Photooxidation of Natural and Waste Cr(III)-Bearing Hydroxides in Water and Ice: Cr(III)-O₂Complex-Mediated Oxidation and the Effect of Natural Organic Matter

The photooxidative dissolution of Cr(III)-bearing hydroxides (Cr(OH)₃and Fe_xCr_1–x(OH)₃) is proposed as a potential abiotic source of Cr(VI) in environments. Compared to the dark oxidation of Cr(OH)₃by 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 H₂O₂that played the dual roles of enhancing Cr(OH)₃oxidation 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)₃can be a source of natural Cr(VI) under realistic environmental conditions. The photooxidative dissolution of Cr(OH)₃is 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 O₂to H₂O₂via •O₂^–. A similar mechanistic behavior was also observed with Fe_xCr_1–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.