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N-nitrosodimethylamine (NDMA) as the most frequently detected disinfection by-product has aroused widespread concern due to its unusually high carcinogenicity, however, there is still limited understanding of its formation mechanisms. In this study, the formation mechanisms of NDMA from some typical hydrazines and hydrazones with high NDMA conversion yields (60%∼90%) during ozonation, i.e., unsymmetrical dimethylhydrazine (UDMH), 1-formyl-2,2-dimethylhydrazine (FDMH), formaldehyde dimethylhydrazone (FDH) and acetone dimethylhydrazone (ADMH), were investigated by using DFT method with the M05 functional. A new NDMA formation mechanism from hydrazines during ozonation was proposed, in which the initial step is hydrogen abstraction rather than previously reported oxygen addition. For hydrazones, the C atom of the -N = C moiety in hydrazones is preferred to be attacked by ozone to generate N,N-dimethylaminonitrene (DMAN), which is an important intermediate in NDMA formation during ozonation. Moreover, the reactivity order of the following H atoms in hydrogen/hydride ion abstraction (HA) by ozone is -NH2 > -N(CH3)2 > -CO-NH ∼ =C(CH3)2 > =CH-. Additionally, formation pathways of some experimentally detected compounds, i.e., HOOOH, HOOH and HCOH, in the ozonation of hydrazine were elucidated in this study. The results are expected to expand our understanding of NDMA formation mechanisms and ozone reaction characteristics.

Recent studies found that ozonation of organic pollutants with dimethylamino groups produces N-nitrosodimethylamine (NDMA) that is highly carcinogenic to humans. However, the formation mechanism of NDMA remains inexplicit, and previously proposed mechanisms are inconsistent with experimental observations. In this study, the formation mechanism of NDMA in ozonation was explored by density functional theory (DFT) calculations, with dimethylamine (DMA) as a model compound. By calculating Gibbs energies and energy barriers, formation of NDMA in ozonation of DMA was observed to proceed through a hydroxylamine mechanism. The calculation results show that hydroxylamine is generated through DMA reacting with hydroxyl radicals (HO•) formed from hydrolysis of ozone. DMA reacting with hydroxylamine can produce unsymmetrical dimethylhydrazine (UDMH), a well-known NDMA precursor. Transformation of UDMH to NDMA is mainly induced by ozone or HO• rather than dissolved oxygen proposed previously. The reaction of DMA and hydroxylamine is pH dependent, with energy barriers increasing from neutral pH to the second pKa of hydroxylamine and then decreasing. This is in accordance with the experimentally observed pH dependence of NDMA yield in ozonation, indicating that the hydroxylamine mechanism is responsible for the NDMA formation in ozonation.

N -nitrosodimethylamine (NDMA) as a disinfection by-product has recently become the focus of considerable research interest due to its unusually high carcinogenicity. In this study, the formation mechanisms of NDMA from dimethylamine (DMA) during chloramination and ozonation were investigated by using the quantum chemical G4 method. The reactivity of haloamines and hydroxylamine reacting with DMA was found in the order: NHCl 2 ∼ NHBrCl (Brˉleaving) > NHBr 2 > NH 2 Cl ∼ NH 2 Br >> NH 2 OH. This offers a theoretical support for the experimentally proposed mechanism that dimethylamine reacts with NHCl 2 rather than NH 2 Cl to form chlorinated unsymmetrical dimethylhydrazine intermediate and the existence of bromochloramine in the presence of bromide during chloramination, and explains the observation that NDMA yield during ozonation is much lower than that during chloramination. Importantly, an N , N -dimethylaminonitrene was found to be a significant intermediate to form NDMA in oxidation reactions by molecular oxygen and ozone. Additionally, results suggest that the amines containing the second nitrogen source directly connecting or close to the N-(CH 3 ) 2 moiety are potential significant NDMA precursors upon ozonation. The findings of this study are helpful for expanding the knowledge of NDMA formation mechanism, and predicting potential NDMA precursors during disinfection.

The paper concerns formation of N-nitrosodimethylamine (NDMA) upon ozonation of dimethylamine (DMA) aqueous solutions. According to current hypothesis ozonated DMA is oxidized to N-dimethylhydroxylamine (DMHA), then to N-methylhydroxylamine (MHA) and finally to hydroxylamine (HA). HA subsequently reacts with the remain part of DMA to form unsymmetrical dimethylhydrazine (UDMH). Finally UDMH undergoes oxidation with ozone to form NDMA. HA is thought to be an important by-product that increases the NDMA formation. We decided to verify the hypothesis by an ozonation of DMA aqueous solutions in the presence of DMHA, MHA and HA. We have clearly proved that ozonation of DMA in the presence of DMHA and/or MHA does not increase NDMA formation. These results do not exclude the possibility of HA formation during DMA ozonation, but unambiguously show that even if HA is formed during this reaction, it does not have any impact on NDMA formation. In authors opinion the formation of MHA and HA is however doubtful since both compounds seem to be rather products of reduction than oxidation. Therefore HA–DMA reaction cannot be responsible for the formation of NDMA when DMA aqueous solution is ozonized.