Visible Light Boosted the Reduction of Cr(VI) in the Absence of Conventional Reductants in Frozen Solutions: The Overlooked Role of Inorganic Anions

The conversion of Cr(VI) in frozen solutions is a mysterious process in the water environment. It has traditionally been believed that the reduction of Cr(VI) can only occur through interaction with traditional reducing agents like natural organic matters (NOMs) and Fe(II), with H₂O merely acting as a solvent. However, this study reveals that visible light can induce the reduction of Cr(VI) by H₂O even without conventional reducing agents. Specifically, within the narrow liquid-like layer between ice crystals, there is a significant increase in concentrations of protons, Cr(VI), and anions (Cl^-, SO₄^2-, and NO₃^-), which promotes the formation of various complexes between Cr(VI) and anions. DFT results reveal 11 types of stable Cr(VI)-anion complexes in solution, with six exhibiting visible light absorption properties. Quadrupole time-of-flight (QTOF) mass spectrometry confirms that the abundance of these six complexes (Cl-CrO₂-Cl, Cr₂O₆-Cl, Cl-Cr₂O₅-Cl, Cr₂O₆-OSO₃H, HO₃SO-Cr₂O₅-OSO₃H, and Cr₂O₆-ONO₂) correlates with the extent to which visible light promotes Cr(VI) reduction, thus highlighting their crucial role as photoreactive intermediate complexes during this conversion process. These findings suggest that H₂O in frozen solutions should no longer be regarded solely as a solvent but also as a reactant, thereby inspiring deeper insights into frozen solution chemistry.