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Formation of iodinated trihalomethanes during breakpoint chlorination of iodide-containing water
Journal of Hazardous Materials 353 (2018) 505–513
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Formation of iodinated trihalomethanes during breakpoint chlorination of iodide-containing water
T
⁎
Zhi Liua,b, Yi-Li Linc, Bin Xua,b, , Chen-Yan Hud, An-Qi Wanga, Ze-Chen Gaoa, Sheng-Ji Xiaa, Nai-Yun Gaoa a
State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China c Department of Safety, Health and Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 824, Taiwan, ROC d College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Iodinated trihalomethanes (I-THMs) Breakpoint chlorination Disinfection by-products (DBPs) Monochloramine Residual oxidant
This study investigated the formation of toxic iodinated trihalomethanes (I-THMs) during breakpoint chlorination of iodide-containing water. Impact factors including I− concentration, natural organic matter (NOM) concentration and type, pH as well as Br−/I− molar ratio were systematically investigated. Moreover, the incorporation of I− into I-THM formation was also calculated. The results showed that I-THM formation varied in different zones of the breakpoint curves. I-THMs increased with increasing chlorine dosage to breakpoint value and then dropped significantly beyond it. Iodoform (CHI3) and chlorodiiodomethane (CHClI2) were the major ITHMs in the pre-breakpoint zone, while dichloroiodomethane (CHCl2I) was the dominant one in the postbreakpoint zone. The formation of I-THMs increased remarkably with I− and dissolved organic carbon (DOC) concentrations. More bromine-containing species were formed as Br−/I− molar ratio increased from 0.5 to 5. In addition, the major I-THM compound shifted from CHCl2I to the more toxic CHClBrI. As pH increased from 6.0 to 8.0, I-THM formation kept increasing in the pre-breakpoint zone and the speciation of I-THMs changed
⁎ Corresponding author at: State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China. E-mail address: [email protected] (B. Xu).
https://doi.org/10.1016/j.jhazmat.2018.04.009 Received 30 August 2017; Received in revised form 2 April 2018; Accepted 4 April 2018 Available online 10 April 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
Journal of Hazardous Materials 353 (2018) 505–513
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alongside the breakpoint curves. The incorporation of I− during breakpoint chlorination was highly dependent on chlorine, I−, and NOM concentrations, NOM type, solution pH and Br−/I− molar ratio.
1. Introduction
monochloramine (Eqs. (1) and (2)).
As one of the main chemical disinfectants, chlorine is economical, flexible and efficient to inactivate pathogenic microorganism in water treatment plants. However, chlorine can also react with organic or inorganic precursors, resulting in the formation of various disinfection byproducts (DBPs) [1]. Chloroform (CHCl3) and other trihalomethanes (THMs), including bromodichloromethane (CHCl2Br), dibromochloromethane (CHClBr2) and bromoform (CHBr3), were identified as DBPs in chlorinated drinking water in 1974, which led to the establishment of relevant regulations in the U.S. in 1979 [2]. In the past 30 years, researches regarding the formation of DBPs as well as the impact of DBPs on human health were flourishing [3]. Although more than 500 DBPs were identified to date [3], extensive investigations were mainly concentrated on the brominated and chlorinated THMs and haloacetic acids (HAAs) [4–6] that were currently regulated by the U.S. Environmental Protection Agency (U.S. EPA) [2]. Iodinated THMs (I-THMs), including iodoform (CHI3), chlorodiiodomethane (CHClI2), dichloroiodomethane (CHCl2I), bromochloroiodomethane (CHClBrI), dibromoiodomethane (CHBr2I) and bromodiiodomethane (CHBrI2), were identified in drinking water as early as 1975 [7]. Although I-THMs are more geno- and cytotoxic than their brominated or chlorinated analogs [8], they have been neither broadly measured nor regulated because their concentrations (maximum level of 19 μg/L) were lower than those of the regulated THMs (maximum level of 164 μg/L) according to a previous study [9]. However, I-THMs, especially CHI3, can result in medicinal odors and tastes in drinking water because of their low threshold concentrations (0.02 to 5 μg/L) [10]. Apart from I-THMs, other iodinated-DBPs (I-DBPs) have also been detected in drinking water in recent years, including iodinated haloacetamides (I-HAcAms) [11], iodinated haloacetic acids (IHAAs) [9] and polar I-DBPs [12]. Total iodine concentrations in the source water usually range from 0.5 to 20 μg/L and can exceed 50 μg/L in certain surface or ground water due to seawater intrusion [13,14]. During the disinfection process using chlorine, monochloramine (NH2Cl) or ozone, iodide (I−) present in source water can be rapidly oxidized to hypoiodous acid (HOI) [10,15,16], which can either be further oxidized to iodate (IO3−) [17], a stable and nontoxic form of iodine in drinking water, or react with natural organic matter (NOM) and result in the formation of various I-DBPs [13,18]. By comparing I-DBP concentrations after disinfection of iodide-containing water using chlorine, ozone, chlorine dioxide and monochloramine, it concludes that monochloramine promotes the formation of I-DBPs, especially CHI3 [19]. The formation of I-THMs during chlorination is affected by many factors, such as ammonia nitrogen, chlorine, I− and Br− concentrations and solution pH [20–22]. Furthermore, the speciation and formation of I-THMs are highly affected by ammonia concentration during breakpoint chlorination of natural water [23]. However, less attention was paid on the formation of I-THMs so far during breakpoint chlorination of natural water. Recently, Zhang et al. [10] and Liu et al. [18] studied the formation of I-THMs in natural water spiked with I− during chlorination and chloramination, respectively. However, the formation of I-THMs in different zones of breakpoint chlorination curve remains unclear. Breakpoint reaction requires a theoretical Cl2: NH3-N molar ratio of 1.5 [24]. The breakpoint reaction is affected by several factors, including temperature, solution pH and chlorine concentration [24]. There are three stages in breakpoint chlorination. The first stage is the reaction between ammonia and hypochlorous acid (HOCl) to yield
Cl2 + H2O → HOCl + H+ + Cl−
(1)
HOCl + NH3 → NH2Cl + H2O
(2)
The second stage is the formation of dichloramine and trichloramine (Eqs. (3)–(4)). HOCl + NH2Cl → NHCl2 + H2O
(3)
HOCl + NHCl2 → NCl3 + H2O
(4)
The third stage is called breakpoint, in which ammonia nitrogen reacts with chlorine completely with minimum free chlorine in water. The reaction can be summarized in Eq. (5). 3HOCl + 2NH3 → N2 + 3H+ + 3Cl− + 3H2O
(5)
Generally, breakpoint chlorination involves the reactions between chlorine and ammonia [24], which eliminates ammonia in raw water [25]. Little information is available regarding the effect of breakpoint chlorination in the treatment of iodide-containing water. Yang et al. [26] reported that the distributions and concentrations of THM and HAA species varied in different zones of the breakpoint curves. Additionally, Charrois and Hrudey [27] indicated that peak N-Nitrosodimethylamine (NDMA) formation occurred in the pre-breakpoint region. To the authors’ best knowledge, no information indicates I-THM formation during breakpoint chlorination. Therefore, the objective of this study was to investigate the formation of I-THMs during breakpoint chlorination of iodide-containing water. The effects of I− and NOM concentrations, NOM type, solution pH, and bromide to iodide molar ratio (Br−/I−) on I-THM formation were evaluated. The results were further calculated for iodine incorporation factor (IIF) to get a deeper insight of I- incorporation in ITHM formation. 2. Materials and methods 2.1. Chemicals All reagents were of analytical grade and used without further purification. Potassium iodide (KI ≥ 99.0%), potassium bromide (KBr ≥ 99.0%) and sodium hypochlorite (NaOCl, 4.00–4.99%) were purchased from Sigma-Aldrich (USA). Five I-THM standards, including CHClI2, CHCl2I, CHClBrI, CHBr2I, and CHBrI2, were purchased from CanSym Chemical Corp. (Canada). Methanol and methyl tert-butyl ether (MTBE) were purchased from J.T. Baker (USA). Ammonium chlorine (NH4Cl), sodium hydroxide (NaOH), sulphuric acid (H2SO4), potassium dihydrogen phosphate (KH2PO4) and humic acid I (HA I) were obtained from Sigma-Aldrich (USA). Suwannee River humic acid II (HA II) and fulvic acid (FA) were obtained from International Humic Substances Society (IHSS). All experiments were carried out using ultrapure water produced from a Milli-Q water purification system (Millipore, USA). 2.2. Experimental procedures 2.2.1. Breakpoint chlorination experiments For breakpoint chlorination, 5 mgC/L NOM (HA I) samples were pre-ammoniated by dosing NH4Cl to 1.4 mg/L (0.1 mM). Then, an aliquot of stock NaOCl solution was immediately spiked into the samples to make the concentration ranging from 1 to 20 mg/L (0.014–0.28 mM). The reaction was last for 2 h at room temperature (25 ± 1 °C) and then 506
