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Iodinated trihalomethane formation during chloramination of iodate-containing waters in the presence of zero valent iron
Water Research 124 (2017) 219e226
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Iodinated trihalomethane formation during chloramination of iodatecontaining waters in the presence of zero valent iron Ying Xia a, Yi-Li Lin b, Bin Xu a, *, Chen-Yan Hu c, Ze-Chen Gao a, Wen-Hai Chu a, Nai-Yun Gao a a
State Key Laboratory of Pollution Control and Resource Reuse, Institute of Disinfection By-product Control in Water Treatment, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 824, Taiwan, ROC c College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 May 2017 Received in revised form 20 July 2017 Accepted 23 July 2017 Available online 24 July 2017
Iodide (I) and iodinated X-ray contrast media (ICM) are the primary iodine sources for the formation of iodinated disinfection byproducts (I-DBPs), and iodate (IO 3 ) is believed to be a desired sink of iodine in water. This study found that highly cytotoxic iodinated trihalomethanes (I-THMs) also can be generated from iodate-containing waters (without any other iodine sources) in the presence of zero valent iron (ZVI) during chloramination, which could be a big issue in the wide usage of iron pipes. The effect of major factors including ZVI dosage, NH2Cl and IO 3 concentrations, initial pH, Br /IO3 molar ratio, phosphate concentration, iron corrosion scales (goethite and hematite) on the formation of I-THMs were investigated. Formation of I-THMs from IO 3 increased with the increase of ZVI dosage, IO3 and NH2Cl concentrations. Chloramines can also remarkably accelerate the reduction of IO 3 by ZVI. Peak I-THM formation was found at pH 8. As the Br/IO 3 molar ratio increased from 0 to 20, I-THM formation considerably enhanced, especially for the bromine-incorporated species. Goethite and hematite enhanced the formation of I-THMs in the presence of ZVI. Additionally, a significant suppression on ITHM formation was observed with the addition of phosphate. Considering that a large number of water distribution networks contain unlined cast iron pipes, transformation of IO 3 in the presence of ZVI during chloramination may contribute to the formation of I-THMs in such systems. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Iodinated trihalomethanes (I-THMs) Iodate (IO 3) Zero valent iron (ZVI) Chloramination Disinfection by-products (DBPs)
1. Introduction The disinfection of drinking-water has been named as a public health triumph of the twentieth century (Richardson, 2003). Before it had been used widely, millions of people died from waterborne illnesses. However, chemical disinfection has also raised an unintended health hazard due to the genotoxic and cytotoxic effects associated with disinfection by-products (DBPs) (Richardson et al., 2007). Over the past decades, regulated DBPs including trihalomethanes (THMs), haloacetic acids (HAAs), bromate and chlorate have aroused wide public concern. However, other emerging iodinated DBPs (I-DBPs) are reported to be more cytotoxic and genotoxic in mammalian cells compared to their chlorinated and brominated analogues (Plewa et al., 2004). For example,
* Corresponding author. E-mail address: [email protected] (B. Xu). http://dx.doi.org/10.1016/j.watres.2017.07.059 0043-1354/© 2017 Elsevier Ltd. All rights reserved.
iodoacetic acid was reported as the most cytotoxic, genotoxic, and potential carcinogenic DBP in mammalian cells (Richardson et al., 2008; Duirk et al., 2011; Richardson and Postigo, 2012; Wei et al., 2013). Additionally, iodinated trihalomethanes (I-THMs) were reported to be attributed to the issues of medicinal or pharmaceutical taste and odor. For instance, the odor and taste threshold concentrations of iodoform are 0.02 and 5 mg/L, respectively (Cancho et al., 2000; Hansson et al., 1987). Therefore, the formation of I-DBPs has raised great concern in recent years (Richardson et al., 2003; Ye et al., 2013; Wang et al., 2014; Zhang et al., 2016). Bichsel and Von Gunten (1999, 2000) reported that chloramination of iodate containing water can form a significant amount of I-THMs due to the formation of active hypolodous acid (HOI) that can react with natural organic matter (NOM), causing the formation of I-THMs. It is a common consensus that the main iodine species in water are iodide (I), iodate (IO 3 ) and iodinated organic compounds such as iodinated X-ray contrast media (ICM) (Bichsel and Von Gunten, 1999; Duirk et al., 2011; Hansen et al., 2011). IO 3 is preferentially
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recommended as an additive to salt for its stability by most health authorities to prevent iodine deficiency (Burgi et al., 2001) because it can rapidly be reduced to I in vivo by glutathione and will not cause toxicological problems (Taurog et al., 1966; Smith et al., 2010). Therefore, IO 3 is a preferred sink of iodine in drinking water (Allard et al., 2013) and has been detected in tap waters with concentrations at level of several to dozens of mg-I/L (Snyder et al., 2005; Gong and Zhang, 2013; Zhang et al., 2016). One of the optimal approaches proposed to mitigate the formation of I-DBP is to ensure full oxidation of the iodinated precursors, such as I (Bichsel and Von Gunten, 1999, 2000) and organo-iodine (Seitz et al., 2008; Wang et al., 2014), to IO 3 (Allard et al., 2015). Large amounts of unlined cast iron pipes have been used in water distribution systems for several centuries throughout the world (Mcneiill and Edwards, 2001; Tang et al., 2006; Husband and Boxall, 2011). However, they are very prone to corrosion, causing the release of ferrous and ferric ions as well as the formation of iron oxides, such as goethite (a-FeOOH) and hematite (Fe2O3), on the top layer of red rust (Tang et al., 2006; Zhu et al., 2014). Recently, chloramines have been utilized as a secondary disinfectant for their long-term stability and less formation of regulated DBPs (Diehl et al., 2000; Seidel et al., 2005; Shen and Andrews, 2011). Humic acid (HA), a major component of NOM, is also ubiquitous in drinking water (Li et al., 2016). Hence, it is an interesting scientific question to understand whether IO 3 can react with zero valent iron (ZVI) and/or its corrosion scales in the presence of chloramine and NOM to form iodinated trihalomethanes (I-THMs). Although IO 3 is widely perceived as nontoxic and stable, the possible trans formation of IO 3 to I by ZVI might make a contribution to the formation of I-THMs in source waters. At present, little is known about which parameters, if any, would affect the formation of ITHM in the presence of chloramine and ZVI. This information is significant to realize the formation of I-THM in both natural and engineered systems. The primary aim of this research was therefore to investigate the formation of I-THMs from IO 3 during chloramination in the presence of ZVI and/or its corrosion scales. Key parameters including ZVI dosage, NH2Cl and IO 3 concentrations, initial pH, Br /IO3 molar ratio and phosphate concentration were studied systematically. 2. Materials and methods
et al., 2012). 2.2. Experimental procedures Batch experiments were performed in duplicate under headspace-free conditions in 40-mL glass screw-cap amber vials with PTFE-lined septa. The vials were then placed into a constanttemperature shaker bath (25 ± 1 C, 150 rpm) in dark for 3 d. After the designed reaction time, 10 mL of the solution was withdrawn with a 20 mL gas-tight syringe into a 25 mL vial and filtered immediately through a glass fiber filter (GF/F, Whatman, USA) to remove ZVI particles before analysis. The filtered solution was quenched using Na2S2O3 with 20% excess of the initial NH2Cl concentration, and the samples were then extracted using MtBE immediately to avoid any chance of I-THM decomposition. Factors including ZVI dosage (0e2.0 g/L), NH2Cl and IO 3 concentrations (0 - 20 mg-Cl2/L and 0e6.35 mg-I/L (0e50 mM), respectively), Br/IO 3 molar ratio (0e20), initial pH (4e10) and phosphate concentration (0e10 mM) on I-THM formation from IO 3 were investigated. Typical experiments were conducted using 50 mL humic acid solutions prepared at DOC concentration of 5.0 mg-C/L and then being spiked with IO 3 , NH2Cl and ZVI to the designed concentrations without pH buffer to avoid any potential interference in the reactions (Fan et al., 2006; Xie, 2005; Xie and Shang, 2005). For the experiments regarding the effect of phosphate concentration on I-THM formation, the initial pH of solutions were adjusted to 7 using small amounts of H2SO4 or NaOH to prevent the change of pH due to the addition of phosphate. For investigating the mass balance of IO 3 transformation by ZVI, IO3 and ZVI initial concentrations were increased to 12.7 mg-I/L (100 mM) and 5 g/L, respectively, to get the signals higher than the detection limits of I and HOI (see Section 2.3). In order to evaluate the risk of I-DBP formation under real drinking water treatment conditions, raw waters from Huangpu River (HR) and Taihu lake as well as the treated water (after coagulation and ultrafiltration but prior to disinfection) from a drinking water treatment plant (DWTP) were collected. The characteristics of the collected water samples were distinctively different, which were summarized in Table S1. The collected samples were filtered through 0.45 mm cellulose acetate membranes (Millipore Corp., Billerica, MA) and stored at 4 C in dark. Only 12.7 mg-I/L (0.1 mM) IO 3 was spiked in the real water samples.
