New Route to Toxic Nitro and Nitroso Products upon Irradiation of Micropollutant Mixtures Containing Imidacloprid: Role of NO_<i>x</i> and Effect of Natural Organic Matter

Abstract

In this study, we reveal the capacity of imidacloprid (a neonicotinoid insecticide) to photoinduce the nitration and nitrosation of three aromatic probes (phenol, resorcinol, and tryptophan) in water. Using a gas-flow reactor and a NO_<i>x</i> analyzer, the production of gaseous NO/NO₂ was demonstrated during irradiation (300-450 nm) of imidacloprid (10^-4 M). Quantum calculations showed that the formation of NO_<i>x</i> 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 to range between 0.08 and 5.1%. The contribution of NO₃^-/NO₂^- to the nitration and nitrosation processes was found to be minor under our experimental conditions. In contrast, the addition of natural organic matter (NOM) significantly enhanced the yields of nitro/nitroso derivatives, likely via the production of triplet excited states (³NOM*) and HO^•. These findings reveal the importance of investigating the photochemical reactivity of water contaminants in a mixture to better understand the cocktail effects on their fate and toxicity.