Chemical Engineering Journal 323 (2017) 312–319

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Chemical Engineering Journal

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Kinetics and modeling of iodoform degradation during UV/chlorineadvanced oxidation process

http://dx.doi.org/10.1016/j.cej.2017.04.0611385-8947/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: tjwenwu@tongji.edu.cn (B. Xu).

An-Qi Wang a, Yi-Li Lin b, Bin Xu a,⇑, Chen-Yan Hu c, Sheng-Ji Xia a, Tian-Yang Zhang a, Wen-Hai Chu a,Nai-Yun Gao a

a State Key Laboratory of Pollution Control and Resource Reuse, Institute of Disinfection By-product Control in Water Treatment, College of Environmental Science andEngineering, Tongji University, Shanghai 200092, PR ChinabDepartment of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Kaohsiung 824, Taiwan, ROCcCollege of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China

h i g h l i g h t s

� UV/chlorination can significantlyaccelerate the degradation of CHI3.

� The rate constant of �OH and CHI3 wasdetermined as 7.7 � 109 M�1 s�1.

� Solution pH has remarkable influenceon the degradation rate of CHI3during UV/chlorination.

� Bicarbonate and NOM can reduce thedegradation rate of CHI3 during UV/chlorination.

� IO3� contributed 13.7% of the total

iodine species after CHI3 degradationduring UV/chlorination.

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

a r t i c l e i n f o

Article history:Received 2 January 2017Received in revised form 16 March 2017Accepted 10 April 2017Available online 13 April 2017

Keywords:Iodinated trihalomethanes (I-THMs)IodoformDegradationUV/chlorineHydroxyl radicalsKinetics

a b s t r a c t

Iodoform (CHI3) is an emerging disinfection by-product (DBP) that may be formed during pre-oxidationor disinfection processes in drinking water treatment. Degradation kinetics, modeling and mechanism ofCHI3 by combined UV/chlorine advanced oxidation processes (AOPs) were studied in this manuscript.CHI3 was effectively removed by UV/chlorine process with the reactions followed pseudo-first orderkinetics. The contributions of direct UV photolysis as well as indirect photolysis (hydroxyl radicals(�OH) to CHI3 degradation during UV/chlorination under different experimental factors were investigatedand determined as 20.3% and 79.7% at pH 5 to 97.1% and 2.9% at pH 9, respectively. Chlorine dosage andCHI3 concentration had slight effects on the contributions of different degradation pathways. NOM andbicarbonate have negative effects on CHI3 degradation. The degradation model of CHI3 during UV/chlo-rine processes was established, and the satisfactory match of the model calculation results and the exper-imental data were found. The reaction rate constant between CHI3 and UV light as well as CHI3 and OHwere determined as 3.43 � 10�3 s�1 and 7.7 � 109 M�1 s�1, respectively. On the basis of the iodine species(such as IO3

�, HOI, I2 and I3�) mass balance analysis, the degradation pathways of CHI3 were proposed and

IO3� contributed 13.7% of the total liberated iodine species during UV/chlorination. These results demon-

strated that UV/chlorination process is a promising AOP technology for the treatment of water containingCHI3.

� 2017 Elsevier B.V. All rights reserved.

A.-Q. Wang et al. / Chemical Engineering Journal 323 (2017) 312–319 313

1. Introduction

Iodoform (CHI3), one of the unregulated iodinated tri-halomethanes (I-THMs), has been identified as an emerging disin-fection by-product (DBP) formed during drinking water treatmentprocesses through the reactions between natural organic matter(NOM) and reactive iodine species, such as HOI [1–3]. Hypoiodousacid (HOI) can be quickly formed during the oxidation of naturaloccurring iodide (I�) in the presence of potassium permanganate,chlorine, or chloramine, which are dosed in pre-oxidation or disin-fection processes in drinking water treatment plants (DWTPs) [3].A certain concentration of CHI3 has been detected in several watersamples from both DWTP and sewage treatment plants in theworld with the levels reported at sub-lg L�1 [2,4–6]. The concen-tration of I-THMs was the highest in drinking water treated withchloramines [2]. For example, one previous study reported that achange from chlorination to chloramination resulted in the forma-tion of 5 lg L�1 CHI3 (I� concentration of 50 lg L�1) [7]. Althoughunregulated I-THMs levels (mean level of mid-lg L�1 and maxi-mum level of 15 lg L�1 in the referred study) [8] observed in drink-ing water are relatively lower than those of the regulated THMs(mean level of 38 lg L�1 and maximum level of 80 lg L�1 in thereferred study) [9], it is generally admitted that I-THMs are morecytotoxic and genotoxic than its regulated chlorinated and bromi-nated analogues [10,11]. For instance, the cytotoxicity of CHI3 isdemonstrated to be 146 times and 60 times higher than that ofCHCl3 and CHBr3, respectively [10]. Moreover, researchers havefound that CHI3 could be responsible for the bad taste and odorissues in drinking water [7] because the odor threshold concentra-tion of CHI3 lies only between 0.03 and 1 lg L�1 [7,12], which issignificantly lower than that of CHCl3 and CHBr3 (300 and 100 lgL�1, respectively) [6]. Therefore, the formation [2,3,13–15] andhealth effects [10] of CHI3 have raised significant concernsrecently.

