The Role of Hydroxylamine as a Nitrification Intermediate in N-Nitrosamine Formation- Indianapolis

The formation of N-nitrosamines, and in particular N-nitrosodimethylamine (NDMA), in drinking water systems that use chloramines is a concern because of their potential carcinogenicity and occurrences in finished waters at toxicologically relevant levels. The widely accepted mechanism for NDMA formation involves a nucleophilic substitution between dichloramine and unprotonated secondary amines to form an unsymmetrical dimethylhydrazine (UDMH) intermediate, followed by UDMH oxidation to form NDMA. This reaction takes days to occur and involves a spin-forbidden reaction, specifically the incorporation of ground-state molecular oxygen as a triplet to form NDMA, which is thermodynamically unfavorable. As such, it is possible that other N-nitrosamine formation pathways exist in chloramination systems, which would have implications for control strategies to curb formation of these compounds. A recent study showed that nitrification in storage facilities and distribution systems can lead to the elevated concentrations of NDMA, but did not elucidate the underlying reaction mechanism. Preliminary experiments for this poster presentation were completed using a chemiluminesence-based total N-nitrosamine (TONO) assay and demonstrated that the reaction between hydroxylamine (NH2OH, a nitrification intermediate formed during ammonia oxidation by nitrifying bacteria) and dimethylamine (DMA, a model NDMA precursor) produces significantly more TONO within hours compared to the reaction between monochloramine and DMA. Upcoming experiments for this poster presentation will focus on results from batch kinetic experiments with DMA, chloramines, and hydroxylamine, performed over a range of pH and dissolved oxygen conditions. A solid-phase extraction (SPE) procedure adapted from EPA Method 521 along with gas chromatography – mass spectrometry (GC-MS) and the TONO assay will be used to measure NDMA and total N-nitrosamines, respectively. Results from these experiments will be used to postulate a novel reaction mechanism for N-nitrosamines, one that likely involves intermediates from the breakdown of monochloramine, the predominant chloramine species present at the pH range of interest in distribution systems (pH 7-9), and one that is especially relevant to systems undergoing nitrification.