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Effects of Dissolved Oxygen and Dimethylamine on Dichloramine Decomposition Products and NDMA Formation Pathways
Citation:
Pham, H., D. Wahman, AND J. Fairey. Effects of Dissolved Oxygen and Dimethylamine on Dichloramine Decomposition Products and NDMA Formation Pathways. 2022 Water Quality & Technology Conference, Cincinnati, OH, November 14 - 17, 2022.
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 dichloramine decomposition and end products in both ambient and low oxygen conditions The long-term importance or significance of the findings: Provides a fundamental understanding of reactive oxygen and nitrogen species that are formed during dichloramine decomposition that have implications in forming disinfection byproducts 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 byproducts in drinking water systems.
Description:
The mechanism for dichloramine decomposition has remained unresolved for over three decades, with previous studies focused on the fate of the chloramine species, free ammonia, and nitrogen gas rather than the relatively minor intermediates and decay products that comprise less than 5 % of the nitrogen mass balance. As a result, reactions in the Unified Model of Chloramine Chemistry associated with dichloramine decomposition and the so-called “unidentified intermediate” are empirical and may not properly account for minor nitrogenous species. A recently published study by this research team (Pham et al., 2021, ES&T, 55(3), 1740–1749) indicated that reactive nitrogen species (RNS) form by dichloramine decomposition, including nitroxyl/nitroxyl anion (herein nitroxyl) and peroxynitrous acid/peroxynitrite (herein peroxynitrite). Nitroxyl was posited to be the “unidentified intermediate” and a direct product of dichloramine hydrolysis, which then reacted with (i) nitroxyl to form nitrous oxide (N2O) or (ii) dissolved oxygen (DO) to form peroxynitrite. In the presence of amine-containing precursors, such as dimethylamine (DMA), peroxynitrite and/or its decay products were demonstrated to react to form N-nitrosodimethylamine (NDMA) and N-nitrodimethylamine (DMNO). This reaction sequence – dichloramine to nitroxyl to peroxynitrite to NDMA – was further supported by experiments with uric acid, a peroxynitrite scavenger which decreased NDMA formation in a dose-response manner at pH 7–10. As DO is needed for peroxynitrite formation, it can impact NDMA yields. However, the effect of DO on dichloramine decomposition products and NDMA formation has not been fully resolved. Few studies have assessed dichloramine decomposition under low DO-conditions, which will be the focus of this presentation. Batch kinetic experiments were completed under ambient and low-DO conditions in waters at pH 9 dosed with preformed dichloramine in the presence and absence of DMA. Yields of chloramine species, N2O, nitrite, nitrate, and NDMA were used along with nitrogen gas yields measured in experiments with 15N-labelled dichloramine to revise and validate the previously empirical reactions in the Unified Model. A companion NDMA kinetic model was formulated in AQUASIM, composed of previously validated models of (i) chloramine decay developed by Jafvert and Valentine (i.e., the updated Unified Model), (ii) nitroxyl chemistry proposed by Lymar and Shafirovich, and (iii) peroxynitrite decomposition developed by Korth and colleagues. The results from the laboratory experiments and numerical modeling facilitated revision of reactions and postulation of an additional N2O formation pathway requiring DO and a previously unknown NDMA formation pathway involving intermediates from chloramine decay but independent of DO. Implications will be discussed in the context of NDMA control strategies for chloramine systems.