Sulfite-Induced Release and Oxidation of Cr(III) in Reduced Chromite Ore Processing Residue under Visible Light: The Critical Role of Fe(IV) Intermediates

Reduced chromite ore processing residue (rCOPR) is vulnerable to the surrounding conditions in environments, which can induce the release and oxidation of Cr(III). This work found the synergistic effect of light and sulfite on destroying rCOPR stability, causing the significant release of Cr(VI), which was 4∼7 times higher than that under single factor action. In Cr_xFe_1–x(OH)₃/sulfite/light system, Cr(VI) release rate could reach 11.11 × 10^–7 M min^–1 g^–1, because of the rapid formation of ·OH, O₂^·–, SO₄^·–, and quadrivalent iron [Fe(IV)]. In addition, this study found that the oxidized chromium residue could continuously release Cr(III) after being transferred to the dark environment. This should be attributed to the obvious change in the iron coordination structure of rCOPR caused by the catalytic reaction of sulfite with light, which greatly weakened the ability of the Fe–O structure to encase chromium elements in a solid. These insights are crucial for predicting the environmental risks of rCOPR in the presence of visible light and sulfur-containing compounds.