chlorine process and formation of iodinated trihalomethanes during post-chlorination

Chemical Engineering Journal 283 (2016) 1090–1096

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Degradation of iohexol by UV/chlorine process and formation of iodinated trihalomethanes during post-chlorination Zhen Wang a, Yi-Li Lin b, Bin Xu a,⇑, Sheng-Ji Xia a, Tian-Yang Zhang a, Nai-Yun Gao a a State Key Laboratory of Pollution Control and Resource Reuse, Key Laboratory of Yangtze River Water Environment, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China b Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 824, Taiwan, ROC

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Iohexol can be effectively degraded

by UV/chlorine process.
Degradation mechanisms of iohexol

involve gradual deiodination and chlorine/hydroxyl addition.
Iohexol degradation was affected by chlorine and bromide concentration as well as pH.
I-THM formation from UV/chlorine treated iohexol favored circumneutral conditions.
UV/chlorine has an advantage over UV in controlling I-THM formation.

a r t i c l e

i n f o

Article history: Received 9 April 2015 Received in revised form 29 July 2015 Accepted 1 August 2015 Available online 22 August 2015 Keywords: UV/chlorine Advanced oxidation process Iodinated contrast media Iodinated disinfection byproduct Kinetics

a b s t r a c t Degradation kinetics of iohexol by UV/chlorine advanced oxidation process (AOP) and the formation of iodinated trihalomethanes (I-THMs) during post-chlorination were investigated in this study. Iohexol, a commonly detected iodinated contrast media in water, can be effectively removed during UV/chlorine process with pseudo-first-order reaction kinetics due to the combination of UV photolysis and oxidation of hydroxyl radicals. The second-order rate constant between iohexol and hydroxyl radicals was determined as 3.8  109 M1 s1 by competition kinetic experiment. Five intermediates were identified by ultra performance liquid chromatography–electrospray ionization-mass spectrometry analysis and degradation pathways of iohexol during UV/chlorine were proposed. Effects of chlorine dose, pH and bromide concentration on iohexol degradation and I-THM formation during post-chlorination were also studied. The results showed that iohexol degradation was accelerated with the increase of chlorine concentration as well as the decrease of pH and bromide concentration. On the other hand, I-THM formation from post-chlorination of UV/chlorine treated iohexol favored relatively low chlorine doses, high bromide concentrations at circumneutral conditions. Raw water experiments showed that I-THM formation after post-chlorination of UV/chlorine-treated iohexol was lower compared to that from UV irradiation, indicating that UV/chlorine is superior to UV alone in controlling I-THM formation. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Pharmaceutical substances are regarded as environmental microcontaminants due to their widespread usage and incomplete ⇑ Corresponding author. Tel.: +86 13918493316. E-mail address: [email protected] (B. Xu). http://dx.doi.org/10.1016/j.cej.2015.08.043 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

removal during wastewater treatment [1]. Among various classes of pharmaceuticals, iodinated X-ray contrast media (ICM) are triiodinated compounds that enhance the contrast between organs and the surrounding tissues and enable visualization of organ details which otherwise could not be investigated [2]. ICM are used in much higher doses than any other intravascular pharmaceuticals, and the worldwide consumption is approximately 3.5  106 kg

