Breakpoint chlorination can be commonly observed in the chlorination of water treatments when ammonia is present. Recently, the simultaneous removal of recalcitrant micropollutants during breakpoint chlorination process has attracted attention. In this study, the major intermediates in the process of breakpoint chlorination for degradation of PPCPs was identified. The degradation of the four model PPCPs (ibuprofen (IBU), tinidazole (TNZ), N, N-Diethyl-3-methyl benzoyl amide (DEET) and ketoprofen (KETO)) at different pH in chlorination ammonia process were simulated, and it was found that the consumption of chlorine in the reaction of dichloramine and hypochlorous acid was positively correlated with the degradation of PPCPs, while the consumption of chlorine in other reactions was very small and insufficient to degrade PPCPs. Therefore, the elementary reactions of hypochlorous acid and dichloramine in Cl/N system were developed in this study, which described the nucleophilic substitution reactions of HOCl with NH<sub>2</sub>Cl and the chlorinated derivatives of NH<sub>2</sub>Cl, and the corresponding hydrolysis reactions. The spontaneity of the reactions was verified by the calculation of Gibbs free energy. Nitrosyl chloride (ClNO) was identified as the highly active intermediate, which has been shown to degrade PPCPs. A certain concentration of ClNO could directly degrade IBU. Interestingly, gold was dissolved in the process of breakpoint chlorination, which indirectly demonstrated ClNO was produced in this process. During the chlorination with ammonia process, the structure of NOM was largely transformed, especially the molecular formula containing N atoms, and less Cl-DBPs were generated compared to chlorination process. Overall, this study presented mechanistic insights on degradation of different PPCPs in the process of breakpoint chlorination.

New insight into the degradation mechanism of PPCPs in the process of breakpoint chlorination

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