Reactive Nitrogen Species Generated from Far-UVC Photolysis of Nitrate Contribute to Pesticide Degradation and Nitrogenous Byproduct Formation

Climate change has resulted in increased use of pesticides and fertilizers in agriculture, leading to elevated pesticide and nitrate levels in aquatic ecosystems that receive agricultural runoff. In this study, we demonstrate that far-UVC (UV₂₂₂) photolysis of nitrate rapidly degrades four pesticides in surface water, with a degradation rate constant 37.1-144.75 times higher than that achieved by UV₂₅₄ photolysis of nitrate. The improved pesticide degradation is due not only to the enhanced direct photolysis by UV₂₂₂ compared to UV₂₅₄ but also to the increased generation of hydroxyl radicals (HO^•) and reactive nitrogen species (e.g., NO₂^• and ONOO^-) in the UV₂₂₂/nitrate process. We determined the innate quantum yields of nitrate photolysis at 222 nm and incorporated these values into a kinetic model, allowing for the accurate prediction of nitrate photodecay and reactive species generation. While reactive nitrogen species predominantly contribute to pesticide degradation in the UV₂₂₂/nitrate process, they also lead to the formation of nitration byproducts. Using stable isotope-labeled nitrate (¹⁵NO₃^-) combined with mass spectrometry, we confirmed that the nitration byproducts are formed from the reactive nitrogen species generated from nitrate photolysis. Additionally, we demonstrate that the UV₂₂₂/nitrate process increases the formation potential of highly toxic nitrogenous chlorinated products (e.g., trichloronitromethane) during postchlorination in real surface water.