Degradation kinetics and N-Nitrosodimethylamine formation during monochloramination of chlortoluron

Science of the Total Environment 417-418 (2012) 241–247

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Degradation kinetics and N-Nitrosodimethylamine formation during monochloramination of chlortoluron Bin Xu a,⁎, Cao Qin a, Chen-Yan Hu b, Yi-Li Lin c,⁎⁎, Sheng-Ji Xia a, Qian Xu a, Seleli Andrew Mwakagenda a, Xiang-yu Bi a, Nai-Yun Gao a a

State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze Aquatic Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China College of Energy and Environment Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China c Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 811, Taiwan, ROC b

a r t i c l e

i n f o

Article history: Received 7 October 2011 Received in revised form 20 December 2011 Accepted 22 December 2011 Available online 23 January 2012 Keywords: Chlortoluron Monochloramine Kinetics Rate constants Disinfection by-products N-Nitrosodimethylamine

a b s t r a c t The degradation of chlortoluron by monochloramination was investigated in the pH range of 4–9. The degradation kinetics can be well described by a second-order kinetic model, first-order in monochloramine (NH2Cl) and first-order in chlortoluron. NH2Cl was found not to be very reactive with chlortoluron, and the apparent rate constants in the studied conditions were 2.5–66.3 M−1 h−1. The apparent rate constants were determined to be maximum at pH 6, minimum at pH 4 and medium at alkaline conditions. The main disinfection byproducts (DBPs) formed after chlortoluron monochloramination were identified by ultra performance liquid chromatography-ESI-MS and GC-electron capture detector. N-Nitrosodimethylamine (NDMA) and 5 volatile chlorination DBPs including chloroform (CF), dichloroacetonitrile, 1,1-dichloropropanone, 1,1,1-trichloropropanone and trichloronitromethane were identified. The distributions of DBPs formed at different solution pH were quite distinct. Concentrations of NDMA and CF were high at pH 7–9, where NH2Cl was the main disinfectant in the solution. NDMA formation during chlortoluron monochloramination with the presence of nitrogenous salts increased in the order of nitrite b nitrate b ammonium for a given monochloramination and chlortoluron concentration. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Pesticides are some of the most commonly detected organic compounds in surface and ground waters, which are dispersed in aqueous medium by direct agricultural runoff or leaching (Canle et al., 2001). Phenylurea based herbicides including chlortoluron, isoproturon and diuron are widely used in the world (Benitez et al., 2009). Many researchers have found that numerous drinking water sources have been contaminated with phenylureas (WHO, 2003; Hazardous Substances Data Bank (HSDB), 2009), which can lead to adverse effects on organisms even at very low concentrations (Javier et al., 2009). European Union has classified some phenylureas (e.g., diuron and isoproturon) as priority hazardous substances (Benitez et al., 2007). Therefore, considerable attention has been focused on the fate of phenylureas in drinking water treatment plants (Kolpin et al., 2002; Squillace et al., 2002). Chlortoluron, a high efficiency herbicide to control annual grasses and broad-leaved weeds, is one of the most frequently detected pesticides in water sources (WHO, 2003). Because of its resistance to chemical, heat and ultraviolet rays, chlortoluron degrades slowly in

⁎ Corresponding author. Tel.: +86 13918493316. E-mail addresses: [email protected] (B. Xu), [email protected] (Y.-L. Lin). 0048-9697/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2011.12.056

water and is quite persistent in the environment (Hazardous Substances Data Bank (HSDB), 2009). An investigation indicated that the half-life of chlortoluron is approximately 80–120 days in water and 30–40 days in soil (WHO, 2003). Even though its solubility in water is low (~70 mg/L, 25 °C) and is only slightly mobile in soil, chlortoluron is frequently detected in surface and ground waters at concentrations ranging from 0.2 to 1.2 μg/L (Hazardous Substances Data Bank (HSDB), 2009). Consequently, an increasing concern has risen about chlortoluron, and it is suggested to be removed using various treatment techniques, such as ozonation (Benitez et al., 2007), photodegradation (Poulain et al., 2003; Lhomme et al., 2005; Amorisco et al., 2006) and nanofiltration (Benitez et al., 2009), which have been demonstrated to be efficient during water treatment processes. Monochloramination has been more and more adopted as a secondary disinfectant to comply with current or future regulations concerning the concentrations of disinfection by-products (DBPs) in drinking water (EPA, 1999) because monochloramine (NH2Cl) is a weaker disinfectant compared to free chlorine and produces less DBPs when reacting with organic precursors (Hua and Reckhow, 2007). NH2Cl is produced by the reaction of hypochlorous acid (HOCl) with ammonia (NH3) and is quantitatively formed at ammonia-to-chlorine ratio (N/Cl) higher than 1 mol/mol at pH 8.5. In the practice of monochloramination, NH2Cl is

