UV induced sulfamethazine and carbamazepine elimination in the presence of nitrate: Roles of reactive species and generation risk of highly toxic DBPs.

This study systematically compared the degradation kinetics, conversion pathways, formation of disinfection by-products (DBPs), and changes in toxicity for sulfamethazine and carbamazepine in UV/nitrate system. Additionally, the study simulated the generation of DBPs in the post-chlorination process after the introduction of bromine ions (Br). The contributions of UV irradiation, hydroxyl radicals (OH), and reactive nitrogen species (RNS) to SMT degradation were determined to be 28.70 %, 11.70 %, and 59.60 %, respectively. The contributions of UV irradiation, OH, and RNS to CBZ degradation were found to be 0.00 %, 96.90 %, and 3.10 %, respectively. A higher dosage of NO facilitated the degradation of both SMT and CBZ. Solution pH posed almost no effect on SMT degradation, while acidic conditions favored CBZ removal. The degradation of SMT was found to be slightly promoted at low concentrations of Cl, while the presence of HCO significantly accelerated the degradation. Cl, as well as HCO, retarded the CBZ degradation. Natural organic matter (NOM) as a free radical scavenger and UV irradiation filter posed a substantial inhibitory effect on the degradation of SMT and CBZ. The degradation intermediates and transformation pathways of SMT and CBZ by UV/NO system were further elucidated. The results showed that the main reaction pathways were bond-breaking reaction, hydroxylation, and nitration/nitrosation reaction. The acute toxicity of most of the intermediates generated during SMT and CBZ degradation was reduced after UV/NO treatment. After treatment of SMT and CBZ in UV/nitrate system, the DBPs generated in subsequent chlorination were mainly trichloromethane and a small amount of nitrogen-containing DBPs. After bromine ions were introduced in UV/NO system, a large amount of the originally generated trichloromethane was converted to tribromomethane.