Article pubs.acs.org/est
Ultraviolet-Induced Effects on Chloramine and Cyanogen Chloride Formation from Chlorination of Amino Acids ShihChi Weng† and Ernest R. Blatchley, III*,†,‡ †
School of Civil Engineering, and ‡Division of Environmental and Ecological Engineering, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *
ABSTRACT: Ultraviolet (UV)-based treatment is commonly used to augment chlorination in swimming pools. However, the effects of combined application of UV254/chlorine on disinfection byproduct (DBP) formation are incompletely defined. To examine this issue, experiments were conducted with amino acids (L-arginine, L-histidine, and glycine) that are representative of those introduced to swimming pools via human body fluids. For each precursor, stepwise experiments were conducted with chlorination and UV254 exposure, with/without post-chlorination. Net formation and decomposition of chloramines and cyanogen chloride (CNCl) were measured for a range of chlorine/precursor (Cl/P) molar ratios and UV254 doses. Substantial production of NH2Cl from L-arginine and L-histidine was observed at Cl/P = 1.0 and 2.0 when postchlorination was applied to UV254-irradiated samples. These results suggested a mechanism of rapid N-chlorination, followed by cleavage of NH3 by UV254 irradiation. CNCl formation was observed from UV254-irradiated samples of L-arginine and L-histidine when Cl/P = 2.0 and 3.0, as well as from glycine for Cl/P ≤ 1. Structurally related precursor compounds were examined for CNCl formation potential in chlorination/UV experiments. CNCl formation was promoted by UV254 exposure of chlorinated imidazole and guanidine compounds, which suggested that these groups contributed to CNCl formation. The results have implications with respect to the application of chlorine and UV for water treatment in swimming pools and other settings, such as water reuse and advanced oxidation processes.
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INTRODUCTION Ultraviolet (UV)-based treatment is commonly used in treatment of swimming pool water. The process accomplishes efficient and rapid disinfection but leaves no residual disinfectant behind. Therefore, pool water treatment with UV is almost always used in conjunction with chlorination to provide a residual disinfectant/oxidant in pool water. The motivations for inclusion of UV in these settings include the ability to inactivate some chlorine-resistant microbes (e.g., Cryptosporidium parvum1) and the ability to reduce the concentration of inorganic chloramines;2,3 the latter issue is relevant with respect to air and water quality in indoor chlorinated swimming pool facilities. In addition, synergistic effects of UV and chlorine as disinfectants have been reported.4 UV irradiation has also been identified as an efficient treatment for nitrosamine compounds,5 which are potent carcinogens that are common in chlorinated water, especially in water from swimming pools.6 UV-based treatment has also been demonstrated to reduce genotoxicity in chlorinated swimming pool water relative to conventional chlorination alone.7 However, there is evidence to indicate that the combined use of UV and chlorine may promote the formation of some disinfection byproducts (DBPs). For example, Soltermann et al. demonstrated the potential for UV irradiation to promote Nnitrosodimethylamine (NDMA) formation in swimming pool situations.8 As such, NDMA is likely to exist as an intermediate in UV/chlorine applications, such as swimming pools. Enhance© 2013 American Chemical Society
ment of DBP formation [e.g., trihalomethanes (THMs) and haloacetic acids (HAAs)] has been reported in swimming pool settings and in drinking water facilities where UV-based treatment has been included.9,10 Shah et al.11 also suggested that the formation of chloropicrin from humic acids was enhanced by combined UV treatment with post-chlorination. Similarly, enhancement of DBP formation by combined UV/ chlorine treatment of waters containing organic N compounds that are known to be present in pools has also been reported.12 Previous studies13−15 have addressed the chlorination of organic N compounds, with particular emphasis on amino acids. These reactions are generally initiated by (rapid) N-chlorination. Chloramines and chloroimine compounds are formed as intermediates during chlorination, followed by relatively slow dechlorination and decarboxylation reactions to from nitriles. Li and Blatchley16 also examined DBP formation from chlorination of nitrous compounds that are known to be present in human body fluids (urine and sweat) and that are likely to be introduced to pools by swimmers. The results of these studies demonstrated that organic nitrogen compounds from human body fluids are important DBP precursors from chlorination. Moreover, DBPs Received: Revised: Accepted: Published: 4269
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Figure 1. Time-course behavior of inorganic chloramines for samples of (a) L-arginine and (b) L-histidine subjected to chlorination for 60 min [initial Cl/P = 1.0, 2.0, and 3.0 (from top to bottom)], followed by UV254 irradiation (solid lines and solid symbols) or UV254 irradiation with post-chlorination (with post CH) at 3 mg/L as Cl2 (dashed lines and open symbols).