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prepared with a small excess of NH3 (N/Cl = 1.1–1.2 mol/mol) to minimize the formation of dichloramine (NHCl2) and trichloramine (NCl3) as well as to prevent the breakpoint reactions (Jafvert and Valentine, 1992). The decay of NH2Cl in drinking water can be due to autodecomposition reactions, direct reactions of NH2Cl with natural organic matters (NOMs) and indirect reactions with NOMs involving free chlorine formed from NH2Cl hydrolysis (Duirk et al., 2005). Although NH2Cl shows more applications in water treatment nowadays, formation of chloramination by-products such as nitrosodimethylamine (NDMA), trihalomethanes (THMs), haloacetic acids (HAAs) and other toxic chloramine derivatives are not well-documented. Recently, many studies focused on chloramination of specific compounds such as resorcinol (Cimetiere et al., 2009), organophosphorus pesticides (Duirk et al., 2010), algal organic matters (Fang et al., 2010) and nitrogenous organic compounds (Yang et al., 2010). The reactivity of such compounds with NH2Cl varies significantly due to chemical structure differences and water matrix parameters including pH, concentration of NH2Cl, reaction time, ammonia addition, and temperature. The objectives of this study were to investigate the kinetics and degradation pathways of chlortoluron during monochloramination, as well as the factors which affect the reaction and formation of DBPs. Moreover, the effects of reaction time, pH, NH2Cl dosages and the presence of different nitrogenous salts on the formation of NDMA were investigated during chlortoluron monochloramination. 2. Materials and methods 2.1. Chemicals Chromatographical purity of chlortoluron (>99.0%) was purchased from Dr. Ehrenstorfer (German) and used without further purification. NDMA standard (>99.5%) and NDMA-d6 (used as internal standard) were obtained from Chem Service, Inc. (Westchester, PA, USA) and Cambridge Isotope Laboratories, Inc. (MA, USA), respectively. THMs mix standard solutions including CF, bromodichloromethane, dibromochloromethane and bromoform were purchased from Supelco (USA). Halogenated volatiles mix standard solutions including bromochloroacetonitrile, dibromoacetonitrile, dichloroacetonitrile (DCAN), 1,1-dichloro-2-propanone (1,1-DCP), 1,1,1-trichloro-2-propanone (1,1,1-TCP), trichloroacetonitrile, and trichloronitromethane (TCNM), were purchased from Sigma-Aldrich (USA). Sodium hypochlorite (NaOCl) solution (available chlorine 4.00–4.99%) and ammonium acetate (≥98%) were purchased from Sigma-Aldrich (USA). Analytical grade reagents including ascorbic acid, ammonium acetate, ammonium chloride (NH4Cl), NaOH, Na2S2O3, KH2PO4, Na2CO3, NaHCO3, CH3COOH, anhydrous Na2SO4, H2SO4, NaNO3 and NaNO2 were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. All solutions were prepared with ultra-pure water produced from a Milli-Q water purification system (Millipore, USA). High performance liquid chromatography (HPLC) grade methanol and methyl tert-butyl ether (MTBE) were obtained from J.T.Baker (USA). NH2Cl solutions were made daily by adding NaOCl solution gently into a stirred NH4Cl solution with the Cl:N molar ratio of at least 1:1.2 to prevent breakpoint chlorination due to local excess of OCl −, and the pH was kept at around 10 to avoid the disproportionation of NH2Cl to NHCl2 (Mitch and Sedlak, 2002). 2.2. Experimental plans Kinetic studies were carried out in a batch reactor (250 mL) equipped with a dispenser which is fixed in a thermostatic culture oscillator with controlled temperature (25±1 °C). Reactions were initiated by adding an aliquot of concentrated NH2Cl solution to the reactor containing buffered chlortoluron solution (9.4 μM in all case). The pH range of the reactions was controlled from 4 to 9, which were buffered using 10 mM acetate (for pH 4–5), 10 mM phosphate (for pH 5–8) and 10 mM