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the detection limit was 0.1 mgC/L. Solution pH was measured using a pH meter (FE20-FiveEasy, Mettler Toledo, Switzerland), which was calibrated using standard buffer solutions. The concentrations of chlorine and monochloramine were determined according to DPD colorimetric method [28]. I− and Br− concentrations were analyzed using an ion chromatograph (ICS-2000, Dionex, USA) equipped with an AS11-HC analytical column and AG11-HC guard column. I-THM quantification method was based upon our previous research with minor modification of the USEPA Method 551.1 [15,34]. Amber glassware was adopted in the experiments because I-THMs are lightly sensitive and can be decomposed over time in water [35]. 10 mL of each sample was extracted with 2 mL MTBE and analyzed using a gas chromatograph (GC-2010, Shimadzu, Japan) that was equipped with an electron capture detector (ECD) and a HP-5 capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness, J&W, USA). The detector and injector temperature were 290 and 200 °C, respectively. The nitrogen carrier gas was at the pressure of 69.8 kPa with the flow rate of 30 mL/ min. The temperature program was set at 40 °C for 10 min, ramped to 260 °C at 15 °C/min and then held for 10 min. Detection limits of CHCl2I, CHClBrI, CHBr2I, CHClI2, CHBrI2, CHI3 and CHCl3 were 0.10, 0.09, 0.08, 0.05, 0.05, 0.08 and 0.10 μg/L, respectively. Quantification limits for CHCl2I, CHClBrI, CHBr2I, CHClI2, CHBrI2, CHI3 and CHCl3 were 0.33, 0.30, 0.27, 0.17, 0.17, 0.27 and 0.33 μg/L, respectively.
the samples were analyzed for total and free chlorine residual concentrations using the DPD titration method [28]. Notably, free bromine, bromamines and iodine formed during chlorination of water samples can also react with DPD, resulting in the interference of chlorine residual measurement using the DPD titration method [29,30]. Therefore, chlorine residuals shown in Figs. 1–6 were displayed as 'residual oxidant'. 2.2.2. I-THM formation experiments I-THM formation experiments were performed in duplicate using 40-mL amber glass vials with PTFE-lined septum screw-caps under headspace-free condition at 25 ± 1 °C. Five influencing factors were studied, including I− concentration (0.64–1.91 mg/L), NOM concentration (3-7 mgC/L) and type (HA I, HA II and FA), solution pH (6.08.0) and Br−/I− molar ratio (0.5–5). For the experiments of different NOM types, HA I, HA II and FA were prepared at the same concentration of 5 mgC/L. Otherwise, HA I was the only NOM that was used in the experiments. Solution pH was controlled at 7.00 ± 0.02 using 10 mM phosphate buffer throughout the experiments except for the varying pH experiments, in which the pH was adjusted using either 1 M NaOH or H2SO4. In a typical run, 1.27 mg/L (10 μM) of I− was spiked into samples containing 5 mgC/L NOM [31] to amplify the formation of I-THMs for quantification in this study. Dosed I− concentration was much higher than the level in natural water. I-THM formation at different reaction time during breakpoint chlorination was presented in Fig. S1. Compared to the formation of I-THMs after 8-h chlorination, half of I-THMs were formed in the first 2-h and their concentrations were enough for detection. Therefore, 2-h reaction time was selected for the I-THM formation experiments. After reaching the planned reaction time (2 h), samples were withdrawn and quenched the residual chlorine using overdosed ascorbic acid ([ascorbic acid]: [chlorine] molar ratio of 1.5) [32,33]. Then the samples were extracted and analyzed for I-THM concentrations immediately to avoid any hydrolytic loss.
3. Results and discussion 3.1. I-THM formation during breakpoint chlorination The concentrations of three I-THMs were measured alongside the breakpoint curves. The Cl2: NH3-N molar ratio was 2.3 to achieve 2-h breakpoint chlorination of the samples. Barrett et al. [36] reported a range of breakpoint values with Cl2: NH3-N molar ratio of 1.4–3.2, which may also depend on the composition of the water samples. As displayed in Fig. 1, total amount of I-THM increased as chlorine concentration reached the breakpoint and then declined significantly as chlorine concentration kept increasing. Among three I-THMs, CHI3 and CHClI2 were the major species in the pre-breakpoint zone, while CHCl2I was the dominant species in the post-breakpoint. It is known that I− can be oxidized to HOI, but can’t be further oxidized to IO3− by NH2Cl.
2.3. Analytical methods NOM concentration was measured using a Shimadzu TOC-VCSH analyzer (Shimadzu, Japan) for dissolved organic carbon (DOC), and
Fig. 1. Effects of chlorine concentration on ITHM formation from I− containing water and residual oxidant concentration during breakpoint chlorination at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, [I−]0 = 1.27 mg/L (10 μM), reaction time = 2 h and 25 °C. Error bars represent the standard deviation of replicate measurements.
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Fig. 2. Effects of I− concentration on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, reaction time = 2 h and 25 °C. (a) [I−]0 = 0.64 mg/L (5 μM), (b) [I−]0 = 1.27 mg/L (10 μM), (c) [I−]0 = 1.91 mg/L (15 μM). Error bars represent the standard deviation of replicate measurements.
Fig. 3. Effects of NOM concentration on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [I−]0 = 1.27 mg/L (10 μM), reaction time = 2 h and 25 °C. (a) [DOC] = 3 mgC/L, (b) [DOC] = 5 mgC/L, (c) [DOC] = 7 mgC/L. Error bars represent the standard deviation of replicate measurements.
However, I− can be oxidized to IO3− by free chlorine, which shows stronger oxidation potential than NH2Cl [16]. As displayed in Fig. S2, NH2Cl is the main oxidant in the pre-breakpoint zone. Consequently, HOI can exist longer to react with NOM during chloramination compared to chlorination [13], which contributes to higher amount of ITHM formation, especially CHI3. With the increasing chlorine concentration, free residual oxidant became the main oxidant in the postbreakpoint zone, while NH2Cl concentration declined to zero (Fig. S2). More I− and HOI could be oxidized to stable IO3− by free chlorine,
resulting in the declination of I-THM formation [19] in the decreasing order of CHI3 > CHClI2 > CHCl2I. 3.2. Effect of I− concentration on I-THM formation The effect of I− concentration on I-THMs formation is displayed in Fig. 2, which indicated similar trend among each other. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination of the samples ranged from 2.2 to 2.3. With increasing I- level, higher chlorine dosage 508
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Fig. 4. Effects of NOM type on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [I−]0 = 1.27 mg/L (10 μM), [DOC] = 5 mgC/L, reaction time = 2 h and 25 °C. (a) HA I, (b) HA II, (c) FA. Error bars represent the standard deviation of replicate measurements.
