Effects of Combined UV and Chlorine Treatment on the Formation of

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Effects of Combined UV and Chlorine Treatment on the Formation of Trichloronitromethane from Amine Precursors Lin Deng,†,‡ Ching-Hua Huang,‡,* and Yung-Li Wang‡ †

Department of Municipal Engineering, Southeast University, Nanjing, 210096, People’s Republic of China School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

‡

S Supporting Information *

ABSTRACT: The objective of this study was to investigate the effects of combined low-pressure ultraviolet (LPUV) irradiation and free chlorination on the formation of trichloronitromethane (TCNM) byproduct from amine precursors, including a commonly used polyamine coagulant aid (poly(epichlorohydrin dimethylamine)) and simple alkylamines dimethylamine (DMA) and methylamine (MA). Results showed that TCNM formation can increase up to 15 fold by combined UV/chlorine under disinfection to advanced oxidation conditions. The enhancement effect is influenced by UV irradiance, chlorine dose, and water pH. Extended reaction time leads to the decay of TCNM by direct photolysis. The combined UV/chlorine conditions significantly promoted degradation of polyamine to generate intermediates, including DMA and MA, which are better TCNM precursors than polyamine, and also facilitated transformation of these amine precursors to TCNM. Under combined UV/chlorine, polyamine degradation was likely promoted by radical oxidation, photodecay of chlorinated polyamine, and chlorine oxidation/substitution. Promoted TCNM formation from primary amine MA was primarily due to radicals’ involvement. Promoted TCNM formation from secondary amine DMA likely involved a combination of radical oxidation, photoenhanced chlorination reactions, and other unknown mechanisms. Insights obtained in this study are useful for reducing TCNM formation during water treatment when both UV and chlorine will be encountered.

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INTRODUCTION Nitrogenous disinfection byproducts (N-DBPs) in drinking water are among emerging DBPs that have generated considerable public concern due to their high toxicity.1,2 The increasing occurrence of N-DBPs in drinking water in recent years has been associated with changes in water disinfection practices as well as lower quality (e.g., higher levels of organic matter and contaminants) of source waters.2 One class of NDBPs that has been frequently detected in drinking water,1 wastewater3 and swimming pool water4 are halonitromethanes (HNMs). HNMs were among 50 high priority DBPs monitored in a 2000−2002 U.S. nationwide survey at drinking water treatment plants (DWTPs), with reported median and maximum levels of total HNMs at 1 and 10 μg/L, respectively.5 Among HNMs, trichloronitromethane (TCNM or chloropicrin) was most commonly detected, with reported concentrations at 0.2−0.5 μg/L in disinfected waters.5 Research using in vitro mammalian cell tests demonstrated that HNMs could be 1−2 orders of magnitude more potent cyctotoxins and genotoxins than currently regulated DBPs, such as trihaolomethanes (THMs) and haloacetic acids (HAAs).6,7 Brominated forms of HNMs are even more toxic than their chlorinated analogues.6 Thus, formation of HNMs in drinking water poses a hazard to public health and the environment. In recent years, due to increasingly stringent regulations over chlorinated DBPs and chlorine-resistant pathogens (e.g., © 2014 American Chemical Society

Cryptosporidium parvum), interest in the application of ultraviolet (UV) light for disinfection has accelerated. A recent survey indicated that more than 28 DWTPs have switched to or adopted UV radiation in primary disinfection in the past decade in the U.S.8 Among the surveyed plants, the use of low-pressure (LP) versus medium-pressure (MP) UV lamps was about 38% and 62%, respectively.8 LPUV lamps emit monochromatically at 254 nm, whereas MPUV lamps emit polychromatic light from 200 to 400 nm.9 In the U.S., UV disinfection is typically followed by chlorination (or chloramination) to ensure the presence of residual chlorine in drinking water delivered to the distribution systems. When chlorine is used for preoxidation prior to UV disinfection, constituents in water can be exposed to residual chlorine and UV simultaneously during the UV process. UV disinfection is applied to chlorinated pool waters and to drinking water containing residual chlorine at households or residential buildings. Furthermore, UV is frequently used in direct combination with other chemical oxidants, including chlorine, to create advanced oxidation processes (AOP) for destruction of organic contaminants.9−11 All of the above Received: Revised: Accepted: Published: 2697

