ARTICLE pubs.acs.org/est
Formation and Decomposition of New and Unknown Polar Brominated Disinfection Byproducts during Chlorination Hongyan Zhai and Xiangru Zhang* Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
bS Supporting Information ABSTRACT: Brominated disinfection byproducts (Br-DBPs) are generally more cytotoxic and genotoxic than their chlorinated analogues. A great portion of total organic bromine in chlorinated drinking water is still unknown and may be ascribed to polar BrDBPs. In this work, a novel approach, precursor ion scan using ultra-performance liquid chromatography/electrospray ionizationtriple quadrupole mass spectrometry, was adopted and further developed for selective detection and identification of polar Br-DBPs, which made it possible to reveal the whole picture of the formation and decomposition of polar Br-DBPs during chlorination. Simulated drinking water samples with chlorine contact times from 1 min to 7 d were analyzed. Many new polar aromatic and unsaturated aliphatic Br-DBPs were detected and tentatively proposed with chemical structures, of which 2,4,6-tribromophenol, 3,5dibromo-4-hydroxybenzoic acid, 2,6-dibromo-1,4-hydroquinone, and 3,3-dibromopropenoic acid were confirmed or identified with authentic standards. It was found that various polar Br-DBPs formed and reached the maximum levels at different chlorine contact times; high molecular weight Br-DBPs might undergo decomposition to relatively low molecular weight Br-DBPs or even finally to haloacetic acids and trihalomethanes. The decomposition of newly detected intermediate Br-DBPs (including molecular ion cluster m/z 345/347/349/351, 2,4,6-tribromophenol, and 3,5-dibromo-4-hydroxybenzoic acid) during chlorination was investigated in detail. The “black box” from the input of “humic substances þ bromide þ chlorine” through the output of “haloacetic acids þ trihalomethanes” was opened to a significant extent.
’ INTRODUCTION Bromide as a natural anion in raw water can be oxidized in chlorination to HOBr, which can react with natural organic matter (NOM) to form brominated disinfection byproducts (Br-DBPs). It has been reported that HOBr acts as a more efficient substitution agent than HOCl.1,2 For the formation of trihalomethanes (THMs) and haloacetic acids (HAAs), HOBr has been found to be 20 times more reactive than HOCl.3 Research has shown that Br-DBPs generally are dozens to hundreds times more cytotoxic and genotoxic than their chlorinated analogues.4-6 For instance, bromoacetic acid is 201.3 times more mutagenic in Salmonella typhimurium stain TA100 and 23.6 times more genotoxic in Chinese hamster ovary cells than chloroacetic acid.5 Hence, much current attention has been focused on Br-DBPs. The complexity of NOM determines a higher magnitude of complexity of the reaction products from NOM, bromide, and chlorine. To date about 110 Br-DBPs have been reported as drinking water DBPs,7-9 and almost all of them are identified by gas chromatography/mass spectrometry with or without derivatization,8,10 which is not amenable to the detection of polar or highly polar Br-DBPs. As a result, a significant portion of total organic bromine (TOBr) formed during chlorination has not been identified or even well characterized.11,12 Since polar Br-DBPs are mixed together with tens of thousands of brominefree organic compounds in a chlorinated water sample, selective detection of polar Br-DBPs in such a complex sample is an arduous issue. Recently, a novel precursor ion scan (PIS) method has been developed for fast selective detection of polar Br-DBPs r 2011 American Chemical Society
