Reinvestigation of the Nitrosamine-Formation Mechanism during

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Environ. Sci. Technol. 2009, 43, 5481–5487

Reinvestigation of the Nitrosamine-Formation Mechanism during Ozonation LEI YANG, ZHONGLIN CHEN,* JIMIN SHEN, ZHENZHEN XU, HENG LIANG, JIAYU TIAN, YUE BEN, XU ZHAI, WENXIN SHI, AND GUIBAI LI State Key Laboratory of Urban Water Resources and Environment, School of Municipal & Environmental Engineering, Harbin Institute of Technology, Harbin, 150090, China

Received February 3, 2009. Revised manuscript received May 29, 2009. Accepted June 1, 2009.

Previous studies have linked nitrosamine formation during ozonation to a nitrosation process in which nitrosation is catalyzed by formaldehyde, a normal byproduct of ozonation. This mechanism cannot explain the increase in N-nitrosodimethylamine (NDMA) formation with an increase of pH. This study reinvestigates the pathway of N-nitrosamine formation during ozonation. Our observations demonstrated the critical importance of some reactive inorganic nitrogenous intermediates, such as hydroxylamine and dinitrogen tetroxide (N2O4). We report two alternative pathways that possibly explain nitrosamine formation during ozonation at neutral and alkaline pH: (i) secondary amine precursors reacting with hydroxylamine to form unsymmetrical dialkylhydrazine intermediates, which are further oxidized to their relevant nitrosamines; and (ii) a nitrosation pathway in which N2O4 acts as the nitrosating reagent. The key variables of pathway (i) (including reaction time,pH,dissolvedoxygen)wereinvestigated.Sincehydroxylamine is a common intermediate of dimethylamine oxidation, it is reasonable to assume that hydroxylamine is a possible inorganic precursor for NDMA formation during oxidation processes using strong oxidants. With an improved understanding of the pathway of nitrosamine formation, it should be apparent that the reactive nitrogenous intermediates play an important role in the N-nitrosamine-formation, so future studies of N-nitrosamine-formation control should be focused on the transformation of nitrogen in water treatment.

Introduction N-nitrosodimethylamine (NDMA), a type of nitrosoamine, is a highly mutagenic compound that is suspected of carcinogenic activity in the human body (1). The formation of NDMA during chlorine disinfection has caused significant concern among drinking-water and wastewater-recycling utilities in recent years (2-7). The U.S. Environmental Protection Agency (USEPA) classifies NDMA as a probable human carcinogen and has estimated that it has a 10-6 cancer risk level in drinking water when present at a concentration of 0.7 ng/L (8). * Corresponding author e-mail: [email protected]; [email protected]; tel: +86-451-86283028; fax: +86-45186283028. 10.1021/es900319f CCC: $40.75

Published on Web 06/12/2009

 2009 American Chemical Society

Dimethylamine (DMA), the most direct and effective precursor of NDMA, can react with nitrate or nitrite in acidic environments (reaction 1), where the yield of NDMA formation was found to reach a maximum at pH 3.4 (9). The reaction is very slow at neutral pH, so it is not the main pathway for NDMA formation in water treatment processes. Formaldehyde, when present, can catalyze the reaction between secondary amines and nitrites, leading to N-nitrosamine formation at higher pH (6.4-11) (10). The yield of Nnitrosamines decreases with increasing pH. H+

(CH3)2NH + NO2 f (CH3)2NNO + H2O

(1)

Monochloramine (NH2Cl) has been identified as the most important oxidant for NDMA formation during chlorine disinfection (3, 11, 12). The formation of NDMA in this environment is consistent with a reaction involving the slow formation of unsymmetrical dimethylhydrazine (UDMH, (CH3)2NNH2) via a nucleophilic substitution reaction between uncharged DMA and NH2Cl, followed by the rapid oxidation of UDMH to a variety of products, including NDMA (reactions 2 and 3). NH2Cl + (CH3)2NH f (CH3)2NNH2 + H+ + Cl-

(2)