carbonate (for pH 8–9) solutions, and pH values were adjusted with small volumes of 0.01, 0.1, or 1 M H2SO4 and/or NaOH. The kinetic reactions were conducted under pseudo-first-order conditions where the initial NH2Cl concentration was in 5 times higher than that of chlortoluron. At different reaction times, 1 mL of solution was rapidly transferred into a HPLC vial containing 50 μL Na2S2O3 (20 mM) to quench the reaction, and stored for further analysis using HPLC-UV. The kinetic experiments proceeded until at least 50% chlortoluron degradation was achieved. The investigation of chlortoluron degradation by-products during monochloramination was conducted at pH 7 under similar reaction conditions to those of kinetic studies. However, the initial concentration of NH2Cl and chlortoluron was set as 2 mM and 94 μM, respectively. One milliliter of samples was withdrawn after 24 h of reaction time and quenched with 100 μL Na2S2O3 (20 mM) for each experiment. The solutions were then analyzed by UPLC-ESI-MS for degradation byproducts to determine the degradation pathways. The analyses of volatile DBPs formation experiments were conducted in duplicate under headspace-free conditions in 40-mL glass of screw-cap vials with PTFE-lined septa. Appropriate NH2Cl or free chlorine (2 mM as Cl2) was applied to chlortoluron solutions (94 μM in all case) under pH 4, 7 and 9 in acetate, phosphate and carbonate buffers (10 mM), respectively. The chlorination reaction was quenched with NH4Cl (20 mM) instead of Na2S2O3 to avoid the interaction with formed nitrogen-disinfection by-products (N-DBPs), and samples were analyzed using GC-ECD right after the extraction by MTBE. The NDMA formation experiments were conducted in triplicate using the method developed by Mitch et al. (2003) with 250 mL amber bottles in a dark area at 25 ± 1 °C to minimize the photolysis of NDMA. Two hundred milliliters of the chlortoluron solution (94 μM in all case) was dosed with 100 mM NH2Cl stock solution prepared fresh daily to make the initial NH2Cl concentration at 0.05– 2.0 mM. The chlortoluron solution was also dosed with 200 mM NaNO3, NaNO2 and NH4Cl, respectively, to make the initial nitrogenous salt concentration at 2 mM in order to study its impact on NDMA formation. The solution pH was buffered at 4, 7 and 9 as the way described previously. At different reaction times (12–240 h), 1 mL of solution was rapidly transferred into a HPLC vial containing 100 μL ascorbic acid (50 mM) to stop the reaction, and then analyzed using UPLC-ESI-MS/MS after adding 100 μL NDMA-d6 (600 μg/L) as an internal standard. 2.3. Analytical methods Chlorine and NH2Cl concentration were analyzed using the N,Ndethyl-p-phenylenediamine (DPD) colorimetric method (APHA et al., 1998). pH was measured using a pH-meter (module PHS-3B, Shanghai LEICI Analysis Instrument Factory, China), which was calibrated on a regular basis using standard pH 4, 6.8 and 9.18 buffers. A Shimadzu 2010 AHT HPLC system (Kyoto, Japan) with an autosampler and an Xterra™ RP 18 column (250 mm× 4.6 mm I.D, 5 μm film thickness, Waters, USA) was used to analyze chlortoluron. The details of chlortoluron detection method were described in our previous study (Xu et al., 2011). The concentrations of volatile DBPs including CF, DCAN, 1,1-DCP, 1,1,1-TCP, and TCNM were analyzed by the USEPA method of 551.1 (USEPA, 1995). Samples were extracted with MTBE, and the extracts were then analyzed by a GC-ECD (GC-2010, Shimadzu, Japan) and a fused silica capillary column (HP-5, 30 m × 0.25 mm I.D., 0.25 μm film thickness, J&W, USA). Non volatile by-products were identified by an UPLC-ESI-MS consisted of an Accela U-HPLC system and a TSQ Quantum mass spectrometer (ESI source, Thermo Scientific Inc., USA) with a reversed-phase Xterra™ MS C18 column (250 mm ×2.1 mm I.D, 5 μm film thickness, Waters, USA). Methanol (50% v/v) and 2 mM ammonium acetate buffer in Milli-Q water (50% v/v) were used as the mobile phase with a flow