conducted with selected organic N compounds that are representative of those introduced to swimming pools via human sweat and urine.
that contain nitrogen (e.g., haloacetamides and haloacetonitriles) tend to be more genotoxic than those without nitrogen.17−19 In the literature, DBP formation resulting from chlorination of reduced N compounds has been relatively well-studied,16,20−24 and some effects of UV irradiation on DBP formation have been reported. However, the effects of combined UV254/chlorine remain poorly defined, especially in relation to reaction mechanisms. To examine these issues, experiments were
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METHODS AND EXPERIMENTS Three amino acids [L-arginine (99%, Aldrich), L-histidine (99%, Sigma-Aldrich), and glycine (99.4%, Sigma)] were used as model precursors to evaluate the effects of UV254 irradiation on DBP 4270
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for NH2Cl, m/z 88 for NCl3, and m/z 89 for NHCl2) and comparisons of measured spectra reported in a previous study.28 Quantification was based on a comparison of the abundance signal(s) at the target m/z and comparison to a standard curve for each pure compound, following the method described by Weaver et al.26 The detection limits of the target compounds were 0.005 mg/L as Cl2 for NH2Cl, 0.01 mg/L as Cl2 for NHCl2, 0.007 mg/L as Cl2 for NCl3, and 0.0009 mg/L for CNCl.
formation in chlorinated water. Structures of precursor compounds used in this study are shown in Figure S1 of the Supporting Information. These compounds were selected for investigation because they represent the majority of amino acids in human sweat and urine.25 They have been demonstrated to be effective precursors to DBPs that are common to chlorinated pools5,16,21,26 and because they include several N-containing moieties that could contribute to N-DBP formation (see Figure S1 of the Supporting Information). Structurally related precursor compounds that included the imidazole group, the guanidine group, and an α-amine moiety or similar structure were also examined (see Figure S1 of the Supporting Information). For each precursor chemical, experiments were conducted using sequential chlorination and UV254 irradiation with and without post-chlorination. UV254 irradiation was accomplished using a flat-plate collimated beam.27 Incident irradiance (200 μW/cm2) was measured by a National Institute of Standards and Technology (NIST)-calibrated radiometer (IL1700, International Light Technologies, Peabody, MA). UV254 irradiation was accomplished by exposing unbuffered aqueous solutions in gastight containers (zero-headspace cuvettes). On the basis of the incident irradiance, the UV254 doses imposed on samples ranged from 120 to 720 mJ/cm2 for exposure times ranging from 10 to 60 min, respectively. The initial target compound concentration in most experiments was 1.8 × 10−5 M at pH 6.9 ± 0.05; pH changed from 6.9 to 6.7 during the course of the experiments. In most cases, aqueous precursor solutions were subjected to pre-/post-chlorination by the addition of NaOCl (10−15% aqueous solution, Aldrich) with initial chlorine/ precursor (Cl/P) molar ratios of 1.0, 2.0, and 3.0 (1.28, 2.56, and 3.83 mg/L as Cl2, respectively). These selected Cl/P molar ratios allowed for the evaluation of the chlorination sequence and the effects of UV254 irradiation on these chlorinated functional groups (α-amine, guandine, and imidazole). The exception to this generalization was glycine, where lower Cl/P ratios were applied (0.25 0.5, and 1.0; equal to 0.32, 0.64, and 1.28 mg/L as Cl2). Post-chlorination (3.0 mg/L as Cl2) was applied to samples following UV254 irradiation, with analysis following immediately (within 30 s of post-chlorination). Reacting solutions were subjected to time-course sampling and analysis. The sequence of chlorination, UV254 irradiation, and post-chlorination was designed to mimic the behavior of precursor chemicals in a chlorinated pool with UV-based treatment. In these systems, chlorinated water from a pool is circulated through a UV system, followed by re-chlorination. Membrane introduction mass spectrometry (MIMS) was used for analysis of volatile DBPs, which included inorganic chloramines (NH2Cl, NHCl2, and NCl3) and cyanogen chloride (CNCl). MIMS was configured within an Agilent GC−MS system [5975C mass-selective detector (MSD) and 6850 GC (Agilent Technologies, Santa Clara, CA)] with a membrane interface that was constructed around small-diameter semipermeatable silicone tubing (0.25 mm inner diameter, 0.47 mm outer diameter, and 60 mm long, Baxter, IL), as described by Shang and Blatchley.28 MIMS was operated under a sample flow rate and an auxiliary gas (helium) flow rate of 0.7 and 0.5 mL/ min, respectively. Standard solutions of target compounds were prepared by mixing free chlorine with ammonium chloride (for NH2Cl, NHCl2, and NCl3) or glycine (for CNCl) under appropriate Cl/P molar ratio and pH conditions, as described by Li and Blatchley16 and Yang and Shang,29 respectively. Compound identification was accomplished by unique fragment ion or molecular ion signals (m/z ratio; m/z 61 for CNCl, m/z 53
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RESULTS AND DISCUSSION L -Arginine and L -histidine were previously identified as precursors for NCl3 and CNCl formation, respectively, from chlorination, but production of these compounds was relatively slow and required a relatively high Cl/P ratio.16 Preliminary experiments involving UV254 irradiation and chlorine coexposure with/without post-chlorination revealed reformation of inorganic chloramines from L-arginine and L-histidine (see Figures S2 and S3 of the Supporting Information, respectively). In addition, CNCl formation was enhanced in samples from both compounds that were subjected to UV254 irradiation. These results were consistent with a previous investigation.12 The objective of this study was to examine this process in greater detail, to allow for the definition of the mechanisms that are responsible for DBP behavior in combined UV254 irradiation and chlorination processes, using amino acids as precursors. Inorganic Chloramine Formation from L-Arginine and L-Histidine. The behavior of inorganic chloramines from Larginine and L-histidine was evaluated stepwise with chlorination/UV254 irradiation, as illustrated in Figure 1. The behaviors of inorganic chloramines were similar from both precursors. When Cl/P = 1.0, only trace quantities of inorganic chloramines were detected to result from chlorination, probably because of the relatively small quantity of free chlorine in the system. UV irradiation of these mixtures also yielded minimal formation of inorganic chloramines. However, post-chlorination of the UVirradiated samples (for initial Cl/P = 1.0) resulted in promotion of NH2Cl formation, with roughly 1 mg/L (as Cl2) being formed from L-arginine, corresponding to a molar yield of 78%. For Lhistidine, NHCl2 was dominant in the post-chlorination experiment (1.2 mg/L as Cl2, with a molar yield of 94%), and substantial production of NH2Cl was also observed (0.6 mg/L as Cl2, with a molar yield of 47%). This behavior was not consistent with chlorination of amino acids at the same total applied chlorine dose, wherein direct production of NCl3 has been reported;16 NH2Cl and NHCl2 were then formed from hydrolysis of NCl3.30 However, in this case, the behavior of inorganic chloramines from post-chlorination of the UVirradiated samples of L-arginine and L-histidine was consistent with chlorination of ammonia, wherein NH2Cl is formed first and was present at the highest concentration. Related postchlorination experiments were conducted with the initial precursor concentration increased to 1.8 × 10−4 M, which was intended to create a nitrogen-rich condition in post-chlorination [the free chlorine/nitrogen molar ratio (Cl/N ≪ 1)]. The results showed that NH2Cl was the only inorganic chloramine that formed from L-arginine and L-histidine in these experiments (see Figures S4 and S5 of the Supporting Information, respectively), which suggested that NH3 was released from the chlorinated forms of both amino acids by UV254 irradiation. These results indicated that, for Cl/P ≤ 1.0, the first step involved chlorine substitution of an amine group to form their respective Nchloramine derivatives. UV254 irradiation of these N-chloramine 4271
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Figure 2. Time-course behavior of CNCl for samples of (a) L-arginine and (b) L-histidine subjected to chlorination for 60 min with initial Cl/P = 1.0, 2.0, and 3.0, followed by UV254 irradiation (solid lines) or UV254 irradiation with post-chlorination (with post CH) at 3 mg/L as Cl2 (dashed lines).