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Figure 1. TCNM formation under combined UV/chlorine treatment: (a) Effect of LPUV irradiance (free chlorine concentration = 7 mg/L); (b) Effect of free chlorine concentration (LPUV = 5.4 mW/cm2). Solutions contained 10 mg/L polyamine initially and were maintained at pH 7.0 and 22 °C.

scenarios present situations in which combined UV and chlorine conditions may be encountered. When combined UV/chlorine is applied, the DBP formation potential will likely differ from that under UV alone or chlorine alone because the formation and transformation mechanisms of precursors and DBPs can differ significantly under the more complex UV/ chlorine conditions. Thus far, there is still limited understanding of the potential impact of combined UV/chlorine conditions on the formation of regulated or emerging DBPs. HNMs contain a nitro group and up to three halogens on the α-cabon. Literature has shown that HNMs may be formed from chlorination or chloramination of various N-containing precursors such as primary amines, amino acids, nucleic acids, and natural organic matter (NOM).12−16 HNM formation can be promoted by preozonation17−19 before chlorination and the presence of nitrite18,20 and bromide18,21 during chlorination. Ozone facilitates HNMs formation by oxidizing the amine functional group to a nitro group.19 Chlorination of nitrite forms nitrating agents such as ClNO2 and N2O4 that promote nitration of precursors to yield HNMs.22 Bromide’s effect is related to formation of HOBr that is a stronger halogenating reagent than HOCl.23 Recently, Reckhow and colleagues24 found that MPUV irradiation of nitrate-containing water followed by chlorination enhanced TCNM formation by up to 550% compared to chlorination alone; LPUV irradiation, in contrast, did not enhance TCNM formation. Shah and colleagues22 observed similar effects that MPUV/postchlorination enhanced TCNM formation in filter water effluents containing nitrate. The authors proposed that UV photolysis of nitrate forms nitrite, NO2• and ONOOH, which all can promote TCNM formation during postchlorination. MPUV is more effective than LPUV because MPUV overlaps better with nitrate or nitrite absorbance bands. Interestingly, Shah and colleagues22 also found that prechlorination followed by MPUV nearly doubled TCNM formation compared to MPUV/postchlorination. In the former, residual chlorine and MPUV existed simultaneously, indicating that combined UV/chlorine conditions promoted HNM formation. Enhanced DBP formation under combined UV/chlorine conditions has also been reported recently for chloroform,25 N-nitrosodimethylamine (NDMA),26 and cyanogen chloride.27,28

The objective of this study was to investigate the effects of combined UV irradiation and free chlorination on the formation of TCNM from amine precursors. Three model amine compounds including poly(epichlorohydrin dimethylamine) (polyamine), dimethylamine (DMA), and methylamine (MA) were studied. The polyamine polymer was selected because it is among the commonly used coagulant and flocculant aids for water and wastewater treatment, and has been shown to be a precursor of N-DBPs such as NDMA.29−31 DMA and MA were selected because they are common organic N compounds found in water sources, are possible degradation products of polyamine, and have been shown to be precursors of TCNM.12,15 LPUV was the choice of UV irradiation in this study because limited research was available on the combined effects of LPUV and free chlorine on TCNM formation. As shown in detail later, TCNM formation was significantly enhanced under combined LPUV/chlorine conditions. The enhanced TCNM formation depended strongly on the UV fluence, chlorine dose, and solution pH. To the best of the authors’ knowledge, this study is among the first to systematically investigate the enhancement effects of combined LPUV/ chlorine on TCNM formation. Results of this study provide new insights on the effects of combined UV/chlorine on DBP formation and can contribute to improvement of control strategies to reduce TCNM formation during drinking water and wastewater treatment.

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MATERIALS AND METHODS Chemicals. Sources of chemicals and reagents are provided in the Supporting Information (SI) Text S1. Experimental Methods and Analyses. All experiments were performed in a magnetically stirred 60-mL cylindrical quartz reactor similar to a previous setup (SI Figure S1).32 UV irradiation was supplied from one side of the quartz reactor by up to three 4-W LP lamps (G4T5 Hg lamps, Philips TUV4W) peaking at 254 nm at ambient temperature (22 °C). The incident light irradiance in the active wavelength region was from 2.0 to 5.4 mW/cm2 to the reactor center, depending on the number of lamps used (corresponding to a photo fluence rate of 3.36−9.07 μEinstein/L-s) measured by a UVX (UVP, U.S.) radiometer. Details of the reaction preparation and monitoring of TCNM, DMA, and MA are described in SI Text S2. 2698

