Freezing switches the predominant mechanism of DOMs-mediated Cr(VI) reduction from complexation and photocatalysis to ice-crystal confinement effect.

The geochemical behavior of hexavalent chromium (Cr(VI)) in cold environments is profoundly influenced by freezing processes. This study reveals a fundamental switching of dissolved organic matter (DOM)-mediated Cr(VI) reduction mechanisms between aqueous and ice phases. In the aqueous phase, the reaction is co-dominated by complexation and photocatalysis: under dark conditions, reduction efficiency strongly correlates with DOM's esterified aromatic structure (EAS; r = 0.99) via ligand-to-metal charge transfer (LMCT); under light, visible light excites DOM to generate reactive species (e.g., triplet-state DOM, ·OH, ·O2⁻), altering pathways and linking to aromaticity (SUVA254). In contrast, the ice phase is governed by the innovative ice-crystal confinement effect. Within the nanoscale quasi-liquid layer (QLL), high bulk polarity DOM and Cr(VI) are anchored at the ice surface, enabling highly efficient electron and proton transfer directly through the interface. Light enhances kinetics but not the pathway, confirming thermodynamic control by confinement. This work proposes ice-crystal confinement catalysis as the dominant mechanism in frozen environments, demonstrating how freezing redirects contaminant transformation via modified microenvironments and mass transfer. These insights provide a critical theoretical foundation for predicting chromium behavior in cold regions and developing effective remediation strategies.