Chlorination of methotrexate in water revisited: Deciphering the kinetics, novel reaction mechanisms, and unexpected microbial risks.

Chlorination of a typical anticancer drug with annually ascending use and global prevalence (methotrexate, MTX) in water has been studied. In addition to the analysis of kinetics in different water/wastewater matrices, high-resolution product identification and in-depth secondary risk evaluation, which were eagerly urged in the literature, were performed. It was found that the oxidation of MTX by free available chlorine (FAC) followed first-order kinetics with respect to FAC and first-order kinetics with respect to MTX. The pH-dependent rate constants (k) ranged from 170.00 M s (pH 5.0) to 2.68 M s (pH 9.0). The moiety-specific kinetic analysis suggested that 6 model substructures of MTX exhibited similar reactivity to the parent compound at pH 7.0. The presence of Br greatly promoted MTX chlorination at pH 5.0-9.0, which may be ascribed to the formation of bromine with higher reactivity than FAC. Comparatively, coexisting I or humic acid inhibited the degradation of MTX by FAC. Notably, chlorination effectively abated MTX in different real water matrices. The liquid chromatography-high resolution mass spectrometry analysis of multiple matrix-mediated chlorinated samples indicated the generation of nine transformation products (TPs) of MTX, among which seven were identified during FAC oxidation for the first time. In addition to the reported electrophilic chlorination of MTX (the major and dominant reaction pathway), the initial attacks on the amide and tertiary amine moieties with C-N bond cleavage constitute novel reaction mechanisms. No genotoxicity was observed for MTX or chlorinated solutions thereof, whereas some TPs were estimated to show multi-endpoint aquatic toxicity and higher biodegradation recalcitrance than MTX. The chlorinated mixtures of MTX with or without Br showed a significant ability to increase the conjugative transfer frequency of plasmid-carried antibiotic resistance genes within bacteria. Overall, this work thoroughly examines the reaction kinetics together with the matrix effects, transformation mechanisms, and secondary environmental risks of MTX chlorination.