Chromite ore processing residue (COPR), the solid waste product from the high-temperature alkaline processing of ferrochromite (FeO center dot Cr2O3), contains Cr(VI) in soluble and insoluble compounds formed in the roasting process. This research investigated tartaric acid in combination with Mn2+ and isopropyl alcohol (IPOH) as co-reductants for reagent- and COPR-derived Cr(VI). The reduction of Cr(VI) by tartaric acid alone at pH 5.0 or greater was negligible; however, in the presence of Mn2+ or IPOH, reduction occurred in hours. Isopropyl alcohol enhanced Cr(VI) reduction, probably via formation of a termolecular complex with the alcohol, tartaric acid, and Cr(VI). In aqueous solutions of reagent-derived Cr(VI) at pH 4, 12 mmol L-1 tartaric acid with 1.0 mmol L-1 Mn2+ or 1.0 mmol L-1 Mn2+ and 0.29 mol L-1 (2% v/v) IPOH reduced 1.0 mmol L-1 Cr(VI) in 48 h. The same treatments at pH 5.5 reduced 0.60 and 0.58 mmol L-1 Cr(VI) (60%) in 96 h, respectively. A minimum half-life of 10.2 h was calculated from first-order rate constants obtained from Mn2+ and IPOH-Mn2+ co-reductant treatments with tartaric acid at pH 4. The most COPR-derived Cr(VI) reduced in suspension was by IPOH and Mn2+ at high acidity (pH 5.8), which reduced 0.52 mmol L-1 (52%) of the COPR-derived soluble Cr(VI) at 96 h. The enhanced reduction of soluble Cr(VI) by tartaric acid by the addition of Mn2+ proceeds within a complex formed by an esterification reaction between tartaric acid and Cr(VI) with Mn2+ bound to tartaric acid. The combined treatment of tartaric acid, IPOH, Mn2+, and a strong acid to lower the pH of COPR-enriched soils would be effective in field applications of this chemistry. By creating a slurry of the field soil with these amendments, mass transfer limitations would be overcome, and Cr(VI) would be reduced to Cr(III) in days.
