A Picture of Polar Iodinated Disinfection Byproducts in Drinking Water

Nov 16, 2009 - European rivers are in the range of 0.5-212 μg/L (1). The two predominant ... meaningful to have a “whole” picture of iodinated DB...
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Environ. Sci. Technol. 2009 43, 9287–9293

A Picture of Polar Iodinated Disinfection Byproducts in Drinking Water by (UPLC/)ESI-tqMS GUOYU DING AND XIANGRU ZHANG* Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong SAR, China

Received June 20, 2009. Revised manuscript received October 23, 2009. Accepted October 30, 2009.

Iodinated disinfection byproducts (DBPs) are generally more toxic than their chlorinated and brominated analogues. Up to date, only a few iodinated DBPs in drinking water have been identified by gas chromatography/mass spectrometry. In this work, a method for fast selective detection of polar iodinated DBPs was developed using an electrospray ionization-triple quadrupole mass spectrometer (ESI-tqMS) by conducting precursor ion scan of iodide at m/z 126.9. With such a method, pictures of polar iodinated DBPs in chlorinated, chloraminated, and chlorineammonia treated water samples were achieved. By coupling stateof-the-art ultra performance liquid chromatography (UPLC) to the ESI-tqMS, structures of 17 iodinated DBPs were tentatively proposed. The results fully demonstrate that, with respect to the DBP number/levels among the three disinfection processes, chloramination generally generated the most/highest iodinated DBPs, chlorination generally produced the fewest/lowest iodinated DBPs, and chlorine-ammonia sequential treatment formed iodinated DBPs lying in between; the numbers of iodinated DBPs in chloraminated Suwannee River Fulvic Acid (SRFA) and Humic Acid (SRHA) were nearly the same, but the levels of aliphatic iodinated DBPs were higher in the chloraminated SRFA while the levels of aromatic iodinated DBPs were higher in the chloraminated SRHA; a couple of nitrogenous iodinated DBPs were found in chloramination and chlorine-ammonia treatment. The ratio of total organic iodine levels in chlorineammonia sequential treatment and chloramination could be expressed as a function of the lag time of ammonia addition.

Introduction The iodine concentrations in major U.S., Canadian, and European rivers are in the range of 0.5-212 µg/L (1). The two predominant forms of iodine in freshwaters are iodide and iodate. The oxidation of iodide by the disinfectants chloramines or chlorine leads to hypoiodous acid, which can react with natural organic matter (NOM) to form iodinated disinfection byproducts (DBPs) (2). Among iodinated trihalomethanes, iodoform is the most undesired due to its unpleasant medicinal taste and odor in drinking waters (3). Recently, Krasner et al. (4) and Plewa et al. (5) reported the discovery of several iodinated haloacids in finished drinking water that used chloramines as disinfectants, including iodoacetic acid, bromoiodoacetic acid, 3-bromo-3-iodopropenoic acid, and 2-iodo-3-methylbutenedioic acid. Bacterial * Corresponding author phone: +852-2358-8479; fax: +852-23581534; E-mail: [email protected]. 10.1021/es901821a CCC: $40.75

