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Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach
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Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach Tingting Gong, Xiangru Zhang* Environmental Engineering Program, Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China
article info
abstract
Article history:
The dissolved iodine species that dominate aquatic systems are iodide, iodate and organo-
Received 30 April 2013
iodine. These species may undergo transformation to one another and thus affect the
Received in revised form
formation of iodinated disinfection byproducts during disinfection of drinking waters or
28 August 2013
wastewater effluents. In this study, a fast, sensitive and accurate method for determining
Accepted 31 August 2013
these iodine species in waters was developed by derivatizing iodide and iodate to organic
Available online xxx
iodine and measuring organic iodine with a total organic iodine (TOI) measurement approach. Within this method, organo-iodine was determined directly by TOI measure-
Keywords:
ment; iodide was oxidized by monochloramine to hypoiodous acid and then hypoiodous
Disinfection byproducts
acid reacted with phenol to form organic iodine, which was determined by TOI measure-
Total organic iodine
ment; iodate was reduced by ascorbic acid to iodide and then determined as iodide. The
TOI
quantitation limit of organo-iodine or sum of organo-iodine and iodide or sum of organo-
Iodide
iodine, iodide and iodate was 5 mg/L as I for a 40 mL water sample (or 2.5 mg/L as I for an
Iodate
80 mL water sample, or 1.25 mg/L as I for a 160 mL water sample). This method was suc-
Waters
cessfully applied to the determination of iodide, iodate and organo-iodine in a variety of water samples, including tap water, seawater, urine and wastewater. The recoveries of iodide, iodate and organo-iodine were 91e109%, 90e108% and 91e108%, respectively. The concentrations and distributions of iodine species in different water samples were obtained and compared. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Iodine is a trace element that exists in natural waters such as seawater, freshwater and rain (Schwehr and Santschi, 2003; Gilfedder et al., 2008, 2009). Iodine is also a biologically essential nutrient for all mammals including humans (Gilfedder et al., 2009), and thus various physiological fluids, such as milk, serum, and urine, contain iodine (Hou et al.,
2000; Verma et al., 1992). Iodine deficiency in the human body causes goitre as well as some other malfunctions and the normative daily iodine intake for an adult has been reported to be 1 mg/kg body weight (Merian et al., 2004). Humans ingest iodine mainly through drinking water and foods. Therefore, to ensure the proper daily iodine intake for humans, iodine has been added to table salts and some other foodstuffs. Owing to the occurrence in natural waters, iodine has also been
* Corresponding author. Tel.: þ852 2358 8479; fax: þ852 2358 1534. E-mail address: [email protected] (X. Zhang). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.08.039
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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detected in tap water (Blount et al., 2010; Pan and Zhang, 2013). Besides, large amounts of iodine-containing organic compounds are used as X-ray contrast media for medical purposes, which add iodine to hospital wastewater (Duirk et al., 2011). Iodine is widely used in the manufacture of chemicals, iodophors, biocides, herbicides, pharmaceuticals and nylon (Pin˜eiro et al., 2011), which ends up in industrial wastewater. The typical iodine concentrations in seawater, freshwater, rain and urine are 45e90, 0.5e20, 0.5e5.0 and 0e300 mg/L, respectively (Whitehead, 1984; Schwehr and ¨ nak et al., 1999). With all Santschi, 2003; Hou et al., 2000; U these natural and artificial iodine sources, wastewater tends to contain a certain level of iodine. The dominant iodine species in waters are iodide, iodate and organo-iodine (Hansen et al., 2011; Gilfedder et al., 2009). Different iodine species may exhibit dramatically different mobility, bioavailability and chemical behaviors in the environment. For example, iodide is rather mobile, while iodate is efficiently sorbed by soils (Gilfedder et al., 2009); iodide, rather than iodate, is the bioavailable iodine form (Eskandari et al., 1997; Ku¨pper et al., 1998); inorganic and organic iodine species have different hydrophilic and biophilic properties (Hu et al., 2005). Furthermore, the “three” dominant iodine species, i.e., iodide, iodate and organo-iodine, may undergo transformation to one another in the environment and during water/wastewater treatment. Nitrate-reducing, iron-reducing and sulfate-reducing bacteria have the potential to reduce iodate to iodide (Farrenkopf et al., 1997; Councell et al., 1997); certain bacteria can oxidize iodide to molecular iodine (Amachi et al., 2005a); iodide undergoes transformation in natural and wastewater systems by fixation on humic substances to form organo-iodine (Ra¨dlinger and Heumann, 2000). Therefore, the study of iodine speciation in waters is of great significance. Drinking water needs to be disinfected during treatment. Wastewater effluents are also subjected to disinfection before discharge to inactivate the microorganisms. During disinfection, iodide can be oxidized by disinfectants to HOI/OI, which further reacts with natural/effluent organic matter to form iodinated disinfection byproducts (DBPs) (Bichsel and von Gunten, 1999, 2000; Krasner et al., 2006; Hua and Reckhow, 2006, 2007; Richardson et al., 2008; Ding and Zhang, 2009; Duirk et al., 2011; Jones et al., 2012; Chu et al., 2012). Iodinated DBPs have drawn more and more concerns due to their significantly higher toxicity than their brominated and chlorinated analogues (Plewa et al., 2004; Cemeli et al., 2006; Richardson et al., 2007, 2008). Since the three iodine species can undergo transformation to one another, all the three iodine species in source water and wastewater need to be determined to predict the iodine levels that are possible to participate in the formation of iodinated DBPs. To date, a number of methods for the determination of iodine species have been reported. However, most of them can determine only one or two iodine species (iodide and/or iodate) (Mishra et al., 2000; Hou et al., 2000). Recently, a couple of methods involving the determination of all the three iodine species (iodide, iodate and organo-iodine) have been reported. In one method, the three iodine species were derivatized to iodide and then iodide was determined by high performance liquid chromatography; however, the derivatization time for
organo-iodine to iodide was more than 20 h, which was timeconsuming (Schwehr and Santschi, 2003). A gas chromatographyemass spectrometry (GCeMS) method has also been developed by derivatizing all the three iodine species to a specific organic iodine derivative followed by GCeMS detection; however, it involved a complex derivatization process for organo-iodine, which was a three-step derivatization; moreover, it required the formation of only one isomer of the iodine derivative, so iodine must react only with the selected iodine scavenger and form only one iodine derivative. Thus the organic compounds which might react with iodine must be removed from the original water samples and the selection of the iodine scavenger was limited to the organic compounds with only one available substitution position for iodine (Zhang et al., 2010a). Ion chromatography with conductivity detection is also not amenable for the detection of inorganic iodine species since conductivity detection responds to all ionic species, resulting in a high quantitation limit (up to 100 mg/L as I). Therefore, the purpose of this study was to develop a method which involves a simple and short derivatization process for each iodine species and provides lower detection limits for those iodine species. In this study, a fast, sensitive and accurate method was developed for the quantitation of iodide, iodate and organoiodine in waters. The developed method was applied to the determination of iodine species in a variety of water samples, including tap water, seawater, urine and wastewater. The concentrations and distributions of iodine species in different water samples were illustrated.
2.
Materials and methods
2.1.
Chemicals and reagents
Potassium iodide (100%) was purchased from BDH. Ascorbic acid (99.7%), potassium iodate (99.7%), ammonium chloride (99.5%) and potassium nitrate (99.0%) were purchased from Riedel-deHae¨n. A sodium hypochlorite stock solution was purchased from Sigma Aldrich and diluted to around 2000 mg/L as Cl2 and periodically standardized by the N,Ndiethyl-p-phenylene diamine (DPD) ferrous titrimetric method (APHA et al., 1995). All other chemicals used in this study were purchased at the highest purities available from Sigma Aldrich. Ultrapure water (18.2 MU cm) was supplied by a NANOpure Diamond purifier system (Barnstead).
2.2.
Preparation of solutions
The iodide and iodate stock solutions (100 mg/L as I) were prepared by dissolving potassium iodide and potassium iodate in ultrapure water, respectively. They were stored in amber glass bottles at 4 C and newly prepared every month. The working solutions of iodide and iodate with lower concentrations were freshly prepared daily by diluting the stock solutions with ultrapure water. An ammonium chloride solution (1.0 g/L as N), a phenol solution (10.0 g/L as C), an ascorbic acid solution (1.0 g/L) and a sodium arsenite solution (13.0 g/L) were prepared. They were stored in amber glass bottles at 4 C. The phenol, ascorbic acid
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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and sodium arsenite solutions were newly prepared every week. A monochloramine solution was prepared just before use by gradually adding the sodium hypochlorite solution to the ammonium chloride solution at a chlorine-to-ammonium molar ratio of 0.8. Iodoacetic acid was dissolved in ultrapure water to prepare standard solutions with total organic iodine (TOI) concentrations of 5, 10, 20, 50 and 100 mg/L as I. A potassium nitrate solution (5000 mg/L as NO 3 ) was prepared as the rinsing solution for removing inorganic halides from activated carbon.
2.3.
