Chromium oxidation potential and related soil characteristics in arable upland soils

Oxidation of Cr(III) to Cr(VI) can increase availability and toxicity of chromium. In this study, the potential of chromium oxidation in arable upland soils was evaluated with a quick test and its relationships with some soil characteristics were investigated. Also, the possibility of Cr(VI) generation was tested in arable upland soils amended with Cr-containing sludge. Thirty-seven out of the forty-five arable upland soils tested could oxidize Cr(III) to Cr(VI). Since manganese (Mn)-oxide is the only known oxidant of chromium in natural conditions, oxidation potential was directly related with Mn-oxide content in the soils. Also, soil pH and organic matter content had a very significant relationship with chromium oxidation in soil. As the pH was higher and the organic matter content lower, the chromium oxidation potential was higher. Soil pH and organic matter seem to be important controlling factors for Mn-oxide content in soils. Manganese-oxides can be dissolved reductively at lower pH and higher organic matter conditions. Soils of high chromium oxidation potential amended with Cr-containing sludge could generate Cr(VI) in laboratory incubation. However, oxidation of the amended Cr(III) was not found in soils of low chromium oxidation potential. In land application of sewage sludges, care should be taken with soil types, because the soils high in Mn-oxides and low in organic matter are able to oxidize chromium. By controlling organic matter content and pH of soils, the risk from oxidized chromium would be reduced in agricultural environments amended with chromium containing wastes.