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 Cl 2 ·L -1 dichloramine (NHCl 2 ), NDMA, nitrous oxide (N 2 O), dissolved oxygen (DO), NHCl 2 , and monochloramine (NH 2 Cl) were kinetically quantified. NHCl 2 , N 2 O, and DO profiles indicated that reactive nitrogen species (RNS) formed during NHCl 2 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 NHCl 2 , NH 2 Cl, NDMA, and DO concentrations and included (1) the unified model of chloramine chemistry revised with HNO as a direct product of NHCl 2 hydrolysis; (2) HNO/NO - then reacting with (i) HNO to form N 2 O, (ii) DO to form ONOOH/ONOO - , or (iii) NHCl 2 or NH 2 Cl 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 NHCl 2 and DMA. Overall, updated NHCl 2 decomposition pathways are proposed, yielding (1) RNS via N H Cl 2 → H N O / N O -→O 2O N O O H / O N O O - and (2) NDMA via O N O O H / O N O O -→ UDMH - Cl or DMA NDMA .