2.1. Chemicals and reagents 2.3. Analytical methods All chemicals were at least of analytical grade except as noted. Commercial 4e4.99% sodium hypochlorite (NaOCl), iodoform (CHI3, 99.0%), ammonium chloride (NH4Cl), NaOH (98%), KH2PO4 (99.0%), KIO3 (99.0%), a-FeOOH, phenol (99.0%), 2-iodophenol (98.0%) and 4-iodophenol (99.0%) and Fe2O3 (99.0%) were purchased from Sigma-Aldrich (USA). The ZVI (99.99%) powder and humic acid (HA) used as the model NOM source in this study were also supplied by Sigma-Aldrich (St Louis, Missouri, USA). Five I-THM standards, including chlorodiiodomethane (CHClI2, 90e95%), dichloroiodomethane (CHCl2I, 95%), bromochloroiodomethane (CHBrClI, 95%), dibromoiodomethane (CHBr2I, 90e95%) and bromodiiodomethane (CHBrI2, 90e95%) were obtained from CanSyn Chemical Corp (Toronto, ON, Canada). Methyl tertbutyl ether (MtBE) was purchased from J.T. Baker (USA). Na2S2O4, Na2S2O3, KBr and H2SO4 were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. All solutions were prepared with ultra-pure water produced using a Milli-Q water purification system (Millipore, USA). A monochloramine (NH2Cl) solution was freshly prepared by adding NaOCl solution slowly into a stirred ammonium chloride (NH4Cl) solution with the Cl2/N molar ratio of 0.8 at pH 8.5 for 0.5e1 h (Xu
DOC was measured using a TOC-VCSH analyzer (Shimadzu, Japan) with the detection limit was 0.1 mg-C/L. Solution pH was measured using a pH meter (FE20-FiveEasy, Mettler Toledo, Switzerland), which was calibrated regularly with standard buffer solutions (pH ¼ 4.01, 7.00, 9.21, Mettler Toledo). Iodate (IO 3 ), iodide (I) and hypolodous acid (HOI) were analyzed using a UPLC (Waters, USA) equipped with an XTerra® MS C18 column (4.6 250 mm i.d., 5 mm film thickness, Waters, USA) and a UV detector. IO 3 was analyzed with the mobile phase of 35% acetonitrile and H3PO4 solution (0.1%), flow rate of 0.80 mL/min, column temperature of 25 C at the wavenumber of 209 nm (Sajonz et al., 2006). The mobile phase for I measurement was consisted of 2%/98% (v/v) acetonitrile and KH2PO4 solution (0.09 M) at a flow rate of 0.80 mL/ min and the wavenumber 223 nm (Tian et al., 2014). HOI was measured using excess phenol to form 2-iodophenol and 4iodophenol, which could be detected using UPLC (Bichsel and Von Gunten, 2000). The retention times for IO 3 , I , 2-iodophenol and 4-iodophenol were 3.24, 4.21, 10.89 and 13.02 min. The injec tion volume was 10 mL, and the detection limits of IO 3 , I and HOI were 10, 5 and 10 mg-I/L, respectively.
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The method for quantifying I-THM was modified from the USEPA Method 551.1 (Munch and Hautman, 1995) in our previous research (Ye et al., 2013, 2014). Samples were extracted using MtBE and the extracts were analyzed using a gas chromatograph (GC2010, Shimadzu, Japan) equipped with an electron capture detector (ECD), a mass spectrometry (MS) and a HP-5 capillary column (30 m 0.25 mm i.d., 0.25 mm film thickness, J&W, USA). One mL of MtBE extract was introduced splitlessly into the GC column. The temperature of the injector and ECD detector was being controlled at 200 and 290 C, respectively with the temperature program being set as: initial temperature of 40 C held for 10 min, ramping to 260 C at the rate of 15 C/min and holding for 10 min (Ye et al., 2013). The detection limits for CHCl2I, CHClBrI, CHBr2I, CHClI2, CHBrI2 and CHI3 were 0.10, 0.09, 0.10, 0.08, 0.10 and 0.08 mg/L, respectively. Monochloramine concentration was determined using the N, N-diethyl-pphenylenediamine (DPD) colorimetric method (APHA, 1998). Additionally, a one-way analysis of variance (ANOVA) test or an independent-samples t-test was conducted using SPSS Statistics (Version 22.0) to determine if the studied factors caused significantly difference in I-THM formation. 2.4. Solid-phase microextraction (SPME) procedures According to previous literature (Allard et al., 2012), SPME was an alternative method to extract and concentrate analytes when the concentration of I-THM was lower than the detection limit of GC-MS. 10 mL water sample was transferred into a 15 mL amber vial and quenched using Na2S2O3 (20% excess of initial NH2Cl concentration) after 3 d of reaction time. Anhydrous sodium sulfate (up to 2.5 g) heated at 400 C for 4 h was selected to salt out and then added to each sample. The sample was immediately stirred at 240 rpm at 65 C with the SPME fiber (DVB/CAR/PDMS, SigmaAldrich, USA) held in the headspace of the vial to allow the adsorption of the analytes. After the adsorption for 40 min, the fiber was placed into GC injector port and desorbed for 2 min at 220 C for analysis using GC-MS in splitless mode (Zhang et al., 2016). The detection limits for CHI3, CHBrI2, CHClI2, CHBr2I, CHClBrI and CHCl2I were 1, 2, 5, 2, 5 and 8 ng/L, respectively. 3. Results and discussion
Fig. 1. Effect of ZVI dosage on I-THM formation from IO 3 in humic acid synthetic water during chloramination. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/ L, initial pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
electrochemical reduction of IO 3 could be consisted of adsorption onto iron surface first and then reduction through electron transfer involving Reactions (1)e(5), where x0 is the reduction potential (Zumdahl and Susan, 2003; Xie, 2005; Wang et al., 2009). IO 3 transformation can be well described using pseudo-first-order reaction kinetics (the observed reaction rate constant, kobs being calculated as 0.006 min1 using the data shown in Fig. S2 with ZVI dosage ¼ 5 g/L and spiked IO 3 ¼ 12.7 mg-I/L). 0
Fe0 4Fe2þ þ 2e x ¼ 0:477V
(1)
Fe0 4Fe3þ þ 3e x0 ¼ 0:337V
(2)
þ 0 2þ IO þ 3H2 O 3 þ 3Fe þ 6H 4I þ 3Fe
(3)
2þ þ 3H2 O4I þ 6Fe3þ þ 6OH IO 3 þ 6Fe
(4)
0 3þ þ 6OH IO 3 þ 2Fe þ 3H2 O4I þ 2Fe
(5)
h i d½IO 3 ¼ kobs IO 3 dt
(6)
3.1. Effect of ZVI dosage on I-THM formation Fig. 1 shows the variation of I-THM formation at different ZVI dosages. As can be seen, all three I-THMs were formed in the presence of ZVI during chloramination of IO 3 -containing water. No I-THMs were detected without ZVI, which indicates that IO 3 is an ultimate stable iodine species in the absence of ZVI during chloramination (Bichsel and Von Gunten, 2000). From Fig. 1, the amount of I-THM (especially CHI3) formation increased abruptly to 113.3e155.6 mg/L as 0.05e2 g/L ZVI was dosed in the solution. According to the former studies, BrO 3 can be electrochemically reduced to Br by ZVI (Westerhoff, 2003; Xie and Shang, 2005), which can be well described using a pseudo-first-order kinetics model (Xie and Shang, 2007). Although little work has been done on the reduction of IO 3 by ZVI, we speculate that the reduction of IO 3 to I is very similar to that of BrO3 to Br by ZVI. In order to further verify this hypothesis, the mass balance of IO 3 transformation by ZVI in Milli-Q water (without NOM and chloramine) was examined with the results shown in Fig. S1 (ZVI dosage ¼ 5 g/L, spiked IO 3 ¼ 12.7 mg-I/L). In Fig. S1, only two iodine species (IO3 and I) were detected during reduction by ZVI. No HOI was detected in the system. IO 3 concentration kept decreasing while I concentration kept increasing as time went by with stable total iodine concentration, which verifies our hypothesis. The
Through Reactions (3)e(5), IO 3 can be reduced to I , the typical iodine source. Meanwhile, NH2Cl can rapidly oxidize I to HOI or other iodine reactive species (Reactions (7) and (8)), which could then react with NOM to form I-THMs (Reactions (9) and (10)) (Hua and Reckhow, 2007; Zhu and Zhang, 2016). Fig. S3 presents the mass balance of total iodine during IO 3 transformation by ZVI and chloramine. With the increase of reaction time, more IO 3 is gradually converted to I and then to HOI during chloramination. As shown in Fig. 1, with the increase of ZVI dosage, available surface area and reaction sites are increasing for IO 3 adsorption and conversion, which could explain the increasing of total I-THM formation, especially CHI3 (Xie and Shang, 2005; Guan et al., 2015). Furthermore, the results indicate that IO 3 is not a stable iodine source as usually expected and can ascribe to I-THM formation during chloramination in the presence of ZVI (Zhang et al., 2016).
