Toluene/ter-Butanol Mixed Solvent for the Selective Extraction of Cr(VI) from Divalent Heavy Metals

Abstract

A solvent-extraction system comprising toluene/ter-butanol (ter-BuOH) mixed solvent as the organic phase was developed to selectively extract Cr(VI) from acidic chloride media in the presence of divalent metals, namely Cd(II), Co(II), Cu(II), Ni(II) and Zn(II) under 5 M CaCl₂ salting-out conditions. Chromium(VI) was selectively extracted as a solvated ion-pair of [ter-BuOH₂⁺·CrO₃Cl^-] at ter-BuOH mole fractions of between 0.1 and 0.6 (9.0 - 57.2% in volume). Divalent metals were extracted at ter-BuOH mole fraction over 0.6 with extraction percents of Co(< 20%), Cu(< 15%), Ni(< 10%) and Zn(< 20%). The concentrations of Ca²⁺, water and ter-BuOH in the organic phase and ter-BuOH in the aqueous phase were determined to find out the effects on the extraction of Cr(VI). The chemical species of Cr(VI) in acidic chloride media containing 5 M CaCl₂ and 0.1 M HCl was confirmed to be the CrO₃Cl^- species. The effects of the acid, salt concentrations in the aqueous phase and the solvent composition of a mixed organic solvent on the extraction of Cr(VI) were evaluated. Based on the above studies, the extraction mechanism was elucidated and the optimum extraction conditions were determined.