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per year [3,4]. Elevated concentrations of ICM are commonly detected in surface waters owing to their stability during conventional wastewater treatment processes [4–7]. In a previous occurrence study involving 23 cities in the United States and Canada, four ICM including iopamidol, iopromide, iohexol, and diatrizoate were found in source waters [5]. ICM in source waters may act as potential precursors of disinfection by-products (DBPs) to react with disinfectants and form highly toxic DBPs such as iodinated DBPs (I-DBPs) and higher molecular weight DBPs of unknown toxicity [8]. For example, Duirk et al. [5] reported the formation of iodinated trihalomethanes (I-THMs) and iodo-acids from ICM containing source water during chlor(am)ination. Wang et al. [9] found the formation of I-THMs from iopamidol favored algae organic precursors in their experiment and Ye et al. [10] presented the formation of I-THMs from five ICM with different characteristics. Moreover, the formation of high molecular weight DBPs from ICM chlorination was partly identified by Wendel et al. recently [11]. It was reported that both cytotoxicity and genotoxicity of chlorinated water were highly correlated with total organic iodine (TOI) while weakly correlated with total organic chlorine (TOCl) [12] because I-DBPs are generally more cytotoxic and/or genotoxic than their chlorinated and brominated analogs [13,14]. Meanwhile, the presence of bromide can lead to the shift of I-THMs towards brominated DBPs [10], which indicated the importance of bromide during the speciation of I-THMs. Joeng [8] investigated the toxicology of five commonly used ICM (iopamidol, diatrizoate, iopromide, iomeprol, and iohexol) during chlorination and found that chlorinated iohexol expressed the highest mammalian cell cytotoxicity and the second highest genotoxicity among the five chlorinated ICM. Moreover, iohexol was the most frequently detected ICM except iopamidol in surface water [5]. Many studies have focused on the degradation of ICM these years. For example, Pereira et al. [15,16] investigated the UV photolysis of pharmaceuticals and estimated the fundamental photolysis parameters. Advanced oxidation processes (AOPs) such as UV/ H2O2, UV/TiO2, O3/H2O2 and photochemical Fe (III)/oxalate also have been studied and proven to be efficient in removing ICM and degradation mechanisms were also studied [17–22]. UV/chlorine process has been considered as a novel AOP method due to the formation hydroxyl radicals (OH) when aqueous chlorine solutions are exposed to UV [23]. Studies have shown that UV/chlorine process was capable of oxidizing many contaminants including nitrobenzene, trichlorethylene, benzotriazole, diclofenac and ronidazole [24–27]. However, to the authors’ best knowledge, no previous work on the degradation of iohexol by UV/chlorine have been found. In addition, the potential risk of degradation products from UV/chlorine treated ICM to form I-DBPs is still in doubt. The objectives of this study were (1) to investigate the degradation kinetics and pathways of iohexol during UV/chlorine process, (2) to evaluate the effect of chlorine dose, solution pH and bromide concentration on the degradation of iohexol during UV/chlorine process and the formation of I-THMs after post-chlorination, and (3) to compare the I-THM formation from iohexol during postchlorination of direct UV and UV/chlorine processes.

2. Materials and methods 2.1. Chemicals and solutions All chemicals were at least of analytical grade except as noted. Iohexol (P99.0%), potassium iodide (KI, P99.0%), potassium bromide (KBr, P99.5%), potassium dihydrogen phosphate (KH2PO4, P99.0%), sodium hydroxide (NaOH, P98.0%), sodium hypochlorite (NaClO, available chlorine 4.00–4.99%), sodium sulfite (Na2SO3,

P98.0%) and iodoform (CHI3) standard were purchased from Sigma–Aldrich (USA). Other five I-THM standards, including chlorodiiodomethane (CHClI2, 90–95%), dichloroiodomethane (CHCl2I, P95%), bromochloroiodomethane (CHBrClI, P95%), dibromoiodomethane (CHBr2I, 90–95%) and bromodiiodomethane (CHBrI2, 90–95%), were obtained from CanSyn Chemical Corporation (Toronto, ON, Canada). Methyl tert-butyl ether (MtBE), methanol and acetonitrile were purchased from J.T. Baker (USA). Formic acid solution (49–51%) was purchased from Fluka (St. Louis, MO, USA). Sulfuric acid (H2SO4) and sodium hyposulfite (Na2S2O3) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and para-chlorobenzoic acid (pCBA) were obtained from Aladdin (Shanghai, China). Stock solutions for iohexol (1 mM) and pCBA (6.4 lM) were prepared with ultra-pure water produced using a Milli-Q water purification system (Millipore, USA). Raw water solutions of Yangtze River and Huangpu River were collected from Yangshupu drinking water treatment plant (YDWTP, Shanghai, China) and Minhang No. 2 drinking water treatment plant (MDWTP, Shanghai, China), respectively, and filtered through 0.45 lm acetate membrane filters (Millipore, USA) immediately. Raw water characteristics are shown in Table 1. All solutions were stored in the dark at 4 °C until usage.

