Effect of bromide on NDMA formation during chloramination of model precursor compounds and natural waters

Water Research 170 (2020) 115323

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Effect of bromide on NDMA formation during chloramination of model precursor compounds and natural waters Wilson Beita-Sandí a, b, *, Cagri Utku Erdem a, Tanju Karanfil a a b

Department of Environmental Engineering and Earth Sciences, Clemson University, Anderson, SC, 29625, United States Research Center of Environmental Pollution (CICA), University of Costa Rica, San Jos
e, 2060, Costa Rica

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 July 2019 Received in revised form 9 November 2019 Accepted 18 November 2019 Available online 24 November 2019

In this work, we investigated the effect of bromide ion (Br
) on NDMA formation using model precursor compounds, wastewater effluents and surface waters. Previous studies showed that Br
reacts with chloramines and forms bromochloramine, a reactive compound responsible for NDMA formation enhancement. Some limitations of those studies were the highest Br
concentrations used, and the limited number of precursors considered. Here, we observed enhancement of NDMA formation from most of the model precursor compounds within the Br
range (0e1000 mg/L) but this effect was suppressed in the presence of NOM. Also, NDMA formation was favored at pH 8 in the presence of Br
compared to pH 6. Nevertheless, Br
suppressed NDMA formation in wastewater effluent samples at low monochloramine doses while no effects were observed in surface waters. © 2019 Elsevier Ltd. All rights reserved.

Keywords: NDMA Bromide NDMA model precursors Natural water Wastewater

1. Introduction N-nitrosodimethylamine (NDMA) is a disinfection byproduct (DBP) formed mainly during the chloramination of water and wastewater (Choi and Valentine, 2002a; Le Roux et al., 2011; Mitch et al., 2003; Mitch and Sedlak, 2004). NDMA is probably carcinogenic to humans with a lifetime cancer risk of one in a million for a concentration of 0.7 ng/L in drinking water (U.S. EPA, 1993). Formation of NDMA during drinking water treatment can be affected by several parameters. For example, NDMA formation increases with increasing chloramine dose, contact time, pH and temperature. NOM in the background water generally suppresses NDMA formation (Chen and Valentine, 2006; Choi and Valentine, 2002b; Hatt et al., 2013; McCurry et al., 2015; Schreiber and Mitch, 2005; Selbes et al., 2018, 2014; 2013; Sgroi et al., 2018, 2014; Shen and Andrews, 2011). Particularly, bromide ion (Br
) is a concern in water treatment because it reacts with natural organic matter (NOM) and disinfectants to form nonenitrogenous DBPs (e.g., bromate, brominated trihalomethanes and haloacetic acids) (Heller-Grossman et al., 1999; Muellner et al., 2007; Richardson et al., 2003). Bromide is a ubiquitous component of fresh and

* Corresponding author. Department of Environmental Engineering and Earth Sciences, Clemson University, Anderson, SC, 29625, United States. E-mail address: [email protected] (W. Beita-Sandí). https://doi.org/10.1016/j.watres.2019.115323 0043-1354/© 2019 Elsevier Ltd. All rights reserved.

seawaters. Concentrations in natural waters (e.g., rivers, groundwaters) range from 2 to 4000 mg/L (Flury and Papritz, 1993; Magazinovic et al., 2004; Soyluoglu et al., 2020) while in seawater between 66,000 and 68,000 mg/L (Magazinovic et al., 2004; Stumm and Morgan, 1996; Wilson et al., 2014). A nationwide study surveyed 23 U.S. wastewater treatment plants (WWTPs) effluents and reported a Br
median concentration (and interquartile range) of 190 (120e240) mg/L (Krasner et al., 2009). Nonetheless, high concentrations of Br
can occur in fresh waters due to anthropogenic activities (e.g., generation of mining tailings, chemical production, textile production, road salt, desalination, fertilizers, natural gas production with hydraulic fracturing and air pollution control methods in coal fired power plants) and seawater intrusion (Flury and Papritz, 1993; Harkness et al., 2015; Mctigue et al., 2014; Watson et al., 2012). For instance, high Br
levels (1000e10,000 mg/L) were detected in process/flowback waters used in hydraulic fracturing operations (Alley et al., 2011; Good and VanBriesen, 2016; Gregory et al., 2011; Harkness et al., 2015; Mctigue et al., 2014; Parker et al., 2014). Elevated Br
concentrations were measured in coastal groundwater and estuary sources due to saltwater intrusion (Ged and Boyer, 2014; Kolb et al., 2017; Krasner et al., 1994; Nair et al., 2016; Tyrovola and Diamadopoulos, 2005; Wu et al., 2013). Bromide is readily oxidized by chloramines to form bromamines and bromochloramine (NHBrCl). Bromamines are less stable than