Fig. 5. Effects of solution pH on the formation of I-THMs and residual oxidant concentration at [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, [I−]0 = 1.27 mg/L (10 μM), reaction time = 2 h and 25 °C. (a) pH = 6.0, (b) pH = 7.0, (c) pH = 8.0. Error bars represent the standard deviation of replicate measurements.
was consumed to oxidize I−, which slightly influenced the breakpoint. However, as displayed in Fig. 2, the amount of residual oxidant increased with I− concentration from 0.64 to 1.91 mg/L (5–15 μM) at the same dosage of chlorine because of the enhancing I2 interference of residual chlorine measurement based on DPD method with increasing I− addition [30]. Total amount of I-THM formation increased with the increasing I− concentration at the same chlorine concentration because the increasing addition of I− could lead to an increase formation of HOI to react with NOM [8,10]. The increase of CHI3, the major compound responsible for the taste and odor problems, was much more significant
compared to the other I-THMs as I− concentration increased in the prebreakpoint zone (Fig. 2(a)–(c)). In contrast, the concentration of CHI3 decreased much faster in the post-breakpoint zone because chlorine was more competitive to incorporate into THMs [13]. Therefore, the dominant I-THM compounds shifted from CHI3 and CHClI2 to CHCl2I at the chlorine dosage higher than breakpoint.
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precursors. The increases of CHCl2I and CHClI2 were higher compared to that of CHI3 with the increasing DOC concentration, which could be due to the fact that triiodo-THMs was less likely to be formed in the presence of excess reaction sites on NOM [18]. In addition, the distribution of CHI3, CHClI2 and CHCl2I formation were similar in Fig. 3 to that in Fig. 2 because of the same distribution of chlorine species alongside the breakpoint curves. 3.4. Effect of NOM type on I-THM formation The influence of NOM type, including HA I, HA II and FA, on the formation of I-THMs was studied. The breakpoint value (Cl2: NH3-N molar ratio) was around 2.3. As displayed in Fig. 4(a)-(c), NOM with high SUVA254 value (HA I > HA II > FA, Table S1) formed much less I-THMs during breakpoint chlorination, especially CHI3 in the postbreakpoint zone. Previous study has demonstrated that NOM with high SUVA254 value (HA I) was easier to undergo substitution reaction both in pre-breakpoint and post-breakpoint zones due to its higher aromaticity, resulting in high concentrations of chlorinated THM precursors [1]. Besides, the competition for monochloramine or chlorine between NOM with high SUVA254 and I− was much fiercer than that with low SUVA254, leading to the fewer formation of HOI [1]. Moreover, NOM with higher SUVA value consumed more chlorine to produce reactive sites for further reaction with HOI [37]. As a consequence, fewer formation of I-THMs from HA I was observed compared to HA II (with midrange SUVA254) and FA (with a low SUVA254 value). Such results were consistent with previous studies [1,18]. Although HOI level was lower in the post-breakpoint zone than that in the pre-breakpoint zone, its reaction rate with THM precursors was faster than HOCl [1] so that more iodinated THMs were preferentially formed, especially CHClI2 at the chlorine dosage of 19 mg/L. 3.5. Effect of solution pH on I-THM formation The effect of solution pH on the formation of I-THMs is displayed in Fig. 5. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination of the samples ranged from 2.2 to 2.7. It was obvious that the breakpoint shifted to higher chlorine dosage as pH increased from 6.0 to 8.0, which corresponded with the theory of breakpoint chlorination [24]. At neutral and alkaline conditions, OCl− is the dominant species with lower oxidizing capacity than HOCl. Therefore, higher dosage of HOCl was required for the complete conversion from NH2Cl to N2 so as to postpone the breakpoint [24]. In Fig. 5(a)–(c), as pH increased from 6.0 to 8.0, the total amount of I-THMs, especially CHI3, increased significantly at the same chlorine dosage in the pre-breakpoint zone. This result is consistent with a previous study in which maximum of total ITHMs formation occurred at pH 8.0 [18], which could be explained by hydrolysis of intermediates and the enolization of THM precursors [13]. Besides, the speciation of I-THMs was highly dependent on pHs alongside the breakpoint curves. It was found that CHI3 was the dominant species in the pre-peak point region for pH 7.0 and 8.0, while CHClI2 was the major species from the peak point to the breakpoint when pH increased from 6.0 to 8.0. In the post-breakpoint zone, as pH ranged from 6.0 to 7.0, CHCl2I was the dominant I-THMs, followed by CHClI2 and CHI3. While at pH 8.0, CHClI2 was the predominant, followed by CHI3 and CHCl2I. The phenomena could be explained by the distribution of iodine and oxidant species. Firstly, the distribution of iodine species is pH-dependent. The conversion between I2 and HOI is described in the following reaction (6) [18]:
Fig. 6. Effects of Br−/I− molar ratio on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, reaction time = 2 h and 25 °C. (a) Br−/I− = 0.5, (b) Br−/I− = 1, (c) Br−/I− = 5. Error bars represent the standard deviation of replicate measurements.
3.3. Effect of NOM concentration on I-THM formation Fig. 3 displays the effect of NOM concentration (expressed as DOC) on I-THM formation. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination of the samples ranged from 2.2 to 2.3. As displayed in Fig. 3, the amount of residual oxidant declined slightly with increasing DOC concentration at the same chlorine dosage. Meanwhile, total amount of I-THM formation increased significantly as DOC concentration increased from 3 to 7 mgC/L, which could serve as I-THM
I2 + H2O ↔ HOI + I− + H+
(6)
HOI is the dominant iodine species at neutral to nearly alkaline conditions, and I2 is the dominant one at acidic conditions [18]. Secondly, the conversion of NH2Cl and the speciation of oxidants are affected by pH. Under alkaline conditions, the dominant NH2Cl with 510
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3.6. Effect of Br−/I− molar ratio on I-THM formation
stronger oxidative power than NHCl2 benefit the formation of I-THMs, especially CHI3 in the pre-breakpoint zone [1]. However, NH2Cl can be hydrolyzed to HOCl with stronger oxidative capacity or transformed to NHCl2 and NCl3 under acidic conditions [28], leading to the fewer formation of HOI and I− incorporation into THMs. Moreover, at a pH value higher than 7.5, the dissociation of HOCl to the less powerful OCl− occurrs [22], which can oxidize less HOI to the stable IO3−. Therefore, at pH > 7.5, more residual HOI can promote the incorporation of I− to THMs and result in the increase of CHI3 and CHClI2 formation.
In source water, Br− could be rapidly oxidized by chlorine and NH2Cl, and incorporate into THMs [5]. In order to clarify the effect of Br− concentration on I-THM formation, appropriate Br− concentrations were spiked into water samples to achieve corresponding Br−/I− molar ratios (0.5, 1 and 5). Although Br−/I− molar ratios of 0.5 and 1 are not realistic under real-world conditions (I− concentration is always much lower than Br− concentration in source water) [38], these ratios were applied to have a better understanding of their impact on the formation
Fig. 7. Evaluation of I− incorporation into I-THMs during breakpoint chlorination at [NH4Cl] = 1.4 mgN/L (0.1 mM), reaction time = 2 h and 25 °C. (a) I− concentration, (b) NOM concentration, (c) NOM type, (d) solution pH, (e) Br−/I− molar ratio. Error bars represent the standard deviation of replicate measurements. 511
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the formation of HOCl, which could oxidize I− to IO3−, resulting in the decrease of HOI to react with NOM in the pre-breakpoint zone, while the dissociation of HOCl to less powerful OCl− was favored at pH 8.0, resulting in less oxidation of I− to IO3− in the post-breakpoint zone. Therefore, the values of IIF also increased with increasing pH (Fig. 7(d)). IIF decreased with increasing SUVA254 value (Fig. 7(c)) because NOM with high-SUVA254 values favored to react with chlorine compared to NOM extracts with low or midrange ones [1], resulting in fewer I-THMs and minimal IIF compared to HA II and FA. Additionally, IIF values decreased with increasing Br−/I− molar ratio (Fig. 7(e)), which were in accordance with a previous study [1]. One reason could be the conversion of I− to IO− 3 by HOBr so as to the low formation of iodinated THMs [1,20]. The other reason could be the strong competition between Br− and I- for NOM so as to weaken the formation of iodinated compounds.
