Article pubs.acs.org/est
Mechanistic Study on the Formation of Cl-/Br-/I-Trihalomethanes during Chlorination/Chloramination Combined with a Theoretical Cytotoxicity Evaluation Sebastien Allard,† Jace Tan,† Cynthia A. Joll,† and Urs von Gunten*,‡,§ †
Curtin Water Quality Research Centre, Department of Chemistry, Curtin University, GPO Box U1987, Perth Western Australia 6845, Australia ‡ Eawag, Swiss Federal Institute of Aquatic Science and Technology, ETH Zürich, 8600 Zürich, Switzerland § School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale Lausanne (EPFL), 1015 Lausanne, Switzerland S Supporting Information *
ABSTRACT: Chlorination followed by chloramination can be used to mitigate the formation of potentially toxic iodinated disinfection byproducts (I-DBPs) while controlling the formation of regulated chloro-bromo-DBPs (Cl-/Br-DBPs). Water samples containing dissolved organic matter (DOM) isolates were subjected to 3 disinfection scenarios: NH2Cl, prechlorination followed by ammonia addition, and HOCl alone. A theoretical cytotoxicity evaluation was carried out based on the trihalomethanes (THMs) formed. This study demonstrates that the presence of bromide not only enhances the yield and rate of iodate formation, it also increases the formation of brominated I-THM precursors. A shift in the speciation from CHCl2I to the more toxic CHBr2I, as well as increased iodine incorporation in THMs, was observed in the presence of bromide. For low bromide concentrations, a decrease in I-THM formation and theoretical cytotoxicity was achieved only for high prechlorination times, while for high bromide concentrations, a short prechlorination time enabled the full conversion of iodide to iodate. For low DOM concentrations or DOM with low reactivity, Br-/I-THMs were preferentially formed for short prechlorination times, inducing high cytotoxicity. However, for high chlorine exposures, the cytotoxicity induced by the formation of regulated THMs might outweigh the benefit of I-THM mitigation. For high DOM concentrations or DOM with higher reactivity, mixed I-THMs were formed together with high concentrations of regulated THMs. In this case, based on the cytotoxicity of the THMs formed, the use of NH2Cl is recommended.
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INTRODUCTION The main chemical disinfectant used worldwide in water treatment is chlorine (HOCl and ClO−; in this paper, “chlorine” will be used to represent both species and an analogous approach will be used for bromine and iodine). Besides disinfection, chlorine reacts with water matrix components,1 e.g. organic (dissolved organic matter (DOM)) or inorganic compounds (mainly bromide (Br−) and iodide (I−)), thereby forming halogenated disinfection byproducts (DBPs).2−6 Some brominated and chlorinated DBPs are regulated, with an important class being the trihalomethanes (THMs).7,8 The THMs are often used as surrogates for halogenated DBPs in general, to ensure that no adverse health effects have to be anticipated. In some areas, where waters contain high concentrations of DBP precursors, alternative disinfectants are currently being used to cope with the stringent regulatory limits. Monochloramine (NH2Cl) is often used as an alternative disinfectant because it is less reactive with DOM and therefore forms lower concentrations of regulated DBPs.9,10 In © 2015 American Chemical Society
practice, NH2Cl is mostly prepared in situ; a free chlorine contact time is allowed before ammonium is added, which reacts as NH3 with HOCl and forms NH2Cl.11 Even though NH2Cl forms smaller concentrations of regulated DBPs, emerging DBPs, such as iodinated DBPs (I-DBPs) or Nnitrosodimethylamine (NDMA), can be produced.5,6,12−14 I-DBPs are of particular concern because they have been reported to be more cytotoxic and genotoxic than the corresponding regulated chlorinated and brominated DBPs.5,14,15 The speciation of halogenated DBPs is of particular importance since each species has a different level of toxicity14 and it is generally accepted that for a similar class of DBPs, the toxicity increases in the order Cl- < Br- < I-DBPs.5,15−18 It has recently been demonstrated that both cytotoxicity and Received: Revised: Accepted: Published: 11105
May 29, 2015 July 31, 2015 August 17, 2015 August 17, 2015 DOI: 10.1021/acs.est.5b02624 Environ. Sci. Technol. 2015, 49, 11105−11114
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Environmental Science & Technology
The reaction solutions were constituted with iodide and bromide at concentrations relevant for drinking waters. Three disinfection scenarios were investigated: (i) concurrent addition of chlorine and ammonia, (ii) prechlorination with varying contact times followed by ammonia addition, and (iii) addition of chlorine only. Subsequently, a theoretical cytotoxicity assessment was carried out, by calculating the overall cytotoxicity from the concentrations of the formed THMs and their known chronic cytotoxicity.
