Triplet chromophoric dissolved organic matter regulating the phototransformation and toxicity of imidacloprid in paddy water

Imidacloprid (IMD) is a neonicotinoid pesticide used ubiquitously in rice cultivation, but its photo-transformation process in rice fields is largely unknown. This study examined the photo-transformation of IMD during rice growth. The degradation rate of IMD was enhanced by 10.5–15.2 times in the presence of different reactive intermediates (RIs) including triplet chromophoric dissolved organic matter (3CDOM*), hydroxyl radicals (•OH), and singlet oxygen (1O2) generated from paddy water under sunlight. The 3CDOM* was the dominant RI and contributed to 88–92% of IMD degradation with a second order reaction rate constant of 3.77 × 108 M−1 s−1, while 1O2 and •OH only contributed 5.7–7.6% and 0.1–0.3% of IMD degradation with second order reaction rate constants of 3.78 × 106 M−1 s−1 and 2.52 × 109 M−1 s−1, respectively. The products of IMD photo-transformation and their potential toxicity were evaluated. We found that the primary denitrified intermediate is highly toxic, and cleavage and ring-opening products are harmful to aquatic animals. In contrast, the hydroxylation and further oxidation pathway associated with 3CDOM* and •OH are detoxification processes for IMD. An APEX model was developed to predict the photodegradation kinetics of IMD in paddy waters at different stages of rice growth. The IMD photodegradation rate is expected to gradually decline with time since rice cultivation, ascribing to the decline of RIs as influenced by the change of DOM characteristics and NO2– concentrations. The findings of this study provide valuable information for understanding the transformation and potential risk of neonicotinoid insecticide during rice growth.