2.2. UV irradiation system The UV irradiation system contained a stainless steel reactor (i. d. = 20 cm, height = 32 cm), a constant temperature water bath (DKB-1915, Jinghong, Shanghai, China) and a diaphragm pump (Model DP0700, Puricom, Taiwan). The temperature of the UV system was controlled at 25 °C by circulating the water in the bath. A 400 mL quartz tube (i.d. = 4.5 cm) and four low pressure (LP) Hg UV lamps (TUV 11 W T5 4P-SE, Philips, Netherlands) were fixed in the center of the UV reactor. The UV intensity was changed with different numbers of UV lamps turned on. The value has been determined in our previous studies using atrazine actinometer (1.53, 3.02, 4.45 and 5.96 mW cm2) [27,28] and monitored using a ultraviolet radiometer (UV-C luxometer, Photoelectric Instrument Factory of Beijing Normal University, Beijing, China).

2.3. Experimental procedures Iohexol is non-volatile [16] and stable without UV irradiation in our preliminary tests (data not shown). Irradiation experiments were performed in the quartz tube containing 200 mL iohexol solution with the initial concentration of 10 lM in all cases. To eliminate the interference of different buffers, solution pH was buffered at pH 5–9 using 10 mM phosphate solution [29] and adjusted using either 0.1/1 M NaOH or H2SO4. Stock chlorine was spiked into solutions to the desired concentration right before UV irradiation. When the sample was irradiated to a certain period, 1 mL solution was rapidly transferred into an ultra-performance liquid chromatography (UPLC) vial and quenched residual chlorine with excess Na2S2O3. Then, the sample was analyzed using UPLC and UPLC – electrospray ionization-mass spectrometry (ESI-MS) immediately.

Table 1 Characteristics of raw waters.

a

Source

DOC (mg C/L)

UV254 (cm1)

SUVAa (L/mg m)

Br (lg/L)

MDWTP YDWTP

4.26 2.00

0.107 0.046

2.51 2.30

92.96 0.157

SUVA: the UV254 absorbance divided by the DOC concentration.

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It was reported the predominant photooxidant produced in the UV/chlorine process were OH radicals at pH > 1 [23]. To determine the reactivity of iohexol with hydroxyl radicals, competition kinetic experiments were carried out by dosing the OH reactive compound, pCBA, which was used as a OH probe because it is known to react fast with OH with negligible rate of direct photolysis [30]. Therefore, the degradation of pCBA in competition kinetic experiments only involves on the reaction of pCBA and OH [31]. For the I-THM formation experiments of post-chlorination, samples dosed with 10 lM iohexol and desired chlorine concentration were firstly exposed to UV at a fluence rate of 3.02 mW cm2 for 10 min, at which time almost all iohexol was degraded according to our preliminary experimental results (data not shown). After UV irradiation was terminated, the solution was transferred and dosed with additional chlorine to meet the concentration of 100 lM for 7 d incubation due to the photolysis of free chlorine during UV/chlorine process [27]. Solution pH was also adjusted to 7 in the pH variable experiment to assure same postchlorination conditions. The incubation experiments were conducted in duplicate under headspace-free conditions in 40-mL amber glass screw-cap vials with PTFE-lined septa at room temperature (25 ± 1 °C).

the modified method were described in our previous study [9]. The quantification limits for I-THMs varied from 0.1 to 1.0 lg/L. 3. Results and discussion 3.1. Degradation kinetics of iohexol during UV/chlorine process Fig. 1 shows the degradation rates of iohexol during chlorination, UV irradiation and UV/chlorine processes. Iohexol is vulnerable during both UV and UV/chlorine processes, while the degradation by chlorine oxidation was negligible with only 0.2% iohexol removal in 5 min, which is in accordance with the ICM chlorination results reported by Wendel et al. [11]. The good linearity of fitted lines in Fig. 1 indicates that both UV and UV/chlorine degradation follow the pseudo first-order kinetics (R2 > 99.0%), and the first-order rate constants were calculated as 0.0095 and 0.0126 s1, respectively. UV/chlorine process is more effective in iohexol degradation due to the formation of the highly reactive OH radicals at pH > 1 [24,27]. Because the rate of iohexol photolysis and iohexol degradation by OH oxidation also follow first-order kinetics (assuming steady-state OH concentration), and iohexol is non-volatile [16], the total rate of iohexol loss can be described in Eqs. (1) and (2) [15,25,34]

