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Comparison of iodinated trihalomethanes formation during aqueous chlor(am)ination of different iodinated X-ray contrast media compounds in the presence of natural organic matter
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Comparison of iodinated trihalomethanes formation during aqueous chlor(am)ination of different iodinated X-ray contrast media compounds in the presence of natural organic matter Tao Ye a, Bin Xu a,*, Zhen Wang a, Tian-Yang Zhang a, Chen-Yan Hu b, Lin Lin a, Sheng-Ji Xia a, Nai-Yun Gao a a
State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze River Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
article info
abstract
Article history:
Iodinated trihalomethanes (I-THMs) formation during chlorination and chloramination of five
Received 16 May 2014
iodinated X-ray contrast media (ICM) compounds (iopamidol, iopromide, iodixanol, histodenz,
Received in revised form
and diatrizoate) in the presence of natural organic matter (NOM) was evaluated and compared.
25 August 2014
Chlorination and chloramination of ICM in the absence of NOM yielded only a trace amount of
Accepted 26 August 2014
I-THMs, while levels of I-THMs were enhanced substantially in raw water samples. With the
Available online 6 September 2014
presence of NOM, the order with respect to the maximum yield of I-THMs observed during
Keywords:
chloramination, I-THM formation was enhanced for hisodenz, iodixanol, diatrizoate, and
Iodinated trihalomethanes
iopromide. The order with respect to the maximum yield of I-THMs observed during chlor-
chlorination was iopamidol >> histodenz > iodixanol > diatrizoate > iopromide. During
Chlor(am)ination
amination was iopamidol > diatrizoate > iodixanol > histodenz > iopromide. With the
Iodinated X-ray contrast media
exception of iopamidol, I-THM formation was favored at relatively low chlorine doses
Disinfection by-products
(100 mM) during ICM chlorination, and significant suppression was observed with high chlorine doses applied (＞100 mM). However, during chloramination, increasing monochloramine dose monotonously increased the yield of I-THMs for the five ICM. During chlorination of iodixanol, histodenz, and diatrizoate, the yields of I-THMs exhibited three distinct trends as the pH increased from 5 to 9, while peak I-THM formation was found at circumneutral pH for chloramination. Increasing bromide concentration not only considerably enhanced the yield of I-THMs but also shifted the I-THMs towards bromine-containing ones and increased the formation of higher bromine-incorporated species (e.g., CHBrClI and CHBr2I), especially in chloramination. These results are of particular interest to understand I-THM formation mechanisms during chlorination and chloramination of waters containing ICM. © 2014 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ86 13918493316; fax: þ86 21 65986313. E-mail address: [email protected] (B. Xu). http://dx.doi.org/10.1016/j.watres.2014.08.044 0043-1354/© 2014 Elsevier Ltd. All rights reserved.
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1.
Introduction
Drinking water disinfection has dramatically reduced the outbreaks of waterborne diseases (e.g., gastroenteritis, cholera and typhoid) (Richardson, 2003; Richardson and Postigo, 2012). However, reactions between dissolved organic matter (DOM) in water and the two widely used low-cost disinfectants, chlorine and chloramines, result in the production of undesired disinfection by-products (DBPs) with potential health concerns (Richardson, 2003; Richardson and Postigo, 2012). Since the first discovery of trihalomethanes (THMs) in chlorinated water in the 1970s (Rook, 1974), DBPs have become a focus of attention in drinking water treatment. Over the past 40 years, regulated DBPs (THMs and haloacetic acid [HAAs]) have been intensively studied, leading to a better understanding of their toxicity, occurrence, and control than other emerging unregulated DBPs, such as iodinated DBPs (IDBPs) (Richardson and Postigo, 2012). In terms of cytotoxicity and genotoxicity, I-DBPs are generally higher than their chlorinated and brominated analogs (Karpel Vel Leitner et al., 1998; Richardson et al., 2008; Plewa and Wagner, 2009). In addition, iodinated trihalomethanes (I-THMs) were found to be associated with unpleasant medicinal and pharmaceutical taste and odor issues (Hansson et al., 1987). Consequently, formation of I-DBPs has given rise to increasing concerns recently (Karpel Vel Leitner et al., 1998; Richardson et al., 2008; Plewa and Wagner, 2009). Normally, I-DBPs are formed by reactions between natural organic matter (NOM) and hypoiodous acid (HOI) produced from the oxidation of iodide (I) (Bichsel and von Gunten, 1999, 2000; Jones et al., 2011, 2012; Ye et al., 2012, 2013). I (from natural sources, sea-water intrusion or brines) present in source waters is believed to be the primary source of iodine in I-DBPs (Bichsel and von Gunten, 1999, 2000; Duirk et al., 2011). Nevertheless, a certain amount of I-DBPs were formed in some drinking water treatment plants (DWTPs) with very low or undetectable levels of I, which implies that iodine sources other than I could also contribute to I-DBP formation (Duirk et al., 2011). Duirk et al. (2011) confirmed that iodine-containing diagnostic pharmaceuticals (i.e., iodinated X-ray contrast media [ICM]) act as an alternative source of iodine and lead to the formation of I-DBPs during disinfection. ICM are widely used for intravascular administration to enable visualization of organ details and enhance the contrast between organs or vessels examined and the surrounding tissues (Putschew et al., 2000). For the sake of minimization of adverse effects on human metabolism, ICM are chemically designed to be inert drugs (Christiansen, 2005). Therefore, they are intrinsically of very low environmental toxicity and optimally do not impose a high risk to human health (StegerHartmann et al., 1999). However, they are too stable and hydrophilic to be effectively removed during conventional wastewater treatment processes, such that they have been found in effluents of wastewater treatment plants, rivers, lakes, and raw drinking water at elevated concentrations (Putschew et al., 2000; Duirk et al., 2011). In the investigation of 10 cities' source waters, four ICM, including iopamidol, iopromide, iohexol, and diatrizoate,
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were found (Duirk et al., 2011). Iopamidol was the most frequently detected ICM with a concentration up to 2700 ng/ L (Duirk et al., 2011). During chlorination and chloramination, hypochlorite (OCl) was thought to be the primary reactive species with iopamidol to form I-DBPs in the presence of NOM (Duirk et al., 2011). A significant amount of IDBPs was formed, up to 212 nM for dichloroiodomethane (CHCl2I) and 3.0 nM for iodoacetic acid, respectively, for chlorination (Duirk et al., 2011). To the best of our knowledge, that is the only work exploring the formation of I-DBPs during disinfection of ICM-containing waters. However, their study was highly focusing on the formation of I-DBPs during disinfection of iopamidol-containing water. Although bromide is usually present in raw water and can play an important role in I-THM formation (Jones et al., 2012), the influence of the presence of bromide was not investigated yet. Since ICM are emerging contaminants and a new source of iodine, the formation of I-THMs from ICM will likely be different from that of I. Thus, it is of great significance to understand the comprehensive factors that influence I-THMs formation during disinfection of waters containing ICM for the sake of developing effective strategies to control I-DBP formation in drinking water treatment. In this study, we systematically investigate the influence of disinfectant dose, reaction pH and bromide (Br) concentration on the formation of I-THMs during chlorination and chloramination of five different commonly used ICM (Table S1, SI) in the presence of NOM. I-THM formation from five ICM during chlorination and chloramination was compared. Different patterns of I-THM formation during chlorination and chloramination of ICM-containing water under the same conditions were the first time being observed in literature.
2.
Materials and methods
2.1.
Chemicals and reagents
All chemicals were at least of analytical grade except as noted. Iopamidol (99.6%), iopromide (98.6%), and iodixanol (100%) were obtained from U.S. Pharmacopeia (Rockville, MD). Commercial 4e4.99% sodium hypochlorite (NaOCl), histodenz, sodium diatrizoate (99.9%), and iodoform (CHI3, 99.0%) as well as potassium bromide (KI 99.0%) were purchased from SigmaeAldrich. I-THMs, including chlorodiiodomethane (CHClI2, 90e95%), dichloroiodomethane (CHCl2I 95%), bromochloroiodomethane (CHBrClI 95%), dibromoiodomethane (CHBr2I, 90e95%) and bromodiiodomethane (CHBrI2, 90e95%), were obtained from CanSyn Chem. Corp. (Toronto, ON, Canada). Four regulated THMs (THM4) mix standard solution, including chloroform (CHCl3), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl) and bromoform (CHBr3), was purchased from Supelco (USA). Methyl tert-butyl ether (MtBE) was purchased from J.T. Baker (USA). All other chemicals were received from Sinopharm Chemical Reagent Co., Ltd. (China) without further purification. All solutions were prepared with ultra-pure water produced from a Milli-Q water purification system (Millipore, USA).
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2.2.