Collection of water samples
A variety of water samples (different aqueous matrices) were collected to test the method. Four tap water samples were collected from four cities in China, including three coastal cities (A, C and D) and one inland city (B). They were immediately transferred to the laboratory in ice coolers by airplane. The chlorine residuals in the four samples were titrated by the DPD ferrous titrimetric method (APHA et al., 1995) and quenched with 105% of the requisite stoichiometric amounts of NaAsO2 (Liu and Zhang, 2012). Three seawater samples were collected from the marine laboratory of the Hong Kong University of Science and Technology (E), Clear Water Bay (F) and Sai Kung (G) in Hong Kong. They were immediately transferred to the laboratory in ice coolers. A urine sample was obtained by collecting urine from five male and five female students followed by a thorough mixing. It was immediately transferred to the laboratory in ice coolers. Four wastewater influent and effluent (undisinfected) samples were collected from two different wastewater treatment plants in Hong Kong, including a saline primary wastewater treatment plant and a saline secondary wastewater treatment plant. They were immediately transferred to the laboratory in ice coolers. All the samples were stored at 4 C to minimize changes in constituents.
2.4. Determination of iodide, iodate and organo-iodine in water samples In this study, a newly developed method for TOI measurement (Pan and Zhang, 2013) was adopted. It involves adsorption of organic iodine onto activated carbon, pyrolysis of adsorbed organic iodine to hydrogen iodide, absorption of hydrogen iodide into ultrapure water, pretreatment of the absorption solution, and off-line separation/detection of iodide in the absorption solution using ultra performance liquid chromatography/electrospray ionization-mass spectrometry (UPLC/ ESI-MS). The analytical scheme for the three iodine species, i.e., iodide, iodate and organo-iodine, is described in Fig. 1. Organo-iodine was measured directly by TOI measurement. Iodide was derivatized to organic iodine and then determined by TOI measurement. Iodate was reduced to iodide and then determined. As described by Pan and Zhang (2013), iodoacetic acid was used as the calibration standard compound for TOI measurement. The iodoacetic acid concentrations for the calibration curve were 5, 10, 20, 50 and 100 mg/L as I. All the
Fig. 1 e Analytical scheme for iodine species.
water samples to be analyzed were firstly filtered with 0.45 mm membranes. For the water samples with high levels of reducing substances (e.g., urine), dilution with ultrapure water was conducted before analysis. To determine iodide in a water sample, a 120 mL aliquot of the water sample was adjusted to pH 7 with sodium hydroxide or nitric acid. A determined amount of monochloramine (Supplementary Information (SI) Table S1) was dosed into the water sample, allowing for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added for a reaction time of 1 h in darkness. Then 105% of the requisite stoichiometric amount of NaAsO2 was added to quench the monochloramine residual in the water sample. Finally, the water sample was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon, 10 mL of the rinsing solution was used to remove inorganic halides, and duplicate measurements were conducted. The procedure for TOI measurement is detailed in the SI. The measured TOI value corresponded to the sum of organo-iodine and iodide in the water sample and the iodide concentration was calculated as the difference between this TOI value and organo-iodine. To determine iodate in a water sample, a 120 mL aliquot of the water sample was adjusted to pH 2 with nitric acid and 4.2 mg/L of ascorbic acid was added to react for 2 min with stirring. After reaction, the water sample was adjusted to pH 7 with sodium hydroxide. A determined amount of monochloramine (SI Table S1) was dosed into the water sample, allowing for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added for a reaction time of 1 h in darkness. Then 105% of the requisite stoichiometric amount of NaAsO2 was added to quench the monochloramine residual in the water sample. Finally, the water sample was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon, 10 mL of the rinsing solution was used to remove inorganic halides, and duplicate measurements were conducted. The measured TOI value corresponded to the sum of organoiodine, iodide and iodate in the water sample and the iodate
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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concentration was calculated as the difference between this TOI value and the sum of organo-iodine and iodide. To determine organo-iodine in a water sample, the water sample was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon, 10 mL of the rinsing solution was used to remove inorganic halides, and duplicate measurements were conducted. To determine either iodide, iodate, or organo-iodine, for water samples with very low concentrations (e.g., <5 mg/L as I), larger volume aliquots (e.g., 80 or 160 mL) were used for the adsorption on the activated carbon while other experimental steps were kept the same.
IO 3 þ 3C6 H8 O6 /I þ 3C6 H6 O6 þ 3H2 O
Since this reduction reaction occurred only in an acidic medium (Verma et al., 1992), it was conducted at pH 2 in this study. According to previous studies (Verma et al., 1992; Mishra et al., 2000), this reduction reaction completed within 2 min with stirring in an acidic medium with various matrices, so each water sample was stirred for 2 min after the addition of ascorbic acid. Any excess of ascorbic acid was subsequently oxidized by monochloramine.
3.1.2.
3.
Results and discussion
3.1.
Method development
3.1.1.
Derivatization of iodide and iodate to organic iodine
For the determination of iodide, the first step was to oxidize iodide to hypoiodous acid only rather than further to iodate. It has been demonstrated that iodide was quickly oxidized by monochloramine to hypoiodous acid but not further to iodate (Bichsel and von Gunten, 1999). Therefore, monochloramine was selected as the oxidizing agent:
G þ NH2 Cl þ Hþ þ H2 O/HOI þ NHþ 4 þ Cl
The kinetics of the reaction has been reported by Bichsel and von Gunten (1999). In this study, the monochloramine concentration involved in the oxidation of iodide was generally around 1 mg/L as Cl2, and pH was around 7, so the theoretical reaction time for oxidizing 99.99% of the iodide to hypoiodous acid was calculated to be 4.5 min. To ensure the complete oxidation, a reaction time of 15 min was adopted for all the water samples. The second step was to derivatize hypoiodous acid to organic iodine. An iodine scavenger was needed to quickly react with hypoiodous acid to form organic iodine derivatives. Among various classes of organic compounds, only aromatic amines and phenols respond to rapid iodination; however, amines can be readily oxidized and thus were not used (Verma et al., 1992). In previous studies, GCeMS was used for the quantitation of organic iodine derivatives, which generally required the formation of only one derivative (Verma et al., 1992; Mishra et al., 2000; Zhang et al., 2010a). In this study, the quantitation of iodide or iodate was done by derivatization to organic iodine and subsequent TOI measurement, thus the iodine derivatives were not limited to only one specific compound (for nearly all iodine-containing organic compounds respond to TOI measurement). Therefore, there was no need to find a compound with only one available substitution position for iodine, which broadened the selection of an iodine scavenger. Phenol, the most common aromatic compound which responds to rapid iodination, was selected in this study. According to Dietrich et al. (1999), the phenol to iodine ratio was high enough that only monoiodophenol could be formed: C6 H5 OH þ HOI/C6 H4 IOH þ H2 O
The kinetics of the reaction has been reported by Bichsel and von Gunten (2000). For the determination of iodate, it was first reduced to iodide by ascorbic acid (C6H8O6):
Selection of monochloramine dose
Monochloramine was selected as the oxidizing agent to oxidize iodide to hypoiodous acid. It was assumed that the highest concentrations of both iodide and iodate in the water samples were 200 mg/L as I. The stoichiometric monochloramine dose for oxidizing 400 mg/L of iodide (the sum of iodide and reduced iodate) was 0.2 mg/L as Cl2. For a real water sample, other components might also consume monochloramine; too low monochloramine doses might lead to incomplete oxidation of the iodide, and too high monochloramine doses could cause decomposition of the TOI. Therefore an appropriate monochloramine dose should be selected for the water sample. In this study, the monochloramine doses in the range of 1 mg/L as Cl2 (i.e., a 5-fold excess) to 20 mg/L as Cl2 (i.e., a 100-fold excess) were tested by using a seawater sample (E) as a representative. Seawater was selected because it contains typical levels of all the three dominant iodine species and a relatively complicated matrix (certain levels of organic compounds and high levels of inorganic ions). The seawater sample was filtered with 0.45 mm membrane, adjusted to pH 7 with nitric acid, and divided into five aliquots (100 mL each). Monochloramine was dosed into the five aliquots (at 1, 2, 5, 10 and 20 mg/L as Cl2) for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added to each aliquot for a reaction time of 1 h in darkness. Then, 105% of the requisite stoichiometric amount of NaAsO2 was added to each aliquot to quench the monochloramine residual. Finally, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon and 10 mL of the rinsing solution was used to remove inorganic halides. The TOI concentrations in the seawater aliquots with monochloramine doses of 1, 2, 5, 10 and 20 mg/L as Cl2 were 22.5, 22.6, 21.0, 19.2 and 17.1 mg/L as I, respectively (SI Fig. S1). The TOI concentration had no significant change when the monochloramine dose increased from 1 to 2 mg/L as Cl2, but it began to decrease significantly when the monochloramine dose was above 2 mg/ L as Cl2. We tested the effect of monochloramine dose on the decomposition of organic iodine (SI), and found that high monochloramine doses caused the decomposition of some organic iodine to inorganic iodine (SI Fig. S2), leading to an underestimation of the iodide concentration. Therefore, an appropriate monochloramine dose that could maintain a
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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monochloramine residual of 1e2 mg/L as Cl2 after 1 h reaction time was selected for each water sample. To achieve a monochloramine residual of 1e2 mg/L as Cl2 after 1 h reaction time, the monochloramine residual in each water sample was measured. The monochloramine residual levels and the corresponding monochloramine doses for all the water samples are shown in SI Table S1. For the determination of iodate in a water sample, a higher monochloramine dose (2 mg/L as Cl2 higher than that for the determination of iodide as shown in SI Table S1) was adopted so that the monochloramine consumption by the excessive ascorbic acid could be compensated.