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chloramines and generally more reactive (Le Roux et al., 2012). Br
enhanced the formation of NDMA during chloramination (Chen ~ as, 2012). However, most of those et al., 2010; Luh and Marin studies were conducted with high Br
levels that are not likely to represent relevant environmental concentrations. For instance, Br
enhanced the formation of NDMA from dimethylamine (DMA) (0.3 mM), initial monochloramine (NH2Cl) concentration of 0.3 mM, pH 7 and Br
ranging from 0 to 24,000 mg/L (Chen et al., 2010). However, NDMA formation from trimethylamine (TMA) was inhibited in the presence of Br
(0e79,904 mg/L) and in similar disinfection conditions to DMA. The formation rate of NDMA during chloramination of DMA was faster in the presence of 2497 mg/L Br
during the first 12 h of reaction time (Chen et al., 2010). Also, the authors found that the greatest NDMA formation occurred at neutral pH, regardless of the absence or presence of Br
. In another study, NDMA formation showed an increase of only 6 ng/L (from 23 to 29 ng/L) in an experiment with natural water in the presence of Br
(0e159,800 mg/L), NH2Cl (70 mg/L as Cl2) and 48 h reaction time (Luo, 2006). Some studies reported enhancement of NDMA formation during chloramination in the presence of Br
. However, typical experimental conditions focused mainly on DMA as a NDMA model precursor and involved high Br
concentrations that are not likely to represent relevant environmental concentrations. Here, we investigated how Br
affects NDMA formation using a wider group of model precursor compounds and water treatment polymers at environmentally relevant concentrations of this ion. Experiments were conducted in distilled and deionized water (DDW), secondary wastewater effluent and surface water samples. Additionally, we investigated the pH and the background dissolved organic carbon (DOC) effects on NDMA formation in the presence of bromide. 2. Materials and methods 2.1. Chemicals and reagents DMA (40%), N,N-dimethylisopropylamine (DMiPA) (>99%), N,Ndimethylaniline (DMAN) (99%), N,N-dimethylbenzylamine (DMBzA) (>99%), NDMA (99.9%), poly(diallyldimethylammonium chloride) (polyDADMAC) (20% w/w), ranitidine (RNTD) (99%), NaBr (min. 99.0%) and TMA (25%) were purchased from SigmaeAldrich (St. Louis, MO, U.S.). Deuterated NDMA (NDMA-d6) (99%) and deuterated N-nitroso-di-n-propylamine (NDPA-d14) (99%) were obtained from Restek (Bellefonte, PA, U.S.). Poly(dimethylamine-coepichlorohydrin) quaternized (polyAMINE) (37.7% w/w) was bought from Scientific Polymer Products, Inc. (Ontario, NY, U.S.). Polyacrylamide (polyACRYL, Sedifloc 400C) was provided by one of the drinking water treatment plants (DWTP). NaOCl solution (5% available chlorine) and (NH4)2SO4 were obtained from Fisher Scientific (Center Valley, PA, U.S.) and VWR (Radnor, PA, U.S.), respectively. Molecular structures of selected precursors are presented in Table 1. 2.2. Chloramination experiments Experiments were conducted in 250 mL amber bottles with 200 nM NDMA model precursors compounds, 0.2 mg/L polyDADMAC, 0.2 mg/L polyamine and 1 mg/L polyACRYL (as active ingredient of the polymer). Formation potential (FP) and uniform formation conditions (UFC) tests were conducted for determining NDMA formation. Briefly, pH of samples was adjusted to 8.0 with phosphate buffer, oxidized with NH2Cl (3 mg/L for UFC and 100 mg/ L for FP) and hold in the dark at room temperature (21  C ± 2  C). The reactions were stopped after 3 d (UFC) and 5 d (FP) by quenching the residual NH2Cl with stoichiometric amounts of

