Environmental Factors Affecting Chromium-Manganese Oxidation-Reduction Reactions in Soil

Disposal of chromium (Cr) hexavalent form, Cr(VI), in soils as additions in organic fertilizers, liming materials or plant nutrient sources can be dangerous since Cr(VI) can be highly toxic to plants, animals, and humans. In order to explore soil conditions that lead to Cr(VI) generation, this study were performed using a Paleudult (Dystic Nitosol) from a region that has a high concentration of tannery operations in the Rio Grande do Sul State, southern Brazil. Three laboratory incubation experiments were carried out to examine the influences of soil moisture content and concentration of cobalt and organic matter additions on soil Cr(VI) formation and release and manganese (Mn) oxide reduction with a salt of chromium chloride (CrCl3) and tannery sludge as inorganic and organic sources of Cr(III), respectively. The amount of Cr(III) oxidation depended on the concentration of easily reducible Mn oxides and the oxidation was more intense at the soil water contents in which Mn(III/IV) oxides were more stable. Soluble organic compounds in soil decreased Cr(VI) formation due to Cr(III) complexation. This mechanism also resulted in the decrease in the oxidation of Cr(III) due to the tannery sludge additions. Chromium(III) oxidation to Cr(VI) at the solid/solution interface involved the following mechanisms: the formation of a precursor complex on manganese (Mn) oxide surfaces, followed by electron transfer from Cr(III) to Mn(III or IV), the formation of a successor complex with Mn(II) and Cr(VI), and the breakdown of the successor complex and release of Mn(II) and Cr(VI) into the soil solution.