However, up to now, only a few studies investigated the degra-dation of CHI3 in drinking water and wastewater treatment pro-cesses [16,17]. Previous studies have identified that CHI3 couldnot be effectively removed by conventional treatment processessuch as coagulation, sedimentation, filtration, and ozonation [18].However, ultraviolet light (UV) and UV-related advanced oxidationprocesses (AOPs) have been identified as effective technology toremove I-THMs in water [17,19]. Xiao et al. [19,20] have investi-gated direct photolysis of I-THMs and reported that I-THMs wentthrough a considerable degradation through photo-cleavage ofthe carbon-halogen bonds, because the molar photon energy ofUV254 (472 kJ mol�1) is significantly higher than that of C-I bond(209 kJ mol�1). UV/H2O2 system has also been currently identifiedas an effective treatment of CHI3 in model natural and surfacewaters, which relies on synergistic effect of

�OH-assisted indirect

photolysis and direct photolysis [5,17]. However, the degradationmechanisms and kinetics of CHI3 during UV/chlorination havenot been fully explored compared to the well-established UV/H2O2 process.

UV/chlorination process is efficient in absorbing UV along withproducing

�OH under photolysis, making it an alternative AOP to

UV/H2O2 process [21–23], which is a widely used AOP techniquein water treatment process. Hypochlorous acid (HOCl) andhypochlorite ion (OCl�) in UV/chlorine system are photolyzed togenerate radicals, including

�OH, �Cl, Cl2� and O

��, while H2O2 is con-sidered to generate radicals for photolysis in UV/H2O2 system,including

�OH and HO2

�. The quantum yield of HOCl, OCl� and

H2O2 are 1.0 ± 0.1 [24], 0.9 ± 0.1 [24] and 1.11 ± 0.7 mol Ein-stein�1 [25], respectively, which are similar at 254 nm with alow-pressure (LP) mercury UV lamp as monochromatic lightsource. However, the availability of free chorine in UV absorbance

is about 3.3 times stronger than H2O2, considering the molarabsorption coefficients are 59 [26], 66 [26] and 19 M�1 cm�1 [27]for HOCl, OCl� and H2O2, respectively. In this regard, many previ-ous studies have shown that UV/chlorine AOP is more efficientthan UV/H2O2 AOP in the degradation of some micropollutantssuch as caffeine [28], ibuprofen [29], trichloroethylene [30], andiopamidole [21]. Furthermore, because the pre-chlorinated waterhas a certain concentration of residual chlorine throughout thewater treatment process, the traditional water treatment processcan be upgraded to UV/chlorine system by adding a compact UVsystem after the filtration process. Recently, a growing number ofresearch groups have studied the kinetic models for UV/chlorina-tion [21,30–32], but the mechanisms still need to be furtherdefined. Therefore, the objectives of this study were (1) to establisha kinetic model to describe the reaction mechanisms of CHI3 duringUV/chlorination with a LP Hg lamp, (2) to understand the effect ofseveral operational factors including pH, chlorine dose, CHI3 con-centration, as well as radical scavengers such as fulvic acid (FA),HCO3

- and Cl- on the UV/chlorine degradation kinetics, and (3) toevaluate the contributions of direct UV irradiation and indirecthydroxyl radical oxidation on CHI3 degradation during UV/chlorineprocess.