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Experimental procedures
Raw water samples were collected from the intake of Yangshupu drinking water treatment plant (YDWTP) in Shanghai, China, and then filtered through 0.45 mm membrane filters (Millipore Corp., USA) immediately and stored in the dark at 4 C until used. Raw water characteristics are shown in Table 1. Chlorination and chloramination of water spiked with different ICM were conducted in duplicate under headspacefree conditions in 40-mL glass screw-cap amber vials with PTFE-lined septa. All samples were buffered with 10 mM phosphate. After being dosed with chlorine or monochloramine, samples were stored at 25 ± 1 C in the dark for 72 h. Impact factors, including initial dosage of chlorine and monochloramine, reaction pH, Br concentration and five different ICM (i.e., iopamidol, iopromide, iodixanol, histodenz, and diatrizoate), were evaluated. Solution pH was adjusted using either 0.01, 0.1, 1 M NaOH or H2SO4. Prior to analysis, aliquots of the reacted solution were withdrawn and the remaining disinfectant was quenched using sodium sulfite (Na2SO3) with 20% excess of initial disinfectant dose after 72 h of reaction time (Duirk et al., 2011). Control experiments in the presence of NOM and without the spiking of ICM as well as in the absence of NOM (i.e., Mill-Q water) and with the spiking of ICM were done with similar procedures. Certain levels of I-THMs were observed during chlorination/chloramination of ICM in Mill-Q water (in the absence of NOM, Table S2, SI) and I-THMs were not detectable without the addition of ICM in raw water samples (in the presence of NOM, data no shown). Chloramination was conducted with preformed monochloramine (NH2Cl) prepared by adding NaOCl solution gently into a stirred ammonium chloride (NH4Cl) solution with the N/ Cl molar ratio of 1.2 at pH 8.5 (Xu et al., 2012). Preformed NH2Cl was prepared freshly before each test. The preparation of dichloramine was shown in the supporting information (Table S6, SI). Chlorine and NH2Cl concentration were analyzed using the N,N-diethyl-p-phenylenediamine (DPD) colorimetric method (APHA et al., 1998).
2.3.
(250 mm 4.0 mm i.d.) and a Dionex AG11-HC guard column (50 mm 4.0 mm i.d.). Method for I-THMs measurements was based on our previous research (Ye et al., 2012, 2013). After quenching, samples were quantified by liquid/liquid extraction with MtBE followed by gas chromatography and electron capture detector (GC/ECD, GC-2010, Shimadzu, Japan) and an HP-5 capillary column (30 m 0.25 mm i.d., 0.25 mm film thickness, J&W, USA). THM4 was determined by GC/ECD after liquid/liquid extraction with MtBE according to USEPA Method 551.1 (USEPA, 1995). The detection limits of I-THMs and THM4 are shown in Table S3, SI.
3.
Results and discussion
3.1.
Effect of chlorine dose on I-THM formation
As shown in Fig. 1, chlorination of raw water spiked with the five different ICM resulted in the formation of appreciable quantities of I-THMs. Compared with controlled experiment results (Table S2, SI), formation of I-THMs was highly enhanced, which indicated that the presence of both ICM and NOM is required for the formation of I-THMs and ICM were the source of iodine (Duirk et al., 2011). An especially higher amount of I-THMs was formed during chlorination of iopamidol-containing water compared to water containing other ICM (Fig. 1). It appears that iopamidol formed a significantly higher amount of I-THMs at high chlorine doses (100 mM), while other ICM formed certain levels of I-THMs at low chlorine doses (100 mM) and I-THM formation was substantially suppressed with chlorine doses higher than 100 mM. Chlorine and chloramines initially react with ICM and lead to the formation of HOI, which subsequently reacts
Analytical methods
The pH was measured with an FE20-FiveEasy pH meter (Mettler Toledo, Switzerland) calibrated regularly using standard buffer solutions (Mettler Toledo; pH ¼ 4.01, 7.00, 9.21). Dissolved organic carbon (DOC) was measured by a Shimadzu TOC-VCSH analyzer (Shimadzu, Japan), and the detection limit was 0.1 mg C/L. UV absorption at 254 nm (UV254) was determined with a UV/Vis spectrophotometer (SQ-4802, UNICO, Shanghai). Br was analyzed using an ion chromatography (Dionex ICS-2000, CA, USA) equipped with a conductivity detector, a Dionex AS11-HC analytical column
Table 1 e Characteristics of raw water from YDWTP. Sample
DOC (mg C/L)
UV254 (1/cm)
SUVAa (L/mg m)
Br (mM)
YDWTP
5.7
0.133
2.3
1.63
a
SUVA (specific ultraviolet absorbance) was calculated from ultraviolet absorbance at 254 nm (UV254) divided by DOC.
Fig. 1 e Effect of chlorine dose on I-THMs formation at 3 d of reaction during chlorination of ICM in the presence of NOM. [ICM] ¼ 10 mM, pH 7, [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar); D, iopromide (the fourth bar); E, iopamidol (the fifth bar).
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with NOM to form I-DBPs (Duirk et al., 2011; Kumkum, 2013). Thus, the different behavior of iopamidol-containing samples seems to suggest that oxidation of iopamidol to HOI occurs at comparatively higher chlorine doses than that of other ICM. However, excess doses of chlorine (>100 mM) may oxidize HOI to iodate (IO 3 ), which could be responsible for the decrease of total I-THMs yield at high chlorine doses (Fig. 1) (Bichsel and von Gunten, 1999; Duirk et al., 2011; Kumkum, 2013). Hence, for the prevention of unpleasant medicinal and pharmaceutical taste and odor associated with I-THMs, a reasonable solution is to increase chlorine dose. Nevertheless, this may enhance the formation of THM4 (Table S4, SI). For example, during chlorination of iopamidol-containing samples, with the increase of chlorine dose from 25 to 250 mM, the sum of THM4 increased from 0.59 to 2.31 mM. CHCl2I and CHBrClI were the only two I-THMs formed with measurable levels during chlorination of diatrizoate, while other ICM could also produce a certain amount of CHBr2I and CHClI2 at relatively low doses of chlorine. CHBrI2 and CHI3 were not detected in all samples during chlorination. With the increase of chlorine dose from 50 to 100 mM, the formation of CHCl2I was significantly enhanced for iopamidol, which makes CHCl2I the dominant I-THMs species at high chlorine doses. Except for iopamidol, it appears that the other four ICM did not favor the formation of I-THMs containing more than one iodine atom (i.e., CHClI2, CHBrI2, and CHI3). For instance, only iopamidol-containing samples could yield CHClI2 with a maximum of 45.8 nM during chlorination, while merely small quantities of CHClI2 were produced in other samples. The differences of the investigated ICM in structure (e.g., side chains) may be partly responsible for this result (Duirk et al., 2011). In addition, the concentration of HOI in these samples was probably too low to lead to a significant incorporation of iodine into the THM precursors. Furthermore, a wide range in CHCl2I concentrations between 19.6 and 198.9 nM was observed in iopamidolcontaining samples. CHCl2I and CHClI2 were the predominant I-THMs formed with chlorine dose higher than 50 mM. Duirk et al. (2011) also reported that CHCl2I and CHClI2 dominated the I-THMs species formed at 100 mM chlorine dose. However, a significantly lower level of CHBrClI was produced in their study. This is probably because Br concentration in our water samples (1.63 mM) is much higher than in their experiments (0.15 mM), and higher Br concentration could lead to higher yields of CHBrClI, which will be discussed in detail later.
3.2.
Effect of monochloramine dose on I-THM formation
Fig. 2 shows the I-THM formation from chloramination of ICM-containing samples. Certain levels of all the six I-THMs were observed during chloramination. I-THM formation monotonously increased with increasing dosage of monochloramine. This is principally because increasing monochloramine dose contributes to the transformation of ICM to HOI (Duirk et al., 2011; Kumkum, 2013). Oxidation of HOI to IO 3 by monochloramine is slow, which permits HOI to persist longer and, in turn, it has more opportunity to react with NOM (Bichsel and von Gunten, 1999, 2000).
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Fig. 2 e Effect of monochloramine dose on I-THMs formation at 3 d of reaction during chloramination of ICM in the presence of NOM. [ICM] ¼ 10 mM, pH 7, [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar); D, iopromide (the fourth bar); E, iopamidol (the fifth bar).