genotoxicity were correlated with total organic iodine (TOI, which is used as a bulk parameter for measuring the sum of iodinated DBPs) but not to total organic chlorine (TOCl).19 However, it is labor intensive to assess the cytotoxicity and genotoxicity of a large number of samples. Therefore, because THMs are typically used as surrogate parameter for halogenated DBPs, the toxicity of disinfected waters can be evaluated based on THM concentrations (including I-THMs) and the relative toxicity of each individual THM.20 This led to a different interpretation of the results since each species has a differing level of toxicity. In iodide-containing waters, NH2Cl promotes the formation of I-DBPs: hypoiodous acid (HOI) is formed21 and reacts with DOM to produce I-DBPs3,14,22,23 since NH2Cl is not able to further oxidize HOI to iodate (IO3−),21 which is a nontoxic and stable form of iodine in drinking water.24 Therefore, under these conditions, HOI remains relatively stable during chloramination and a high formation of I-DBPs is expected. Several options have been investigated to reduce the formation of I-DBPs. Recently, it was demonstrated that preozonation can avoid the formation of I-DBPs25 since the oxidation of I− to IO3− is very fast.21 Another option is the application of prechlorination followed by chloramination (chlorination/ chloramination process),26,27 in which chlorine is followed by ammonia addition to form NH2Cl. A prechlorination time is beneficial, since, in contrast to NH2Cl, HOCl oxidizes iodide to iodate,21 thereby minimizing the formation of I-DBPs. After prechlorination, ammonia is added to form NH2Cl, which mitigates the formation of regulated DBPs. The formation of IDBPs in the chlorination/chloramination process depends strongly on the concentration of bromide. It reacts with HOCl to form HOBr,28 which accelerates the oxidation of HOI to iodate,29 and influences the formation and speciation of ITHMs.26,27,29 HOBr oxidizes HOI to iodate by a bromidecatalyzed process with a higher second order rate constant than HOCl.29 In addition, HOBr is more reactive toward phenolic moieties within DOM than HOCl30−32 and it may produce higher concentrations of brominated precursors available for mixed I-THM formation. Whereas in the absence of bromide just three I-THMs are formed during chlorination (CHCl2I, CHClI2, and CHI3), its presence leads to three additional ITHMs (CHBr2I, CHBrI2 and CHBrClI) (in the current study, the THMs include these six Cl-/Br-/I-THMs and the four regulated THMs (4 Cl-/Br-THMs)). Even though the effect of inorganic precursors (Br−, I−) on THM formation has been investigated in several studies,26,27,29 the type and concentration of organic precursor, e.g. DOM, is less well understood and may affect the fate of iodine in the chlorination/chloramination process. It has been shown that for waters with low aromaticity (i.e., DOM with low specific UV absorbance at 254 nm (SUVA254)), the I-THM formation was higher from preformed NH2Cl than from prechlorination followed by ammonia addition.27,33 An opposite trend was observed for waters with high aromaticity (high SUVA254), in which the highest formation of I-THMs was observed from prechlorination compared to preformed NH2Cl.27 The aim of this study was to elucidate the fate of iodine in the chlorination/chloramination process. In contrast to previous studies in which natural waters were spiked with iodide and/or bromide at various concentrations,26,27,29 in the current investigation various DOM extracts from the International Humic Substance Society (IHSS) were used to avoid interferences from other components of natural water matrices.
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MATERIALS AND METHODS Reagents and Analytical Methods. All experiments were carried out with ultrapure water from an ELGA purification system (resistivity of 18.2 MΩ cm). All chemicals used were of the highest analytical grade (AR grade ≥99%). DOM samples (Nordic Reservoir (NR), Suwannee River (SR)) and a fulvic acid sample (Pony Lake (PL)) with SUVA254 values of 4.85, 4.45, and 3.06 L/(mg C·m), respectively, were obtained from the International Humic Substance Society (for details see Text S1). Iodide, bromide, and iodate were analyzed simultaneously via ion chromatography using a Dionex ICS3000 (AG9HC/ AS9HC) followed by a postcolumn reaction.34 The limits of quantification were 5 μg/L for iodide, 2 μg/L for bromide, and 1 μg/L for iodate. The 10 THMs, i.e. Cl-/Br-/I-THMs, were analyzed simultaneously by headspace solid-phase microextraction gas chromatography−mass spectrometry (for details see Text S2).35 Error bars correspond to duplicate analyses on each sample. The pH of the solutions was controlled by a 5 mM phosphate buffer (pH 8). Standard stock solutions of chlorine were prepared from a reagent grade sodium hypochlorite solution (10% active chlorine) and calibrated weekly by iodometric titration. Working solutions were prepared daily and measured with the DPD colorimetric method, using a SHIMADZU UV Pharmaspec 1700 spectrophotometer.36 The DOC concentration of the samples was determined by the UV/persulfate oxidation method, using a Shimadzu TOC Analyzer. Experimental Procedure. Preliminary experiments were carried out to determine the chlorine demand for each synthetic water and to ensure an oxidant residual after the prechlorination step. For each set of experiments, aliquots of aqueous solutions of DOM, iodide, and bromide were added into a 500 mL volumetric flask containing ultrapure water with 5 mM phosphate buffer to achieve the following concentrations: 1−4 mg C/L of DOM, 0.5 μM (63 μg/L) iodide, and 1−25 μM (80 μg/L to 2 mg/L) bromide. Each synthetic water solution was then divided into eight 50 mL samples. Part of the remaining bulk solution was used as a “blank” to confirm the initial concentrations of the precursors. The eight samples were placed on a multistirrer plate and stirred vigorously. The reaction was initiated by the addition of chlorine and the stirring was stopped once the addition was complete. The chlorine reaction was quenched after various reaction times (2, 5, 10, 20, and 30 min) by addition of an aliquot of NH4Cl under vigorous stirring (5 times molar excess with respect to the initial HOCl concentration) thereby forming NH2Cl. Just before quenching, an aliquot of the solution was analyzed with the DPD method to estimate the residual oxidant concentration. In this paper, the residual oxidant concentration is expressed as HOX, being the sum of the concentrations of chlorine, bromine, and iodine. After quenching with NH4Cl, the stirring was stopped and the samples were covered with parafilm. It was verified that THM loss by volatilization was 11106