2. Materials and methods

2.1. Chemicals

All chemicals, except specially stated, were at least of analyticallevel. CHI3 (99%), 1,2-dibromopropane (97%), methanol (HPLCgrade), nitrobenzene (NB), phenol, 2-iodophenol, 4-iodophenol,NaOCl solution (4.00–4.99% available chlorine), NaHCO3 (�99%),NaOH (�98%), and KH2PO4 (�99.0%) were purchased fromSigma-Aldrich (USA). Extraction solvent methyl tert-butyl ether(MtBE) was obtained from J.T. Baker (USA). Na2S2O3, NaCl andH2SO4 were purchased from Sinopharm Chemical Reagent Co.,Ltd. (Shanghai, China). All solutions and samples were preparedusing ultra-pure water produced using a Milli-Q water purificationsystem (Millipore, USA). The NOM stock solution was prepared bydissolving an aliquot of Suwannee River Fulvic acid (FA) Standard Ⅱsolids (Cat. No. 2S101F, International Humic Substance Society)into Milli-Q water and filterting through 0.45 lm cellulose acetatemembranes (Anpel Co. Ltd, Shanghai, China). Then the stock solu-tion was stored at 4 �C in the dark prior to use.

2.2. UV system

UV irradiation experiments were conducted in a UV irradiationdevice, which was equipped with a 11W LP Hg UV lamp (254 nm,4P-SE, Philips). The setup of the device is shown in Fig. S1 in Sup-porting Information (SI). The UV fluence rate value (I0) and theeffective path length (b) were determined as 3.25 � 10�7 Einsteins�1 L�1 and 8.2 cm respectively, using atrazine and hydrogen per-oxide as actinometers [27,33]. Detailed information was providedin Text S1 and Fig. S2 in SI.

2.3. Experimental procedures

To investigate the influence of chlorine concentration, solutionpH and radical scavengers on CHI3 degradation during UV/chlori-nation, a 600 mL testing solution was prepared containing 100 lgL�1 CHI3, 0–50 mg L�1 free chlorine, 0–5 mM chloride, 0–10 mgL�1 FA or 0–4 mM bicarbonate, buffered at pH 5–9 with 10 mMphosphate buffer, and then equally transferred to 12 array quartztubes fixed in a circle around the UV lamp (Fig. S1). Samples fromeach tube were collected in sequence at different irradiation time,

Table 1Principle reactions during UV/chlorination.

No. Reaction Rate constant Ref.

1 HOCl + hv? �OH + Cl� /HOCl = 1.0 mol Einstein�1 [26]2 OCl� + hv? O�� + Cl� /OCl

� = 1.0 mol Einstein�1 [26]3 CHI3 + hv? products /CHI3 = 0.51 mol Einstein�1 This study4 O�� + H2O? �OH + OH� k1 = 1.8 � 106 M�1 s�1 [38]5 HOCl, OCl� + H+ pKa1 = 7.5 [26]6 �OH, O�� + H+ pKa2 = 11.9 [38]7 �OH + HOCl? ClO� + H2O k2 = 2.0 � 109 M�1 s�1 [44]8 �OH + OCl� ? ClO� + OH� k3 = 8.8 � 109 M�1 s�1 [43]9 �OH + OH� ? O�� + H2O k4 = 1.3 � 1010 M�1 s�1 [38]10 �OH + CHI3 ? products k5 = 7.7 � 109 M�1 s�1 This study11 �OH + Cl� ? HOCl� k6 = 4.3 � 109 M�1 s�1 [48]12 �OH + HCO3

� ? CO3� + H2O k7 = 8.5 � 106 M�1 s�1 [47]

13 �OH + NOM? products k8 = 2.5 � 104 (mg/L)�1 s�1 [43]14 HOCl� ? �OH + Cl� k9 = 6.1 � 109 s�1 [49]

314 A.-Q. Wang et al. / Chemical Engineering Journal 323 (2017) 312–319

quenched with Na2S2O3 at concentration as 1.2 times of the initialchlorine concentration, and then extracted for the analysis ofremaining CHI3 concentration within 2 h. Duplicate experimentswere conducted and the error bars in all data plots representedone standard deviation of these duplicate analytical results.

2.4. Analytical methods

CHI3 was quantified following the method modified from USEPAMethod 551.1 [34]. Samples were extracted using MtBE and theconcentration of CHI3 was analyzed using a gas chromatography(GC-2010, Shimadzu, Japan) equipped with a mass spectrometrydetector (MS) and a HP-5 capillary column (30 m � 0.25 mm i.d.,0.25 lm film thickness, J&W, USA). 1,2-dibromopropane, whichwas prepared in methanol, was used as the internal standard(IS). The concentrations of degradation products (I�, IO3

� and HOI)were detected using an UPLC, which was equipped with a UVdetector and an XTerra� MS C18 column (5 lm, 4.6 � 250 mm,Waters, USA) [2,35,36]. HOI is less stable than I� and IO3

