Freeze-induced degradation of organic micropollutants by chlorine

Freeze-accelerated reactions mainly arise from the freeze-concentration effect, where solutes are enriched in unfrozen liquid channels between ice crystals. While chlorine (HOCl) is more abundant than iodine and bromine in environments, its reactivity under freezing conditions remains largely unexplored. This study investigates the degradation of carbamazepine (CBZ) by HOCl in ice. Under pH 6.5 at −25 ℃, approximately 90 % of CBZ was degraded within 10 min, whereas negligible degradation occurred at room temperature, attributable to increased HOCl concentration in the freeze-concentrated solution. The roles of reactant concentration, pH, freeze-thaw cycling, temperature, and coexisting substances were systematically evaluated. Humic acid and nitrite inhibited CBZ degradation by scavenging HOCl, while chloride enhanced it via Cl 2 formation. Fourteen degradation products were identified with pathways involving deamidation, hydroxylation, decarboxylation, etc. Ecological Structure Activity Relationships (ECOSAR)-based toxicity predictions revealed that while some products were less toxic than CBZ, others exhibited comparable or higher chronic toxicity. This study establishes a freeze-induced oxidative degradation pathway for organic micropollutants by HOCl, highlighting its environmental relevance in cold regions such as frozen water infrastructure, atmospheric droplets, and ice-containing systems with residual chlorine. The findings underscore the importance of evaluating both degradation efficiency and by-product toxicity under freeze-induced conditions.