Figure 3. Time-course behavior of CNCl for samples of (a) guanidine compounds or structurally related compound and (b) imidazole compounds subjected to chlorination for 60 min with Cl/P = 1.0 (top) and Cl/P = 2.0 (bottom), followed by UV254 irradiation (solid lines) or UV254 irradiation with post-chlorination (with post CH) at 3 mg/L as Cl2 (dashed lines).
chloramine concentrations in a manner that was consistent with the previously reported photodecay of these compounds.2 In post-chlorination experiments, NH2Cl was still the dominant inorganic chloramine, as observed in the experiments when Cl/P
derivatives caused N−Cl cleavage and yielded NH3, which was converted to inorganic chloramines by post-chlorination. For Cl/P = 2.0, inorganic chloramines were formed by chlorination. UV254 irradiation resulted in decreases of inorganic 4272
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formed directly from chlorination, and the concentration decreased with UV exposure. On other hand, there was no obvious trend for NH2Cl and NHCl2 formations in these experiments. From Figure 3a, compounds with a guanidine moiety (guanidine and guanidineacetic acid) showed trace formation of CNCl to result from chlorination. The rate and yield of CNCl from these compounds were lower than those from glycine, which is known to be an efficient precursor for CNCl.21 Substantial CNCl formation was observed from four guanidine compounds when UV254 irradiation was applied to chlorinated samples, with molar yields as high as 11% for guanidine at Cl/P = 2.0. Post-chlorination of UV-irradiated samples did not contribute substantially to CNCl formation. This suggested that CNCl formation was promoted by direct photolysis of chlorinated guanidine. On the other hand, only trace quantities of CNCl were generated during the 60 min chlorination period from acetamidine and acetamide. After UV254 irradiation, CNCl formation was promoted, even under the condition of Cl/P = 1.0. The results suggested that UV254 irradiation of the chlorinated guanidine group resulted in the formation of CNCl. Moreover, the results also indicated that the CNCl formation potential of the guanidine group is greater than those of acetamidine and acetamide (guanidine > acetamidine > acetamide; Figure 3a). Collectively, the results showed the CNCl formation from UV254 irradiation of chlorinated guanidine compounds, which was consistent with the hypothesized mechanism presented previously. From Figure 3b, all compounds with the imidazole structure showed slight formation of CNCl during chlorination when Cl/P = 1.0 and 2.0; the behavior of CNCl formation from the imidazole compounds was similar to that of the guanidine compounds. Substantial CNCl formation was observed from the chlorinated forms of the three imidazole compounds when UV254 irradiation was applied on chlorinated samples, with a molar yield as high as 5.4% for 4-methylimidazole at Cl/P = 2.0. Postchlorination of the UV-irradiated samples did not enhance CNCl formation, which indicated that CNCl formation was promoted by direct photolysis of chlorinated imidazole. In addition to the formation of the inorganic chloramines and CNCl, the formation of CH3NCl2 was observed from 1methyimidazole (see Figure S9 of the Supporting Information, with a molar yield up to 23% at Cl/P = 2.0); however, the ability to produce CNCl from 1-methyimidazole did not decrease (in comparison to imidazole and 4-methylimidazole), which suggested that the nitrogen atom in CNCl was not produced from the nitrogen connected to the methyl group. Formation of CNCl from Glycine. Glycine has been identified as an efficient precursor for CNCl formation from chlorination.31 Glycine has also been reported to be present in human urine at a higher concentration than the other amino acids;25 therefore, the behavior of glycine in response to UV/ chlorine exposure is likely to be important in swimming pools. CNCl formation potential from glycine was examined using a similar protocol as described above, except that lower Cl/P ratios were evaluated. The reason for the lower Cl/P ratio is that the reaction between free chlorine and glycine is fast, such that the chlorinated intermediate (chloroglycine) is rapidly converted to CNCl when free chlorine is present in the system (when Cl/P > 1, even in the absence of UV254 radiation). The low Cl/P ratio (Cl/P < 1) allowed for the formation of chlorinated intermediates from glycine, as well as the evaluation of the UV-