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NH2 Cl þ I þ H2 O/HOI þ Cl þ NH3
(7)
NH2 Cl þ HOI/NHClI þ H2 O
(8)
HOI þ NOM/I THMs
(9)
NHClI þ NOM/I THMs þ NH2 Cl
(10)
3.2. Effect of NH2Cl and IO 3 concentrations on I-THM formation As shown in Fig. 2, the total formation of I-THM monotonously increased from 0 to 142.4 mg/L with increasing NH2Cl dosage from 0 to 20 mg-Cl2/L, which could be explained by more transformation of I to HOI or other reactive species by NH2Cl in the presence of ZVI (Figs. S1 and S3) that could react with NOM to generate more ITHMs. No I-THMs were detected without NH2Cl, which suggests that no intermediate product HOI were formed during the reduction of IO 3 by ZVI, which matches the results presented in Fig. S1. Moreover, compared with the results shown in Figs S1 and S3, the presence of chloramines can accelerate the transformation of IO 3 to I by ZVI (Fig. S3) compared with the control experiment without chloramines (Fig. S1). This phenomenon could be explained by more I consumption by chloramines to produce HOI or trace amounts of other iodine reactive species (e.g., NHClI) in the solution so as to facilitate the reduction of IO 3 by ZVI (Zhu and Zhang, 2016). The concentrations of CHCl2I, CHClI2 and CHI3 increased from 0 to 17.4 mg/L, 1.6e48.8 mg/L and 2.0e63.6 mg/L, respectively, as NH2Cl concentration increased from 1 to 20 mg-Cl2/L. CHI3 was the dominant I-THM species, which agrees with other studies regarding chloramination of I-containing waters (Bichsel and Von Gunten, 1999, 2000; Hua and Reckhow, 2007). Fig. 3 shows the effect of IO 3 concentration on I-THM formation during chloramination in humic acid synthetic water. As IO 3 concentration increased from 0 to 6.35 mg-I/L. I-THM formation increased rapidly from 0 to 220.8 mg/L (Fig. 3), especially for the species of CHI3 (from 0 to 166.84 mg/L) and CHClI2 (from 0 to
Fig. 2. Effect of NH2Cl concentration on I-THM formation from IO 3 during chloramination in humic acid synthetic water in the presence of ZVI. (Conditions: DOC ¼ 5.0 mg-C/L, IO3 concentration ¼ 1.27 mg-I/L, ZVI dosage ¼ 0.5 g/L, initial pH ¼ 6.7 ± 0.1 without buffer, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
Fig. 3. Effect of IO 3 concentration on I-THM formation from IO3 during chloramination in humic acid synthetic water. (Conditions: DOC ¼ 5.0 mg-C/L, ZVI dosage ¼ 0.5 g/L, initial pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
43.92 mg/L) because of the increasing transformation to I by ZVI (Fig. S1, Reactions (3)e(5)). Therefore, special attention should be paid to the treatment of IO 3 -containing waters during chloramination in cast iron pipes. 3.3. Effect of initial pH on I-THM formation Fig. 4 shows the effect of initial pH on I-THM formation during chloramination in humic acid synthetic water. As shown in Fig. 4, initial pH had a strong impact on the formation and distribution of I-THMs during chloramination of IO 3 -containing water. I-THM formation was minimum (37.2 mg/L) at pH 4, gradually increased to a maximum of 171.2 mg/L at pH 8, and finally decreased to 137.8 mg/L at pH 10. This trend is consistent with a previous study
Fig. 4. Effect of initial pH on I-THM formation from IO 3 in humic acid synthetic water during chloramination in the presence of ZVI. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/L, ZVI dosage ¼ 0.5 g/L, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
Y. Xia et al. / Water Research 124 (2017) 219e226
investigating I-THM formation during chloramination of ICMcontaining water (Ye et al., 2014). The reactions between chloramines and NOM involve monochloramine (NH2Cl), dichloramine (NHCl2), nitrogen trichloride (NCl3) and free chlorine, which exist synchronously and vary in concentration under different pH values (Jafvert and Valentine, 1992; USEPA, 1999). With the decrease of pH values, the decomposition and hydrolysis of NH2Cl will lead to the formation of NHCl2 (for 4 < pH < 7.2), NCl3 (for pH < 4), and free chlorine (USEPA, 1999), which could further oxide HOI to stable IO 3 (Bichsel and Von Gunten, 1999, 2000). Therefore, the substantial inhibition was found as pH decreased from 8 to 4. Moreover, NH2Cl is stable and the major species at pH > 8, which can only oxidize I to the reactive HOI without further oxidation to the stable IO 3 (Bichsel and Von Gunten, 1999; USEPA, 1999). As a result, alkaline conditions are more favorable for I-THM formation. Hua and Reckhow (2008) also demonstrated that the formation of I-THMs increased as pH increased from 5 to 10 during iodide chloramination. Nevertheless, a slight but not statistically significant decrease in I-THM formation (p ¼ 0.1787, Table S3) was observed when the pH further increased from 8 to 10. Indeed, several earlier research have suggested that BrO 3 reduction by ZVI is inhibited at pH > 7 (Xie and Shang, 2007; Wang et al., 2009; Zhang et al., 2014). As pH increased, the Reactions (3)e(5) proceed from right to left, which together with the precipitation of the formed iron-hydroxide on the surface of ZVI would hinder IO 3 reduction (Xie, 2005; Zhang et al., 2014). Another explanation for the suppression is the ionization of ZVI at pH 7e11, causing electron repulsion between ZVI and IO 3 so as to decrease the reduction of IO3 (Wang et al., 2009). Because high I-THM concentration was detected at circumneutral pH (Fig. 4), special attention should be paid to the risk of I-THM formation when water containing IO 3 is chloraminated and distributed in the systems using iron materials. 3.4. Effect of bromide and phosphate concentrations on I-THM formation Bromide (Br) naturally exists in the raw water and groundwater of coastal cities due to saltwater intrusion. Richardson et al. (2008) reported that the concentrations of Brand I ranged from
Fig. 5. Effect of Br/IO 3 molar ratio on I-THM formation from IO3 in humic acid synthetic water. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/ L(10 mM), ZVI dosage ¼ 0.5 g/L, initial pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
223
24 to 1120 mg/L and 0.4e104.2 mg/L, respectively, but the molar ratio of Br/I varied a lot from locations to locations. Therefore, the effect of Br/IO 3 molar ratio on I-THM formation from IO3 -containing water was investigated in this study, and the results are shown in Fig. 5. In general, increasing initial Br concentration enhance both the amount and species of I-THMs remarkably, especially for the brominated I-THMs (CHBr2I and CHBrI2), while the chlorinated I-THMs (CHCl2I and CHClI2) decreased as the Br/ IO 3 molar ratio increased from 0 to 20. One reason is the oxidation of Br by the ionized monochlorammonium ion (NH3Clþ), which leads to the production of NHBrCl (main species), NH2Br and NHBr2 through a series of reactions (Eqs. (S1)e(S9) in Table S2) (Trofe et al., 1980; Vikesland et al., 2001; Roux et al., 2012). At the same time, small amounts of HOBr could be produced by the hydrolysis and displacement of chloramines and bromamines (see Table. S2) (Bousher et al., 1989; Roux et al., 2012; Zhai et al., 2014). Hence, with the increase of Br, more brominated oxidant species (NHBrCl, HOBr, NH2Br and NHBr2) will be formed to react with NOM, leading to increasing amounts of brominated I-THMs (Jones et al., 2012). Besides, CHI3 was the predominant I-THM species regardless of the Br/IO 3 molar ratio, which was also reported by Hua and Reckhow (2008) for the chloramination of iodine-containing samples in the presence of Br. The result suggests that in the presence of sufficient HOI, iodine substitutes more efficiently in THM precursors than chlorine (Hua et al., 2006). It also suggests that HOI is more reactive than brominated oxidant species in substitution and addition reactions to form I-THMs during chloramination (Bichsel and Von Gunten, 2000; Hua et al., 2006). The impact of phosphate concentration on I-THM formation was investigated because phosphate is an essential nutrient needed for organisms in most ecosystems and is a common nonpoint-source pollutant in surface waters (Zhang et al., 2009). The experiments were performed at pH 7 after spiking phosphate and the results are shown in Fig. 6. I-THM formation significantly decreased from 126.5 to 10.4 mg/L as phosphate concentration increased from 0 to 10 mM, indicating that phosphate can substantially inhibit I-THM formation. Previous studies have reported that phosphate is a strong inhibitor in bromate, nitrate, and arsenic reduction by ZVI (Chunming Su and Puls, 2001; Su and Puls, 2004; Fan et al., 2006).
Fig. 6. Effect of phosphate concentration on I-THM formation from IO 3 in humic acid synthetic water. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/L, ZVI dosage ¼ 0.5 g/L, initial pH ¼ 7.0 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
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Xie and Shang (2007) demonstrated that BrO 3 reduction by ZVI is a surface-mediated process with possible adsorption onto iron surface. The primary reduction of IO 3 by ZVI was very similar to that of BrO 3 according to Section 3.1. Phosphate is a known inner-sphere complex-forming anion that is strongly adsorbed to iron surfaces or is coprecipitated to form discrete solid phases on the surfaces of ZVI, which can compete with IO 3 for adsorption sites and inhibit electron transfer from ZVI to IO 3 (Roberts et al., 2004; Xie and Shang, 2007). Therefore, the sharply decline of I-THM formation with increasing phosphate concentration is ascribable to decreasing efficiency of IO 3 reduction to I by ZVI.