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prechlorination, was observed for short prechlorination times (2 and 5 min, see below for explanation) followed by a decrease for longer prechlorination times (10−30 min). The concentrations of CHCl2I and CHClI2 were very similar in all the chlorine followed by ammonium addition experiments. For other DOM extracts, SR and NR (Figures S1b, c), a slight increase of CHCl2I was observed with increasing prechlorination time. Overall, the concentration of CHI3, as well as the total I-THM concentration, increased for short prechlorination times (see below for explanation). This means that not only the competition between DOM and chlorine for reaction with HOI has to be taken into consideration but also the formation of THM precursors from reaction of chlorine with DOM. When low concentrations of bromide were present in the solution, the same trend was observed, although the speciation of I-THMs shifted from Cl-/I-THMs to Br-/I-THMs (Figures S1a-r). Impact of Bromide/Iodide Ratio on Formation and Speciation of I-THMs. To better understand the influence of bromide, the concentration of bromide was varied from 0 to 25 μM while the iodide concentration was kept constant ([I−]0 = 0.5 μM). This led to molar bromide/iodide ratios ranging from 0 to 50, comparable to a recent survey in US waters.37 As shown in Figure 2a−c, the addition of bromide to the system significantly affected the speciation of I-THMs (for a complete data set, see Figures S1a-r). In the absence of bromide, the main I-THM was found to be CHCl2I (Figure 2a). When the bromide concentration was increased to a bromide/iodide ratio of 10, mixed Cl-/Br-/ITHMs were formed at differing concentrations depending on the prechlorination time (Figure 2b). However, none of the 6 ITHMs dominated the speciation. When the bromide/iodide ratio was increased to 50, Br-/I-THMs clearly became the major species with highest concentrations of CHBr2I. This observation shows that bromine produced I-THM precursors at a higher rate and yield than chlorine, even though chlorine was in excess. This is consistent with previous reports that bromine is generally a better agent for electrophilic substitution and addition than chlorine.32,38 The evolution of the percent iodine-incorporation in ITHMs as a function of the bromide/iodide ratio is shown in Figure 3 for experiments with NR (see Figure S2a,b for corresponding experiments with PL and SR). The percent iodine-incorporation in I-THMs based on the initial iodide concentration is shown as a function of the oxidant (HOX) exposure expressed in mg/L × min. The oxidant exposure is determined from the integrated oxidant−time curve at the time-point of quenching. This allows a comparison between different experiments even if they have been carried out with differing oxidant doses. Low and High Oxidant Exposures: Absence of Bromide. In the absence of bromide, the percent iodine-incorporation in ITHMs was almost constant over the entire range of HOX exposures (Figures 3 and S2a,b). In this case, HOI is quite stable in solution (half-life of 50 min for 2 mg Cl2/L) because bromine is not present to oxidize HOI to iodate (Scheme 1). Therefore, the formation of iodate is low and the benefit of sequestering the iodine as iodate might be compensated by the formation of THM precursors (Cl-/I-DOM1 in Scheme 1). This is illustrated by the high formation of CHCl2I for all prechlorination times in Figures S1b,c. Low HOX Exposures: Presence of Bromide. For low oxidant exposures in the presence of bromide, an increase in the iodine incorporation in THMs was observed (Figures 3 and S2a,b). In
negligible by comparing the THM concentration of a freshly prepared 5 μg/L standard to the same standard covered with parafilm and left on the bench for 24 h (RSD < 7%). For the sample to be treated with monochloramine without prechlorination, the desired amount of NH4Cl (5 times molar excess with respect to the HOCl concentration) was added prior to chlorine addition to quantitatively form NH2Cl under vigorous stirring during chlorine addition. For the addition of chlorineonly experiment, no NH4Cl was added. After 24 h, the samples withdrawn for analysis of THMs were quenched with an aqueous solution of ascorbic acid,2 while the samples for iodate analysis were quenched with sulfite. Iodate was analyzed within 24 h, even though it was verified that the concentration measured directly after quenching with NH4Cl was identical to that measured in the sample collected after 24 h.
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RESULTS AND DISCUSSION Formation and Speciation of I-THMs in the Absence of Bromide: Influence of the Prechlorination Time. The formation of I-THMs from NH2Cl without prechlorination, chlorine followed by ammonia addition at various prechlorination times, and chlorine-only is shown in Figure 1. The concentration and speciation of the I-THMs are depicted for treatment of Pony Lake Fulvic Acid (PL) (2 mg C/L), with I− (0.5 μM) at pH 8.0 and a total initial oxidant concentration equivalent to 2 mg Cl2/L (28 μM), after 24h contact time, in absence of bromide. During chloramination without prechlorination, the main I-THM formed was CHI3 with a concentration of 2.4 nM (0.94 μg/L) (Figure 1). This result is expected since HOI was the predominant halogenating agent present. Low concentrations of CHCl2I and CHClI2 were also detected. Conversely, in the chlorine-only experiment, when chlorine was the main oxidant, the opposite trend was observed with highly chlorinated I-THMs being the most abundant. CHCl2I dominated the speciation of the I-THMs with a concentration of 7.0 nM (1.5 μg/L), while CHClI2 was also formed at a lower concentration (Figure 1). CHI3 was formed at trace level in this case because the chlorine concentration was much higher than the HOI concentration, which did not enable triple iodination to CHI3. Furthermore, HOI is also oxidized to iodate under these conditions. Surprisingly, an increase in the CHI3 concentration in the chlorine followed by ammonium addition experiments, compared to the experiment with NH2Cl without