Published on Web 11/16/2009

 2009 American Chemical Society

studies have shown that iodoacetic acid is 2.6 and 523.3 times more mutagenic in Salmonella typhimurium strain TA100 than bromoacetic acid and chloroacetic acid, respectively. Mammalian cell studies have shown that iodoacetic acid is 3.2 and 287.5 times more cytotoxic in Chinese hamster ovary cells than bromoacetic acid and chloroacetic acid, respectively; and iodoacetic acid is 2.0 and 47.2 times more genotoxic in Chinese hamster ovary cells than bromoacetic acid and chloroacetic acid, respectively (5). Richardson et al. (6) reported that diiodoacetic acid is approximately 2 and 34 times more cytotoxic in Chinese hamster ovary cells than dibromoacetic acid and dichloroacetic acid, respectively; with the exception of iodoform, iodo-trihalomethanes are more cytotoxic than the regulated trihalomethanes, but they are much less cytotoxic than iodo-acids. All these facts seem to suggest that iodinated DBPs, especially polar ones, warrant more attention. As an emerging and novel research area, drinking water iodinated DBPs have been reported occasionally in recent years (4-7). Up to date, however, only a few iodinated DBPs have been discovered by gas chromatography/mass spectrometry (GC/MS) (4). The limitation of GC/MS may have restrained the research in this area. Many iodinated DBPs would be expected to be highly polar and hard to detect by GC/MS. Thus, liquid chromatography/MS or liquid direct infusion-MS such as electrospray ionization (ESI)-MS would be a logical choice for detecting polar/highly polar iodinated DBPs. Because chlorinated humic substances is an extremely complex mixture that may contain thousands of major components (8), how to differentiate iodine-containing DBPs from iodine-free components in a complex water sample is a challenging issue. Zhang et al. (8, 9) reported methods for fast selective detection of polar chlorinated and brominated DBPs in drinking water using an ESI-triple quadrupole mass spectrometer (ESI-tqMS). The mass spectrometer permits the use of precursor ion scan (PIS) mode, which can detect the entire precursor ions that generate a specific fragment ion, e.g., chlorine-containing and bromine-containing DBPs can be selectively detected by performing PISs of Cl- (m/z 35) and Br- (m/z 79 or 81), respectively. Likewise, it is anticipated that a similar method based on the ESI-tqMS PIS of I- (m/z 126.9) could be used for fast selective detection of iodinated DBPs. During disinfection with chemical oxidants, naturally occurring iodide can be oxidized to HOI which can be further oxidized to IO3- or react with NOM to form iodinated DBPs. Bichsel and von Gunten (2) successfully determined the kinetics of HOI oxidation reactions, and predicted that the “probability” of the formation of iodinated organic DBPs during drinking water disinfection increases in the order O3 < Cl2 < NH2Cl, and the formation of iodinated organic DBPs in ozonation is rather unlikely. Later, Krasner et al. (4) reported four iodoacids in finished drinking water from a plant that used chloramines only, and suggested that it should be possible to limit the formation of iodinated DBPs through a sufficient free-chlorine contact time before the addition of ammonia to form chloramines. Hua et al. (7) measured the total organic iodine (TOI) during chlorination. Because only a few iodinated DBPs are known, and known iodinated DBPs account for only a small fraction of TOI, it would be meaningful to have a “whole” picture of iodinated DBPs so that the formation of iodinated DBPs in different disinfection processes could be fully characterized and compared. The objectives of this work were to develop a method for fast selective detection of polar iodine-containing compounds using the ESI-tqMS PIS of m/z 126.9; to apply the method VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. (a) Structures, formulas and m/z values of standard compounds in the standard solution. Spectra of the ESI-tqMS PIS m/z 126.9 of the standard solution at different cone voltages (b) 10 V, (c) 20 V, (d) 30 V, and (e) 40 V. All the spectra were normalized to the same y-axis scale. for selective detection of polar iodinated DBPs in simulated drinking water, with/without the aid of ultra performance liquid chromatography (UPLC); and further, to investigate the formation of iodinated DBPs with different disinfectants and carbon sources. Theoretical Isotopic Abundances for PIS of I-. The natural isotopic abundances of bromine and chlorine provide essential information in predicting the numbers of bromine and chlorine atoms in a compound. The theoretical abundance for PISs m/z 79 and 81 has been reported (9). The theoretical isotopic abundance for PIS m/z 126.9 was developed in this work. For a compound containing m bromine atoms, n chlorine atoms and p iodine atoms (BrmClnIp), it should have a total of m+n+1 isotopic peaks in PIS m/z 126.9. The abundance of the Nth isotopic peak in PIS m/z 126.9 (AN,pre126.9) can be described as follows: min(N-1,m)

AN,pre126.9 )

∑

k N-k-1 -(N-k-1) Cm Cn 3

(1)

k)max(N-n-1,0)

where m is the number of bromine atoms in the compound, and n is the number of chlorine atoms in the compound; k is the number of 81Br in the Nth isotopic peak, N - k - 1 is the number of 37Cl in the Nth isotopic peak, and k is any integer between max (N - n - 1, 0) and min(N - 1, m); Ckm and CnN - k - 1 are the binomial coefficients. Equation 1 is coincidently the same as the one for calculating the isotopic abundance in full scan; p is not involved in the equation because iodine has only one natural isotope. By comparing the isotopic abundance in PIS m/z 126.9 and the theoretical one, the numbers of chlorine and bromine atoms in a compound can be determined. 9288