B. Xu et al. / Science of the Total Environment 417-418 (2012) 241–247

rate of 0.25 mL/min. Operating parameters of the ESI conditions were described in our previous study (Xu et al., 2011). MS chromatograms were obtained in both total ion current (TIC) and SIM mode. The full scans (m/z 50–400) and specific ions for mass spectra acquisition were obtained. NDMA was analyzed using UPLC-ESI-MS (ESI source, TSQ Quantum, Thermo Fisher Scientific Inc., USA) with a Thermos Hypersil GOLD C8 column (150 mm × 2.1 mm I.D, 3 μm film thickness, Thermo Fisher Scientific Inc., USA). Methanol (solvent A) and 2 mM ammonium acetate buffer in Milli-Q water (solvent B) were used as the mobile phase with a flow rate of 0.15 mL/min. The gradient elution program was as follows: linear from 10% of A to 45% B in 4 min, to 100% of A at 5 min, then isocratic until 10 min, and finally returned to 10% of A in 1 min. The ion spray voltage was operated at 4500 V with a source temperature of 350 °C. The MRM transitions were m/z 75–43 (quantification) and m/z 75–58 (confirmation) for NDMA, and m/z 81–46 for NDMA-d6. Instrument detection limit was 2 μg/L for NDMA. 3. Results and discussion 3.1. Reaction kinetics For chlortoluron monochloramination, the degradation kinetics exhibited a pseudo-first-order dependence on the chlortoluron concentration, as demonstrated by the linear time-course plot of ln([chlortoluron]0/[chlortoluron]), where [chlortoluron]0 and [chlortoluron] is the initial concentration of chlortoluron and concentration of chlortoluron at different reaction times, respectively. Fig. 1 shows the pseudo-first-order kinetic plot of chlortoluron monochloramination at pH 7 with different NH2Cl concentrations (34.7 to 278.0 μM). These plots were linear and the correlation coefficients were superior to 0.92 for all reactions. Therefore, the overall reaction rate can be expressed as following equations. d½chlortoluron ¼ −kobs ½chlortoluron dt

ð1Þ

where kobs was the pseudo-first order rate constant and can be calculated from the slope of the fitted lines in Fig. 1. The kobs increased from 0.0021 to 0.0162 h−1 with initial NH2Cl concentration increased from 34.7 to 277.9 μM. In the upper-right part of Fig. 1, the relationship of kobs to the initial concentration of NH2Cl ([NH2Cl]0) is linear (R2 = 0.99), which suggests that the rate of chlortoluron monochloramination is first-order with respect to NH2Cl concentration. Therefore, the monochloramination of

243

chlortoluron follows second-order kinetics, first-order relative to the NH2Cl concentration and to the chlortoluron concentration. As a result, the kinetics of monochloramination of chlortoluron can be described as Eq. (2). d½chlortoluron ¼ −kapp ½chlortoluron½NH 2 Cl dt

ð2Þ

kapp is the apparent second-order rate constant of chlortoluron monochloramination at specific pH. The relation between kapp and kobs can be expressed as: kapp ¼ kobs =½NH 2 Cl:

ð3Þ

3.2. Effect of pH on chlortoluron monochloramination It is well known that NH2Cl is unstable by nature and can undergo a series of auto-decomposition reactions (Valentine and Jafvert, 1988), which is a relatively complex process and highly related to the pH value of the solution (Valentine and Jafvert, 1988). Therefore, the effect of solution pH was investigated in pH 4–9 with similar initial concentrations of NH2Cl and chlortoluron (138.9 and 9.4 μM, respectively), and the experimental results confirmed a pseudo-first-order kinetics (data not shown). Fig. 2 shows the effect of pH on kapp of chlortoluron monochloramination and chlorination, respectively. The reactivity of chlortoluron with both chlorine and NH2Cl was low (small kapp values in Fig. 2) compared to that of ametryn (Xu et al., 2009) and bisphenol A (Gallard et al., 2004) during chlorination. The kapp of chlortoluron monochloramination was below 70 M−1 h−1 under pH 4–9 and was maximum at pH 6, minimum at pH 4, and medium at alkaline conditions, which is generally smaller than that of chlortoluron chlorination. The pH dependency of the kapp values of chlortoluron monochloramination exhibits a different pattern from that of chlorination. As reported in our previous study (Xu et al., 2011), the predominant reactions of chlortoluron chlorination were found to be the acid-catalyzed reaction of chlortoluron with HOCl, reaction of chlortoluron with HOCl and OCl−. However, monochloramination of chlortoluron underwent complex reactions including chloramination and chlorination due to the auto-decomposition of NH2Cl. Previous works found that NHCl2 and NH2Cl usually coexist in chloramine solution. NHCl2 can form by either NH2Cl hydrolysis (Eqs. (4)–(6)) (Vikesland et al., 2001) or acid catalyzed NH2Cl