sodium thiosulfate anhydrous. NDMA-d6 was added (40 ng/L) as a surrogate to the quenched samples before extraction. 2.3. NDMA analysis Samples were extracted using solid-phase extraction cartridges and analyzed for NDMA following U.S. Environmental Protection Agency Method 521 with minor modifications (U.S. EPA, 2004). One microliter of the samples and the standards were injected on a Agilent 7890B (Santa Clara, CA, USA) Gas Chromatograph (GC) equipped with an Agilent DB-1701 (30 m  0.25 mm x 1.00 mm) column, and coupled with an Agilent 7000C triple quadrupole mass spectrometer (MS/MS) (Beita-Sandí et al., 2016; McDonald et al., 2012). NDMA concentration was normalized to the NDMA-d6 recovery. Multiple reaction monitoring transitions, specific dwell times and collision energies of the analytes are presented in Table S2. Additional details regarding NDMA testing procedure can be found in Text S1 of the Supporting Information. More details about DOC, dissolved nitrogen (DN) and other analytical methods are provided in and Table S3 of the Supporting Information. Unless otherwise specified, error bars in the graphs represent the standard deviation of two injections of one sample. 2.4. Water samples Water samples were collected from five locations in South Carolina, U.S., and classified as surface waters (SW) or secondary wastewater effluents (WW). SWe1 was collected from a lake, SWe2 and SWe3 were sampled from a DWTP. SWe2 was collected after the conventional clarification processes while SWe3 in the influent of that DWTP. The WW samples were collected from the secondary effluent of (i) a municipal wastewater treatment plant (WWe1); (ii) a municipal/industrial (WWe2) and (iii) a municipal (WWe3). Selected characterization parameters of the tested water samples are provided in Table 2. Upon arrival to the laboratory, samples were filtered through 0.45 mm pore size Whatman™ Polycap 150 TC filters (Pittsburgh, PA, U.S.) and stored (for 3 d maximum) at 4  C until experiments were conducted. 3. Results and discussion 3.1. Effect of bromide on NDMA formation in secondary wastewater effluent and surface water samples SW and WW samples (Table 2) were amended with Br
at concentrations ranging from naturally occurring up to 1000 mg/L and then chloraminated to investigate the effect of Br
on the formation of NDMA. The concentration of NDMA was normalized to the concentration of NDMA in the absence of Br
(C0) and the results are shown in Figs. 1 and 2. NDMA formation in the absence of Br
during UFC chloramination in SW samples was relatively low (<10 ng/L), indicating the lack of a significant amount of NDMA precursors. On the other hand, NDMA formation FP conditions were relatively higher, this is 30 ng/L (SWe1), 21 ng/L (SWe2) and 42 ng/L (SWe2). In SW samples, NDMA formation (with 3 or 100 mg/L NH2Cl as Cl2) was not enhanced with increasing Br
concentration (Fig. 1). NDMA yields (in the absence of Br
) in WW (Fig. 2) were significantly higher than in SW samples, ranging from 145 to 190 ng/L and 433e1620 ng/L for UFC and FP, respectively. In the presence of Br
, NDMA formation was suppressed with NH2Cl at 3 mg/L as Cl2 with increasing Br
concentration. However, no changes were observed at 100 mg/L as Cl2. Bromide decreased NDMA formation in WW samples but caused no effect in SW waters. Since NH2Cl/Br
mass ratios were

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Table 1 Molecular structures of selected precursors.

Table 2 Characterization of wastewater effluents (WW) and surface water (SW) samples. Parameter

WW-1

WW-2

SW-1

SW-2

SW-3

Ammonia (mg/L as NHþ 4 eN) Bromide (mg/L) 2þ Calcium (mg/L as Ca ) DN (mg/L as N) DOC (mg/L) Nitrate (mg/L as NO
3 eN) Nitrite (mg/L as NO
2 eN) pH Sulfate (mg/L) SUVA254 (L/mgem)