3.5. Effect of ZVI and its iron corrosion scales on I-THM formation As we all known, unlined cast iron pipes have been used to transport portable water for a long time (Mcneiill and Edwards, 2001), which are very prone to electrochemical corrosion and are usually covered with deposits of corrosion products as time went by (Sarin et al., 2004). Typical iron corrosion products may be constitutive of goethite (a-FeOOH), lepidocrocite (g-FeOOH), hematite (Fe2O3) and magnetite (Fe3O4) (Wang et al., 2012). Also, the outer passive layer (red rust) of ZVI is mainly composed of FeOOH and Fe2O3 (Tang et al., 2006). Thereby, the effect of typical corrosion scales (a-FeOOH and Fe2O3) on the formation of I-THMs in the presence of ZVI were investigated, and the results are shown in Fig. 7. From Fig. 7, statistically significant increases in I-THM formation (134% and 127%, p < 0.01 in Table S3) were observed after the incorporation of a-FeOOH or Fe2O3 compared to that with ZVI alone. Nevertheless, no I-THMs formed in the presence of a-FeOOH or Fe2O3 alone, which confirms that these iron corrosion scales are not able to reduce IO 3 to I without ZVI. Former studies have demonstrated that IO3 can adsorb onto the surface of a-FeOOH and €tmark Fe2O3 by complexation reactions (Couture and Seitz, 1983; Sa et al., 1996; Nagata and Fukushi, 2010). Concomitantly, hydroxyl ion was released into solution through ligand exchange reactions during IO 3 adsorption to iron oxides, resulting in an increase in solution pH. Couture and Seitz (1983) interpreted that IO 3 could be
Fig. 7. Effect of typical corrosion scales on I-THM formation from IO 3 in humic acid synthetic water during chloramination. (Conditions: a-FeOOH ¼ 0.1 g/L, Fe2O3 ¼ 0.1 g/ L, DOC ¼ 5.0 mg-C/L, IO3 concentration ¼ 1.27 mg-I/L, pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
strongly adsorbed onto Fe2O3 by forming inner-sphere species (FeOIO2 bond), thereby releasing hydroxyl ion and leading to an increase in pH so as to the enhancing I-THM formation as discussed in previous Section 3.3. On the other hand, the adsorption of IO 3 on ZVI would lead to the release of ferrous ions (Reaction (3)). The ferrous ions could be absorbed and incorporated into the lattices of a-FeOOH and Fe2O3, which were then being converted to semiconductive goethite and hematite so as to permit electron transfer to IO 3 (Liu et al., 2006). Moreover, some researchers found that iron oxides could oxidize NOM into much smaller precursors and modify active sites on NOM (Hassan et al., 2006; Hu et al., 2016), which could enhance the formation of I-THM. Actually, the mechanism for enhancing I-THM formation by a-FeOOH and Fe2O3 is not distinct, there is still a need for further investigation. Also, the effects of some other typical corrosion scales are unknown and deserve further research. 3.6. I-THM formation from IO 3 in real drinking water treatment conditions The experimental conditions (NH2Cl, DOC and IO 3 concentrations) in the previous sections were expanded appropriately to emphasize the effects of the investigated factors on I-THM formation as well as to get detectable analytical results, as many other researchers did in the literature (Wang et al., 2014; Zhang et al., 2016). However, in real drinking water treatment processes, the NH2Cl dosage, DOC concentration and IO 3 level are usually less than 10 mg-Cl2/L (USEPA, 1999), 5 mg-C/L and several to dozens of mg-I/L, respectively (Snyder et al., 2005; Gong and Zhang, 2013). Therefore, additional experiments using low DOC, NH2Cl and IO 3 concentrations to address the importance of I-THM formation were conducted, and the results are shown in Fig. 8. As can be seen in Fig. 8, I-THM formation from IO 3 spiked real waters (12.7 mg-I/L) was detected during monochloramination for 3 days as 356.4 and 654.5 ng/L in HA synthetic waters (Situations A and B in Fig. 8), 2427.7 and 2176.3 ng/L in HR and DWTP raw waters (Situations C and D in Fig. 8) and 1950 ng/L in DWTP treated water. The yields of I-THMs in real water samples (Situations C, D, E) were much higher
Fig. 8. I-THM formation under realistic drinking water treatment conditions in humic acid (HA) synthetic, raw and treated waters from drinking water treatment plant (DWTP) with 12.7 mg-I/L IO 3 spiked. (A) HA synthetic water, DOC ¼ 1.20 mg-C/L; (B) HA synthetic water, DOC ¼ 4.04 mg-C/L; (C) Huangpu River raw water, DOC ¼ 4.04 mgC/L, Br ¼ 102.1 mg/L; (D) Raw water from DWTP, DOC ¼ 2.86 mg-C/L, Br ¼ 82.7 mg/L; (E) Treated water from DWTP, DOC ¼ 2.31 mg-C/L, Br ¼ 80.6 mg/L (Conditions: ZVI dosage ¼ 0.1 g/L, NH2Cl dosage ¼ 5 mg-Cl2/L, chloramination time ¼ 3 d and temperature ¼ 25 ± 1 C).
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than those in HA synthesized samples (Situations A and B), even for the raw waters with the same DOC concentration (Situations B and C). This can be explained by more easy incorporation of low-SUVA NOM (Fig. 8 and Table S1) with iodine to generate more I-THMs (Jones et al., 2012). Moreover, the total formation of I-THMs in the DWTP treated water decreased slightly (10.4%) compared to that in the DWTP raw water (Situations D and E). Therefore, it is crucial to adopt suitable technical treatment for the control of I-THM formation in DWTPs. 4. Conclusions This study reveals that highly cytotoxic I-THMs can be generated from IO 3 -containing water during chloramination in the presence of ZVI by the pathways of IO 3 reduction by ZVI and oxidation of reduction product (e.g., I) by chloramines in the following. I-THM formation can be further enhanced by increasing ZVI dosage, IO 3 and NH2Cl concentrations, Br /IO3 molar ratio as well as the presence of NOM and iron corrosion scales (goethite and hematite). As initial pH increased from 4 to 10, I-THM formation from IO 3 showed an increasing then slightly decreasing trend with the maximum at pH 8. The presence of phosphate caused a sharp decline of I-THM formation from IO 3 during chloramination, which is due to the competition of adsorption sites on ZVI surface with IO 3 and the hindrance of electron transfer from ZVI to IO 3 . Further more, chloramines can accelerate the conversion from IO 3 to I remarkably, which can be explained by the more consumption of I by chloramines to generate HOI or other reactive iodine species so as to accelerate the reduction of IO 3 by ZVI. Overall, this study highlights the potential for significant I-THM formation during chloramination of IO 3 -containing water in the presence of ZVI in water distribution systems using cast iron pipes. Further research regarding technical applications to remove IO 3 (and I ) is highly required for the prevention and control of I-THM formation. Acknowledgments This study was supported in part by the Natural Science Foundation of China (Nos. 51678354 and 51478323), the National Major Science and Technology Project of China (No. 2017ZX07207004), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRK16005) and the Ministry of Science and Technology, Taiwan (MOST-104-2221-E-327-001-MY3). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.watres.2017.07.059. References Allard, S., Charrois, J.W.A., Joll, C.A., Heitz, A., 2012. Simultaneous analysis of 10 trihalomethanes at nanogram per liter levels in water using solid-phase microextraction and gas chromatography mass-spectrometry. J. Chromatogr. A 1238, 15e21. Allard, S., Nottle, C.E., Chan, A., Joll, C., Gunten, U., 2013. Ozonation of iodidecontaining waters: selective oxidation of iodide to iodate with simultaneous minimization of bromate and I-THMs. Water Res. 47 (6), 1953e1960. Allard, S., Tan, J., Joll, C.A., Gunten, U., 2015. Mechanistic study on the formation of Cl-/Br-/I-trihalomethanes during chlorination/chloramination combined with a theoretical cytotoxicity evaluation. Environ. Sci. Technol. 49 (18), 11105e11114. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th Edition-4500-NO3-D Nitrate Electrode Method. American Public Health Association, Washington, DC. Bichsel, Y., Von Gunten, U., 1999. Oxidation of iodide and hypoiodous acid in the disinfection of natural waters. Environ. Sci. Technol. 33 (22), 4040e4045. Bichsel, Y., Von Gunten, U., 2000. Formation of iodo-trihalomethanes during disinfection and oxidation of iodide-containing waters. Environ. Sci. Technol. 34 (13), 2784e2791.
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Effects of ferrous ions on the reductive dechlorination of trichloroethylene by zero-valent iron. J. Hazard. Mater. 136 (3), 706e713. Mcneiill, L.S., Edwards, M., 2001. Iron pipe corrosion in distribution systems. Water Works Assoc. 93 (7), 88e100. Munch, D.J., Hautman, D.P., 1995. U.S. EPA Method 551.1: Determination of Chlorination Disinfection Byproducts, Chlorinated Solvents, and Halogenated Pesticides/Herbicides in Drinking Water by Liquid-liquid Extraction and Gas Chromatography with Electron-capture Detection. U.S. EPA, Cincinnati, OH. Nagata, T., Fukushi, K., 2010. Prediction of iodate adsorption and surface speciation on oxides by surface complexation modeling. Geochim. Cosmochim. Acta 74 (21), 6000e6013. Plewa, M.J., Wagner, E.D., Richardson Jr., S.D., Thruston, A., Woo, Y.T., Mckague, A.B., 2004. Chemical and biological characterization of newly discovered iodoacid drinking water disinfection byproducts. Environ. Sci. Technol. 38 (18), 4713e4722. Richardson, S.D., 2003. Disinfection by-products and other emerging contaminants in drinking water. Trends Anal. Chem. 22 (10), 666e684. Richardson, S.D., Plewa, M.J., Wagner, E.D., Schoeny, R., Demarini, D.M., 2007. Occurrence, genotoxicity, and carcinogenicity of regulated and emerging disinfection by-products in drinking water: a review and roadmap for research. Mutat. Res. Rev. Mutat. Res. 636 (3), 178e242. Richardson, S.D., Fasano, F., Ellington, J.J., Crumley, F.G., Buettner, K.M., Evans, J.J., Blount, B.C., Silva, L.K., Waite, T.J., Luther, G.W., McKague, A.B., Miltner, R.J., Wagner, E.D., Plewa, M.J., 2008. Occurrence and mammalian cell toxicity of iodinated disinfection byproducts in drinking water. Environ. Sci. Technol. 42 (22), 8330e8338. Richardson, S.D., Postigo, C., 2012. Drinking Water Disinfection By-products. Emerging Organic Contaminants and Human Health. Springer, pp. 93e137.
