Freezing enhanced transformation of sulfamethoxazole by MnO2

The cryosphere plays a critical role in global biogeochemical cycles, however, the kinetic behavior and environmental fate of pollutants in ice remain insufficiently understood. In this study, we investigated for the first time the effect of freezing and syringic acid (SYA) on the transformation of sulfamethoxazole (SMX) mediated by manganese dioxide (MnO 2 ) under slightly acidic conditions. Freezing significantly accelerated SMX removal, reached 23.1% within 4 h in the absence of SYA, whereas this removal was negligible in aqueous solution. The presence of SYA further promoted the transformation of SMX, achieving 81.3% removal in ice compared to 46.5% in aqueous solution. We proposed that this enhanced transformation was driven by synergistic effects of solute aggregation and localized pH drop between ice crystals, arising from the freeze-concentration effect. These processes collectively elevated the redox activity of MnO 2 , and significantly promoted the rate of SMX transformation. Under this condition, SYA was first oxidized by MnO 2 to quinone, which subsequently underwent nucleophilic addition with SMX to form coupling products. These findings underscore that natural ice matrices serve as a potential medium for organic matter transformation, thereby exerting a profound influence on the environmental fate of sulfonamide antibiotics in polar and cold regions.