0.1 43 13 4.3 3.5 17 <0.002 7.8 20 2.1

<0.02 62 47 23.5 5.5 12 <0.002 6.7 22 2.1

<0.02 17 7 0.2 1.9 8 <0.002 7.4 24 1.4

0.39 52 20 0.6 4.8 0.7 <0.002 7.5 63 1.4

0.16 52 18 0.4 15 0.8 <0.002 7.8 11 4.0

the same in SW and WW samples; thus, other components in water need to be considered to explain the observed Br
effect on NDMA formation in WW samples. In the literature, studies reported that Br
enhanced NDMA yields from chloramination of DMA (Chen

~ as, 2012). However, competition for et al., 2010; Luh and Marin NH2Cl of background constituents in the water was minimized since those experiments were conducted in DDW. The presence of a complex background matrix that includes NOM, soluble microbial products (SMPs), and a wider range of NDMA precursors may explain the different trends observed in this study. In the presence of Br
, NH2Cl decays as Br
is oxidized by NH2Cl to bromochloramine (NHBrCl) (Bousher et al., 1989; Gazda and Margerum, 1994; Le Roux et al., 2012; Trote et al., 1980). Firstly, the ionization of NH2Cl forms monochlorammonium ion (NH3Clþ) (Eq. 1, Table S1). NH3Clþ, an intermediate compound, oxidizes Br
and leads to the production of bromamines (Eq. 2, Table S1) (Vikesland et al., 2001). Additionally, the hydrolysis of NH2Cl forms free chlorine that can also further oxidize Br
to hypobromous acid (HOBr) (Eq. 4, 5 in Table S1). However, in typical chloramination conditions, the oxidation of Br
by NH3Clþ usually dominates. Both NH2Br and HOBr undergo further rapid reactions to produce mixed bromochloramines, primarily bromochloramine (NHBrCl) (Eq. 3,

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electronegativity of the brominated nitrogen atom, thus favoring the nucleophilic substitution with the DMA moiety (Le Roux et al., 2012). However, an increase in the bromochloramine concentrations did not form more NDMA in the wastewater effluent samples. It is reasonable to think that bromochloramine reacted with effluent organic matter (EfOM ¼ NOM plus SMPs) rather than with NDMA precursors and consequently decreased the formation of NDMA. 3.2. Effect of bromide on the formation of NDMA from model precursor compounds

Fig. 1. Effect of Br
on NDMA formation in surface water (SW) samples. Experimental conditions A) [NDMA]UFC, SW-1 ¼ 9 ng/L, [NDMA]UFC, SW-2 ¼ 5 ng/L, [NDMA]UFC, SW3 ¼ 10 ng/L, [NH2Cl]0 ¼ 3 mg/L as Cl2, phosphate buffer ¼ 2 mM, contact time ¼ 3 d and B) [NDMA]FP, SW-1 ¼ 30 ng/L, [NDMA]FP, SW-2 ¼ 42 ng/L, [NDMA]UFC, SW-3 ¼ 21 ng/L, [NH2Cl]0 ¼ 100 mg/L as Cl2, phosphate buffer ¼ 8 mM, contact time ¼ 5 d. In both experiments the temperature was (21 ± 2)  C and the pH ¼ 8.

Fig. 2. Effect of Br
on NDMA formation in secondary wastewater effluents (WW) samples. Experimental conditions A) [NDMA]UFC, WW-1 ¼ 179 ng/L, [NDMA]UFC, WW2 ¼ 145 ng/L, [NH2Cl]0 ¼ 3 mg Cl2/L, phosphate buffer ¼ 2 mM, contact time ¼ 3 d and B) [NDMA]FP, WW-1 ¼ 433 ng/L, [NDMA]FP, WW-2 ¼ 1278 ng/L, [NH2Cl]0 ¼ 100 mg Cl2/L, phosphate buffer ¼ 8 mM, contact time ¼ 5 d. In both experiments the temperature was (21 ± 2)  C and the pH ¼ 8.

