Unravelling molecular transformation of dissolved organic matter in UV/Dichloroisocyanurate, UV/Chlorine, and UV/Chloramine processes using high Resolution mass spectrometry

This study utilized Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to investigate the molecular-level transformation of dissolved organic matter (DOM) during treatment with the novel UV/dichloroisocyanurate (NaDCC) process. Comparisons were made with conventional UV/hypochlorite (NaClO) and UV/monochloramine (NH 2 Cl) processes to assess the feasibility and environmental risks of UV/NaDCC. Results showed that UV/NaDCC and UV/NaClO had comparable and stronger DOM mineralization effects than UV/NH 2 Cl. Notably, UV/NaDCC treatment more effectively reduced DOM aromaticity, fluorescence, and apparent molecular weight, producing more oxidized and saturated molecules. Kendrick mass defect (KMD) analysis indicated that demethylation and carboxylation reactions played key roles in UV/NaDCC treatment, supported by a greater number of precursor-product pairs. The UV/NaDCC system produced substantial chlorinated and N-containing byproducts (Cl-BPs and N-BPs). Among the Cl-BPs, species formed via + 2Cl reaction ( e. g. , +H 2 Cl 2 O 2 , +Cl 2 O) were dominant, similar to the UV/NaClO system. The N-BPs with higher nitrogen content ( e. g. , N3 and N4) were more abundant than in UV/NH 2 Cl. The UV/NaDCC system produced the highest concentrations of TCM and DCAN, whereas the UV/NaClO system yielded the highest concentration of DCAA. Toxicity tests revealed that UV/NaDCC effluent displayed the highest toxicity. To the best of our knowledge, this study provides the first molecular-level insight into DOM transformation in UV/NaDCC treatment, offering theoretical and practical guidance for its application.