3.1.3.
Selection of phenol dose
Phenol was selected as the iodine scavenger to react with hypoiodous acid to form organic iodine. The stoichiometric phenol dose for derivatizing 400 mg/L of iodide to organic iodine was 0.2 mg/L of phenol as C. For a real water sample, other components might also consume phenol; too low phenol doses might lead to low reaction rates as well as formation of volatile organic iodine species (Bichsel and von Gunten, 2000), and too high phenol doses could affect the adsorption efficiency of TOI onto the activated carbon. Therefore, an appropriate phenol dose should be selected for the water sample. In this study, to enhance the reaction rate and prevent the formation of volatile organic iodine species, higher phenol doses in the range of 20 mg/L as C (i.e., a 100fold excess) to 100 mg/L as Cl2 (i.e., a 500-fold excess) were tested by using a seawater sample (E) as a representative. The seawater sample was filtered with 0.45 mm membrane and adjusted to pH 7 with nitric acid. It was divided into five aliquots (100 mL each). Then, 2 mg/L of monochloramine as Cl2 was dosed into each aliquot for a reaction time of 15 min in darkness. Subsequently, phenol was added to the aliquots (at 20, 40, 60, 80 and 100 mg/L of phenol as C) for a reaction time of 1 h in darkness. Next, 105% of the requisite stoichiometric amount of NaAsO2 was added to each aliquot to quench the monochloramine residual. Finally, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon and 10 mL of the rinsing solution was used to remove inorganic halides. The TOI concentrations in the seawater aliquots with phenol doses of 20, 40, 60, 80 and 100 mg/L as C were 22.6, 22.4, 22.9, 22.3 and 22.5 mg/L as I, respectively (SI Fig. S3). Therefore, the phenol dose had no effect on the derivatization of iodide and 20 mg/L of phenol as C was thus selected for all the water samples.
3.1.4.
Selection of ascorbic acid dose
Ascorbic acid was selected as the reducing agent to reduce iodate to iodide. The stoichiometric ascorbic acid dose for reducing 200 mg/L of iodate as I to iodide was 0.83 mg/L. For a real water sample, other components might also consume ascorbic acid; too low ascorbic acid doses might lead to incomplete reduction of iodate, and too high ascorbic acid doses could affect the adsorption efficiency of TOI onto the activated carbon. Therefore an appropriate ascorbic acid dose should be selected for the water sample. In this study, the ascorbic acid doses in the range of 4.2 mg/L (i.e., a 5-fold
5
excess) to 84.0 mg/L (i.e., a 100-fold excess) were tested by using a seawater sample (E) as a representative. The seawater sample was filtered with 0.45 mm membrane and adjusted to pH 2 with nitric acid. It was divided into five aliquots (100 mL each). Then, ascorbic acid was added into the aliquots (at 4.2, 8.4, 21.0, 42.0 and 84.0 mg/L) to react for 2 min with stirring. After reaction, the water sample was adjusted to pH 7 with sodium hydroxide. Then, 4, 6, 11, 19 and 36 mg/L of monochloramine as Cl2 (considering the monochloramine consumption by the excessive ascorbic acid) was dosed into the five aliquots, respectively, for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added for a reaction time of 1 h in darkness. Then 105% of the requisite stoichiometric amount of NaAsO2 was added to quench the monochloramine residual in each aliquot. Finally, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon and 10 mL of the rinsing solution was used to remove inorganic halides. The TOI concentrations in the seawater aliquots with ascorbic acid doses of 4.2, 8.4, 21.0, 42.0 and 84.0 mg/L were 44.7, 44.6, 45.4, 45.0 and 44.9 mg/L as I, respectively (SI Fig. S4). Therefore, the ascorbic acid dose had no effect on the reduction of iodate and 4.2 mg/L of ascorbic acid was thus selected for all the water samples.
3.1.5. Selection of reaction time for the reaction between phenol and hypoiodous acid In this study, the phenol dose was 20 mg/L as C; according to Bichsel and von Gunten (2000), the reaction time for transforming 99.99% of hypoiodous acid to organic iodine was calculated to be less than 1 min. This reaction between phenol and hypoiodous acid in ultrapure water completed in 1 min (SI Fig. S5a), which was in accord with the theoretical calculation. However, for real water samples, the reaction rate might be affected by matrices. The reaction completed within 1 h for the seawater sample (SI Fig. S5b). Accordingly, a reaction time of 1 h was adopted for all the water samples.
3.1.6.
Selection of rinsing solution volume
Inorganic halides such as chloride, bromide and iodide may be adsorbed on the activated carbon to some extent. Positive interferences could result if inorganic halides are not fully removed from the activated carbon. Inorganic halides on the activated carbon are generally removed by rinsing with a concentrated nitrate solution (APHA et al., 1995). According to the TOI measurement method by Pan and Zhang (2013), the normally adopted rinsing solution volume was 10 mL. For water samples containing high levels of inorganic halides (e.g., seawater), a small rinsing solution volume may cause incomplete removal of inorganic halides while a large rinsing solution volume may cause desorption of some organic halogens that are weakly adsorbed on the activated carbon. Therefore, an appropriate rinsing solution volume was required to remove only the inorganic halides but not the organic halogens from the activated carbon. To test the effect of rinsing solution volume on TOI measurement, a seawater sample (E) was selected as a representative. The seawater sample was filtered with 0.45 mm
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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membrane, adjusted to pH 7 with nitric acid, and divided into five aliquots (120 mL each). Subsequently, 20 mg/L of phenol as C was added to each aliquot for a reaction time of 1 h in darkness. Then, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which an 80 mL aliquot of the water sample was used for the adsorption on the activated carbon and different volumes (10, 20, 30, 40 and 50 mL) of the rinsing solution were used for rinsing inorganic halides from the activated carbon. The TOI concentrations in the seawater aliquots with rinsing solution volumes of 10, 20, 30, 40 and 50 mL were 4.5, 4.4, 4.3, 4.5 and 4.5 mg/L as I, respectively (SI Fig. S6). Therefore, rinsing solution volume did not show a significant effect on TOI measurement and 10 mL was thus selected for all the water samples.
3.2. Detection limit, quantitation limit and precision of the method For either organo-iodine, sum of organo-iodine and iodide or sum of organo-iodine, iodide and iodate, the method detection limit and the method quantitation limit for a 40 mL water sample were 3.7 and 5.0 mg/L as I, respectively, which were the same as those in the TOI measurement (Pan and Zhang, 2013). For some water samples with very low concentrations (e.g., <5 mg/L as I), a larger volume aliquot (e.g., 80 or 160 mL) was used for adsorption. The detection limit of this method was comparable or even lower compared with those of previous methods, e.g., 8.1 mg/L (Blount and Blasini, 2006) and 9 mg/L (Rong and Takeuchi, 2004) for iodide.
The precision of the method was specified by the relative standard deviations (RSDs), which were calculated from analyzing seven replicates of the standard solutions containing 5 mg/L of iodide, iodate and iodoacetic acid as I. The calculated RSDs were 2.9%, 1.3% and 3.4% for iodide, iodate and organo-iodine, respectively. The RSDs of this method were <3.5%, which were comparable to those of the previous methods, e.g., <5.2% (Blount and Blasini, 2006) and <3% (Schwehr and Santschi, 2003).
3.3.
Recoveries of iodide, iodate and organo-iodine
The recoveries of iodide, iodate and organo-iodine were determined by spiking the standard compounds into the water samples. As illustrated in Tables 1 and 2, the recoveries of iodide and iodate were 91e109% and 90e108%, respectively, indicating that both iodide and iodate were well recovered with this method. The recoveries of organo-iodine were 91e108%, which were the same as those in the TOI measurement by Pan and Zhang (2013).
3.4. Iodide, iodate and organo-iodine measurement of different water samples To verify the method, different types of water samples were determined and the results are shown in Table 1, Table 2, and SI Table S2. The UPLC/ESI-MS selected ion recording (SIR) chromatograms of m/z 126.9 of a representative water sample (seawater E) are shown in SI Fig. S7. For all the water samples, satisfactory recoveries of iodide and iodate were obtained.