= 1.0. For Cl/P = 3.0, the concentration of inorganic chloramines increased during chlorination and decreased in subsequent UV254 irradiation. Post-chlorination of these solutions indicated the reformation of the inorganic chloramines; however, the behavior of inorganic chloramines in post-chlorination was different when Cl/P = 3.0 than when Cl/P = 1.0 and 2.0. Specifically, at this higher Cl/P ratio, NH2Cl was not the dominant inorganic chloramine. The results suggested that NH3 release by UV254 irradiation was suppressed in the Cl/P = 3.0 experiment, relative to experiments conducted at Cl/P = 1.0 or 2.0. The decrease of NH3 release was assumed to be attributable to the formation of a dichloro-amine group. N−Cl bond cleavage of dichloro-amine groups has been shown to result in the formation of chlorinated imine compounds (R−CN−Cl13,21), which would also imply less NH3 formation. In separate experiments, amino acid compounds with α-amine groups (L-alanine and Lnorvaline, with molecular structures shown in Figure S1 of the Supporting Information and data shown in Figure S6 of the Supporting Information) were also examined with the same treatment sequence, and the behavior of inorganic chloramines under Cl/P = 1.0 and 2.0 was consistent with the observation of L-arginine or L-histidine, supporting the suppression of NH3 formation. Formation of CNCl from L-arginine and L-histidine. In the stepwise chlorination and UV254 experiments of L-arginine and L-histidine, the formation of CNCl was observed during UV irradiation, as illustrated in Figure 2. In all cases, only trace quantities of CNCl were detected during chlorination, but substantial formation of CNCl was observed with UV254 irradiation of chlorinated samples for Cl/P = 2.0 and 3.0 (0.07 mg/L for L-arginine, with a molar yield of 6%, and 0.17 mg/L for L-histidine, with a molar yield of 15%, under Cl/P = 3.0). As described above, when Cl/P = 1.0, chlorination was believed to take place predominantly on the α-amine group of L-arginine or L-histidine, where the N-monochloramine derivatives release NH3 as a result of UV254 irradiation. When Cl/P was increased to 2.0 and 3.0, there was sufficient free chlorine to promote chlorination of another nitrous group (guanidine group for Larginine and imidazole group for L-histidine); under these circumstances, rapid CNCl formation was observed to result from UV254 irradiation of chlorinated samples. This observation suggested that CNCl formation attributable to UV254 irradiation was associated with the chlorination of a nitrous functional group other than the α-amine. Thus, it was hypothesized that the chloro-guanidine group of L-arginine and chloro-imidazole group of L-histidine were responsible for CNCl formation with UV254 irradiation. Formation of CNCl from Structurally Related Compounds. Several compounds containing the guanidine group structure (guanidine, guanidineacetic acid, acetamidine, and acetamide) as well as several imidazole compounds (imidazole, 1-methylimidazole, and 4-methylimidazole) were examined for CNCl formation potential. Molecular structures of these precursor compounds are shown in Figure S1 of the Supporting Information. The experiments were conducted in a manner of stepwise exposure to chlorination and UV254 irradiation with/ without post-chlorination, as described previously. CNCl formation resulting from sequential chlorination/UV254 irradiation of guanidine compounds and imidazole compounds are shown in panels a and b of Figure 3, respectively. The formation of inorganic chloramines from guanidine compounds and imidazole compounds is illustrated in Figures S7 and S8 of the Supporting Information, respectively. For most cases, NCl3 was 4273
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induced effects on chlorinated compounds. Figure 4 illustrates CNCl formation from glycine with chlorination for 60 min
Scheme 1. Proposed UV-Induced Mechanisms of Chlorinated (a) L-Arginine and (b) L-Histidine
Figure 4. CNCl formation from chlorination and UV254 irradiation experiments of glycine under Cl/P = 0.25, 0.5, and 1.0.
