Novel insights into the photochemical transformation of neonicotinoid insecticides: Potential involvement of adjuvants

Abstract

Commercial pesticide formulations often contain both the active ingredient and different adjuvants, the potential impacts of adjuvants on pesticides’ transformation have been barely considered. In this study, effects of typical adjuvants (i.e. hexadecyl trimethyl ammonium bromide (CTAB), hexadecyl trimethyl ammonium chloride (CTAC) and p-Octyl polyethylene glycol phenyl ether (TX-100)) on the photodegradation of four selected neonicotinoid insecticides (NIs), including thiamethoxam (TMX), dinotefuran (DIN), imidacloprid (IMP) and acetamiprid (ACP) were systematically investigated. Specifically, CTAB and CTAC exhibited minimal or no impact on the photolysis of four NIs, while TX-100 significantly inhibited their decay. Such inhibitory effect could mainly be attributed weakening the formation of key transient intermediates, such as the corresponding radical cation (NI•+), radical anion (NI•-) as well as hydrated electron (eaq-). Photoproducts identification revealed a novel transformation pathway of NIs in the presence of TX-100, characterized by de-chain reactions, resulting in the formation of photostable dimeric photoproducts. These dimers exhibited varying toxicity: for TMX, DIN and IMP, their corresponding dimers showed increased toxicity compared to their parent compounds, whereas for ACP, they displayed decreased toxicity. These provide important information for the evaluation the environmental fates and toxicology of formulaic NIs.