using an electrospray ionization-triple quadrupole mass spectrometer (ESI-tqMS).13 Because bromine-containing compounds may generate bromide ions (79Br- and 81Br-) in the collision chamber of the mass spectrometer, by performing PIS of m/z 79 or 81, the mass spectrometer can detect almost all polar brominecontaining compounds in a sample, which makes it possible to investigate the formation and decomposition of new and unknown polar Br-DBPs during chlorination. The major bromine-containing molecular ions picked out by the ESI-tqMS PISs of m/z 79 and 81 can then be analyzed by product ion scans to obtain structural information. One problem related to direct infusion ESI-tqMS product ion scans is the possible presence of diverse molecular ions with an identical m/z value in a sample. When direct infusion ESI-tqMS product ion scans are conducted for such molecular ions, their fragmented ions may be mixed together and sometimes cannot be used for proposing structures. In this study, to overcome this kind of possible overlap, state-of-the-art ultra-performance liquid chromatography (UPLC) was coupled with the ESI-tqMS for preseparation, which should aid in better proposing or identifying structures of unknown polar Br-DBPs. More importantly, once the information on the formation and decomposition of polar Br-DBPs (especially on the intermediate halogenated DBPs) during chlorination is obtained, it may help Received: October 12, 2010 Accepted: January 30, 2011 Revised: January 12, 2011 Published: February 16, 2011 2194
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Environmental Science & Technology to resolve the mechanistic gap between NOM and final halogenated DBPs in chlorination. Since THMs were first reported as DBPs in drinking water in 1974,14 there has been an enormous research effort to explore the formation mechanisms of halogenated DBPs.15 Rook14 first hypothesized that THMs in chlorinated drinking water are produced by reactions between chlorine and NOM. However, the structure of NOM is so complex that most researchers use model compounds to reveal structures or functional groups responsible for the formation of THMs (refs 15 and 16 and references therein). One of the most widely accepted mechanisms is the “haloform reaction”, which involves an electrophilic addition to the R-carbon of an enolizable carbonyl compound.17 Reckhow and Singer18 studied the chlorination of many aromatic and aliphatic model compounds and found out that chloroform, trichloroacetic acid, and dichloroacetic acid were common products. Accordingly, they pioneeringly proposed a generalized conceptual model for the formation mechanism of both THMs and HAAs: predominant oxidation reactions of chlorine with NOM lead to more oxygenated functional groups such as β-diketone-type moieties; the activated carbon in β-diketone-type moiety would quickly become fully substituted by chlorine; hydrolysis would then occur rapidly to form a ketone-type intermediate with dichlorinated methyl on one side and one R group on the other side of the carbonyl group; if the R group was OH or OR, then the hydrolysis reaction would produce dichloroacetic acid; otherwise, the intermediate would be further chlorinated to a base-hydrolyzable trichloromethyl species. The specific research on the formation mechanism of BrDBPs in chlorination is rare, and bromine is believed to react with NOM in the same way as chlorine.19 This historical summary shows that the chlorination studies with model compounds provide some important insight into the formation mechanism of halogenated DBPs. However, most model compounds may be neither a part of NOM nor real intermediate DBPs in chlorination. There is still a doubt whether the reactions of chlorine and model compounds occur in chlorination of drinking water. The objectives of this study were to investigate the formation and decomposition of polar Br-DBPs during chlorination, and to identify the intermediate halogenated DBPs during chlorination, which may help to fill in the gap between NOM and final halogenated DBPs such as THMs and HAAs in chlorination. As a prerequisite for identifying new polar Br-DBPs, the separation characteristics of polar bromine-containing compounds in UPLC were examined.
