Selective extraction of Hg(II), Cd(II) and Zn(II) ions from aqueous media by a green chemistry procedure using aqueous two-phase systems

Abstract

A simple aqueous two-phase systems of polyethylene glycol (PEG)/sulphate salt (Na2SO4 or (NH4)2SO4) was proposed for the selective extraction of Hg(II), Cd(II) and Zn(II) ions from aqueous media, in presence of iodide as extracting agents. The extraction efficiency of metal ions from salt-rich phase to the polymer-rich phase was investigated as a function of the amount of added iodide extractant, nature of inorganic sulphate used as phase-forming salt, the volume ration between phases of aqueous two-phase system and the pH of salt solution. Maximum extraction efficiency of considered metal ions was obtained in the aqueous polyethylene glycol/ammonia sulphate two-phase system and a volume ration of the phases equal with 1.0. On the other hand, in mentioned experimental conditions, at iodide concentration higher than 0.06molL−1, the Hg(II) ions are quantitatively extracted (>99%) into polymer-rich phase, at all studied values of salt solution pH, while a quantitative extraction of Cd(II) ions was obtained only in strong acid media (salt solution pH lower than 3.0). The extraction efficiency of Zn(II) ions is insignificant (<15%) for all values of salt solution pH and iodide concentration added as extractant, they preferring to remain in the salt-rich phase of extraction system. Therefore, by adequate selection of the experimental conditions, and efficient and selective extraction of these metal ions could be obtained, and a separation procedure in two steps is proposed. The separation factors calculated for each case (SHg/Cd=(1.48±0.13)×103; SHg/Zn=(2.99±0.21)×103 and SCd/Zn=(3.16±0.17)×103, respectively) indicate that the proposed procedure is efficient and has a large viability for the selective extraction of Hg(II) and Cd(II) ions in presence of Zn(II) ions. In addition, this extraction procedure has the main advantage that is environmental safe.