It is worthy to note that trace levels of CHI3 could only be produced at relatively higher monochloramine doses for all samples. This could possibly be accounted for by the more HOI needed to produce CHI3 which contains three iodine atoms. During chloramination of iodide-containing waters, CHI3 normally predominates the I-THMs species formed (Bichsel and von Gunten, 2000; Hua and Reckhow, 2007; Duirk et al., 2011), while chlorination shifts the product distribution from CHI3 to the mixed I-THMs (e.g., CHClI2) (Bichsel and von Gunten, 2000). However, during chlorination/chloramination of ICM-containing water, it seems that chlorination does not definitely result in the production of more mixed I-THMs than chloramination, especially at high disinfectant doses (Figs. 1 and 2). Actually, chloramination of ICM-containing water could also produce significant levels of mixed I-THMs. For example, diatrizoate-containing water in chloramination yielded appreciable amounts of mixed I-THMs (i.e., CHClI2, CHBrClI, CHClI2, CHBr2I, and CHBrI2), while three of these mixed I-THMs (CHClI2, CHBr2I, and CHBrI2) were not detected in chlorination (Figs. 1 and 2). During chlorination of iopamidol, CHCl2I was the dominant I-THMs species. However, a similar amount of CHCl2I, CHBrClI, and CHClI2 was formed during chloramination of iopamidol. It is therefore possible to indicate that chlorination/chloramination of ICM-containing water differ with those of iodide-containing water in their ITHM formation mechanisms and pathways. With the increase of monochloramine dose from 25 to 250 mM, I-THMs, which contain two iodine atoms (i.e., CHClI2 and CHBrI2), showed a more pronounced increasing trend than I-THMs with only one iodine atom (i.e., CHCl2I, CHBrClI, and CHBr2I) for all the ICM. For instance, during chloramination of histodenz, yields of I-THMs increased approximately 4
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times for CHCl2I, 2 times for CHBrClI, and 16 times for CHBr2I, while yields of CHClI2 and CHBrI2 increase nearly 24 and 50 times, respectively. This behavior could be attributed to the increased levels of HOI as monochloramine dose increased. It should also be noted that the percentage that chlorinated ITHMs occupied of the total I-THMs declined gradually with the increase of monochloramine dose (Table S5, SI). Take iodixanol for an example. At monochloramine dose of 25 mM, chlorinated I-THMs (i.e., CHCl2I) accounted for all the I-THMs formed. And this proportion dropped to about 79.0% at monochloramine dose of 250 mM (Table S5 C, SI). These consistent trends shared by all the samples may imply that I-THMs tend to be formed through a similar pathway for the five investigated ICM during chloramination. When comparing I-THM formation between chlorination and chloramination of the ICM-containing waters, it is evident that higher amounts of I-THMs were generally formed at relatively low chlorine doses (100 mM) during chlorination (except iopamidol), while chloramination produced larger amounts of I-THMs at high monochloramine doses (100 mM). As demonstrated previously, high doses of chlorine could oxidize HOI to IO 3 . However, high doses of monochloramine not only increased the formation of HOI, but it also enhanced the availability of OCl due to the hydrolysis of monochloramine to free chlorine (HOCl/OCl). In terms of the yield of total I-THMs, chlorination of diatrizoate, histodenz, iodixanol, and iopromide achieved maximum yields of 63.8, 97.6, 84.3, 47.1 nM, respectively, which were significantly lower than that of iopamidolcontaining samples (276.1 nM). With respect to chloramination, the maximum value of iopamidol-containing samples (254.4 nM) was also comparatively higher than those of diatrizoate, histodenz, iodixanol, and iopromide (206.2, 145.1, 153.3, 111.7 mg/L, respectively). The differences in I-THMs yield demonstrate that free chlorine and chloramines reactivity with different ICM may vary significantly due to their structural variation of the side chains on the aromatic rings (Table S1, SI) (Duirk et al., 2010, 2011). During chlorination and chloramination, formation of I-THMs in iopamidol-containing water is more favorable than in water containing other four investigated ICM. Therefore, more attention should be paid to the treatment of iopamidol-containing water during disinfection since iopamidol was the ICM most frequently detected (Duirk et al., 2011) and I-DBPs have already been identified in chlorination and chloramination after iopamidol photolysis (Tian et al., 2014).
3.3.
Effect of pH on I-THM formation
Formation of I-THMs in water containing iopromide and iopamidol were not further investigated because chlorination/ chloramination of iopamidol-containing water had been intensively studied by Duirk et al. (2011), and relatively lower amounts of I-THMs were formed in iopromide-containing samples in our investigation. As shown in Fig. 3a and b, the reaction pH had a strong impact on the formation and distribution of I-THMs during chlorination/chloramination of ICMcontaining water. During chlorination, increasing pH from 5 to 9 consistently promoted the formation of THM4 in samples containing
Fig. 3 e Effect of pH on I-THMs formation at 3 d of reaction during chlorination (a) and chloramination (b) of ICM in the presence of NOM. [ICM] ¼ 10 mM, [Cl2] ¼ 50 mM (a) or [NH2Cl] ¼ 250 mM (b), [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar).
diatrizoate, histodenz, and iodixanol (Fig. S1, SI), but total ITHMs yields did not behave similarly in this trend (Fig. 3). For diatrizoate, the yield of total I-THMs decreased gradually from 77.6 nM at pH 5 to 21.3 nM at pH 9. However, a reverse trend was observed in iodixanol-containing samples that it rose progressively from 56.3 nM at pH 5 to 103.5 nM at pH 9. The yield in histodenz-containing samples had a minimum value of 53.4 nM at pH 5, reached the maximum value of 98.1 nM at pH 7, and finally dropped to 65.2 nM at pH 9. To some extent, these inconsistent behaviors among the three ICM could be attributable to the distribution of HOCl/OCl at different pH (Hua and Reckhow, 2007; Duirk et al., 2011). For diatrizoate, the higher levels of I-THMs formed at lower pH was probably because the protonation of diatrizoate at lower pH enhanced its reactivity with chlorine. It should be mentioned that pH also influences the reaction rate of HOI with humic substance (Bichsel and von
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Gunten, 1999, 2000; Ye et al., 2012, 2013). In addition, pH values can affect the formation of individual DBP (Hua and Reckhow, 2007). Incidentally, the concentrations of THM4 were significantly high compared to I-THMs (Fig. 3 and S1, SI). The reaction between chloramines and NOM to form DBPs involves monochloramine, dichloramine (NHCl2), and free chlorine, which exist simultaneously and vary in concentration with respect to pH values (Jafvert and Valentine, 1992; USEPA, 1999; Greyshock and Vikesland, 2006). A noteworthy result was that comparatively more I-THMs were formed at circumneutral pH (5.5e7.4) for all the ICM-containing samples during chloramination (Fig. 3b), which was consistent with the observation reported by Duirk et al. (2011) where higher levels of I-DBPs were formed at pH <8.5. In all likelihood, this result is ascribable to the auto-decomposition and hydrolysis of NH2Cl to form dichloramine and free chlorine (HOCl/OCl) at pH <8 (Jafvert and Valentine, 1992; USEPA, 1999; Greyshock and Vikesland, 2006; Duirk et al., 2011). OCl was demonstrated by Duirk et al. (2011) to be the primary reactive species with ICM. Furthermore, lowering pH to 5 leads to the enhancement of dichloramine as a result of the acid-catalyzed monochloramine disproportionation reaction (Duirk et al., 2010; Jafvert and Valentine, 1992). Interestingly, a noticeable amount of I-THMs could be produced at pH 5. Extra experiments were done to investigate the formation of I-THMs at pH 5 during dichloramination of ICM. It was found that dichoramine could also lead to the formation of an appreciable amount of I-THMs (Table S6, SI). Our result may infer that OCl and dichloramine are both likely to react concurrently with ICM to contribute to the generation of I-THMs. At pH 9, monochloramine is stable and the major species (Jafvert and Valentine, 1992; Kirmeyer et al., 1993; USEPA, 1999). Notice that CHCl2I and CHClI2 were the only two I-THMs formed and I-THM formation were substantially suppressed at pH 9. This may suggest that the reactivity of monochloramine with ICM may be low and monochloramine alone is unlikely to favor the formation of I-THMs in our study (Duirk et al., 2011). It should also be noted that pH in the range of 5e9 may also have an effect on the reactivity of NOM, which could influence the formation of I-THMs during chlorination/chloramination of ICM-containing water (Hua and Reckhow, 2007). Consequently, based on above discussion and analysis, a reasonable conclusion could be made that formation of I-THMs during chloramination of ICM-containing water highly depends on the availability of dichloramine and free chlorine generated by auto-decomposition and hydrolysis of monochloramine. This could also explain why increasing monochloramine dose consistently increased I-THM formation (Fig. 2). Increasing monochloramine dose results in an increasing amount of dichloramine and free chlorine, which can effectively react with ICM and lead to the formation of HOI. However, the increased free chlorine (HClO/OCl) is unlikely to further oxidize HOI to IO 3 because only trace levels of free chlorine can be present (Wu et al., 2003). Therefore, a considerable amount of I-THMs was formed during chloramination (Fig. 2).
3.4.
Effect of bromide on I-THM formation
Fig. 4a and b illustrates the differences in species and concentrations of I-THMs when ICM-containing water was spiked
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Fig. 4 e Effect of bromide concentration on I-THMs formation at 3 d of reaction during chlorination (a) and chloramination (b) of ICM in the presence of NOM. [ICM] ¼ 10 mM, [Cl2] ¼ 50 mM (a) or [NH2Cl] ¼ 250 mM (b), [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar).