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Materials and Methods Materials. A standard solution was composed of 1 mg/L each of six iodine-containing standard compounds in 1:1 water/ acetonitrile. The standard compounds (Figure 1a) included iodoacetic acid (TCI America), 3,5-diiodosalicylic acid, 3,5diiodo-L-tyrosine and 2,4,6-triiodophenol (ChemService), and (E)-3-(5-iodo-2-furyl)prop-2-enoic acid and 3-iodophthalic acid (Maybridge). These standard compounds were chosen because most polar iodinated DBPs in drinking water are believed to be iodinated acids or phenols, which could obtain their carboxyl or phenol groups by transfer from humic substances or by oxidation of humic substances during disinfection. Ultrapure water (18.2 MΩ/cm) was supplied by a NANOpure Diamond purifier system (Barnstead). Suwannee River Fulvic Acid (SRFA, 2S101F) and Humic Acid (SRHA, 2S101H) were purchased from the International Humic Substances Society (IHSS). The standard solution was diluted and mixed with an SRFA stock solution to generate a test solution (for verifying the method) that contained 0.10 mg/L each of the six iodine-containing standard compounds and 3 mg/L SRFA as C. Simulated Drinking Water Sample Preparation. Simulated water samples were prepared by dissolving 3 mg/L SRFA or SRHA as C, 90 mg/L NaHCO3 as CaCO3 and 200 µg/L (1.58 µM) potassium iodide as I-. The relatively high iodide level was used to amplify iodinated DBPs so that unknown polar iodinated DBPs can be selectively detected and identified by the (UPLC/)ESI-tqMS PIS method. The rationale for using this iodide level is detailed in the Supporting Information (SI). Hua et al. (7) have used 10-30 µM of iodide in their study for better observation of iodinated DBPs. Bromide was not added so as to simplify the reaction mixture. To compare the formation of iodinated DBPs in disinfected simulated drinking waters, four sets of samples were prepared as follows.

The simulated water sample with SRFA was added with 5.0 mg/L sodium hypochlorite as Cl2 (coded as “SRFA+Cl2”). The simulated water sample with SRFA or SRHA was added with 5.0 mg/L monochloramine as Cl2 (coded as “SRFA+ NH2Cl” or “SRHA+NH2Cl”, respectively). Monochloramine was prepared just before use by reacting ammonium chloride and sodium hypochlorite solutions in a chlorine-to-ammonia ratio of 0.8 mol/mol. The simulated water sample with SRFA was added with 5 mg/L sodium hypochlorite as Cl2, followed 65 s later by the addition of 4.71 mg/L ammonium chloride (coded as “SRFA+Cl2+NH4+”). SRFA was chosen to compare the formation of iodinated DBPs from different disinfection processes, whereas SRHA and SRFA were used to compare the formation of iodinated DBPs from different carbon sources. All the samples were kept in darkness at ambient temperature for 5 days and then quenched with stoichiometric amounts of 0.1 M NaAsO2 according to the measured chlorine residues. During the 5 day contact time, the pH values of all the samples ranged from 8.7 to 8.2. Sample Pretreatment. The simulated drinking water samples were pretreated with the same procedure as for polar brominated DBPs (9). Briefly, 2 L of each sample was first acidified to pH e 0.5 with sulfuric acid. After saturated with sodium sulfate, the water sample was extracted with 200 mL methyl tert-butyl ether. The organic layer was transferred to a rotary evaporator to concentrate to 1 mL. The 1 mL solution in methyl tert-butyl ether was mixed with 10 mL acetonitrile, and the mixture was sparged back to 1 mL with ultra high purity nitrogen gas. The 1 mL solution in acetonitrile was stored at 4 °C and diluted with 1 mL ultrapure water just before MS analysis. To determine whether there were any impurities in the solvents or any artifacts during the extraction and concentration, control samples were generated by repeating the same procedure with the simulated water samples without disinfection. ESI-tqMS Analysis. Each pretreated sample was analyzed with a Waters Acquity ESI-tqMS (Waters), whose parameters were optimized with the standard solution. By setting PIS m/z 126.9, all the electrospray-ionizable iodine-containing compounds were expected to be detected. The parameters of the instrument were set as follows: sample flow rate via an infusion pump 10 µL/min, ESI negative mode, source temperature 120 °C, desolvation temperature 350 °C, desolvation gas flow 650 L/h, cone gas flow 50 L/h, collision gas (Argon) 0.25 mL/min, LM resolution 15, and HM resolution 15. For all the PISs, the data collection mode was multichannel analysis, which can greatly enhance precursor ion intensities by accumulating multiple scans and eliminate possible ion intensity fluctuation in a single scan. UPLC/ESI-tqMS Analysis. A Waters UPLC system was coupled to the Waters Acquity ESI-tqMS (UPLC/ESI-tqMS, Waters). Five µL of a pretreated sample was injected in the UPLC. The chromatographic separation was achieved by an HSS T3 column (2.1 × 50 mm, 1.8 µm particle size, Waters). A gradient eluent of acetonitrile/water (v/v) from 10/90 to 90/10 in the first 12 min followed by an isocratic eluent of 10/90 acetonitrile/water for 3 min was applied at a flow rate of 0.60 mL/min. The ESI-tqMS PIS of m/z 126.9 was conducted to detect iodine-containing compounds. The parameters for the UPLC PIS were set the same as those for the direct infusion PIS except that higher desolvation temperature (400 °C) and desolvation gas flow (800 L/h) were used. For an iodinated molecular ion detected by the PIS, the UPLC/ESI-tqMS selected ion recording (SIR) mode was applied to confirm the retention time (RT) of the molecular ion; then, product ion scans were conducted at the specific RT to gain fragment information of the molecular ion. TOI Measurement. A 50 mL aliquot of each simulated drinking water sample was adjusted to pH 2 with concentrated nitric acid, and then passed through two consecutive