Figure 1. Formation of I-THMs in the chlorination/chloramination process. Influence of prechlorination time (data for NH2Cl (no prechlorination) and HOCl only are also shown). Pony Lake Fulvic Acid (PL), DOC = 2 mg C/L, [I−]0 = 0.5 μM, [HOCl]o = 2 mg Cl2/L (28 μM), [NH4Cl] = 150 μM for quenching, pH 8.0. 11107
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Figure 2. Formation and speciation of I-THMs for differing precursor scenarios. Row 1. Influence of bromide concentration. Nordic Reservoir DOM (NR), DOC = 2 mg C/L, [I−]0 = 0.5 μM, pH 8.0. (a) [Br−]0 = 0, [HOCl]o = 2 mg Cl2/L (28 μM), [NH4Cl] = 150 μM for quenching, (b) [Br−]0 = 5 μM, [HOCl]o = 2 mg Cl2/L (28 μM), [NH4Cl] = 150 μM for quenching, (c) [Br−]0 = 25 μM, [HOCl]o = 3 mg Cl2/L (42 μM), [NH4Cl] = 225 μM for quenching. Row 2. Influence of DOM concentration. Pony Lake Fulvic Acid (PL), [I−]0 = 0.5 μM, [Br−]0 = 5 μM, pH 8.0. (d) DOC = 1 mg C/L, [HOCl]o = 2 mg Cl2/L (28 μM), [NH4Cl] = 150 μM for quenching, (e) DOC = 2 mg C/L, [HOCl]o = 3 mg Cl2/L (42 μM), [NH4Cl] = 225 μM for quenching, (f) DOC = 4 mg C/L, [HOCl]o = 4 mg Cl2/L (56 μM), [NH4Cl] = 300 μM for quenching. Row 3. Influence of DOM type. DOC = 2 mg C/L, [I−]0 = 0.5 μM, [Br−]0 = 1 μM, pH 8.0, [HOCl]o = 2 mg Cl2/L (28 μM), [NH4Cl] = 150 μM for quenching. (g) Nordic Reservoir DOM (NR), (h) Suwanee River DOM (SR), (i) Pony Lake Fulvic Acid (PL).
bromide/iodide ratios (Figures 3 and S2a,b). Therefore, one can assume that using a high prechlorination time allows efficient mitigation of iodinated organic compounds in both cases. As shown in Figures S3a−c, the yield of iodate increased with increasing bromide concentration. This is due to the fact that for high bromide concentration, most of the iodide is quickly converted to iodate, while for low bromide concentrations, a larger fraction of iodide is available as iodine and may be incorporated into DOM as organic iodine but does not necessarily lead to the formation of I-THMs. Influence of DOM Concentration on the Formation of I-THMs and Iodate. As shown in Scheme 1, DOM either quenches the oxidants (HOX) by halogenation (electrophilic aromatic substitution or electrophilic addition, DOM1) or reduction (electron transfer, DOM2) reactions or acts as a precursor for I-THM formation (Cl-/Br-/I-DOM1).26,27,32 To elucidate the role of DOM during chlorination of iodidecontaining waters, experiments were carried out with various types and concentrations of DOM. These experiments were designed with an excess of HOCl, similar to realistic conditions in water treatment where a disinfectant residual is needed, i.e., the chlorine dose was proportional to the DOM concentration. As shown in Figures 2d−f (see Figure S4a−r for a complete data set), the concentration of PL DOM affected both the speciation and yield of I-THMs. The maximum concentration of individual I-THMs for a bromide/iodide ratio of 10 decreased with increasing DOM concentration, with a maximum concentration for CHI3 of 29 nM at 1 mg C/L, 14 nM at 2 mg C/L, and 3 nM at 4 mg C/L (Figures 2d−f). For a
the presence of bromide, HOBr is formed upon chlorine addition and reacts with both DOM and inorganic iodine species. HOBr is known to oxidize HOI to iodate. In excess of chlorine, this is a bromide-catalyzed reaction.29 This has also been verified in the present study (Figure S3a−c), where the rate and yield of iodate formation increased with increasing initial bromide concentration for all experimental conditions. Thereafter, at the time of ammonia addition, HOBr is quenched and NH2Br is formed (see Scheme 1).39 HOBr is then not available to catalyze the oxidation of HOI to iodate. If there is residual HOI at this point, it becomes the main halogenating agent since it does not react with ammonia (Scheme 1).21 Therefore, HOI can react with the precursors formed from the reactions of HOCl/HOBr with DOM leading to I-(Cl-/Br-)THMs. At this stage, there is not much information on the reactivity of bromamines with DOM.38 High HOX Exposures: Presence of Bromide. For high oxidant exposures, the competition between THM precursors or HOBr for reaction with HOI becomes more important. Under these conditions, HOBr, being in large excess, can react to an important extent with both DOM and HOI, leading to a high extent of DOM bromination and high degree of transformation of HOI to iodate, thereby reducing the availability of HOI to form I-THMs (and in general I-DBPs) (Scheme 1). This explains the decrease in the percent iodineincorporation in I-THMs for longer HOX exposures and high bromide/iodide ratios (Figures 3 and S2a,b). It should be noted that for high HOX exposures, the iodine incorporation in THMs was comparable for low and high 11108
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before iodate formation or a triple halogenation of THM precursors can occur. Even though the same I-THM concentration was measured for high HOX exposures (Figure S5a,b), a higher formation of iodinated organic moieties (nonTHMs) is expected when the DOM concentration is high. Influence of DOM Type on the Formation of I-THMs and Iodate. The influence of the type of DOM on the formation of I-THMs was studied by using various DOM extracts (Figures 2g−i and S1a−r). As shown in Figures 2g−i, for a bromide/iodide ratio of 2, the SR and NR extracts exhibited similar patterns of I-THM formation, with a high formation of Cl-/I-THMs (mainly CHCl2I), while for the PL extract, the speciation was dominated by highly iodinated THMs (CHI3 and CHClI2). Increasing the bromide concentration attenuated the differences observed among the extracts (Figures S1a−r). For high bromide concentrations (bromide/ iodide ratio of 50; Figure S1p−r), the speciation was dominated by CHBr2I and no differences were observed among SR, NR, and PL in terms of the speciation of I-THMs. However, a