Phenylalanine or nitrate as the major nitrogen source? Occurrence and formation of overlooked yet toxic nonhalogenated nitro(so)-byproducts in UV/Chlorine-treated drinking water.

Nitrogenous disinfection byproducts (DBPs) have raised significant concerns due to their relatively high toxicity, yet the fate and toxicity of nonhalogenated nitro(so)-byproducts (NHNBPs) during UV/chlorine treatment in the presence of nitrogen sources remains poorly understood. In this study, 19 NHNBPs were identified by ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry for the first time. Nine among these were detected in UV/chlorine and chlorinated tap water samples at concentrations ranging from 152 to 480 ng/L. Then, the formation pathways of NHNBPs during UV/chlorine treatment with NO3- was investigated using phenylalanine as the model precursor. The results revealed that nitrophenols form via the simultaneous generation of mono-/di-nitrophenols, whereas hydroxy‑mono-/di-nitrobenzoic acids undergo sequential nitration. This mechanistic divergence suggested differing nitrogen sources: Mononitrophenols primarily originated from organic nitrogen, while NO3- was essential for dinitrophenol formation. Both nitrogen sources contributed selectively to hydroxy‑(di)nitrobenzoic acid generation. Cytotoxicity assays using Chinese Hamster Ovary cells showed that the toxicity of NHNBPs was higher than 2,6-dichlorophenol. The developed quantitative structure-activity relationship model demonstrated the importance of cellular uptake efficiency and electrophilic reactivity in the toxicity mechanisms. These findings highlight that previously overlooked nonhalogenated nitro-byproducts should be a concern in drinking water.