Degradation of Organic Micropollutants in UV/NH₂Cl Advanced Oxidation Process

Monochloramine (NH₂Cl) can be irradiated by UV to create an advanced oxidation condition (i.e., UV/NH₂Cl) for the elimination of organic micropollutants (OMPs) from source water. However, information in retrospective studies was scarce on how UV/NH₂Cl performance would be affected by the water matrix and OMP molecular structures. In this study, the degradation of five representative OMPs, including triclosan, carbamazepine, sulfamethoxazole, estradiol (E2), and ethinylestradiol (EE2), was examined in different water matrices. All OMPs were rapidly removed by UV/NH₂Cl but exhibited different degradation mechanisms. Although •OH, •Cl, and direct photolysis mainly contributed to the overall degradation of OMPs in buffered nanopure water, the contribution of reactive nitrogen species (RNS) generated from the photolysis of NH₂Cl was not negligible in the degradation of E2 and EE2. A phenolic group was identified as the moiety reactive toward RNS. Based on quantitative analysis of the impact on OMP degradation from cosolutes (including Cl^–, HCO₃^–, NOM) as well as pH and NH₂Cl doses, we developed a kinetic model for the prediction of OMP degradation in complex water matrices. In environmental water matrices, the performance and radical contributions in UV/NH₂Cl and UV/H₂O₂ systems were taken into comparison, which showed faster degradation of OMPs and a more significant contribution of CO₃^•– in the UV/NH₂Cl process.