higher absolute formation of CHBr2I was observed for SR and NR compared to PL. It has been demonstrated that DOM with high SUVA254 (SR and NR) is more prone to reaction with chlorine due to a higher aromaticity,40 forming high concentrations of chlorinated THM precursors and eventually leading to chlorinated ITHMs as shown by the trend of CHCl2I vs prechlorination time (Figures S1b,c). In contrast, for DOM with a lower aromatic content (PL), the number of reactive sites is limited, thus HOI out-competes HOCl because it reacts faster with the THM precursor sites41 and highly iodinated THMs are preferentially formed (Figure S1a, TOC art). Furthermore, a higher chlorine consumption is expected by direct reaction for DOM extracts with high SUVA254 (SR, NR) as compared to DOM extracts with midrange SUVA254 (PL) (Figure S7). Therefore, less chlorine is available for oxidation of HOI to iodate for SR and NR when compared to PL. Hence, while only 40% (0.2 μM) of the initial iodine is oxidized to iodate for SR and NR, 80% (0.4 μM) is converted to iodate for PL for an oxidant exposure of ∼40 mg/L × min (Figure S8). Overall, a
Figure 3. Influence of the bromide/iodide ratio on the evolution of percent iodine-incorporation in I-THMs as a function of the oxidant (HOX) exposure. Nordic Reservoir DOM (NR), DOC = 2 mg C/L, [I−]0 = 0.5 μM, [Br−]0 = 0−25 μM, [HOCl]o = 2−4 mg Cl2/L (28− 56 μM), [NH4Cl] = 150−300 μM for quenching, pH 8.
low DOM concentration (i.e., 1 mg C/L), the speciation was dominated by the Br- and I-THMs, while almost no Cl-/ITHMs were formed. Cl-/I-THMs started to be formed for a DOM concentration of 2 mg C/L at a lower concentration than the Br-/I-THMs. When increasing the DOM concentration to 4 mg C/L, Cl-/I-THMs were the dominant species of a mixture of Cl-/Br-/I-THMs. Therefore, increasing the DOM concentration in the HOCl−Br−I−system increases the diversity of THMs, eventually leading to a chlorine-dominated system (TOC art). Previous studies have demonstrated that HOBr is more reactive toward phenolic moieties or halogen-reactive sites within DOM than HOCl22,30,31 and it has also recently been demonstrated that HOBr produces I-THM precursors at a higher rate and yield than chlorine even when chlorine is in large excess.29 For low DOM concentrations, the reactive sites are primarily reacting with HOBr and HOI, forming brominated DOM moieties (and eventually Br-/I-THMs). For high DOM concentrations, HOCl will also react with the remaining HOX-reactive sites and mixed Cl-/Br-/I-THMs and ultimately Cl-/I-THMs are formed because the reaction of DOM with chlorine will outcompete the reaction of chlorine with bromide to form HOBr. As illustrated in Figures S5a and b for bromide/iodide ratios of 2 and 10, respectively, the iodine incorporation in THMs decreased with increasing DOM concentration for low HOX exposures with a maximum of 38% iodine-incorporation in THMs for 2 min prechlorination (bromide/iodide ratio of 10 and 1 mg C/L, Figure S5b). When the prechlorination time increased, the difference between the experiments diminished up to a point where the iodine incorporation was similar for all DOM concentrations at high HOX exposures (Figures S5a,b). It should be noted that for experiments with high bromide concentrations, most of the iodide is converted to iodate by HOBr even for low prechlorination times, which leads to minimal formation of I-THMs. It seems promising to increase the prechlorination time up to a point where the formation of ITHMs is controlled. However, this optimum strongly depends on the DOM concentration. As shown in Figure S6, the yield of iodate decreased with increasing DOM concentration. For low DOM concentrations, most of the iodide is converted to iodate, whereas for high DOM concentrations, the available iodine is distributed over many reactive DOM sites and is depleted
Scheme 1. Main reactions involved in the chlorination/ chloramination process in the presence of DOM, bromide, and iodide. DOM1: DOM fraction reacting by electrophilic aromatic substitution and electrophilic addition with HOX. DOM2: DOM fraction reacting by electron transfer with HOX
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further investigate the interdependence of the iodine species, the iodine incorporation in THMs was compared to the iodate formation (Figure S10a). For high bromide concentrations and/or high prechlorination times, most of the iodide was converted to iodate and only a low formation of I-THMs was observed (Figure S10a). However, for low bromide concentrations and/or low prechlorination times, the iodide oxidation to iodate was less pronounced and concomitantly the formation of I-THMs was more important. Figure S10a shows that iodate formation is not inversely correlated to I-THM formation. Conversely, the formation of CHI3 is strongly inversely correlated to the formation of iodate (Figure S10b). Similar to previously published data,42 CHI3 represented around 80% of the I-THMs formed when chloramine was used as the sole oxidant for iodide-containing solutions (i.e., no iodate was formed). When a prechlorination step was applied, the percent CHI3 of I-THMs decreased with increasing iodate formation. This observation can be explained by the fact that both CHI3 formation and iodate are controlled solely by HOI, which can react with DOM to CHI3 or be further oxidized to iodate. However, since the formation of CHI3 is not proportional to the total I-THM formation, this correlation has only limited value for an overall assessment. Cytotoxicity Evaluation by Calculations. In practice, ammonia is added to chlorinated water to keep the concentrations of regulated DBPs below the drinking water standards, thereby minimizing the overall toxicity. Although a prechlorination step (partially) oxidizes iodide to the nontoxic iodate and limits the formation of I-DBPs, it may produce higher concentrations of regulated DBPs, mainly Cl-/Br-DBPs. To determine whether the use of the chlorination/chloramination process is beneficial, the overall chronic cytotoxicity based on the measured THM concentrations was evaluated. As described in a recent publication,20 the C50 value related to each THM was used to calculate the relative cytotoxicity