New route to toxic nitro and nitroso products upon irradiation of micropollutants mixtures containing imidacloprid: role of NOx and effect of natural organic matter

International audienceIn this study we reveal the capacity of imidacloprid (a neonicotinoid insecticide) to photoinduce nitration and nitrosation of three aromatic probes (phenol, resorcinol, tryptophan) in water. Using a gas-flow reactor and NOx analyzer, the production of gaseous NO/NO2 was demonstrated during irradiation (300-450 nm) of imidacloprid (10-4 M). Quantum calculations showed that formation of NOx proceeds via homolytic cleavage of the RN-NO2 bond in the triplet state. In addition to gaseous NO/NO2, nitrite and nitrate were also detected in water, with the following mass balance: 40±8% for NO2, 2±0.5% for NO, 52±5% for NO3- and 16±2% for NO2-. The formation of nitro/nitroso probe derivatives was evidenced by high resolution mass spectrometry and their yields were found be ranging between 0.08% and 5.1%. The contribution of NO3-/NO2- to the nitration and nitrosation processes was found minor under our experimental conditions. In contrast, the addition of natural organic matter (NOM) enhanced significantly the yields of nitro/nitroso derivatives, likely via production of triplet excited states (3NOM*) and HO.. These findings reveal the importance of investigating the photochemical reactivity of water contaminants in mixture to better understand the cocktail effects on their fate and toxicity