Table S1). Additional fast decomposition reactions of NHBrCl lead to formation of Br
(Eq. 6) (Bousher et al., 1989). Overall, Br
acts as a catalytic in the decomposition of NH2Cl. Increasing Br
results in less NH2Cl available and more highly reactive NHBrCl formed. However, in the presence of NOM a series of reactions of NH2Cl with NOM provides a more complex scenario. Wastewater effluent samples contain more reactive NDMA reactive precursors than surface waters. It has been demonstrated that NH2Cl decay rate increases with increasing Br
concentration at the same time that ~ as, 2014). NHBrCl is bromochloramine increases (Luh and Marin expected to produce more NDMA than NH2Cl due to the higher

The effect of Br
concentration (0e1000 mg/L) on NDMA yield was assessed by conducting experiments with nine NDMA model precursors compounds and UFC and FP chloramination. NDMA model precursor compounds concentrations were 200 nM, it was expected that in these conditions NH2Cl decayed predominantly by self-decomposition and by Br
oxidation. Moreover, the effect of background NOM and/or EfOM was minimized by running the experiments in DDW; thus, formation of NDMA occurred from the reaction of precursors and haloamines added or formed during the reaction. NDMA formation was normalized to the concentration of NDMA formed in the absence of Br
as shown in Fig. 3 ([NH2Cl] ¼ 3 mg/L as Cl2). The presence of Br
enhanced the formation of NDMA in solutions of the selected NDMA model precursors compounds in the range of Br
concentrations studied (0e1000 mg/L). DMiPA, DMBzA and RNTD exhibited the highest NDMA molar conversion yield 37.9%, 73.1%, and 78.8%, respectively. Low yield NDMA precursor compounds such as DMA, TMA and DMAN had molar conversion rates (<1%) similar to that found in literature (Selbes et al., 2013). However, of the three compounds, DMA and TMA rendered the higher increase of NDMA in the presence of Br
which was about twoefold. NHBrCl is expected to form more NDMA than NH2Cl due to the higher electronegativity of the brominated nitrogen atom, consequently favoring the nucleophilic substitution with DMA (Le Roux et al., 2012). Despite the high conversion rate to NDMA of DMBzA and RNTD, Br
did not enhance the formation of NDMA. This can be explained by the fast and high molar conversion rate into NDMA upon chloramination of these two compounds (Le Roux et al., 2012, 2011; Shen and Andrews, 2011; Spahr et al., 2017). From the three tested polymers, only polyDADMAC and polyACRYL were affected by the presence of Br
. A second set of experiments was conducted with the NDMA model precursors compounds under FP conditions. The results (Fig. 4) indicate that the formation of NDMA from the selected precursors remained relatively constant without any change with increasing Br
concentrations when the ratio of NH2Cl to Br
was increased. Even though bromochloramine is more reactive than NH2Cl, it is also less stable than NH2Cl. Due to the abundance of NH2Cl in FP conditions, the bromamines and bromochloramine can be deactivated by reacting with NH2Cl (Le Roux et al., 2012). NDMA yield from chloramination ([NH2Cl] ¼ 175 mg/L as Cl2) of DMA was almost double in the presence of 79,904 mg/L Br
(NH2Cl/ Br
molar ratio of 2.5:1) and 24 h of reaction time (Le Roux et al., 2012). In contrast, in our study molar ratios of NH2Cl/Br
were 3.4:1 and 114:1 with 1000 mg/L Br
for UFC and FP conditions, respectively. These findings emphasize the importance of the NH2Cl to Br
ratio. An additional experiment was conducted with only DMA and TMA. The solutions containing the amines were oxidized with NH2Cl at 3 mg/L as Cl2 and 3 d of reaction time. Bromide concentration ranged from 0 to 32,000 mg/L (Fig. 5). Such high

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Fig. 3. Effect of Br
on NDMA formation from model precursors compounds. Experimental conditions: [NDMA]0, DMA ¼ 23 ng/L, [NDMA]0, TMA ¼ 32 ng/L, [NDMA]0, DMiPA ¼ 2950 ng/ L, [NDMA]0, DMAN ¼ 19 ng/L, [NDMA]0, DMBzA ¼ 10835 ng/L, [NDMA]0, RNTD ¼ 11670 ng/L, [NDMA]0, polyDADMAC ¼ 27 ng/L, [NDMA]0, polyAMINE ¼ 120 ng/L, [NDMA]0, polyACRYL ¼ 7 ng/L, [DMA]0 ¼ [TMA]0 ¼ [DMiPA]0 ¼ [DMAN]0 ¼ [DMBzA]0 ¼ [RNTD]0 ¼ 0.0002 mM, [polyDADMAC]0 ¼ [polyAMINE]0 ¼ 0.2 mg/L, [polyACRYL]0 ¼ 1 mg/L, [NH2Cl]0 ¼ 3 mg Cl2/L, pH ¼ 8, phosphate buffer ¼ 2 mM, contact time ¼ 3 d, temperature ¼ (21 ± 2)  C.

