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Detection, identification and formation of new iodinated disinfection byproducts in chlorinated saline wastewater effluents Tingting Gong, Xiangru Zhang* Environmental Engineering Program, Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong SAR, China

article info

abstract

Article history:

The use of seawater for toilet flushing introduces high levels of inorganic ions, including

Received 30 June 2014

iodide ions, into a city's wastewater treatment systems, resulting in saline wastewater

Received in revised form

effluents. Chlorination is widely used in disinfecting wastewater effluents owing to its low

24 September 2014

cost and high efficiency. During chlorination of saline wastewater effluents, iodide may be

Accepted 25 September 2014

oxidized to hypoiodous acid, which may further react with effluent organic matter to form

Available online 5 October 2014

iodinated disinfection byproducts (DBPs). Iodinated DBPs show significantly higher toxicity than their brominated and chlorinated analogues and thus have been drawing increasing

Keywords:

concerns. In this study, polar iodinated DBPs were detected in chlorinated saline waste-

Disinfection byproducts

water effluents using a novel precursor ion scan method. The major polar iodinated DBPs

DBPs

were identified and quantified, and their organic precursors and formation pathways were

Saline wastewater effluents

investigated. The formation of iodinated DBPs under different chlorine doses and contact

Chlorination

times was also studied. The results indicated that a few polar iodinated DBPs were generated in the chlorinated saline primary effluent, but few were generated in the chlorinated saline secondary effluent. Several major polar iodinated DBPs in the chlorinated saline primary effluent were proposed with structures, among which a new group of polar iodinated DBPs, iodo-trihydroxybenzenesulfonic acids, were identified and quantified. The organic precursors of this new group of DBPs were found to be 4-hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene, and the formation pathways of these new DBPs were tentatively proposed. Both chlorine dose and contact time affected the formation of iodinated DBPs in the chlorinated saline wastewater effluents. © 2014 Elsevier Ltd. All rights reserved.

1.

Introduction

To lower the local freshwater demand and conserve the limited freshwater resources, Hong Kong has implemented the use of seawater for toilet flushing territory-wide since the

* Corresponding author. Tel.: þ852 2358 8479; fax: þ852 2358 1534. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.watres.2014.09.041 0043-1354/© 2014 Elsevier Ltd. All rights reserved.

1950s (Tang et al., 2007). A few other coastal cities or countries, including Avalon City, Marshall Islands and Kiribati, have also adopted this practice (Boehm et al., 2009; Mirti and Davies, 2005). But seawater toilet flushing introduces high levels of inorganic ions, including bromide and iodide ions, into the wastewater treatment systems, resulting in saline wastewater

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effluents. Chlorination is the most widely used method of disinfecting wastewater effluents owing to its low cost and high efficiency. In chlorination of saline wastewater effluents, bromide/iodide ions can be oxidized by chlorine or chloramines to hypobromous/hypoiodous acid, which may further react with effluent organic matter (EfOM) to form brominated/ iodinated disinfection byproducts (DBPs) (Ding et al., 2013). When the brominated/iodinated DBPs are discharged into marine water, they may pose adverse impacts on the marine ecosystem (Yang and Zhang, 2013; Liu and Zhang, 2014). A previous study has reported the formation of brominated DBPs in chlorination of saline wastewater effluents (Ding et al., 2013), but the formation of iodinated DBPs in chlorinated saline wastewater effluents has never been reported. Recently, iodinated DBPs have been drawing increasing concerns due to their relatively high toxicity. Previous studies have reported that iodinated DBPs are generally several to hundreds of times more cytotoxic and genotoxic than their brominated and chlorinated analogues (Plewa et al., 2008; Cemeli et al., 2006; Richardson et al., 2007, 2008). Our group has shown recently that iodinated DBPs (including emerging iodinated phenolic DBPs) presented substantially higher developmental toxicity and growth inhibition than their brominated and chlorinated analogues (Yang and Zhang, 2013; Liu and Zhang, 2014). Therefore, iodinated DBPs in chlorinated saline wastewater effluents are of great toxicological significance to the marine ecosystem and should be investigated. Gas chromatography-electron capture detection and gas chromatography-mass spectrometry have been used for detecting and identifying iodinated DBPs in drinking waters (Bichsel and von Gunten, 2000; Plewa et al., 2004; Krasner et al., 2006; Hua and Reckhow, 2007; Richardson et al., 2008; Duirk et al., 2011; Jones et al., 2012). However, since these techniques are less amenable to polar/semi-polar iodinated DBPs, only a few iodinated DBPs have been detected and identified in drinking waters, including mainly iodo-trihalomethanes, iodoacids and iodo-amides (Brass et al., 1977; Bichsel and von Gunten, 2000; Cancho et al., 2000; Plewa et al., 2004, 2008; Richardson, 2003; Richardson et al., 2008; Duirk et al., 2011; Chu et al., 2012). Owing to the complex constituents of EfOM in saline wastewater effluents (Ding et al., 2013), new polar iodinated DBPs may be generated in chlorinated saline wastewater effluents and they may escape detection by both techniques. Recently, a novel precursor ion scan (PIS) method has been developed for the fast selective detection of polar iodinated DBPs using an electrospray ionization-triple quadrupole mass spectrometer (ESI-tqMS) (Ding and Zhang, 2009). By setting PIS of m/z 126.9, almost all polar iodinated DBPs in a sample can be selectively detected. The principle of the PIS method is illustrated in Supplementary Information (SI) Section 1. Further, by coupling it with ultra performance liquid chromatography (UPLC) for preseparation, the ESI-tqMS can be used for identifying unknown polar iodinated DBPs. Total organic iodine (TOI) is another important collective parameter indicating the formation of all (nonpolar and polar) iodinated DBPs as a whole. A new TOI measurement approach involving UPLC/ESI-MS for detection of iodide has recently been developed by Pan and Zhang (2013). The purposes of this study were thus to detect polar iodinated DBPs in chlorinated saline wastewater effluents, to