with varying concentrations of bromide during chlorination and chloramination. Generally, increasing initial bromide levels could enhance the formation of total I-THMs. Increasing bromide concentration also notably shifted the I-THMs towards bromine-containing ones and increased the formation of higher bromine-incorporated species (e.g., CHBrClI and CHBr2I), especially during chloramination. During chlorination, with the increase of bromide concentration, I-THMs formed in samples containing diatrizoate and iodixanol increased gradually from the minimums of 57.6 and 84.8 nM at no Br added to the maximums of 69.3 and 183.4 mg/L at 15 mM Br, respectively, while the yield of I-THMs in histodenz-containing samples increased from its minimum of 97.6 nM to its peak value 169.0 nM at 10 mM Br and then declined to 107.1 nM at 15 mM Br (Fig. 4a). An increase in Br
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can increase the level of bromine species (i.e., HOBr/OBr), which then, along with the residual chlorine, reacts with precursors to produce mixed I-THMs (Liang and Singer, 2003). The proportion that the brominated I-THMs comprised of total I-THMs increased substantially and then reached a plateau with increasing levels of bromide for all the samples (Table S7, SI). Furthermore, the increased formation of I-THMs with increasing bromide concentration may also give an indication that free bromine (HOBr/OBr) is likely to react with ICM similarly to the one proposed for free chlorine (Duirk et al., 2011). However, it appears that bromide concentration did not have a significant effect on the formation of CHBr2I and CHClI2, both of which tended to be formed at trace levels, and both CHBrI2 and CHI3 were not detected. With the presence of relatively high bromide levels (e.g., 8 mM), chlorination of ICM-containing waters significantly favored the formation of CHBrClI (Table S8, SI). This is presumably because HOCl, HOBr and HOI were in intense competition with each other to incorporate into I-THM precursors in our system, which makes it difficult to achieve the incorporation of more than one chlorine, bromine, or iodine atom into I-THMs. During chloramination, with increasing bromide concentration, I-THMs formed in samples containing diatrizoate, histodenz, and iodixanol kept increasing gradually to their maximums of 240.2, 173.3 and 197.4 nM, respectively (Fig. 4b). Two possible reasons may account for this result. One is the oxidation of bromide by monochloramine through a series of reactions forms bromamines and bromochloramine (NHBrCl) (Trofe et al., 1980; Bousher et al., 1989), which have been demonstrated to react with NOM to form brominated DBPs (Heller-Grossman et al., 1999). Alternatively, trace amounts of HOBr could be produced by the decomposition of chloramines and bromamines (Trofe et al., 1980; Bousher et al., 1989; Le Roux et al., 2012). The increase of bromide concentration increased the bromine-containing oxidants (i.e., HOBr, NHBrCl, or other bromamines). Hence, the remarkably enhanced formation of I-THMs due to the addition of bromide may support the fact that chloramines (e.g., NH2Cl) is less reactive with ICM than bromine-containing oxidants. Of particular interest was the finding that individual ITHMs showed distinct formation trends during chloramination. With respect to all the samples, as bromide concentration increased, formation of CHCl2I, CHCl2 and CHI3 decreased gradually, while yields of CHBr2I kept increasing. The formation of CHBrClI showed an increasing and then decreasing trends, whilst the increase of bromide concentration had no evident influence on the formation of CHBrI2. Of note, with increased bromide concentration, I-THMs containing more bromine atoms (i.e., CHBr2I, CHBrClI, and CHBrI2) were formed in higher amounts than I-THMs containing chlorine atoms (i.e., CHCl2I and CHClI2), suggesting that the incorporation of bromine into I-THMs was easier than chlorine. In terms of bromide substitution in THM4, bromide incorporation factor (BIF) was used (Symons et al., 1996). In our study, I-THMs were included in the ten THMs (THM10, including THM4 and six I-THMs) and BIF was calculated according to Eqs. (1) and (2). As bromine incorporation into THM10 increased, the value of BIF also increased (Table S9, SI). Increasing bromide concentration may have increased the bromamines species (e.g., NHBrCl) or HOBr thereby leading to
higher bromine substitution degrees of THM10 (Trofe et al., 1980; Uyak and Toroz, 2007; Le Roux et al., 2012). Interestingly, BIF in chloramination was smaller than its corresponding value in chlorination. This is probably because chlorine is more efficient than chloramines in oxidizing bromide to HOBr. We also tried to use bromine substitution factor (BSF) introduced by Hua et al. to investigate the bromine substitution of THM10 (Hua et al., 2006). The ratio of Br/Cl2 was calculated and summarized in Fig. S2, SI. By and large, the relationship between BSF and Br/Cl2 consumption molar ratio falls into the curvilinear trend showed by Hua et al. (2006). BIF ¼
THMBr ðmMÞ THM10 ðmMÞ
THMBr ¼ 1 CHCl2 Br þ 1 CHBrClI þ 1 CHBrI2 þ 2 CHClBr2 þ 2 CHBr2 I þ 3 CHBr3
4.
(1)
(2)
Practical implication
The results from our study suggest that the formation of ITHMs during chlorination and chloramination of ICMcontaining water is influenced by several factors (disinfectant dose, reaction pH, and bromide concentration, etc.). Since ICM may not be present at high levels in raw waters (Duirk et al., 2011), a feasible method to control I-THM formation is to increase chlorine dose. However, this could result in an increased formation of THM4. Chloramines may be a preferable alternative because both THM4 and I-THMs are suppressed at relatively low chloramines doses. However, chloramination is favorable for the formation of I-DBPs when naturally occurring iodide is present in raw water (Bichsel and von Gunten, 2000). Thus, attention should also be paid to the treatment of iodide when iodide and ICM are present simultaneously. The pH also showed a significant effect on I-THM formation. Both chlorination and chloramination produced a considerable amount of I-THMs at circumneutral pH. Bromide ion is present in all natural waters. Increasing bromide levels in source waters poses a challenge to drinking water treatment utilities in that formation of I-THMs could be promoted during chlorination/chloramination. Therefore, one effective strategy is to apply an appropriate dose of chlorine or chloramines. Furthermore, different ICM also differed in their potential for I-THM formation. Chlorination/chloramination of the most frequently detected ICM, iopamidol, resulted in the formation of a substantial amount of I-THMs compared with other investigated ICM. Thus, more attention should be paid to the disinfection of waters containing iopamidol. Both chlorination and chloramination of ICM did not favor the formation of CHI3. Formation of CHI3 may not be an issue during disinfection of ICM-containing waters.
5.
Conclusions
The formation of ICM during chlor(am)ination of five ICM (i.e., iopamidol, diatrizoate, iodixanol, histodenz, and iopromide) was invesitagted. In the presence of NOM, chlor(am)ination of ICM-containing water can lead to the formation of a
w a t e r r e s e a r c h 6 6 ( 2 0 1 4 ) 3 9 0 e3 9 8
substantial amount of I-THMs. Chlorine and monochloramine dose, pH, as well as bromide concentration showed a significant effect on the formation of I-THMs. Among the five investigated ICM, iopamidol was found to yield the maximum I-THMs during chlor(am)ination. For the other four ICM, low chlorine doses (100 mM) was favorable for I-THM formation, while I-THM formation was highly suppressed at high chlorine doses (＞100 mM). During chloramination, I-THM formation increased with the increase of monochloramine dose for all the five ICM. With the increase of pH from 5 to 9, chlorination of iodixanol, histodenz, and diatrizoate showed three different formation patterns. During chloramination, a relatively higher amount of I-THMs was observed at circumneutral pH. Dichloramine was found to be reactive with ICM at low pH (pH 5) and result in the formation of a certain amount of I-THMs, while the reactivity of monochloramine with ICM should be low as only a small quantity of I-THMs was formed at pH 9 where monochloramine was the major chloramines species. With the presence of a relatively high bromide concentration, not only considerable enhancement of I-THMs yield but also the shift of ITHMs towards bromine-containing ones were observed. The increased level of bromine species (i.e., HOBr/OBr formed during chlorination, HOBr, and bromamines and bromochloramine formed during chloramination) could be responsible for these results.
Acknowledgments This study was supported in part by the Natural Science Foundation of China (No. 51278352 and 41301536), the Fundamental Research Funds for the Central Universities in China, and the National Major Science and Technology Project of China (No. 2012ZX07404004 and 2012ZX07408001). We appreciate the critical comments from the anonymous reviewers. We also would like to thank Dr. Danmeng Shuai (GWU) for reviewing and discussion.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2014.08.044.