concentrations of Br
are not expected to occur in source waters but were used to illustrate how Br
can increase the formation of NDMA in waters that contain precursors that generally have low NDMA conversion yield. Under these conditions, NDMA rose linearly as the Br
increased. NDMA formation from DMA was enhanced by 25efold and by 45efold from TMA. 3.3. Effect of the pH on NDMA formation in the presence of bromide in selected model precursors compounds The effect of pH on the formation of NDMA from DMA, TMA, polyDADMAC and polyACRYL was studied in the presence of 1000 mg/L Br
and NH2Cl at 3 mg/L as Cl2. The pH was controlled with 0.002 M phosphate buffer at 6, 7 and 8. NDMA formation was normalized relative to the concentration at pH 6, and the results are shown in Fig. 6. All the investigated compounds exhibited an increase, to a different extent, in the concentration of NDMA formed as the pH increased. Particularly, TMA and polyDADMAC showed the higher rate of NDMA formation, while polyACRYL was not

considerably affected by the changes in the pH. NDMA formation enhancement with increasing pH from 6 to 8 in the presence of Br
during chloramination was attributed to the formation of the highly reactive bromochloramine. Bromochlor~ as, amine was found to be more stable at pH 8 than 6 (Luh and Marin 2014). Thus, the higher formation of NDMA at pH 8 compared to pH 6 was attributed to the increased stability of bromochloramine. At pH 6, bromochloramine rapidly decomposes, decreasing the conversion rate of NDMA precursors to NDMA. Also, at pH 8 the ratio of the deprotonated amine is higher than at lower pH values, consequently favoring the reaction kinetics towards the NDMA formation (Schreiber and Mitch, 2006; Selbes et al., 2018; Shen and Andrews, 2013). 3.4. Effect of NOM on the formation of NDMA in selected model precursor compounds and treated water It is important to understand the effect of Br
on the formation of NDMA from selected NDMA model precursors in the presence of

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Fig. 4. Effect of Br
on NDMA formation from model precursors compounds. Experimental conditions: [NDMA]0, DMA ¼ 456 ng/L, [NDMA]0, TMA ¼ 774 ng/L, [NDMA]0, ¼ 5600 ng/L, [NDMA]0, DMAN ¼ 213 ng/L, [NDMA]0, DMBzA ¼ 9700 ng/L, [NDMA]0, RNTD ¼ 11293 ng/L, [NDMA]0, polyDADMAC ¼ 87 ng/L, [NDMA]0, polyAMINE ¼ 526 ng/L, [NDMA]0, polyACRYL ¼ 13 ng/L, [DMA]0 ¼ [TMA]0 ¼ [DMiPA]0 ¼ [DMAN]0 ¼ [DMBzA]0 ¼ [RNTD]0 ¼ 200 nM, [polyDADMAC]0 ¼ [polyAMINE]0 ¼ 0.2 mg/L, [polyACRYL]0 ¼ 1 mg/L, [NH2Cl]0 ¼ 100 mg Cl2/L, pH ¼ 8, phosphate buffer ¼ 8 mM, contact time ¼ 5 d, temperature ¼ (21 ± 2)  C. DMiPA

NOM due to the water treatment practical implications. The effect of NOM on the formation of NDMA from DMA, TMA, polyDADMAC, polyACRYL was evaluated in the presence of 1000 mg/L Br
. The initial concentrations were [DMA] and [TMA] ¼ 200 nM, [polyDADMAC] ¼ 0.2 mg/L and [polyACRYL] ¼ 1.0 mg/L. Untreated water from a DWTP was adjusted with DDW to the appropriate DOC concentration. The results are presented in Fig. 7. NDMA formation was normalized to the concentration of NDMA in the presence of Br
and no DOC. The formation of NDMA from DMA, TMA and polyDADMAC was suppressed by >70% at 0.94 mg C/L, while polyACRYL was less impacted (~25%) by the presence of NOM. After 1.9 mg/L DOC, the formation of NDMA remained constant for the four compounds. The difference in the molecular structures of the model precursors did not cause changes in the pattern of NDMA formation. Under the experimental conditions, it was clear that increasing DOC concentration suppressed the formation of NDMA from the selected NDMA precursor compound, but the effect was stronger for DMA, TMA and polyDADMAC. Bromide enhanced the formation