identify the major polar iodinated DBPs, to quantify those iodinated DBPs, to determine their organic precursors and formation pathways, and to study the formation of iodinated DBPs under different chlorine doses and contact times.

2.

Materials and methods

2.1.

Chemicals and reagents

Potassium iodide (100%) was purchased from BDH. Ammonium chloride (99.5%) and potassium nitrate (99.0%) were €n. A sodium hypochlorite stock purchased from Riedel-deHae solution was purchased from SigmaeAldrich and it was diluted to around 2000 mg/L as Cl2 and standardized by the N,N-diethyl-p-phenylene diamine ferrous titrimetric method every month (APHA et al., 1995). 4-Hydroxybenzenesulfonic acid solution (65% wt. in H2O) and 1,2,3-trihydroxybenzene (99%) were purchased from SigmaeAldrich. All other chemicals used in this study were purchased at the highest purities available from SigmaeAldrich. Ultrapure water (18.2 MU cm) was supplied by a NANOpure Diamond purifier system (Barnstead).

2.2.

Preparation of solutions

A potassium iodide solution (100 mg/L as I), an ammonium chloride solution (1.0 g/L as N), a sodium sulfite solution (12.6 g/ L), and a sodium arsenite solution (13.0 g/L) were prepared. They were stored in amber glass bottles at 4  C and newly prepared every week. Stock solutions of 4-hydroxybenzenesulfonic acid (100 mg/L) and 1,2,3-trihydroxybenzene (100 mg/L) were prepared just before use. Iodoacetic acid was dissolved in ultrapure water to prepare standard solutions with TOI concentrations of 5, 10, 20, 50 and 100 mg/L as I. A potassium nitrate solution (5000 mg/L as NO 3) was prepared as the rinsing solution for removing inorganic halides from activated carbon in TOI measurement.

2.3. Collection, characterization and storage of the saline wastewater effluents Two undisinfected saline wastewater effluent samples (24-h composite) were collected from a saline primary wastewater treatment plant and a saline secondary wastewater treatment plant. For the two wastewater treatment plants, their influents were the same or similar as they both were domestic wastewater. The collected effluent samples were immediately transported to the lab in ice packs. Some chemical characteristics of the collected saline wastewater effluents were measured immediately. The pH, dissolved organic carbon (DOC), ammonium and bromide concentrations were measured with a pH meter (Thermo Orion), a total organic carbon analyzer (Shimadzu), a fast injection analyzer (QuickChem), and an ion chromatograph (Dionex), respectively. The iodide concentration was measured following the method by Gong and Zhang (2013). The collected saline wastewater effluents were adjusted to pH 2 with sulfuric acid and stored at 4  C to minimize changes in components. Before the experiment, they were brought to room temperature, adjusted to

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their original pH values with NaOH, and filtered with 11 mm filter papers to remove large particles.

2.4.

Chlorination of the saline wastewater effluents

Aliquots (1 L) of the saline wastewater effluents were disinfected by dosing certain amounts of NaOCl (18 and 10 mg/L as Cl2 for the saline primary and secondary effluents, respectively). For the saline primary effluent, the chlorine dose adopted in the wastewater treatment plant was 18 mg/L as Cl2; for the saline secondary effluent, chlorination was not conducted in the wastewater treatment plant, and thus a moderate chlorine dose capable of achieving the disinfection criterion was chosen in this study (Ding et al., 2013). Chlorination was conducted in headspace-free amber glass bottles. After a contact time of 30 min, the chlorine residual in each aliquot was measured (APHA et al., 1995) and dechlorinated with 105% of the requisite stoichiometric amount of Na2SO3, which is commonly used in wastewater treatment as a dechlorinating agent. Aliquots (1 L) of the filtered saline wastewater effluents without chlorination were used as control. To investigate the formation of iodinated DBPs under different chlorine doses, chlorine doses of 0 (undisinfected), 6, 12, 18, 24, 30 and 36 mg/L as Cl2 with a contact time of 30 min were adopted for the saline primary effluent, while chlorine doses of 0 (undisinfected), 3, 6, 9, 12, 15 and 18 mg/L as Cl2 with a contact time of 30 min were adopted for the saline secondary effluent. To investigate the formation of iodinated DBPs under different contact times, contact times of 10, 20, 30, 60, 90, 120 and 180 min with a chlorine dose of 18 mg/L as Cl2 were adopted for both effluents. These chlorine doses and contact times were selected since they were capable of achieving the corresponding disinfection criteria for the two types of saline wastewater effluents.