’ EXPERIMENTAL METHODS Materials. The separation characteristics of polar brominecontaining compounds in UPLC were examined using 21 polar bromine-containing standard compounds, whose sources, molecular ion m/z values, and pKa values are listed in Table S1 in the Supporting Information (SI). A standard compound 1,4-benzoquinone (Acros) was used to prepare dibromo-1,4-hydroquinone (as described in the SI). Suwannee River humic acid (SRHA) was purchased from the International Humic Substances Society. A stock solution containing 2000 mg/L each of four regulated THMs was purchased from Supelco. HPLCgrade methanol, acetonitrile, and methyl tert-butyl ether (MtBE) were purchased from Aldrich. A stock solution of hypochlorite was prepared by absorption of ultrahigh-purity chlorine gas with a sodium hydroxide solution and standardized by the DPD
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ferrous titrimetric method.20 Ultrapure water (18.2 MΩ/cm) was supplied by a NANOpure system (Barnstead). Simulated Drinking Water Sample Preparation. Simulated raw water was prepared with ultrapure water containing 3.0 mg/ L SRHA as C, 90.0 mg/L NaHCO3 as CaCO3, and 2.0 mg/L NaBr as Br- (a high level of bromide found in several drinking water supplies in different parts of the world). Hua et al.12 have used 2.4 mg/L of bromide in their studies for better observation of Br-DBPs. The simulated raw water was chlorinated to prepare a series of simulated drinking water samples with chlorine contact times of 1 min, 1 h, 3 h, 9 h, 12 h, 1 d, 5 d, and 7 d (i.e., chlorinated SRHA samples). For all the samples, chlorination was conducted with chlorine dosage of 5.0 mg/L as Cl2 in darkness at ambient temperature. The chlorine dose of 5.0 mg/L as Cl2 was chosen for better observation of new and unknown polar Br-DBPs. Immediately after chlorine addition, all the samples were adjusted to pH ∼7.5 with 1.8 M hydrochloric acid. After a given chlorine contact time, residual chlorine in a sample was quenched immediately with 120% of the requisite stoichiometric dose of NaAsO2. To determine whether there were any impurities in the reagents or any artifacts in the disinfection and subsequent pretreatment, one control sample was generated by repeating the same procedure with aforementioned chlorinated SRHA samples without chlorination. Simulated Drinking Water Sample Pretreatment. Procedures for water sample pretreatment are based on a previous study.13 Briefly, a 1-L water sample was adjusted to pH 0.5 with 7:3 (v/v) concentrated sulfuric acid/water and was added with 100 g of Na2SO4. Then, the sample was extracted with 100 mL of MtBE. After extraction, the MtBE layer was transferred to a rotary evaporator and concentrated to 0.5 mL. The 0.5 mL solution in MtBE was mixed with 20 mL of acetonitrile, and the mixture was rotoevaporated back to 0.5 mL. The 0.5 mL solution in acetonitrile was stored at 4 °C. Prior to (UPLC/)ESI-tqMS analyses, the 0.5 mL solution was diluted with ultrapure water to 1 mL. (UPLC/)ESI-tqMS Analysis. The pretreated samples were analyzed by a Waters Acquity ESI-tqMS. The ESI-tqMS parameters were optimized and set as follows: sample flow rate via an infusion pump 10 μL/min, ESI negative mode, capillary voltage 2.9 kV, cone voltage 15 V, source temperature 110 °C, desolvation temperature 300 °C, desolvation gas 650 L/h, cone gas 50 L/h, mass resolution 15 (1-unit resolution); collision energy 15-35 eV (depending on analytes) and collision gas (Argon) 0.25 mL/min for PISs and product ion scans. By setting the ESItqMS PISs of m/z 79 and 81, all