�, whichcould be decomposed into I� or oxidized into IO3

� during the stor-age or UPLC analysis. Therefore, HOI was quenched with phenolsand was detected as iodophenol using UPLC [2] in this study. Themethod detection limit (MDL) of CHI3, I�, IO3

�, and HOI were0.08, 10, 5 and 10 lg L�1, respectively. The concentration ofdissolved organic carbon (DOC) was analyzed using a TOC-VCSH(Shimadzu, Japan) with the MDL as 0.1 mg-C L�1. The concentra-tion of chlorine was calibrated following the colorimetric methodusing the reagent N, N-diethyl-p-phenylenediamine (DPD) [37].The pH was detected with a pH meter (FE20-FiveEasy, MettlerToledo, Switzerland), which was regularly calibrated using stan-dard buffer solutions.

2.5. Kinetic modeling

When considering the direct photolysis, the quantum yield (U)of CHI3 is an essential parameter, which indicates the efficiency ofCHI3 decomposition by the absorbed photons. The quantum yield(mol�Einstein�1) is defined as the number of destroyed target com-pound divided by the number of photons absorbed by the system.

Based on the theory of Beer-Lambert Law, which is the basic lawof light absorption, the degradation kinetics of target organic com-pound during UV irradiation process can be expressed in the fol-lowing Eq. (1) [27,38] :

� dCt

dt¼ UI0½1� expð�2:303beCtÞ� ð1Þ

where Ct represents the concentration (M) of the organic compoundat reaction time t (s), U represents the quantum yield (mol�Ein-stein�1) of the organic compound, I0 is the UV fluence rate value(Einstein s�1 L�1) at the wavelength 254 nm, b is the effective opti-cal path length (cm) of the photoreactor, and e is the molar absorp-tion coefficient of the organic compound at the wavelength 254 nm(M�1 cm�1).

During UV/chlorination, the overall degradation of CHI3 wascontributed by direct photolysis, as well as indirect photolysis byhydroxyl radicals (discussions were shown in the following Sec-tion 3.1), and the degradation kinetics is expressed in the followingEq. (2):

d½CHI3�=dt ¼ �kobs½CHI3� ¼ �ðkdobs þ kidobsÞ½CHI3� ð2Þwhere kobs = the overall pseudo-first-order rate constant of CHI3degradation; kdobs = the pseudo-first-order rate constant duringdirect photolysis; kiobs = the pseudo-first-order rate constant duringindirect photolysis.

A degradation kinetic model was established in this study tosimulate CHI3 degradation based on the hypothesis that CHI3 is

degraded mainly by direct (UV) and indirect (�OH radicals) photol-

ysis during UV/chlorination. Table 1 shows the possible photo-chemical reactions involving in CHI3 degradation during UV/chlorination as well as their rate/equilibrium constants. k5, whichrepresents the rate constant of CHI3 degradation by

�OH radicals,

was calculated by solving a series of differential equations(detailed information was provided in the following Section 3.1.2,Eqs. (5)–(8)) using MATLAB (MathWorks) with the function ode15s,and the 95% confidence intervals was obtained using the functionnlparci. The modeling concentrations of CHI3,

�OH, HOCl and OCl�

during UV/chlorination with the initial reactant concentrationswere obtained using the tool SimBiology with the function ode15sand 10�6 as the absolute tolerance.

3. Results and discussion

3.1. Kinetic modeling

Fig. 1 shows the pseudo first-order rate results of CHI3 degrada-tion during chlorination, UV irradiation and UV/chlorine. CHI3could be rapidly degraded through both UV and UV/chlorine pro-cesses. As shown in Fig. 1, CHI3 degraded rapidly during UV/chlori-nation with a removal of more than 98.8% within 7 min, while thedegradation during chlorination was minimal with only 2.9% CHI3reduction within 10 min. The fitted lines in Fig. 1 shows a good lin-earity, indicating that CHI3 degradation during both UV and UV/chlorine processes followed pseudo first-order kinetics. The calcu-lated first-order rate constants of CHI3 degradation during UV andUV/chlorine processes were as 0.0034 and 0.0113 s�1, respectively,indicating that CHI3 degradation was more effective during UV/chlorine process. In UV/chlorine system, both non-selective

�OH

and selective�Cl were generated. A scavenging experiment was

conducted to examine the role of radicals in CHI3 degradation byusing NB as the radical scavenger, which reacted fast with