2.5. Pretreatment of the chlorinated saline wastewater effluents The samples were pretreated according to a previously developed procedure (Ding and Zhang, 2009; Xiao et al., 2010, 2012). In brief, a sample was adjusted to pH 0.5 with sulfuric acid and saturated through the addition of Na2SO4. The sample was then extracted with methyl tert-butyl ether (MtBE). The volume of MtBE was one-tenth of that of the sample. After extraction, the MtBE layer was transferred to a rotary evaporator and evaporated to 1 mL. The 1 mL solution in MtBE was mixed with 20 mL of acetonitrile and then rotoevaporated to 0.5 mL. The 0.5 mL solution in acetonitrile was diluted with 0.5 mL of ultrapure water and filtered through a 0.45 mm membrane before (UPLC/)ESI-tqMS analysis. To determine whether there were any iodine-containing compounds in the undisinfected saline wastewater effluents or any artifacts during pretreatment, the control samples were also pretreated following the same procedure.

2.6.

(UPLC/)ESI-tqMS analysis

A Waters Acquity ESI-tqMS was used to analyze the pretreated samples by direct infusion PIS m/z 126.9. The parameters of

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the instrument were set as follows: sample flow rate via an infusion pump 10 mL/min, ESI negative mode, capillary voltage 2.8 kV, cone voltage 20 V, 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, collision energy 40 eV, LM resolution 15, and HM resolution 15 (Ding and Zhang, 2009). For all the PISs, the data collection mode was Multi-Channel Analysis. A Waters UPLC system was coupled to the ESI-tqMS (UPLC/ ESI-tqMS). Five mL of a pretreated sample was injected into the UPLC system. The chromatographic separation was achieved by an HSS T3 column (2.1  100 mm, 1.8 mm particle size, Waters). A gradient eluent of acetonitrile and water was applied at a flow rate of 0.50 mL/min. The composition of acetonitrile/water changed linearly from 10/90 to 90/10 in the first 12 min, and then returned to 10/90 in 0.1 min, which was held for 2.9 min for re-equilibration. The parameters for the UPLC/ESI-tqMS were set the same as those for the direct infusion ESI-tqMS except that a higher desolvation temperature (400  C) and a higher desolvation gas flow (800 L/h) were applied. To identify an iodine-containing molecular ion detected by the direct infusion PIS m/z 126.9, the retention time of the molecular ion was confirmed by UPLC/ESI-tqMS multiple reaction monitoring (MRM). Then, product ion scans of the molecular ion were conducted at the specific retention time to gain the fragment information needed for proposing a structure. Finally, the proposed structure was confirmed using the corresponding standard compound.

2.7. Synthesis and isolation of the newly identified group of polar iodinated DBPs As described later in the results and discussion section, the four major ions/ion clusters in the PIS m/z 126.9 spectrum of the chlorinated saline primary effluent were proposed to be 2-iodo-3,4,5-trihydroxybenzenesulfonic acid, 2-chloro-6iodo-3,4,5-trihydroxybenzenesulfonic acid, 2-bromo-6-iodo3,4,5-trihydroxybenzenesulfonic acid, and 2,6-diiodo-3,4,5trihydroxybenzenesulfonic acid. Since the corresponding standard compounds were not commercially available, they were synthesized in the lab. To obtain the organic precursor (3,4,5-trihydroxybenzenesulfonic acid), 3.783 g of 1,2,3trihydroxybenzene was dissolved in 100 mL of 96% H2SO4 and kept at 25  C for 48 h for sulfonation reaction  et al., 1995). The solid product (3,4,5(Veselinovic trihydroxybenzenesulfonic acid) was separated by filtering the solution through a fine filter with pore sizes of 4e5.5 mm and stored at 4  C before use. Then, two samples (samples 1 and 2) were prepared for synthesis. For sample 1, 0.75 g of 3,4,5-trihydroxybenzenesulfonic acid and 0.92 g of iodine (at a 3,4,5-trihydroxybenzenesulfonic acid-to-iodine molar ratio of 1:1) were dissolved in 300 mL of acetonitrile. For sample 2, 0.75 g of 3,4,5-trihydroxybenzenesulfonic acid and 1.84 g of iodine (at a 3,4,5-trihydroxybenzenesulfonic acid-to-iodine molar ratio of 1:2) were dissolved in 300 mL of acetonitrile. Each sample was heated at 90  C until the acetonitrile was completely evaporated. The remaining solid was dissolved in 300 mL of ultrapure water and subjected to ultrasonication for 15 min (to ensure the solid was completely

