Kinetics, pathways and toxicity evaluation of neonicotinoid insecticides degradation via UV/chlorine process

Abstract

Although UV/chlorine advanced oxidation process has been employed to treat contaminants, the roles of secondary radicals are generally negligible. The comprehensive evaluation of secondary radicals on contaminants degradation remains deficient. In this study, two typical neonicotinoid insecticides, imidacloprid (IMD) and thiacloprid (THIA), were degraded via UV/chlorine process. It is for the first time found that the existence of Cl− significantly promoted THIA degradation during UV/chlorine process, attributed to the formation of Cl2−. In addition, the effects of pH, fulvic acid (FA), and bicarbonate (HCO3–) on the degradation of IMD and THIA were evaluated. Low pH was in favor of the degradation of IMD and THIA, while both FA and HCO3– had slight inhibition effects. Meanwhile, the degradation of IMD and THIA via UV/chlorine process was inhibited in real matrices. The existence of Br− heavily suppressed the degradation of THIA via UV/chlorine process. The initial transformation pathways of IMD and THIA involved electron transfer, H-abstraction, and then disproportionation or the methylene bridge cleavage, yielding hydroxyl and keto derivatives as well as free amine. Although the mineralization for IMD (29%) and THIA (35%) was low, the acute toxicity to Vibrio fischeri for the treated samples was lower than that for corresponding parent compounds. This study comprehensively evaluated the degradation of IMD and THIA via UV/chlorine process, highlighting Cl2− as an important reactive species for THIA elimination.