�OH

(k�OH-NB = 3.9 � 109 M�1 s�1) but its reactions with �Cl was negligi-ble [39]. The NB (50 lM) was in excess concentration compared toCHI3 (0.25 lM). As shown in Fig. 1, when NB was dosed, the scav-enging of

�OH reduced the degradation efficiency of CHI3 during

UV/chlorine process to a comparable level of UV irradiation alone.It proved that the degradation of CHI3 during UV/chlorine processwas mainly due to the effect of UV and

�OH radicals. Although �Cl

was also a strong oxidant with standard electrode potential of2.2–2.6 V [40], and that the photochemically generated

�Cl could

regenerate HOCl [41], the effect of�Cl on CHI3 degradation during

UV/chlorine process is negligible according to the results of scav-enging experiment. The other radicals, such as

�ClO and

�O�, only

had a negligible effect on CHI3 degradation during UV/chlorine pro-

Fig. 1. Pesudo-first-order kinetics plot of CHI3 during chlorination, UV irradiation,and UV/chlorine processes ([CHI3]0 = 100 lg L�1, [Chlorine]0 = 20 mg L�1, pH = 7,temperature = 22 ± 1 �C).

A.-Q. Wang et al. / Chemical Engineering Journal 323 (2017) 312–319 315

cess because their reactivity with organic compounds was muchlower [31,32].

3.1.1. Degradation kinetics of CHI3 during direct UV photolysisIntegrating the above Eq. (2) in Section 2.5 leads to Eq. (3):

ln10ebC0 � 1

10ebCt � 1

!¼ 2:303UI0bet ¼ kobst ð3Þ

where C0 is the initial concentration (M) of the organic compoundand kobs represents the observed first order rate constant (s�1).Because the direct photolysis of CHI3 follows first-order kinetics[19], the quantum yield of CHI3 can be determined using the follow-ing Eq. (4):

U ¼ kobs2:303I0be

ð4Þ

Fig. S3 in SI shows the degradation of CHI3 during direct UVphotolysis. The kobs was calculated as 3.43 � 10�3 s�1. Substitutingthe values of e (1100 M�1 cm�1 for CHI3) [19], I0, and b, which wereobtained in this study as 3.25 � 10�7 Einstein s�1 L�1 and 8.2 cmrespectively, the quantum yield of CHI3 was calculated as0.51 mol Einstein�1.

Fig. 2. The experimental and predicted values of the overall pseudo-first-order rateconstants of CHI3 degradation by direct and indirect photolysis at pH 5–9 duringUV/chlorination ([CHI3]0 = 100 lg L�1, [Chlorine]0 = 20 mg L�1, tempera-ture = 22 ± 1 �C). Error bars indicate one standard deviation of duplicate

3.1.2. Degradation kinetics of CHI3 during UV/chlorinationBased on the reactions listed in Table 1, as well as the assump-

tion that�OH radicals was the dominant radicals, a kinetic model of

CHI3 degradation during UV/chlorination was established, and aseries of differential equations can be derived and shown in the fol-lowing Eqs. (5)–(8):

d½HOCl�=dt ¼ �ruv;HOCl � ruv;OCl� � k2½�OH�½HOCl� � k3½�OH�½OCl��ð5Þ

d½CHI3�=dt ¼ �ruv;CHI3 � k5½�OH�½CHI3� ð6Þ

d½�OH�=dt ¼ ruv;HOCl þ k1½O���½H2O� � k2½�OH�½HOCl�� k3½�OH�½OCl�� � k4½�OH�½OH��� k5½�OH�½CHI3� �

Xi

kSi½�OH�½Si� ð7Þ

d½O���=dt ¼ ruv;OCl� � k1½O���½H2O� þ k4½�OH�½OH�� ð8Þ

where [Si] is the concentration of radical scavengers in the solution(such as chloride, NOM and alkalinity) and kSi represents thesecond-order rate constant of reactions between scavengers andhydroxyl radicals [42,43]. The calculations of ruv, HOCl, ruv, OCl

� , andruv,CHI3 are expressed in Text S2 in SI.

Based on the established kinetic model in this study, the rateconstant k5 for the reaction between

�OH radicals and CHI3 was cal-

culated as 7.7 � 109 M�1 s�1 by solving Eqs. (5)–(8) using the func-tion of ode15s of MATLAB, which is close to the value8.9 � 109 M�1 s�1 reported in a previous publication regardingCHI3 degradation during UV/H2O2 process [17]. This value wasused for the determination of the pseudo-first-order rate constantsof CHI3 in model calculation during UV/chlorination. The contribu-tions of direct and indirect photolysis to CHI3 were calculated usingMATLAB with the concentration profiles of

�OH and CHI3, and the

results are shown in Text S3 and Table S1 in SI.