associated with the formation of each THM (see Text S3 and Table S1). The C50 value is the concentration of each individual THM inducing 50% reduction in the density of Chinese Hamster Ovary cells treated for 72 h. The C50 values for each THM are summarized in Table S1. An example of the calculation for the data shown in Figure 4a (this corresponds to the data in Figures 2b for I-THMs and S11a for Cl-/Br-THMs) is also given in Table S1. The relative cytotoxicity of each compound is represented by the fraction of the THM concentration and the C50 values. The cytotoxicity of regulated THMs (Cl-/BrTHMs, black bars) and iodinated THMs (Cl-/Br-/I-THMs, gray bars) were summed to derive a total relative cytotoxicity value based on all THMs present in a sample. This allows a comparison of the unitless THM-associated total relative cytotoxicity resulting from various experimental conditions or various treatment options. Influence of DOM Type. The influence of the DOM type on THM-associated total relative cytotoxicity was studied for NR, PL, and SR DOM (Figures 4a−c). The calculated cytotoxicity induced by the I-THMs was influenced by the DOM type (Figures 4a−c, gray bars). Similar to the percent iodineincorporation in THMs (Figures 3 and S2a,b), the calculated total relative cytotoxicity induced by the I-THMs increased for low prechlorination times for NR, PL, and SR. Increasing the prechlorination time to 5 min strongly decreased the calculated cytotoxicity based on I-THMs for PL since most of the iodide was converted to iodate (Figure 4b, diamonds). The contribution of I-THMs to cytotoxicity became negligible for
lower rate and yield of iodate formation was observed for extracts with high SUVA254 (SR, NR) compared to extracts with midrange SUVA254 (PL) (Figure S3a−c). As shown in Figure S9a, the percent iodine-incorporation in I-THMs increased up to ∼14% for low HOX exposures and a bromide/iodide ratio of 5 for all DOM extracts. For high oxidant exposures, two different trends were observed. For SR and NR, the percent iodine-incorporation in I-THMs plateaued or slightly decreased (∼10−13%). HOI is available to react with chlorinated THM precursor moieties, thereby forming Cl-/ITHMs. This led to a high percent iodine-incorporation in ITHMs even for high HOX exposures (Figures S9a, S2b, and 3). In contrast, for PL, a significant decrease of iodine incorporation to ∼2% was observed when increasing the prechlorination time. A large portion of the iodine is oxidized to iodate under these conditions, which leads to a decrease of the percent iodine-incorporation in I-THMs for a longer HOX exposure (Figures S9a and S2a). Figure S9b shows the percent iodine-incorporation in ITHMs vs SUVA254 for the sample without prechlorination (NH2Cl alone) and the highest prechlorination time of 30 min. In accordance with previous studies carried out with raw and postcoagulated waters and different DOM fractions,26,33 the extracts with high SUVA254 (SR and NR) form less I-THMs than PL (midrange SUVA254,) when exposed to preformed monochloramine (HOX exposure = 0 mg/L × min). In this case, HOI is the main oxidant, and therefore, for high SUVA254 DOM with a high concentration of reactive sites, iodine reacts with many sites and there is not enough iodine residual to further iodinate the single and double iodinated sites to form CHI3. In contrast, for midrange SUVA254 DOM with a low concentration of reactive sites, the iodination of the DOM moieties is significant and leads to the formation of CHI3. For a HOX exposure > ≈10 mg/L × min (pre-Cl2 time = 30 min in Figure S9b), an opposite trend was observed. The extracts with high SUVA254 (SR and NR) formed more I-THMs than PL. During the prechlorination step, a large amount of Cl-/Br-/IDBP precursors are formed since there are many reactive sites. HOCl/HOBr also react with HOI to a lower extent because they are consumed by the DOM. Therefore, after quenching HOCl/HOBr with ammonia, HOI is available for reaction with Cl-/Br-/I-DBP precursors, leading to the formation of I-(Cl-/ Br-)THMs. As a result, for high SUVA254 DOM (SR and NR), a lower yield of iodate and higher iodine incorporation in THMs was observed under these experimental conditions. For midrange SUVA254 DOM (PL) (with low concentration of reactive sites), most sites were fully chlorinated/brominated before they could react with HOI. This effect was further reinforced by a high degree of conversion of iodine to iodate. Efficiency of the Chlorination/Chloramination Process for Mitigation of I-THMs. One of the aims of this study was an improved understanding of the chlorination/chloramination process as a mitigation strategy for I-DBPs. Prechlorination times were optimized with the intent to convert all the iodide to iodate before ammonia addition, thereby reducing the formation of I-DBPs. However, as shown and discussed in Figures 3, S2a,b (varying bromide concentrations), and S5a,b (varying DOM concentrations), a comparable percent iodineincorporation in I-THMs was measured under various experimental conditions, while the iodate yield varied significantly (Figures S3a−c, S6). This observation is even more important if we consider that the formation of I-THMs represents only a fraction of the total I-DBPs formed. To 11110
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Figure 4. Calculated total relative cytotoxicity based on the THM concentrations divided by the C50 value for individual THMs (Σ([THM]i/C50,i × 106), see Table S1 for calculations) and iodate yield ([IO3−]/[I−]0). (a) Nordic Reservoir DOM (NR), [Br−]0 = 5 μM, [HOCl]o = 2 mg Cl2/L (28 μM), [NH4Cl] = 150 μM for quenching, (see Figure 2b for I-THMs and Figure S11a for Cl-/Br-THMs); (b) Pony Lake Fulvic Acid (PL), [Br−]0 = 5 μM, [HOCl]o = 3 mg Cl2/L (42 μM), [NH4Cl] = 225 μM for quenching (see Figure 2e for I-THMs and Figure S11b for Cl-/Br-THMs); (c) Suwannee river DOM (SR), [Br−]0 = 5 μM, [HOCl]o = 2 mg Cl2/L (28 μM), [NH4Cl] = 150 μM for quenching, (see Figure S1k for I-THMs and Figure S11c for Cl-/Br-THMs); (d) Pony Lake Fulvic Acid (PL), [Br−]0 = 0−25 μM, [HOCl]o = 2−3 mg Cl2/L (28−42 μM), [NH4Cl] = 150−225 μM for quenching, prechlorination time 10 min. For all samples DOC = 2 mg C/L, [I−]0 = 0.5 μM, pH 8.0.
