Updated Reaction Pathway for Dichloramine Decomposition: Formation of Reactive Nitrogen Species and...

The N-nitrosodimethylamine (NDMA) formation pathway in chloraminated drinking water remains unresolved. In pH 7-10 waters amended with 10 microM total dimethylamine and 800 microeq 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 $NH{\mathrm{Cl}}_2\to HNO/N{O}^{-}\stackrel{O_2}{\to }ONOOH/ONO{O}^{-}$ and (2) NDMA via $ONOOH/ONO{O}^{-}\stackrel{\mathrm{UDMH}-\mathrm{Cl}\mathrm{or}\mathrm{DMA}}{\to }\mathrm{NDMA}$.