3.2. Effect of pH on CHI3 degradation

Fig. S4 shows that CHI3 degradation rate decreased as pH valueincreased from 5 to 9 during UV/chlorination. The experimentalvalues of overall pseudo-first-order rate constants of CHI3 degrada-tion (kobs) as well as the predicted ones at pH 5–9 are shown inFig. 2. Overall, the model predictions of the contributions of indi-rect photolysis (

�OH radicals) and UV photolysis matched well with

the experimental data. In Fig. 2, as pH increased from 5 to 9, theexperimental kobs decreased from 1.4 � 10�2 to 4.2 � 10�3 s�1

due to the decrease of indirect photolysis (from 1.1 � 10�2 to9.6 � 10�5 s�1), while the contribution of direct UV photolysis onlychanged slightly (ranged between 2.9 � 10�3 to 4.3 � 10�3 s�1).The results can be explained by the strong impact of pH on the dis-sociation of HOCl/OCl� (pKa = 7.5), which led to a further effect onthe

�OH consumption by chlorine species. The rate constants of

�OH radical scavenged by HOCl and OCl- are 2.0 � 109 and8.8 � 109 M�1 s�1, respectively [44,45] (Rxns. (7) and (8) inTable 1), so OCl� shows stronger

�OH scavenging ability than HOCl.

At pH > 7.5, OCl� is the main chlorine species, which becomes thedominant

�OH scavenger. In addition, the photolysis of OCl� can

produce O�� and Cl� rather than

�OH (Rxn. (2) in Table 1), which

reduced the yield of�OH during UV/chlorination. Therefore, more

�OH can be remained in the solution during HOCl photolysis at

experiments.

316 A.-Q. Wang et al. / Chemical Engineering Journal 323 (2017) 312–319

acidic conditions than OCl� photolysis at alkaline conditions. Con-sidering that the operating conditions of most water treatmentprocesses are circumneutral, the following experiments were con-ducted at pH 7.

3.3. Effect of chlorine dosage on CHI3 degradation

Fig. S5 shows that the degradation of CHI3 increased withincreasing chlorine dosage during UV/chlorination. The experi-mental values of the overall pseudo-first-order rate constants ofCHI3 degradation as well as the predicted ones at different chlorinedosages (0–50 mg L�1) at pH 7 are shown in Fig. 3, in which themodeling results again matched well with the experimental data.As shown in Fig. 3, the experimental kobs increased from3.4 � 10�3 to 1.5 � 10�2 s�1 as chlorine dosage increased from 0to 50 mg L�1, which could be explained by the increasing yield of�OH during UV/chlorination (Rxn. (1) in Table 1). This result couldalso been explained by the increasing pseudo-first-order rate con-stant of

�OH and CHI3, which increased from 5.4 � 10�3 to

9.1 � 10�3 s�1 as the dosage of chlorine increased from 10 to50 mg L�1. Although typical chlorine dosage applied for disinfec-tion is 2–3 mg L�1 in drinking water treatment plants [46], theresults in Fig. 3 indicate that an increasing chlorine dosage to morethan 10 mg L�1 could be beneficial to enhance the degradation ofCHI3. Higher chlorine dosage (>30 mg L�1) did not increase thepseudo-first-order rate constant of

�OH reacting with CHI3 because

excess chlorine could act as a�OH scavenger (Rxns. (7) and (8) in

Table 1). In addition, high chlorine concentration could act as acompetitive UV absorber due to the screening effect [26], whichmight reduce the UV light absorbed by CHI3.

3.4. Effect of radical scavengers on CHI3 degradation

Chloride (Cl�), bicarbonate (HCO3�) and natural organic matter

(NOM) are natural occurring radical scavengers with the rate con-stants reacting with

�OH as 4.3 � 109 M�1 s�1, 8.5 � 106 M�1 s�1

and 2.5 � 104 (mg/L)-1 s�1, respectively [43,47,48]. The effect ofthese radical scavengers during UV/chlorination was evaluated atpH 7, with chlorine dosage as 20 mg L�1 and initial CHI3 concentra-tion as 100 lg L�1, and the results are shown in Figs. S6–S8 in SI.Experimental and the predicted pseudo-first-order rate constants

