•OH Dominates the Dark Oxidation of Elemental Mercury at the Liquid–Ice Interface during Environmentally Relevant Freeze–Thaw Cycles

Oxidation of elemental mercury (Hg(0)) is critical in Hg long-range transport and deposition. Freeze–thaw cycles commonly occur in the atmosphere, surface water, and soil in cold regions; however, their effects on Hg(0) oxidation, particularly during environmentally relevant freeze–thaw processes, remain unknown. Here, we investigated the freeze-induced dark oxidation of Hg(0) by potential reactive oxygen species under controlled freeze–thaw conditions. Freeze–thaw cycles (25 to −20 to 25 °C and 10 to −10 to 10 °C) accelerated the oxidation of Hg(0) compared with constant 25/10 °C, especially in acidic solutions (pH 4.0). Subsequent characterization and radical quenching experiments confirmed that the spontaneously produced hydroxyl radical (•OH) was the predominant oxidant responsible for Hg(0) oxidation. Notably, hydrogen peroxide (H2O2) loading at environmental levels did not accelerate Hg(0) oxidation, suggesting that the observed oxidation is distinct from the mechanism of the freeze-concentration effect (Hg(0) and H2O2 enrichment increase the oxidation rate) as previously documented. This work demonstrated a novel pathway for Hg(0) oxidation driven by the spontaneous production of •OH at the liquid–ice interface, independent of H2O2. This research highlights that the interfacial •OH production can affect the environmental behavior of Hg(0) with important implications for understanding its fate, particularly in the atmosphere.