Oxidation of trivalent chromium induced by unsaturated oils: A pathway for hexavalent chromium formation in soil.

The kinetics and mechanisms of the oxidation of Cr(III) in soil contaminated by unsaturated oils were investigated. Batch experiments were performed with unsaturated oils, namely, fish oil, hydrogenated lard oil, rapeseed oil, and caster oil. Impacts of environmental parameters, including temperature, soil pH, UV irradiation, oil content, and soil moisture content were examined. Results showed that oxidation of Cr(III) in oil-treated soils was accompanied by the formation of Cr(VI), which first increased and then decreased. Changes in the peroxide values of oils and the production of hexanal in the soil indicated that hydroperoxide was closely related to the formation of Cr(VI). tert-Butylhydroperoxide, as a model molecule of hydroperoxide, significantly enhanced the oxidation of Cr(III) in water. This result further showed that hydroperoxides were responsible for the oxidation of Cr(III). Native soil substances, such as organic matter, Fe(II), and microbes, and the decomposition products of hydroperoxides, such as aldehydes, could reduce Cr(VI). The change in Cr(VI) content in the soil resulted from the competition between the oxidation of Cr(III) and the reduction of Cr(VI). High temperature, high soil pH, UV irradiation, and low soil moisture content could facilitate the oxidation of Cr(III), which is of environmental significance.