Fig. 3. The experimental and predicted values of the overall pseudo-first-order rateconstants of CHI3 degradation by direct and indirect photolysis at free chlorineconcentration of 0–50 mg L�1 during UV/chlorination ([CHI3]0 = 100 lg L�1, pH = 7,temperature = 22 ± 1 �C). Error bars indicate one standard deviation of duplicateexperiments.

of CHI3 degradation in the presence of these radical scavengersare shown in Fig. 4. The overall degradation rate constant of CHI3remained stable (1.1 � 10�2 s�1, Fig. 4(a)) as chloride concentra-tion increased form 0 to 5 mM (Fig. S6 in SI), which means thatchloride concentration has negligible influence on degradation rateof CHI3 during UV/chlorination. The rate constant of reactionbetween

�OH and CHI3 remained 7.6 � 10�3 s�1 at different chlo-

Fig. 4. The experimental and predicted values of the overall pseudo-first-order rateconstants of CHI3 degradation by direct and indirect photolysis at different chloride(a), bicarbonate (b), and NOM (c) concentrations during UV/chlorination([CHI3]0 = 100 lg L�1, [Chlorine]0 = 20 mg L�1, pH = 7, temperature = 22 ± 1 �C).Error bars indicate one standard deviation of duplicate experiments.

Fig. 5. The experimental and predicted values of the overall pseudo-first-order rateconstants of CHI3 degradation by direct and indirect photolysis at CHI3 concentra-tions of 100–400 lg L�1 during UV/chlorination ([Chlorine]0 = 20 mg L�1, pH = 7,temperature = 22 ± 1 �C). Error bars indicate one standard deviation of duplicateexperiments.

Fig. 6. Evolution of iodine species during CHI3 degradation during UV/chlorinationas a function of reaction time ([CHI3]0 = 81.5 lM-I L�1, [Chlorine]0 = 20 mg L�1,pH = 7, temperature = 22 ± 1 �C). Error bars indicate one standard deviation ofduplicate experiments.

A.-Q. Wang et al. / Chemical Engineering Journal 323 (2017) 312–319 317

ride concentrations (Fig. 4(a)), which can be explained by thereversible reactions of Cl� and HOCl� (Rxns. (11) and (14) inTable 1). The rate constants of the forward (Rxn. (11)) and back-ward reactions (Rxn. (14)) are 4.3 � 109 M�1 s�1 and6.1 � 109 s�1, respectively [48,49], which means that the scaveng-ing effect of Cl� on

�OH is counterbalanced by the generation of

�OH from the decomposition of HOCl�.

Because the dissociation constant (pKa) of H2CO3/HCO3� and

HCO3�/CO3

2� were 6.3 and 10.3, respectively [50], HCO3� was the

predominant species at pH 7. Fig. 4(b) shows the experimental val-ues of the overall pseudo-first-order rate constants of CHI3 degra-dation as well as the predicted ones at bicarbonate concentrationsof 0–4 mM. The degradation rate of CHI3 decreased as bicarbonateconcentration increased from 0 to 4 mM (SI Fig. S7), covering thecommon range of bicarbonate concentrations in surface waters(0.3 to 2 mM) [51]. As the concentration of bicarbonate increasedfrom 0 to 4 mM, the overall degradation rate constant of CHI3decreased from 1.1 � 10�2 to 8.8 � 10�3 s�1. The indirect photoly-sis degradation rate constant showed a decrease from 7.6 � 10�3 to5.8 � 10�3 s�1 when 0.5 mM bicarbonate was dosed. However, thedegradation rate constant of direct photolysis showed a plateauwith the increase of bicarbonate concentration. Thus the negativeeffect of bicarbonate on CHI3 degradation mainly results from thedecrease of the indirect photolysis degradation rate constant. Thisresult could be explained by the scavenging effect of HCO3

� (Rxn.(12) in Table 1). The reaction between

�OH and HCO3

� results inthe formation of CO3

� [47], which is a less reactive species and donot react with micropollutants [43].

As NOM concentration increased from 0 to 10 mg L�1, a nega-tive effect on the degradation of CHI3 was observed (Fig. S8 inSI). When NOM concentration increased from 0 to 10 mg L�1, theoverall degradation rate constant decreased from 1.1 � 10�2 to2.8 � 10�3 s�1, and the contribution of

�OH decreased from

7.6 � 10�3 to 2.0 � 10�3 s�1 (Fig. 4(c)). The contribution of directphotolysis also showed a decreasing trend from 3.4 � 10�3 s�1 to0.8 � 10�3 s�1. These results show that NOM could act as a strongradical scavenger in CHI3 degradation during UV/chlorination. Twoexplanations relevant to the property of NOM may contribute theresults. The first one is that NOM can compete with CHI3 to con-sume