FIGURE 2. Optimum collision energies in the ESI-tqMS PIS m/z 126.9 for the standard compounds in the standard solution. activated carbon microcolumns (Mitsubishi). After adsorption, the carbon microcolumns were rinsed with 5 mL of 5000 mg/L KNO3 as NO3- to remove iodide, and subsequently subjected to pyrolysis at 1000 °C with an AQF-100 automatic quick furnace (Mitsubishi). The gases from the pyrolysis were absorbed in 5 mL ultrapure water (10). The iodide concentration in the absorption solution was analyzed with the ESItqMS (Waters) using the SIR mode by selecting I- at m/z 126.9. Potassium iodide at different levels was spiked to the absorption solution to correct possible matrix effects. The detection limit for TOI was 0.57 µg/L as I. For each simulated drinking water sample, triplicate aliquots were analyzed to obtain an average TOI concentration.

Results and Discussion Method Development. Key parameters that affect the ESItqMS PIS of m/z 126.9 include cone voltage, collision energy and capillary voltage. They were examined in detail with the standard solution. According to previous studies of chlorinated and brominated DBPs, it was an essential step to generate true molecular ions with relatively high intensities in the ESI source (8, 9). SI Figure S1 shows the molecular ion intensities in PIS m/z 126.9 at different cone voltages. The intensity of each molecular ion increased with cone voltage until a maximum, and then decreased with increasing cone voltage. Notably, when cone voltage exceeded a certain level, some standard compounds decomposed at the sample cone (Figure 1): e.g., when cone voltage increased to 30 V, C7H3I2O3- (m/z 389) and C8H4IO4- (m/z 291) began to lose one carboxylic group to form C6H3I2O- (m/z 345) and C7H4IO2- (m/z 247), respectively. Accordingly, to maintain relatively high molecular ion intensities and to avoid the decomposition of iodine-containing molecular ions at the sample cone, cone voltages between 15 and 20 V should be selected. It has been reported that the molecular ion intensities of brominated compounds in PISs m/z 79 and 81 could be maximized at lower collision energies than those of chlorinated compounds in PISs m/z 35 and 37 (8, 9). Since the bond strength of CsI is lower than those of CsBr and CsCl, and the bond length of CsI is longer than those of CsBr and CsCl (11), the optimum collision energy, at which the maximum molecular ion intensity occurs in the PIS, should be lower for iodinated compounds, e.g., the optimum collision energy for ICH2COO- was lower than those of BrCH2COO- and ClCH2COO-. Of great interest was the finding that, as shown in Figure 2 and SI Figure S2, the optimum collision energies for the iodinated compounds increased almost proportionally with their molecular weights VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Spectra of the ESI-tqMS PIS m/z 126.9 under collision energy 40 eV for the simulated drinking water samples (a) “SRFA+Cl2”, (b) “SRFA+NH2Cl”, (c) “SRHA+NH2Cl”, and (d) “SRFA+Cl2+NH4+”; all the spectra were normalized to the same y-axis scale. (e) Proposed structures; “*” means “or its isomers”. (MWs). Consequently, there are two practicable ways in choosing collision energy for iodinated compounds: to obtain certain intensities for all compounds, a moderate collision energy around 30 eV should be used; alternatively, a low collision energy around 20 eV is chosen for low MW compounds, and a high collision energy around 40 eV is chosen for relatively high MW compounds. Although high collision energies over 40 eV were tested, high molecular weight DBPs with m/z > 500 Da were quite low in intensities and thus were not included. It is of note that the ion with m/z 126.9 might also correspond to Cl2CHCOO- (a common DBP in chlorinated drinking water), but it should not affect the selective detection of iodinated DBPs in PIS m/z 126.9. According to a previous study, Cl2CHCOO- will break down to Cl2CH- and Cl- as collision energy exceeds 15 eV (9), and thus any possible Cl2CHCOO--containing molecular ions should not be detected by PIS m/z 126.9 at the chosen collision energies (g20 eV). If bromide is present in raw water, BrClCHCOOH could be generated during chlorination or chloramination. This DBP may be dissociated to form BrClCH- (m/z 126.9) in the collision chamber, and thus be detected by PIS m/z 126.9 at collision energy 20 eV, but the spike from BrClCH- can be readily eliminated by performing PIS m/z 126.9 at higher collision energy like 40 eV or differentiated by performing PIS m/z 128.9, or PISs m/z 79 and 81. Additional information can be found in SI Figure S3. SI Figure S4 shows the variation of molecular ion intensities in PIS m/z 126.9 with capillary voltage. With the increase of capillary voltage from 2.6 to 3.2 kV, the ion intensity of m/z 291 increased slightly, and the ion intensities of other standard compounds increased to a maximum and then decreased with capillary voltage. Thus, capillary voltages between 2.8 and 3.0 kV should be appropriate. 9290

