Freezing-enhanced peracetic acid oxidation of 4-chlorophenol in the presence of bromide: Kinetics, free-bromine formation, and disinfection by-products

This study demonstrates the efficacy of the peracetic acid (PAA)/bromide (Br - ) system for degrading 4-chlorophenol (4-CP) under freezing conditions, highlighting both performance and associated byproduct risks. Freezing significantly enhanced 4-CP removal across a range of Br - concentrations, driven by freeze-concentration effects. The reaction followed pseudo-first-order kinetics, with the rate constant ( k ) increasing with Br - concentrations. This enhancement is attributed to solute enrichment within the unfrozen brine phase, which promotes bromine activation and accelerates oxidation kinetics. Mechanistic analyses indicate that free bromine specie (HOBr) is responsible for 4-CP transformation, leading to the formation of brominated intermediates. In addition to enhanced contaminant removal, freezing also promoted the formation of brominated disinfection by-products (DBPs), including haloacetic acids (HAAs) and trihalomethanes (THMs). Notably, the dichloroacetic acid (DCAA) yields increased more than twentyfold compared to room temperature. These results demonstrate a dual effect of freezing in bromide-containing systems, simultaneously enhancing pollutant degradation and DBP formation. The findings highlight the importance of considering freezing-induced chemical processes in cold-region water treatment and environmental risk assessment.