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Iodinated trihalomethane formation during chloramination of iodatecontaining waters in the presence of zero valent iron Ying Xia a, Yi-Li Lin b, Bin Xu a, *, Chen-Yan Hu c, Ze-Chen Gao a, Wen-Hai Chu a, Nai-Yun Gao a a
State Key Laboratory of Pollution Control and Resource Reuse, Institute of Disinfection By-product Control in Water Treatment, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 824, Taiwan, ROC c College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 May 2017 Received in revised form 20 July 2017 Accepted 23 July 2017 Available online 24 July 2017
Iodide (I) and iodinated X-ray contrast media (ICM) are the primary iodine sources for the formation of iodinated disinfection byproducts (I-DBPs), and iodate (IO 3 ) is believed to be a desired sink of iodine in water. This study found that highly cytotoxic iodinated trihalomethanes (I-THMs) also can be generated from iodate-containing waters (without any other iodine sources) in the presence of zero valent iron (ZVI) during chloramination, which could be a big issue in the wide usage of iron pipes. The effect of major factors including ZVI dosage, NH2Cl and IO 3 concentrations, initial pH, Br /IO3 molar ratio, phosphate concentration, iron corrosion scales (goethite and hematite) on the formation of I-THMs were investigated. Formation of I-THMs from IO 3 increased with the increase of ZVI dosage, IO3 and NH2Cl concentrations. Chloramines can also remarkably accelerate the reduction of IO 3 by ZVI. Peak I-THM formation was found at pH 8. As the Br/IO 3 molar ratio increased from 0 to 20, I-THM formation considerably enhanced, especially for the bromine-incorporated species. Goethite and hematite enhanced the formation of I-THMs in the presence of ZVI. Additionally, a significant suppression on ITHM formation was observed with the addition of phosphate. Considering that a large number of water distribution networks contain unlined cast iron pipes, transformation of IO 3 in the presence of ZVI during chloramination may contribute to the formation of I-THMs in such systems. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Iodinated trihalomethanes (I-THMs) Iodate (IO 3) Zero valent iron (ZVI) Chloramination Disinfection by-products (DBPs)
1. Introduction The disinfection of drinking-water has been named as a public health triumph of the twentieth century (Richardson, 2003). Before it had been used widely, millions of people died from waterborne illnesses. However, chemical disinfection has also raised an unintended health hazard due to the genotoxic and cytotoxic effects associated with disinfection by-products (DBPs) (Richardson et al., 2007). Over the past decades, regulated DBPs including trihalomethanes (THMs), haloacetic acids (HAAs), bromate and chlorate have aroused wide public concern. However, other emerging iodinated DBPs (I-DBPs) are reported to be more cytotoxic and genotoxic in mammalian cells compared to their chlorinated and brominated analogues (Plewa et al., 2004). For example,
* Corresponding author. E-mail address: [email protected] (B. Xu). http://dx.doi.org/10.1016/j.watres.2017.07.059 0043-1354/© 2017 Elsevier Ltd. All rights reserved.
iodoacetic acid was reported as the most cytotoxic, genotoxic, and potential carcinogenic DBP in mammalian cells (Richardson et al., 2008; Duirk et al., 2011; Richardson and Postigo, 2012; Wei et al., 2013). Additionally, iodinated trihalomethanes (I-THMs) were reported to be attributed to the issues of medicinal or pharmaceutical taste and odor. For instance, the odor and taste threshold concentrations of iodoform are 0.02 and 5 mg/L, respectively (Cancho et al., 2000; Hansson et al., 1987). Therefore, the formation of I-DBPs has raised great concern in recent years (Richardson et al., 2003; Ye et al., 2013; Wang et al., 2014; Zhang et al., 2016). Bichsel and Von Gunten (1999, 2000) reported that chloramination of iodate containing water can form a significant amount of I-THMs due to the formation of active hypolodous acid (HOI) that can react with natural organic matter (NOM), causing the formation of I-THMs. It is a common consensus that the main iodine species in water are iodide (I), iodate (IO 3 ) and iodinated organic compounds such as iodinated X-ray contrast media (ICM) (Bichsel and Von Gunten, 1999; Duirk et al., 2011; Hansen et al., 2011). IO 3 is preferentially
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recommended as an additive to salt for its stability by most health authorities to prevent iodine deficiency (Burgi et al., 2001) because it can rapidly be reduced to I in vivo by glutathione and will not cause toxicological problems (Taurog et al., 1966; Smith et al., 2010). Therefore, IO 3 is a preferred sink of iodine in drinking water (Allard et al., 2013) and has been detected in tap waters with concentrations at level of several to dozens of mg-I/L (Snyder et al., 2005; Gong and Zhang, 2013; Zhang et al., 2016). One of the optimal approaches proposed to mitigate the formation of I-DBP is to ensure full oxidation of the iodinated precursors, such as I (Bichsel and Von Gunten, 1999, 2000) and organo-iodine (Seitz et al., 2008; Wang et al., 2014), to IO 3 (Allard et al., 2015). Large amounts of unlined cast iron pipes have been used in water distribution systems for several centuries throughout the world (Mcneiill and Edwards, 2001; Tang et al., 2006; Husband and Boxall, 2011). However, they are very prone to corrosion, causing the release of ferrous and ferric ions as well as the formation of iron oxides, such as goethite (a-FeOOH) and hematite (Fe2O3), on the top layer of red rust (Tang et al., 2006; Zhu et al., 2014). Recently, chloramines have been utilized as a secondary disinfectant for their long-term stability and less formation of regulated DBPs (Diehl et al., 2000; Seidel et al., 2005; Shen and Andrews, 2011). Humic acid (HA), a major component of NOM, is also ubiquitous in drinking water (Li et al., 2016). Hence, it is an interesting scientific question to understand whether IO 3 can react with zero valent iron (ZVI) and/or its corrosion scales in the presence of chloramine and NOM to form iodinated trihalomethanes (I-THMs). Although IO 3 is widely perceived as nontoxic and stable, the possible trans formation of IO 3 to I by ZVI might make a contribution to the formation of I-THMs in source waters. At present, little is known about which parameters, if any, would affect the formation of ITHM in the presence of chloramine and ZVI. This information is significant to realize the formation of I-THM in both natural and engineered systems. The primary aim of this research was therefore to investigate the formation of I-THMs from IO 3 during chloramination in the presence of ZVI and/or its corrosion scales. Key parameters including ZVI dosage, NH2Cl and IO 3 concentrations, initial pH, Br /IO3 molar ratio and phosphate concentration were studied systematically. 2. Materials and methods
et al., 2012). 2.2. Experimental procedures Batch experiments were performed in duplicate under headspace-free conditions in 40-mL glass screw-cap amber vials with PTFE-lined septa. The vials were then placed into a constanttemperature shaker bath (25 ± 1 C, 150 rpm) in dark for 3 d. After the designed reaction time, 10 mL of the solution was withdrawn with a 20 mL gas-tight syringe into a 25 mL vial and filtered immediately through a glass fiber filter (GF/F, Whatman, USA) to remove ZVI particles before analysis. The filtered solution was quenched using Na2S2O3 with 20% excess of the initial NH2Cl concentration, and the samples were then extracted using MtBE immediately to avoid any chance of I-THM decomposition. Factors including ZVI dosage (0e2.0 g/L), NH2Cl and IO 3 concentrations (0 - 20 mg-Cl2/L and 0e6.35 mg-I/L (0e50 mM), respectively), Br/IO 3 molar ratio (0e20), initial pH (4e10) and phosphate concentration (0e10 mM) on I-THM formation from IO 3 were investigated. Typical experiments were conducted using 50 mL humic acid solutions prepared at DOC concentration of 5.0 mg-C/L and then being spiked with IO 3 , NH2Cl and ZVI to the designed concentrations without pH buffer to avoid any potential interference in the reactions (Fan et al., 2006; Xie, 2005; Xie and Shang, 2005). For the experiments regarding the effect of phosphate concentration on I-THM formation, the initial pH of solutions were adjusted to 7 using small amounts of H2SO4 or NaOH to prevent the change of pH due to the addition of phosphate. For investigating the mass balance of IO 3 transformation by ZVI, IO3 and ZVI initial concentrations were increased to 12.7 mg-I/L (100 mM) and 5 g/L, respectively, to get the signals higher than the detection limits of I and HOI (see Section 2.3). In order to evaluate the risk of I-DBP formation under real drinking water treatment conditions, raw waters from Huangpu River (HR) and Taihu lake as well as the treated water (after coagulation and ultrafiltration but prior to disinfection) from a drinking water treatment plant (DWTP) were collected. The characteristics of the collected water samples were distinctively different, which were summarized in Table S1. The collected samples were filtered through 0.45 mm cellulose acetate membranes (Millipore Corp., Billerica, MA) and stored at 4 C in dark. Only 12.7 mg-I/L (0.1 mM) IO 3 was spiked in the real water samples.