electrospray-ionizable Br-DBPs should be detected. For all the ESI-tqMS PISs and product ion scans, the data collection mode was Multi-Channel Analysis, which can greatly enhance precursor ion intensities by accumulating multiple scans and eliminate possible ion intensity fluctuation in a single run. A Waters UPLC system was coupled to the Waters Acquity ESI-tqMS (UPLC/ESI-tqMS). Six μL of a pretreated sample was injected in the UPLC. The UPLC separation was carried out with an HSS T3 column (50 2.1 mm, 1.8 μm particle size, Waters). Two different gradient eluents were tested. One eluent was composed of methanol and water. The composition of methanol/water (v/v) changed linearly from 5/95 to 90/10 in the first 8 min, and then returned in 0.10 min to 5/95, which was held for 2.9 min for re-equilibration; the flow rate was 0.40 mL/min and the column temperature was 35 °C. The other eluent was 2195
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Environmental Science & Technology composed of acetonitrile and water. The composition of acetonitrile/water (v/v) changed linearly from 5/95 to 90/10 in the first 9 min, and then returned in 0.10 min to 5/95, which was held for 2.9 min for re-equilibration; the flow rate was 0.50 mL/min and the column temperature was 20 °C. The MS parameters for the UPLC/ESI-tqMS were set the same as those for the ESItqMS except for the following: source temperature 150 °C, desolvation temperature 400 °C, desolvation gas 800 L/h, mass resolution 15 for full scan and 13.5 (1.5-unit resolution) for PISs, multiple reaction monitoring (MRM) and product ion scans; dwell time 0.05 s for MRM. For a brominated molecular ion detected by the PIS, the UPLC/ESI-tqMS MRM mode was applied to confirm the retention time (RT) of the molecular ion; a product ion scan was conducted at the specific RT to gain fragment information of the molecular ion for proposing a structure; then, the corresponding standard compound was used to confirm or identify the proposed structure. Reactions of Newly Detected or Identified DBPs with Chlorine. As described later in Results and Discussion, one group of intermediate Br-DBPs in the chlorinated SRHA samples coeluted at RT around 2.2 min, corresponding to ion clusters m/ z 345/347/349/351, 301/303/305/307, and 257/259/261/263. These intermediate Br-DBPs in the 1-h chlorinated SRHA sample were isolated by collecting the UPLC fraction corresponding to their RTs of 1.9-2.3 min. For each UPLC run, 6 μL of the 1-h chlorinated SRHA sample was injected and 0.16 mL of the fraction was collected; after 50 UPLC runs, 8.0 mL of the fraction was collected. Three 2-mL aliquots of the collected fraction were diluted with ultrapure water to 100-mL solutions, which were allowed to react with different doses of chlorine (pH 7.5-8.5 after chlorine addition). The reaction conditions and pretreatment procedures are detailed in the SI. Also as described later in Results and Discussion, 2,4,6tribromophenol and 3,5-dibromo-4-hydroxybenzoic acid were two newly confirmed/identified Br-DBPs in the chlorinated SRHA samples. The standard compounds of the two Br-DBPs were purchased and further purified with the UPLC. The purified standard compounds were then allowed to react with chlorine (pH 7-8 after chlorine addition) without spiked bromide unless otherwise stated. Complete information on the purification and concentration of the standard compounds and on the reactions of the purified standard compounds with chlorine can be found in the SI. Other Analytical Procedures. THMs were extracted and analyzed according to USEPA Method 551.121 as described in the SI. Total organic chlorine (TOCl) and TOBr were determined using a precombustion station (AQF-100, Mitsubishi) with an online ion chromatography system (ICS-90, Dionex).20,22
