Probing nitro(so) and chloro byproducts and their precursors in natural organic matter during UV/NH 2 Cl treatment by FT-ICR MS with machine learning insights

The UV/monochloramine (UV/NH 2 Cl) process, while efficiently eliminating micropollutants, produces toxic byproducts. This study utilized Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to investigate molecular-level changes in natural organic matter (NOM) and to disclose formation pathways of nitro (so) and chloro byproducts in the UV/NH 2 Cl process. The UV/NH 2 Cl process significantly increased the saturation and oxidation levels and altered the elemental composition of NOM. Using 15 N labeling and a screening workflow, nitro(so) byproducts with nitrogen originating from inorganic sources (i.e., reactive nitrogen species (RNS) and/or NH 2 Cl) were found to exhibit total intensities comparable to those from NOM. RNS, rather than NH 2 Cl, played a significant role in incorporating nitrogen into NOM. Through linkage analysis, nitro(so) addition emerged as an important reaction type among the 25 reaction types applied. By using phenol as a representative model compound, the nitro byproducts were confirmed to be mainly generated through the oxidation of nitroso byproducts instead of nitration. Machine learning and SHAP analysis further identified the major molecular indices distinguishing nitro(so) and chloro precursors from non-precursors. This study enhances our fundamental understanding of the mechanisms driving the generation of nitro(so) and chloro byproducts from their precursors in complex NOM during the UV/NH 2 Cl process.