2.1. Chemicals and reagents 2.3. Analytical methods All chemicals were at least of analytical grade except as noted. Commercial 4e4.99% sodium hypochlorite (NaOCl), iodoform (CHI3, 99.0%), ammonium chloride (NH4Cl), NaOH (98%), KH2PO4 (99.0%), KIO3 (99.0%), a-FeOOH, phenol (99.0%), 2-iodophenol (98.0%) and 4-iodophenol (99.0%) and Fe2O3 (99.0%) were purchased from Sigma-Aldrich (USA). The ZVI (99.99%) powder and humic acid (HA) used as the model NOM source in this study were also supplied by Sigma-Aldrich (St Louis, Missouri, USA). Five I-THM standards, including chlorodiiodomethane (CHClI2, 90e95%), dichloroiodomethane (CHCl2I, 95%), bromochloroiodomethane (CHBrClI, 95%), dibromoiodomethane (CHBr2I, 90e95%) and bromodiiodomethane (CHBrI2, 90e95%) were obtained from CanSyn Chemical Corp (Toronto, ON, Canada). Methyl tertbutyl ether (MtBE) was purchased from J.T. Baker (USA). Na2S2O4, Na2S2O3, KBr and H2SO4 were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) without further purification. All solutions were prepared with ultra-pure water produced using a Milli-Q water purification system (Millipore, USA). A monochloramine (NH2Cl) solution was freshly prepared by adding NaOCl solution slowly into a stirred ammonium chloride (NH4Cl) solution with the Cl2/N molar ratio of 0.8 at pH 8.5 for 0.5e1 h (Xu
DOC was measured using a TOC-VCSH analyzer (Shimadzu, Japan) with the detection limit was 0.1 mg-C/L. Solution pH was measured using a pH meter (FE20-FiveEasy, Mettler Toledo, Switzerland), which was calibrated regularly with standard buffer solutions (pH ¼ 4.01, 7.00, 9.21, Mettler Toledo). Iodate (IO 3 ), iodide (I) and hypolodous acid (HOI) were analyzed using a UPLC (Waters, USA) equipped with an XTerra® MS C18 column (4.6 250 mm i.d., 5 mm film thickness, Waters, USA) and a UV detector. IO 3 was analyzed with the mobile phase of 35% acetonitrile and H3PO4 solution (0.1%), flow rate of 0.80 mL/min, column temperature of 25 C at the wavenumber of 209 nm (Sajonz et al., 2006). The mobile phase for I measurement was consisted of 2%/98% (v/v) acetonitrile and KH2PO4 solution (0.09 M) at a flow rate of 0.80 mL/ min and the wavenumber 223 nm (Tian et al., 2014). HOI was measured using excess phenol to form 2-iodophenol and 4iodophenol, which could be detected using UPLC (Bichsel and Von Gunten, 2000). The retention times for IO 3 , I , 2-iodophenol and 4-iodophenol were 3.24, 4.21, 10.89 and 13.02 min. The injec tion volume was 10 mL, and the detection limits of IO 3 , I and HOI were 10, 5 and 10 mg-I/L, respectively.
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The method for quantifying I-THM was modified from the USEPA Method 551.1 (Munch and Hautman, 1995) in our previous research (Ye et al., 2013, 2014). Samples were extracted using MtBE and the extracts were analyzed using a gas chromatograph (GC2010, Shimadzu, Japan) equipped with an electron capture detector (ECD), a mass spectrometry (MS) and a HP-5 capillary column (30 m 0.25 mm i.d., 0.25 mm film thickness, J&W, USA). One mL of MtBE extract was introduced splitlessly into the GC column. The temperature of the injector and ECD detector was being controlled at 200 and 290 C, respectively with the temperature program being set as: initial temperature of 40 C held for 10 min, ramping to 260 C at the rate of 15 C/min and holding for 10 min (Ye et al., 2013). The detection limits for CHCl2I, CHClBrI, CHBr2I, CHClI2, CHBrI2 and CHI3 were 0.10, 0.09, 0.10, 0.08, 0.10 and 0.08 mg/L, respectively. Monochloramine concentration was determined using the N, N-diethyl-pphenylenediamine (DPD) colorimetric method (APHA, 1998). Additionally, a one-way analysis of variance (ANOVA) test or an independent-samples t-test was conducted using SPSS Statistics (Version 22.0) to determine if the studied factors caused significantly difference in I-THM formation. 2.4. Solid-phase microextraction (SPME) procedures According to previous literature (Allard et al., 2012), SPME was an alternative method to extract and concentrate analytes when the concentration of I-THM was lower than the detection limit of GC-MS. 10 mL water sample was transferred into a 15 mL amber vial and quenched using Na2S2O3 (20% excess of initial NH2Cl concentration) after 3 d of reaction time. Anhydrous sodium sulfate (up to 2.5 g) heated at 400 C for 4 h was selected to salt out and then added to each sample. The sample was immediately stirred at 240 rpm at 65 C with the SPME fiber (DVB/CAR/PDMS, SigmaAldrich, USA) held in the headspace of the vial to allow the adsorption of the analytes. After the adsorption for 40 min, the fiber was placed into GC injector port and desorbed for 2 min at 220 C for analysis using GC-MS in splitless mode (Zhang et al., 2016). The detection limits for CHI3, CHBrI2, CHClI2, CHBr2I, CHClBrI and CHCl2I were 1, 2, 5, 2, 5 and 8 ng/L, respectively. 3. Results and discussion
Fig. 1. Effect of ZVI dosage on I-THM formation from IO 3 in humic acid synthetic water during chloramination. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/ L, initial pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
electrochemical reduction of IO 3 could be consisted of adsorption onto iron surface first and then reduction through electron transfer involving Reactions (1)e(5), where x0 is the reduction potential (Zumdahl and Susan, 2003; Xie, 2005; Wang et al., 2009). IO 3 transformation can be well described using pseudo-first-order reaction kinetics (the observed reaction rate constant, kobs being calculated as 0.006 min1 using the data shown in Fig. S2 with ZVI dosage ¼ 5 g/L and spiked IO 3 ¼ 12.7 mg-I/L). 0
Fe0 4Fe2þ þ 2e x ¼ 0:477V
(1)
Fe0 4Fe3þ þ 3e x0 ¼ 0:337V
(2)
þ 0 2þ IO þ 3H2 O 3 þ 3Fe þ 6H 4I þ 3Fe
(3)
2þ þ 3H2 O4I þ 6Fe3þ þ 6OH IO 3 þ 6Fe
(4)
0 3þ þ 6OH IO 3 þ 2Fe þ 3H2 O4I þ 2Fe
(5)
h i d½IO 3 ¼ kobs IO 3 dt
(6)
3.1. Effect of ZVI dosage on I-THM formation Fig. 1 shows the variation of I-THM formation at different ZVI dosages. As can be seen, all three I-THMs were formed in the presence of ZVI during chloramination of IO 3 -containing water. No I-THMs were detected without ZVI, which indicates that IO 3 is an ultimate stable iodine species in the absence of ZVI during chloramination (Bichsel and Von Gunten, 2000). From Fig. 1, the amount of I-THM (especially CHI3) formation increased abruptly to 113.3e155.6 mg/L as 0.05e2 g/L ZVI was dosed in the solution. According to the former studies, BrO 3 can be electrochemically reduced to Br by ZVI (Westerhoff, 2003; Xie and Shang, 2005), which can be well described using a pseudo-first-order kinetics model (Xie and Shang, 2007). Although little work has been done on the reduction of IO 3 by ZVI, we speculate that the reduction of IO 3 to I is very similar to that of BrO3 to Br by ZVI. In order to further verify this hypothesis, the mass balance of IO 3 transformation by ZVI in Milli-Q water (without NOM and chloramine) was examined with the results shown in Fig. S1 (ZVI dosage ¼ 5 g/L, spiked IO 3 ¼ 12.7 mg-I/L). In Fig. S1, only two iodine species (IO3 and I) were detected during reduction by ZVI. No HOI was detected in the system. IO 3 concentration kept decreasing while I concentration kept increasing as time went by with stable total iodine concentration, which verifies our hypothesis. The
Through Reactions (3)e(5), IO 3 can be reduced to I , the typical iodine source. Meanwhile, NH2Cl can rapidly oxidize I to HOI or other iodine reactive species (Reactions (7) and (8)), which could then react with NOM to form I-THMs (Reactions (9) and (10)) (Hua and Reckhow, 2007; Zhu and Zhang, 2016). Fig. S3 presents the mass balance of total iodine during IO 3 transformation by ZVI and chloramine. With the increase of reaction time, more IO 3 is gradually converted to I and then to HOI during chloramination. As shown in Fig. 1, with the increase of ZVI dosage, available surface area and reaction sites are increasing for IO 3 adsorption and conversion, which could explain the increasing of total I-THM formation, especially CHI3 (Xie and Shang, 2005; Guan et al., 2015). Furthermore, the results indicate that IO 3 is not a stable iodine source as usually expected and can ascribe to I-THM formation during chloramination in the presence of ZVI (Zhang et al., 2016).
