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

Abstract

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 and (2) NDMA via .

Review of chloramine decay models in drinking water system

“…12 They suggested that the autodecomposition model can adequately predict the monochloramine decay with NOM concentration below 3.5 mg L −1 . Similarly, Pham et al 46 validated the autodecomposition model for different pH conditions and found to accurately simulate the total chlorine, monochloramine and dichloramine profiles for pH range of 7-9. However, there was slight mismatch of simulated monochloramine profile at pH of 10.…”

“…However, for the same reaction, Morris and Isaac 40 provided a different rate coefficient which was 1.5 × 10 10 M −1 h −1 . The reactions and rate coefficients for reaction (7) and ( 8) were further revised by Pham et al 46 by assuming the nitroxyl (HNO) to be the unidentified product. The revised reactions are:…”

“…However, at pH of 9 and 10, the model underpredict the measured NDMA concentration, indicating that the reaction kinetics in the proposed pathway were not robust. Considering this limitation, Pham et al 46 proposed additional pathways for NDMA formation that adequately simulated the monochloramine, dichloramine, NDMA and DO concentrations within the pH range of 7–10. In the proposed pathway, HNO and nitroxyl anion (NO − ) are initially formed through dichloramine hydrolysis.…”

Emerging investigator series: contributions of reactive nitrogen species to transformations of organic compounds in water: a critical review

“…The NDMA yield also decreased by approximately 50% in the presence of uric acid at pH 9. 122 These ndings indicate that RNS generated by dichloramine decomposition may play a role in N-DBP formation, although the relevance compared to other reaction pathways likely depends on conditions including pH and the concentration of dissolved oxygen. -derived RNS reactions are unlikely to contribute signicantly to organic contaminant transformation compared to reactions with ozone and cOH.…”

“…Recently, RNS from dichloramine decomposition were implicated in the formation of N-nitrosodimethylamine (NDMA), a highly toxic N-DBP. 122 Under typical drinking water chloramination conditions, NDMA formation is thought to occur via the reaction of dichloramine with amine precursors. 123,124 In a parallel reaction pathway, dichloramine hydrolysis produces nitroxyl (HNO), which reacts with dissolved oxygen to form peroxynitrous acid (ONOOH).…”