0.02

3.5

Chlorination

2.5 2

0.01 0.005 0 0

1.5

50 100 150 200 250 300 [NH2Cl]0 (µ M)

[NH Cl] = 34.7 µ M 2

1

0

[NH Cl] = 69.5 µ M 2

0

log (kapp) (M-1h-1)

0.015 kobs (h-1)

ln([chlortoluron]0 / [chlortoluron])

4 3

3 2.5 2 1.5 Monochloramination

1

[NH Cl] =104.2 µ M 2

0.5

0

[NH Cl] =138.9 µ M 2 2

0

0

40

80

120

160

200

240

0.5

0

[NH Cl] =277.9 µ M 0

280

320

0

4

5

6

7

8

9

pH

Time (h) Fig. 1. Pseudo-first-order kinetic plot of chlortoluron monochloramination at 25 ± 1 °C, pH 7, [chlortoluron]0 = 9.40 μM and five different initial NH2Cl concentrations.

Fig. 2. Effect of pH on the apparent second-order rate constant of chlortoluron monochloramination. Data of chlortoluron chlorination was from Xu et al. (2011).

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disproportionation reaction (Eqs. (7)–(8)) (Valentine and Jafvert, 1988). NH2 Cl þ H2 O→HOCl þ NH3

ð4Þ

Table 1 Summary of the main derivatives of chlortoluron formed during monochloramination. Retention time (min)

MH+ ion (m/z)

2.4

263

Derivative (R = −NH-CO-N(CH3)2) OH H3C

2.4

HOCl þ NH2 Cl→NHCl2 þ H2 O

297

ð5Þ

R Cl

Cl

Cl

OH

Cl

Cl

H3C

2.4

R

80

CH3 Cl N CH3

NH2 Cl þ NH2 Cl→NHCl2 þ NH3

ð6Þ

2.4

217

NO 2 H3C

NO2 Cl

2.6 þ

þ

NH2 Cl þ H ⇌NH3 Cl

ð7Þ

247

H3C

RCl Cl

3.3

281

Cl H3C

RCl Cl

4.2 þ

NH3 Cl þ NH2 Cl→NHCl2 þ NH3 þ H

þ

229

ð8Þ

The relative importance of these reactions on chlortoluron degradation is dependent on factors such as pH, ionic strength, temperature and alkalinity of solution (Vikesland et al., 2001). In Fig. 2, the lowest kapp at pH 4 could be explained by the low concentration of NH2Cl to react with chlortoluron because most of NH2Cl was transformed to NHCl2 under acidic conditions (Eqs. (6) and (7)). However, kapp increases substantially with increasing pH up to 6.0, which could be explained by HOCl formation from the hydrolysis of NH2Cl (Deinzer et al., 1978). In the studies of Losito et al. (2000) and Acero et al. (2007), chlorine has been shown to be capable of reacting with phenylurea herbicides including chlortoluron, diuron and isopropturon in water treatment processes, which was also demonstrated in Fig. 2 that HOCl is more reactive with chlortoluron than NH2Cl. As pH increased from 6.0 to 9.0, the observed rate of chlortoluron loss decreased due to less NH2Cl autodecomposition to form chlorine under alkaline condition (Vikesland et al., 2001).