of I-THMs. Fig. 6 displays the concentrations and speciation of I-THMs at molar ratios from 0.5 to 5. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination ranged from 2.2 to 2.3 in this study. There was no significant change in the breakpoint for varying Br− addition. However, the concentrations and species of I-THMs greatly differentiated from those observed in parts 3.1-3.5. It could be explained by the change and distribution of oxidant species in different zones alongside the breakpoint curves. In the pre-breakpoint zone, the major species were CHI3 and CHClI2 (Fig. 6(a)–(b)). With the increasing molar ratio of Br−/I− from 1 to 5, the formation of bromine-containing I-THMs (i.e., CHClBrI, CHBr2I and CHBrI2) was relatively higher at the same chlorine concentration due to increasing reaction between Br− and NH2Cl, one of the major oxidants in the pre-breakpoint zone. Moreover, in Fig. S3, an increase of Iconcentration was observed with increasing Br−/I− molar ratio, indicating the hindrance of I− incorporation by Br− addition. The reactions between NH2Cl and Br− can yield bromamine (NH2Br) and bromochloramine (NHBrCl) [39–41], which were confirmed as important oxidants of NOM [42], resulting in the formation of bromine-containing THMs. It is worth noting that the formation of bromine-containing ITHMs increased remarkably in the post-breakpoint zone, especially CHClBrI. The reasons were as follows. HOBr formed through reactions between chlorine and Br− [1] can oxidize HOI to the stable IO3-, leading to the decrease of I− incorporation into THM formation. This is the reason why I− concentration was decreased and I-THMs were also low (Fig. S3). Besides, Br− could be recycled by chlorine to HOBr, resulting in further electrophilic substitution of NOM moieties, which led to complete bromine incorporation into NOM [43]. Due to higher reactivity of HOBr toward NOM than that of HOCl, HOBr favored the formation of bromine-substituted THMs [10]. Therefore, the increasing Br- concentration promoted the formation of bromine-containing THMs.
4. Conclusions The formation and speciation of I-THMs varied in different zones alongside the breakpoint curves. I-THM formation increased with increasing chlorine concentration in the pre-breakpoint zone and then decreased significantly in the post-breakpoint zone. Among the three detective I-THMs, CHI3 and CHClI2 were the major compounds in the pre-breakpoint zone, while CHCl2I was the dominant one in the postbreakpoint zone. I-THM formation increased with increasing I− concentration, NOM concentration and solution pH, but decreased with increasing SUVA254 value. As Br−/I− molar ratio increased from 0.5 to 5, the formation of bromine-containing I-THMs increased, and the decrease of iodinated THMs, especially CHI3, was observed. In addition, a shift of the major species from CHCl2I to the more toxic CHClBrI was found in the presence of Br− in the post-breakpoint zone. For all the influencing factors, IIF decreased with increasing chlorine concentration during the breakpoint chlorination, especially in the post-breakpoint zone. In sum, the formation and transformation of I-THMs alongside the breakpoint curves was confirmed. It is efficient to decrease the formation of I-THMs by controlling the disinfection conditions at acidic or nearly neutral pHs. Moreover, an appropriate chlorine concentration was crucial to control the formation of DBPs, especially I-THMs in the field application.
3.7. Evaluation of I− incorporation into I-THMs during breakpoint chlorination In the presence of I−, the incorporation of I− into THMs occurs during chlorination. In order to assess iodine substitution during I-THM formation, iodine incorporation factor (IIF) was calculated in this study by the formula shown in the following Eq. (7), which is similar to the calculation of bromine incorporation factor [44]:
Acknowledgments
3
IIF =
∑ j j[THM]j TTHM
This study was supported in part by the Natural Science Foundation of China (Nos. 51778444, 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 in Taiwan (MOST-104-2221-E-327-001-MY3).
(7)
where [THM]j is the concentration of THM compound (μM) with j iodine atoms. TTHM refers to the total molar concentrations of THMs, which could range from 0 to 3 according to the values of bromine incorporation factor [45–47]. The higher the IIF values, the more the THM compounds shift to the iodinated ones. Fig. 7(a)–(e) displays the effect of I− concentration, NOM concentration and type, solution pH as well as Br−/I− molar ratio on the variation of IIF values during breakpoint chlorination. IIF values decreased significantly as chlorine concentration increased from 1 to 20 mg/L, especially in the post-breakpoint zone. As illustrated in Section 3.1, NH2Cl was the major oxidant in the pre-breakpoint zone, which promoted I-THM formation [13]. Besides, HOI could be rapidly oxidized by the increasing concentration of free chlorine beyond the breakpoint, resulting in the significant declination of IIF. As displayed in Fig. 7(a), the values of IIF increased with increasing I− concentration at the same chlorine dosage, which could be explained as higher I− consumption by the oxidants, resulting in an increasing amount of HOI that might react with NOM to form I-THMs. The values of IIF also increased with increasing DOC concentration (Fig. 7(b)) because of the increasing concentration of THM precursors. Besides, the hydrolysis of NH2Cl was prone to occur at acidic conditions, leading to
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.04.009. References [1] S. Allard, J. Tan, C.A. Joll, U. von Gunten, Mechanistic study on the formation of Cl-/Br-/I-trihalomethanes during chlorination/chloramination combined with a theoretical cytotoxicity evaluation, Environ. Sci. Technol. 49 (2015) 11105–11114. [2] USEPA, National interim primary drinking water regulations: control of trihalomethanes in drinking water, Fed. Regist. 44 (1979) 68624. [3] S.D. Richardson, Disinfection by-products and other emerging contaminants in drinking water, Trac-Trends Anal. Chem. 22 (2003) 666–684. [4] T.A. Bellar, J.J. Lichtenberg, R.C. Kroner, The occurrence of organohalides in chlorinated drinking waters, J. Am. Water Works Assoc. 66 (1974) 703–706. [5] J.J. Rook, Formation of haloforms during chlorination of natural waters, Water Treat. Exam. 23 (1974) 234–243.