Table 1 e Iodide concentrations, recoveries and RSDs for different water samples (n [ 2). Sample
Iodide (mg/L as I)
Iodide ultrapure water solution (50 mg/L as I) Tap water Aa Tap water A þ 20 mg/L iodide as Ia Tap water Ba Tap water B þ 20 mg/L iodide as Ia Tap water Ca
52.5 (105%)
b
RSD (%)
Sample
Iodide (mg/L as I)
RSD (%)
2.9
Seawater G
32.1
1.1
0.1 21.3 (106%)
1.8 1.9
Seawater G þ 20 mg/L iodide as I Urine (diluted by 10 times)a
52.4 (102%) 4.0
1.5 3.1
0.3 21.9 (108%)
3.2 1.7
Urine (diluted by 10 times) þ 20 mg/L iodide as Ia Saline primary wastewater influent
22.2 (91%) 36.2
1.5 2.2
0.4
1.2
54.5 (92%)
1.7
Tap water C þ 20 mg/L iodide as Ia Tap water Da
22.1 (109%)
2.4
Saline primary wastewater influent þ 20 mg/L iodide as I Saline primary wastewater effluent
26.4
0.4
0.4
2.0
46.4 (100%)
3.0
Tap water D þ 20 mg/L iodide as Ia Seawater E
22.1 (109%)
1.3
Saline primary wastewater effluent þ 20 mg/L iodide as I Saline secondary wastewater influent
23.1
1.1
18.1
3.0
44.4 (107%)
0.6
Seawater E þ 20 mg/L iodide as I Seawater F
38.8 (104%)
2.9
Saline secondary wastewater influent þ 20 mg/L iodide as I Saline secondary wastewater effluent
5.0
1.5
13.8
0.8
Saline secondary wastewater effluent þ 20 mg/L iodide as I
25.7 (104%)
0.7
Seawater F þ 20 mg/L iodide as I
32.6 (94%)
1.0
a A 160 mL aliquot was used for each TOI measurement. b Data listed in all the brackets were the calculated iodide recoveries of the water samples.
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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w a t e r r e s e a r c h x x x ( 2 0 1 3 ) 1 e1 0
Table 2 e Iodate concentrations, recoveries and RSDs for different water samples (n [ 2). Sample
Iodate (mg/L as I)
RSD (%)
Sample
Iodate (mg/L as I)
RSD (%)
Iodate ultrapure water solution (50 mg/L as I) Tap water Aa Tap water A þ 20 mg/L iodate as Ia Tap water Ba
48.8 (98%)b
0.1
Seawater G
16.3
2.0
6.8 27.0 (101%) 10.6
1.9 1.5 0.4
34.7 (92%) n.d.c 18.1 (91%)
0.9 1.2 1.5
Tap water B þ 20 mg/L iodate as Ia Tap water Ca
30.7 (101%) 4.7
1.9 2.1
3.0 24.1 (106%)
1.9 2.1
Tap water C þ 20 mg/L iodate as Ia
24.9 (101%)
0.3
10.9
3.2
Tap water Da
7.1
1.1
30.8 (100%)
3.1
Tap water D þ 20 mg/L iodate as Ia
28.7 (108%)
2.3
8.9
1.1
Seawater E
21.7
1.3
29.3 (102%)
2.6
Seawater E þ 20 mg/L iodate as I
40.0 (92%)
1.7
4.1
0.2
Seawater F
20.0
0.6
Seawater G þ 20 mg/L iodate as I Urine (diluted by 10 times)a Urine (diluted by 10 times) þ20 mg/L iodate as Ia Saline primary wastewater influent Saline primary wastewater influent þ 20 mg/L iodate as I Saline primary wastewater effluent Saline primary wastewater effluent þ 20 mg/L iodate as I Saline secondary wastewater influent Saline secondary wastewater influent þ 20 mg/L iodate as I Saline secondary wastewater effluent Saline secondary wastewater effluent þ 20 mg/L iodate as I
25.3 (106%)
2.7
Seawater F þ 20 mg/L iodate as I
37.9 (90%)
1.7
a A 160 mL aliquot was used for each TOI measurement. b Data listed in all the brackets were the calculated iodate recoveries of the water samples. c Not detected.
The concentrations and distributions of the three iodine species in different water samples are shown in Fig. 2. Several important points extracted from the results are described as follows. The total iodine concentrations in tap waters A, B, C and D were 8.2, 12.5, 6.5 and 12.9 mg/L as I, respectively, which
Fig. 2 e Concentrations of iodine species in different water samples.
were comparable since their source waters were all river waters. The tap water samples contained all the three iodine species, and the predominant species was iodate. According to the chlorine residual titration, all the four tap water samples contained free chlorine residuals in the range of 0.3e0.5 mg/L as Cl2. Due to the fast oxidation of iodide to iodate by free chlorine, it is rational that iodate was the dominant iodine species in these tap water samples. The total iodine concentrations in the three seawater samples were 44.2, 39.5 and 51.2 mg/L as I, respectively, which were generally in agreement with previous reports that the total iodine concentration in seawater should be 45e90 mg/L as I (Whitehead, 1984; Schwehr and Santschi, 2003). The seawater samples contained all the three iodine species, i.e., iodide, iodate and organo-iodine. In seawaters E and F, the dominant iodine species was iodate, which agreed with some of previous studies, with the conclusion that the predominant iodine species in seawater was the thermodynamically stable iodate (Ito, 1997; Schwehr and Santschi, 2003). However, since seawaters E and F were both surface seawater samples, the concentration differences between iodide and iodate were not significant (Ito, 1997; Schwehr and Santschi, 2003). Interestingly, it was found that in seawater G, the dominant iodine species was iodide rather than iodate. It has been demonstrated that iodide could be produced by biologically mediated reduction of iodate and was favorable under reducing conditions (Rong et al., 2007). Therefore, it is possible that iodide becomes dominant in some seawater samples, especially in coast, estuary and bay waters (Rong et al., 2007); some
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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researchers also reported that iodide dominated in their seawater samples (Hou et al., 1999; Hansen et al., 2011). Since seawater G was a surface seawater sample taken from an area near the pier, it was probably affected by human activities, resulting in a high level of iodide. The total iodine concentration in the urine sample was determined to be 54.9 mg/L as I. The urine sample contained only iodide and organo-iodine, while the predominant species was iodide with a concentration of 39.7 mg/L as I. One study also reported similar results that iodide was the dominant iodine species in the urine samples while iodate was undetectable (Zhang et al., 2010b). It is reasonable that iodide was the main species in urine, owing to high levels of reducing substances in it. The iodine in urine finally ends up in domestic wastewater. The total iodine concentrations in the saline primary and secondary wastewater influents were 44.6 and 37.0 mg/L as I, respectively, which were quite similar. The relatively high total iodine levels in the saline wastewater influents were mainly ascribed to the use of seawater for toilet flushing, which introduced iodine from seawater to the wastewaters. Besides, some physiological fluids (e.g., milk, serum and urine), tap water, sewage from kitchen (containing table salts and some foodstuffs) and hospital wastewater (containing X-ray contrast media) might also contribute to the iodine in the wastewater influents (Hou et al., 2000; Verma et al., 1992; Blount et al., 2010; Pan and Zhang, 2013; Duirk et al., 2011). In both saline wastewater influents, iodide was the dominant species with concentrations even higher than some of the seawater samples, indicating that besides seawater, some other wastewater sources also contributed to the iodide in these wastewater influents. The total iodine concentrations in the saline primary and secondary wastewater effluents were 44.4 and 27.6 mg/L as I, respectively. Interestingly, the predominant iodine species in the saline primary wastewater effluent was still iodide while that of the saline secondary wastewater effluent became organoiodine, which was in accord with a previous study showing that the transformation of iodide to organo-iodine was enhanced by microorganisms (Ra¨dlinger and Heumann, 2000). The high iodide concentration in the saline primary wastewater effluent might cause the formation of more iodinated DBPs during wastewater disinfection. Moreover, it was found that iodine was partially removed during both primary and secondary treatment; the total iodine removal efficiencies of the saline primary and secondary wastewaters were calculated to be 0.45% and 25.4%, respectively, indicating that secondary treatment resulted in a much higher removal efficiency for iodine. It can be ascribed to two reasons: firstly, some microorganisms tend to accumulate iodide in their cells (Amachi et al., 2005b) and thus the iodine can be removed by accumulating in the activated sludge during secondary treatment; secondly, the transformation of iodide to organo-iodine is enhanced by microorganisms (Ra¨dlinger and Heumann, 2000) and therefore the mobile iodide can be transformed to organo-iodine during secondary treatment followed by adsorption on the sediments. The significant decrease of iodide and increase of organo-iodine in the saline secondary wastewater after treatment supported these two assumptions.
4.
Conclusions
A fast, sensitive and accurate method was developed for determining the three dominant iodine species in waters, i.e., iodide, iodate and organo-iodine. The new method involved derivatization of iodide and iodate to organic iodine and measurement of TOI. The developed method was applied to the determination of iodide, iodate and organo-iodine in a variety of water samples, and meaningful results with good recoveries and low RSDs were achieved. Interestingly, it was found that the iodide concentration in the saline primary wastewater effluent of Hong Kong was significantly higher than the saline secondary wastewater effluent, which might cause the formation of more iodinated DBPs in the saline primary wastewater effluent during wastewater disinfection. Furthermore, iodine was partially removed during both primary and secondary wastewater treatment while secondary treatment showed a significantly higher removal efficiency than primary treatment. This new method makes possible the accurate determination of relatively low concentrations of iodide, iodate, and organo-iodine in waters, and provides a sensitive tool to evaluate the mass balances of iodine and the transformation among the three different iodine species during disinfection as well as any other environmental processes.
