Updated Reaction Pathway for Dichloramine Decomposition: Formation of Reactive Nitrogen Species and N‑Nitrosodimethylamine

The N-nitrosodimethylamine (NDMA) formation pathway in chloraminated drinking water remains unresolved. In pH 7–10 waters amended with 10 μM total dimethylamine and 800 μeq Cl₂·L^–1 dichloramine (NHCl₂), NDMA, nitrous oxide (N₂O), dissolved oxygen (DO), NHCl₂, and monochloramine (NH₂Cl) were kinetically quantified. NHCl₂, N₂O, and DO profiles indicated that reactive nitrogen species (RNS) formed during NHCl₂ decomposition, including nitroxyl/nitroxyl anion (HNO/NO^–) and peroxynitrous acid/peroxynitrite anion (ONOOH/ONOO^–). Experiments with uric acid (a ONOOH/ONOO^– scavenger) implicated ONOOH/ONOO^– as a central node for NDMA formation, which were further supported by the concomitant N-nitrodimethylamine formation. A kinetic model accurately simulated NHCl₂, NH₂Cl, NDMA, and DO concentrations and included (1) the unified model of chloramine chemistry revised with HNO as a direct product of NHCl₂ hydrolysis; (2) HNO/NO^– then reacting with (i) HNO to form N₂O, (ii) DO to form ONOOH/ONOO^–, or (iii) NHCl₂ or NH₂Cl to form nitrogen gas; and (3) NDMA formation via ONOOH/ONOO^– or their decomposition products reacting with (i) dimethylamine (DMA) and/or (ii) chlorinated unsymmetrical dimethylhydrazine (UDMH-Cl), the product of NHCl₂ and DMA. Overall, updated NHCl₂ decomposition pathways are proposed, yielding (1) RNS via NHCl2→HNO/NO−→O2ONOOH/ONOO− and (2) NDMA via ONOOH/ONOO−→UDMH−ClorDMANDMA.