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

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 < 6.0), while under alkaline conditions (pH = 8.0–9.4), Mn(II)-catalyzed oxidation plays a dominant role. In anerobic zones, Mn(II) is the dominant species of Mn, thus H2O2, which is mainly produced by the serpentinization process, becomes the main oxidant involved in Cr(III) oxidation. In addition to direct oxidation, H2O2 can also indirectly oxidize Cr(III) during the catalysis of Mn(II) under alkaline conditions. In surface soils, •OH can be generated by the photolysis of Fe(OH)2+, and it can further oxidize Cr(III) to Cr(VI). Moreover, photolysis of [Cr(OH)n]3-n can transform Cr(III) into Cr(II), which is more active and can be further oxidized into Cr(VI) by O2. In groundwater, both MnOx and H2O2 may oxidize Cr(III) in unconfined aquifers groundwater. In confined aquifers, H2O2 becomes the main oxidant for Cr(III) under alkaline conditions. In addition to in situ oxidation, the transformations of generated Cr(VI) during migration are also reviewed.