OH H3C

R Cl

4.2

297

OH H3C

RCl Cl

14.8

168

Cl

H3C

N C O Cl

17.4

213

H3C

R Cl

17.4

281

Cl H3C

R Cl

30.1

247

Cl Cl

H3C

R Cl

31.5

172

H 3C

NO2 Cl

34.6

247

Cl H3C

R Cl

3.3. Degradation pathways of chlortoluron during monochloramination The concentration of chlortoluron decreased gradually during monochloramination and transferred into some degradation by-products, which were identified using UPLC-ESI-MS. Table 1 summarized the detected derivatives of chlortoluron (MH+ at m/z 213) after 24 h reaction time during monochloramination at pH 7, and most of which are chloro/hydroxylated degradation by-products. The ions of m/z 229 and 263 are monohydroxy and chlorohydroxy derivatives of chlortoluron, respectively. The m/z ions of 247 corresponded to three well separated isomers of monochloro derivatives of chlortoluron, while the m/z ions of 281 and 297 corresponded to two well separated isomers of dichloro and dichloromonohydroxy derivatives of chlortoluron, respectively. The extracting peak intensities for the m/z 229, 247, 281, 263 and 297 ions are also reported as non-volatile intermediates during chlortoluron chlorination at pH 7 using LC-ESI-MSn (Zambonin et al., 2000). Along with hydroxylation and substitution reactions, chlortoluron was also subject to oxidation reaction during monochloramination happening on the aromatic ring and ureic side-chain, and four other derivatives were identified (m/z 168, 172, 217 and 80). The UPLC-ESI-MS chromatogram obtained by extracting the m/z 229, 213, 247, 263, 281, 297,168 and 80 ions after 24 h of monochloramination of chlortoluron at pH 7 is shown in Supplementary Fig. S1. Followed by these processes, some hazardous DBPs were formed, which will be discussed in the following section. The pathways of chlortoluron degradation and DBP formation during monochloramination were proposed and shown in Fig. 3.

3.4. DBP formation during chlorination and monochloramination of chlortoluron It is well documented that solution pH can affect chlorine dissociation and NH2Cl auto-decomposition (Duirk et al., 2005; Acero et al., 2007). Thus, it is expected that the formation of DBPs varied significantly with pH during both chlorination and monochloramination. Fig. 4 presents the DBPs formed after monochloramination and chlorination of chlortoluron, respectively, at pH 4, 7 and 9 after 168 h. In general, CF was the most abundant volatile DBP formed after either chlorination or monochloramination at all tested pH. Concentration of CF increased with increasing pH during chlorination with a maximum of 2758.3 μg/ L (molar yield as 0.24 mol/mol) at pH 9. However, during monochloramination of chlortoluron, CF concentration increased from pH 4 to 7, and then decreased as pH increased to 9. The maximum concentration of CF during chlortoluron monochloramination was 159.5 μg/L (molar yield as 0.014 mol/mol) at pH 7, which was 16.1 times lower than that during chlortoluron chlorination. The higher CF concentration during chlortoluron monochloramination at neutral pH could be explained by the presence of HOCl from NH2Cl hydrolysis, which was much more reactive with chlortoluron than NH2Cl itself (higher kapp in Fig. 2). However, four of the six detected DBPs (DCAN, 1,1-DCP, 1,1,1-TCP and NDMA) formed at higher levels in chlortoluron monochloramination than in chlortoluron chlorination at pH 7 and 9 (Fig. 4(a)–(c)). Among these DBPs, particular concern was focused on NDMA as an

B. Xu et al. / Science of the Total Environment 417-418 (2012) 241–247

HOCl

HOCl

+

+

or

NH2Cl m/z=213

245

NH2Cl m/z=247

m/z=247

m/z=229

NH2Cl HOCl

NH2Cl HOCl Further reactions

+ m/z=281

m/z=281

HOCl

HOCl

NH2Cl

NH2Cl m/z=168

HOCl

HOCl

NH2Cl

NH2Cl m/z=80

m/z=263

m/z=263

+ m/z=297

m/z=297

+ m/z=217

m/z=172

+ NDMA

+ CF

+ DCAN

1,1,1-TCP

+ 1,1-DCP

+ TCNM

Fig. 3. Proposed pathways of chlortoluron degradation and DBP formation during monochloramination (R = \NH\CO\N(CH3)2).

