traditional liquid-liquid extraction. In this work, the extracting behavior of thallium(III) in the polyethylene glycol-based aqueous two-phase system (PEG–ATPs) was thoroughly investigated. The PEG-ATPs used in this study contains 2.5 M (NH4)2SO4 as a phase forming salt and 30% (v/v) PEG(4 0 0) solution. Thallium(III) partitioning was evaluated by turning the processing parameters including the concentration of chlorine, solution pH, reaction time, temperature and initial metal ion concentration. It was found that the efficient extraction of Tl ions mainly depends on the concentration of chlorine. By increasing the chlorine concentration or decreasing the solution pH, the extraction of Tl ions increased markedly. Na2SO3 is demonstrated to be an excellent agent for the back extraction of PEG-Tl(III); a back extraction rate of 76% yielded. Temperature shows a significant impact on the exaction of Tl(III) (97.0% of Tl(III) extracted within 3.0 min reaction at 320 K), but a minor effect on the extraction of other ions (e.g., NH4+, Zn2+, Mn2+, Sb3+, Fe3+, Pb2+ and Bi3+, etc.) in the system. Results from conductivity test, infrared spectroscopy, and thermogravimetry demonstrated that the complexation reaction between PEG and Tl (III) can form a complex structure with oxygen atoms in the polyethylene glycol molecule (PEG(4 0 0)-Tl(III)). One Tl(III) ions coordinated with four PEG structural units for the novel structure. Overall, PEG is an environmental-friendly and efficient extraction agent for the removal of thallium from wastewater.