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Bousher, A., Brimblecombe, P., Midgley, D., 1989. Kinetics of reactions in solutions containing monochloramine and bromide. Water Res. 23, 1049e1058. Christiansen, C., 2005. X-ray contrast mediaean overview. Toxicology 209, 185e187. Duirk, S.E., Desetto, L.M., Davis, G.M., Lindell, C., Cornelison, C.T., 2010. Chloramination of organophosphorus pesticides found in drinking water sources. Water Res. 44, 761e768. Duirk, S.E., Lindell, C., Cornelison, C.C., Kormos, J., Ternes, T.A., Attence-Ramos, M., Osiol, J., Wagner, E.D., Plewa, M.J., Richardson, S.D., 2011. Formation of toxic iodinated disinfection by-products from compounds used in medical imaging. Environ. Sci. Technol. 45, 6845e6854. Greyshock, A.E., Vikesland, P.J., 2006. Triclosan reactivity in chloraminated waters. Environ. Sci. Technol. 40, 2615e2622. Hansson, R.C., Henderson, M.J., Jack, P., Taylor, R.D., 1987. Iodoform taste complaints in chloramination. Water Res. 21, 1265e1271. Heller-Grossman, L., Idin, A., Limoni-Relis, B., Rebhun, M., 1999. Formation of cyanogen bromide and other volatile DBPs in the disinfection of bromide-rich lake water. Environ. Sci. Technol. 33, 932e937. Hua, G., Reckhow, D.A., 2007. Characterization of disinfection by product precursors based on hydrophobicity and molecular size. Environ. Sci. Technol. 41, 3309e3315. Hua, G., Reckhow, D.A., Kim, J., 2006. Effect of bromide and iodide on the formation and speciation of disinfection byproducts during chlorination. Environ. Sci. Technol. 40, 3050e3056. Jafvert, C.T., Valentine, R.L., 1992. Reaction scheme for the chlorination of ammoniacal water. Environ. Sci. Technol. 26, 577e586. Jones, D.B., Saglam, A., Triger, A., Song, H., Karanfil, T., 2011. ITHMs formation and speciation: preformed monochloramine versus prechlorination followed by ammonia addition. Environ. Sci. Technol. 45, 10429e10437. Jones, D.B., Saglam, A., Song, H., Karanfil, T., 2012. The impact of bromide/iodide concentration and ratio on iodinated trihalomethane formation and speciation. Water Res. 46, 11e20. Karpel Vel Leitner, N., Vessella, J., Dore, M., Legube, B., 1998. Chlorination and formation of organoiodinated compounds: the important role of ammonia. Environ. Sci. Technol. 32, 1680e1685. Kirmeyer, G.J., Foust, G.W., Pierson, G.L., Simmler, J.J., LeChevallier, M.W., 1993. Optimizing Chloramine Treatment. American Water Works Association, Denver, CO. Kumkum, P., 2013. Iodinated Pharmaceuticals as Precursors to Total Organic Halogen Formation in the Presence of Chlorinated Oxidants and Absence of Natural Organic Matter (Electronic Thesis or Dissertation). Retrieved from. https://etd. ohiolink.edu/. , J.-P., 2012. Formation of NDMA and Le Roux, J., Gallard, H., Croue halogenated DBPs by chloramination of tertiary amines: the influence of bromide ion. Environ. Sci. Technol. 46, 1581e1589. Liang, L., Singer, P.C., 2003. Factors influencing the formation and relative distribution of haloacetic acids and trihalomethanes in drinking water. Environ. Sci. Technol. 37, 2920e2928. Plewa, M.J., Wagner, E.D., 2009. Quantitative Comparative Mammalian Cell Cytotoxicity and Genotoxicity of Selected Classes of Drinking Water Disinfection Byproducts. American Water Works Research Foundation, Denver, CO. Putschew, A., Wischnack, S., Jekel, M., 2000. Occurrence of triiodinated X-ray contrast agents in the aquatic environment. Sci. Total Environ. 255, 129e134. Richardson, S.D., 2003. Disinfection by-products and other emerging contaminants in drinking water. Trends Anal. Chem. 22 (10), 666e684. 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., Wanger, D.E.,
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Available online at www.sciencedirect.com
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Comparison of iodinated trihalomethanes formation during aqueous chlor(am)ination of different iodinated X-ray contrast media compounds in the presence of natural organic matter Tao Ye a, Bin Xu a,*, Zhen Wang a, Tian-Yang Zhang a, Chen-Yan Hu b, Lin Lin a, Sheng-Ji Xia a, Nai-Yun Gao a a
State Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of Yangtze River Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
article info
abstract
Article history:
Iodinated trihalomethanes (I-THMs) formation during chlorination and chloramination of five
Received 16 May 2014
iodinated X-ray contrast media (ICM) compounds (iopamidol, iopromide, iodixanol, histodenz,
Received in revised form
and diatrizoate) in the presence of natural organic matter (NOM) was evaluated and compared.
25 August 2014
Chlorination and chloramination of ICM in the absence of NOM yielded only a trace amount of
Accepted 26 August 2014
I-THMs, while levels of I-THMs were enhanced substantially in raw water samples. With the
Available online 6 September 2014
presence of NOM, the order with respect to the maximum yield of I-THMs observed during
Keywords:
chloramination, I-THM formation was enhanced for hisodenz, iodixanol, diatrizoate, and
Iodinated trihalomethanes
iopromide. The order with respect to the maximum yield of I-THMs observed during chlor-
chlorination was iopamidol >> histodenz > iodixanol > diatrizoate > iopromide. During
Chlor(am)ination
amination was iopamidol > diatrizoate > iodixanol > histodenz > iopromide. With the
Iodinated X-ray contrast media
exception of iopamidol, I-THM formation was favored at relatively low chlorine doses
Disinfection by-products
(100 mM) during ICM chlorination, and significant suppression was observed with high chlorine doses applied (＞100 mM). However, during chloramination, increasing monochloramine dose monotonously increased the yield of I-THMs for the five ICM. During chlorination of iodixanol, histodenz, and diatrizoate, the yields of I-THMs exhibited three distinct trends as the pH increased from 5 to 9, while peak I-THM formation was found at circumneutral pH for chloramination. Increasing bromide concentration not only considerably enhanced the yield of I-THMs but also shifted the I-THMs towards bromine-containing ones and increased the formation of higher bromine-incorporated species (e.g., CHBrClI and CHBr2I), especially in chloramination. These results are of particular interest to understand I-THM formation mechanisms during chlorination and chloramination of waters containing ICM. © 2014 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: þ86 13918493316; fax: þ86 21 65986313. E-mail address: [email protected] (B. Xu). http://dx.doi.org/10.1016/j.watres.2014.08.044 0043-1354/© 2014 Elsevier Ltd. All rights reserved.
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1.
Introduction
Drinking water disinfection has dramatically reduced the outbreaks of waterborne diseases (e.g., gastroenteritis, cholera and typhoid) (Richardson, 2003; Richardson and Postigo, 2012). However, reactions between dissolved organic matter (DOM) in water and the two widely used low-cost disinfectants, chlorine and chloramines, result in the production of undesired disinfection by-products (DBPs) with potential health concerns (Richardson, 2003; Richardson and Postigo, 2012). Since the first discovery of trihalomethanes (THMs) in chlorinated water in the 1970s (Rook, 1974), DBPs have become a focus of attention in drinking water treatment. Over the past 40 years, regulated DBPs (THMs and haloacetic acid [HAAs]) have been intensively studied, leading to a better understanding of their toxicity, occurrence, and control than other emerging unregulated DBPs, such as iodinated DBPs (IDBPs) (Richardson and Postigo, 2012). In terms of cytotoxicity and genotoxicity, I-DBPs are generally higher than their chlorinated and brominated analogs (Karpel Vel Leitner et al., 1998; Richardson et al., 2008; Plewa and Wagner, 2009). In addition, iodinated trihalomethanes (I-THMs) were found to be associated with unpleasant medicinal and pharmaceutical taste and odor issues (Hansson et al., 1987). Consequently, formation of I-DBPs has given rise to increasing concerns recently (Karpel Vel Leitner et al., 1998; Richardson et al., 2008; Plewa and Wagner, 2009). Normally, I-DBPs are formed by reactions between natural organic matter (NOM) and hypoiodous acid (HOI) produced from the oxidation of iodide (I) (Bichsel and von Gunten, 1999, 2000; Jones et al., 2011, 2012; Ye et al., 2012, 2013). I (from natural sources, sea-water intrusion or brines) present in source waters is believed to be the primary source of iodine in I-DBPs (Bichsel and von Gunten, 1999, 2000; Duirk et al., 2011). Nevertheless, a certain amount of I-DBPs were formed in some drinking water treatment plants (DWTPs) with very low or undetectable levels of I, which implies that iodine sources other than I could also contribute to I-DBP formation (Duirk et al., 2011). Duirk et al. (2011) confirmed that iodine-containing diagnostic pharmaceuticals (i.e., iodinated X-ray contrast media [ICM]) act as an alternative source of iodine and lead to the formation of I-DBPs during disinfection. ICM are widely used for intravascular administration to enable visualization of organ details and enhance the contrast between organs or vessels examined and the surrounding tissues (Putschew et al., 2000). For the sake of minimization of adverse effects on human metabolism, ICM are chemically designed to be inert drugs (Christiansen, 2005). Therefore, they are intrinsically of very low environmental toxicity and optimally do not impose a high risk to human health (StegerHartmann et al., 1999). However, they are too stable and hydrophilic to be effectively removed during conventional wastewater treatment processes, such that they have been found in effluents of wastewater treatment plants, rivers, lakes, and raw drinking water at elevated concentrations (Putschew et al., 2000; Duirk et al., 2011). In the investigation of 10 cities' source waters, four ICM, including iopamidol, iopromide, iohexol, and diatrizoate,
391
were found (Duirk et al., 2011). Iopamidol was the most frequently detected ICM with a concentration up to 2700 ng/ L (Duirk et al., 2011). During chlorination and chloramination, hypochlorite (OCl) was thought to be the primary reactive species with iopamidol to form I-DBPs in the presence of NOM (Duirk et al., 2011). A significant amount of IDBPs was formed, up to 212 nM for dichloroiodomethane (CHCl2I) and 3.0 nM for iodoacetic acid, respectively, for chlorination (Duirk et al., 2011). To the best of our knowledge, that is the only work exploring the formation of I-DBPs during disinfection of ICM-containing waters. However, their study was highly focusing on the formation of I-DBPs during disinfection of iopamidol-containing water. Although bromide is usually present in raw water and can play an important role in I-THM formation (Jones et al., 2012), the influence of the presence of bromide was not investigated yet. Since ICM are emerging contaminants and a new source of iodine, the formation of I-THMs from ICM will likely be different from that of I. Thus, it is of great significance to understand the comprehensive factors that influence I-THMs formation during disinfection of waters containing ICM for the sake of developing effective strategies to control I-DBP formation in drinking water treatment. In this study, we systematically investigate the influence of disinfectant dose, reaction pH and bromide (Br) concentration on the formation of I-THMs during chlorination and chloramination of five different commonly used ICM (Table S1, SI) in the presence of NOM. I-THM formation from five ICM during chlorination and chloramination was compared. Different patterns of I-THM formation during chlorination and chloramination of ICM-containing water under the same conditions were the first time being observed in literature.
2.
Materials and methods
2.1.