(CH3)2NNH2 + NH2Cl + H2O f (CH3)2NNO + NH+ 4 + Cl (3)

Recently, the critical importance of dichloramine (NHCl2) and dissolved oxygen in NDMA formation was discovered (13). Schreiber and Mitch proposed a new pathway of NDMA formation in which NHCl2 reacts with DMA to form chlorinated unsymmetrical dimethylhydrazine (UDMH-Cl, (CH3)2NNHCl) intermediates (reaction 4), which can be oxidized by chloramines or dissolved oxygen to form NDMA. The N-Cl linkage in UDMH-Cl enables the incorporation of dissolved oxygen to give NDMA, so NDMA formation occurs readily by this pathway. NHCl2 + (CH3)2NH f (CH3)2NNHCl + H+ + Cl-

(4)

The results of studies performed by Mun ˜ oz and von Sonntag (14) and von Gunten (15), indicate that ozonation does not lead to NDMA formation. But Andrzejewski et al. (16, 17) detected NDMA after ozonation and other oxidation treatments of aqueous solutions of DMA. Although the yield of NDMA during ozonation (below 0.4% in relation to DMA) (17) is much less than the yield during chloramination, it should be significant if there are reasonable concentrations of DMA in the untreated water. Andrzejewski et al. (17) explained the NDMA formation as a product of formaldehydecatalyzed nitrosation, which included multistep reactions as described by Keefer and Roller (10), because of the presence of nitrite in the postreaction mixture. The formaldehydecatalyzed pathway can explain NDMA formation at pH > 6.4, but it fails to explain the increases in yield of NDMA as pH increased during ozonation. In our earlier research on NDMA formation during denitrification process of wastewater treatment, we found that NDMA can be detected in the absence of nitrite (not added in the prereaction mixture and undetected during the reaction), but that the yield of NDMA was an order of magnitude smaller than when nitrite was present. Without nitrite, NDMA could not form via the nitrosation pathway, so there might be other mechanisms which could explain the NDMA formation observed during the denitrification process. VOL. 43, NO. 14, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Hydroxylamine (NH2OH), as a common intermediate of nitrogen transformation processes, has a molecule structure similar to monochloramine. Hence, organic precursors of NDMA, such as DMA, may react with hydroxylamine to form NDMA via an analogous pathway during the chloramination process. Secondary amines are easily oxidized to hydroxylamines, although yields are generally poor due to overoxidation (18, 19). Kim and Choi (20) proposed a mechanism of photocatalytic degradation of aliphatic amines with TiO2. They indicated that hydroxylamines were the intermediates of the transformation of organic amines into inorganic products, during which hydroxyl radicals played an important role. During ozonation, aliphatic amines react with both ozone and hydroxyl radicals, so it is expected that hydroxylamines will be formed. The aim of this paper is to reinvestigate the mechanisms of N-nitrosamine formation during ozonation as proposed by Andrzejewski et al. (17). We conducted a series of experiments using DMA as a model precursor for NDMA formation to investigate the possibility that nitrosation occurs in neutral and alkaline media. These results showed that the yield of NDMA during ozonation could not be explained completely by the nitrosation pathway. We then focused on investigating the hypothesis that NDMA is also a product of the reaction of hydroxylamine with DMA in the absence of nitrite. While detailed mechanistic studies have yet to be done, our results suggest that the formation of secondary N-nitrosamines by a reaction between the relevant secondary amines (such as DMA, diethylamine, methylethylamine) and hydroxylamine via the UDMH pathway could explain the observed increase in NDMA production during ozonation as pH increases.