2.4. Analytical methods

In

ð1Þ

kOH ¼ kOHIOX  ½OHss

ð2Þ

where kT is the total first-order rate constant of iohexol degradation during UV/chlorine process, kUV is the first-order photolysis rate constant (s1) of iohexol, kOH is the first-order rate constant (s1) of iohexol degradation by OH oxidation, kOH-IOX is the secondorder rate constant (M1 s1) between iohexol and OH, t is the exposure time (s), and [OH] SS is the steady-state concentration of  OH (M). The second-order rate constant for the reaction between iohexol and OH radicals was determined by applying a competing kinetic method using pCBA based on previous work [35]. The second-order rate constant was calculated by Eqs. (2)–(4)

kpCBA ¼ kOHpCBA  ½OHss kOHIOX ¼ kOHpCBA 

ð3Þ

kOH kT  kUV ¼ kOHpCBA  kpCBA kpCBA

ð4Þ

0 -0.5

t

0

In ([iohexol] / [iohexol] )

Chlorine concentrations were analyzed with the N,N-diethyl-pphenylenediamine (DPD) colorimetric standard method [32]. A Shimadzu TOC-VCSH analyzer (Shimadzu, Japan) was used to measure the dissolved organic carbon (DOC) concentration of raw waters and UV irradiated iohexol solution. As a water quality parameter which correlates with aromatic compounds, UV254 was measured using a spectrophotometer at the absorbance wavelength of 254 nm (SQ-4802 UNICO, Shanghai). Bromide was analyzed using an ion chromatography (Dionex ICS-2000, USA) equipped with a conductivity detector, a Dionex AS11-HC analytical column (250 mm  4.0 mm i.d.) and a Dionex AG11-HC guard column (50 mm  4.0 mm i.d.). Solution pH was monitored using a pH meter (FE20-FiveEasy, Mettler Toledo, Switzerland), which was calibrated regularly with standard buffer solutions (Mettler Toledo, USA). Iohexol, pCBA and iodide released from photolysis of iohexol were analyzed using a UPLC (Waters, USA) equipped with a XTerraR MS C18 column (4.6  250 mm i.d., 5 mm film thickness, Waters, USA) and a UV detector at the wavelength 245 nm, 235 nm and 226 nm, respectively. The mobile phase for iohexol analysis was consisted of 5%/95% (v/v) acetonitrile and Milli-Q water at a flow rate of 0.80 mL/min. As for pCBA analysis, the mobile phase was changed to 35%/65% (v/v) acetonitrile and Milli-Q water at a flow rate of 0.50 mL/min. The injection volume was 10 lL. The detection limits of iohexol and pCBA were 10 and 5 lg/L, respectively. Details of iodide analysis using UPLC were described elsewhere [28]. Degradation intermediates of iohexol during UV/chlorine process were identified using a UPLC–ESI-MS system consisted of an Accela UHPLC system, a TSQ Quantum mass spectrometer (ESI source, Thermo Scientific Inc., USA) and a XTerraR MS C18 column (250 mm  2.1 mm, i.d., 5 mm film thickness, Waters, USA). The mobile phase was consisted of 20%/80% (v/v) acetonitrile and 0.1% formic acid solution at a flow-rate of 0.30 mL/min. MS chromatograms were obtained both in total ion current (TIC) mode for mass spectra acquisition and the selected ion monitoring (SIM) mode. I-THMs were extracted using MtBE and analyzed with a gas chromatograph (GC-2010, Shimadzu, Japan) equipped with an electron capture detector and a HP-5 capillary column (30 m  0.25 mm i.d., 0.25 lm film thickness, J&W, USA) according to the method modified from USEPA method 551.1 [33]. Details of

½IOXt ¼ kT t ¼ ðkUV þ kOH Þt ½IOX0

Chlorination UV irradiation UV/chlorine

-1 -1.5 -2 -2.5 -1

2

k = 0.0095 s , R =0.991 UV k = 0.0126 s-1, R2=0.995 T

-3 -3.5 -4

0

50

Fig. 1. Degradation of iohexol irradiation, and UV/chlorine sity = 3.02 mW cm2, HOCl tion = 10 lM, 10 mM phosphate

100

150 Time (s)

200

250

300

as a function of time during chlorination, UV processes, respectively at pH 7. UV intenconcentration = 100 lM, iohexol concentrabuffer, temperature = 25 °C.

