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Advancing N-Nitrosodimethylamine (NDMA) Formation in Chloramine Systems - Reactive Nitrogen Species and Mass Balances from Dichloramine Decay
Citation:
Fairey, J., H. Pham, AND D. Wahman. Advancing N-Nitrosodimethylamine (NDMA) Formation in Chloramine Systems - Reactive Nitrogen Species and Mass Balances from Dichloramine Decay. 18th INTERNATIONAL CONFERENCE ON CHEMISTRY AND THE ENVIRONMENT, Venice, ITALY, June 11 - 15, 2023.
Impact/Purpose:
The environmental or health problem addressed by the study: Understanding dichloramine decomposition A general description of the work and results: Experiments have been conducted to evaluate existing models for dichloramine decomposition, including generating nitrogen mass balances with and without oxygen present The long-term importance or significance of the findings: Update our understanding of minor products of dichloramine decomposition that may impact disinfection byproduct formation Who would be interested in or could apply the results (e.g. program or regional partners, general public, local communities): Researchers and drinking water utilities trying to minimize disinfection byproduct formation.
Description:
The reaction mechanism for dichloramine decomposition has remained unresolved for several decades. Previous studies have focused on the fate of the chloramine species, free ammonia, and nitrogen gas rather than the minor intermediates and decay products that comprise less than 5% of the nitrogen mass balance under typical chloramination conditions. We recently showed that nitroxyl formed by dichloramine hydrolysis and was the key missing intermediate in the N-nitrosodimethylamine (NDMA) formation pathway (Pham et. 2021). In this reaction scheme, nitroxyl reacts with dissolved oxygen (DO) to form peroxynitrite which reacts with dimethylamine (DMA) to form NDMA. We revised the Unified Model (UF) of Chloramine Chemistry to include nitroxyl as the product of dichloramine hydrolysis and added existing reaction schemes for nitroxyl and peroxynitrite chemistry. The resultant UF+RNS model kinetically simulated dichloramine decay, monochloramine formation, DO decomposition, and NDMA formation at pH 7–10. However, nitrous oxide formation—a product of nitroxyl reacting with itself followed by a dehydration reaction—was undersimulated at pH 7–9 and oversimulated at pH 10, prompting additional studies to assess the nitroxyl fate during dichloramine decay. Nitrogen and oxygen mass balances were performed under ambient and low DO conditions in the presence and absence of DMA. This presentation will focus on key findings from these mass balances, which include (1) a revision to nitroxyl chemistry to better simulate nitrous oxide formation at pH 7–10 while maintaining fits for dichloramine, monochloramine, DO, and NDMA and (2) identification of peroxynitrite decay products that react with DMA to form NDMA. These findings help resolve the dichloramine decomposition pathway and its role in NDMA formation in chloramine systems and shed light on the reaction pathway for the so-called Unidentified Product of chloramine chemistry in the absence of DMA. This Unidentified Product contains nitrogen, chlorine, and oxygen and is an inorganic disinfection byproduct of unknown health significance. The practical implications of these findings will be discussed in terms of chloramine formation and stability