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RESULTS

TCNM Formation from Polyamine under Combined UV/Chlorine. To evaluate the effects of combined UV/ chlorine on the formation of TCNM, synthetic waters with slightly elevated precursor concentrations (10 mg/L of polyamine, pH = 7.0) and a range of UV irradiance (2.0−5.4 mW/cm2) and fluence (0−9712 mJ/cm2) were employed to generate significant TCNM for easier detection and assessment. As shown in Figure 1a, with chlorine (7 mg/L) only, a low level (0.24−0.38 μg/L) of TCNM was formed from the polyamine solution, but the TCNM concentration remained approximately the same over time. These results suggest that TCNM was formed very quickly from the reaction of polyamine with free chlorine and additional reaction time did not lead to more TCNM formation. In contrast, when the polyamine solution was exposed to both LPUV and free chlorine, TCNM formation was significantly higher (up to 1.06, 1.56, or 1.91 μg/L with increasing UV irradiance) and varied dramatically over time. TCNM formation rose quickly to its maximum at around 5 min, and then decreased significantly from 5 to 30 min to finally reach a negligible level ( DMA > polyamine under the combined UV/ chlorine conditions. Compared to TCNM formation under combined UV/ chlorine, the formation of TCNM from DMA and MA under free chlorination only was considerably lower, reaching only up to 1.70 and 7.27 μg/L, respectively, in 30 min (Figure 6b,c). Overall, the data showed that the yield of TCNM formation by free chlorination only was MA > DMA > polyamine (SI Table S1).

free chlorine and LPUV (5.4 mW/cm2) irradiation at pH 7.0. The above amine concentrations were selected so that a comparable organic-N molar concentration (around 72.9 μM) was achieved in all reactors for easier comparison. All three precursors yielded TCNM and the maximum concentration of TCNM (at 2−4 min) was 1.97, 2.94, and 7.47 μg/L for polyamine, DMA, and MA, respectively (Figure 4). The results indicate that the TCNM formation potential followed the trend of MA > DMA > polyamine (SI Table S1) under combined UV/chlorine conditions.

Figure 4. Effect of polyamine, DMA, and MA on the formation of TCNM in solutions (pH = 7.0, T = 22 °C) that contained 7 mg/L free chlorine under LPUV (5.4 mW/cm2) irradiation.

Experiments were also conducted to monitor the generation of DMA and MA from degradation of polyamine under combined UV/chlorine using 10 mg/L polyamine, 9.8 mg/L free chlorine and LPUV (5.4 mW/cm2) irradiation at pH 7.0. As Figure 5 shows, with free chlorine only, the generation of DMA rose from 5.56 to 14.66 μg/L in 22 min, and then dropped to 9.91 μg/L from 22 to 30 min, whereas the generation of MA rose from 1.17 to 4.09 μg/L in 22 min, and then dropped to 2.67 μg/L from 22 to 30 min. Under combined UV/chlorine conditions, much more DMA and MA were generated. The concentrations of DMA and MA rose quickly and then decreased over time, reaching a maximum concentration of 52.68 and 137.78 μg/L, respectively, at around 2700

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Figure 5. Effect of LPUV (5.4 mW/cm2) irradiation on the formation of (a) DMA and (b) MA in solutions that contained 9.8 mg/L free chlorine and 10 mg/L polyamine at pH = 7.0 and T = 22 °C.

Figure 6. Effect of TBA (1 g/L) on the formation of TCNM in solutions that contained (a) 10 mg/L polyamine, (b) 10 mg/L DMA, or (c) 10 mg/ L MA, and 10 mg/L free chlorine under LPUV (5.4 mW/cm2) irradiation. The solution was at pH 7.0 and 22 °C.