Acknowledgment This work was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Projects No. 622412 and No. RPC11EG16).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.08.039.
references
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach Tingting Gong, Xiangru Zhang* Environmental Engineering Program, Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong, China
article info
abstract
Article history:
The dissolved iodine species that dominate aquatic systems are iodide, iodate and organo-
Received 30 April 2013
iodine. These species may undergo transformation to one another and thus affect the
Received in revised form
formation of iodinated disinfection byproducts during disinfection of drinking waters or
28 August 2013
wastewater effluents. In this study, a fast, sensitive and accurate method for determining
Accepted 31 August 2013
these iodine species in waters was developed by derivatizing iodide and iodate to organic
Available online xxx
iodine and measuring organic iodine with a total organic iodine (TOI) measurement approach. Within this method, organo-iodine was determined directly by TOI measure-
Keywords:
ment; iodide was oxidized by monochloramine to hypoiodous acid and then hypoiodous
Disinfection byproducts
acid reacted with phenol to form organic iodine, which was determined by TOI measure-
Total organic iodine
ment; iodate was reduced by ascorbic acid to iodide and then determined as iodide. The
TOI
quantitation limit of organo-iodine or sum of organo-iodine and iodide or sum of organo-
Iodide
iodine, iodide and iodate was 5 mg/L as I for a 40 mL water sample (or 2.5 mg/L as I for an
Iodate
80 mL water sample, or 1.25 mg/L as I for a 160 mL water sample). This method was suc-
Waters
cessfully applied to the determination of iodide, iodate and organo-iodine in a variety of water samples, including tap water, seawater, urine and wastewater. The recoveries of iodide, iodate and organo-iodine were 91e109%, 90e108% and 91e108%, respectively. The concentrations and distributions of iodine species in different water samples were obtained and compared. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Iodine is a trace element that exists in natural waters such as seawater, freshwater and rain (Schwehr and Santschi, 2003; Gilfedder et al., 2008, 2009). Iodine is also a biologically essential nutrient for all mammals including humans (Gilfedder et al., 2009), and thus various physiological fluids, such as milk, serum, and urine, contain iodine (Hou et al.,
2000; Verma et al., 1992). Iodine deficiency in the human body causes goitre as well as some other malfunctions and the normative daily iodine intake for an adult has been reported to be 1 mg/kg body weight (Merian et al., 2004). Humans ingest iodine mainly through drinking water and foods. Therefore, to ensure the proper daily iodine intake for humans, iodine has been added to table salts and some other foodstuffs. Owing to the occurrence in natural waters, iodine has also been
* Corresponding author. Tel.: þ852 2358 8479; fax: þ852 2358 1534. E-mail address: [email protected] (X. Zhang). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.08.039
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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detected in tap water (Blount et al., 2010; Pan and Zhang, 2013). Besides, large amounts of iodine-containing organic compounds are used as X-ray contrast media for medical purposes, which add iodine to hospital wastewater (Duirk et al., 2011). Iodine is widely used in the manufacture of chemicals, iodophors, biocides, herbicides, pharmaceuticals and nylon (Pin˜eiro et al., 2011), which ends up in industrial wastewater. The typical iodine concentrations in seawater, freshwater, rain and urine are 45e90, 0.5e20, 0.5e5.0 and 0e300 mg/L, respectively (Whitehead, 1984; Schwehr and ¨ nak et al., 1999). With all Santschi, 2003; Hou et al., 2000; U these natural and artificial iodine sources, wastewater tends to contain a certain level of iodine. The dominant iodine species in waters are iodide, iodate and organo-iodine (Hansen et al., 2011; Gilfedder et al., 2009). Different iodine species may exhibit dramatically different mobility, bioavailability and chemical behaviors in the environment. For example, iodide is rather mobile, while iodate is efficiently sorbed by soils (Gilfedder et al., 2009); iodide, rather than iodate, is the bioavailable iodine form (Eskandari et al., 1997; Ku¨pper et al., 1998); inorganic and organic iodine species have different hydrophilic and biophilic properties (Hu et al., 2005). Furthermore, the “three” dominant iodine species, i.e., iodide, iodate and organo-iodine, may undergo transformation to one another in the environment and during water/wastewater treatment. Nitrate-reducing, iron-reducing and sulfate-reducing bacteria have the potential to reduce iodate to iodide (Farrenkopf et al., 1997; Councell et al., 1997); certain bacteria can oxidize iodide to molecular iodine (Amachi et al., 2005a); iodide undergoes transformation in natural and wastewater systems by fixation on humic substances to form organo-iodine (Ra¨dlinger and Heumann, 2000). Therefore, the study of iodine speciation in waters is of great significance. Drinking water needs to be disinfected during treatment. Wastewater effluents are also subjected to disinfection before discharge to inactivate the microorganisms. During disinfection, iodide can be oxidized by disinfectants to HOI/OI, which further reacts with natural/effluent organic matter to form iodinated disinfection byproducts (DBPs) (Bichsel and von Gunten, 1999, 2000; Krasner et al., 2006; Hua and Reckhow, 2006, 2007; Richardson et al., 2008; Ding and Zhang, 2009; Duirk et al., 2011; Jones et al., 2012; Chu et al., 2012). Iodinated DBPs have drawn more and more concerns due to their significantly higher toxicity than their brominated and chlorinated analogues (Plewa et al., 2004; Cemeli et al., 2006; Richardson et al., 2007, 2008). Since the three iodine species can undergo transformation to one another, all the three iodine species in source water and wastewater need to be determined to predict the iodine levels that are possible to participate in the formation of iodinated DBPs. To date, a number of methods for the determination of iodine species have been reported. However, most of them can determine only one or two iodine species (iodide and/or iodate) (Mishra et al., 2000; Hou et al., 2000). Recently, a couple of methods involving the determination of all the three iodine species (iodide, iodate and organo-iodine) have been reported. In one method, the three iodine species were derivatized to iodide and then iodide was determined by high performance liquid chromatography; however, the derivatization time for
organo-iodine to iodide was more than 20 h, which was timeconsuming (Schwehr and Santschi, 2003). A gas chromatographyemass spectrometry (GCeMS) method has also been developed by derivatizing all the three iodine species to a specific organic iodine derivative followed by GCeMS detection; however, it involved a complex derivatization process for organo-iodine, which was a three-step derivatization; moreover, it required the formation of only one isomer of the iodine derivative, so iodine must react only with the selected iodine scavenger and form only one iodine derivative. Thus the organic compounds which might react with iodine must be removed from the original water samples and the selection of the iodine scavenger was limited to the organic compounds with only one available substitution position for iodine (Zhang et al., 2010a). Ion chromatography with conductivity detection is also not amenable for the detection of inorganic iodine species since conductivity detection responds to all ionic species, resulting in a high quantitation limit (up to 100 mg/L as I). Therefore, the purpose of this study was to develop a method which involves a simple and short derivatization process for each iodine species and provides lower detection limits for those iodine species. In this study, a fast, sensitive and accurate method was developed for the quantitation of iodide, iodate and organoiodine in waters. The developed method was applied to the determination of iodine species in a variety of water samples, including tap water, seawater, urine and wastewater. The concentrations and distributions of iodine species in different water samples were illustrated.
2.
Materials and methods
2.1.
Chemicals and reagents
Potassium iodide (100%) was purchased from BDH. Ascorbic acid (99.7%), potassium iodate (99.7%), ammonium chloride (99.5%) and potassium nitrate (99.0%) were purchased from Riedel-deHae¨n. A sodium hypochlorite stock solution was purchased from Sigma Aldrich and diluted to around 2000 mg/L as Cl2 and periodically standardized by the N,Ndiethyl-p-phenylene diamine (DPD) ferrous titrimetric method (APHA et al., 1995). All other chemicals used in this study were purchased at the highest purities available from Sigma Aldrich. Ultrapure water (18.2 MU cm) was supplied by a NANOpure Diamond purifier system (Barnstead).
2.2.
Preparation of solutions
The iodide and iodate stock solutions (100 mg/L as I) were prepared by dissolving potassium iodide and potassium iodate in ultrapure water, respectively. They were stored in amber glass bottles at 4 C and newly prepared every month. The working solutions of iodide and iodate with lower concentrations were freshly prepared daily by diluting the stock solutions with ultrapure water. An ammonium chloride solution (1.0 g/L as N), a phenol solution (10.0 g/L as C), an ascorbic acid solution (1.0 g/L) and a sodium arsenite solution (13.0 g/L) were prepared. They were stored in amber glass bottles at 4 C. The phenol, ascorbic acid
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3
and sodium arsenite solutions were newly prepared every week. A monochloramine solution was prepared just before use by gradually adding the sodium hypochlorite solution to the ammonium chloride solution at a chlorine-to-ammonium molar ratio of 0.8. Iodoacetic acid was dissolved in ultrapure water to prepare standard solutions with total organic iodine (TOI) concentrations of 5, 10, 20, 50 and 100 mg/L as I. A potassium nitrate solution (5000 mg/L as NO 3 ) was prepared as the rinsing solution for removing inorganic halides from activated carbon.
2.3.
