Rotating Ring-Disk Electrode and Quantum-Chemical Study of the

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Rotating Ring-Disk Electrode and Quantum Chemical Study of the Electrochemical Reduction of Monoiodoacetic Acid and Iodoform Jing Ma, Mingquan Yan, Andrey Mikhailocich Kuznetsov, Aleksey Nikolajevich Masliy, Guodong Ji, and Gregory V. Korshin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03951 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 24, 2015

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Environmental Science & Technology

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Rotating Ring-Disk Electrode and Quantum Chemical

2

Study of the Electrochemical Reduction of

3

Monoiodoacetic Acid and Iodoform

4

Jing Ma‡, Mingquan Yan‡*, Andrey M. Kuznetsov&, Aleksey N. Masliy &, Guodong Ji , Gregory

5

V. Korshin†

‡

‡

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Department of Environmental Engineering, College of Environmental Sciences and

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Engineering, Peking University, the Key Laboratory of Water and Sediment Sciences, Ministry

8

of Education, Beijing 100871, China

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†

Department of Civil and Environmental Engineering, Box 352700 University of Washington,

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Seattle, WA 98195-2700 United States

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& Department of Inorganic Chemistry, Kazan National Research Technological University, K.

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Marx Street 68, Russian Federation 420015

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*Corresponding author: Mingquan Yan, Department of Environmental Engineering, College of

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Environmental Sciences and Engineering, Peking University, Beijing 100871, China; Tel: +86

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10 62755914-81, Fax: +86 10 62756526. E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: This study examined the electrochemical (EC) reduction of monoiodoacetic acid

20

(MIAA) and iodoform (CHI3) which are typical iodine-containing disinfection by-products (I-

21

DBPs). Experiments carried out using the method of rotating ring-disk electrode (RRDE) with

22

gold working electrode showed that the reduction of CHI3 and MIAA is diffusion-controlled.

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MIAA diffusion coefficient was determined to be (1.86±0.24)·10-5 cm2 s-1. The yield of the

24

iodide ion formed as a result of MIAA or CHI3 reduction was affected by the presence of

25

dissolved organic matter (DOM) and resorcinol. Increasing concentrations of DOM or resorcinol

26

did not affect the EC reduction of the examined I-DBPs but the formation of iodide was

27

suppressed. This indicated that free iodine •I was formed as a result of the first step in the EC

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reduction of MIAA and CHI3. This also indicated that the pathway of the EC reduction of MIAA

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and CHI3 was different from that typical for the reduction of Br- and Cl-containing DBPs in

30

which case Br- or Cl- tend to be formed as a result of the electron transfer. Quantum-chemical

31

(QC) calculations confirmed the thermodynamic likelihood of and possible preference to the

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formation of free iodine species as a result of the EC reduction of MIAA, CHI3 and other I-

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DBPs.

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Keywords: disinfection by-products; dissolved organic matter; electrochemical reduction;

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iodine; iodoform; monoiodoacetic acid; rotating ring-disk electrode

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2


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Introduction

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Chloramination of drinking water that is frequently employed to suppress the formation of

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chlorine- and bromine-containing trihalomethanes (THMs) and haloacetic acids (HAAs) is

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known to promote the formation of iodine-containing DBPs (I-DBPs)

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occurrence of I-DBP in the U.S. and Canada demonstrates that iodo-HAAs and iodo-THMs are

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present in water produced by most of the treatment plants that use chloramine as disinfectant,

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with reported high concentrations of 1.7 µg L-1 for monoiodoacetic acid (MIAA), 10.2 µg L-1 for

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bromochloroiodomethane, and 7.9 µg L-1 for dichloroiodomethane

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demonstrated that MIAA was 3 and 287 times more cytotoxic than monobromoacetic and

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monochloroacetic acid, respectively

47

than CHBr3 and CHCl3, respectively 4.