emergent DBP in drinking water and wastewater treatment processes (Mitch et al., 2003). Chlortoluron, as one potential precursor of NDMA, generated more NDMA during monochloramination than during chlorination in our experiments, which confirmed the results reported by Schreiber and Mitch (2006) that NDMA can be produced during chloramination of secondary amines, such as dimethylamine (DMA). As shown in Fig. 4, concentration of NDMA during chlortoluron chlorination was below detection limit. However, the use of NH2Cl greatly increases NDMA formation, which may be attributable to the capability of NH2Cl to provide more nitrogen source for the nitroso group in NDMA (Choi and Valentine, 2002). Previous studies demonstrated that maximum NDMA formation rate during monochloramination of DMA and wastewater effluent was between pH 6 and 9 (Choi and Valentine, 2002; Mitch et al., 2003). Similar results were observed during monochloramination of chlortoluron in our study, with NDMA concentration of 5.5 μg/L, 23.0 μg/L and 21.3 μg/L (molar yield as 0.79 mmol/mol, 3.3 mmol/mol and 3.1 mmol/mol) at pH 4, 7 and 9, respectively. The results were also consistent with the reports of diuron chloramination indicating that NDMA yield was greatest at circumneutral pH and lower under acidic or basic conditions (Chen, 2009). As for the formation of TCNM, shifting from chlorine to NH2Cl of chlortoluron produced significantly less TCNM at pH 4 and 7. However, the TCNM concentration at pH 9 increased a lot compared to those at lower pH values and was comparable between chlorination and monochloramination of chlortoluron. Since NH2Cl can contribute the nitrogen source to N-DBPs (Yang et al., 2010), higher concentration of DCAN was found during chlortoluron monochloramination than during chlorination. Highest concentration of DCAN was 79.1 μg/L (molar yield as 7.65 mmol/mol) at pH 7 for monochloramination and 37.7 μg/L (molar yield as 3.64 mmol/mol) at pH 4 for chlorination, respectively. Due to base-catalyzed hydrolysis decomposition (Yang et al., 2007), DCAN concentration decreased to 0.59 μg/L and 26.0 μg/L at pH 9 during chlorination and monochloramination, respectively.

The yield of 1,1-DCP and 1,1,1-TCP followed an increasing and then decreasing pattern as pH increased from 4 to 9 during monochloramination of chlortoluron, with maximum concentration of 50.1 μg/L (molar yield as 4.20 mmol/mol) and 47.8 μg/L (molar yield as 3.15 mmol/mol) at pH 7, respectively. However, 1,1-DCP and 1,1,1-TCP concentrations decreased with increasing pH during chlorination, which imply that the formation pathways of 1,1-DCP and 1,1,1-TCP are different from monochloramination and chlorination of chlortoluron. It was noted that 1,1-DCP and 1,1,1-TCP concentrations were highest at pH 4 during chlorination (23.3 μg/L and 189.8 μg/L), which indicated their slow decomposition rate under acidic conditions. The decrease of 1,1-DCP and 1,1,1-TCP concentrations during monochloramination and chlorination at pH 9 could be attributed to base-catalyzed hydrolysis decomposition (Yang et al., 2007). 3.5. Effects of reaction time, NH2Cl concentration and coexisting nitrogenous salts on NDMA formation during chlortoluron monochloramination In order to understand the change of NDMA formation under different water matrix parameters during chlortoluron monochloramination, the effect of reaction time, NH2Cl concentration and coexisting nitrogenous salts was evaluated. Fig. 5(a) shows the results of time-dependent NDMA formation and the degradation of chlortoluron during monochloramination. Almost 98% of chlortoluron was oxidized rapidly at the beginning of the reaction (chlortoluron concentration below 1 mg/L after 48 h). However, the NDMA concentration increased gradually and reached a maximum of 23.3 μg/L at the end of reaction time (240 h), which suggests a complex reaction pathway involving the formation of other intermediates that potentially postpones the formation rate of NDMA (Chen and Young, 2008). Chen and Young (2008) also observed the increase of NDMA concentration with time increase during diuron chlorination of 7 days. Fig. 5(b) shows the relationship between NH2Cl concentration and NDMA formation during chlortoluron monochloramination at pH 7 after 168 h of reaction time. As expected, NDMA formation increased

300 Monochloramination

CF concentration (µ g/L)

250

250

Chlorination

200

200

150

150

100

100

pH 4 50

0

50

CF

DCAN 1,1-DCP TCNM 1,1,1-TCP NDMA

Other DBPs concentration (µ g/L)

300

60

pH 7 40

1000

0

20

CF

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Fig. 4. DBP formation during chlorination and monochloramination of chlortoluron at pH 4 (a), 7 (b) and 9 (c). [Chlortoluron]0 = 94 μM, [HOCl]0 or [NH2Cl]0 = 2 mM, reaction time = 168 h.

significantly from 3.4 to 23.0 μg/L as NH2Cl concentration increased from 0.05 mM to 2 mM, which suggests that NH2Cl was an effective NDMA-forming oxidant. To investigate the relative health risks posed by NDMA formation with the presence of different nitrogenous species (nitrite, nitrate or ammonium), experiments were conducted on a given chlortoluron and NH2Cl concentration after 168 h of reaction time, and the results were presented in Fig. 5(c). NDMA concentration formation during chlortoluron monochloramination with the presence of nitrogenous