Materials and Methods Materials. All experiments were conducted using deionized water. Dimethylamine hydrochloride (99%, Acros Organics), methylethylamine (94%, MEA, Acros Organics), diethylamine (g99%, DEA, Acros Organics), hydroxylamine hydrochloride (g98.5%, Bechmark, China), and sodium nitrite (g97%, Bodi, China) were used without further purification. Supelco N-nitrosodimethylamine (200 µg/mL in methanol, NDMA), AccuStandard nitrosomethylethylamine (1.0 mg/mL in methanol, NMEA) and Sigma-Aldrich nitrosodiethylamine (g99.0%, NDEA) were used as standards without further purification. Solution pH was adjusted by the addition of sodium hydroxide or hydrochloric acid. Experimental Procedures. Unless otherwise specified, all nitrosamine formation experiments were conducted in sealed 250 mL bottles, and pH was adjusted using 3 M NaOH. The solutions were reacted for 24-36 h in the dark at 25 °C before being analyzed for nitrosamine concentration. The reactions were quenched by Na2SO3 as terminator. The experiments investigating the influence of pH and dissolved oxygen (DO) on NDMA formation were performed in parallel tests (n ) 3). In the DO experiments, deionized water was bubbled with nitrogen gas or oxygen gas to attain a specified DO concentration before the reagents were added. The ozonated water used in the ozonation experiment was prepared at room temperature (25 °C) in a 1 L reactor, which had been modified from a flat-bottomed flask. Ozone was produced by a laboratory ozonizer (DHX-SS-1G, Harbin Jiu Jiu Electrochemistry Engineering Ltd., China) supplying dry oxygen. The ozone was introduced into the reactor and the magnetic stirrer was then switched on. Dissolved ozone was measured by the Indigo method (21). Water containing ozone at a specified concentration was transferred to 250 mL bottles containing a diluted stock solution of DMA. The pH was adjusted using NaOH and HCl solutions. The reactions were halted after 24 h by adding 0.1 M Na2S2O3 solution to quench the ozone. 5482

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Analysis. NDMA, NMEA, and NDEA were detected by a high-performance liquid chromatography (HPLC, LC-10A, Shimadzu, Japan) method developed by Chen et al. (22). A C-18 column (5 µm particles, 4.6 × 150 mm, Agela Technologies Inc.) was used to separate peaks in the LC before injection into the ultraviolet detector. The mobile phase was methanol-water (5:95, v/v) at a flow rate of 1.0 mL/min. The injection volume was 100 µL. NDMA, NMEA, and NDEA were detected by the ultraviolet detector at a wavelength of 228 nm. The practical method detection limit (MDL) was 0.1 µg/L. Hydroxylamine was qualitatively detected using HPLC with a fluorometric detector (23). 4-Nitrobenzaldehyde was introduced as a derivatization reagent for UDMH analysis (24), and the derivatization products were qualitatively detected by a Finnigan LCQ DECA XPMAX capillary HPLC iontrap MS system operated in the ESI+ mode. Spectra were obtained by performing full-scan MS within the m/z range of 50-500. The mobile phase was 10 mM ammonium acetate in methanol-water solution (methanol/water ) 7:3, v/v) at a flow rate of 0.2 mL/min. Dissolved oxygen concentration was detected using a dissolved oxygen analyzer (Level2, WTW, Germany).

Results and Discussion Investigation of the Nitrosation Pathway during Ozonation. Andrzejewski et al. (17) used the formaldehyde-catalyzed pathway proposed by Keefer and Roller (10) to explain the formation of NDMA during ozonation at alkaline pH, while ozone does not actually participate in the nitrosation process. However, as discussed in the introduction, the observations made by Andrzejewski were not completely consistent with those of Keefer and Roller. If the yield of NDMA decreases with increasing pH, over the pH range 7-11, during nitrosation catalyzed by formaldehyde (which would be consistent with the results indicated by Keefer and Roller (10)), then the catalytic nitrosation pathway may not be the main pathway for NDMA formation during ozonation at neutral and alkaline pH. Therefore, at the beginning of this study, experiments were designed to examine the variation in NDMA formation via formaldehyde-catalyzed nitrosation under a range of experimental conditions. These conditions and the results of the experiments are shown in the Supporting Information (SI) as Table S1. After 24 h, only 2 µg/L NDMA was detected while concentrations of formaldehyde, DMA, and nitrite at pH 7.0 were 1.0, 0.3, and 0.3 mM, respectively. The ratio of DMA converted to NDMA was only 0.009%. At pH > 7, no NDMA was detected after 24 h. That is, nitrosation catalyzed by formaldehyde can occur at neutral pH, but that process cannot explain the increase in NDMA formation as pH increases during ozonation. It is possible that some other reactive nitrogenous intermediate is responsible for NDMA formation during ozonation. Andrzejewski (17) indicated that hydroxyl radicals might play a significant role in NDMA formation during ozonation, especially at alkaline pH. It is well-known that hydroxyl radicals are readily formed under alkaline conditions due to the decomposition of ozone accelerated by hydroxide ions (25), so more DMA could be degraded into nitrite and other nitrogenous products under these conditions. Nitrite can be oxidized rapidly by hydroxyl radicals (k1 ) 1.0 ( 0.1 × 1010 M-1 s-1 (26)), as shown in reaction 5. k1

