Reaction mechanisms of main free radicals (HO•, Cl• and ClO•) with carbamazepine in the UV/Chlorine process

Carbamazepine (CBZ) is a ubiquitous persistent micropollutant in water. This study evaluated the degradation of CBZ by UV/Chlorine process. The influencing factors, such as chlorine dosage, solution pH, and water matrix constituents (Cl − , HCO 3 – and NOM) on CBZ removal were systematically investigated. The reactive HO•, Cl• and ClO• species were mainly responsible for the degradation of CBZ, and their contributions determined by probes (Benzoic acid, Nitrobenzene and 1,4-Dimethoxybenzene) varied with pH. Theoretical quantum calculations involving a total of 79 transition states suggested that HO• attack CBZ by hydrogen atom transfer (HAT) and radical adduct formation (RAF) mechanisms, while Cl• and ClO• proceed mainly through HAT and RAF mechanism, respectively. The reaction rates between CBZ and free radicals were calculated by evaluating the energy barriers and transition states. Furthermore, the proposed formation pathways and reaction mechanisms of main hydroxylated products and chlorine substituted products were unequivocally authenticated by transition states calculations. Overall, this study combined experimental and computational approaches to provide an in-depth degradation mechanism of CBZ in UV/Chlorine process.