N-nitrosodimethylamine (NDMA) formation and its mitiga tion during chlorination and ozonation of N,N dimethylhydrazine compounds: Reaction kinetics, mechanisms and implications for NDMA formation control

Abstract

N-nitrosodimethylamine (NDMA) is a potent probable human carcinogen and can be formed as a disinfection byproduct in oxidative drinking water and wastewater treatment. Hydrazine, hydrazide, and hydrazone with N,N-dimethyl groups have been identified as potent NDMA precursors during ozonation with molar yields of up to 94%. Some of these compounds are used in specific industries and have a potential to be discharged into sewage-impacted drinking water sources, which can be a concern for drinking water and wastewater utilities using ozonation. In this study, kinetics, mechanisms, and NDMA formation during ozonation and chlorination of selected N,N-dimethylhydrazine compounds were investigated in order to develop NDMA formation control strategy in water and wastewater treatment. The reaction of ozone with daminozide (DMZ), which is a model compound of N,N-dimethylhydrazine compound, was rapid (k > 105 M-1 s-1) and produced NDMA with a molar yield of 80%. OH radicals (•OH) and superoxide radicals (O2-•) were formed in the reaction of ozone with these compounds, indicating that the transformation pathway includes heterolytic cleavages of the primary ozone adduct. On the other hand, the reaction of •OH with DMZ was relatively slow (k = 3.4 × 109 M-1 s-1 at pH 7) and did not produce NDMA, which could explain the enhanced NDMA formation during ozonation in presence of tert-butanol as •OH scavenger. Reaction mechanisms are proposed in which an ozone adduct is formed at the nitrogen next to N,N-dimethylamine which decomposes via homolytic and heterolytic cleavages of the -N+-O-O-O- bond, forming NDMA as a final product. From the rapid and efficient conversion of N,N-dimethylhydrazine compounds into NDMA, significant NDMA formation is expected to be inevitable during ozonation of the waters containing these precursors, which could be demonstrated in the ozone and O3/H2O2 experiments in wastewater effluent matrices. Pre-chlorination, which is an option for bromate mitigation during ozonation, was tested as the NDMA formation mitigation strategy for post-ozonation of waters containing N,N-dimethylhydrazine compounds. Chlorine showed moderate reactivity to DMZ (k = 102 M-1 s-1 at pH 7) and the chlorination of DMZ formed negligible NDMA (molar yield of < 0.1%). In addition, the chlorinated intermediates/products of DMZ produced little NDMA upon subsequent ozone treatment. Therefore, pre-chlorination was found to be effective in controlling NDMA formation in post-ozone treatment of waters containing N,N-dimethylhydrazine compounds.