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where kpCBA is the first-order rate constant of pCBA in the competition system and kOH-pCBA is the second-order rate constant of pCBA calculated as 4.5  109 M1 s1 by Zona et al. [36]. The calculated value of kOH-IOX is 3.8  109 M1 s1 in this study (based on Eq. (4) and data shown in Fig. S1), which was similar to the values obtained by Jeong et al. ((3.20 ± 0.13)  109 M1 s1) [21] and Pereira et al. (3.81  109 M1 s1) [16]. Nevertheless, the generation of OH during UV/chlorine did not appreciably enhance the degradation of iohexol (Fig. 1). Limited benefits of photo-Fenton reaction were also observed for iohexol degradation during UV/H2O2 [16]. This phenomenon can be explained by the efficient direct photolysis of iohexol (about 95% degradation in 5 min) and the effects of light screening by chlorine/H2O2 [37]. During the degradation of iohexol by UV/chlorine process, five intermediates were identified using UPLC–ESI-MS with both SIM and TIC modes, and the chromatograms as well as fragment information were provided in Fig. S2 and Table S1, respectively, of Supplementary material. Moreover, the destruction pathways of iohexol during UV/chlorine processes were proposed (Fig. S3, SI). According to the information in Table S1 and Fig. S3, the degradation of iohexol involves hydrogen atom abstraction reactions from side chains, gradual deiodination, hydroxyl addition, chlorine substitution, and a side chain may also be substituted by hydroxyl. 3.2. Effect of chlorine dose on iohexol degradation during UV/chlorine and I-THM formation after post-chlorination Considering the formation of hydroxyl radicals is originated from chlorine photolysis, effect of chlorine dose in UV/chlorine process was examined and the results were shown in Fig. 2 and Table 2. The data in Fig. 2 show good linearity and the slopes of the fitted lines are the kT of iohexol during UV/chlorine and summarized in Table 2. As shown in Fig. 2 and Table 2, higher chlorine concentration leads to faster degradation of iohexol. As chlorine dose reached 500 lM, iohexol was completely degraded in 4 min. Because chlorine can hardly oxidize iohexol (Fig. 1), the acceleration of iohexol decomposition should be owing to the increase of hydroxyl radicals, which expressed as [OH]SS in Eq. (2). In a previous study, the release of iodide was detected during UV irradiation of iopamidol, another kind of ICM [28], which also happened during UV irradiation of iohexol in our study with the data shown in Fig. S4, SI. Moreover, the release of iodide increased with the decrease of iohexol during UV irradiation. The released

Table 2 Pseudo-first-order rate constant of iohexol during UV/chlorine process at different experimental conditions. Chlorine dose (lM)

Bromide (lM)

pH

kT (s1)

0 50 100 200 500 100 100 100 100 100 100 100 100 100

0 0 0 0 0 0 0 0 0 10 25 50 100 200

7 7 7 7 7 5 6 8 9 7 7 7 7 7

0.0095 0.0108 0.0126 0.0152 0.0178 0.0174 0.0156 0.0109 0.0104 0.0123 0.0115 0.0094 0.0091 0.0084

[Iohexol]0 = 10 lM, UV fluence rate = 3.02 mW cm2, temperature = 25 ± 1 °C.

iodide can be oxidized to hypoiodous acid (HOI) by chlorine, and then the reactive iodine species would react with organic precursors provided by iohexol to form I-THMs [38]. For example, iohexol degradation reactions such as –OH substitution of side chain during UV/chlorine can provide organic carbon source for I-DBPs formation. Fig. 3 illustrated the effect of chlorine doses on the formation of I-THMs after post-chlorination of UV/chlorine-treated iohexol. Interestingly, although iohexol degraded faster with increasing chlorine concentration, the formation of all I-THM species decreased. When chlorine concentration increased from 12.5 to 200 lM, the concentration of CHCl2I, CHClI2 and CHI3 decreased from 67.5, 18.4 and 18.0 lg/L to 10.5, 5.7 and 3.6 lg/L, respectively. It was reported that the half-life of HOI is 8 min at chlorine concentration of 2 mg/L as Cl2 (28 lM) at pH 9 [38]. Iodide released from iohexol can easily be oxidized to HOI and be further oxidized to iodate in the presence of chlorine, which is a desired sink of iodide because iodate is stable and cannot react with NOM to form I-DBPs [38]. In this study, UV light and iodide would compete to react with chlorine, and excess doses of chlorine may oxidize HOI to iodate during the UV/chlorine AOP, which could be responsible for the decrease of total I-THMs yield after post-chlorination.