• OH• + NO2 f NO2 + OH

(5)

Nitrite radicals (NO•2) can react with a second nitrite radical to form N2O4 (k2 ) 9 × 108 M-1 s-1, k-2 ) 1.38 × 104 M-1 s-1), a reactive nitrogen-oxygen species (27), as shown in reaction 6.

k2

2NO•2 798 N2O4

(6)

k-2

N2O4 exists in two isomeric forms, ON-ONO2 and O2N-NO2, which are very effective nitrosating and nitrating agents, respectively. The nitrosation of DMA by asymmetrical N2O4 (ON-ONO2) was proven to contribute to NDMA formation (reaction 7) (27), while NO2- can be regenerated by the hydrolysis of N2O4 (reaction 8). However, the hydrolysis rate of N2O4 is slower than the reaction rate of N2O4 with most amines. Additionally, the formation of N2O4 during the oxidation of nitrite to nitrate by free chlorine was once used to explain the enhanced nitrosation observed in the presence of HOCl (28). (CH3)2NH + ONONO2 f (CH3)2NNO + HONO2 (7) + N2O4 + H2O T NO2 + NO3 + 2H

(8)

As pH increases, hydroxide ions in water will accelerate the decomposition of ozone to form more hydroxyl radicals (25), leading to more DMA transforming to hydroxylamines, which will be further oxidized into NO2- and other products. The formation of N2O4 could be restrained by the competition between NO2- and other compounds, such as hydroxylamines, for the hydroxyl radicals. For instance, the rate constant of the reaction between hydroxyl radicals and hydroxylamine is 9.5 × 109 M-1 s-1 (29), which is comparable to the k1 of reaction 5. So the yield of N2O4 would not be increased evidently with the increasing pH. Furthermore, the rate of N2O4 hydrolysis also increases at alkaline pH. Therefore, the nitrosation pathway via N2O4 still cannot completely explain the high yield of N-nitrosamine during ozonation under alkaline conditions. NDMA Formation through the Hydroxylamine Pathway. An experiment was designed to find out whether NDMA could be formed by the reaction of DMA and hydroxylamine. As the hydroxylamine concentration increased from 0.05 to 2 mM (pH ) 7.0, 24 h), the yield of NDMA increased from 1 to 2.5 µg/L, representing no more than 0.011% of DMA converted into NDMA (Figure 1A). Another experiment considered the effect of reaction time on NDMA formation (0.3 mM DMA, 2 mM hydroxylamine). In that experiment, approximately 2.4 µg/L NDMA was formed in the first 24 h, after which the yield increased more slowly (Figure 1B). NDMA formation during this process may occur via an analogous pathway involving UDMH (reaction 9), given that hydroxylamine has a structure similar to monochloramine.

FIGURE 1. (A) Effect of hydroxylamine dosage on NDMA formation ([DMA] ) 0.3 mM, pH ) 7.0, T ) 25 °C). (B) Effect of reaction time on NDMA formation ([NH2OH] ) 2 mM, [DMA] ) 0.3 mM, pH ) 7.0, T ) 25 °C).

