Structural dependent degradation of histamine H₂-receptor antagonists by UV/NH₂Cl: Reactive species contribution and the role of carbonate ions on ·NO generation

Histamine H₂-receptor antagonists (HRAs) are a group of widely used gastrointestinal drugs that have been frequently detected in natural environments. UV/chloramine (NH₂Cl) is a widely used disinfection process in water treatment, while the performance of HRAs degradation by UV/NH₂Cl is still unclear. In this work, the degradation kinetics, mechanism and disinfection byproducts formation of HRAs by UV/NH₂Cl were studied. The degradation of HRAs, including ranitidine (RNTD), nizatidine (NZTD), roxatidine (RXTD), cimetidine (CMTD) and famotidine (FMTD) was fast in the UV/NH₂Cl process, and ·OH, Cl· and ·NO contributed to their degradation. CMTD had the highest rate constants with ·OH, Cl· and ·NO. RXTD, NZTD and FMTD were more reactive with ·OH, while RNTD was more selective towards Cl·. The contribution of ·OH, Cl· and reactive nitrogen species to HRAs degradation in water matrices was further quantified by kinetic modeling. Interestingly, carbonate species were found to promote the yield of ·NO. Further investigation through sensitivity analysis of the modeling reactions concluded the sensitive reactions for ·NO promotion. The structure–activity assessment domenstrated that the reactivity of thioether sulfur, CMTD-C₃ and RXTD-N₄₈ and piperidine rings resulted in the difference of degradation rates. Hydroxylation, nitrosation, chlorine substitution and oxygen atom transfer were the primary pathways for HRAs degradation. The formation of NDMA might be a prevalent problem along HRAs’ degradation in the UV/NH₂Cl process, and the formation potential of NDMA is dependent on the types of amine groups in HRAs. This work provides an effective strategy to eliminate HRAs, revealed the NDMA formation potential in HRAs, and emphasized the role of carbonate species in ·NO generation.