Interfacial photoreactions of Cr(VI) and oxalate on lepidocrocite surface under oxic and acidic conditions: Reaction mechanism and potential implications for contaminant degradation in surface waters

The transformation of contaminants by photocatalysts under light illumination has been extensively studied in engineered water systems. However, the interfacial photoreaction mechanism for the reduction of chromium (Cr(VI)) on natural iron (Fe) oxides in acidic and oxic surface waters associated with soil remain unclear. Herein, the photochemical behavior of Cr(VI) and oxalate on lepidocrocite surfaces was investigated to disclose the natural detoxification process of Cr(VI). Kinetic results showed that Cr(VI) was quickly reduced in the system with lepidocrocite and oxalate under light illumination, while no Cr(VI) reduction was observed in the dark. No obvious secondary Fe oxides and Cr-containing precipitates were found, but the newly generated Cr(III) was evenly distributed on the lepidocrocite surface or entered into the lepidocrocite defects. Cr(VI) and oxalate can form ternary complexes with lepidocrocite, and Cr(VI) reduction and the oxidation of oxalate to carbon dioxide occurred simultaneously on the lepidocrocite surface. Amongst the reactions involving lepidocrocite, oxalate, and Cr(VI), the oxidation of oxalate was attributed to reactive oxygen species such as H2O2, OH, and ·O2−, while Cr(VI) reduction was mainly caused by photogenerated electrons, CO2−, H2O2, Fe(II), and through the direct electron transfer in the Cr(VI)-oxalate complex. The findings of the Cr(VI) molecular-scale photoreduction mechanism on mineral surfaces would have implications for interpreting the natural attenuation of Cr(VI) toxicity in natural surface waters associated with soil.