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dissolved). Sample 1 was then divided into three aliquots (aliquots 1, 2 and 3). For aliquot 1, 105% of the requisite stoichiometric amount of Na2SO3 was added to reduce the residual I2, and 2-iodo-3,4,5-trihydroxybenzenesulfonic acid was obtained. For aliquot 2, 0.13 g of NaOCl as Cl2 (at a 3,4,5trihydroxybenzenesulfonic acid-to-Cl2 molar ratio of 1:1.5) was added and the solution was heated at 100  C for 20 min. Then, 105% of the requisite stoichiometric amount of Na2SO3 was added to reduce the residuals I2 and Cl2. Accordingly, 2chloro-6-iodo-3,4,5-trihydroxybenzenesulfonic acid was obtained. For aliquot 3, 0.097 g of Br2 (at a 3,4,5trihydroxybenzenesulfonic acid-to-Br2 molar ratio of 1:0.5) was added and the solution was heated at 100  C for 20 min. Then, 105% of the requisite stoichiometric amount of Na2SO3 was added to reduce the residuals I2 and Br2. Thus, 2-bromo6-iodo-3,4,5-trihydroxybenzenesulfonic acid was obtained. For sample 2, 105% of the requisite stoichiometric amount of Na2SO3 was added to reduce the residual I2, and 2,6-diiodo3,4,5-trihydroxybenzenesulfonic acid was obtained. The adopted molar ratios of the reactants were the optimized ones. The synthesized samples were pretreated following the same procedure as described in Section 2.5. Each pretreated synthesized sample was analyzed by UPLC/ESI-tqMS MRM to confirm the retention time of the corresponding standard compound, and then the standard compound was isolated from the pretreated synthesized sample with the UPLC system. For each UPLC run, 7.5 mL of the pretreated synthesized sample was injected into the system and the fraction of the standard compound was collected in the corresponding retention time range. The fraction of each standard compound was collected following the same procedure.

2.8. Quantification of the newly identified group of polar iodinated DBPs The collected fraction of a standard compound (described in Section 2.7) was analyzed by the UPLC/ESI-tqMS PIS m/z 126.9 to detect all the iodine-containing compounds in it. If the standard compound was the only iodine-containing compound in the collected fraction, then 4 mL of the collected fraction was diluted to 200 mL with ultrapure water and subjected to TOI measurement following the method by Pan and Zhang (2013). Eighty mL of each sample was adsorbed on activated carbon and duplicate measurements were conducted. According to the TOI concentration, the concentration of the standard compound in the collected fraction was calculated. Then, the chlorinated saline primary effluent sample was spiked with different levels of the standard compound and pretreated following the same procedure as described in Section 2.5. Duplicate samples were prepared and measured.

2.9. Organic precursors of the newly identified group of polar iodinated DBPs

solution of a certain concentration was prepared by diluting the stock solution with ultrapure water. Certain amounts of iodide and ammonium chloride were added to the solution and the pH of the solution was adjusted using H2SO4 or NaOH. Then, a certain amount of NaOCl was added for chlorination. After a contact time of 30 min, the sample was dechlorinated with a certain amount of Na2SO3. The concentrations and pH values of different samples are shown in Tables S1eS7 in the SI. For 4-hydroxybenzenesulfonic acid, three series of samples were prepared by varying the 4-hydroxybenzenesulfonic acid concentration (A Series, SI Table S1), the iodide concentration (B Series, SI Table S2) and the pH (C Series, SI Table S3). For 1,2,3-trihydroxybenzene, four series of samples were prepared by varying the 1,2,3-trihydroxybenzene concentration (D Series, SI Table S4), the iodide concentration (E Series, SI Table S5), the pH (F Series, SI Table S6) and the sulfite concentration for dechlorination (G Series, SI Table S7). The samples were pretreated following the same procedure as described in Section 2.5.

2.9.2. Comparison of sulfite and arsenite as the dechlorinating agent To examine whether sulfite was a source of the sulfo group in the identified group of iodinated DBPs, another dechlorinating agent, namely arsenite (Liu and Zhang, 2013), was chosen for comparison. Two chlorinated 1,2,3trihydroxybenzene samples were prepared. One was the same as the F-2 sample in Section 2.9.1. Briefly, 30 mg/L of iodide and 25 mg/L of ammonium as N were added to 100 mL of the 1,2,3-trihydroxybenzene solution (0.01 mg/L). The pH of the solution was adjusted to 10 with NaOH. Then, 18 mg/L of NaOCl as Cl2 was added for chlorination. After a contact time of 30 min, the sample was dechlorinated with 18 mg/L of Na2SO3 as Cl2. For the other sample, arsenite was used for dechlorination instead of sulfite while other conditions were the same. Similarly, a chlorinated saline primary effluent sample dechlorinated with sulfite and another dechlorinated with arsenite were prepared. The chlorine dose was 18 mg/L as Cl2, and the contact time was 30 min. The samples were pretreated following the same procedure as described in Section 2.5.

2.9.3. Organic precursors in the undisinfected saline wastewater effluents To examine whether 4-hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene were present in the undisinfected saline primary or secondary effluent, UPLC/ESI-tqMS selected ion monitorings (SIMs) of m/z 173 and 125 were conducted for the undisinfected saline primary or secondary effluent sample, the water solution containing 1 mg/L of 4hydroxybenzenesulfonic acid or 1,2,3-trihydroxybenzene, and the undisinfected saline primary or secondary effluent sample spiked with 4-hydroxybenzenesulfonic acid or 1,2,3trihydroxybenzene.

2.9.1. Chlorination of 4-hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene

2.10.

4-Hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene were allowed to react with chlorine. One hundred mL of 4hydroxybenzenesulfonic acid or 1,2,3-trihydroxybenzene

TOI measurement was carried out following a previously developed approach (Pan and Zhang, 2013; Li et al., 2010). For each sample, duplicate measurements were conducted.

TOI measurement

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3.

