Far UVC photolysis of PDS and HO in saline water generates disinfection By-products.

Far-UVC-driven (222 nm) advanced oxidation processes (UV/AOPs) have gained increasing attentions for water decontamination owing to their high yields of reactive species and rapid degradation rate constants of emerging contaminants. However, the formation of toxic disinfection by-products (DBPs) during UV/AOP treatments of saline water remains unexplored. In this study, we demonstrated that DBPs generated in UV/PDS and UV/HO treatment of saline water with the presence of 0.5 M Cl was 3.55-4.50 and 1.47-1.61 times higher, respectively, compared to those generated in UV/PDS and UV/HO. Adsorbable organic halogen (AOX) generated in UV/AOPs was also higher than UV/AOPs. Notably, DBPs and AOX generated from Suwannee River fulvic acid (SRFA) in UV/PDS treatment were 32.33 times and 6.14 times higher than in UV/HO, primarily due to the elevated concentration of reactive halogen species and the accumulation of hypochlorous acid/ hypohalite. Moreover, the presence of Br⁻ in saline water led to the conversion of chlorinated DBPs into brominated DBPs in both UV/PDS and UV/HO, significantly increasing the overall toxicity associated with DBPs. Increasing the pH from 5.0 to 9.0 was found to reduce DBP-associated toxicity. This study highlights the formation of toxic DBPs in UV/AOPs and provides theoretical foundation for the selection and regulation of UV/AOPs from the perspective of DBP control.