Chemicals and reagents
All chemicals were at least of analytical grade except as noted. Iopamidol (99.6%), iopromide (98.6%), and iodixanol (100%) were obtained from U.S. Pharmacopeia (Rockville, MD). Commercial 4e4.99% sodium hypochlorite (NaOCl), histodenz, sodium diatrizoate (99.9%), and iodoform (CHI3, 99.0%) as well as potassium bromide (KI 99.0%) were purchased from SigmaeAldrich. I-THMs, including chlorodiiodomethane (CHClI2, 90e95%), dichloroiodomethane (CHCl2I 95%), bromochloroiodomethane (CHBrClI 95%), dibromoiodomethane (CHBr2I, 90e95%) and bromodiiodomethane (CHBrI2, 90e95%), were obtained from CanSyn Chem. Corp. (Toronto, ON, Canada). Four regulated THMs (THM4) mix standard solution, including chloroform (CHCl3), bromodichloromethane (CHBrCl2), dibromochloromethane (CHBr2Cl) and bromoform (CHBr3), was purchased from Supelco (USA). Methyl tert-butyl ether (MtBE) was purchased from J.T. Baker (USA). All other chemicals were received from Sinopharm Chemical Reagent Co., Ltd. (China) without further purification. All solutions were prepared with ultra-pure water produced from a Milli-Q water purification system (Millipore, USA).
392
2.2.
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Experimental procedures
Raw water samples were collected from the intake of Yangshupu drinking water treatment plant (YDWTP) in Shanghai, China, and then filtered through 0.45 mm membrane filters (Millipore Corp., USA) immediately and stored in the dark at 4 C until used. Raw water characteristics are shown in Table 1. Chlorination and chloramination of water spiked with different ICM were conducted in duplicate under headspacefree conditions in 40-mL glass screw-cap amber vials with PTFE-lined septa. All samples were buffered with 10 mM phosphate. After being dosed with chlorine or monochloramine, samples were stored at 25 ± 1 C in the dark for 72 h. Impact factors, including initial dosage of chlorine and monochloramine, reaction pH, Br concentration and five different ICM (i.e., iopamidol, iopromide, iodixanol, histodenz, and diatrizoate), were evaluated. Solution pH was adjusted using either 0.01, 0.1, 1 M NaOH or H2SO4. Prior to analysis, aliquots of the reacted solution were withdrawn and the remaining disinfectant was quenched using sodium sulfite (Na2SO3) with 20% excess of initial disinfectant dose after 72 h of reaction time (Duirk et al., 2011). Control experiments in the presence of NOM and without the spiking of ICM as well as in the absence of NOM (i.e., Mill-Q water) and with the spiking of ICM were done with similar procedures. Certain levels of I-THMs were observed during chlorination/chloramination of ICM in Mill-Q water (in the absence of NOM, Table S2, SI) and I-THMs were not detectable without the addition of ICM in raw water samples (in the presence of NOM, data no shown). Chloramination was conducted with preformed monochloramine (NH2Cl) prepared by adding NaOCl solution gently into a stirred ammonium chloride (NH4Cl) solution with the N/ Cl molar ratio of 1.2 at pH 8.5 (Xu et al., 2012). Preformed NH2Cl was prepared freshly before each test. The preparation of dichloramine was shown in the supporting information (Table S6, SI). Chlorine and NH2Cl concentration were analyzed using the N,N-diethyl-p-phenylenediamine (DPD) colorimetric method (APHA et al., 1998).
2.3.
(250 mm 4.0 mm i.d.) and a Dionex AG11-HC guard column (50 mm 4.0 mm i.d.). Method for I-THMs measurements was based on our previous research (Ye et al., 2012, 2013). After quenching, samples were quantified by liquid/liquid extraction with MtBE followed by gas chromatography and electron capture detector (GC/ECD, GC-2010, Shimadzu, Japan) and an HP-5 capillary column (30 m 0.25 mm i.d., 0.25 mm film thickness, J&W, USA). THM4 was determined by GC/ECD after liquid/liquid extraction with MtBE according to USEPA Method 551.1 (USEPA, 1995). The detection limits of I-THMs and THM4 are shown in Table S3, SI.
3.
Results and discussion
3.1.
Effect of chlorine dose on I-THM formation
As shown in Fig. 1, chlorination of raw water spiked with the five different ICM resulted in the formation of appreciable quantities of I-THMs. Compared with controlled experiment results (Table S2, SI), formation of I-THMs was highly enhanced, which indicated that the presence of both ICM and NOM is required for the formation of I-THMs and ICM were the source of iodine (Duirk et al., 2011). An especially higher amount of I-THMs was formed during chlorination of iopamidol-containing water compared to water containing other ICM (Fig. 1). It appears that iopamidol formed a significantly higher amount of I-THMs at high chlorine doses (100 mM), while other ICM formed certain levels of I-THMs at low chlorine doses (100 mM) and I-THM formation was substantially suppressed with chlorine doses higher than 100 mM. Chlorine and chloramines initially react with ICM and lead to the formation of HOI, which subsequently reacts
Analytical methods
The pH was measured with an FE20-FiveEasy pH meter (Mettler Toledo, Switzerland) calibrated regularly using standard buffer solutions (Mettler Toledo; pH ¼ 4.01, 7.00, 9.21). Dissolved organic carbon (DOC) was measured by a Shimadzu TOC-VCSH analyzer (Shimadzu, Japan), and the detection limit was 0.1 mg C/L. UV absorption at 254 nm (UV254) was determined with a UV/Vis spectrophotometer (SQ-4802, UNICO, Shanghai). Br was analyzed using an ion chromatography (Dionex ICS-2000, CA, USA) equipped with a conductivity detector, a Dionex AS11-HC analytical column
Table 1 e Characteristics of raw water from YDWTP. Sample
DOC (mg C/L)
UV254 (1/cm)
SUVAa (L/mg m)
Br (mM)
YDWTP
5.7
0.133
2.3
1.63
a
SUVA (specific ultraviolet absorbance) was calculated from ultraviolet absorbance at 254 nm (UV254) divided by DOC.
Fig. 1 e Effect of chlorine dose on I-THMs formation at 3 d of reaction during chlorination of ICM in the presence of NOM. [ICM] ¼ 10 mM, pH 7, [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar); D, iopromide (the fourth bar); E, iopamidol (the fifth bar).
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with NOM to form I-DBPs (Duirk et al., 2011; Kumkum, 2013). Thus, the different behavior of iopamidol-containing samples seems to suggest that oxidation of iopamidol to HOI occurs at comparatively higher chlorine doses than that of other ICM. However, excess doses of chlorine (>100 mM) may oxidize HOI to iodate (IO 3 ), which could be responsible for the decrease of total I-THMs yield at high chlorine doses (Fig. 1) (Bichsel and von Gunten, 1999; Duirk et al., 2011; Kumkum, 2013). Hence, for the prevention of unpleasant medicinal and pharmaceutical taste and odor associated with I-THMs, a reasonable solution is to increase chlorine dose. Nevertheless, this may enhance the formation of THM4 (Table S4, SI). For example, during chlorination of iopamidol-containing samples, with the increase of chlorine dose from 25 to 250 mM, the sum of THM4 increased from 0.59 to 2.31 mM. CHCl2I and CHBrClI were the only two I-THMs formed with measurable levels during chlorination of diatrizoate, while other ICM could also produce a certain amount of CHBr2I and CHClI2 at relatively low doses of chlorine. CHBrI2 and CHI3 were not detected in all samples during chlorination. With the increase of chlorine dose from 50 to 100 mM, the formation of CHCl2I was significantly enhanced for iopamidol, which makes CHCl2I the dominant I-THMs species at high chlorine doses. Except for iopamidol, it appears that the other four ICM did not favor the formation of I-THMs containing more than one iodine atom (i.e., CHClI2, CHBrI2, and CHI3). For instance, only iopamidol-containing samples could yield CHClI2 with a maximum of 45.8 nM during chlorination, while merely small quantities of CHClI2 were produced in other samples. The differences of the investigated ICM in structure (e.g., side chains) may be partly responsible for this result (Duirk et al., 2011). In addition, the concentration of HOI in these samples was probably too low to lead to a significant incorporation of iodine into the THM precursors. Furthermore, a wide range in CHCl2I concentrations between 19.6 and 198.9 nM was observed in iopamidolcontaining samples. CHCl2I and CHClI2 were the predominant I-THMs formed with chlorine dose higher than 50 mM. Duirk et al. (2011) also reported that CHCl2I and CHClI2 dominated the I-THMs species formed at 100 mM chlorine dose. However, a significantly lower level of CHBrClI was produced in their study. This is probably because Br concentration in our water samples (1.63 mM) is much higher than in their experiments (0.15 mM), and higher Br concentration could lead to higher yields of CHBrClI, which will be discussed in detail later.
3.2.
Effect of monochloramine dose on I-THM formation
Fig. 2 shows the I-THM formation from chloramination of ICM-containing samples. Certain levels of all the six I-THMs were observed during chloramination. I-THM formation monotonously increased with increasing dosage of monochloramine. This is principally because increasing monochloramine dose contributes to the transformation of ICM to HOI (Duirk et al., 2011; Kumkum, 2013). Oxidation of HOI to IO 3 by monochloramine is slow, which permits HOI to persist longer and, in turn, it has more opportunity to react with NOM (Bichsel and von Gunten, 1999, 2000).
