Analytical Sciences

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

Sethsiri S. SAMARATUNGA, Jun NISHIMOTO, and Masaaki TABATA

Department of Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan

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.

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