A review of the formation of Cr(VI) via Cr(III) oxidation in soils and groundwater

Abstract Hexavalent chromium (Cr(VI)) is a widespread, toxic contaminant in the environment. Owing to the increasingly strict requirements for green production and pollution emission, Cr(VI) generation via Cr(III) oxidation is gradually becoming the primary source of Cr(VI) in soils and groundwater instead of direct emissions. Herein, the formation of Cr(VI) by Cr(III) oxidation in soils and groundwater is systematically reviewed. The main oxidants of Cr(III) in soils are manganese oxides (MnOx), hydrogen peroxide (H2O2), and photochemical generated radicals, which respectively dominate in aerobic, anaerobic, and surface zones of soils. In aerobic zones, dissolved Cr(III) can be oxidized on the surface of MnOx under acidic conditions (pH