Beyond Freeze Concentration: Interfacial Processes Shape Cr(VI) Reduction by Organic Acids in Frozen Systems

Ice-phase Cr(VI) reduction is commonly attributed to freeze concentration and proton enrichment, implicitly assuming solution-like reaction mechanisms. However, this study reveals that interfacial processes serve as a critical complement to freeze concentration and strongly influence reaction pathways in ways not explained by concentration enrichment alone. By investigating nine structurally diverse organic acids under frozen conditions and comparing them with aqueous systems, we observed no reduction at ambient temperature but pronounced, structure-dependent behaviors in frozen solution. Organic acids were categorized into three classes: Class I (e.g., tartaric acid) promoted Cr(VI) reduction in the dark via five-membered ring complexes and showed light enhancement correlated with TD-DFT excitation energies (R2 = 0.93), consistent with photoinduced ligand-to-metal charge transfer (LMCT). Class II (maleic acid) reduced Cr(VI) only under visible light through a dehydration-hydration-chromate ester pathway. Strikingly, Class III acids (e.g., acetic acid) suppressed reduction, likely by competitively adsorbing on ice surfaces, disrupting proton mobility and blocking catalytic sites, as supported by CP2K/DFT calculations. These findings underscore that freeze concentration alone is insufficient to explain pathway selection and that interfacial processes make an essential contribution by modulating coordination environments and proton-coupled electron transfer in frozen systems. This work redefines frozen-phase reactivity, highlighting the imperative of interfacial processes in predicting contaminant transformations in cold environments.