Nitrate Enhanced\nSulfamethoxazole Degradation by 222\nnm Far-UVC Irradiation: Role of Reactive Nitrogen Species

The application of 222 nm far-UVC irradiation for degrading\norganic\nmicropollutants in water shows promise. Nitrate (NO<sub>3</sub><sup>–</sup>), found in nearly all water bodies, can significantly\nimpact the performance of 222 nm far-UVC-driven systems. This work\nwas the first to investigate the effect of NO<sub>3</sub><sup>–</sup> on sulfamethoxazole (SMX) photodegradation at 222 nm, finding that\nNO<sub>3</sub><sup>–</sup> significantly enhances SMX degradation\nin different dissociated forms. Besides the hydroxyl radical (<sup>•</sup>OH), reactive nitrogen species (RNS) also played important\nroles in SMX degradation. With increasing NO<sub>3</sub><sup>–</sup> concentration, the RNS contribution to SMX degradation decreased\nfrom 25.7 to 8.6% at pH 3 but increased from 1.5 to 24.7% at pH 7,\nsince the deprotonated SMX with electron-rich groups reacted more\neasily with RNS. The transformation mechanisms of SMX involving isomerization,\nbond cleavage, hydroxylation, nitrosation, and nitration processes\nwere proposed. <sup>15</sup>N isotope labeling experiments showed\nthat the RNS-induced nitrated products even became the major products\nof SMX in the 222 nm far-UVC/NO<sub>3</sub><sup>–</sup> system\nat pH 7 and exhibited a higher toxicity than SMX itself. Further research\nis necessary to avoid or eliminate these toxic byproducts. This study\nprovides valuable insights for guiding the utilization of 222 nm far-UVC\nfor treating antibiotics in NO<sub>3</sub><sup>–</sup>-containing\nwater.