Extent of oxidation of Cr(III) to Cr(VI) under various conditions pertaining to natural environment

Abstract

Calculations show that oxidation of chromium oxide (Cr 2O 3) by oxygen and oxidation of chromium hydroxide (Cr(OH) 3) by manganese dioxide (MnO 2) are thermodynamically feasible in both aerobic and mildly anoxic environments. Experiments were carried out to determine the rate and extent of chromium oxidation under various conditions, i.e., when Cr 2O 3 was heated in the presence of oxygen, when Cr(OH) 3 and MnO 2 mixtures were suspended in aerobic or anoxic aqueous media at various pH values, when Cr(OH) 3 and MnO 2 mixtures interacted in moist aerobic conditions and when chromium assumed to be Cr(OH) 3 and manganese assumed to be MnO 2 interacted in the presence of competing electron donors/acceptors, as is the case in chromium-contaminated sludge. Results indicate that trivalent chromium in Cr 2O 3 could be readily converted to hexavalent chromium at a temperature range of 200–300 °C, with conversion rates of up to 50% in 12 h. In aqueous media, Cr(OH) 3 was slowly converted to dissolved Cr(VI) in the presence of MnO 2, both in aerobic and anoxic conditions, with conversion rates of up to 1% in 60 days. In moist aerobic conditions and in the presence of MnO 2, Cr(OH) 3 slowly converted to hexavalent chromium, with up to 0.05% conversion observed in 90 days. Chromium oxidation also occurred in sludge samples, especially under aerobic conditions. However, such transformation was found to be transitory, with the Cr(VI) formed being ultimately reduced back to Cr(III) due to the presence of various reducing agents in the sludge. Nevertheless since up to 17% conversion of Cr(III) to Cr(VI) occurred in sludge under aerobic conditions by 30 days, there is real danger under field conditions of spreading Cr(VI) pollution due to possible intervening rainfall, runoff and percolation.