Enhanced freezing-induced carbamazepine degradation by bromate under solar irradiation via the formation of hypobromous acid and hydroxyl radical

Ice is a crucial medium in cold regions and plays an important role in the transformation of pollutants. When waters receiving treated wastewater freeze in cold regions during winter, the emerging contaminant carbamazepine (CBZ) and the disinfection by-product bromate (BrO3-) can coexist in ice. However, their interaction in ice remains poorly understood. Here, CBZ degradation by BrO3- in ice was investigated via a simulation experiment. Results showed that 96% of CBZ was degraded by BrO3- after 90 min in ice in dark, while the degradation was negligible in water. The time required for nearly 100% CBZ degradation by BrO3- in ice under solar irradiation was 22.2% shorter than in dark. The production of hypobromous acid (HOBr) was responsible for the gradually accelerated CBZ degradation rate in ice. The HOBr generation time in ice under solar irradiation was 50% shorter than in dark. The formation of HOBr and hydroxyl radical by the direct photolysis of BrO3- under solar irradiation enhanced the CBZ degradation in ice. CBZ was mainly degraded by deamidation, decarbonylation, decarboxylation, hydroxylation, molecular rearrangement, and oxidation reactions. Further more, 18.5% of degradation products exhibited lower toxicity than their parent CBZ. This work can provide new insights into the environmental behaviors and fate of emerging contaminants in cold regions.