393
Fig. 2 e Effect of monochloramine dose on I-THMs formation at 3 d of reaction during chloramination of ICM in the presence of NOM. [ICM] ¼ 10 mM, pH 7, [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar); D, iopromide (the fourth bar); E, iopamidol (the fifth bar).
It is worthy to note that trace levels of CHI3 could only be produced at relatively higher monochloramine doses for all samples. This could possibly be accounted for by the more HOI needed to produce CHI3 which contains three iodine atoms. During chloramination of iodide-containing waters, CHI3 normally predominates the I-THMs species formed (Bichsel and von Gunten, 2000; Hua and Reckhow, 2007; Duirk et al., 2011), while chlorination shifts the product distribution from CHI3 to the mixed I-THMs (e.g., CHClI2) (Bichsel and von Gunten, 2000). However, during chlorination/chloramination of ICM-containing water, it seems that chlorination does not definitely result in the production of more mixed I-THMs than chloramination, especially at high disinfectant doses (Figs. 1 and 2). Actually, chloramination of ICM-containing water could also produce significant levels of mixed I-THMs. For example, diatrizoate-containing water in chloramination yielded appreciable amounts of mixed I-THMs (i.e., CHClI2, CHBrClI, CHClI2, CHBr2I, and CHBrI2), while three of these mixed I-THMs (CHClI2, CHBr2I, and CHBrI2) were not detected in chlorination (Figs. 1 and 2). During chlorination of iopamidol, CHCl2I was the dominant I-THMs species. However, a similar amount of CHCl2I, CHBrClI, and CHClI2 was formed during chloramination of iopamidol. It is therefore possible to indicate that chlorination/chloramination of ICM-containing water differ with those of iodide-containing water in their ITHM formation mechanisms and pathways. With the increase of monochloramine dose from 25 to 250 mM, I-THMs, which contain two iodine atoms (i.e., CHClI2 and CHBrI2), showed a more pronounced increasing trend than I-THMs with only one iodine atom (i.e., CHCl2I, CHBrClI, and CHBr2I) for all the ICM. For instance, during chloramination of histodenz, yields of I-THMs increased approximately 4
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times for CHCl2I, 2 times for CHBrClI, and 16 times for CHBr2I, while yields of CHClI2 and CHBrI2 increase nearly 24 and 50 times, respectively. This behavior could be attributed to the increased levels of HOI as monochloramine dose increased. It should also be noted that the percentage that chlorinated ITHMs occupied of the total I-THMs declined gradually with the increase of monochloramine dose (Table S5, SI). Take iodixanol for an example. At monochloramine dose of 25 mM, chlorinated I-THMs (i.e., CHCl2I) accounted for all the I-THMs formed. And this proportion dropped to about 79.0% at monochloramine dose of 250 mM (Table S5 C, SI). These consistent trends shared by all the samples may imply that I-THMs tend to be formed through a similar pathway for the five investigated ICM during chloramination. When comparing I-THM formation between chlorination and chloramination of the ICM-containing waters, it is evident that higher amounts of I-THMs were generally formed at relatively low chlorine doses (100 mM) during chlorination (except iopamidol), while chloramination produced larger amounts of I-THMs at high monochloramine doses (100 mM). As demonstrated previously, high doses of chlorine could oxidize HOI to IO 3 . However, high doses of monochloramine not only increased the formation of HOI, but it also enhanced the availability of OCl due to the hydrolysis of monochloramine to free chlorine (HOCl/OCl). In terms of the yield of total I-THMs, chlorination of diatrizoate, histodenz, iodixanol, and iopromide achieved maximum yields of 63.8, 97.6, 84.3, 47.1 nM, respectively, which were significantly lower than that of iopamidolcontaining samples (276.1 nM). With respect to chloramination, the maximum value of iopamidol-containing samples (254.4 nM) was also comparatively higher than those of diatrizoate, histodenz, iodixanol, and iopromide (206.2, 145.1, 153.3, 111.7 mg/L, respectively). The differences in I-THMs yield demonstrate that free chlorine and chloramines reactivity with different ICM may vary significantly due to their structural variation of the side chains on the aromatic rings (Table S1, SI) (Duirk et al., 2010, 2011). During chlorination and chloramination, formation of I-THMs in iopamidol-containing water is more favorable than in water containing other four investigated ICM. Therefore, more attention should be paid to the treatment of iopamidol-containing water during disinfection since iopamidol was the ICM most frequently detected (Duirk et al., 2011) and I-DBPs have already been identified in chlorination and chloramination after iopamidol photolysis (Tian et al., 2014).
3.3.
Effect of pH on I-THM formation
Formation of I-THMs in water containing iopromide and iopamidol were not further investigated because chlorination/ chloramination of iopamidol-containing water had been intensively studied by Duirk et al. (2011), and relatively lower amounts of I-THMs were formed in iopromide-containing samples in our investigation. As shown in Fig. 3a and b, the reaction pH had a strong impact on the formation and distribution of I-THMs during chlorination/chloramination of ICMcontaining water. During chlorination, increasing pH from 5 to 9 consistently promoted the formation of THM4 in samples containing
Fig. 3 e Effect of pH on I-THMs formation at 3 d of reaction during chlorination (a) and chloramination (b) of ICM in the presence of NOM. [ICM] ¼ 10 mM, [Cl2] ¼ 50 mM (a) or [NH2Cl] ¼ 250 mM (b), [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar).
diatrizoate, histodenz, and iodixanol (Fig. S1, SI), but total ITHMs yields did not behave similarly in this trend (Fig. 3). For diatrizoate, the yield of total I-THMs decreased gradually from 77.6 nM at pH 5 to 21.3 nM at pH 9. However, a reverse trend was observed in iodixanol-containing samples that it rose progressively from 56.3 nM at pH 5 to 103.5 nM at pH 9. The yield in histodenz-containing samples had a minimum value of 53.4 nM at pH 5, reached the maximum value of 98.1 nM at pH 7, and finally dropped to 65.2 nM at pH 9. To some extent, these inconsistent behaviors among the three ICM could be attributable to the distribution of HOCl/OCl at different pH (Hua and Reckhow, 2007; Duirk et al., 2011). For diatrizoate, the higher levels of I-THMs formed at lower pH was probably because the protonation of diatrizoate at lower pH enhanced its reactivity with chlorine. It should be mentioned that pH also influences the reaction rate of HOI with humic substance (Bichsel and von
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Gunten, 1999, 2000; Ye et al., 2012, 2013). In addition, pH values can affect the formation of individual DBP (Hua and Reckhow, 2007). Incidentally, the concentrations of THM4 were significantly high compared to I-THMs (Fig. 3 and S1, SI). The reaction between chloramines and NOM to form DBPs involves monochloramine, dichloramine (NHCl2), and free chlorine, which exist simultaneously and vary in concentration with respect to pH values (Jafvert and Valentine, 1992; USEPA, 1999; Greyshock and Vikesland, 2006). A noteworthy result was that comparatively more I-THMs were formed at circumneutral pH (5.5e7.4) for all the ICM-containing samples during chloramination (Fig. 3b), which was consistent with the observation reported by Duirk et al. (2011) where higher levels of I-DBPs were formed at pH <8.5. In all likelihood, this result is ascribable to the auto-decomposition and hydrolysis of NH2Cl to form dichloramine and free chlorine (HOCl/OCl) at pH <8 (Jafvert and Valentine, 1992; USEPA, 1999; Greyshock and Vikesland, 2006; Duirk et al., 2011). OCl was demonstrated by Duirk et al. (2011) to be the primary reactive species with ICM. Furthermore, lowering pH to 5 leads to the enhancement of dichloramine as a result of the acid-catalyzed monochloramine disproportionation reaction (Duirk et al., 2010; Jafvert and Valentine, 1992). Interestingly, a noticeable amount of I-THMs could be produced at pH 5. Extra experiments were done to investigate the formation of I-THMs at pH 5 during dichloramination of ICM. It was found that dichoramine could also lead to the formation of an appreciable amount of I-THMs (Table S6, SI). Our result may infer that OCl and dichloramine are both likely to react concurrently with ICM to contribute to the generation of I-THMs. At pH 9, monochloramine is stable and the major species (Jafvert and Valentine, 1992; Kirmeyer et al., 1993; USEPA, 1999). Notice that CHCl2I and CHClI2 were the only two I-THMs formed and I-THM formation were substantially suppressed at pH 9. This may suggest that the reactivity of monochloramine with ICM may be low and monochloramine alone is unlikely to favor the formation of I-THMs in our study (Duirk et al., 2011). It should also be noted that pH in the range of 5e9 may also have an effect on the reactivity of NOM, which could influence the formation of I-THMs during chlorination/chloramination of ICM-containing water (Hua and Reckhow, 2007). Consequently, based on above discussion and analysis, a reasonable conclusion could be made that formation of I-THMs during chloramination of ICM-containing water highly depends on the availability of dichloramine and free chlorine generated by auto-decomposition and hydrolysis of monochloramine. This could also explain why increasing monochloramine dose consistently increased I-THM formation (Fig. 2). Increasing monochloramine dose results in an increasing amount of dichloramine and free chlorine, which can effectively react with ICM and lead to the formation of HOI. However, the increased free chlorine (HClO/OCl) is unlikely to further oxidize HOI to IO 3 because only trace levels of free chlorine can be present (Wu et al., 2003). Therefore, a considerable amount of I-THMs was formed during chloramination (Fig. 2).