Collection of water samples
A variety of water samples (different aqueous matrices) were collected to test the method. Four tap water samples were collected from four cities in China, including three coastal cities (A, C and D) and one inland city (B). They were immediately transferred to the laboratory in ice coolers by airplane. The chlorine residuals in the four samples were titrated by the DPD ferrous titrimetric method (APHA et al., 1995) and quenched with 105% of the requisite stoichiometric amounts of NaAsO2 (Liu and Zhang, 2012). Three seawater samples were collected from the marine laboratory of the Hong Kong University of Science and Technology (E), Clear Water Bay (F) and Sai Kung (G) in Hong Kong. They were immediately transferred to the laboratory in ice coolers. A urine sample was obtained by collecting urine from five male and five female students followed by a thorough mixing. It was immediately transferred to the laboratory in ice coolers. Four wastewater influent and effluent (undisinfected) samples were collected from two different wastewater treatment plants in Hong Kong, including a saline primary wastewater treatment plant and a saline secondary wastewater treatment plant. They were immediately transferred to the laboratory in ice coolers. All the samples were stored at 4 C to minimize changes in constituents.
2.4. Determination of iodide, iodate and organo-iodine in water samples In this study, a newly developed method for TOI measurement (Pan and Zhang, 2013) was adopted. It involves adsorption of organic iodine onto activated carbon, pyrolysis of adsorbed organic iodine to hydrogen iodide, absorption of hydrogen iodide into ultrapure water, pretreatment of the absorption solution, and off-line separation/detection of iodide in the absorption solution using ultra performance liquid chromatography/electrospray ionization-mass spectrometry (UPLC/ ESI-MS). The analytical scheme for the three iodine species, i.e., iodide, iodate and organo-iodine, is described in Fig. 1. Organo-iodine was measured directly by TOI measurement. Iodide was derivatized to organic iodine and then determined by TOI measurement. Iodate was reduced to iodide and then determined. As described by Pan and Zhang (2013), iodoacetic acid was used as the calibration standard compound for TOI measurement. The iodoacetic acid concentrations for the calibration curve were 5, 10, 20, 50 and 100 mg/L as I. All the
Fig. 1 e Analytical scheme for iodine species.
water samples to be analyzed were firstly filtered with 0.45 mm membranes. For the water samples with high levels of reducing substances (e.g., urine), dilution with ultrapure water was conducted before analysis. To determine iodide in a water sample, a 120 mL aliquot of the water sample was adjusted to pH 7 with sodium hydroxide or nitric acid. A determined amount of monochloramine (Supplementary Information (SI) Table S1) was dosed into the water sample, allowing for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added for a reaction time of 1 h in darkness. Then 105% of the requisite stoichiometric amount of NaAsO2 was added to quench the monochloramine residual in the water sample. Finally, the water sample was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon, 10 mL of the rinsing solution was used to remove inorganic halides, and duplicate measurements were conducted. The procedure for TOI measurement is detailed in the SI. The measured TOI value corresponded to the sum of organo-iodine and iodide in the water sample and the iodide concentration was calculated as the difference between this TOI value and organo-iodine. To determine iodate in a water sample, a 120 mL aliquot of the water sample was adjusted to pH 2 with nitric acid and 4.2 mg/L of ascorbic acid was added to react for 2 min with stirring. After reaction, the water sample was adjusted to pH 7 with sodium hydroxide. A determined amount of monochloramine (SI Table S1) was dosed into the water sample, allowing for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added for a reaction time of 1 h in darkness. Then 105% of the requisite stoichiometric amount of NaAsO2 was added to quench the monochloramine residual in the water sample. Finally, the water sample was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon, 10 mL of the rinsing solution was used to remove inorganic halides, and duplicate measurements were conducted. The measured TOI value corresponded to the sum of organoiodine, iodide and iodate in the water sample and the iodate
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concentration was calculated as the difference between this TOI value and the sum of organo-iodine and iodide. To determine organo-iodine in a water sample, the water sample was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon, 10 mL of the rinsing solution was used to remove inorganic halides, and duplicate measurements were conducted. To determine either iodide, iodate, or organo-iodine, for water samples with very low concentrations (e.g., <5 mg/L as I), larger volume aliquots (e.g., 80 or 160 mL) were used for the adsorption on the activated carbon while other experimental steps were kept the same.
IO 3 þ 3C6 H8 O6 /I þ 3C6 H6 O6 þ 3H2 O
Since this reduction reaction occurred only in an acidic medium (Verma et al., 1992), it was conducted at pH 2 in this study. According to previous studies (Verma et al., 1992; Mishra et al., 2000), this reduction reaction completed within 2 min with stirring in an acidic medium with various matrices, so each water sample was stirred for 2 min after the addition of ascorbic acid. Any excess of ascorbic acid was subsequently oxidized by monochloramine.
3.1.2.
3.
Results and discussion
3.1.
Method development
3.1.1.
Derivatization of iodide and iodate to organic iodine
For the determination of iodide, the first step was to oxidize iodide to hypoiodous acid only rather than further to iodate. It has been demonstrated that iodide was quickly oxidized by monochloramine to hypoiodous acid but not further to iodate (Bichsel and von Gunten, 1999). Therefore, monochloramine was selected as the oxidizing agent:
G þ NH2 Cl þ Hþ þ H2 O/HOI þ NHþ 4 þ Cl
The kinetics of the reaction has been reported by Bichsel and von Gunten (1999). In this study, the monochloramine concentration involved in the oxidation of iodide was generally around 1 mg/L as Cl2, and pH was around 7, so the theoretical reaction time for oxidizing 99.99% of the iodide to hypoiodous acid was calculated to be 4.5 min. To ensure the complete oxidation, a reaction time of 15 min was adopted for all the water samples. The second step was to derivatize hypoiodous acid to organic iodine. An iodine scavenger was needed to quickly react with hypoiodous acid to form organic iodine derivatives. Among various classes of organic compounds, only aromatic amines and phenols respond to rapid iodination; however, amines can be readily oxidized and thus were not used (Verma et al., 1992). In previous studies, GCeMS was used for the quantitation of organic iodine derivatives, which generally required the formation of only one derivative (Verma et al., 1992; Mishra et al., 2000; Zhang et al., 2010a). In this study, the quantitation of iodide or iodate was done by derivatization to organic iodine and subsequent TOI measurement, thus the iodine derivatives were not limited to only one specific compound (for nearly all iodine-containing organic compounds respond to TOI measurement). Therefore, there was no need to find a compound with only one available substitution position for iodine, which broadened the selection of an iodine scavenger. Phenol, the most common aromatic compound which responds to rapid iodination, was selected in this study. According to Dietrich et al. (1999), the phenol to iodine ratio was high enough that only monoiodophenol could be formed: C6 H5 OH þ HOI/C6 H4 IOH þ H2 O
The kinetics of the reaction has been reported by Bichsel and von Gunten (2000). For the determination of iodate, it was first reduced to iodide by ascorbic acid (C6H8O6):
Selection of monochloramine dose
Monochloramine was selected as the oxidizing agent to oxidize iodide to hypoiodous acid. It was assumed that the highest concentrations of both iodide and iodate in the water samples were 200 mg/L as I. The stoichiometric monochloramine dose for oxidizing 400 mg/L of iodide (the sum of iodide and reduced iodate) was 0.2 mg/L as Cl2. For a real water sample, other components might also consume monochloramine; too low monochloramine doses might lead to incomplete oxidation of the iodide, and too high monochloramine doses could cause decomposition of the TOI. Therefore an appropriate monochloramine dose should be selected for the water sample. In this study, the monochloramine doses in the range of 1 mg/L as Cl2 (i.e., a 5-fold excess) to 20 mg/L as Cl2 (i.e., a 100-fold excess) were tested by using a seawater sample (E) as a representative. Seawater was selected because it contains typical levels of all the three dominant iodine species and a relatively complicated matrix (certain levels of organic compounds and high levels of inorganic ions). The seawater sample was filtered with 0.45 mm membrane, adjusted to pH 7 with nitric acid, and divided into five aliquots (100 mL each). Monochloramine was dosed into the five aliquots (at 1, 2, 5, 10 and 20 mg/L as Cl2) for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added to each aliquot for a reaction time of 1 h in darkness. Then, 105% of the requisite stoichiometric amount of NaAsO2 was added to each aliquot to quench the monochloramine residual. Finally, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon and 10 mL of the rinsing solution was used to remove inorganic halides. The TOI concentrations in the seawater aliquots with monochloramine doses of 1, 2, 5, 10 and 20 mg/L as Cl2 were 22.5, 22.6, 21.0, 19.2 and 17.1 mg/L as I, respectively (SI Fig. S1). The TOI concentration had no significant change when the monochloramine dose increased from 1 to 2 mg/L as Cl2, but it began to decrease significantly when the monochloramine dose was above 2 mg/ L as Cl2. We tested the effect of monochloramine dose on the decomposition of organic iodine (SI), and found that high monochloramine doses caused the decomposition of some organic iodine to inorganic iodine (SI Fig. S2), leading to an underestimation of the iodide concentration. Therefore, an appropriate monochloramine dose that could maintain a
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monochloramine residual of 1e2 mg/L as Cl2 after 1 h reaction time was selected for each water sample. To achieve a monochloramine residual of 1e2 mg/L as Cl2 after 1 h reaction time, the monochloramine residual in each water sample was measured. The monochloramine residual levels and the corresponding monochloramine doses for all the water samples are shown in SI Table S1. For the determination of iodate in a water sample, a higher monochloramine dose (2 mg/L as Cl2 higher than that for the determination of iodide as shown in SI Table S1) was adopted so that the monochloramine consumption by the excessive ascorbic acid could be compensated.