’ RESULTS AND DISCUSSION Characterization of the UPLC Separation of Polar Bromine-Containing Compounds. The separation of 21 polar
bromine-containing standard compounds with the UPLC was evaluated with two gradient eluents. The methanol/water gradient eluent was found to give better separation for most of the polar bromine-containing compounds than the acetonitrile/ water gradient eluent. Furthermore, more molecular ion clusters of polar DBPs in the chlorinated SRHA samples were detected with the methanol/water gradient eluent. Accordingly, unless otherwise stated, the RTs of standard compounds and
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compounds in the chlorinated SRHA samples were obtained with the methanol/water gradient eluent. Table S1 in the SI lists the RTs of 21 polar bromine-containing standard compounds under the UPLC setting. It was found that the RTs of these bromine-containing compounds had a direct relationship with their structures. Generally, the RTs followed the trend bromine-containing aliphatic acids < bromine-containing benzoic acids < bromine-containing phenols. The whole RT range can be roughly divided into three domains: 0-2.5 min for bromine-containing aliphatic acids, 2.5-6.0 min for brominecontaining benzoic acids, and 6.0-8.0 min for bromine-containing phenols. When a polar bromine-containing compound contained more than one polar functional group, its RT could become shorter and move to an adjacent domain. The relationship of RT to structure was also manifested by that of RT to pKa, i.e., generally the higher the pKa value, the longer the RT. The specific RT domain for a specific type of bromine-containing compounds provided an important reference in proposing structures for newly detected Br-DBPs. Identification of New Polar Br-DBPs in Chlorinated SRHA Samples. The 1-h and 5-d chlorinated SRHA samples represented the samples with high levels of relatively high and low molecular weight (MW) Br-DBPs, respectively. By combining the pairs of ion clusters in the ESI-tqMS PIS spectra of m/z 79 and 81 (SI Figure S1a-b), many polar Br-DBPs were selectively detected in the 1-h chlorinated SRHA sample (Table 1), including ion clusters m/z 363/365/367/369, 345/347/349/351, 215/ 217/219, 202/204, 193/195, 171/173/175, 159/161, 141/143, and 137/139, and ion clusters centered at m/z 321/323, 303/ 305, 297/299, 275/277, 251/253, and 231/233. Ion clusters m/z 149/151, 161/163/165/167, and 205/207/209/211 were significant in the ESI-tqMS PIS spectra of m/z 79 and 81 of the 5-d chlorinated SRHA sample (SI Figure S1c-d). Ion clusters m/z 215/217/219, 193/195, 171/173/175, and 137/139 should correspond to dibromoacetic acid, 2-bromobutenedioic acid, bromochloroacetic acid, and bromoacetic acid, respectively.13 The other ions or clusters were analyzed by the ESI-tqMS product ion scans to obtain structural information. As illustrated in the SI, ion clusters m/z 363/365/367/369, 205/207/209/ 211, 161/163/165/167, and 149/151 in the 1-h or 5-d chlorinated SRHA sample were proposed as tribromopentenedioic acid, dibromochloroacetic acid, bromodichloroacetic acid, and bromopropenoic acid, respectively (SI Figure S2). Ion clusters m/z 303/305, 287/289, 279/281, 259/261, 243/245, and 233/ 235 were not intense in the ESI-tqMS PISs of m/z 79 and 81, but the multiple losses of 44 (CO2) in their product ion scan spectra