Results and discussion

3.1.

Characteristics of the saline wastewater effluents

The chemical characteristics of the two saline wastewater effluents are given in Table S8 in the SI. The DOC concentration of the saline secondary effluent (6.9 mg/L as C) was much lower than that of the saline primary effluent (18.9 mg/L as C) due to the significant DOC removal by secondary treatment. The saline secondary effluent was well nitrified with an ammonium concentration of 0.7 mg/L as N, while the saline primary effluent was poorly nitrified with an ammonium concentration as high as 24.5 mg/L as N. The bromide concentrations of the two effluents (27.9 and 27.0 mg/L in the saline primary and secondary effluents, respectively) were comparable due to the same percentage (around 30%) of seawater in the wastewaters. However, the iodide concentration of the saline primary effluent (26.4 mg/ L) was much higher than that of the saline secondary effluent (5.0 mg/L) as a result of the much higher removal efficiency of iodine by secondary treatment (Gong and Zhang, 2013).

3.2. Detection of polar iodinated DBPs in the chlorinated saline wastewater effluents According to the method developed by Ding and Zhang (2009), all electrospray-ionizable iodinated DBPs can be selectively detected by the PIS m/z 126.9. For the iodinated DBPs that also contain bromine/chlorine atoms, by comparing the isotopic peak abundances in the PIS m/z 126.9 spectrum to the theoretical ones, the exact numbers of bromine and chlorine atoms in the compounds can be determined. Fig. 1a shows the PIS m/z 126.9 spectrum of the chlorinated saline primary effluent. A few ions/ion clusters with relatively high intensities can be seen, including m/z 269/271/273, 317/ 319, 331, 365/367, 409/411 and 457, indicating that several polar iodinated DBPs were generated in this effluent. Fig. 1b displays the PIS m/z 126.9 spectrum of the chlorinated saline secondary effluent. Few peaks with relatively high intensities can be seen, indicating that few polar iodinated DBPs were generated in this effluent. Such a big contrast in the formation of iodinated DBPs between the primary and secondary effluents may be owing to three reasons. First, the ammonium concentration of the saline primary effluent was 24.5 mg/L as N. When the effluent was dosed with 18 mg/L of chlorine, the chlorine reacted with ammonium immediately to form monochloramine and thus the main disinfectant was monochloramine. For the saline secondary effluent, the ammonium concentration was only 0.7 mg/L as N. When this effluent was dosed with 10 mg/L of chlorine, free chlorine was the main disinfectant. It has been reported that monochloramine favors the formation of iodinated DBPs (Bichsel and von Gunten, 1999; Richardson et al., 2008; Ding and Zhang, 2009). Second, the concentration of iodide (inorganic precursor) in the saline secondary effluent was significantly lower than that in the saline primary effluent (SI Table S8). Third, the saline secondary effluent had a significantly lower concentration of DOC (organic precursors) than the saline primary effluent (SI

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Fig. 1 e (a) PIS m/z 126.9 spectrum of the chlorinated saline primary effluent. The chlorine dose was 18 mg/L as Cl2 and the contact time was 30 min. (b) PIS m/z 126.9 spectrum of the chlorinated saline secondary effluent. The chlorine dose was 10 mg/L as Cl2 and the contact time was 30 min. The y-axes of charts a and b are on the same scale (1.49 £ 106). The structures of the proposed and identified ions/ion clusters in the chlorinated saline primary effluent are listed on the top. *: Proposed structures. Table S8). Of note is that Ca2þ and Mg2þ ions in saline wastewater effluents (originally occurring in seawater) may affect the formation of iodinated DBPs mainly because such divalent metal ions can bind on chromophores of organic matter (Yan and Korshin, 2014; Yan et al., 2013; Liu et al., 2007) and thus affect the reactivity of organic matter with halogens during chlorination of saline wastewater effluents. In this study, the iodinated DBPs in the chlorinated “real” saline wastewater effluents (containing Ca2þ and Mg2þ ions) were examined, so the results should have reflected the formation of iodinated DBPs in real cases.

3.3. Identification of the major polar iodinated DBPs in the chlorinated saline primary effluent Through UPLC/ESI-tqMS MRM and product ion scans, several major ions/ion clusters in the PIS m/z 126.9 spectrum of the chlorinated saline primary effluent were identified. The corresponding chemical structures are shown in Fig. 1. Identification of the ion cluster with m/z 365/367 is exemplified here. Fig. 2 shows the MRM chromatogram (365 / 126.9, 367 / 126.9) and the product ion scan spectra of ion cluster m/z 365/367 of the sample. The relatively long retention time (6.47 min) indicated that this compound was likely aromatic (Zhai and Zhang, 2011). An isotopic peak abundance ratio of m/z 365 to 367 at 3:1 was observed in the MRM spectrum, indicating that this compound contained one chlorine atom. In the product ion scan spectra of both m/z 365 and 367, a loss of 80 was observed, which

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confirmed that the compound with m/z 365/367 in the sample was 2-chloro-6-iodo-3,4,5-trihydroxybenzenesulfonic acid. Similarly, the ions/ion clusters with m/z 331, 409/411 and 457 were identified to be 2-iodo-3,4,5-trihydroxybenzenesulfonic acid, 2-bromo-6-iodo-3,4,5-trihydroxybenzenesulfonic acid and 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid, respectively. Details pertaining to the identification of the three compounds are described in SI Sections 2e4 and Figs. S1eS3. In this way, a new group of polar iodinated DBPs referred to as iodo-trihydroxybenzenesulfonic acids were identified in the chlorinated saline primary effluent. Ion clusters with m/z 269/ 271/273 and 317/319 were also tentatively proposed to be chlorobromoiodomethanol and chlorodiiodomethanol, respectively (SI section 5).