followed by UV254 irradiation for 60 min under Cl/P = 0.25, 0.5, and 1.0. During chlorination, free chlorine was depleted immediately (data not shown) and the CNCl yield increased with the Cl/P ratio. With UV254 irradiation, enhancement of CNCl formation was observed without the addition of free chlorine. Only trace quantities of NH2Cl were formed during the chlorination period under Cl/P ≤ 1, and corresponding photodecay of NH2Cl was also observed during the UV irradiation period (data shown in Figure S10 of the Supporting Information). In the study by Na and Olson,21 a mechanism of CNCl formation from chlorination of glycine was proposed in which glycine is chlorinated to form N-dichloroglycine, followed by sequential dechlorination to yield a chloro-imine intermediate. The imine compound then decayed to yield cyanide, which is rapidly chlorinated to yield CNCl. Dechlorination of Ndichloroglycine was reported to limit the overall rate of CNCl formation. UV-induced cleavage of the N−Cl bond has been demonstrated previously for inorganic and organic N compounds.2,3,12 Collectively, this information suggests that UV254 irradiation enhances CNCl formation from glycine by promoting the rate-limiting process (dechlorination). Proposed Mechanisms for Volatile DBP Formation from L-Arginine and L-Histidine. Hypothesized mechanisms to describe the behavior of L-arginine and L-histidine when subjected to chlorination/UV254 irradiation are presented in Scheme 1. With regard to L-arginine, under Cl/P = 1.0, most free chlorine is expected to participate in rapid N-chlorination of the α-amine group, resulting in the formation of N-chloro-arginine (compound I in Scheme 1a); this would leave little free chlorine to participate in other reactions. With UV254 irradiation of the chlorinated L-arginine, the N−Cl bond of the chloro-amine group is cleaved to yield an aminyl radical,2 followed by hydrolysis and NH3 release. Post-chlorination of the resulting mixture will result in inorganic chloramine formation (predominantly NH2Cl). When the Cl/P ratio was increased to 2.0, the guanidine group in L-arginine experienced chlorine substitution to yield dichloroarginine, with one chlorine on the α-amine group and one chlorine on the terminal amine (as shown by compound II in Scheme 1a). Pattison and Davies32 indicated that the reaction
rate constant of free chlorine with amino acids is higher than with guanidine compounds, which is consistent with the Nchlorination sequence presented in Scheme 1a. When the dichloro compound was subjected to UV254 irradiation, the chlorinated α-amine group underwent N−Cl bond cleavage releasing NH3, which then yielded inorganic chloramines upon re-chlorination, as described above. CNCl formation was promoted by UV254-induced N−Cl bond cleavage from chloroguanidine. The chlorine atom on CNCl could come from chloramines (both organic and inorganic) or the chlorine radical from photolysis of chloramines.2 When the Cl/P ratio increased to 3.0, the additional free chlorine led to the formation of a trichloro-arginine (compound III in Scheme 1a), which lost the ability to release NH3 with UV254 irradiation. Presumably, the dichloro-amine group would still undergo N−Cl cleavage by UV254 irradiation forming a chloro-imine intermediate, which, in turn, will yield nitrile compounds, as suggested by Shah et al.11 L-Histidine is believed to have followed a similar reaction mechanism when subjected to chlorination and UV irradiation. When Cl/P = 1.0, the α-amine group in L-histidine was chlorinated to yield chloro-histidine (compound I in Scheme 1b). UV254 irradiation would promote N−Cl bond cleavage to yield an aminyl intermediate, which then undergoes hydrolysis to yield NH3. When Cl/P = 2.0, the imidazole group started to be chlorinated and formed dichloro-histidine (monochloro substitution on the α-amine group and imidazole group; compound II in Scheme 1b). When dichloro-histidine was subjected to UV 254 irradiation, NH 3 and CNCl were released from monochloro-amine and monochloro-imidazole, respectively. When Cl/P = 3.0, trichloro-histidine was formed, which is 4274
