The Role of Chloramine Species in NDMA Formation Rate
- Meric Selbes, Daekyun Kim, and Tanju Karanfil
Recent studies have shown that nitrosamines is a class of emerging disinfection by-products with cancer risk at very low concentrations (i.e., ng/L). Amines present in source waters are found to be precursors of nitrosamines, especially N-nitrosodimethylamine (NDMA) which is the most commonly detected species. Thus, understanding NDMAs formation rates is critical to develop strategies for controlling its levels in distribution systems.
The main objectives of this study were to investigate: (i) the role of chloramine species (i.e., mono- and di-) in the formation of NDMA over reaction time; (ii) the factors that affect chloramine speciation (i.e., pH, sulfate, and natural organic matter [NOM]); and (iii) the reactivity of chloramine species in natural waters. Five amines (Dimethylamine, trimethylamine, dimethylisopropylamine [DMiPA], dimethylbenzylamine, and ranitidine [RNTD]) were selected for this study and exposed to chloramine with different monochloramine to dichloramine ratios. Chloraminated sample bottles were opened at 3, 6, 12, 24, 48, 72, and 120 hours, extracted, and analyzed for NDMA using GC/MS/MS. RNTD whose reaction is preferred with monochloramine had relatively fast NDMA formation and reached its maximum yield (~80%) within 24 hours. On the other hand, DMiPA whose reaction is preferred with dichloramine had relatively slow NDMA formation rates. Especially, under dichloramine limited conditions NDMA yields from the amines preferring dichloramine were almost negligible. For DMiPA, the NDMA formation decreased from ~80% to ~0.3% when dichloramine was limited. Thus, NDMA formations from the amines preferring dichloramine were controlled by decomposition of the monochloramine to dichloramine.
The results indicate the NDMA formation during chloramination is highly dependent on the precursor’s structure. The presence of NOM created a competition with amines for dichloramine and thus decreased the NDMA formation rates and overall yields. Consequently, drastic decreases were observed from the amines preferring dichloramine. On the other hand, almost no NOM interference was observed in NDMA formation from the amines preferring monochloramine. In NDMA formation, pH influences both chloramine speciation and protonation state of the amine. Thus, more profound effect was observed on DMiPA which is dichloramine sensitive, whereas RNTD did not show a distinct difference. Lastly, the presence of sulfate that can increase the chloramine decomposition was found to increase NDMA formation from DMiPA, but had no effect on RNTD. In selected natural waters, low levels (i.e., a few ng/L) of NDMA formation by monochloramine were recorded. Dichloramine was found to be the dominant species responsible for NDMA formation in those waters. These findings indicate that NDMA formation is relatively slow due to limited availability of dichloramine in natural waters, but it could be further formed in distribution systems as water age increases.
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