�OH radicals (Rxn. (13) in Table 1) with the rate constant of

2.5 � 104 (mg/L)�1 s�1 [43]. The other reason is that NOM absorbsUV light at 254 nmwith a molar absorption coefficient at 3.08 (mg/L)�1 m�1 [52]. The filtering effect of NOM could result in the reduc-tion of UV light absorption by CHI3 and chlorine species, whichcould lead to decreasing reaction rates of direct photolysis andindirect photolysis. The UV absorption effect of NOM was shownin the model as expressed in Text S2 in SI. Because NOM is ubiqui-tous in aquatic environment, the significant impact of NOM on UV/chlorination of drinking water should be worth special attention.

3.5. Effect of CHI3 concentration on its photodegradation rate

Fig. S9 in SI shows the degradation of CHI3 during UV/chlorina-tion, which decreased as initial CHI3 concentrations increased from100 to 400 lg L�1. Fig. 5 shows the overall pseudo-first-order rateconstants of CHI3 degradation, which decreased from 1.1 � 10�2 to9.0 � 10�3 s�1 as initial CHI3 concentrations increased from 100 to400 lg L�1. The contributions of

�OH and UV photolysis to CHI3

degradation remained stable as 64% and 36%, respectively. Theseresults show that increasing initial concentration of CHI3 had anegative effect on the degradation rate constant of CHI3, whichmight be due to larger molar absorption coefficient of CHI3(1100 M�1cm�1) than chlorine species (59 M�1 cm�1 for HOCland 66 M�1 cm�1 for OCl�) [19,26], resulting in the reduction ofUV light absorbed by chlorine so as to the decrease of

�OH radicals

produced during UV/chlorination.

3.6. Identification of degradation products

The degradation products of CHI3 UV/chlorination were ana-lyzed using UPLC, and the evolution of detected iodine species(I�, IO3

� and HOI) as a function of reaction time were illustratedin Fig. 6. As shown in Fig. 6, the concentration of CHI3 kept decreas-ing from the beginning to 30 min. However, the concentration of I�

increased rapidly within the first 3 min and then reached a plateaubetween 15 to 30 min. Within the first 15 min, HOI and I2 concen-tration increased to 17.0 lM and then decreased gradually to16.2 lM at 30 min. At the end of the reaction, IO3

� was detectedand contributed 13.7% of the concentration of total liberated iodinespecies. IO3

� is a desired stable sink of iodine species in drinkingwater treatment process because it is proven to be nontoxicin vivo due to its quick reduction to I� by glutathione [53]. How-ever, compared with UV/chlorination, no IO3

� was observed duringUV process of CHI3 degradation [19] and only less than 2% of total

318 A.-Q. Wang et al. / Chemical Engineering Journal 323 (2017) 312–319

iodine species was observed as IO3� during UV/H2O2 process [17].

Different from the degradation products during UV/chlorination,I� was the dominant degradation product of CHI3 during UV andUV/H2O2 processes, which could be further oxidized to HOI bychlorine or chloramine, resulting in the formation of I-DBPs. Thus,compared with UV and UV/H2O2, UV/chlorination process is apromising AOP technology for the treatment of water containingCHI3.

4. Conclusions

UV/chlorine AOP is becoming more common in water treatmentfacilities for its fast reaction rate and non-selective oxidation. Thedegradation of CHI3 during UV/chlorination was fast due to indirectphotolysis (k5 = 7.7 � 109 M�1 s�1) and direct photolysis(kobs = 3.43 � 10�3 s�1). Higher chlorine concentration and lowerpH are beneficial for CHI3 degradation during UV/chlorination,while NOM and bicarbonate have negative effects on CHI3 degrada-tion. Chloride concentration did not apparently affect the degrada-tion rate. IO3

� contributed 13.7% of the total liberated iodine speciesduring UV/chlorination of CHI3. The satisfactory match of themodel calculation results and the experimental data indicates theapplicability the established degradation model to real water treat-ment situations, which along with the studied degradation kineticscould provide a reference for the selection of operating parameters.

Acknowledgments

This study was supported in part by the Natural Science Foun-dation of China (Nos. 51478323 and 51678354), the National MajorScience and Technology Project of China (No. 2015ZX07406004),State Key Laboratory of Pollution Control and Resource ReuseFoundation (No. PCRRK16005) and the Ministry of Science andTechnology in Taiwan (MOST-104-2221-E-327-001-MY3).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2017.04.061.

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