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Fig. 5. Effects of reaction time (a), NH2Cl concentration (b), and the presence of nitrogenous salts (c) on NDMA formation with [chlortoluron]0 = 94 μM, pH = 7. (a) [NH2Cl]0 = 2 mM; (b) reaction time = 168 h; (c) concentration of each nitrogenous salts was 2 mM, reaction time = 168 h.

salts increased in the order of nitrite b nitrate b ammonium. The highest NDMA concentration with the presence of ammonium (36.2 μg/L compared to 23.0 μg/L without coexistence of nitrogenous salts) could be attributed to the inhibition of NH2Cl decay (Vikesland et al., 2001). The presence of nitrate can also increase NDMA formation to 26.7 μg/L by increasing N2O4 concentration via the reaction between NO + and NO3−, which can act as the nitrosating reagent (Choi and Valentine, 2003; Yang et al., 2009). When chlortoluron

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monochloramination was proceeded in the presence of nitrite, NDMA formation decreased to as low as 5.1 μg/L due to scavenging of NH2Cl by nitrite (Mitch and Sedlak, 2002). 4. Conclusions (1) The kinetics of chlortoluron monochloramination can be well described by a second-order kinetics model, first-order in NH2Cl and first-order in chlortoluron. The monochloramination of chlortoluron is a slow process under circumneutral pH range (pH 4–9). The apparent rate constants were 2.5–66.3 M−1 h−1, with a minimum at pH 4, a maximum at pH 6 and a medium at alkaline conditions. (2) The main degradation by-products during chlortoluron monochloramination were identified by UPLC-ESI-MS and GC-ECD including CF, DCAN, 1,1-DCP, TCNM, 1,1,1-DCP and NDMA. Degradation pathways of chlortoluron monochloramination were proposed. (3) CF was the major DBP formed after chlorination and monochloramination of chlortoluron. DCAN, 1,1-DCP, 1,1,1-TCP and NDMA formed at higher levels in chlortoluron monochloramination than in chlorination at pH 7 and 9. (4) Increasing reaction time and NH2Cl dosage increased the formation of NDMA during chlortoluron monochloramination. The presence of nitrogenous salts including ammonium, nitrite and nitrate can affect NDMA formation. Nitrite can significantly suppress the formation of NDMA compared to the conditions with or without the presence of nitrate and nitrite. Supplementary materials related to this article can be found online at doi:10.1016/j.scitotenv.2011.12.056. Acknowledgments This study was supported in part by the High Technology Research and Development (863) Program (No. 2008AA06Z302) in China, Natural Science Foundation of China (No. 51078280), the Shanghai Rising-Star Program (No. 11QA1407000), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, China, Foundation of the State Key Laboratory of Pollution Control and Resource Reuse (No. PCRRY11006) and the National Science Council of Taiwan (NSC-99-2221-E-327-019). References Acero JL, Real FJ, Benitez FJ, Gonzalez M. Kinetics of reactions between chlorine or bromine and the herbicides diuron and isoproturon. J Chem Technol Biotechnol 2007;82: 214–22. Amorisco A, Losito I, Carbonara T, Palmisano F, Zambonin PG. Photocatalytic degradation of phenyl-urea herbicides chlortoluron and chloroxuron: characterization of the by-products by liquid chromatography coupled to electrospray ionization tandem mass spectrometry. Rapid Commun Mass Spectrom 2006;20:1569–76. APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater. 20th ed. Washington, DC, USA: American Public Health Assoc; 1998. Benitez FJ, Real FJ, Acero JL, Garcia C. Kinetics of the transformation of phenyl-urea herbicides during ozonation of natural waters: rate constants and model predictions. Water Res 2007;41:4073–84. Benitez FJ, Acero JL, Real FJ, Garcia C. Removal of phenyl-urea herbicides in ultrapure water by ultrafiltration and nanofiltration processes. Water Res 2009;43:267–76. Canle M, Rodriguez S, Rodriguez-Vazquez LF, Santaballa JA, Steenken S. First stages of photodegradation of the urea herbicides fenuron, monuron and diuron. J Mol Struct 2001;565:133–9. Chen WH, Young TM. Influence of nitrogen source on NDMA formation during chlorination of diuron. Water Res 2009;43:3047–56.

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