Separation of Cr(VI) from Cr(III) in CaCl2 aqueous salt medium using hydrocarbon (as toluene, n-hexane, n-heptane or n-octane) mixed solvents of ter-butanol

A simple and selective solvent extraction system was developed using mixed solvent of ter-butanol(ter-BuOH), namely toluene/ter-BuOH, n-hexane/ter-BuOH, n-heptane/ter-BuOH and n-octane/ter-BuOH to separate Cr(VI) from Cr(III). Toluene/ter-BuOH mixed system excelled the others. Chromium(VI) was extracted in the presence of Cr(III) into the mixed solvents at the mole fraction of ter-BuOH (χt-but) in the range of 0.1 and 0.5 (9.0–47.1% in volume) after addition of CaCl2 (5 M). The extraction was repeated twice for quantitative extraction of Cr(VI). Chromium(III) was not extracted in the above range. The distribution ratio of ter-BuOH and the concentrations of water and CaCl2 in the organic phase were determined and their influences on the extraction of Cr(VI) were assessed. When χt-but = 0, neither Cr(VI) nor Cr(III) was extracted. The chemical species in the aqueous phase was confirmed to be the CrO3Cl− species. When [salts]aq or [acid]aq = 0, the separation of Cr(VI) from Cr(III) was not achieved. This study revealed that Cr(VI) was extracted as a solvated ion-pair complex of [ter-BuOH2+·CrO3Cl−]. The optimum conditions for the separation of Cr(VI) from Cr(III) are [CaCl2]aq = 5 M, [HCl]aq = 0.1 M and χt-but = 0.3 (27.6% in volume). Further, the extracted Cr(VI) was stripped by the addition of a 2 M NaOH aqueous solution. The present method was applied to the separation and determination of Cr(VI) and Cr(III) in two industrial effluent samples. The relative standard deviations (n = 14) of Cr(VI) and Cr(III) were 4.45 and 2.86% for Company A, and 5.99 and 3.31% for Company B.