revealed that they could be highly polar Br-DBPs with poly carboxylic groups (SI Figure S3). Ion clusters m/z 141/143, 159/ 161, and 202/204 might correspond to adducts [Br- þ H2CO3], [Br- þ H2CO3 þ H2O] and [Br- þ 3CH3CN], respectively. UPLC/ESI-tqMS full scan and product ion scans were also performed to identify unknown polar Br-DBPs (Table 1). Ion cluster m/z 345/347/349/351 was prominent in the chlorinated SRHA samples. According to its isotopic ratios in the ESI-tqMS PISs of m/z 79 and 81 (SI Figure S1) and its fragmentation pattern in the UPLC/ESI-tqMS product ion scans (SI Figure S4), ion cluster m/z 345/347/349/351 was tentatively proposed as C6H5O2Br3 (or C5HO3Br3). Ion clusters m/z 257/259/261/ 263 and 301/303/305/307, which almost coeluted with ion cluster m/z 345/347/349/351 in the UPLC and had a fragmentation pattern similar to ion cluster m/z 345/347/349/351 (SI Figure S5), were proposed as C6H5O2BrCl2 (or C5HO3BrCl2) 2196
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Table 1. Ion Clusters of Polar Br-DBPs in the Chlorinated SRHA Samples m/z
formula or structure
occurrencea
RT (min)
127/129/131
dichloroacetic acid (confirmed)
0.79
a, b, c, d, e, f, g, h
137/139
bromoacetic acid (confirmed)
0.78
a, b, c, d, e, f, g, h
149/151
bromopropenoic acid (proposed)
--b
a, b, c, d, e, f, g, h
161/163/165/167
bromodichloroacetic acid (proposed)
--b
a, b, c, d, e, f, g, h
171/173/175
bromochloroacetic acid (confirmed)
0.80
a, b, c, d, e, f, g, h
191/193/195
containing 1Clþ1Br
2.28
b, c, d, e
193/195
2-bromobutenedioic acid (confirmed)
0.79
a, b, c, d, e, f, g, h
205/207/209/211 215/217/219
dibromochloroacetic acid (proposed) dibromoacetic acid (confirmed)
--b 0.83
a, b, c, d, e, f, g, h a, b, c, d, e, f, g, h
221/223/225
containing 1Clþ1Br
4.81
b
225/227/229
containing 2Br
3.34
b
227/229/231
3,3-dibromopropenoic acid (confirmed)
0.80
b, c, d, e, f, g, h
229/231/233
containing 2Br
2.11
g, h
235/237/239
containing 1Clþ1Br
2.50
b, c, d, e
257/259/261/263
C5HO3BrCl2 or C6H5O2BrCl2 (proposed)
2.00
a, b, c, d, e, f
265/267/269 265/267/269/271
2,6-dibromo-1,4-hydroquinone (confirmed) containing 1Clþ2Br
5.07 1.92
b, c, d b, c, d
269/271/273
containing 2Br
3.41
b, c, d, e, f
275/277/279/281
containing 2Clþ1Br
1.49
b, c, d, e
275/277/279/281
containing 3Br
3.77
b, c, d, e
277/279/281
3,5-dibromo-4-hydroxybenzaldehyde (proposed)
4.02
b, c, d, e, f, g, h
279/281/283
containing 2Br
2.18
b, c, d, e
279/281/283
containing 2Br
2.70
b, c, d
293/295/297 293/295/297/299
3,5-dibromo-4-hydroxybenzoic acid (confirmed) containing 3Br
3.08 0.70
b, c, d, e, f, g, h b, c, d, e
299/301/303/305
containing 1Clþ2Br
6.60
b, c
301/303/305/307
C5HO3Br2Cl or C6H5O2Br2Cl (proposed)
2.10
a, b, c, d, e, f, g, h
313/315/317/319
containing 3Br
3.61
b, c, d, e
319/321/323/325
containing 1Clþ2Br
1.49
a, b, c, d, e
325/327/329/331
containing 3Br
5.48
b, c, d, e
327/329/331/333
2,4,6-tribromophenol (confirmed)
7.77
b, c, d, e, f, g, h
333/335/337/339 343/345/347/349
containing 3Br containing 3Br
3.12 6.60
b, c, d, e, f b, c
345/347/349/351
C5HO3Br3 or C6H5O2Br3 (proposed)
2.20
a, b, c, d, e, f, g, h
345/347/349/351
containing 3Br
1.67
b, c, d, e
345/347/349/351
containing 3Br
5.26
b, c, d, e, f
363/365/367/369
tribromopentenedioic acid (proposed)
1.49
a, b, c, d, e, f
369/371/373/375/377
containing 4Br
5.81
b, c, d, e, f
391/393/395/397
containing 3Br
2.35
b, c, d, e
a
a, b, c, d, e, f, g, and h indicate the chlorinated SRHA samples with chlorine contact times of 1 min, 1 h, 3 h, 9 h, 12 h, 1 d, 3 d, and 5 d, respectively. b Observed in direct infusion ESI-tqMS only.