3.4. Quantification of the newly identified iodinated DBPs in the chlorinated saline primary effluent

Fig. 2 e UPLC/ESI-tqMS MRM (365 / 126.9, 367 / 126.9) chromatograms of (a) 2-chloro-6-iodo-3,4,5trihydroxybenzenesulfonic acid, (b) the chlorinated saline primary effluent sample, and (c) the sample mixed with 2chloro-6-iodo-3,4,5-trihydroxybenzenesulfonic acid. The y-axes of charts a‒c are on the same scale. UPLC/ESI-tqMS product ion scan spectra of (d) m/z 365 of the chlorinated saline primary effluent sample, (e) m/z 367 of the chlorinated saline primary effluent sample, (f) m/z 365 of 2chloro-6-iodo-3,4,5-trihydroxybenzenesulfonic acid, and (g) m/z 367 of 2-chloro-6-iodo-3,4,5trihydroxybenzenesulfonic acid.

was proposed to be a sulfo group. Since this ion cluster was detected by PIS m/z 126.9, it should contain at least one iodine atom. After subtraction of one benzene ring, one chlorine atom, one sulfo group and one iodine atom from m/z 365/367, the remaining part was 48, for which a reasonable combination should be three oxygen atoms. Therefore, the ion cluster with m/z 365/367 was proposed to be 2-chloro-6-iodo-3,4,5trihydroxybenzenesulfonic acid or its isomers. The standard compound of 2-chloro-6-iodo-3,4,5-trihydroxybenzenesulfonic acid was synthesized in the lab. Fig. 2 also displays the UPLC/ ESI-tqMS MRM (365 / 126.9, 367 / 126.9) chromatograms of a 2-chloro-6-iodo-3,4,5-trihydroxybenzenesulfonic acid standard solution as well as the sample spiked with 2-chloro-6iodo-3,4,5-trihydroxybenzenesulfonic acid, and the product ion scan spectra of ion cluster m/z 365/367 of the 2-chloro-6iodo-3,4,5-trihydroxybenzenesulfonic acid standard solution. The same retention time and product ion scan spectra

For each identified iodinated DBP, its concentration in the chlorinated saline primary effluent was determined. The quantification of 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid is exemplified here. The PIS m/z 126.9 spectrum of the collected fraction of 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid is shown in SI Fig. S4b. Only one intensive peak (m/z 457) can be seen in the PIS m/z 126.9 spectrum, indicating that 2,6-diiodo3,4,5-trihydroxybenzenesulfonic acid was the only iodinecontaining compound in the collected fraction. Therefore, the concentration of 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid in the collected fraction could be determined by analyzing the TOI concentration in it. The TOI concentration was measured to be 240 mg/L as I, and thus the concentration of 2,6diiodo-3,4,5-trihydroxybenzenesulfonic acid in the collected fraction was calculated to be 433 mg/L. By spiking the sample with different levels of the standard compound, the concentration of 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid in the chlorinated saline primary effluent was determined to be 0.91 mg/L (SI Fig. S4c). Similarly, the concentrations of 2-iodo-3,4,5trihydroxybenzenesulfonic acid, 2-chloro-6-iodo-3,4,5trihydroxybenzenesulfonic acid and 2-bromo-6-iodo-3,4,5trihydroxybenzenesulfonic acid in this effluent were determined to be 0.57, 0.43 and 1.07 mg/L, respectively. The quantification of the three compounds is described in SI sections 6e8 and Figs. S5eS7. Duplicate measurements were conducted for the four DBPs, with relative standard deviations below 7% and recoveries ranging from 91% to 103%. Most recently, it has been reported that iodinated DBPs presented substantially higher developmental toxicity and growth inhibition than their brominated and chlorinated analogues, and halogenated phenolic DBPs generally presented substantially higher developmental toxicity and growth inhibition than halogenated aliphatic DBPs (Yang and Zhang, 2013; Liu and Zhang, 2014). Since these newly identified iodo-trihydroxybenzenesulfonic acids belong to “iodinated phenolic DBPs”, their comparative toxicity and environmental stability (e.g., in receiving marine water) should be of concern and interest. However, these studies cannot be conducted at this stage as there are no purified standard compounds available.