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DISCUSSION Results of this study clearly showed that combined UV/ chlorine can enhance the formation of TCNM from polyamine, DMA and MA compared to free chlorination only. The enhanced TCNM formation is, however, highly dependent on reaction timemost pronounced in the first several minutes, followed by a decrease in TCNM concentration over time due to the photodegradation of TCNM. Compared to free chlorination only, the combination of UV and free chlorine enhanced formation of TCNM from polyamine, DMA, and MA by as much as 537%, 831%, and 446%, respectively, under the employed experimental conditions (Figures 1 and 6). The enhanced TCNM formation from polyamine by combined UV/chlorine compared to free chlorination only is likely due to two main causes: (1) the combined UV/chlorine significantly facilitates degradation of polyamine to generate MA, DMA, and other intermediates that are even better TCNM precursors than polyamine; and (2) the combined UV/ chlorine also facilitates transformation of intermediates, including MA and DMA, to TCNM. Results of this study differ from those reported previously by Reckhow and colleagues24 and Shah and colleagues.22 The above studies reported that MPUV pretreatment followed by postchlorination significantly increased TCNM formation in drinking waters that contained nitrate and organic matter; in contrast, LPUV pretreatment did not have the same enhancement effect. Shah and colleagues 22 did observe that prechlorination followed by MPUV, in which residual chlorine was present during MPUV treatment, increased TCNM formation even more than MPUV pretreatment followed by postchlorination. The current study differs from the previous studies because (1) nitrate was not amended to any of the

reaction solutions; and (2) the impact of combined UV and free chlorine was investigated under LPUV. Previous studies10,39,40 have shown that several reactions including generation of •OH and •Cl radicals can occur in the combined UV/chlorine process in aqueous systems: Cl 2 + H 2O ↔ HOCl + HCl

(1)

HOCl ↔ H+ + OCl−

(2)

(pK a = 7.6 at 20°C)

HOCl + UV photons → •OH + •Cl

(3)

OCl− + UV photons → •O− + •Cl

(4)

• −

O + H 2O → •OH + OH−

(5)

•

Cl + H 2O → •OH + Cl− + H+

(6)

•

Cl + Cl− ↔ Cl 2•−

(7) •

Once formed from photolysis of free chlorine, Cl radical can react with water to produce more •OH radical (reaction 6) or combine with excess chloride ion, if present in the solution, to form Cl2•− radical anion (reaction 7).38,41 The generation of reactive radical species renders the combined UV/chlorine an AOP. Polyamine’s structure adsorbs negligible UV radiation at 254 nm, yet its degradation to release DMA and MA is significantly enhanced under combined UV/chlorine compared to free chlorination only (Figure 5). Hydroxyl and chlorine radicals generated under UV/chlorine are strongly electrophilic species and can attack organic substrates by H-abstraction and electron transfer.38,41,42 Literature is abundant for reactions involving • OH but limited for •Cl. Previous research has reported that, 2701

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Figure 7. Proposed degradation schemes of polyamine under combined UV/chlorine conditions to release DMA and MA.

Figure 8. Proposed possible reaction schemes of TCNM formation from primary and secondary amines under combined UV/chlorine conditions. Note: Other possible mechanisms are discussed in the text.

compared to •OH, •Cl is comparable in reacting with uncharged alcohols and more reactive in reacting with anionic carboxylic acids, primarily via H-abstraction.43 Studies regarding reactions of •Cl with amine compounds are scarce. Nevertheless, considering the similarity between •OH and •Cl and their coexistence in the system, a likely degradation pathway of polyamine by these radicals to release DMA and MA is

proposed in Figure 7 Scheme I, on the basis of known oxidation of amines by •OH radicals.42 Oxidation of polyamine by free chlorine alone releases some amounts of DMA and MA (Figure 5). As illustrated in Figure 7 Scheme II, Park and colleagues29 have proposed the degradation mechanism of polyamine by free chlorine/ chloramine, in which polymer’s terminal tertiary amine groups are chlorinated. Under combined UV/chlorine, a third reaction 2702

