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 δ-MnO2 was investigated in batch reaction systems at 25 °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 δ-MnO2. 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.