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3.5. Organic precursors and formation pathways of the newly identified group of polar iodinated DBPs in the chlorinated saline primary effluent Based on the structures of the newly identified group of DBPs (containing three hydroxyl groups and one sulfo group attached to the benzene ring), linear alkylbenzene sulfonates, 3,4,5-trimethylbenzenesulfonic acid, 4-hydroxybenzenesulfonic acid and 1,2,3trihydroxybenzene were proposed to be the organic precursors. These possible organic precursors were chlorinated to examine whether they were capable of forming this group of DBPs. To simplify the systems, only iodide was added before chlorination while bromide was not. Therefore, only 2-iodo-3,4,5-trihydroxybenzenesulfonic acid and 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid were the target products in these samples. It was found that only 4hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene were capable of generating this group of DBPs. The results are described in SI section 9 and Figs. S8eS11 and the proposed formation pathways are shown in Fig. 3. The B-1 sample, with a 4-hydroxybenzenesulfonic acid concentration of 0.1 mg/L, an iodide concentration of 100 mg/L, an ammonium concentration of 25 mg/L as N, a pH value of 7, and a chlorine dose of 18 mg/L as Cl2, showed the highest formation of 2-iodo-3,4,5-trihydroxybenzenesulfonic acid and 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid among all the chlorinated 4-hydroxybenzenesulfonic acid samples. The F-2 sample, with a 1,2,3-trihydroxybenzene concentration of 0.01 mg/L, an iodide concentration of 30 mg/L, an ammonium concentration of 25 mg/L as N, a pH value of 10, a chlorine dose of 18 mg/L as Cl2, and a sulfite concentration of 18 mg/L as Cl2 for dechlorination, showed the highest formation of 2-iodo-3,4,5-trihydroxybenzenesulfonic acid and 2,6-diiodo-3,4,5-trihydroxybenzenesulfonic acid among all the chlorinated 1,2,3-trihydroxybenzene samples. The MRM chromatograms of the B-1 and F-2 samples are shown in SI Fig. S12de and Fig. S12ij, respectively. In Fig. S12d, the peak at 3.47 min represents 2-iodo-3,4,5-

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trihydroxybenzenesulfonic acid; in Fig. S12e, the peak at 4.73 min represents 2,6-diiodo-3,4,5trihydroxybenzenesulfonic acid. In Fig. S12i, the peak at 3.49 min represents 2-iodo-3,4,5-trihydroxybenzenesulfonic acid; in Fig. S12j, the peak at 4.75 min represents 2,6-diiodo3,4,5-trihydroxybenzenesulfonic acid. The results indicated that both 4-hydroxybenzenesulfonic acid and 1,2,3trihydroxybenzene, as the organic precursors, were capable of generating this new group of iodinated DBPs. The presence of 4-hydroxybenzenesulfonic acid and 1,2,3trihydroxybenzene in the undisinfected saline primary effluent was verified. The SIMs of m/z 173 of 4hydroxybenzenesulfonic acid, the undisinfected saline primary effluent sample and the sample spiked with 4hydroxybenzenesulfonic acid are shown in SI Fig. S12ac. The compound with m/z 173 at 0.61 min in the sample was confirmed to be 4-hydroxybenzenesulfonic acid due to the same retention time (0.61 min). Similarly, the SIMs of m/z 125 of 1,2,3-trihydroxybenzene, the undisinfected saline primary effluent sample and the sample spiked with 1,2,3trihydroxybenzene are shown in SI Fig. S12fh. The compound with m/z125 at 1.11 min in the sample was confirmed to be 1,2,3-trihydroxybenzene due to the same retention time (1.11 min). Therefore, both 4-hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene were present in the undisinfected saline primary effluent and thus could be the organic precursors of the new group of iodinated DBPs. Since the new group of iodinated DBPs were not generated in the chlorinated saline secondary effluent, this effluent probably did not contain the two organic precursors. SI Fig. S13 shows the SIMs of the undisinfected saline primary and secondary effluent samples. The peaks of 4-hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene can only be seen in the SIMs of the undisinfected saline primary effluent, indicating that the two organic precursors were only present in this effluent but not in the undisinfected saline secondary effluent. Since 1,2,3-trihydroxybenzene itself does not contain a sulfo group, the source of the sulfo group of the formed iodinated DBPs in the chlorinated 1,2,3-trihydroxybenzene

Fig. 3 e Proposed formation pathways of the newly identified group of polar iodinated DBPs from the organic precursors of (a) 4-hydroxybenzenesulfonic acid, and (b) 1,2,3-trihydroxybenzene.

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samples was proposed to be sulfite, which was used as the dechlorinating agent. SI Fig. S14 shows the MRM chromatograms of the chlorinated 1,2,3-trihydroxybenzene samples using either sulfite or arsenite as the dechlorinating agent. The results indicated that only when sulfite was used as the dechlorinating agent were 2-iodo-3,4,5trihydroxybenzenesulfonic acid and 2,6-diiodo-3,4,5trihydroxybenzenesulfonic acid formed. Therefore, the proposed formation pathway from 1,2,3-trihydroxybenzene is reasonable. The pathway was further verified by dechlorinating the chlorinated saline primary effluent using either sulfite or arsenite as the agent. SI Fig. S15 shows the PIS m/z 126.9 spectra of the chlorinated saline primary effluent samples dechlorinated with either sulfite or arsenite. The peak intensities of the four identified ions/ion clusters (m/z 331, 365/367, 409/411 and 457) in the PIS m/z 126.9 spectrum of the sample dechlorinated with sulfite were significantly higher than those of the sample dechlorinated with arsenite, indicating that sulfite (as the dechlorinating agent) favored the formation of the four iodinated DBPs, which also supports the proposed formation pathway of this group of iodinated DBPs from 1,2,3trihydroxybenzene.