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NH2 Cl þ I þ H2 O/HOI þ Cl þ NH3
(7)
NH2 Cl þ HOI/NHClI þ H2 O
(8)
HOI þ NOM/I THMs
(9)
NHClI þ NOM/I THMs þ NH2 Cl
(10)
3.2. Effect of NH2Cl and IO 3 concentrations on I-THM formation As shown in Fig. 2, the total formation of I-THM monotonously increased from 0 to 142.4 mg/L with increasing NH2Cl dosage from 0 to 20 mg-Cl2/L, which could be explained by more transformation of I to HOI or other reactive species by NH2Cl in the presence of ZVI (Figs. S1 and S3) that could react with NOM to generate more ITHMs. No I-THMs were detected without NH2Cl, which suggests that no intermediate product HOI were formed during the reduction of IO 3 by ZVI, which matches the results presented in Fig. S1. Moreover, compared with the results shown in Figs S1 and S3, the presence of chloramines can accelerate the transformation of IO 3 to I by ZVI (Fig. S3) compared with the control experiment without chloramines (Fig. S1). This phenomenon could be explained by more I consumption by chloramines to produce HOI or trace amounts of other iodine reactive species (e.g., NHClI) in the solution so as to facilitate the reduction of IO 3 by ZVI (Zhu and Zhang, 2016). The concentrations of CHCl2I, CHClI2 and CHI3 increased from 0 to 17.4 mg/L, 1.6e48.8 mg/L and 2.0e63.6 mg/L, respectively, as NH2Cl concentration increased from 1 to 20 mg-Cl2/L. CHI3 was the dominant I-THM species, which agrees with other studies regarding chloramination of I-containing waters (Bichsel and Von Gunten, 1999, 2000; Hua and Reckhow, 2007). Fig. 3 shows the effect of IO 3 concentration on I-THM formation during chloramination in humic acid synthetic water. As IO 3 concentration increased from 0 to 6.35 mg-I/L. I-THM formation increased rapidly from 0 to 220.8 mg/L (Fig. 3), especially for the species of CHI3 (from 0 to 166.84 mg/L) and CHClI2 (from 0 to
Fig. 2. Effect of NH2Cl concentration on I-THM formation from IO 3 during chloramination in humic acid synthetic water in the presence of ZVI. (Conditions: DOC ¼ 5.0 mg-C/L, IO3 concentration ¼ 1.27 mg-I/L, ZVI dosage ¼ 0.5 g/L, initial pH ¼ 6.7 ± 0.1 without buffer, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
Fig. 3. Effect of IO 3 concentration on I-THM formation from IO3 during chloramination in humic acid synthetic water. (Conditions: DOC ¼ 5.0 mg-C/L, ZVI dosage ¼ 0.5 g/L, initial pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
43.92 mg/L) because of the increasing transformation to I by ZVI (Fig. S1, Reactions (3)e(5)). Therefore, special attention should be paid to the treatment of IO 3 -containing waters during chloramination in cast iron pipes. 3.3. Effect of initial pH on I-THM formation Fig. 4 shows the effect of initial pH on I-THM formation during chloramination in humic acid synthetic water. As shown in Fig. 4, initial pH had a strong impact on the formation and distribution of I-THMs during chloramination of IO 3 -containing water. I-THM formation was minimum (37.2 mg/L) at pH 4, gradually increased to a maximum of 171.2 mg/L at pH 8, and finally decreased to 137.8 mg/L at pH 10. This trend is consistent with a previous study
Fig. 4. Effect of initial pH on I-THM formation from IO 3 in humic acid synthetic water during chloramination in the presence of ZVI. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/L, ZVI dosage ¼ 0.5 g/L, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
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investigating I-THM formation during chloramination of ICMcontaining water (Ye et al., 2014). The reactions between chloramines and NOM involve monochloramine (NH2Cl), dichloramine (NHCl2), nitrogen trichloride (NCl3) and free chlorine, which exist synchronously and vary in concentration under different pH values (Jafvert and Valentine, 1992; USEPA, 1999). With the decrease of pH values, the decomposition and hydrolysis of NH2Cl will lead to the formation of NHCl2 (for 4 < pH < 7.2), NCl3 (for pH < 4), and free chlorine (USEPA, 1999), which could further oxide HOI to stable IO 3 (Bichsel and Von Gunten, 1999, 2000). Therefore, the substantial inhibition was found as pH decreased from 8 to 4. Moreover, NH2Cl is stable and the major species at pH > 8, which can only oxidize I to the reactive HOI without further oxidation to the stable IO 3 (Bichsel and Von Gunten, 1999; USEPA, 1999). As a result, alkaline conditions are more favorable for I-THM formation. Hua and Reckhow (2008) also demonstrated that the formation of I-THMs increased as pH increased from 5 to 10 during iodide chloramination. Nevertheless, a slight but not statistically significant decrease in I-THM formation (p ¼ 0.1787, Table S3) was observed when the pH further increased from 8 to 10. Indeed, several earlier research have suggested that BrO 3 reduction by ZVI is inhibited at pH > 7 (Xie and Shang, 2007; Wang et al., 2009; Zhang et al., 2014). As pH increased, the Reactions (3)e(5) proceed from right to left, which together with the precipitation of the formed iron-hydroxide on the surface of ZVI would hinder IO 3 reduction (Xie, 2005; Zhang et al., 2014). Another explanation for the suppression is the ionization of ZVI at pH 7e11, causing electron repulsion between ZVI and IO 3 so as to decrease the reduction of IO3 (Wang et al., 2009). Because high I-THM concentration was detected at circumneutral pH (Fig. 4), special attention should be paid to the risk of I-THM formation when water containing IO 3 is chloraminated and distributed in the systems using iron materials. 3.4. Effect of bromide and phosphate concentrations on I-THM formation Bromide (Br) naturally exists in the raw water and groundwater of coastal cities due to saltwater intrusion. Richardson et al. (2008) reported that the concentrations of Brand I ranged from
Fig. 5. Effect of Br/IO 3 molar ratio on I-THM formation from IO3 in humic acid synthetic water. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/ L(10 mM), ZVI dosage ¼ 0.5 g/L, initial pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
223
24 to 1120 mg/L and 0.4e104.2 mg/L, respectively, but the molar ratio of Br/I varied a lot from locations to locations. Therefore, the effect of Br/IO 3 molar ratio on I-THM formation from IO3 -containing water was investigated in this study, and the results are shown in Fig. 5. In general, increasing initial Br concentration enhance both the amount and species of I-THMs remarkably, especially for the brominated I-THMs (CHBr2I and CHBrI2), while the chlorinated I-THMs (CHCl2I and CHClI2) decreased as the Br/ IO 3 molar ratio increased from 0 to 20. One reason is the oxidation of Br by the ionized monochlorammonium ion (NH3Clþ), which leads to the production of NHBrCl (main species), NH2Br and NHBr2 through a series of reactions (Eqs. (S1)e(S9) in Table S2) (Trofe et al., 1980; Vikesland et al., 2001; Roux et al., 2012). At the same time, small amounts of HOBr could be produced by the hydrolysis and displacement of chloramines and bromamines (see Table. S2) (Bousher et al., 1989; Roux et al., 2012; Zhai et al., 2014). Hence, with the increase of Br, more brominated oxidant species (NHBrCl, HOBr, NH2Br and NHBr2) will be formed to react with NOM, leading to increasing amounts of brominated I-THMs (Jones et al., 2012). Besides, CHI3 was the predominant I-THM species regardless of the Br/IO 3 molar ratio, which was also reported by Hua and Reckhow (2008) for the chloramination of iodine-containing samples in the presence of Br. The result suggests that in the presence of sufficient HOI, iodine substitutes more efficiently in THM precursors than chlorine (Hua et al., 2006). It also suggests that HOI is more reactive than brominated oxidant species in substitution and addition reactions to form I-THMs during chloramination (Bichsel and Von Gunten, 2000; Hua et al., 2006). The impact of phosphate concentration on I-THM formation was investigated because phosphate is an essential nutrient needed for organisms in most ecosystems and is a common nonpoint-source pollutant in surface waters (Zhang et al., 2009). The experiments were performed at pH 7 after spiking phosphate and the results are shown in Fig. 6. I-THM formation significantly decreased from 126.5 to 10.4 mg/L as phosphate concentration increased from 0 to 10 mM, indicating that phosphate can substantially inhibit I-THM formation. Previous studies have reported that phosphate is a strong inhibitor in bromate, nitrate, and arsenic reduction by ZVI (Chunming Su and Puls, 2001; Su and Puls, 2004; Fan et al., 2006).
