Reactive\nNitrogen Species Generated from Far-UVC Photolysis\nof Nitrate Contribute to Pesticide Degradation and Nitrogenous Byproduct\nFormation

Climate change has resulted in increased\nuse of pesticides and\nfertilizers in agriculture, leading to elevated pesticide and nitrate\nlevels in aquatic ecosystems that receive agricultural runoff. In\nthis study, we demonstrate that far-UVC (UV<sub>222</sub>) photolysis\nof nitrate rapidly degrades four pesticides in surface water, with\na degradation rate constant 37.1–144.75 times higher than that\nachieved by UV<sub>254</sub> photolysis of nitrate. The improved pesticide\ndegradation is due not only to the enhanced direct photolysis by UV<sub>222</sub> compared to UV<sub>254</sub> but also to the increased\ngeneration of hydroxyl radicals (HO<sup>•</sup>) and reactive\nnitrogen species (e.g., NO<sub>2</sub><sup>•</sup> and ONOO<sup>–</sup>) in the UV<sub>222</sub>/nitrate process. We determined\nthe innate quantum yields of nitrate photolysis at 222 nm and incorporated\nthese values into a kinetic model, allowing for the accurate prediction\nof nitrate photodecay and reactive species generation. While reactive\nnitrogen species predominantly contribute to pesticide degradation\nin the UV<sub>222</sub>/nitrate process, they also lead to the formation\nof nitration byproducts. Using stable isotope-labeled nitrate (<sup>15</sup>NO<sub>3</sub><sup>–</sup>) combined with mass spectrometry,\nwe confirmed that the nitration byproducts are formed from the reactive\nnitrogen species generated from nitrate photolysis. Additionally,\nwe demonstrate that the UV<sub>222</sub>/nitrate process increases\nthe formation potential of highly toxic nitrogenous chlorinated products\n(e.g., trichloronitromethane) during postchlorination in real surface\nwater.