Nitrate Enhanced Sulfamethoxazole Degradation by 222 nm Far-UVC Irradiation: Role of Reactive Nitrogen Species

The application of 222 nm far-UVC irradiation for degrading organic micropollutants in water shows promise. Nitrate (NO₃^–), found in nearly all water bodies, can significantly impact the performance of 222 nm far-UVC-driven systems. This work was the first to investigate the effect of NO₃^– on sulfamethoxazole (SMX) photodegradation at 222 nm, finding that NO₃^– significantly enhances SMX degradation in different dissociated forms. Besides the hydroxyl radical (^•OH), reactive nitrogen species (RNS) also played important roles in SMX degradation. With increasing NO₃^– concentration, the RNS contribution to SMX degradation decreased from 25.7 to 8.6% at pH 3 but increased from 1.5 to 24.7% at pH 7, since the deprotonated SMX with electron-rich groups reacted more easily with RNS. The transformation mechanisms of SMX involving isomerization, bond cleavage, hydroxylation, nitrosation, and nitration processes were proposed. ¹⁵N isotope labeling experiments showed that the RNS-induced nitrated products even became the major products of SMX in the 222 nm far-UVC/NO₃^– system at pH 7 and exhibited a higher toxicity than SMX itself. Further research is necessary to avoid or eliminate these toxic byproducts. This study provides valuable insights for guiding the utilization of 222 nm far-UVC for treating antibiotics in NO₃^–-containing water.