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believed to have contained a dichlorinated α-amine and monochloro-imidazole. UV254 irradiation of trichloro-histidine led to the formation of CNCl and a chlorinated imine, following a similar pattern to that of L-arginine (as described above). The results of this study suggested the potential for CNCl formation from guanidine compounds (L-arginine) and imidazole compounds (L-histidine) with chlorination/UV254 irradiation treatment. UV254 irradiation appears to promote N−Cl bond cleavage, which has been identified as a rate-limiting step formation of several halogenated N-DBPs.21,22 In this case, it appears that acceleration of N−Cl bond cleavage is responsible for enhancement of CNCl formation. Post-chlorination caused inorganic chloramine reformation, which was attributable to NH3 release caused by UV254 irradiation of the monochloroamine group. However, it should be noted that the results of this work did not allow for additional details of the reaction mechanisms to be defined. It appears that UV irradiation promotes N−Cl bond cleavage, which has been identified as the rate-limiting step in halonitrile formation.21 However, it is unclear if the formation of CNCl follows the same pathway as defined for reactions involving chlorination only (no UV). The results of this work have allowed for definition of a possible mechanism of DBP formation from the combined application of chlorine and UV. The results also indicated that the combined use of UV254/chlorine treatment in swimming pools may promote the formation of CNCl from several organic N precursors that are present in human body fluids, such as sweat and urine. However, CNCl is likely to behave as an intermediate when free chlorine is present because CNCl oxidation is catalyzed by hypochlorite.21 Therefore, the increase of CNCl formation by UV-based treatment is likely to lead to higher chlorine demand (i.e., faster chlorine consumption) when UVbased treatment is implemented. However, it is important to note that the maintenance of a free chlorine residual is variable among public swimming pools.26,33 In pools where the free chlorine concentration is near the limit of detection, inclusion of UV-based treatment could lead to a substantial increase of the CNCl concentration. The results of this work suggest that maintenance of a free chlorine residual becomes increasingly important in systems where UV-based treatment is implemented. The results of this work have implications with respect to the application of chlorine and UV for water treatment in swimming pool facilities and in other settings where these two forms of treatment may be used together, including reuse applications and advanced oxidation processes (UV/chlorine AOPs). UV/ chlorine as an AOP system has not been studied extensively, with only a few studies being published in recent years12,34,35 Although Sichel et al.35 demonstrated improved efficiency of degradation of emerging contaminants by UV/chlorine relative to UV/H2O2, only the feasibility of UV/chlorine as an AOP was evaluated. The mechanism of enhancement of contaminant degradation by UV/chlorine was not clearly addressed. According to the results of this study, the N−Cl bond is susceptible to UV254 irradiation; it may be possible to use these reactions to promote the degradation of N-containing organic compounds (including many emerging contaminants) in water. In addition, Plewa et al.36 demonstrated that inclusion of UV irradiation in drinking water treatment with chlorination decreases genotoxicity.
Article
ASSOCIATED CONTENT
S Supporting Information *
Structures of precursor compounds used in this study (Figure S1), formation of inorganic chloramines, CNCHCl2, and CNCl from L-arginine (Figure S2) and L-histidine (Figure S3) with sequential treatments of chlorination (CH), UV254 irradiation (UV), and post-chlorination (with post CH), time-course behavior of inorganic chloramines for samples of L-arginine (Figure S4) and L-histidine (Figure S5), time-course behavior of inorganic chloramines for samples of (a) L-alanine and (b) Lnorvaline (Figure S6), time-course behavior of inorganic chloramines from guanidine compounds (Figure S7) and imidazole compounds (Figure S8), CH3NCl2 formation from chlorination/UV irradiation and post-chlorination (with post CH) experiments of 1-methylimidazole (Figure S9), and NH2Cl formation from chlorination/UV254 irradiation experiments of glycine (Figure S10). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 1-765-494-0316. Fax: 1-765-494-0395. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support for this work was provided by grants from the National Swimming Pool Foundation, Engineered Treatment Systems, LLC, and American Chemistry CouncilChlorine Chemistry Division.
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REFERENCES
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Environmental Science & Technology
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dx.doi.org/10.1021/es400273w | Environ. Sci. Technol. 2013, 47, 4269−4276