Since chlorine is more electron withdrawing than the hydroxyl group, less UDMH will be formed via nucleophilic substitution on the amine group of hydroxylamine than chloramine, leading to less NDMA formation. With an increase in ozone concentration, more DMA can be oxidized into hydroxylamine, leading to more NDMA formation. The phenomenon is consistent with the observations of Andrzejewski, who found that the yield of NDMA increased with increases in ozone/DMA ratio (17). NH2OH + (CH3)2NH f (CH3)2NNH2 + H2O

(9)

To validate our hypothesis that NDMA is also a possible product of the reaction between hydroxylamine and DMA via the UDMH pathway, we need first to verify the formation of UDMH in the reaction solution containing hydroxylamine and DMA. A diluted reaction solution (12 h, pH 11.06) of hydroxylamine (100 mM) and DMA (100 mM) was derivatized with 4-nitrobenzaldehyde and then detected by ESI+ -MS. If UDMH is formed in the reaction solution containing hydroxylamine and DMA, then the reaction between UDMH and 4-nitrobenzaldehyde (OdCHArNO2, M1), shown in reaction 10, will occur and the UDMH derivative product, 1,4-nitrobenzaldehyde dimethylhydrazone ((CH3)2NNd CHArNO2, M2), can be used as the surrogate for UDMH qualitative analysis (24). (CH3)2NNH2 + O ) CHArNO2 T (CH3)2NN ) CHArNO2 + H2O (10) The solution was measured without separation by chromatogram column. The total ion chromatograms (TIC) with background subtracted is shown in Figure 2A. During ESI+MS analysis, small polar or basic molecules can produce [M + H]+/[M + Na]+ by proton/cation attachment, or dimers such as [2M + H]+/[2M + Na]+ (30). As shown in Figure 2B, [M2 + 1]+ was detected. The derivatization of UDMH needs excess M1 for the synthesis of M2, so [M1 + 1]+ and [2M1 + 1]+ (dimer of M1), m/z 152 and m/z 303 respectively, were also detected. We assumed that M2 was able to form dimers during the ESI+-MS process in the same way as M1. This hypothesis was proven by the detection of [2M2 + 1]+. According to the results shown in Figure 2B, M2 can also form a covalent dimer ([2(M2-1) + 1]+). The detection of the three M2 quantification ions, [M2 + 1]+, [2M2 + 1]+ and [2(M2-1) + 1]+, is proof of the formation of UDMH during the reaction between DMA and hydroxylamine. The extracted ion chromatograms of these four ions are also shown in Figure 2A. As shown in Figure 2B, we can also find m/z 367 and m/z 409, which may be [M1 + M2 + 23]+ and [2M2 + 23]+, respectively. There are still some ions we cannot define which may be products of DMA and hydroxylamine, or derivatization products of M1. Effect of pH on the Formation of NDMA via the Hydroxylamine Pathway. Since hydroxylamine can react with DMA to form NDMA, we attempted to find more evidence to confirm hydroxylamine as a possible precursor of NDMA during ozonation. According to Andrzejewski et al. (17), pH is an important factor in NDMA formation during ozonation. The experiments reported in this section were designed to determine whether pH also plays an important role in the hydroxylamine pathway. The pH of a series of 1 mM hydroxylamine solutions was adjusted to 4.65, 6.04, 6.79, 7.20, 10.52, 11.06, and 11.27, respectively. After 0.3 mM DMA was added, the solutions were mixed completely then reacted in the dark for 24 h. As shown in Figure 3, the yield of NDMA increased with increasing pH, and the yield of NDMA was lower than the detection limit under acidic conditions. The formation of UDMH is a well-known alkaline catalytic reaction (31). At pH > pKa,DMA ) 10.7, unprotonated DMA is VOL. 43, NO. 14, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (A) Total ion chromatograms (TIC) and extracted ion chromatograms for ESI+-MS analysis of DMA (100 mM) and hydroxylamine (100 mM) reaction products (12 h, pH ) 11.06) with a 4-nitrobenzaldehyde derivatization. (B) ESI+-MS (50-500) of compounds with retention time RT ) 0.12-0.85 min. able to react with free hydroxylamine to form UDMH, while hydroxylamine can undergo acidic ionization into NH2O- in the presence of strong alkali (32) (reaction 11). We therefore assumed that the maximum yield of NDMA would not occur at the highest pH. However, our results were not consistent with this hypothesis. NH2OH + OH- f NH2O- + H2O (11) This difference may have been caused by the order in which the reagents were added to the reaction solution in our study. The stock solution of DMA was prepared using dimethylamine hydrochloride, so protonated DMA (DMAH+) was the major species added to the reaction solution. DMA-H+ should react with NH2O- more readily than DMA. We hypothesized that the reaction of NH2O- and DMA-H+ can also produce UDMH (reaction 12), which can be further oxidized into NDMA. NH2O- + (CH3)2NH+ (12) 2 f (CH3)2NNH2 + H2O We found that other secondary aliphatic amines, such as DEA and MEA, could also react with hydroxylamine to form 5484

