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 (NO), 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 NO on sulfamethoxazole (SMX) photodegradation at 222 nm, finding that NO 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 NO 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. N isotope labeling experiments showed that the RNS-induced nitrated products even became the major products of SMX in the 222 nm far-UVC/NO 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 NO-containing water.