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Under cone voltage 20 V, capillary voltage 3.0 kV and collision energy 20 eV, the test solution that contained 0.10 mg/L each of the six iodine-containing standard compounds and 3 mg/L SRFA as C was analyzed by PIS m/z 126.9. The spectrum in SI Figure S5 demonstrates that all the iodinecontaining compounds in the test solution were detected by this method. Therefore, the appropriate instrument conditions were as follows: cone voltage 20 V, capillary voltage 3.0 kV, and collision energies 20 eV (for low MW compounds) and 40 eV (for relatively high MW compounds). Water Analysis. SI Figure S6 and Figure 3 display the spectra of PIS m/z 126.9 of the simulated drinking water samples under collision energies 20 and 40 eV, respectively. These spectra disclosed for the first time the pictures of iodinated DBPs, which contained abundant information on iodinated DBPs from various disinfection processes and carbon sources. Some important points extracted from the pictures are described as follows. First of all, numerous new iodine-containing DBPs were detected in the simulated drinking water samples. All these iodinated DBP peaks should be real because they were reproducible and had significantly higher intensities (1 × 105-1.7 × 106) than those in control samples (1 × 103-1 × 104). Additional information can be found in the SI Figure S7. The major iodine-containing molecular ions in the spectra of PIS m/z 126.9 included m/z 171, 175/177, 185, 197, 199, 207/209, 215, 219/221, 225, 241, 243, 247, 251, 255, 257, 259, 263/265, 267, 267/269, 271, 285, 291, 293, 311, 317, 319, 321, 323, 335, 351, 367, 373, 381, 389, 390, 391, 393, 403, 405, 417, 419, 421, 433, 435, 447, 449, 461, 463, 473, 475,487, 489, 491, 499, etc. Except for a couple of known molecular ions (such as m/z 185 and 219/221 corresponded to iodoacetic acid and chloroiodoacetic acid, respectively), all the other ions

