Chapter 9
The Role of Pre-Oxidation in Controlling NDMA Formation: A Review Downloaded by UNIV OF PITTSBURGH on May 15, 2016 | http://pubs.acs.org Publication Date (Web): August 24, 2015 | doi: 10.1021/bk-2015-1190.ch009
Meric Selbes,1 Malcolm Glenn,2 and Tanju Karanfil*,3 1Hazen
and Sawyer, Environmental Engineers and Scientists, Fairfax, Virginia 22030, U.S.A. 2Alliance Consulting Engineers, Columbia, South Carolina 29202, U.S.A. 3Department of Environmental Engineering and Earth Sciences, coulombslemson University, Anderson, South Carolina 29625, U.S.A. *E-mail:
[email protected].
N-nitrosodimethylamine (NDMA), a probable human carcinogen, is a disinfection by-product (DBP) that has been detected in chloraminated drinking water systems. Options for NDMA control include the physical removal of either precursors or precursor deactivation prior to chloramination. This review summarizes some of the recent findings related to the control of NDMA formation with commonly used oxidants in drinking water treatment. Each oxidant (chlorine, chlorine dioxide, ozone) has the potential to reduce NDMA formation with varying degree of efficiencies depending on the precursors in source waters. In general, ozone and chlorine are effective oxidants for controlling NDMA precursors, likely due to their high reaction rate constants with amines. In some cases, the oxidants themselves have been shown to increase the NDMA formation. The selection of pre-oxidant type, dose and application location for NDMA control would site-specific. Utilities intend on using pre-oxidation as a strategy for NDMA control should also the impacts on the formation of regulated DBPs and other unintended consequences that can be associated with those oxidants.
© 2015 American Chemical Society Karanfil et al.; Recent Advances in Disinfection By-Products ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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Introduction N-nitrosodimethylamine (NDMA), a probable human carcinogen, is a disinfection by-product (DBP) that has been detected in chloraminated drinking water systems. Due to their adverse health effects, methods to control the formation of nitrosamines during water treatment have gained increased attention during the past decade. These methods mainly involve either the removal of NDMA precursors or their transformation into less reactive forms. Since most of the nitrosamine precursors have been linked low molecular weight hydrophilic compounds (1), their removal from water is challenging. Pre-oxidation (e.g., using chlorine, ozone, or chlorine dioxide) prior to chloramination can be a viable approach for some water utilities to control the NDMA levels. However, the use of oxidants should be optimized for a given source water and water treatment process configuration since these oxidants are also associated with the formation of both regulated DBPs and NDMA (without chloramination). The objective of this chapter is to provide a literature review on the oxidant applications for controlling NDMA formation during drinking water treatment.
The Role of Oxidants/Disinfectants in NDMA Formation Several pathways have been proposed for the formation of nitrosamines. In drinking water, formation of nitrosamines with chloramines is practically most important pathway. It has been shown that mono- and dichloramines can lead to formation nitrosamines from DMA, tertiary or quaternary amines with DMA moieties, natural organic matter, polyelectrolytes, ion-exchange resins, fungicides, pesticides, herbicides, pharmaceuticals, personal care products, and wastewater effluent impacted waters. In addition to these chloramines (2–11),. The molecular structures of precursors mentioned in this review are given in Table 1. Besides chloramine, some other oxidants/disinfectants may lead to NDMA formation (Figure 1).
Chlorine The first pathway that has been suggested to form NDMA is through nitrosation of nitrogen-containing compounds. Nitrosation involves introducing a nitroso group (-NO) into an organic compound causing the formation of nitroso compounds. By N-nitrosation of nitrogen-containing organic compounds, nitrosamines are formed. Possible nitrosating agents include nitrous acid (HNO2), nitrogen oxides (N2O3, N2O4), and some others (4). For example, the nitrosation reaction induced by nitrite is as follows: “Under acidic conditions, nitrite (NO2-) is transformed to nitrous acid (HNO2), which is not stable in aqueous solution but decomposes either to the nitrosyl cation (NO+) or to dinitrogen trioxide (N2O3). Both of which are capable of reacting with nitrogen-containing organic compounds to form nitrosamines (4). 152 Karanfil et al.; Recent Advances in Disinfection By-Products ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
Downloaded by UNIV OF PITTSBURGH on May 15, 2016 | http://pubs.acs.org Publication Date (Web): August 24, 2015 | doi: 10.1021/bk-2015-1190.ch009
Table 1. Some of the Structures of NDMA Precursors
153 Karanfil et al.; Recent Advances in Disinfection By-Products ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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Figure 1. NDMA formation and the effect of pre-oxidation.
Choi and Valentine (12) proposed that NDMA is formed by nitrosation of dimethylamine (DMA) with nitrite that was catalyzed by free chlorine. Specifically, they attributed NDMA formation to the formation of a dinitrogen tetraoxide (N2O4) intermediate, which is the product of oxidation of nitrite by free chlorine. This intermediate is a very effective nitrosating agent with its formation more favorable at neutral pH, which is typical in water treatment facilities, in contrast to the formation of N2O3, which occurs at low pH. Although the reaction is rapid, the yields are approximately two orders of magnitude lower than that of the chloramination pathway (9). Due to reliance on nitrite for NDMA formation, this pathway has been suggested for NDMA formation in chlorinated wastewater effluents and natural-bodied, recreational waters (13, 14). Chlorine Dioxide Andrzejewski and Nawrocki (15) have shown that chlorine dioxide reaction with DMA led to the formation of NDMA. The highest observed yield was 0.2% molar conversion under very high chlorine dioxide concentrations (i.e., 36 mg/L) at pH 8.0. Compared to chloramination or other oxidants, however, this much yield is expected to be negligible under drinking water treatment conditions. Ozone Ozonation of DMA forms NDMA but yields generally are 50% (5, 18, 19). The ozonation of N,N-Dimethylsulfamide (DMS), a transformation product of the fungicide tolylfluanide, was observed to form NDMA at a 52% yield (18). Lastly, the ozonation of polyDADMAC, a polymer used in water treatment plants also formed NDMA (20). These results clearly indicate that the ozonation of polyDADMAC can release the DMA moiety while concurrently forming hydroxylamines. The simultaneous reaction of these two products could form UDMH, at which point the formed UDMHs would be converted to NDMA in the presence of ozone. Permanganate Andrzejewski and Nawrocki (21) have shown that potassium permanganate reaction with DMA led to the formation of NDMA. The highest observed yield was 0.04% molar conversion under very high permanganate concentrations (i.e., 700 mg/L) and alkali pH conditions (pH>8.35). Compared to the chloramination or other oxidants, this much lower yield is expected to be negligible under actual drinking water treatment conditions. UV Irradiation Ultraviolet (UV) irradiation of nitrite at