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guanidine group of L-arginine and the chloro-imidazole group of L-histidine played an important role in CNCl formation. In the recent work that reported enhanced N-nitrosamine formation from secondary amines under UV/chlorine, Soltermann and colleagues26 hypothesized that N-nitrosamine formation occurred via the reaction of nitric oxide or peroxynitrite with the secondary aminyl radical, which were products from photolysis of NH2Cl and chlorinated secondary amines, respectively. However, it remains unclear at present how UV-induced photodecay of chlorinated amines may lead to TCNM, and more research is needed. It should also be noted that DMA and MA are probably not the only breakdown intermediates of polyamine for TCNM formation. In fact, the levels of DMA and MA detected from polyamine (Figure 5) could not fully account for the level of TCNM detected if the same TCNM formation yields from DMA and MA alone (SI Table S1) were assumed. Thus, TCNM formation from polyamine under UV/chlorine is likely more complicated than those proposed in Figures 7 and 8, including other unknown mechanisms. Furthermore, this study employed the chlorine to amine molar ratio (Cl/N) of 0.43− 1.92 (SI Table S1), lower than the chlorine to precursor molar ratios of 1−3.5 employed in the previous studies.26−28 In this study, both parent and chlorinated amines were present when UV was applied and thus might be susceptible to all the proposed reaction pathways. At higher chlorine to precursor molar ratios, most precursors are chlorinated and photodecay of chlorinated precursors plays a major role.26−28 The results associated with the effects of UV irradiance and chlorine concentration (Figure 1) are consistent with expectation, because combined UV/chlorine at a higher UV irradiance or a higher free chlorine concentration is likely to generate more reactive radical species and thus facilitate more polyamine degradation and TCNM formation. A higher chlorine concentration could also generate more chlorinated polyamine that was susceptible to photodecay. For the effect of pH, chlorine is present as HOCl and OCl− species in aqueous solutions depending on water pH (reaction 2). Both species absorb UV radiation, with maximum absorbance at around 235 and 292 nm for HOCl and OCl−, respectively.9,37 Although OCl− has a stronger molar absorptivity than HOCl at the absorbance peak,37 the two species absorb light and undergo photolysis at comparable rates near 254 nm.47 Even so, the generation of •OH from UV photolysis of free chlorine was greater at pH 5 than at pH 10. The scavenging effect of OCl− for •OH may contribute to the lower •OH yield at the higher pH.40 In free chlorination only, a lower pH is also better because HOCl is a stronger oxidant than OCl− to oxidize organics like polyamine.45 The potential effect of pH on the photodecay of chlorinated polyamine and other amine intermediates is currently unclear, although pH had little effect on the photodecay of inorganic chloramines.44 Overall, the results showed that more TCNM formation from polyamine occurred at lower pH (Figure 3); this trend was actually the opposite of the trend for TCNM formation from MA by chlorination only, which increased with increasing pH.12,48 Last, the significant drop off of TCNM concentration at the later reaction time in this study is due to direct photolysis of TCNM. Fang and colleagues34 observed gradual degradation (1st-oder rate constant k = 8−9 × 10−3 min−1) of TCNM under LPUV in aqueous solutions using a lower UV irradiance than this study (0.323 versus 3.36−9.07 μE/L-s) and reported a quantum yield of around 0.46 for TCNM. Using a similar

scheme must also be considered, namely the photodegradation of chloramine intermediates. Studies have shown that UV irradiation can induce N−Cl bond cleavage in inorganic chloramines such as NH2Cl, NHCl2, and NCl3 to yield aminyl (•NH2) and •Cl radicals.44 The aminyl radicals can further react with dissolved oxygen in water to form peroxyl radical intermediates (NH2O2•) and finally lead to nitrite, nitrate, and N2O products. Promoted N−Cl bond cleavage by UV irradiation has also been shown in organic chloramines.26−28 Thus, the chlorinated polyamine may be susceptible to similar photodecay (Figure 7 Scheme III). The resultant polyamine aminyl radical may further decompose via pathways proposed in Scheme I to lead to DMA and MA. The generated •Cl may contribute to additional degradation of polyamine via Scheme I. On the basis of the results in Figure 5, Schemes I and III played a greater role than Scheme II in the degradation of polyamine under combined UV/chlorine. To assess why combined UV/chlorine enhances TCNM formation from DMA and MA, the formation mechanisms of TCNM by chlorination only and under the coexistence of chlorine, radical species and UV need to be considered. For TCNM formation from free chlorination only, previous research12 has proposed that MA is first rapidly transformed to dichlorinated-MA, which is further oxidized to form a nitroalkane. Sequential additions of Cl[+] to the methyl group of nitroalkane to eventually yield TCNM. Cl[+1] additions proceed more rapidly with increasing chlorine substitution as the electron-withdrawing chlorines enhance the acidity of halonitroalkanes. On the basis of the above pathway, it is reasonable to propose that radical species such as •OH and •Cl radicals generated under combined UV/chlorine can more efficiently oxidize chlorinated-MA to nitroalkane intermediate, and thus promote TCNM formation (Figure 8 Scheme I). This scheme is a major pathway of TCNM formation from primary amine MA under combined UV/chlorine because adding radical quencher effectively blocked TCNM formation (Figure 6c). Although •Cl may also contribute to chlorination of the methyl group of MA (and DMA), this possibility cannot be confirmed based on the current data and merits further investigation. For secondary amine DMA, adding TBA only partially blocked TCNM formation (Figure 6b), suggesting that other mechanisms that did not depend on •OH and •Cl were also operative in TCNM formation from DMA under combined UV/chlorine. On the basis of the knowledge on chlorination of amines,15,45 chlorination of DMA can form a methyl-imine (CH3−NCH2) intermediate. As proposed in Figure 8 Scheme II, the methyl-imine intermediate may undergo two possible further transformations: (1) hydrolysis to yield MA; or (2) chlorination. After hydrolysis the formed MA follows the proposed Scheme I in Figure 8 to form TCNM. For (2), chlorination leads to additions of Cl[+] to the imine carbon similar to that previously proposed.18,46 Sequential chlorination steps leads to trichloromethylamine, which is oxidized to form TCNM. It is important to point out that the chlorinated amine and imine intermediates proposed in Figure 8 Scheme II may be affected by UV irradiation. Absorption of UV radiation may activate these chlorinated amines and imines in ways that render them more likely to follow the reaction paths to form TCNM, or induce N−Cl bond cleavage. In the recent work that reported enhanced CNCl formation from L-arginine and Lhistidine under UV/chlorine, Weng and colleagues28 proposed that UV254-induced N−Cl bond cleavage from the chloro2703