FIGURE 4. (a) Chromatogram of the UPLC/ESI-tqMS PIS m/z 126.9 of Sample “SRHA+NH2Cl”. Spectra of the UPLC/ESI-tqMS PIS m/z 126.9 of Sample “SRFA+NH2Cl” at different RTs (b) 0.66 min, (c) 0.84 min, (d) 3.36 min, and (e) 4.33 min. might be new iodinated DBPs that have not been reported previously.TheseiodinatedDBPshavelongeludedresearchers. With the aid of UPLC, the structures of many new iodinated DBPs were tentatively proposed. Due to the complex nature of chlorinated/chloraminated humic substances, one peak in the spectra of PIS m/z 126.9 might possibly correspond to two or more isobaric iodinated compound homologues. To avoid this kind of overlapping, UPLC was coupled with the ESI-tqMS by setting PIS m/z 126.9 for detection. Figure 4a shows the chromatogram of the “SRFA+NH2Cl” sample that was analyzed with the UPLC/ ESI-tqMS by setting PIS m/z 126.9. Each peak in the chromatogram should correspond to one (or more) iodinated DBP. After integration of the chromatogram at different RTs, the corresponding spectra were obtained as exemplified in Figure 4b-d, which shows that the sample was separated to a great extent. At RT 0.66 min, there were mainly ions m/z 219/221 and 241. At RTs 0.84 and 3.36 min, there were ions m/z 311 and 373, respectively. At RT 4.33 min, ion m/z 389 was dominant. Then the UPLC/ESI-tqMS SIR mode by selecting a specific molecular ion was applied to confirm the RT of the molecular ion so that product ion scans could be conducted at the specific RT. SI Figure S8 shows the UPLC/ ESI-tqMS SIR (m/z 219) chromatogram and product ion scan spectrum of ion m/z 219 at RT 0.66 min. The chromatogram confirmed that ion m/z 219 was retained mainly at 0.66 min. The product ions of m/z 219 contained m/z 175 (loss of CO2 from the molecular ion), m/z 140 (loss of 35Cl from 175), and m/z 127 (I-). Based on the same scheme in interpreting product ion scan spectra of chlorinated or brominated analogues (9), the structure of this molecular ion should be ClIHCCOOH, which has been identified as a DBP during cooking with iodized table salt (12). SI Figure S9a shows the

SIR chromatogram of m/z 241, appearing at RT 0.66 min. SI Figure S9b shows the product ion scan spectrum of m/z 241 at RT 0.66 min, which contained product ions m/z 197 (loss of CO2 from the molecular ion), m/z 153 (loss of CO2 from m/z 197), and m/z 127 (I-). The only reasonable structures should be two isomers (Z)-iodobutenedioic acid and (E)iodobutenedioic acid. The identification of iodinated molecular ions with m/z 373, 389, and 311 is shown in SI Figures S10-S12. Similarly, for a number of other iodinated molecular ions with relatively high intensities, their structures were tentatively proposed as shown in Figure 3e. All these may be new iodinated DBPs that have not been reported yet. Second, the number and levels of iodinated DBPs in the chloraminated SRFA sample were generally significantly greater than those in the chlorinated SRFA sample, and the number and levels of iodinated DBPs in the chlorineammonia treated sample lied in between. Different iodinated compounds may have different ESI efficiencies, but for the same iodinated compound, the ESI efficiency should be approximately a constant under the same instrument settings, and thus the intensity of an iodinated molecular ion in PIS m/z 126.9 should be a good indicator for the level of the corresponding iodinated compound in a sample. The differences among iodinated DBPs in the three samples are manifested in Figure 3a, b, and d. The intensities of iodinated molecular ions were generally greater in the chloraminated SRFA sample than in the chlorinated SRFA sample, but there were exceptions: e.g., the intensities of ions m/z 171, 199, 207/209, 215, 243, 251, 255, 257, 259, 287, 295, 297, and 341 were lower in the chloraminated SRFA sample than in the chlorinated SRFA sample (SI Figure S6a and b). Ions m/z 171, 215, and 259 might correspond to ICOO- (iodoformic acid), ICOO-+CO2 (adduct) and ICOO-+2CO2 (adduct), VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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respectively; ion cluster m/z 207/209 might correspond to ICOO-+HCl (adduct). Third, the number of iodinated DBPs in the chloraminated SRFA and SRHA samples were nearly the same, but the levels of iodinated DBPs in the two samples were somewhat different. The iodinated DBPs in the two samples are compared as shown in Figure 3b and c. For most iodinated molecular ions (including ions m/z 197, 241, 267, 311, 323, 335, 351, 379, 381, 393, 405, 433, 435, 449, 461, 473, 475, 487, 489, 491, etc), their intensities were greater in the chloraminated SRFA sample than in the chloraminated SRHA sample. These iodinated DBPs generally appeared in the UPLC chromatograms with short RTs, and seemed to consist of mainly aliphatic compounds, e.g., m/z 197, 241, 255, 267, 311, 323, and 351 might correspond to ICHdCHCOO-, HOOCCIdCHCOO-, HOOCCIdC(CH3)COO-, I2CH-, I2CHCOO-, I2C ) CHCOO-, and CH2dC(CH2I)CHICOO-, respectively. For some iodinated molecular ions (including ions m/z 175/177, 219/221, 263/265, 267, 269, 305, 307, 317, 325, 367, 391, etc), their intensities were almost the same in the chloraminated SRFA sample as in the chloraminated SRHA sample. For some iodinated molecular ions (including ions m/z 237, 247, 271, 291, 294, 373, 389, 417, etc), their intensities were lower in the chloraminated SRFA sample than in the chloraminated SRHA sample. These iodinated DBPs appeared in the UPLC chromatograms with relatively long RTs, and seemed to consist of mainly aromatic compounds, e.g., m/z 247, 291, 373, 389, and 417 might correspond to 4-iodobenzoic acid, 3-iodophthalic acid, 2,4-diiodobenzoic acid, 5,6-diiodosalicylic acid, and 5,6-diiodo-3-ethylsalicylic acid, respectively. The presence of higher levels of aromatic iodinated DBPs in the chloraminated SRHA may be ascribed to the higher aromatic content in SRHA. According to the IHSS (13), the aromatic contents in SRHA and SRFA are 31% and 22%, respectively. Some aromatic structures in SRHA or SRFA may pass on to those iodinated DBPs during chloramination. Fourth, only a couple of nitrogenous iodinated DBPs were detected in the chloraminated and chlorine-ammonia treated samples. The nitrogenous molecular ions (mainly including ions m/z 294 and 390), which are assumed to be singly charged, are characterized by their even-numbered m/z values; their structures have yet to be proposed due to weak product ion scan spectra. Nitrogenous DBPs are generally believed to be more toxic than carbon-based DBP analogues (14). Fifth, the isotopic abundances in the spectra of PIS m/z 126.9 match the theoretical calculations. For instance, the ion clusters with m/z 219/221 was proved to be chloroiodoacetic acid by the UPLC/ESI-tqMS product ion scans. Since this compound contains one chlorine atom, according to eq 1, A1,pre126.9 and A2,pre126.9 can be calculated to be 1 and 1 /3, respectively, with a ratio of 3:1. This ratio matches what was observed in the spectra of PIS m/z 126.9 (Figure 3 and SI Figure S6). The ion clusters with m/z 175/177 and 263/265 corresponded to ClICH- and HOOCCClICOO-, respectively. They had isotopic abundance ratios of 3:1 in the spectra of PIS m/z 126.9 (Figure 3 and SI Figure S6), which also match the theoretical calculations. Last but not the least, low and high collision energies in PIS m/z 126.9 favored the detection of low and relatively high MW iodinated DBPs, respectively. The molecular ion intensities of low MW DBPs (with m/z < 260) in PIS m/z 126.9 were stronger under collision energy 20 eV (SI Figure S6). The molecular ion intensities of relatively high MW DBPs (with m/z > 260) in PIS m/z 126.9 were stronger under collision energy 40 eV (Figure 3). This is consistent with the observation from the standard compounds. Total ion intensity (TII), which is defined as the summation of all the ion intensities from m/z 128 to 500 in the spectrum of PIS m/z 126.9, may indicate the total polar iodinated DBPs 9292