and C6H5O2Br2Cl (or C5HO3Br2Cl), respectively. Complete information for proposing the formulas is presented in the SI. Another ion cluster, m/z 277/279/281 at RT 4.02 min, was tentatively proposed as 3,5-dibromo-4-hydroxybenzaldehyde (SI Figure S6). Structures of ion clusters m/z 293/295/297, 227/229/231, 327/329/331/333, and 265/267/269 were proposed and confirmed by authentic standard compounds. Figure 1 shows the UPLC/ESI-tqMS MRM chromatograms of a 3,5-dibromo-4hydroxybenzoic acid standard solution, the 1-h chlorinated SRHA sample spiked with 3,5-dibromo-4-hydroxybenzoic acid, and the 1-h chlorinated SRHA sample, as well as the product ion scan spectra of ion cluster m/z 293/295/297 of the 1-h
chlorinated SRHA sample and the 3,5-dibromo-4-hydroxybenzoic acid standard solution. The same RTs and identical product ion scan spectra confirmed that ion cluster m/z 293/295/297 at RT 2.78 min was 3,5-dibromo-4-hydroxybenzoic acid, the concentration of which in the 1-h chlorinated SRHA sample was 0.35 μg/L (obtained by spiking different levels of the standard compound in the sample). Ion cluster m/z 293/295/297 at 3.69 min could be an isomer of 3,5-dibromo-4-hydroxybenzoic acid. It is of note that the acetonitrile/water gradient eluent was used in the identification of ion cluster m/z 293/295/297 because it gave a narrower peak for this compound. To our knowledge, 3,5-dibromo-4-hydroxybenzoic acid was identified for the first time as a chlorination DBP. Similarly, by comparing 2197
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Figure 1. (a-c) UPLC/ESI-tqMS MRM (293f79, 295f79/81, 297f81) chromatograms of 3,5-dibromo-4-hydroxybenzoic acid, the 1-h chlorinated SRHA sample mixed with 3,5-dibromo-4-hydroxybenzoic acid, and the 1-h chlorinated SRHA sample, respectively; (d-f) UPLC/ESI-tqMS product ion scan spectra of m/z 293, 295, and 297 of the 1-h chlorinated SRHA sample, respectively; (g) UPLC/ESI-tqMS product ion scan spectrum of m/z 295 of 3,5-dibromo-4hydroxybenzoic acid.
with corresponding standard compounds, ion clusters m/z 227/ 229/231 at RT 0.80 min and 327/329/331/333 at RT 7.77 min were confirmed to be 3,3-dibromopropenoic acid and 2,4,6tribromophenol (SI Figure S7), the concentrations of which in the 1-h chlorinated SRHA sample were 0.15 and 0.45 μg/L, respectively. Ion cluster m/z 265/267/269 at RT 5.07 min were confirmed to be 2,6-dibromo-1,4-hydroquinone by comparing with the product, 2,6-dibromo-1,4-hydroquinone, which was generated from chlorination of 1,4-benzoquinone with the presence of bromide. Such a confirmation is detailed in the SI (Figure S8). The three compounds have been reported to be drinking water DBPs.9,23,24 In this study, 2,4,6-tribromophenol and 3,5-dibromo-4-hydroxybenzoic acid contributed, respectively, 0.04% and 0.02% to the TOBr (836 μg/L as Br in the 1-h chlorinated SRHA sample). Since 2,4,6-tribromophenol and 3,5-dibromo-4-hydroxybenzoic acid were just two of 17 polar aromatic Br-DBPs with RTs in the range of 2.5-8.0 min, and the unidentified polar aromatic Br-DBPs had intensities similar to 2,4,6-tribromophenol and
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3,5-dibromo-4-hydroxybenzoic acid, all the polar aromatic BrDBPs might contribute ∼0.5% to the TOBr in the 1-h chlorinated SRHA sample. It has been reported that a high percentage of the total organic halogen is moderate and/or high in MW (e.g., >1000 Da),25 but the concentration of each of those DBP species may be one or more magnitudes lower. Formation and Decomposition of Entire Polar Br-DBPs in Chlorinated SRHA Samples. With the new PIS approach, “snapshots” of entire polar Br-DBPs in a water sample basically can be taken at any moment. Figure 2a-h show the ESI-tqMS PIS spectra of m/z 79 of the chlorinated SRHA samples with different chlorine contact times. The ion clusters in the low m/z range (e.g., m/z 171/173 for bromochloroacetic acid, 193 for 2-bromobutenedioic acid, and 215/217 for dibromoacetic acid) formed quickly and kept increasing in intensity until a contact time of 5 d. The ion clusters in the relatively high m/z range increased within short contact times and then gradually decreased in intensity with increasing contact time. Meanwhile, the chlorine equivalent residual decreased rapidly from 1 min (3.25 mg/L as Cl2) to 9 h (0.05 mg/L as Cl2) and was