a prechlorination time >5 min. For NR and SR, the calculated contribution of I-THMs to the relative cytotoxicity slowly decreased for prechlorination times >2 min in accordance with a partial oxidation of iodide to iodate (Figures 4a,c, diamonds). In agreement with the formation of Cl-/Br-THMs (Figure S11a−c), the cytotoxicity induced by the regulated THMs increased with increasing prechlorination time for NR, PL, and SR (Figures 4a−c, black bars). When increasing the prechlorination contact time, HOCl and HOBr have more time to react with organic precursors and, therefore, the formation of Cl-/Br-THMs and their contribution to cytotoxicity increases. However, similar to the I-THMs, the speciation is mainly driven by highly brominated THMs for PL, while mixed Cl-/Br-THMs are formed for NR and SR (Figures S11a−c). It is interesting to note that the application of chlorine to PL clearly reduces the total relative cytotoxicity by efficiently oxidizing iodide to iodate, therefore reducing the formation of I-THMs, while the contribution of regulated THMs to cytotoxicity remained relatively low (Figure 4b). Furthermore, for PL, the application of chlorine led to a lower cytotoxicity compared to the application of preformed monochloramine if the I-THMs are considered. In contrast, for NR and SR, the decrease in the total relative cytotoxicity induced by the oxidation of iodide to iodate (diamonds), and the concomitant mitigation of I-THMs, during the prechlorination step was compensated and in some cases even outweighed by the total
relative cytotoxicity induced by the formation of regulated THMs. On the basis of the total relative cytotoxicity calculations, NH2Cl alone is recommended for these types of DOM. This difference between the DOM extracts demonstrates that the type of DOM and its reactivity with chlorine and bromine is a decisive factor. For highly reactive DOM (with high SUVA254), the chlorination/chloramination process does not seem to be efficient in reducing the total THM-related cytotoxicity, while for DOM with lower reactivity (midrange SUVA254), it is a good option for mitigating the formation of ITHMs and thereby reducing the total relative cytotoxicity. Influence of DOM Concentration. The influence of the DOM concentration on the calculated total relative cytotoxicity is shown in Figures S12a,b for PL in the absence of bromide. The total relative cytotoxicity induced by the regulated THMs exhibited a strong dependence on the DOM concentration with an increase in total relative cytotoxicity for increasing DOM concentrations. For higher DOM concentration, a higher formation of regulated THMs was observed (CHCl3 in this case, because [Br−] = 0) when exposed to an excess of chlorine. A different pattern was observed in the presence of bromide (Figure S13a). When the bromide concentration increased, the differences in calculated cytotoxicity gradually decreased and a similar total relative cytotoxicity was observed for all DOM concentrations (1−4 mg C/L) at a bromide/iodide ratio of 20 (Figure S13a). In contrast, in absence of bromide, the 11111
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Implications for Mitigation Strategies for I-DBPs. The experimental results of the chlorination/chloramination process indicate that the prechlorination time, the bromide concentration, and the DOM type and concentration are critical to the extent of formation and speciation of I-THMs and iodate under typical drinking water treatment conditions. For waters with low bromide concentrations, a short prechlorination time enhances the formation of I-THMs and the total relative cytotoxicity, therefore, a longer contact time is recommended to allow the full conversion of iodide to iodate and mitigate the formation of I-DBPs. For waters containing high bromide concentrations, short prechlorination times can lead to a full conversion of iodide to iodate, resulting in a low formation of ITHMs and a reasonable formation of Cl-/Br-THMs. In contrast, high free chlorine contact times and the related formation of Br-THMs might outweigh the benefits of limited formation of I-THMs, such that a higher total relative cytotoxicity results. For waters with low DOM concentrations, a short prechlorination time enhances the formation of I-THMs and the total relative cytotoxicty, except for high bromide concentrations. No firm conclusion can be drawn from the experiments carried out with high DOM concentrations because a high iodine incorporation into DOM is expected for all treatment scenarios possibly leading to the formation of other I-DBPs. In the case of highly reactive DOM (high SUVA254), a prechlorination step is not beneficial and the application of preformed monochloramine is recommended even though formation of the more toxic I-THMs will be favored. However, the distribution of HOI over many reactive sites will limit the overall I-THM formation. For water containing DOM with low aromatic content (midrange SUVA254), a prechlorination step is beneficial, although the contact time with chlorine needs to be adjusted for the bromide and DOM concentrations to ensure full oxidation of iodide to iodate for an optimal mitigation of I-THMs.