3.1.3.
Selection of phenol dose
Phenol was selected as the iodine scavenger to react with hypoiodous acid to form organic iodine. The stoichiometric phenol dose for derivatizing 400 mg/L of iodide to organic iodine was 0.2 mg/L of phenol as C. For a real water sample, other components might also consume phenol; too low phenol doses might lead to low reaction rates as well as formation of volatile organic iodine species (Bichsel and von Gunten, 2000), and too high phenol doses could affect the adsorption efficiency of TOI onto the activated carbon. Therefore, an appropriate phenol dose should be selected for the water sample. In this study, to enhance the reaction rate and prevent the formation of volatile organic iodine species, higher phenol doses in the range of 20 mg/L as C (i.e., a 100fold excess) to 100 mg/L as Cl2 (i.e., a 500-fold excess) were tested by using a seawater sample (E) as a representative. The seawater sample was filtered with 0.45 mm membrane and adjusted to pH 7 with nitric acid. It was divided into five aliquots (100 mL each). Then, 2 mg/L of monochloramine as Cl2 was dosed into each aliquot for a reaction time of 15 min in darkness. Subsequently, phenol was added to the aliquots (at 20, 40, 60, 80 and 100 mg/L of phenol as C) for a reaction time of 1 h in darkness. Next, 105% of the requisite stoichiometric amount of NaAsO2 was added to each aliquot to quench the monochloramine residual. Finally, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon and 10 mL of the rinsing solution was used to remove inorganic halides. The TOI concentrations in the seawater aliquots with phenol doses of 20, 40, 60, 80 and 100 mg/L as C were 22.6, 22.4, 22.9, 22.3 and 22.5 mg/L as I, respectively (SI Fig. S3). Therefore, the phenol dose had no effect on the derivatization of iodide and 20 mg/L of phenol as C was thus selected for all the water samples.
3.1.4.
Selection of ascorbic acid dose
Ascorbic acid was selected as the reducing agent to reduce iodate to iodide. The stoichiometric ascorbic acid dose for reducing 200 mg/L of iodate as I to iodide was 0.83 mg/L. For a real water sample, other components might also consume ascorbic acid; too low ascorbic acid doses might lead to incomplete reduction of iodate, and too high ascorbic acid doses could affect the adsorption efficiency of TOI onto the activated carbon. Therefore an appropriate ascorbic acid dose should be selected for the water sample. In this study, the ascorbic acid doses in the range of 4.2 mg/L (i.e., a 5-fold
5
excess) to 84.0 mg/L (i.e., a 100-fold excess) were tested by using a seawater sample (E) as a representative. The seawater sample was filtered with 0.45 mm membrane and adjusted to pH 2 with nitric acid. It was divided into five aliquots (100 mL each). Then, ascorbic acid was added into the aliquots (at 4.2, 8.4, 21.0, 42.0 and 84.0 mg/L) to react for 2 min with stirring. After reaction, the water sample was adjusted to pH 7 with sodium hydroxide. Then, 4, 6, 11, 19 and 36 mg/L of monochloramine as Cl2 (considering the monochloramine consumption by the excessive ascorbic acid) was dosed into the five aliquots, respectively, for a reaction time of 15 min in darkness. Subsequently, 20 mg/L of phenol as C was added for a reaction time of 1 h in darkness. Then 105% of the requisite stoichiometric amount of NaAsO2 was added to quench the monochloramine residual in each aliquot. Finally, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which a 40 mL aliquot of the water sample was used for the adsorption on the activated carbon and 10 mL of the rinsing solution was used to remove inorganic halides. The TOI concentrations in the seawater aliquots with ascorbic acid doses of 4.2, 8.4, 21.0, 42.0 and 84.0 mg/L were 44.7, 44.6, 45.4, 45.0 and 44.9 mg/L as I, respectively (SI Fig. S4). Therefore, the ascorbic acid dose had no effect on the reduction of iodate and 4.2 mg/L of ascorbic acid was thus selected for all the water samples.
3.1.5. Selection of reaction time for the reaction between phenol and hypoiodous acid In this study, the phenol dose was 20 mg/L as C; according to Bichsel and von Gunten (2000), the reaction time for transforming 99.99% of hypoiodous acid to organic iodine was calculated to be less than 1 min. This reaction between phenol and hypoiodous acid in ultrapure water completed in 1 min (SI Fig. S5a), which was in accord with the theoretical calculation. However, for real water samples, the reaction rate might be affected by matrices. The reaction completed within 1 h for the seawater sample (SI Fig. S5b). Accordingly, a reaction time of 1 h was adopted for all the water samples.
3.1.6.
Selection of rinsing solution volume
Inorganic halides such as chloride, bromide and iodide may be adsorbed on the activated carbon to some extent. Positive interferences could result if inorganic halides are not fully removed from the activated carbon. Inorganic halides on the activated carbon are generally removed by rinsing with a concentrated nitrate solution (APHA et al., 1995). According to the TOI measurement method by Pan and Zhang (2013), the normally adopted rinsing solution volume was 10 mL. For water samples containing high levels of inorganic halides (e.g., seawater), a small rinsing solution volume may cause incomplete removal of inorganic halides while a large rinsing solution volume may cause desorption of some organic halogens that are weakly adsorbed on the activated carbon. Therefore, an appropriate rinsing solution volume was required to remove only the inorganic halides but not the organic halogens from the activated carbon. To test the effect of rinsing solution volume on TOI measurement, a seawater sample (E) was selected as a representative. The seawater sample was filtered with 0.45 mm
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membrane, adjusted to pH 7 with nitric acid, and divided into five aliquots (120 mL each). Subsequently, 20 mg/L of phenol as C was added to each aliquot for a reaction time of 1 h in darkness. Then, each aliquot was subjected to TOI measurement following the method by Pan and Zhang (2013), in which an 80 mL aliquot of the water sample was used for the adsorption on the activated carbon and different volumes (10, 20, 30, 40 and 50 mL) of the rinsing solution were used for rinsing inorganic halides from the activated carbon. The TOI concentrations in the seawater aliquots with rinsing solution volumes of 10, 20, 30, 40 and 50 mL were 4.5, 4.4, 4.3, 4.5 and 4.5 mg/L as I, respectively (SI Fig. S6). Therefore, rinsing solution volume did not show a significant effect on TOI measurement and 10 mL was thus selected for all the water samples.
3.2. Detection limit, quantitation limit and precision of the method For either organo-iodine, sum of organo-iodine and iodide or sum of organo-iodine, iodide and iodate, the method detection limit and the method quantitation limit for a 40 mL water sample were 3.7 and 5.0 mg/L as I, respectively, which were the same as those in the TOI measurement (Pan and Zhang, 2013). For some water samples with very low concentrations (e.g., <5 mg/L as I), a larger volume aliquot (e.g., 80 or 160 mL) was used for adsorption. The detection limit of this method was comparable or even lower compared with those of previous methods, e.g., 8.1 mg/L (Blount and Blasini, 2006) and 9 mg/L (Rong and Takeuchi, 2004) for iodide.
The precision of the method was specified by the relative standard deviations (RSDs), which were calculated from analyzing seven replicates of the standard solutions containing 5 mg/L of iodide, iodate and iodoacetic acid as I. The calculated RSDs were 2.9%, 1.3% and 3.4% for iodide, iodate and organo-iodine, respectively. The RSDs of this method were <3.5%, which were comparable to those of the previous methods, e.g., <5.2% (Blount and Blasini, 2006) and <3% (Schwehr and Santschi, 2003).
3.3.
Recoveries of iodide, iodate and organo-iodine
The recoveries of iodide, iodate and organo-iodine were determined by spiking the standard compounds into the water samples. As illustrated in Tables 1 and 2, the recoveries of iodide and iodate were 91e109% and 90e108%, respectively, indicating that both iodide and iodate were well recovered with this method. The recoveries of organo-iodine were 91e108%, which were the same as those in the TOI measurement by Pan and Zhang (2013).
3.4. Iodide, iodate and organo-iodine measurement of different water samples To verify the method, different types of water samples were determined and the results are shown in Table 1, Table 2, and SI Table S2. The UPLC/ESI-MS selected ion recording (SIR) chromatograms of m/z 126.9 of a representative water sample (seawater E) are shown in SI Fig. S7. For all the water samples, satisfactory recoveries of iodide and iodate were obtained.
