Effects of Amine Structure, Chloramine Species and Oxidation Strategies on N-Nitrosodimethylamine
- Meric Selbes - Hazen and Sawyer
To comply with the increasingly stringent disinfection by-product (DBP) regulations (trihalomethanes [THMs] and haloacetic acids [HAAs]) in the United States, many water treatment plants have been switching from chlorination to chloramination in the last decade. However, chloramination leads to the formation of nitrosamines, which are classified as probable human carcinogens at concentrations as low as 0.2 ng/L. In particular, N-nitrosodimethylamine (NDMA) is the most frequently detected nitrosamine in distribution systems in the United States. Nitrosamines (including NDMA) have been recently highlighted for possible regulatory action in the near future. However, there is still much more to learn about the precursors and formation pathways of nitrosamines (especially NDMA). The main objective of this research was to systematically examine nitrosamines formation mechanisms from amines and examine the interactions of these precursors with different oxidants. Specifically, the research focused on: (i) the formation of nitrosamines (especially NDMA) from amino acids (AAs), (ii) the roles of amines structure on NDMA formation during chloramination, (iii) the importance of chloramine species in the NDMA formation, and (iv) the interaction of various precursors with different oxidants (chlorine, chlorine dioxide and ozone) and their consequent effect on NDMA formation. For practical applications, the key findings from this study are summarized as:
(i) Nine AAs were selected initially as nitrosamine precursors, and tested under different oxidation conditions for their formation of nitrosamines. NDMA and other nitrosamines yields of all nine AAs during chloramination were below the minimum reporting levels. Although AAs are known to form other DBPs (i.e., THMs, HAAs), they did not appear to be an important contributor to nitrosamines formation.
(ii) Due to very low conversion yields of nitrosamines, the research focus was directed towards tertiary amines which are more reactive nitrosamine precursors. Since, NDMA is the most frequently detected nitrosamine, potential NDMA precursors were selected to investigate the effect of tertiary amine structure in NDMA formation under chloramination conditions. Dimethylamine (DMA) and 20 different tertiary aliphatic and aromatic amines were carefully examined based on their functional groups attached to the basic DMA structure. The results indicated a wide range (0.02% to 83.9%) of NDMA yields indicating the importance of the structure of tertiary amines, and both stability and electron distribution of the leaving group of tertiary amines on NDMA formation. DMA moieties associated with branched alkyls or benzyl like groups had very high NDMA formation yields (>25%). Especially, strategies for controlling the discharge of those types of contaminants would lead to decreases in NDMA precursor’s levels in source waters.
(iii) The precursor’s structure also influences the chloramine species (mono- vs. di-) responsible for NDMA formation. The role of chloramine species in NDMA formation as a function of time was evaluated for five amines and natural waters, and exposed to varying levels of chloramine with different ratios of mono/dichloramine. Amines (e.g., ranitidine) that prefer monochloramine reacted relatively fast to form NDMA and reached the maximum yield within 24 hours. On the other hand, the NDMA formation from amines (e.g., DMA) that prefer dichloramine was relatively slow due to limited availability of dichloramine. Investigating two case studies showed that some NDMA can be formed by monochloramine; however, dichloramine was observed to be the dominant species responsible for NDMA formation in both systems. Thus, utilities may opt to minimize the formation of dichloramine in their distribution systems (e.g., maintaining higher pH) to control NDMA formation. However, it should be noted that dichloramine may not be the only reactive species and depending on the structures of precursors monochloramine might be responsible for NDMA formation.
(iv) The interaction of fifteen NDMA precursors with different pre-oxidants (chlorine, chlorine dioxide and ozone) prior to chloramination was investigated as a control technique to minimize NDMA formation. The results have shown that pre-oxidation strategies can be an effective method for utilities to control NDMA formation. Chlorine has shown reduction in NDMA formation for most of the precursors (except polymers). The use of chlorine dioxide could also be effective in reducing NDMA formation for source waters that contain precursors with high NDMA yields (>5%). However, it can be detrimental, increasing NDMA formation if the precursor’s yield is less than <1.0%, due to the release of the DMA moiety. Similar to chlorine dioxide, the use of ozone as a pre-oxidant has a potential for contrasting outcomes. While ozone may stimulate NDMA formation from specific structures (i.e., polymers, amides or hydrazines), the simultaneously produced hydroxyl radicals would work against this effect. The effectiveness of these two pre-oxidants is highly dependent on the characteristics of the precursors existing in source waters.
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