3.6. Formation of iodinated DBPs in the chlorinated saline wastewater effluents under different chlorine doses and contact times The formation of iodinated DBPs in the chlorinated saline primary effluent under different chlorine doses is exemplified here. To investigate the formation of iodinated DBPs in the chlorinated saline primary effluent under different chlorine doses, seven samples were prepared. The chlorine doses were 0, 6, 12, 18, 24, 30 and 36 mg/L as Cl2, and the contact time was 30 min. Fig. 4ag shows the PIS m/z 126.9 spectra of the seven samples. Several ions/ion clusters with relatively high intensities can be seen in these spectra, including m/z 269/271/ 273, 317/319, 331, 365/367, 409/411 and 457, the structures of which have been proposed or identified in Section 3.3 above. The intensities of these major peaks increased as the chlorine dose rose from 0 to 24 mg/L before decreasing when the chlorine dose exceeded 24 mg/L. The TOI concentrations of the seven samples were 7.2, 7.7, 12.4, 13.6, 14.4, 12.8 and 12.2 mg/L as I. The TOI concentration in the undisinfected saline primary effluent (corresponding to the sample with a chlorine dose of 0 mg/L as Cl2) was 7.2 mg/L as I. Fig. 4h presents the TOI concentrations and total ion intensities (TIIs) of the seven samples. The TII was calculated by summing up the peak intensities from m/z 128 to 600 in the PIS m/z 126.9 spectrum (indicating the formation of polar iodinated DBPs). The TOI concentrations showed the same trend as the TIIs, demonstrating that the formation of iodinated DBPs increased as the chlorine dose rose from 0 to 24 mg/L before decreasing when the chlorine dose exceeded 24 mg/L. During chlorination of the saline primary effluent, the following three reactions were involved: NHþ 4 þ HOCl/NH2 Cl

(1)

I þ NH2Cl / HOI

(2)

Fig. 4 e PIS m/z 126.9 spectra of the chlorinated saline primary effluent samples under chlorine doses of (a) 0 mg/ L, (b) 6 mg/L, (c) 12 mg/L, (d) 18 mg/L, (e) 24 mg/L, (f) 30 mg/L, and (g) 36 mg/L as Cl2, and a contact time of 30 min. The yaxes of charts a‒g are on the same scale (7.06 £ 105). (h) TOI concentrations and TIIs of the seven samples.

EfOM þ HOI/Iodinated DBPs

(3)

The ammonium concentration of the saline primary effluent was 24.5 mg/L as N. For all the tested chlorine doses (0e36 mg/L as Cl2), the dosed chlorine reacted with ammonium in the effluent immediately to form monochloramine. As the chlorine dose increased, the monochloramine concentration also increased. Since the molar concentration of monochloramine (mg/L level) was much higher than that of iodide (mg/L level), the iodide could be completely oxidized by monochloramine to hypoiodous acid. An increase in the monochloramine concentration caused reaction (2) and in turn reaction (3) to accelerate. Therefore, the formation of iodinated DBPs increased initially with increasing chlorine dose, but it subsided as the chlorine dose continued to rise

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beyond a threshold. This finding might be ascribed to two reasons. First, some of the EfOM might have reacted with the high level of monochloramine and thus the organic precursors for the formation of iodinated DBPs were reduced. Second, the formed iodinated DBPs might have also reacted with the high level of monochloramine to undergo transformation and decomposition. The formation of iodinated DBPs in the chlorinated saline primary effluent under different contact times and the formation of iodinated DBPs in the chlorinated saline secondary effluent under different chlorine doses and contact times are described in SI sections 10e12 and Figs. S16eS18. The results indicated that both chlorine dose and contact time affected the formation of iodinated DBPs.

4.

Conclusions

Polar iodinated DBPs were detected in chlorinated saline wastewater effluents. A number of polar iodinated DBPs were generated in the chlorinated saline primary effluent, but few were generated in the chlorinated saline secondary effluent, owing possibly to the significantly higher ammonium, iodide and DOC concentrations in the former. The major polar iodinated DBPs in the chlorinated saline primary effluent were identified to be a new group of polar iodinated DBPs, iodotrihydroxybenzenesulfonic acids. The concentrations of 2iodo-3,4,5-trihydroxybenzenesulfonic acid, 2-chloro-6-iodo3,4,5-trihydroxybenzenesulfonic acid, 2-bromo-6-iodo-3,4,5trihydroxybenzenesulfonic acid and 2,6-diiodo-3,4,5trihydroxybenzenesulfonic acid in the chlorinated saline primary effluent were determined to be 0.57, 0.43, 1.07 and 0.91 mg/L, respectively. The organic precursors of this new group of DBPs were found to be 4-hydroxybenzenesulfonic acid and 1,2,3-trihydroxybenzene, and the formation pathways of these new DBPs were tentatively proposed. The results also indicated that sulfite as the dechlorinating agent favored the formation of this new group of DBPs. Both chlorine dose and contact time affected the formation of iodinated DBPs in the chlorinated saline wastewater effluents. Finally, the comparative toxicity and environmental stability of this new group of iodinated DBPs deserve study.

Acknowledgement This work was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Projects 623409 and RPC11EG16).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2014.09.041.

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