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[6] T. Bond, J. Huang, M.R. Templeton, N. Graham, Occurrence and control of nitrogenous disinfection by- products in drinking water e a review, Water Res. 45 (2011) 4341–4354. [7] R.C. Hansson, M.J. Henderson, P. Jack, R.D. Taylor, Iodoform taste complaints in chloramination, Water Res. 21 (1987) 1265–1271. [8] M.J. Plewa, E.D. Wagner, S.D. Richardson, A.D. Thruston, Y.T. Woo, A.B. McKague, Chemical and biological characterization of newly discovered lodoacid drinking water disinfection byproducts, Environ. Sci. Technol. 38 (2004) 4713–4722. [9] S.W. Krasner, H.S. Weinberg, S.D. Richardson, S.J. Pastor, R. Chinn, M.J. Sclimenti, G.D. Onstad, A.D. Thruston, Occurrence of a new generation of disinfection byproducts, Environ. Sci. Technol. 40 (2006) 7175–7185. [10] T.Y. Zhang, B. Xu, C.Y. Hu, Y.L. Lin, L. Lin, T. Ye, F.X. Tian, A comparison of iodinated trihalomethane formation from chlorine, chlorine dioxide and potassium permanganate oxidation processes, Water Res. 68 (2015) 394–403. [11] W.H. Chu, N.Y. Gao, D.Q. Yin, S.W. Krasner, M.R. Templeton, Trace determination of 13 haloacetamides in drinking water using liquid chromatography triple quadrupole mass spectrometry with atmospheric pressure chemical ionization, J. Chromatogr. A. 1235 (2012) 178–181. [12] G.Y. Ding, X.R. Zhang, A picture of polar iodinated disinfection byproducts in drinking water by (UPLC/)ESI-tqMS, Environ. Sci. Technol. 43 (2009) 9287–9293. [13] Y. Bichsel, U. von Gunten, Formation of iodo-trihalomethanes during disinfection and oxidation of iodide containing waters, Environ. Sci. Technol. 34 (2000) 2784–2791. [14] Y. Muramatsu, K.H. Wedepohl, The distribution of iodine in the earth's crust, Chem. Geol. 147 (1998) 201–216. [15] T. Ye, B. Xu, Y.L. Lin, C.Y. Hu, L. Lin, T.Y. Zhang, N.Y. Gao, Formation of iodinated disinfection by-products during oxidation of iodide-containing waters with chlorine dioxide, Water Res. 47 (2013) 3006–3014. [16] Y. Bichsel, U. von Gunten, Oxidation of iodide and hypoiodous acid in the disinfection of natural waters, Environ. Sci. Technol. 33 (1999) 4040–4045. [17] S. Allard, C.E. Nottle, A. Chan, C. Joll, U. von Gunten, Ozonation of iodide-containing waters: selective oxidation of iodide to iodate with simultaneous minimization of bromate and I-THMs, Water Res. 47 (2013) 1953–1960. [18] S.G. Liu, Z.L. Li, H.Y. Dong, B.A. Goodman, Z.M. Qiang, Formation of iodo-trihalomethanes, iodo-acetic acids, and iodo-acetamides during chloramination of iodide-containing waters: factors influencing formation and reaction pathways, J. Hazard. Mater. 321 (2017) 28–36. [19] G. Hua, D.A. Reckhow, Comparison of disinfection byproduct formation from chlorine and alternative disinfectants, Water Res. 41 (2007) 1667–1678. [20] D.B. Jones, A. Saglam, H.C. Song, T. Karanfil, The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation, Water Res. 46 (2012) 11–20. [21] G. Hua, D.A. Reckhow, J. Kim, Effect of bromide and iodide ions on the formation and speciation of disinfection byproducts during chlorination, Environ. Sci. Technol. 40 (2006) 3050–3056. [22] J. Criquet, S. Allard, E. Salhi, C.A. Joll, A. Heitz, U. von Gunten, Iodate and iodotrihalomethane formation during chlorination of iodide-containing waters: role of bromide, Environ. Sci. Technol. 46 (2012) 7350–7357. [23] N.K.V. Leitner, J. Vessella, M. Dore, B. Legube, Chlorination and formation of organoiodinated compounds: the importance role of ammonia, Environ. Sci. Technol. 32 (1998) 1680–1685. [24] K. Driss, M. Bouhelassa, Modeling drinking water chlorination at the breakpoint: I. Derivation of breakpoint reactions, Desalin. Water Treat. 52 (2014) 5757–5768. [25] M.J. McGuire, Eight revolutions in the history of US drinking water disinfection, J. Am. Water Work Assoc. 98 (2006) 123–149. [26] X. Yang, C. Shang, J.C. Huang, DBP formation in breakpoint chlorination of wastewater, Water Res. 39 (2005) 4755–4767. [27] J.W. Charrois, S.E. Hrudey, Breakpoint chlorination and free-chlorine contact time:
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Formation of iodinated trihalomethanes during breakpoint chlorination of iodide-containing water
T
⁎
Zhi Liua,b, Yi-Li Linc, Bin Xua,b, , Chen-Yan Hud, An-Qi Wanga, Ze-Chen Gaoa, Sheng-Ji Xiaa, Nai-Yun Gaoa a
State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China c Department of Safety, Health and Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung 824, Taiwan, ROC d College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Iodinated trihalomethanes (I-THMs) Breakpoint chlorination Disinfection by-products (DBPs) Monochloramine Residual oxidant
This study investigated the formation of toxic iodinated trihalomethanes (I-THMs) during breakpoint chlorination of iodide-containing water. Impact factors including I− concentration, natural organic matter (NOM) concentration and type, pH as well as Br−/I− molar ratio were systematically investigated. Moreover, the incorporation of I− into I-THM formation was also calculated. The results showed that I-THM formation varied in different zones of the breakpoint curves. I-THMs increased with increasing chlorine dosage to breakpoint value and then dropped significantly beyond it. Iodoform (CHI3) and chlorodiiodomethane (CHClI2) were the major ITHMs in the pre-breakpoint zone, while dichloroiodomethane (CHCl2I) was the dominant one in the postbreakpoint zone. The formation of I-THMs increased remarkably with I− and dissolved organic carbon (DOC) concentrations. More bromine-containing species were formed as Br−/I− molar ratio increased from 0.5 to 5. In addition, the major I-THM compound shifted from CHCl2I to the more toxic CHClBrI. As pH increased from 6.0 to 8.0, I-THM formation kept increasing in the pre-breakpoint zone and the speciation of I-THMs changed
⁎ Corresponding author at: State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China. E-mail address: [email protected] (B. Xu).
https://doi.org/10.1016/j.jhazmat.2018.04.009 Received 30 August 2017; Received in revised form 2 April 2018; Accepted 4 April 2018 Available online 10 April 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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alongside the breakpoint curves. The incorporation of I− during breakpoint chlorination was highly dependent on chlorine, I−, and NOM concentrations, NOM type, solution pH and Br−/I− molar ratio.
1. Introduction
monochloramine (Eqs. (1) and (2)).
As one of the main chemical disinfectants, chlorine is economical, flexible and efficient to inactivate pathogenic microorganism in water treatment plants. However, chlorine can also react with organic or inorganic precursors, resulting in the formation of various disinfection byproducts (DBPs) [1]. Chloroform (CHCl3) and other trihalomethanes (THMs), including bromodichloromethane (CHCl2Br), dibromochloromethane (CHClBr2) and bromoform (CHBr3), were identified as DBPs in chlorinated drinking water in 1974, which led to the establishment of relevant regulations in the U.S. in 1979 [2]. In the past 30 years, researches regarding the formation of DBPs as well as the impact of DBPs on human health were flourishing [3]. Although more than 500 DBPs were identified to date [3], extensive investigations were mainly concentrated on the brominated and chlorinated THMs and haloacetic acids (HAAs) [4–6] that were currently regulated by the U.S. Environmental Protection Agency (U.S. EPA) [2]. Iodinated THMs (I-THMs), including iodoform (CHI3), chlorodiiodomethane (CHClI2), dichloroiodomethane (CHCl2I), bromochloroiodomethane (CHClBrI), dibromoiodomethane (CHBr2I) and bromodiiodomethane (CHBrI2), were identified in drinking water as early as 1975 [7]. Although I-THMs are more geno- and cytotoxic than their brominated or chlorinated analogs [8], they have been neither broadly measured nor regulated because their concentrations (maximum level of 19 μg/L) were lower than those of the regulated THMs (maximum level of 164 μg/L) according to a previous study [9]. However, I-THMs, especially CHI3, can result in medicinal odors and tastes in drinking water because of their low threshold concentrations (0.02 to 5 μg/L) [10]. Apart from I-THMs, other iodinated-DBPs (I-DBPs) have also been detected in drinking water in recent years, including iodinated haloacetamides (I-HAcAms) [11], iodinated haloacetic acids (IHAAs) [9] and polar I-DBPs [12]. Total iodine concentrations in the source water usually range from 0.5 to 20 μg/L and can exceed 50 μg/L in certain surface or ground water due to seawater intrusion [13,14]. During the disinfection process using chlorine, monochloramine (NH2Cl) or ozone, iodide (I−) present in source water can be rapidly oxidized to hypoiodous acid (HOI) [10,15,16], which can either be further oxidized to iodate (IO3−) [17], a stable and nontoxic form of iodine in drinking water, or react with natural organic matter (NOM) and result in the formation of various I-DBPs [13,18]. By comparing I-DBP concentrations after disinfection of iodide-containing water using chlorine, ozone, chlorine dioxide and monochloramine, it concludes that monochloramine promotes the formation of I-DBPs, especially CHI3 [19]. The formation of I-THMs during chlorination is affected by many factors, such as ammonia nitrogen, chlorine, I− and Br− concentrations and solution pH [20–22]. Furthermore, the speciation and formation of I-THMs are highly affected by ammonia concentration during breakpoint chlorination of natural water [23]. However, less attention was paid on the formation of I-THMs so far during breakpoint chlorination of natural water. Recently, Zhang et al. [10] and Liu et al. [18] studied the formation of I-THMs in natural water spiked with I− during chlorination and chloramination, respectively. However, the formation of I-THMs in different zones of breakpoint chlorination curve remains unclear. Breakpoint reaction requires a theoretical Cl2: NH3-N molar ratio of 1.5 [24]. The breakpoint reaction is affected by several factors, including temperature, solution pH and chlorine concentration [24]. There are three stages in breakpoint chlorination. The first stage is the reaction between ammonia and hypochlorous acid (HOCl) to yield
Cl2 + H2O → HOCl + H+ + Cl−
(1)
HOCl + NH3 → NH2Cl + H2O
(2)
The second stage is the formation of dichloramine and trichloramine (Eqs. (3)–(4)). HOCl + NH2Cl → NHCl2 + H2O
(3)
HOCl + NHCl2 → NCl3 + H2O
(4)
The third stage is called breakpoint, in which ammonia nitrogen reacts with chlorine completely with minimum free chlorine in water. The reaction can be summarized in Eq. (5). 3HOCl + 2NH3 → N2 + 3H+ + 3Cl− + 3H2O
(5)
Generally, breakpoint chlorination involves the reactions between chlorine and ammonia [24], which eliminates ammonia in raw water [25]. Little information is available regarding the effect of breakpoint chlorination in the treatment of iodide-containing water. Yang et al. [26] reported that the distributions and concentrations of THM and HAA species varied in different zones of the breakpoint curves. Additionally, Charrois and Hrudey [27] indicated that peak N-Nitrosodimethylamine (NDMA) formation occurred in the pre-breakpoint region. To the authors’ best knowledge, no information indicates I-THM formation during breakpoint chlorination. Therefore, the objective of this study was to investigate the formation of I-THMs during breakpoint chlorination of iodide-containing water. The effects of I− and NOM concentrations, NOM type, solution pH, and bromide to iodide molar ratio (Br−/I−) on I-THM formation were evaluated. The results were further calculated for iodine incorporation factor (IIF) to get a deeper insight of I- incorporation in ITHM formation. 2. Materials and methods 2.1. Chemicals All reagents were of analytical grade and used without further purification. Potassium iodide (KI ≥ 99.0%), potassium bromide (KBr ≥ 99.0%) and sodium hypochlorite (NaOCl, 4.00–4.99%) were purchased from Sigma-Aldrich (USA). Five I-THM standards, including CHClI2, CHCl2I, CHClBrI, CHBr2I, and CHBrI2, were purchased from CanSym Chemical Corp. (Canada). Methanol and methyl tert-butyl ether (MTBE) were purchased from J.T. Baker (USA). Ammonium chlorine (NH4Cl), sodium hydroxide (NaOH), sulphuric acid (H2SO4), potassium dihydrogen phosphate (KH2PO4) and humic acid I (HA I) were obtained from Sigma-Aldrich (USA). Suwannee River humic acid II (HA II) and fulvic acid (FA) were obtained from International Humic Substances Society (IHSS). All experiments were carried out using ultrapure water produced from a Milli-Q water purification system (Millipore, USA). 2.2. Experimental procedures 2.2.1. Breakpoint chlorination experiments For breakpoint chlorination, 5 mgC/L NOM (HA I) samples were pre-ammoniated by dosing NH4Cl to 1.4 mg/L (0.1 mM). Then, an aliquot of stock NaOCl solution was immediately spiked into the samples to make the concentration ranging from 1 to 20 mg/L (0.014–0.28 mM). The reaction was last for 2 h at room temperature (25 ± 1 °C) and then 506
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the detection limit was 0.1 mgC/L. Solution pH was measured using a pH meter (FE20-FiveEasy, Mettler Toledo, Switzerland), which was calibrated using standard buffer solutions. The concentrations of chlorine and monochloramine were determined according to DPD colorimetric method [28]. I− and Br− concentrations were analyzed using an ion chromatograph (ICS-2000, Dionex, USA) equipped with an AS11-HC analytical column and AG11-HC guard column. I-THM quantification method was based upon our previous research with minor modification of the USEPA Method 551.1 [15,34]. Amber glassware was adopted in the experiments because I-THMs are lightly sensitive and can be decomposed over time in water [35]. 10 mL of each sample was extracted with 2 mL MTBE and analyzed using a gas chromatograph (GC-2010, Shimadzu, Japan) that was equipped with an electron capture detector (ECD) and a HP-5 capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness, J&W, USA). The detector and injector temperature were 290 and 200 °C, respectively. The nitrogen carrier gas was at the pressure of 69.8 kPa with the flow rate of 30 mL/ min. The temperature program was set at 40 °C for 10 min, ramped to 260 °C at 15 °C/min and then held for 10 min. Detection limits of CHCl2I, CHClBrI, CHBr2I, CHClI2, CHBrI2, CHI3 and CHCl3 were 0.10, 0.09, 0.08, 0.05, 0.05, 0.08 and 0.10 μg/L, respectively. Quantification limits for CHCl2I, CHClBrI, CHBr2I, CHClI2, CHBrI2, CHI3 and CHCl3 were 0.33, 0.30, 0.27, 0.17, 0.17, 0.27 and 0.33 μg/L, respectively.
the samples were analyzed for total and free chlorine residual concentrations using the DPD titration method [28]. Notably, free bromine, bromamines and iodine formed during chlorination of water samples can also react with DPD, resulting in the interference of chlorine residual measurement using the DPD titration method [29,30]. Therefore, chlorine residuals shown in Figs. 1–6 were displayed as 'residual oxidant'. 2.2.2. I-THM formation experiments I-THM formation experiments were performed in duplicate using 40-mL amber glass vials with PTFE-lined septum screw-caps under headspace-free condition at 25 ± 1 °C. Five influencing factors were studied, including I− concentration (0.64–1.91 mg/L), NOM concentration (3-7 mgC/L) and type (HA I, HA II and FA), solution pH (6.08.0) and Br−/I− molar ratio (0.5–5). For the experiments of different NOM types, HA I, HA II and FA were prepared at the same concentration of 5 mgC/L. Otherwise, HA I was the only NOM that was used in the experiments. Solution pH was controlled at 7.00 ± 0.02 using 10 mM phosphate buffer throughout the experiments except for the varying pH experiments, in which the pH was adjusted using either 1 M NaOH or H2SO4. In a typical run, 1.27 mg/L (10 μM) of I− was spiked into samples containing 5 mgC/L NOM [31] to amplify the formation of I-THMs for quantification in this study. Dosed I− concentration was much higher than the level in natural water. I-THM formation at different reaction time during breakpoint chlorination was presented in Fig. S1. Compared to the formation of I-THMs after 8-h chlorination, half of I-THMs were formed in the first 2-h and their concentrations were enough for detection. Therefore, 2-h reaction time was selected for the I-THM formation experiments. After reaching the planned reaction time (2 h), samples were withdrawn and quenched the residual chlorine using overdosed ascorbic acid ([ascorbic acid]: [chlorine] molar ratio of 1.5) [32,33]. Then the samples were extracted and analyzed for I-THM concentrations immediately to avoid any hydrolytic loss.
3. Results and discussion 3.1. I-THM formation during breakpoint chlorination The concentrations of three I-THMs were measured alongside the breakpoint curves. The Cl2: NH3-N molar ratio was 2.3 to achieve 2-h breakpoint chlorination of the samples. Barrett et al. [36] reported a range of breakpoint values with Cl2: NH3-N molar ratio of 1.4–3.2, which may also depend on the composition of the water samples. As displayed in Fig. 1, total amount of I-THM increased as chlorine concentration reached the breakpoint and then declined significantly as chlorine concentration kept increasing. Among three I-THMs, CHI3 and CHClI2 were the major species in the pre-breakpoint zone, while CHCl2I was the dominant species in the post-breakpoint. It is known that I− can be oxidized to HOI, but can’t be further oxidized to IO3− by NH2Cl.
