Chlorination of Clothianidin During Disinfection: Kinetics, Pathways, and Toxicity

Neonicotinoid pesticides are a typical category of emerging hazardous micropollutants, and chlorine (Cl₂) is a widely used disinfectant that readily induces the chlorination of organic micropollutants. This study systematically investigated the chlorination kinetics and transformation pathways of a representative neonicotinoid pesticide (clothianidin, CLO) and evaluated the cytotoxicity variation via Chinese Hamster Ovary (CHO) cell assays. CLO chlorination followed second-order kinetics, with a first-order dependence on both CLO and Cl₂ concentrations, and the apparent rate constant (k_app) value was measured to be 1.758 × 10^-4 μM^-1 h^-1, at a pH of 7.0. The CLO chlorination initially accelerated and then retarded with the increase in pH. The same tendency was involved in the yield of disinfection byproducts (i.e., trihalomethanes and haloacetic acids). Dissolved organic matter was also a crucial factor inhibiting the chlorination of CLO. The reaction of CLO⁺ with HOCl was more prevalent than between CLO⁺ with ClO^-, wherein HOCl likely exerts electrophilic attack either after 2-nitroguanidine hydrolysis or directly at the nitrogen sites of secondary amines. Cell exposure results revealed that the chronic cytotoxicity of CLO decreased significantly after chlorination. This study helps to the mechanistic understanding of neonicotinoid transformation during water disinfection, and provides a valuable reference for the control of neonicotinoid pesticides in drinking water.