70 [HOCl]=0 µM, R 2=0.991

5

CHCl 2I

60

[HOCl]=50 µM, R 2=0.999

CHClI2

2

[HOCl]=100 µM, R =0.951

CHI3

50 I-THMs (µg/L)

[HOCl]=200 µM, R 2=0.997

t

In ( [iohexol] / [iohexol] )

4

2 [HOCl]=500 µM, R =0.987

0

3

2

40 30 20 10

1

0 0

0

50

100

150 Time (s)

200

250

300

Fig. 2. Pseudo-first-order kinetics plot of iohexol degradation with different chlorine doses during UV/chlorine process at pH 7. UV intensity = 3.02 mW cm2, iohexol concentration = 10 lM, 10 mM phosphate buffer, temperature = 25 °C.

12.5

100 25 50 Chlorine concentration (µM)

200

Fig. 3. Effect of chlorine doses on the formation of I-THMs after post-chlorination of UV/chlorine-treated iohexol at pH 7. UV intensity = 3.02 mW cm2, iohexol concentration = 10 lM, 10 mM phosphate buffer, temperature = 25 °C, incubation time = 7 d. Error bars represent one standard deviation of duplicate measurements.

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3.3. Effect of pH on iohexol degradation during UV/chlorine and I-THM formation after post-chlorination

3.4. Effect of bromide concentration on iohexol degradation during UV/ chlorine and I-THM formation after post-chlorination The effect of bromide concentration on the degradation of iohexol during UV/chlorine process was also studied, and the kT

pH 5 pH 7 pH 9

0.9

0

0.8 TOC / TOC

0.7

t

Degradation of iohexol during UV/chlorine was carried out at pH 5–9 (data not shown), and the first-order rate constant was calculated and summarized in Table 2. The reaction rate has a negative correlation with the increase of pH, which can be attributed to the change of kOH because we found that kUV is independent of solution pH during UV irradiation (Fig. S5, SI). Many studies have proven the ability of free chlorine species (OCl and HOCl) to produce OH is not the same [23,31,39]. Furthermore, Nowell and Hoigne reported that OCl yield 0.1 OH while HOCl yield 0.9 from photolysis [23]. Therefore, the degradation rates of iohexol at low pHs are relatively high because HOCl is the dominant species at acidic conditions (HOCl pKa = 7.5), leading to the increase of hydroxyl radicals. Fig. 4 shows I-THMs formation at pH 5–9 after postchlorination of UV/chlorine treated iohexol. The I-THM formation exhibited an increasing followed by a decreasing phase as pH increased from 5 to 9. I-THM formation was favored under circumneutral condition, reaching 54.4 lg/L in total at pH 7, while the concentration is 30.1 lg/L at pH 5 and 39.5 lg/L at pH 9. As shown in Fig. 5, the mineralization of iohexol as a function of pH was quantified by measuring the decrease of TOC concentration to explain the I-THM formation trend. The mineralization of iohexol was enhanced at low pH and 60% TOC was oxidized to CO2 after 10 min at pH 5. Therefore, from pH 5–7, I-THMs increased because the increasing organic carbon provided more and more organic precursors to form I-THMs. However, AOP can not only mineralize iohexol but also oxidize high molecular weight substances to low molecular weight ones, which can react with HOI easily [21,40]. At pH 7, 8, and 9, the TOC was relatively high but the proportion of low molecular weight organics declined with increasing pH due to the reduced production of hydroxyl radicals, resulting in the decrease of I-THM formation.

