Various disinfection byproducts (DBPs) form during the process of chlorination disinfection,posing potential threats to drinking water safety and human health.Sulfamethazine (SMT),the most commonly used and frequently detected veterinary antibiotic,was investigated in detail with regard to its transformation and kinetics in reactions with free available chlorine (FAC).Using liquid chromatography coupled to quadrupole time-of-flight tandem mass spectrometry,several DBPs were identified based on different confidence levels,and a variety of reaction types,including desulfonation,S-N cleavage,hydroxylation,and chlorine substitution,were proposed.The kinetic experiments indicated that the reaction rate was FAC-and pH-dependent,and SMT exhibits low reactivity toward FAC in alkaline conditions.The DBPs exhibited a much higher acute toxicity than SMT,as estimated by quantitative structure activity relationship models.More importantly,we observed that the FAC-treated SMT reaction solution might increase the genotoxic potential due to the generation of DBPs.This investigation provides substantial new details related to the transformation of SMT in the chlorination disinfection process.