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approach and molar absorptivity as Fang and colleagues,34 a quantum yield calculated based on results of this study (kobs = 4.14 × 10−2 min−1, UV irradiation intensity = 9.07 μE/L-s, and effective path length = 2.16 cm) was around 0.184. Although TCNM is susceptible to degradation by radical species such as • OH,49 •OH appeared to play an insignificant role in the photolysis of TCNM under the experimental conditions employed in this study (Figure 2). The study by Cole and colleagues49 did find that the reaction rate of TCNM with •OH was slow (kapp = 4.97 × 107 M−1·s−1) compared to many organic compounds. Shah and colleagues22 evaluated the photolysis of TCNM under MPUV in both deionized water and deionized water spiked with humic acid and nitrate and found that indirect photolysis of TCNM was negligible. The photolysis products of TCNM were not monitored in this study. Photolysis of TCNM in the gas phase generates nitrosyl chloride (NOCl) and phosgene (Cl2CO).33 Phosgene is unstable in water and hydrolyzes to CO2 and HCl.50 Fang and colleagues34 detected chloride, nitrate, and nitrite among the products in photolysis of TCNM in aqueous systems. The authors proposed homolytic cleavage at either the C−Cl bond or the C−N bond to form a halogen radical or nitrite radical, which may react with water to produce HCl or HNO3. Other radical products (•CCl3 and •CCl2NO2) may react with water to produce CCl3OH and CCl2NO2OH, which then hydrolyze to form HCl, HNO3, formaldehyde, and formic acid.49,51 Overall, results of this study indicate that enhanced TCNM formation from amine precursors may be an undesirable effect when applying combined UV/chlorine treatment. The combined UV/chlorine treatment can increase TCNM formation by 2-15 fold compared to free chlorination only under disinfection to AOP conditions (300−1620 mJ/cm2 when maximum TCNM concentration was observed) employed in this study. Results obtained in this study for three amine precursors, representing primary, secondary, and complex macromolecular amines, improve the understanding of how amine precursors may be influenced by combined UV/ chlorine and change their tendency to form TCNM. Further research on the effects of combined UV/chlorine on other nitrogen-containing precursors and natural organic matter in real water matrices for TCNM formation is warranted.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ph.D. Programs Foundation of Ministry of Education of China (No. 20100092120018) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (No. 1105007001) for L.D. is gratefully acknowledged. The time and insightful recommendations of three reviewers are highly appreciated.

