The Impact of Dissolved Organic Matter on Photodegradation Rates, Byproduct Formations, and Degradation Pathways for Two Neonicotinoid Insecticides in Simulated River Waters

The influences of dissolved organic matter (DOM) on neonicotinoid photochemical degradation and product formation in natural waters remain unclear, potentially impacting the sustainability of river systems. Therefore, our overall objective was to investigate the photodegradation mechanisms and phototransformation byproducts of two neonicotinoid pesticides, imidacloprid and thiamethoxam, under simulated sunlight at the microcosm scale, to assess the implications of DOM for insecticide degradation in rivers. Direct and indirect photolysis were investigated using twelve water matrices to identify possible reaction pathways with two DOM sources and three quenching agents. Imidacloprid, thiamethoxam, and potential degradants were measured, and reaction pathways identified. The photodegradation rates for imidacloprid (0.156 to 0.531 h⁻¹) and thiamethoxam (0.027 to 0.379 h⁻¹) were measured. The Mississippi River DOM with 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy resulted in rapid formation of imidacloprid desnitro and imidacloprid urea as compared to other treatments. These observations indicate that the production of reactive oxygen species has the potential to influence the photodegradation of imidacloprid, via indirect photolysis, resulting in the formation of degradation products (e.g., imidacloprid desnitro) potentially harmful to non-target species. The findings offer insight into the potential role DOM in river systems has on sustainable water quality related to these two neonicotinoid degradation pathways and byproduct formations.