3.4.
Effect of bromide on I-THM formation
Fig. 4a and b illustrates the differences in species and concentrations of I-THMs when ICM-containing water was spiked
395
Fig. 4 e Effect of bromide concentration on I-THMs formation at 3 d of reaction during chlorination (a) and chloramination (b) of ICM in the presence of NOM. [ICM] ¼ 10 mM, [Cl2] ¼ 50 mM (a) or [NH2Cl] ¼ 250 mM (b), [buffer] ¼ 10 mM, temperature ¼ 25 ± 1 C. Error bars represent one standard deviation of replicate measurements. A, diatrizoate (the first bar); B, hiztodenz (the second bar); C, iodixanol (the third bar).
with varying concentrations of bromide during chlorination and chloramination. Generally, increasing initial bromide levels could enhance the formation of total I-THMs. Increasing bromide concentration also notably shifted the I-THMs towards bromine-containing ones and increased the formation of higher bromine-incorporated species (e.g., CHBrClI and CHBr2I), especially during chloramination. During chlorination, with the increase of bromide concentration, I-THMs formed in samples containing diatrizoate and iodixanol increased gradually from the minimums of 57.6 and 84.8 nM at no Br added to the maximums of 69.3 and 183.4 mg/L at 15 mM Br, respectively, while the yield of I-THMs in histodenz-containing samples increased from its minimum of 97.6 nM to its peak value 169.0 nM at 10 mM Br and then declined to 107.1 nM at 15 mM Br (Fig. 4a). An increase in Br
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can increase the level of bromine species (i.e., HOBr/OBr), which then, along with the residual chlorine, reacts with precursors to produce mixed I-THMs (Liang and Singer, 2003). The proportion that the brominated I-THMs comprised of total I-THMs increased substantially and then reached a plateau with increasing levels of bromide for all the samples (Table S7, SI). Furthermore, the increased formation of I-THMs with increasing bromide concentration may also give an indication that free bromine (HOBr/OBr) is likely to react with ICM similarly to the one proposed for free chlorine (Duirk et al., 2011). However, it appears that bromide concentration did not have a significant effect on the formation of CHBr2I and CHClI2, both of which tended to be formed at trace levels, and both CHBrI2 and CHI3 were not detected. With the presence of relatively high bromide levels (e.g., 8 mM), chlorination of ICM-containing waters significantly favored the formation of CHBrClI (Table S8, SI). This is presumably because HOCl, HOBr and HOI were in intense competition with each other to incorporate into I-THM precursors in our system, which makes it difficult to achieve the incorporation of more than one chlorine, bromine, or iodine atom into I-THMs. During chloramination, with increasing bromide concentration, I-THMs formed in samples containing diatrizoate, histodenz, and iodixanol kept increasing gradually to their maximums of 240.2, 173.3 and 197.4 nM, respectively (Fig. 4b). Two possible reasons may account for this result. One is the oxidation of bromide by monochloramine through a series of reactions forms bromamines and bromochloramine (NHBrCl) (Trofe et al., 1980; Bousher et al., 1989), which have been demonstrated to react with NOM to form brominated DBPs (Heller-Grossman et al., 1999). Alternatively, trace amounts of HOBr could be produced by the decomposition of chloramines and bromamines (Trofe et al., 1980; Bousher et al., 1989; Le Roux et al., 2012). The increase of bromide concentration increased the bromine-containing oxidants (i.e., HOBr, NHBrCl, or other bromamines). Hence, the remarkably enhanced formation of I-THMs due to the addition of bromide may support the fact that chloramines (e.g., NH2Cl) is less reactive with ICM than bromine-containing oxidants. Of particular interest was the finding that individual ITHMs showed distinct formation trends during chloramination. With respect to all the samples, as bromide concentration increased, formation of CHCl2I, CHCl2 and CHI3 decreased gradually, while yields of CHBr2I kept increasing. The formation of CHBrClI showed an increasing and then decreasing trends, whilst the increase of bromide concentration had no evident influence on the formation of CHBrI2. Of note, with increased bromide concentration, I-THMs containing more bromine atoms (i.e., CHBr2I, CHBrClI, and CHBrI2) were formed in higher amounts than I-THMs containing chlorine atoms (i.e., CHCl2I and CHClI2), suggesting that the incorporation of bromine into I-THMs was easier than chlorine. In terms of bromide substitution in THM4, bromide incorporation factor (BIF) was used (Symons et al., 1996). In our study, I-THMs were included in the ten THMs (THM10, including THM4 and six I-THMs) and BIF was calculated according to Eqs. (1) and (2). As bromine incorporation into THM10 increased, the value of BIF also increased (Table S9, SI). Increasing bromide concentration may have increased the bromamines species (e.g., NHBrCl) or HOBr thereby leading to
higher bromine substitution degrees of THM10 (Trofe et al., 1980; Uyak and Toroz, 2007; Le Roux et al., 2012). Interestingly, BIF in chloramination was smaller than its corresponding value in chlorination. This is probably because chlorine is more efficient than chloramines in oxidizing bromide to HOBr. We also tried to use bromine substitution factor (BSF) introduced by Hua et al. to investigate the bromine substitution of THM10 (Hua et al., 2006). The ratio of Br/Cl2 was calculated and summarized in Fig. S2, SI. By and large, the relationship between BSF and Br/Cl2 consumption molar ratio falls into the curvilinear trend showed by Hua et al. (2006). BIF ¼
THMBr ðmMÞ THM10 ðmMÞ
THMBr ¼ 1 CHCl2 Br þ 1 CHBrClI þ 1 CHBrI2 þ 2 CHClBr2 þ 2 CHBr2 I þ 3 CHBr3
4.
(1)
(2)
Practical implication
The results from our study suggest that the formation of ITHMs during chlorination and chloramination of ICMcontaining water is influenced by several factors (disinfectant dose, reaction pH, and bromide concentration, etc.). Since ICM may not be present at high levels in raw waters (Duirk et al., 2011), a feasible method to control I-THM formation is to increase chlorine dose. However, this could result in an increased formation of THM4. Chloramines may be a preferable alternative because both THM4 and I-THMs are suppressed at relatively low chloramines doses. However, chloramination is favorable for the formation of I-DBPs when naturally occurring iodide is present in raw water (Bichsel and von Gunten, 2000). Thus, attention should also be paid to the treatment of iodide when iodide and ICM are present simultaneously. The pH also showed a significant effect on I-THM formation. Both chlorination and chloramination produced a considerable amount of I-THMs at circumneutral pH. Bromide ion is present in all natural waters. Increasing bromide levels in source waters poses a challenge to drinking water treatment utilities in that formation of I-THMs could be promoted during chlorination/chloramination. Therefore, one effective strategy is to apply an appropriate dose of chlorine or chloramines. Furthermore, different ICM also differed in their potential for I-THM formation. Chlorination/chloramination of the most frequently detected ICM, iopamidol, resulted in the formation of a substantial amount of I-THMs compared with other investigated ICM. Thus, more attention should be paid to the disinfection of waters containing iopamidol. Both chlorination and chloramination of ICM did not favor the formation of CHI3. Formation of CHI3 may not be an issue during disinfection of ICM-containing waters.
5.
Conclusions
The formation of ICM during chlor(am)ination of five ICM (i.e., iopamidol, diatrizoate, iodixanol, histodenz, and iopromide) was invesitagted. In the presence of NOM, chlor(am)ination of ICM-containing water can lead to the formation of a
w a t e r r e s e a r c h 6 6 ( 2 0 1 4 ) 3 9 0 e3 9 8
substantial amount of I-THMs. Chlorine and monochloramine dose, pH, as well as bromide concentration showed a significant effect on the formation of I-THMs. Among the five investigated ICM, iopamidol was found to yield the maximum I-THMs during chlor(am)ination. For the other four ICM, low chlorine doses (100 mM) was favorable for I-THM formation, while I-THM formation was highly suppressed at high chlorine doses (＞100 mM). During chloramination, I-THM formation increased with the increase of monochloramine dose for all the five ICM. With the increase of pH from 5 to 9, chlorination of iodixanol, histodenz, and diatrizoate showed three different formation patterns. During chloramination, a relatively higher amount of I-THMs was observed at circumneutral pH. Dichloramine was found to be reactive with ICM at low pH (pH 5) and result in the formation of a certain amount of I-THMs, while the reactivity of monochloramine with ICM should be low as only a small quantity of I-THMs was formed at pH 9 where monochloramine was the major chloramines species. With the presence of a relatively high bromide concentration, not only considerable enhancement of I-THMs yield but also the shift of ITHMs towards bromine-containing ones were observed. The increased level of bromine species (i.e., HOBr/OBr formed during chlorination, HOBr, and bromamines and bromochloramine formed during chloramination) could be responsible for these results.
Acknowledgments This study was supported in part by the Natural Science Foundation of China (No. 51278352 and 41301536), the Fundamental Research Funds for the Central Universities in China, and the National Major Science and Technology Project of China (No. 2012ZX07404004 and 2012ZX07408001). We appreciate the critical comments from the anonymous reviewers. We also would like to thank Dr. Danmeng Shuai (GWU) for reviewing and discussion.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2014.08.044.
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