1

0.6 0.5 0.4

0

2

4

6 Time (min)

8

10

Fig. 5. Mineralization of iohexol as a function of time and pH (5, 7 and 9) during UV/chlorine process. UV intensity = 3.02 mW cm2, iohexol concentration = 10 lM, 10 mM phosphate buffer, HOCl concentration = 100 lM, temperature = 25 °C. Error bars represent one standard deviation of duplicate measurements.

was calculated and summarized in Table 2. As can be seen from Table 2, the first-order rate constant decreased from 0.0126 to 0.0084 s1 as bromide concentration increased from 0 to 200 lM. In the presence of chlorine and hydroxyl radicals, bromide can be easily oxidized to HOBr and further oxidized to bromate [41,42]. Hence, iohexol and bromide would compete for oxidizing agents (hydroxyl radical and chlorine), leading to the decrease of kOH as more OH radical was utilized by bromide with elevating bromide concentration. Fig. 6 illustrated the differences in the formation and speciation of I-THMs when samples were spiked with varying concentrations of bromide before UV/chlorine process. Generally, increasing initial bromide levels could enhance the formation of total I-THMs at low bromide concentrations (<50 lM), and total I-THMs concentration was relatively stable when bromide dose was high (>50 lM). Adding 50 lM bromide resulted in 118.5 lg/L I-THMs forming, which was the highest in the testing range. Furthermore, I-THM species with more iodine atoms (CHClI2, CHBrI2 and iodoform) increased gradually with increasing bromide concentration, while CHCl2I

35 70

CHCl 2I CHClI2

50

20 15 10

CHClBrI CHBr I

1.9

CHClI

1.8

2

I-THMs (µg/L)

I-THMs (µg/L)

25

2

CHBrI 2 40

1.7

CHI3 1.6

30 1.5 20

5 0

60

CHI3

2 ISF

1.4

10

5

6

7 pH

8

9

Fig. 4. Effect of solution pH on the formation of I-THMs after post-chlorination of UV/chlorine-treated iohexol at pH 7. UV intensity = 3.02 mW cm2, iohexol concentration = 10 lM, 10 mM phosphate buffer, HOCl concentration = 100 lM, temperature = 25 °C, incubation time = 7 d. Error bars represent one standard deviation of duplicate measurements.

ISF

30

CHCl 2I

0

1.3 10

25 50 100 Bromide concentration (µM)

200

Fig. 6. Effect of bromide concentration on the formation of I-THMs after postchlorination of UV/chlorine treated iohexol at pH 7. UV intensity = 3.02 mW cm2, iohexol concentration = 10 lM, 10 mM phosphate buffer, HOCl concentration = 100 lM, temperature = 25 °C, incubation time = 7 d. Error bars represent one standard deviation of duplicate measurements.

Z. Wang et al. / Chemical Engineering Journal 283 (2016) 1090–1096

80

YDWTP raw water

and iohexol can be completely removed in 10 min. On the contrary, iodide and reactive iodine species remained in the UV/chlorinetreated solution would less than that in UV irradiated solution, due to the OH radical oxidation of iodide. It is worthy that natural organic matter (NOM) enhanced I-THM formation significantly to 151.5 and 247.6 lg/L in YDWTP and MDWTP, respectively (Fig. 7), while only 54.4 lg/L I-THMs formed in the absence of NOM during post-chlorination of UV/chlorinetreated iohexol (Fig. 4). One explanation could be the presence of organic precursors which is essential for I-THM formation [5]. The other explanation would be the partition of bromide to form brominated I-THMs including CHBr2I, CHClBrI and CHBrI2, which were not detected in the synthesized water without bromide (Figs. 3 and 4) and were accounted for 61.4% and 55.6% of the total I-THMs in YDWTP and MDWTP, respectively (Fig. 7).

UV UV/chlorine

60

I-THMs (µg/L)

40 20

100 80 60

MDWTP raw water

40 20 CHCl I 2

CHClBrI CHBr I CHClI CHBrI 2 2 2

CHI

3

Fig. 7. Comparison of I-THM formation after post-chlorination of UV and UV/ chlorine treated iohexol containing raw waters from YDWTP and MDWTP, respectively, at pH 7. UV intensity = 3.02 mW cm2, iohexol concentration = 10 lM, 10 mM phosphate buffer, HOCl concentration = 100 lM, temperature = 25 °C, incubation time = 7 d. Error bars represent one standard deviation of duplicate measurements.

kept decreasing, suggesting that individual halogen atoms showed distinct incorporation trends during the post-chlorination. To get a deeper insight into the I-THM speciation, iodine substitution factor (ISF) was adopted for the analysis of the data in Fig. 6, which is defined as the ratio of the molar concentration of iodine incorporated into a given class of DBP to the total molar concentration of halogen atoms in that class [43]. In this study, ISF was presented in Eq. (5)