of NDMA from the selected compounds (Fig. 3) in the absence of NOM. However, such conditions are not expected to happen during water treatment. NOM is a ubiquitous component of natural waters, NH2Cl decay in the presence of NOM increases via two pathways, auto decomposition and oxidation of NOM (Chen and Valentine, 2006). In the conditions of our experiments, bromamines species and to a lesser extent HOBr are likely to form through the oxidation of Br
by NH2Cl. In the presence of NOM, HOBr participates in oxidation/substitution reactions resulting in a mixture of halogenated DBPs that leads to the loss of NH2Cl (Duirk and Valentine, 2007). The oxidation of Br
by NH2Cl forms different active forms of bromine in the þ1 valence state. These bromine active species may also react with NOM resulting in the formation of brominated and mixed chlorinated-brominated DBPs. Two major reactions in the oxidation of Br
are of importance. First, the ionization of NH2Cl forms the monochlorammonium ion (NH3Clþ) that later oxidizes Br
to bromochloramine (NHBrCl) (Duirk and Valentine, 2007; Le Roux et al., 2012; Trote et al., 1980). Second, the hydrolysis of

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Fig. 5. Effect of high concentrations of Br
on NDMA formation from DMA and TMA. Experimental conditions: [DMA]0 ¼ [TMA]0 ¼ 200 nM, [NH2Cl]0 ¼ 3.0 mg/L as Cl2, phosphate buffer ¼ 2 mM, contact time ¼ 3 d, temperature ¼ (21 ± 2)  C.

NH2Cl forms hypochlorous acid that further oxidize Br
to HOBr (Valentine and Jafvert, 1988). It should be noted that NOM competes for chloramines species (Schreiber and Mitch, 2007). Thus, all bromine species formed as a consequence of the oxidation of Br
either by NH2Cl or hypochlorous acid can also oxidize NOM (Duirk et al., 2005; Duirk and Valentine, 2007; Vikesland et al., 2001). Thus, there is a strong competition between NDMA precursors and NOM for all chloramine or bromine active species. Consequently, it is postulated that the increase in the DOC concentration results in the formation of more chlorinated/brominated DBPs and a decrease in the overall formation of NDMA. An additional experiment was conducted with treated water (SWe2) to assess the effect of Br
on NDMA formation during chloramination in the presence of NOM. The treated water was diluted with DDW to 2.0 mg/L DOC to simulate conditions of a finished water. The samples were amended with DMA and TMA at

Fig. 6. Effect of pH and Br
on NDMA formation from DMA, TMA, polyDADMAC and polyACRYL. Experimental conditions: [DMA]0 ¼ [TMA]0 ¼ 200 nM, [polyDADMAC]0 ¼ 0.2 mg/L, [polyACRYL]0 ¼ 1 mg/L, [Br
]0 ¼ 1000 mg/L, [NH2Cl]0 ¼ 3 mg/L as Cl2, phosphate buffer ¼ 8 mM, contact time ¼ 3 d, temperature ¼ (21 ± 2)  C.

7

Fig. 7. Effect of DOC on NDMA formation from DMA, TMA, polyDADMAC and polyACRYL. Experimental conditions: [DMA]0 ¼ [TMA]0 ¼ 200 nM, [polyDADMAC]0 ¼ 0.2 mg/L, [polyACRYL]0 ¼ 1 mg/L, [Br
]0 ¼ 1000 mg/L, [NH2Cl]0 ¼ 3 mg/L as Cl2, phosphate buffer ¼ 8 mM, contact time ¼ 3 d, temperature ¼ (21 ± 2)  C.