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the relevant nitrosamines. To more easily determine any changes in nitrosamine formation, we adjusted the pH of some reaction solutions to 11.06 using NaOH. The results are shown in the Supporting Information as Figure S1. The yields of all three nitrosamines (NDMA, NDEA, and NMEA) increased as a function of increases in hydroxylamine concentration (from 0.05 to 1.0 mM). The results also showed that the maximum nitrosamine yield occurred close to a secondary amine/hydroxylamine ratio of approximately 1 (Figure 4). The formation amount of the three nitrosamines decreased rapidly as the ratio of secondary aliphatic amine to hydroxylamine was further increased beyond 1. Effect of Dissolved Oxygen on NDMA Formation via the Hydroxylamine Pathway. Dissolved oxygen (DO) plays an important role in NDMA formation through the UDMH pathway during chloramination. UDMH can be oxidized into NDMA by DO at a rate of k3 ) 0.3 ( 0.1 M-1 S1- (reaction 13) (13). Since chloramines are stronger oxidants than DO, DO was not considered to be the main factor influencing NDMA formation during the chloramination process.

FIGURE 3. Influence of pH on NDMA formation via the hydroxylamine pathway ([NH2OH] ) 1 mM, [DMA] ) 0.2 mM, T ) 25 °C). Error bars represent one standard deviation (n ) 3).

FIGURE 5. Influence of dissolved oxygen on NDMA formation ([hydroxylamine] ) [DMA] ) 0.3 mM, pH ) 11.06, T ) 25 °C). Error bars represent one standard deviation (n ) 3). limit at pH 2.5 and 5.6, while 0.2 µg/L NDMA formed at pH 3.0 (shown in Figure 6B). We supposed that nitrosation should be the major pathway for NDMA formation during ozonation. As pH increased from 7.0 to 10.0, NDMA formation rose slightly from 0.97 to 2.02 µg/L, but the yield of NDMA decreased to 1.91 µg/L when pH increased to 11.6. The hydroxylamine pathway can be used to explain this phenomenon under neutral or alkali conditions. As pH increases, more hydroxide ions in solution will accelerate the decomposition of ozone to form hydroxyl radicals (reactions 14 and 15) (25), which then oxidize the amine into more hydroxylamines.

FIGURE 4. Influence of secondary aliphatic amine to hydroxylamine ratio on N-nitrosamine formation ([hydroxylamine] ) 0.3 mM, pH ) 11.06, T ) 25 °C). k3

(CH3)2NNH2 + O2 f (CH3)2NNO + H2O

(13)

