Visible Light Accelerates Cr(III) Release and Oxidation in Cr–Fe Chromite Residues: An Overlooked Risk of Cr(VI) Reoccurrence

The reduced chromite ore processing residue (rCOPR) deposited in environments is susceptible to surrounding factors and causes reoccurrence of Cr­(VI). However, the impact of natural sunlight on the stability of rCOPR is still unexplored. Herein, we investigated the dissolution and transformation behaviors of Cr­(III)–Fe­(III) hydroxide, a typical Cr­(III)-containing component in rCOPR, under visible light. At acidic conditions, the release rate of Cr­(III) under illumination markedly increased, up to 7 times higher than that in the dark, yet no Cr­(VI) was produced. While at basic conditions, only Cr­(VI) was obtained by photo-oxidation, with an oxidation rate of ∼7 times higher than that by δ-MnO2 under dark conditions at pH 10, but no reactive oxygen species was generated. X-ray absorption near-edge structure and density functional theory analyses reveal that coexisting Fe in the solid plays a critical role in the pH-dependent release and transformation of Cr­(III), where photogenerated Fe­(II) accelerates Cr­(III) produced at acidic conditions. Meanwhile, at basic conditions, the production of intermediate Cr­(III)–Fe­(III) clusters by light leads to the oxidation of Cr­(III) into Cr­(VI) through the nonradical “metal-to-metal charge transfer” mechanism. Our study provides a new insight into Cr­(VI) reoccurrence in rCOPR and helps in predicting its environmental risk in nature.