A comparative study of oxidation of Cr(III) in aqueous ions, complex ions and insoluble compounds by manganese-bearing mineral (birnessite).

Manganese oxides are considered to be main oxidants resulting in transformation of Cr(III) to Cr(VI) in soils. Oxidation of aqueous Cr(III), Cr(III)-EDTA and insoluble species of Cr(III), such as Cr(OH)3, CrFe(OH)6 and CrPO4, by delta-MnO2 was investigated in batch reaction systems at 25 degrees C and different pH values to predict the potential for Cr(III) oxidation in soil environments. Results indicate that Cr(III) can be rapidly oxidized to Cr(VI) at the beginning of the reaction; however, Mn(II) is produced and fills the adsorption sites on the manganese oxide surface. As a result, produced Mn(II) greatly slows Cr(III) oxidation by delta-MnO(2). Lower pH and higher concentration of manganese oxide markedly enhance the rate and extent of aqueous Cr(III) oxidation. The oxidation of Cr(III)-EDTA by manganese oxide is significantly affected by the chelating time between Cr(III) and EDTA and the molar ratio of EDTA to Cr(III). The formed complex ions of Cr(III)-EDTA are hardly oxidized by manganese oxide and no Cr(VI) was detected in a pH range of 5-6. The rate and extent of oxidation of Cr(OH)3 and CrFe(OH)6 by manganese oxide decrease with pH increasing from 2 to 4. No release of Cr(VI) was observed in the suspension of CrFe(OH)6 and manganese oxide at pH 4 and in the suspension of CrPO4 and manganese oxide at all pH levels tested. The results demonstrate that the order of stability of Cr(III) in these precipitates is CrPO4>CrFe(OH)6>Cr(OH)3 in the presence of manganese oxide.