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FIGURE 5. (a) TII levels of the simulated drinking water samples in the ESI-tqMS PIS m/z 126.9 under collision energies 20 eV (CE20) and 40 eV (CE40), and the ratios of the TII levels under the two collision energies. (b) TOI levels in the simulated drinking water samples. in a sample. Figure 5a shows the TII levels of the four samples. Under collision energy 20 eV, the ratio of the TII levels in samples “SRFA+Cl2”, “SRFA+NH2Cl”, “SRHA+NH2Cl”, and “SRFA+Cl2+NH4+” was 0.80:1.0:0.80:0.66. Under collision energy 40 eV, the ratio of the TII levels in samples “SRFA+Cl2”, “SRFA+NH2Cl”, “SRHA+NH2Cl”, and “SRFA+Cl2+NH4+” was 0.48:1.0:0.83:0.65. Because the TII levels under collision energies 20 and 40 eV represent low and relatively high MW iodinated DBPs, respectively, the ratio of the TII levels under collision energies 20 and 40 eV (CE20/CE40) may be used to indicate the distribution of low and relatively high MW iodinated DBPs in a sample: the higher the ratio, the lower the average MW. The CE20/CE40 ratios in samples “SRFA+Cl2”, “SRFA+NH2Cl”, “SRHA+NH2Cl”, and “SRFA+ Cl2+NH4+” were 2.29, 1.42, 1.34, and 1.45, respectively. These ratios suggest that the average MW of iodinated DBPs in chlorination was lower than that in chloramination, and the average MW of iodinated DBPs in chloraminated SRHA was higher than that in chloraminated SRFA, which can be ascribed to the higher oxidizing power of chlorine (15) and the higher average MW of SRHA, respectively. Besides, a comparison of Figure 5a and b suggests that the TII level was an indicator for the TOI level. Notably, if bromide is present in raw water, bromochloroacetic acid (m/z 171) may be detected by PIS m/z 126.9 under low collision energy, so its intensity should be subtracted from TII. TOI indicates the total concentration of all iodinated organic DBPs in a sample. Figure 5b shows the TOI levels in each sample. The ratio of the TOI levels in samples “SRFA+Cl2”, “SRFA+NH2Cl”, “SRHA+NH2Cl”, and “SRFA+ Cl2+NH4+” was 0.32:1.0:0.82:0.75. In consideration of either the TII levels under collision energy 20 eV, or the TII levels under collision energy 40 eV, or the TOI levels, the total levels of iodinated DBPs in the samples were in the order “SRFA+NH2Cl” > “SRFA+Cl2+ NH4+” > “SRFA+Cl2”. Both chlorine and chloramine can

