Wenhui Gan, Tom Bond, Xin Yang, Paul Westerhoff

N-nitrosodimethylamine (NDMA) is an emerging disinfection by-product, and we show that use of chlorine dioxide (ClO2) has the potential to increase NDMA formation in waters containing precursors with hydrazine moieties. NDMA formation was measured after oxidation of 13 amines by monochloramine, ClO2 and pre-treatment with ClO2 followed by post-monochloramination. Daminozide, a plant growth regulator, was found to yield 5.01±0.96% NDMA upon reaction with ClO2, although no NDMA was recorded during chloramination. The reaction rate was estimated to be ~0.0085 s-1 and based upon our identification by mass spectrometry spectra of intermediates the reaction likely proceeds via the hydrolytic release of unsymmetrical dimethylhydrazine (UDMH), with the hydrazine structure a key intermediate in NDMA formation. The presence of UDMH was confirmed by gas chromatography-mass spectrometry (GC-MS) analysis. For 10 of the 13 compounds, ClO2 pre-oxidation reduced NDMA yields compared with monochloramination alone, which is explained by our measured release of DMA. This work shows potential pre-oxidation strategies to control NDMA formation may not impact all organic precursors uniformly, so differences might be source specific depending upon occurrence of different precursors in source waters. For example, daminozide is a plant regulator so drinking water that intakes heavily influenced by upstream agricultural runoff could be at risk.