Fig. 6. Effect of phosphate concentration on I-THM formation from IO 3 in humic acid synthetic water. (Conditions: DOC ¼ 5.0 mg-C/L, IO 3 concentration ¼ 1.27 mg-I/L, ZVI dosage ¼ 0.5 g/L, initial pH ¼ 7.0 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
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Xie and Shang (2007) demonstrated that BrO 3 reduction by ZVI is a surface-mediated process with possible adsorption onto iron surface. The primary reduction of IO 3 by ZVI was very similar to that of BrO 3 according to Section 3.1. Phosphate is a known inner-sphere complex-forming anion that is strongly adsorbed to iron surfaces or is coprecipitated to form discrete solid phases on the surfaces of ZVI, which can compete with IO 3 for adsorption sites and inhibit electron transfer from ZVI to IO 3 (Roberts et al., 2004; Xie and Shang, 2007). Therefore, the sharply decline of I-THM formation with increasing phosphate concentration is ascribable to decreasing efficiency of IO 3 reduction to I by ZVI.
3.5. Effect of ZVI and its iron corrosion scales on I-THM formation As we all known, unlined cast iron pipes have been used to transport portable water for a long time (Mcneiill and Edwards, 2001), which are very prone to electrochemical corrosion and are usually covered with deposits of corrosion products as time went by (Sarin et al., 2004). Typical iron corrosion products may be constitutive of goethite (a-FeOOH), lepidocrocite (g-FeOOH), hematite (Fe2O3) and magnetite (Fe3O4) (Wang et al., 2012). Also, the outer passive layer (red rust) of ZVI is mainly composed of FeOOH and Fe2O3 (Tang et al., 2006). Thereby, the effect of typical corrosion scales (a-FeOOH and Fe2O3) on the formation of I-THMs in the presence of ZVI were investigated, and the results are shown in Fig. 7. From Fig. 7, statistically significant increases in I-THM formation (134% and 127%, p < 0.01 in Table S3) were observed after the incorporation of a-FeOOH or Fe2O3 compared to that with ZVI alone. Nevertheless, no I-THMs formed in the presence of a-FeOOH or Fe2O3 alone, which confirms that these iron corrosion scales are not able to reduce IO 3 to I without ZVI. Former studies have demonstrated that IO3 can adsorb onto the surface of a-FeOOH and €tmark Fe2O3 by complexation reactions (Couture and Seitz, 1983; Sa et al., 1996; Nagata and Fukushi, 2010). Concomitantly, hydroxyl ion was released into solution through ligand exchange reactions during IO 3 adsorption to iron oxides, resulting in an increase in solution pH. Couture and Seitz (1983) interpreted that IO 3 could be
Fig. 7. Effect of typical corrosion scales on I-THM formation from IO 3 in humic acid synthetic water during chloramination. (Conditions: a-FeOOH ¼ 0.1 g/L, Fe2O3 ¼ 0.1 g/ L, DOC ¼ 5.0 mg-C/L, IO3 concentration ¼ 1.27 mg-I/L, pH ¼ 6.7 ± 0.1 without buffer, NH2Cl dosage ¼ 10 mg-Cl2/L, chloramination time ¼ 3 d, temperature ¼ 25 ± 1 C).
strongly adsorbed onto Fe2O3 by forming inner-sphere species (FeOIO2 bond), thereby releasing hydroxyl ion and leading to an increase in pH so as to the enhancing I-THM formation as discussed in previous Section 3.3. On the other hand, the adsorption of IO 3 on ZVI would lead to the release of ferrous ions (Reaction (3)). The ferrous ions could be absorbed and incorporated into the lattices of a-FeOOH and Fe2O3, which were then being converted to semiconductive goethite and hematite so as to permit electron transfer to IO 3 (Liu et al., 2006). Moreover, some researchers found that iron oxides could oxidize NOM into much smaller precursors and modify active sites on NOM (Hassan et al., 2006; Hu et al., 2016), which could enhance the formation of I-THM. Actually, the mechanism for enhancing I-THM formation by a-FeOOH and Fe2O3 is not distinct, there is still a need for further investigation. Also, the effects of some other typical corrosion scales are unknown and deserve further research. 3.6. I-THM formation from IO 3 in real drinking water treatment conditions The experimental conditions (NH2Cl, DOC and IO 3 concentrations) in the previous sections were expanded appropriately to emphasize the effects of the investigated factors on I-THM formation as well as to get detectable analytical results, as many other researchers did in the literature (Wang et al., 2014; Zhang et al., 2016). However, in real drinking water treatment processes, the NH2Cl dosage, DOC concentration and IO 3 level are usually less than 10 mg-Cl2/L (USEPA, 1999), 5 mg-C/L and several to dozens of mg-I/L, respectively (Snyder et al., 2005; Gong and Zhang, 2013). Therefore, additional experiments using low DOC, NH2Cl and IO 3 concentrations to address the importance of I-THM formation were conducted, and the results are shown in Fig. 8. As can be seen in Fig. 8, I-THM formation from IO 3 spiked real waters (12.7 mg-I/L) was detected during monochloramination for 3 days as 356.4 and 654.5 ng/L in HA synthetic waters (Situations A and B in Fig. 8), 2427.7 and 2176.3 ng/L in HR and DWTP raw waters (Situations C and D in Fig. 8) and 1950 ng/L in DWTP treated water. The yields of I-THMs in real water samples (Situations C, D, E) were much higher
Fig. 8. I-THM formation under realistic drinking water treatment conditions in humic acid (HA) synthetic, raw and treated waters from drinking water treatment plant (DWTP) with 12.7 mg-I/L IO 3 spiked. (A) HA synthetic water, DOC ¼ 1.20 mg-C/L; (B) HA synthetic water, DOC ¼ 4.04 mg-C/L; (C) Huangpu River raw water, DOC ¼ 4.04 mgC/L, Br ¼ 102.1 mg/L; (D) Raw water from DWTP, DOC ¼ 2.86 mg-C/L, Br ¼ 82.7 mg/L; (E) Treated water from DWTP, DOC ¼ 2.31 mg-C/L, Br ¼ 80.6 mg/L (Conditions: ZVI dosage ¼ 0.1 g/L, NH2Cl dosage ¼ 5 mg-Cl2/L, chloramination time ¼ 3 d and temperature ¼ 25 ± 1 C).
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than those in HA synthesized samples (Situations A and B), even for the raw waters with the same DOC concentration (Situations B and C). This can be explained by more easy incorporation of low-SUVA NOM (Fig. 8 and Table S1) with iodine to generate more I-THMs (Jones et al., 2012). Moreover, the total formation of I-THMs in the DWTP treated water decreased slightly (10.4%) compared to that in the DWTP raw water (Situations D and E). Therefore, it is crucial to adopt suitable technical treatment for the control of I-THM formation in DWTPs. 4. Conclusions This study reveals that highly cytotoxic I-THMs can be generated from IO 3 -containing water during chloramination in the presence of ZVI by the pathways of IO 3 reduction by ZVI and oxidation of reduction product (e.g., I) by chloramines in the following. I-THM formation can be further enhanced by increasing ZVI dosage, IO 3 and NH2Cl concentrations, Br /IO3 molar ratio as well as the presence of NOM and iron corrosion scales (goethite and hematite). As initial pH increased from 4 to 10, I-THM formation from IO 3 showed an increasing then slightly decreasing trend with the maximum at pH 8. The presence of phosphate caused a sharp decline of I-THM formation from IO 3 during chloramination, which is due to the competition of adsorption sites on ZVI surface with IO 3 and the hindrance of electron transfer from ZVI to IO 3 . Further more, chloramines can accelerate the conversion from IO 3 to I remarkably, which can be explained by the more consumption of I by chloramines to generate HOI or other reactive iodine species so as to accelerate the reduction of IO 3 by ZVI. Overall, this study highlights the potential for significant I-THM formation during chloramination of IO 3 -containing water in the presence of ZVI in water distribution systems using cast iron pipes. Further research regarding technical applications to remove IO 3 (and I ) is highly required for the prevention and control of I-THM formation. Acknowledgments This study was supported in part by the Natural Science Foundation of China (Nos. 51678354 and 51478323), the National Major Science and Technology Project of China (No. 2017ZX07207004), State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRK16005) and the Ministry of Science and Technology, Taiwan (MOST-104-2221-E-327-001-MY3). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.watres.2017.07.059. References Allard, S., Charrois, J.W.A., Joll, C.A., Heitz, A., 2012. Simultaneous analysis of 10 trihalomethanes at nanogram per liter levels in water using solid-phase microextraction and gas chromatography mass-spectrometry. J. Chromatogr. A 1238, 15e21. Allard, S., Nottle, C.E., Chan, A., Joll, C., Gunten, U., 2013. Ozonation of iodidecontaining waters: selective oxidation of iodide to iodate with simultaneous minimization of bromate and I-THMs. Water Res. 47 (6), 1953e1960. Allard, S., Tan, J., Joll, C.A., Gunten, U., 2015. Mechanistic study on the formation of Cl-/Br-/I-trihalomethanes during chlorination/chloramination combined with a theoretical cytotoxicity evaluation. Environ. Sci. Technol. 49 (18), 11105e11114. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th Edition-4500-NO3-D Nitrate Electrode Method. American Public Health Association, Washington, DC. Bichsel, Y., Von Gunten, U., 1999. Oxidation of iodide and hypoiodous acid in the disinfection of natural waters. Environ. Sci. Technol. 33 (22), 4040e4045. Bichsel, Y., Von Gunten, U., 2000. Formation of iodo-trihalomethanes during disinfection and oxidation of iodide-containing waters. Environ. Sci. Technol. 34 (13), 2784e2791.
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