The influence of DO on NDMA formation was examined at pH 11.06 and the results are presented in Figure 5. The results indicate that there is a certain DO value at which the maximum concentration of NDMA is formed. As DO increased from 1.21 to 8.75 mg/L, the yield of NDMA increased due to the oxidation of UDMH. But as DO increased further from 8.75 to 30.0 mg/L, NDMA yield decreased from 44.87 to 2.21 µg/L. This decline may have been caused by competition between hydroxylamine and UDMH for oxygen. As a substance that can be both oxidized and reduced, hydroxylamine may react with oxygen to form nitrite and nitrate, thereby decreasing the amount of hydroxylamine available as an inorganic precursor of NDMA. Proposed Mechanisms of NDMA Formation during Ozonation. A possible full-scale mechanism for NDMA formation during ozonation is proposed for the pH range 2.5-11.6. Mirvish (9) reported that NDMA could be readily formed via the reaction of nitrite and DMA at acidic pH, where the maximum yield occurred at pH 3.4 (reaction 1), similar results were observed in our experiments (shown in Figure 6A). During ozonation, the yield of NDMA was below our detection

O3 + OH- f HO2 + O2

(14)

· · O3 + HO2 f OH + O2 + O2

(15)

The reactive intermediate UDMH is formed via a nucleophilic substitution reaction between DMA (DMA-H+) and NH2OH (NH2O-). Strongly alkaline conditions (pH > 11.0) are not favorable for NDMA formation during ozonation, although we hypothesized that the hydroxylamine pathway is an alkaline catalytic reaction. Our results were consistent with the hypothesis proposed in our discussion of the effect of pH. Furthermore, more hydroxyl radicals can be formed at alkaline pH, which will consume more hydroxylamine and convert it into nitrite, N2O, N2, and other products (32). Some nitrosating reagents, such as N2O4, can be formed by hydroxyl radicals and nitrite, so nitrosation can still be a pathway for NDMA formation at neutral or even alkaline pH. Based on the results of this and other related studies (10, 16, 17, 20, 31, 32), a diagram of the proposed mechanism is shown in Scheme 1. The transformation of nitrogen is a complex process with many intermediates and products, some of which are reactive precursors of N-Nitrosamines. Since hydroxylamine is a common intermediate of DMA oxidation, the oxidant could be ozone, ClO2, potassium permanganate, or potassium ferrate, among others. It is thus reasonable to assume that hydroxylamine is an important inorganic precursor for NDMA formation during oxidation processes. Just as Andrzejewski mentioned in recent research (33), the formation of NDMA by DMA oxidation might occur as a multistage reaction: reactive organic substances and nitrogenous inorganic precursors form in the first stage, then NDMA is formed by the further reaction of the intermediates with DMA. Even though the formation of hydroxylamine seems to be a reasonable explanation for the yield of NDMA during oxidation, hydroxylamine is unlikely to be the only VOL. 43, NO. 14, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Influence of pH on NDMA formation during nitrosation (A) ([DMA] ) 20 mM, [NO2-] ) 100 mM, T ) 25 °C); and ozonation (B) ([DMA] ) 0.5 mM, [O3] ) 0.67 mg/L, T ) 25 °C).

SCHEME 1. Proposed Reaction Pathways for NDMA Formation during Ozonation of Dimethylamine

inorganic precursor, since nitrosation via the reaction of a nitrosating reagent (NO-X) with DMA is also an effective pathway for NDMA formation. Alternatively, hydroxylamine could be an intermediate in the reaction of hydroxide with chloramine solutions, as shown in reaction 16. Hydroxylamine can be oxidized rapidly by chloramine into nitrogen and other products (reaction 17) (34). The possibility that hydroxylamine acts as the Nnitrosamine precursor during chloramination can be investigated in future studies. 5486

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NH2Cl + OH- f NH2OH + Cl-

(16)

NH2Cl + 2NH2OH + OH- f NH3 + N2 + 3H2O + Cl(17) The most effective organic precursor of NDMA in natural waters is still unknown, so studies into the occurrence and control of inorganic nitrogenous substances, such as nitrite, nitrate, ammonia, and hydroxylamine, will be helpful in reducing nitrosamine formation during water treatment.

Acknowledgments This project was supported by the National Natural Science Foundation of China (50578052, 50638020, and 50821002).

Supporting Information Available Conditions and results of the experiments on NDMA formation via nitrosation catalyzed by formaldehyde at neutral and alkaline pH (Table S1); the influence of concentration of hydroxylamine on N-nitrosamine formation at pH 11.06 (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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