P3 ISF ¼

1095

n¼1 n  CHCl3mn Brm In

I  THM6 CHCl2 I þ CHBr2 I þ CHClBrI þ 2  ðCHClI2 þ CHBrI2 Þ þ 3  CHI3 ¼ CHCl2 I þ CHBr2 I þ CHClBrI þ CHClI2 þ CHBrI2 þ CHI3 ð5Þ

As shown in Fig. 6, ISF constantly increased with elevating bromide dose, which indicated that iodine atom dominates the substitution reaction to form I-THMs. Iodoform became the dominant species instead of CHCl2I when the bromide level lifted from 10 lM to 200 lM. As discussed, both the consumption of OH radicals and chlorine by bromide would increase with bromide spiked to the samples. Thus, iodate formation from iodide and HOI in AOP may fall due to the lack of oxidizing agents (OH radicals and chlorine) caused by competition. In this way, more reactive iodine was utilized during the post-chlorination, contributing to the rise of ISF. 3.5. Comparison of UV and UV/chlorine process on the formation of ITHMs after post-chlorination Two raw waters spiked 10 lM iohexol were studied for the formation of I-THMs after post-chlorination of direct UV and UV/chlorine treated iohexol. As shown in Fig. 7, it is obvious that all concentrations of the detected I-THM species were lower during post-chlorination of UV/chlorine than of UV/irradiation alone, which suggests that UV/chlorine has an advantage over UV irradiation in reducing I-THM formation. The total concentrations of ITHMs after post-chlorination of UV treated iohexol were 235.5 and 326.0 lg/L for YDWTP and MDWTP, respectively, which decreased to 151.5 and 247.6 lg/L after post-chlorination of UV/ chlorine. Although UV/chlorine process is more effective on iohexol degradation, the iodide released from the two techniques would not show big differences because UV exposure is effective enough

4. Conclusions (1) Iohexol could be degraded effectively by both UV exposure and UV/chlorine AOP while the chlorine oxidation is negligible. UV/chlorine AOP is more effective in destroying iohexol due to the formation of hydroxyl radicals. The second-order rate constant between OH radicals and iohexol was calculated as 3.8  109 M1 s1. (2) Higher chlorine concentration could degrade iohexol more efficiently while I-THM formation after post-chlorination of UV/chlorine treated iohexol decreased with the increase of chlorine dose. (3) Degradation of iohexol during UV/chlorine process is pH dependent and the first-order rate constant increased at low pHs. However, I-THM formation exhibited an increasing followed by a decreasing phase as pH increased from 5 to 9. (4) The increase of bromide concentration slowed down the destruction of iohexol during UV/chlorine but enhanced ITHM formation after post-chlorination. (5) Raw water experiments revealed that I-THM formation from UV/chlorine-treated iohexol was lower than that from UV irradiation, indicating that UV/chlorine AOP has an advantage over UV in controlling I-THM formation.

Acknowledgments This study was supported in part by the Natural Science Foundation of China (Nos. 51278352 and 51478323), the Fundamental Research Funds for the Central Universities in China, the National Major Science and Technology Project of China (Nos. 2012ZX07404004 and 2012ZX07408001) and the Ministry of Science and Technology in Taiwan (MOST-103-2221-E-327-003). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2015.08.043. References [1] T.A. Ternes, A. Joss, H. Siegrist, Peer reviewed: scrutinizing pharmaceuticals and personal care products in wastewater treatment, Environ. Sci. Technol. 38 (2004) 392A–399A. [2] A. Putschew, S. Wischnack, M. Jekel, Occurrence of triiodinated X-ray contrast agents in the aquatic environment, Sci. Total Environ. 255 (2000) 129–134. [3] C. Christiansen, X-ray contrast media—an overview, Toxicology 209 (2005) 185–187. [4] S. Pérez, D. Barceló, Fate and occurrence of X-ray contrast media in the environment, Anal. Bioanal. Chem. 387 (2007) 1235–1246. [5] S.E. Duirk, C. Lindell, C.C. Cornelison, J. Kormos, T.A. Ternes, M. Attene-Ramos, J. Osiol, E.D. Wagner, M.J. Plewa, S.D. Richardson, Formation of toxic iodinated

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