Table 1 e Iodide concentrations, recoveries and RSDs for different water samples (n [ 2). Sample
Iodide (mg/L as I)
Iodide ultrapure water solution (50 mg/L as I) Tap water Aa Tap water A þ 20 mg/L iodide as Ia Tap water Ba Tap water B þ 20 mg/L iodide as Ia Tap water Ca
52.5 (105%)
b
RSD (%)
Sample
Iodide (mg/L as I)
RSD (%)
2.9
Seawater G
32.1
1.1
0.1 21.3 (106%)
1.8 1.9
Seawater G þ 20 mg/L iodide as I Urine (diluted by 10 times)a
52.4 (102%) 4.0
1.5 3.1
0.3 21.9 (108%)
3.2 1.7
Urine (diluted by 10 times) þ 20 mg/L iodide as Ia Saline primary wastewater influent
22.2 (91%) 36.2
1.5 2.2
0.4
1.2
54.5 (92%)
1.7
Tap water C þ 20 mg/L iodide as Ia Tap water Da
22.1 (109%)
2.4
Saline primary wastewater influent þ 20 mg/L iodide as I Saline primary wastewater effluent
26.4
0.4
0.4
2.0
46.4 (100%)
3.0
Tap water D þ 20 mg/L iodide as Ia Seawater E
22.1 (109%)
1.3
Saline primary wastewater effluent þ 20 mg/L iodide as I Saline secondary wastewater influent
23.1
1.1
18.1
3.0
44.4 (107%)
0.6
Seawater E þ 20 mg/L iodide as I Seawater F
38.8 (104%)
2.9
Saline secondary wastewater influent þ 20 mg/L iodide as I Saline secondary wastewater effluent
5.0
1.5
13.8
0.8
Saline secondary wastewater effluent þ 20 mg/L iodide as I
25.7 (104%)
0.7
Seawater F þ 20 mg/L iodide as I
32.6 (94%)
1.0
a A 160 mL aliquot was used for each TOI measurement. b Data listed in all the brackets were the calculated iodide recoveries of the water samples.
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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Table 2 e Iodate concentrations, recoveries and RSDs for different water samples (n [ 2). Sample
Iodate (mg/L as I)
RSD (%)
Sample
Iodate (mg/L as I)
RSD (%)
Iodate ultrapure water solution (50 mg/L as I) Tap water Aa Tap water A þ 20 mg/L iodate as Ia Tap water Ba
48.8 (98%)b
0.1
Seawater G
16.3
2.0
6.8 27.0 (101%) 10.6
1.9 1.5 0.4
34.7 (92%) n.d.c 18.1 (91%)
0.9 1.2 1.5
Tap water B þ 20 mg/L iodate as Ia Tap water Ca
30.7 (101%) 4.7
1.9 2.1
3.0 24.1 (106%)
1.9 2.1
Tap water C þ 20 mg/L iodate as Ia
24.9 (101%)
0.3
10.9
3.2
Tap water Da
7.1
1.1
30.8 (100%)
3.1
Tap water D þ 20 mg/L iodate as Ia
28.7 (108%)
2.3
8.9
1.1
Seawater E
21.7
1.3
29.3 (102%)
2.6
Seawater E þ 20 mg/L iodate as I
40.0 (92%)
1.7
4.1
0.2
Seawater F
20.0
0.6
Seawater G þ 20 mg/L iodate as I Urine (diluted by 10 times)a Urine (diluted by 10 times) þ20 mg/L iodate as Ia Saline primary wastewater influent Saline primary wastewater influent þ 20 mg/L iodate as I Saline primary wastewater effluent Saline primary wastewater effluent þ 20 mg/L iodate as I Saline secondary wastewater influent Saline secondary wastewater influent þ 20 mg/L iodate as I Saline secondary wastewater effluent Saline secondary wastewater effluent þ 20 mg/L iodate as I
25.3 (106%)
2.7
Seawater F þ 20 mg/L iodate as I
37.9 (90%)
1.7
a A 160 mL aliquot was used for each TOI measurement. b Data listed in all the brackets were the calculated iodate recoveries of the water samples. c Not detected.
The concentrations and distributions of the three iodine species in different water samples are shown in Fig. 2. Several important points extracted from the results are described as follows. The total iodine concentrations in tap waters A, B, C and D were 8.2, 12.5, 6.5 and 12.9 mg/L as I, respectively, which
Fig. 2 e Concentrations of iodine species in different water samples.
were comparable since their source waters were all river waters. The tap water samples contained all the three iodine species, and the predominant species was iodate. According to the chlorine residual titration, all the four tap water samples contained free chlorine residuals in the range of 0.3e0.5 mg/L as Cl2. Due to the fast oxidation of iodide to iodate by free chlorine, it is rational that iodate was the dominant iodine species in these tap water samples. The total iodine concentrations in the three seawater samples were 44.2, 39.5 and 51.2 mg/L as I, respectively, which were generally in agreement with previous reports that the total iodine concentration in seawater should be 45e90 mg/L as I (Whitehead, 1984; Schwehr and Santschi, 2003). The seawater samples contained all the three iodine species, i.e., iodide, iodate and organo-iodine. In seawaters E and F, the dominant iodine species was iodate, which agreed with some of previous studies, with the conclusion that the predominant iodine species in seawater was the thermodynamically stable iodate (Ito, 1997; Schwehr and Santschi, 2003). However, since seawaters E and F were both surface seawater samples, the concentration differences between iodide and iodate were not significant (Ito, 1997; Schwehr and Santschi, 2003). Interestingly, it was found that in seawater G, the dominant iodine species was iodide rather than iodate. It has been demonstrated that iodide could be produced by biologically mediated reduction of iodate and was favorable under reducing conditions (Rong et al., 2007). Therefore, it is possible that iodide becomes dominant in some seawater samples, especially in coast, estuary and bay waters (Rong et al., 2007); some
Please cite this article in press as: Gong, T., Zhang, X., Determination of iodide, iodate and organo-iodine in waters with a new total organic iodine measurement approach, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.08.039
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researchers also reported that iodide dominated in their seawater samples (Hou et al., 1999; Hansen et al., 2011). Since seawater G was a surface seawater sample taken from an area near the pier, it was probably affected by human activities, resulting in a high level of iodide. The total iodine concentration in the urine sample was determined to be 54.9 mg/L as I. The urine sample contained only iodide and organo-iodine, while the predominant species was iodide with a concentration of 39.7 mg/L as I. One study also reported similar results that iodide was the dominant iodine species in the urine samples while iodate was undetectable (Zhang et al., 2010b). It is reasonable that iodide was the main species in urine, owing to high levels of reducing substances in it. The iodine in urine finally ends up in domestic wastewater. The total iodine concentrations in the saline primary and secondary wastewater influents were 44.6 and 37.0 mg/L as I, respectively, which were quite similar. The relatively high total iodine levels in the saline wastewater influents were mainly ascribed to the use of seawater for toilet flushing, which introduced iodine from seawater to the wastewaters. Besides, some physiological fluids (e.g., milk, serum and urine), tap water, sewage from kitchen (containing table salts and some foodstuffs) and hospital wastewater (containing X-ray contrast media) might also contribute to the iodine in the wastewater influents (Hou et al., 2000; Verma et al., 1992; Blount et al., 2010; Pan and Zhang, 2013; Duirk et al., 2011). In both saline wastewater influents, iodide was the dominant species with concentrations even higher than some of the seawater samples, indicating that besides seawater, some other wastewater sources also contributed to the iodide in these wastewater influents. The total iodine concentrations in the saline primary and secondary wastewater effluents were 44.4 and 27.6 mg/L as I, respectively. Interestingly, the predominant iodine species in the saline primary wastewater effluent was still iodide while that of the saline secondary wastewater effluent became organoiodine, which was in accord with a previous study showing that the transformation of iodide to organo-iodine was enhanced by microorganisms (Ra¨dlinger and Heumann, 2000). The high iodide concentration in the saline primary wastewater effluent might cause the formation of more iodinated DBPs during wastewater disinfection. Moreover, it was found that iodine was partially removed during both primary and secondary treatment; the total iodine removal efficiencies of the saline primary and secondary wastewaters were calculated to be 0.45% and 25.4%, respectively, indicating that secondary treatment resulted in a much higher removal efficiency for iodine. It can be ascribed to two reasons: firstly, some microorganisms tend to accumulate iodide in their cells (Amachi et al., 2005b) and thus the iodine can be removed by accumulating in the activated sludge during secondary treatment; secondly, the transformation of iodide to organo-iodine is enhanced by microorganisms (Ra¨dlinger and Heumann, 2000) and therefore the mobile iodide can be transformed to organo-iodine during secondary treatment followed by adsorption on the sediments. The significant decrease of iodide and increase of organo-iodine in the saline secondary wastewater after treatment supported these two assumptions.
4.
Conclusions
A fast, sensitive and accurate method was developed for determining the three dominant iodine species in waters, i.e., iodide, iodate and organo-iodine. The new method involved derivatization of iodide and iodate to organic iodine and measurement of TOI. The developed method was applied to the determination of iodide, iodate and organo-iodine in a variety of water samples, and meaningful results with good recoveries and low RSDs were achieved. Interestingly, it was found that the iodide concentration in the saline primary wastewater effluent of Hong Kong was significantly higher than the saline secondary wastewater effluent, which might cause the formation of more iodinated DBPs in the saline primary wastewater effluent during wastewater disinfection. Furthermore, iodine was partially removed during both primary and secondary wastewater treatment while secondary treatment showed a significantly higher removal efficiency than primary treatment. This new method makes possible the accurate determination of relatively low concentrations of iodide, iodate, and organo-iodine in waters, and provides a sensitive tool to evaluate the mass balances of iodine and the transformation among the three different iodine species during disinfection as well as any other environmental processes.
Acknowledgment This work was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Projects No. 622412 and No. RPC11EG16).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.08.039.
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