The oxidative effect of duckweed pond DOM and its mechanism in the photodegradation of neonicotinoid insecticides.

Neonicotinoid pesticides (NNIs) are widely used worldwide and commonly detected in natural aquatic systems and in engineered systems, including urban or agriculture ponds. We investigated the photoreactivity of DOM and the photodegradation of NNIs in duckweed ponds (DWP) water under simulated sunlight. The molecular composition of DOM in DWP water typically contained more than 65% bulk compositional measurements of lignin-like, terrestrially derived molecules, revealed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). These compounds serve as critical candidates for the oxidizing ³DOM* in the DWP water. The steady-state concentrations of triplet-excited state DOM (³DOM*), singlet oxygen (¹O₂) and hydroxyl radical (^·OH) were measured 6.00 × 10^-14 M, 5.42 × 10^-13 M, and 8.33 × 10^-15 M, respectively, under 5 mg/L [DOC]. Enhanced removal rates of thiamethoxam (TMX) and dinotefuran (DIN) in the irradiated DWP water relative to the purity water demonstrated the importance of indirect photolysis pathways involving photochemically produced ³DOM*and ¹O₂. The findings demonstrated that in DWP water, the photolysis rate constants (k) of TMX and DIN increased to 0.3573 h^-1 and 0.3237 h^-1, respectively. Degradation of imidacloprid was not significantly promoted through the photochemical production of ³DOM* and ¹O₂. Results from this study underscore the role of DOM as photosensitizer in limiting the persistence of NNIs in duckweed ponds through photochemical reactions.