200 nM. Bromide was spiked at 0, 250, 500 and 1000 mg/L. NDMA was measured and NDMA molar yields were calculated, results are presented in Fig. 8. Bromide enhanced NDMA formation from DMA, particularly at high Br
levels (i.e., 1000 mg/L). For instance, NDMA molar yields were (1.26 ± 0.02) % vs. (1.47 ± 0.02) % in the absence of Br
and with 1000 mg/L. This is a 17% increase from (187 ± 3) ng/L to (217 ± 3). However, TMA showed no variation with increasing Br
concentration. NDMA molar yields from TMA were (0.76 ± 0.02) % within the studied Br
range. Though, in the absence of NOM the formation of NDMA from DMA and TMA increased by 88% and 110%, respectively. Bromide is not effectively removed during conventional water treatment processes and Br
at high concentrations is a cause of

Fig. 8. Effect of Br
on NDMA formation in a treated source water spiked with DMA and TMA. Experimental conditions: [DOC]0 ¼ 2 mg/L, [DMA]0 ¼ [TMA]0 ¼ 200 nM, [Br¡]0 ¼ 0, 250, 500 and 1000 mg/L, [NH2Cl]0 ¼ 3 mg/L as Cl2, phosphate buffer ¼ 8 mM, contact time ¼ 3 d, temperature ¼ (21 ± 2)  C. Error bars represent the standard deviation of duplicate samples and two injections of each of the samples (n ¼ 4).

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concern, because it can lead to higher formation of NDMA. Overall, the results show that enhancement of NDMA formation due to presence of Br
is NDMAeprecursor specific and is only relevant at high concentrations of Br
. It is also important to note that the presence of NOM increases the competition with NDMA precursors for active brominated haloamines. 4. Conclusions Previous studies showed that Br
enhanced NDMA formation from a limited number of NDMA precursors investigated and using high Br
concentrations, magnifying its effect. Although Br
is a ubiquitous component in natural and wastewater effluents, the reported concentrations (e.g., <4000 mg/L) in the literature are lower than those used in such studies. Consistent with the literature, we also found that Br
enhanced NDMA formation from model precursors compounds. Since these experiments were conducted in deionized water, the effect of NOM was controlled. However, NDMA formation increase due to the presence of Br
and NOM is not observed. NOM competes with NDMA precursors for chloramines or bromochloramines species. In real waters, increasing Br
concentration (0e1000 mg/L) in surface waters caused no enhancement on the formation of NDMA either with NH2Cl at 3 or 100 mg/L as Cl2. In contrast, Br
suppressed NDMA formation with increasing Br
concentration in wastewater effluent samples at NH2Cl concentration of 3 mg/L as Cl2. An effect not observed at 100 mg Cl2/L. It is possible that bromochloramine reacted with NOM rather than with NDMA precursors and consequently decreased the formation of NDMA. Therefore, enhancement of NDMA formation might not be as relevant considering environmental Br
levels and treated water DOC conditions. Nonetheless, it is important to note that increasing concentrations of Br
can occur due to other anthropogenic activities and saltwater intrusion. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to thank the water and wastewater treatment plants for providing samples. Wilson Beita-Sandí was partially supported by the University of Costa Rica. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.watres.2019.115323. References Alley, B., Beebe, A., Rodgers, J., Castle, J.W., 2011. Chemical and physical characterization of produced waters from conventional and unconventional fossil fuel resources. Chemosphere 85, 74e82. https://doi.org/10.1016/j.chemosphere.2011.05.043. Beita-Sandí, W., Ersan, M.S., Uzun, H., Karanfil, T., 2016. Removal of N-nitrosodimethylamine precursors with powdered activated carbon adsorption. Water Res. 88, 711e718. https://doi.org/10.1016/j.watres.2015.10.062. Bousher, A., Brimblecombe, P., Midgley, D., 1989. Kinetics of reactions in solutions containing monochloramine and bromide. Water Res. 23, 1049e1058. https:// doi.org/10.1016/0043-1354(89)90180-2. Chen, Z., Valentine, R.L., 2006. Modeling the formation of N -nitrosodimethylamine (NDMA) from the reaction of natural organic matter (NOM) with monochloramine y. Environ. Sci. Technol. 40, 7290e7297. https://doi.org/10.1021/ es0605319. Chen, Z., Yang, L., Zhai, X., Zhao, S., Li, A., Shen, J., 2010. N-nitrosamine formation

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