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(1) Krasner, S. W.; Weinberg, H. S.; Richardson, S. D.; Pastor, S. J.; Chinn, R.; Sclimenti, M. J.; Onstad, G. D.; Thruston, A. D. Occurrence of a new generation of disinfection byproducts. Environ. Sci .Technol. 2006, 40 (23), 7175−7185. (2) Bond, T.; Huang, J.; Templeton, M. R.; Graham, N. Occurrence and control of nitrogenous disinfection by-products in drinking waterA review. Water Res. 2011, 45, 4341−4354. (3) Krasner, S. W.; Westerhoff, P.; Chen, B. Y.; Rittmann, B. E.; Nam, S. N.; Amy, G. Impact of wastewater treatment processes on organic carbon, organic nitrogen, and DBP precursors in effluent organic matter. Environ. Sci. Technol. 2009, 43 (8), 2911−2918. (4) Liviac, D.; Wagner, E. D.; Mitch, W. A.; Altonji, M. J.; Plewa, M. J. Genotoxicity of water concentrates from recreational pools after various disinfection methods. Environ. Sci. Technol. 2010, 44 (9), 3527−3532. (5) Weinberg, H. S.; Krasner, S. W.; Richardson, S. D.; Thruston, A. D. The Occurrence of Disinfection By-Products (DBPs) of Health Concern in Drinking Water: Results of a Nationwide DBP Occurrence Study; Athens, GA, 2002. (6) Plewa, M. J.; Wagner, E. D.; Jazwierska, P.; Richardson, S. D.; Chen, P. H.; McKague, A. B. Halonitromethane drinking water disinfection by-products: Chemical characterization and mammalian cell cytotoxicity and genotoxicity. Environ. Sci. Technol. 2004, 38, 62− 68. (7) Muellner, M. G.; Wagner, E. D.; Mccalla, K.; Richardson, S. D.; Woo, Y.-T.; Plewa, M. J. Haloacetonitriles vs. regulated haloacetic acids: Are nitrogen-containing DBPs more toxic? Environ. Sci. Technol. 2007, 41, 645−651. (8) Dotson, A. D.; Rodriguez, C.; Linden, K. G. UV Disinfection implementation status in U.S. water treatment plants. J. Am. Water Works Assoc. 2012, 104 (3), 77−78. (9) Wang, D.; Bolton, J. R.; Hofmann, R. Medium pressure UV combined with chlorine advanced oxidation for trichloroethylene destruction in a model water. Water Res. 2012, 46, 4677−4686. (10) Jin, J.; Gamal El-Din, M.; Bolton, J. R. Assessment of the UV/ chlorine process as an advanced oxidation process. Water Res. 2011, 45, 1890−1896. (11) Sichel, C.; Garcia, C. A. K. Feasibility studies: UV/chlorine advanced oxidation treatment for the removal of emerging contaminants. Water Res. 2011, 45, 6371−6380. (12) Joo, S. H.; Mitch, W. A. Nitrile, aldehyde, and halonitroalkane formation during chlorination/chloramination of primary amines. Environ. Sci. Technol. 2007, 41, 1288−1296. (13) Lee, W.; Westerhoff, P.; Croue, J.-P. Dissolved organic nitrogen as a precursor for chloroform, dichloroacetonitrile, N-nitrosodimethylamine, and trichloronitromethane. Environ. Sci. Technol. 2007, 41, 5485−5490. (14) Yang, X.; Fan, C.; Shang, C.; Zhao, Q. Nitrogenous disinfection byproducts formation and nitrogen origin exploration during chloramination of nitrogenous organic compounds. Water Res. 2010, 44, 2691−2702. (15) Shah, A. D.; Mitch, W. A. Halonitroalkanes, halonitriles, haloamides, and N-nitrosamines: A critical review of nitrogenous disinfection by-products formation pathways. Environ. Sci. Technol. 2012, 46, 119−131.

ASSOCIATED CONTENT

S Supporting Information *

Chemicals and reagents; experimental methods and analyses; additional references; illustration of experimental set-up (Figure S1); relationship between the maximum TCNM formation (at ∼5 min reaction time) and the applied LPUV irradiance under combined UV/chlorine conditions (Figure S2); relationship between the maximum TCNM formation (at ∼5 min reaction time) and the applied free chlorine concentration from polyamine under combined UV/chlorine conditions (Figure S3); and comparison of the yield of TCNM formation from MA, DMA, and polyamine under free chlorination only and combined LPUV/chlorine (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 404-894-7694; fax: 404-385-7087; e-mail: ching-hua. [email protected]. 2704

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dx.doi.org/10.1021/es404116n | Environ. Sci. Technol. 2014, 48, 2697−2705