calculated total relative cytotoxicity related to I-THMs drastically decreased with increasing DOM concentration (Figure S12b) for all prechlorination times. As shown in Figures S5a,b, the percent iodine-incorporation in THMs was higher for low DOM concentrations. Furthermore, for low DOM concentrations, the speciation was dominated by CHI3, while for high DOM concentrations, CHCl2I was the dominant species (Figures S4a−c). As CHI3 is ∼60 times more cytotoxic than CHCl2I and being the major species formed during experiments with low DOM concentration, this results in a higher total relative cytotoxicity. Similar to the regulated THMs, in the presence of bromide, the cytotoxicity related to the I-THMs was not affected by the DOM concentration (Figure S13b) since most of the iodide was oxidized to iodate for low prechlorination times. However, as for the experiments without bromide, for NH2Cl alone, the cytotoxicity related to ITHMs was dominated by CHI3 and decreased with increasing DOM concentration because of the distribution of iodine among more reactive sites, making tri-iodination unlikely. These results clearly highlight the fact that the DOM concentration is a key parameter and that THM concentrations alone are not sufficient to assess the toxicity of the finished water. Influence of Bromide Concentration. The bromide concentration is another important factor affecting the calculated total relative cytotoxicity. As shown in Figure 4d, for DOM with low aromatic content (PL), a high calculated total relative cytotoxicity was observed for a bromide/iodide ratio of 2 due to the presence of both HOI and Br-DOM moieties leading to a high formation of mixed Br-/I-THMs. The cytotoxicity decreased for bromide/iodide ratios of 5 and 10, due to catalytic oxidation of iodide to iodate (diamonds) and thus a reduction in the formation of I-THMs. In this case, the calculated total relative cytotoxicity became lower than for the experiment without bromide (bromide/iodide ratio of 0). For bromide/iodide ratios >10, the increased calculated total relative cytotoxicity induced by the Br-THMs outweighs the benefit of I-THM mitigation. For NR and SR (Figures S14a,b), the presence of low bromide concentrations (bromide/iodide ratios 2−10) enhanced the total relative cytotoxicity for ITHMs, while the cytotoxicity related to the regulated THMs remained almost constant. For bromide/iodide ratios >10, the calculated total relative cytotoxicity of the regulated THMs increased and exceeded the cytotoxicity induced by the ITHMs since most of the iodide was converted to iodate. Overall, for highly reactive DOM (high SUVA254) (NR, SR, Figure S14a,b), it can be concluded that the presence of bromide increases the cytotoxicity (compared to bromide/ iodide ratio of 0). It was recently demonstrated that the cytotoxicity and genotoxicity of water treated by HOCl or NH2Cl was driven by the formation of Br-/I-DBPs measured as total organic halogens.19 In this study, only THMs were measured and assessed with respect to cytotoxicity. In the case of monochloramine, as there was no formation of iodate, a high level of iodine incorporation into DOM was expected and other IDBPs may form,43 such as iodoacetic acid which is highly cytotoxic and genotoxic with a C50 of 2.95 × 10−6 M (∼22 times more cytotoxic than CHI3).5,14 Therefore, in future studies, it is important to consider the toxicity (if known) of all formed DBPs when assessing the potential health risk of a treatment scenario.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02624. Text S1. Dissolved organic matter extracts from the International Humic Substances Society. Text S2. Analysis of regulated and iodinated THMs. Figure S1. Influence of the bromide−iodide ratio, prechlorination time, and DOM type on I-THMs formation and speciation. Figure S2. Influence of bromide concentration on the evolution of % iodine-incorporation in I-THMs. Figure S3. Influence of the bromide−iodide ratio on iodate formation for SR, NR, and PL. Figure S4. Influence of the bromide−iodide ratio, prechlorination time, and DOM concentration on I-THMs formation and speciation. Figure S5. Influence of DOM concentration on the evolution of % iodine-incorporation in ITHMs. Figure S6. Influence of DOM concentration on iodate formation. Figure S7. Oxidant consumption for Pony Lake Fulvic Acid (PL), Suwanee River DOM (SR) and Nordic Reservoir DOM (NR). Figure S8. Influence of DOM type on iodate formation. Figure S9. Influence of DOM type on the evolution of % iodine-incorporation in I-THMs. Figure S10. Iodine speciation in the chlorination/chloramination process. Text S3. Methodology for toxicity calculation based on THMs. Table S1. 11112
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Example of cytotoxicity evaluation based on THM formation. Figure S11. Formation of Cl-/Br-THMs. Figure S12. Influence of DOM concentration on the toxicity evaluation based on THMs concentration and C50 values. Figure S13. Influence of DOM concentration on the calculated total relative cytotoxicity based on THM concentrations and C50 values. Figure S14. Influence of the DOM type and bromide concentration on the calculated total relative cytotoxicity for individual THMs and iodate yields. (PDF)
AUTHOR INFORMATION
Corresponding Author
*Phone: +41 58 765 5270; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Andrew Chan for his assistance in the laboratory. Funding and support was provided by the Australian Research Council (ARC LP100100285), Water Corporation (WA), Curtin University, Eawag, and Water Research Australia.
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