Photooxidation of Natural and Waste Cr(III)-Bearing Hydroxides in Water and Ice: Cr(III)-O₂ Complex-Mediated Oxidation and the Effect of Natural Organic Matter

The photooxidative dissolution of Cr(III)-bearing hydroxides (Cr(OH)3 and FexCr1-x(OH)3) is proposed as a potential abiotic source of Cr(VI) in environments. Compared to the dark oxidation of Cr(OH)3 by O2, photoirradiation enhanced Cr(VI) production in both aqueous and frozen solutions. In the presence of natural organic matter (NOM), the photoproduction of Cr(VI) was significantly promoted in aqueous solution but hindered in frozen solution. Photosensitized NOM induced the production of H2O2 that played the dual roles of enhancing Cr(OH)3 oxidation in aqueous solution and accelerating Cr(VI) reduction in frozen solution. The in situ generated Cr(VI) in the Cr(OH)3/NOM/UV system was gradually accumulated during the repeated cycles of aqueous-frozen phase transitions and light-dark variations, which confirms that the stable Cr(OH)3 can be a source of natural Cr(VI) under realistic environmental conditions. The photooxidative dissolution of Cr(OH)3 is proposed to involve the complex formation between dissolved Cr(OH)4- and O2, which subsequently undergoes the photoinduced charge transfer, resulting in the oxidation of Cr(III) to Cr(VI) with the concurrent reduction of O2 to H2O2 via •O2-. A similar mechanistic behavior was also observed with FexCr1-x(OH)3 (x = 0.3, 0.5, and 0.7) for the photooxidative production of Cr(VI). This study unveils an underappreciated pathway of Cr(VI) production from natural and waste Cr(III)-bearing hydroxides.