oxidize I- rapidly to HOI, which can then react with NOM to form iodinated DBPs. However, HOCl/OCl- can further oxidize HOI to iodate, which is stable and unreactive toward NOM, with a reaction rate constant of 8.2-52 M-1 s-1 (2). The reaction rate constant of NH2Cl and HOI is smaller than 2 × 10-3 M-1s-1 (2), whereas that of HOI and NOM is 0.1-0.4 M-1 s-1 (16). It means that the half-life of HOI during chloramination is much longer than that during chlorination, leading to the formation of more iodinated DBPs during chloramination than during chlorination (4, 16, 17). An attractive disinfection process is the chlorine-ammonia sequential treatment, which makes full use of the advantages of chlorine and chloramine. During the lag time of ammonia addition, chlorine can perform efficient disinfection and convert partial iodide to iodate. After ammonia addition, chlorine reacts much faster with ammonia than with HOI, resulting in the formation of chloramine, which can reduce the formation of chlorinated/brominated DBPs (18) and maintain stable chlorine residual. By controlling the lag time, the disinfection efficiency and the levels of chlorinated/ brominated DBPs can be controlled; especially, the ratio of TOI levels in chlorine-ammonia sequential treatment and chloramination (TOICl2,NH+4 /TOINH2Cl) can be controlled, which is critical in controlling the levels of iodinated DBPs. Therefore, a simple equation was developed below to estimate the ratio. During chlorination, I- is oxidized to HOI immediately after chlorine addition (eq 2). In the pH range of 8.2-8.9, the reaction of HOI with ClO- is dominant, and is first order in ClO- (2). Compared with iodate formation, the consumption of HOI by NOM (kHOI+NOM ) 0.1-0.4 M-1 s-1) can be neglected. HClO + I- f HOI + Cl- kHOCl+I- ) 4.3 × 108M-1s-1 (2) (ref 2) + HOI + 2ClO- f IO3 + 2Cl + H kOCl-+HOI )

52 ( 5 M-1s-1 (3) (ref 2) Based on eqs 2 and 3, the ratio can be obtained: TOICl2,NH4+ /TOINH2Cl )

CT,OCl,i - 3CT,OI,i × 100% ACT,OCl,i - (2 + A)CT,OI,i (4)

where CT,OCl,i is the chlorine dose, CT,OI,i is the initial iodide concentration, A is exp((CT,OCl,i - 3CT,OI,i)kOCl-+HOIRHOIROCl-t), t is the lag time, RHOI is [H+ ]/([H+] + ka,HOI), and ROCl-is ka,HOCl/ ([H+] + ka,HOCl). The derivation of eq 4 is detailed in the SI. According to eq 4, TOICl2,NH4+/TOINH2Cl with a lag time of 65 s can be calculated to be 80%, which is a good estimation of the ratio (75%) as observed in Figure 5b. In drinking water treatment utilities within the areas with iodide-rich raw waters, sequential additions of chlorine and ammonia are suggested in order to achieve sufficient disinfection and minimize the formation of iodinated DBPs as well as chlorinated/brominated DBPs. Time interval between chlorine and ammonia additions may be determined by the composition and toxicity of halogenated DBPs.

Acknowledgments This work was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKUST 623409). We are grateful to the

editor and three anonymous referees for constructive critique and useful suggestions.

Supporting Information Available Additional details and Figures S1-S12. This material is available free of charge via the Internet at http://pubs.acs.org.

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