2.3. Analytical methods NOM concentration was measured using a Shimadzu TOC-VCSH analyzer (Shimadzu, Japan) for dissolved organic carbon (DOC), and
Fig. 1. Effects of chlorine concentration on ITHM formation from I− containing water and residual oxidant concentration during breakpoint chlorination at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, [I−]0 = 1.27 mg/L (10 μM), reaction time = 2 h and 25 °C. Error bars represent the standard deviation of replicate measurements.
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Fig. 2. Effects of I− concentration on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, reaction time = 2 h and 25 °C. (a) [I−]0 = 0.64 mg/L (5 μM), (b) [I−]0 = 1.27 mg/L (10 μM), (c) [I−]0 = 1.91 mg/L (15 μM). Error bars represent the standard deviation of replicate measurements.
Fig. 3. Effects of NOM concentration on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [I−]0 = 1.27 mg/L (10 μM), reaction time = 2 h and 25 °C. (a) [DOC] = 3 mgC/L, (b) [DOC] = 5 mgC/L, (c) [DOC] = 7 mgC/L. Error bars represent the standard deviation of replicate measurements.
However, I− can be oxidized to IO3− by free chlorine, which shows stronger oxidation potential than NH2Cl [16]. As displayed in Fig. S2, NH2Cl is the main oxidant in the pre-breakpoint zone. Consequently, HOI can exist longer to react with NOM during chloramination compared to chlorination [13], which contributes to higher amount of ITHM formation, especially CHI3. With the increasing chlorine concentration, free residual oxidant became the main oxidant in the postbreakpoint zone, while NH2Cl concentration declined to zero (Fig. S2). More I− and HOI could be oxidized to stable IO3− by free chlorine,
resulting in the declination of I-THM formation [19] in the decreasing order of CHI3 > CHClI2 > CHCl2I. 3.2. Effect of I− concentration on I-THM formation The effect of I− concentration on I-THMs formation is displayed in Fig. 2, which indicated similar trend among each other. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination of the samples ranged from 2.2 to 2.3. With increasing I- level, higher chlorine dosage 508
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Fig. 4. Effects of NOM type on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [I−]0 = 1.27 mg/L (10 μM), [DOC] = 5 mgC/L, reaction time = 2 h and 25 °C. (a) HA I, (b) HA II, (c) FA. Error bars represent the standard deviation of replicate measurements.
Fig. 5. Effects of solution pH on the formation of I-THMs and residual oxidant concentration at [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, [I−]0 = 1.27 mg/L (10 μM), reaction time = 2 h and 25 °C. (a) pH = 6.0, (b) pH = 7.0, (c) pH = 8.0. Error bars represent the standard deviation of replicate measurements.
was consumed to oxidize I−, which slightly influenced the breakpoint. However, as displayed in Fig. 2, the amount of residual oxidant increased with I− concentration from 0.64 to 1.91 mg/L (5–15 μM) at the same dosage of chlorine because of the enhancing I2 interference of residual chlorine measurement based on DPD method with increasing I− addition [30]. Total amount of I-THM formation increased with the increasing I− concentration at the same chlorine concentration because the increasing addition of I− could lead to an increase formation of HOI to react with NOM [8,10]. The increase of CHI3, the major compound responsible for the taste and odor problems, was much more significant
compared to the other I-THMs as I− concentration increased in the prebreakpoint zone (Fig. 2(a)–(c)). In contrast, the concentration of CHI3 decreased much faster in the post-breakpoint zone because chlorine was more competitive to incorporate into THMs [13]. Therefore, the dominant I-THM compounds shifted from CHI3 and CHClI2 to CHCl2I at the chlorine dosage higher than breakpoint.
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precursors. The increases of CHCl2I and CHClI2 were higher compared to that of CHI3 with the increasing DOC concentration, which could be due to the fact that triiodo-THMs was less likely to be formed in the presence of excess reaction sites on NOM [18]. In addition, the distribution of CHI3, CHClI2 and CHCl2I formation were similar in Fig. 3 to that in Fig. 2 because of the same distribution of chlorine species alongside the breakpoint curves. 3.4. Effect of NOM type on I-THM formation The influence of NOM type, including HA I, HA II and FA, on the formation of I-THMs was studied. The breakpoint value (Cl2: NH3-N molar ratio) was around 2.3. As displayed in Fig. 4(a)-(c), NOM with high SUVA254 value (HA I > HA II > FA, Table S1) formed much less I-THMs during breakpoint chlorination, especially CHI3 in the postbreakpoint zone. Previous study has demonstrated that NOM with high SUVA254 value (HA I) was easier to undergo substitution reaction both in pre-breakpoint and post-breakpoint zones due to its higher aromaticity, resulting in high concentrations of chlorinated THM precursors [1]. Besides, the competition for monochloramine or chlorine between NOM with high SUVA254 and I− was much fiercer than that with low SUVA254, leading to the fewer formation of HOI [1]. Moreover, NOM with higher SUVA value consumed more chlorine to produce reactive sites for further reaction with HOI [37]. As a consequence, fewer formation of I-THMs from HA I was observed compared to HA II (with midrange SUVA254) and FA (with a low SUVA254 value). Such results were consistent with previous studies [1,18]. Although HOI level was lower in the post-breakpoint zone than that in the pre-breakpoint zone, its reaction rate with THM precursors was faster than HOCl [1] so that more iodinated THMs were preferentially formed, especially CHClI2 at the chlorine dosage of 19 mg/L. 3.5. Effect of solution pH on I-THM formation The effect of solution pH on the formation of I-THMs is displayed in Fig. 5. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination of the samples ranged from 2.2 to 2.7. It was obvious that the breakpoint shifted to higher chlorine dosage as pH increased from 6.0 to 8.0, which corresponded with the theory of breakpoint chlorination [24]. At neutral and alkaline conditions, OCl− is the dominant species with lower oxidizing capacity than HOCl. Therefore, higher dosage of HOCl was required for the complete conversion from NH2Cl to N2 so as to postpone the breakpoint [24]. In Fig. 5(a)–(c), as pH increased from 6.0 to 8.0, the total amount of I-THMs, especially CHI3, increased significantly at the same chlorine dosage in the pre-breakpoint zone. This result is consistent with a previous study in which maximum of total ITHMs formation occurred at pH 8.0 [18], which could be explained by hydrolysis of intermediates and the enolization of THM precursors [13]. Besides, the speciation of I-THMs was highly dependent on pHs alongside the breakpoint curves. It was found that CHI3 was the dominant species in the pre-peak point region for pH 7.0 and 8.0, while CHClI2 was the major species from the peak point to the breakpoint when pH increased from 6.0 to 8.0. In the post-breakpoint zone, as pH ranged from 6.0 to 7.0, CHCl2I was the dominant I-THMs, followed by CHClI2 and CHI3. While at pH 8.0, CHClI2 was the predominant, followed by CHI3 and CHCl2I. The phenomena could be explained by the distribution of iodine and oxidant species. Firstly, the distribution of iodine species is pH-dependent. The conversion between I2 and HOI is described in the following reaction (6) [18]:
Fig. 6. Effects of Br−/I− molar ratio on the formation of I-THMs and residual oxidant concentration at pH 7.0, [NH4Cl] = 1.4 mgN/L (0.1 mM), [DOC] = 5 mgC/L, reaction time = 2 h and 25 °C. (a) Br−/I− = 0.5, (b) Br−/I− = 1, (c) Br−/I− = 5. Error bars represent the standard deviation of replicate measurements.
