Kinetics and Mechanistic Investigation of the Photocatalytic Degradation of Clothianidin

: Clothianidin was selected as a model compound for kinetics and mechanistic investigation of the photocatalytic degradation of nicotine insecticides. We assessed the feasibility of the degradation process, and systematically explored the effects of catalyst amount, reaction temperature, substrate concentration, initial solution pH value, and the presence of inorganic anions and cations on the reaction. The results revealed a Langmuir-Hinshelwood interaction of pseudo-first-order kinetics, showing a rate constant of 0.0506 min-1. The photocatalytic degradation rate was the highest when the amount of catalyst (TiO2) was 3 g·L-1. It was observed that an increase of reaction temperature or lowering of substrate concentration was beneficial to clothianidin degradation. Additionally, the initial pH value of the solution had an obvious effect on the degradation process. Strong acid and alkali were not conducive and the degradation rate constant was the largest at pH=5. Furthermore, most of the inorganic anions and cations significantly hindered the degradation of clothianidin. Finally, the degradation intermediates were identified by GC/MS, and the reaction pathways at the initial stage of the photocatalytic process were proposed:hydroxylation of parent compound, cleavage of N-N bond in the nitroguanidine group, and cleavage of the C-N bond connected to the nitroguanidine and thiazole rings.