3.3. Effect of NOM concentration on I-THM formation Fig. 3 displays the effect of NOM concentration (expressed as DOC) on I-THM formation. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination of the samples ranged from 2.2 to 2.3. As displayed in Fig. 3, the amount of residual oxidant declined slightly with increasing DOC concentration at the same chlorine dosage. Meanwhile, total amount of I-THM formation increased significantly as DOC concentration increased from 3 to 7 mgC/L, which could serve as I-THM
I2 + H2O ↔ HOI + I− + H+
(6)
HOI is the dominant iodine species at neutral to nearly alkaline conditions, and I2 is the dominant one at acidic conditions [18]. Secondly, the conversion of NH2Cl and the speciation of oxidants are affected by pH. Under alkaline conditions, the dominant NH2Cl with 510
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3.6. Effect of Br−/I− molar ratio on I-THM formation
stronger oxidative power than NHCl2 benefit the formation of I-THMs, especially CHI3 in the pre-breakpoint zone [1]. However, NH2Cl can be hydrolyzed to HOCl with stronger oxidative capacity or transformed to NHCl2 and NCl3 under acidic conditions [28], leading to the fewer formation of HOI and I− incorporation into THMs. Moreover, at a pH value higher than 7.5, the dissociation of HOCl to the less powerful OCl− occurrs [22], which can oxidize less HOI to the stable IO3−. Therefore, at pH > 7.5, more residual HOI can promote the incorporation of I− to THMs and result in the increase of CHI3 and CHClI2 formation.
In source water, Br− could be rapidly oxidized by chlorine and NH2Cl, and incorporate into THMs [5]. In order to clarify the effect of Br− concentration on I-THM formation, appropriate Br− concentrations were spiked into water samples to achieve corresponding Br−/I− molar ratios (0.5, 1 and 5). Although Br−/I− molar ratios of 0.5 and 1 are not realistic under real-world conditions (I− concentration is always much lower than Br− concentration in source water) [38], these ratios were applied to have a better understanding of their impact on the formation
Fig. 7. Evaluation of I− incorporation into I-THMs during breakpoint chlorination at [NH4Cl] = 1.4 mgN/L (0.1 mM), reaction time = 2 h and 25 °C. (a) I− concentration, (b) NOM concentration, (c) NOM type, (d) solution pH, (e) Br−/I− molar ratio. Error bars represent the standard deviation of replicate measurements. 511
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the formation of HOCl, which could oxidize I− to IO3−, resulting in the decrease of HOI to react with NOM in the pre-breakpoint zone, while the dissociation of HOCl to less powerful OCl− was favored at pH 8.0, resulting in less oxidation of I− to IO3− in the post-breakpoint zone. Therefore, the values of IIF also increased with increasing pH (Fig. 7(d)). IIF decreased with increasing SUVA254 value (Fig. 7(c)) because NOM with high-SUVA254 values favored to react with chlorine compared to NOM extracts with low or midrange ones [1], resulting in fewer I-THMs and minimal IIF compared to HA II and FA. Additionally, IIF values decreased with increasing Br−/I− molar ratio (Fig. 7(e)), which were in accordance with a previous study [1]. One reason could be the conversion of I− to IO− 3 by HOBr so as to the low formation of iodinated THMs [1,20]. The other reason could be the strong competition between Br− and I- for NOM so as to weaken the formation of iodinated compounds.
of I-THMs. Fig. 6 displays the concentrations and speciation of I-THMs at molar ratios from 0.5 to 5. The Cl2: NH3-N molar ratio to achieve 2-h breakpoint chlorination ranged from 2.2 to 2.3 in this study. There was no significant change in the breakpoint for varying Br− addition. However, the concentrations and species of I-THMs greatly differentiated from those observed in parts 3.1-3.5. It could be explained by the change and distribution of oxidant species in different zones alongside the breakpoint curves. In the pre-breakpoint zone, the major species were CHI3 and CHClI2 (Fig. 6(a)–(b)). With the increasing molar ratio of Br−/I− from 1 to 5, the formation of bromine-containing I-THMs (i.e., CHClBrI, CHBr2I and CHBrI2) was relatively higher at the same chlorine concentration due to increasing reaction between Br− and NH2Cl, one of the major oxidants in the pre-breakpoint zone. Moreover, in Fig. S3, an increase of Iconcentration was observed with increasing Br−/I− molar ratio, indicating the hindrance of I− incorporation by Br− addition. The reactions between NH2Cl and Br− can yield bromamine (NH2Br) and bromochloramine (NHBrCl) [39–41], which were confirmed as important oxidants of NOM [42], resulting in the formation of bromine-containing THMs. It is worth noting that the formation of bromine-containing ITHMs increased remarkably in the post-breakpoint zone, especially CHClBrI. The reasons were as follows. HOBr formed through reactions between chlorine and Br− [1] can oxidize HOI to the stable IO3-, leading to the decrease of I− incorporation into THM formation. This is the reason why I− concentration was decreased and I-THMs were also low (Fig. S3). Besides, Br− could be recycled by chlorine to HOBr, resulting in further electrophilic substitution of NOM moieties, which led to complete bromine incorporation into NOM [43]. Due to higher reactivity of HOBr toward NOM than that of HOCl, HOBr favored the formation of bromine-substituted THMs [10]. Therefore, the increasing Br- concentration promoted the formation of bromine-containing THMs.
4. Conclusions The formation and speciation of I-THMs varied in different zones alongside the breakpoint curves. I-THM formation increased with increasing chlorine concentration in the pre-breakpoint zone and then decreased significantly in the post-breakpoint zone. Among the three detective I-THMs, CHI3 and CHClI2 were the major compounds in the pre-breakpoint zone, while CHCl2I was the dominant one in the postbreakpoint zone. I-THM formation increased with increasing I− concentration, NOM concentration and solution pH, but decreased with increasing SUVA254 value. As Br−/I− molar ratio increased from 0.5 to 5, the formation of bromine-containing I-THMs increased, and the decrease of iodinated THMs, especially CHI3, was observed. In addition, a shift of the major species from CHCl2I to the more toxic CHClBrI was found in the presence of Br− in the post-breakpoint zone. For all the influencing factors, IIF decreased with increasing chlorine concentration during the breakpoint chlorination, especially in the post-breakpoint zone. In sum, the formation and transformation of I-THMs alongside the breakpoint curves was confirmed. It is efficient to decrease the formation of I-THMs by controlling the disinfection conditions at acidic or nearly neutral pHs. Moreover, an appropriate chlorine concentration was crucial to control the formation of DBPs, especially I-THMs in the field application.
3.7. Evaluation of I− incorporation into I-THMs during breakpoint chlorination In the presence of I−, the incorporation of I− into THMs occurs during chlorination. In order to assess iodine substitution during I-THM formation, iodine incorporation factor (IIF) was calculated in this study by the formula shown in the following Eq. (7), which is similar to the calculation of bromine incorporation factor [44]:
Acknowledgments
3
IIF =
∑ j j[THM]j TTHM
This study was supported in part by the Natural Science Foundation of China (Nos. 51778444, 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 in Taiwan (MOST-104-2221-E-327-001-MY3).
(7)
where [THM]j is the concentration of THM compound (μM) with j iodine atoms. TTHM refers to the total molar concentrations of THMs, which could range from 0 to 3 according to the values of bromine incorporation factor [45–47]. The higher the IIF values, the more the THM compounds shift to the iodinated ones. Fig. 7(a)–(e) displays the effect of I− concentration, NOM concentration and type, solution pH as well as Br−/I− molar ratio on the variation of IIF values during breakpoint chlorination. IIF values decreased significantly as chlorine concentration increased from 1 to 20 mg/L, especially in the post-breakpoint zone. As illustrated in Section 3.1, NH2Cl was the major oxidant in the pre-breakpoint zone, which promoted I-THM formation [13]. Besides, HOI could be rapidly oxidized by the increasing concentration of free chlorine beyond the breakpoint, resulting in the significant declination of IIF. As displayed in Fig. 7(a), the values of IIF increased with increasing I− concentration at the same chlorine dosage, which could be explained as higher I− consumption by the oxidants, resulting in an increasing amount of HOI that might react with NOM to form I-THMs. The values of IIF also increased with increasing DOC concentration (Fig. 7(b)) because of the increasing concentration of THM precursors. Besides, the hydrolysis of NH2Cl was prone to occur at acidic conditions, leading to
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