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Free chlorine tends to oxidize the background iodide rapidly to iodate thus suppressing the

49

formation of I-DBPs. In contrast, chloramine oxidizes the iodide to free iodine species (e.g.,

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HOI, OI-, I2, I3- and others) that react with dissolved organic matter (DOM) forming I-DBPs

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Sedimentation, filtration, ozonation, membrane filtration and direct UV photolysis do not appear

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to remove I-DBPs effectively 9-12 although advanced oxidation processes are reportedly efficient

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in degrading I-DBPs

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other processes allow removing as high as 70% of Cl- and Br-containing THMs and ca. 60% of

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HAAs

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bromine-containing DBPs and other organic contaminants has been also examined in prior

57

research

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products and intermediates 16-18, 20. CHBr3 has been shown to be more amenable to reduction than

14, 15

13

5

2, 4

1-3

. The survey of the

. Previous studies have

while CHI3 was 60 times and 146 times more cytotoxic

6-8

.

. Sorption on activated carbon and combinations of this treatment with

. An alternative treatment via electrochemical (EC) reduction of chlorine- and

16-20

. It has demonstrated that halogenated species can be reduced to dehalogenated

3



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CHBr2Cl due to the relative strength of the C-Cl bond compared with that of the C-Br bond (397

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and 280 kJ mol-1, respectively), and the order of THM dehalogenation preference was

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established as CHBr3 > CHBr2Cl > CHBrCl2 > CHCl3 11. Bromine-containing HAAs have a

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much higher EC activity than their chlorinated analogues 21 and these compounds can undergo a

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relatively facile EC degradation 22.

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Prior research of oxidative and reductive degradation of I-DBPs is limited

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OI- and I3-and IO3- were observed to form in the photodegradation of CHCl2I and CHI3

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Photochemical transformations of I-DBPs can also lead to the formation of iso-iodoform formed

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in some cases via the recombination of •I and •CHI2 radical intermediates 27, 28.

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Despite the extent of these studies, effects of typical aquatic species, for instance DOM on

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processes governing the degradation of I-DBPs and other I-containing organic contaminants

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remain to be elucidated in more detail. In this paper, we used the EC method of rotating ring-disk

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electrode (RRDE) to explore the EC reduction of I-DBPs exemplified by the typically occurring

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MIAA and CHI3. RRDE is a well-attested electrochemical (EC) method that utilizes an

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arrangement of two electrodes whose surfaces form a disc and a coaxially positioned ring,

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respectively

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controlled independently. Rotation of the ring/disc system causes soluble intermediates and/or

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reactive products generated at the disc to be convectively transported to the ring whose EC

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control allows quantifying the current associated with the reactive species generated at the disc,

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and its dependence on the rotation rate, EC potentials of the ring and the disc, and other system

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conditions. In all, RRDE method is suitable for in situ probing EC and other properties of

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reactive intermediates and products generated in EC-controlled redox reactions

24

23-26

. For instance I·, 13, 23, 24

.

. These electrodes are placed in the same solution but their EC potentials are

4


24

. Reductive

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transformations of MIAA, CHI3 and other I-DBPs can take place in reactors designed to remove

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I-DBPs, or they can occur in water distribution systems on surfaces of plumbing materials.

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Outcomes of such processes can be a function of the EC potential of the system, mass transfer

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limitations, concentration and reactivity of DOM and other parameters

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goals of this study were: i) to determine effects of kinetic and mass transfer limitations on the EC

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reduction of MIAA and CHI3; ii) to elucidate effects of DOM on the EC behavior of these DBPs

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and iii) explore intrinsic mechanisms of these processes.

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Materials and Methods

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Reagents and chemicals. Chemicals were AR grade unless otherwise mentioned. All solutions

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were prepared using Milli-Q water (18.2 MΩ cm-1, Millipore Corp., MA, USA). MIAA was

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purchased from Chengdu Huaxia Chemical Co., Ltd. CHI3 and resorcinol were purchased from

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Sinopharm Chemical Reagent Co. Ltd, respectively. 7.868 g L-1 MIAA stock solution was

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prepared in DI water. 2 g L-1 of CHI3 stock solutions were prepared in methanol right before the

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experiments. Suwannee River fulvic acid, SRFA (1S101F), Suwannee River natural organic

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matter, SRNOM (1R101N) and Nordic Reservoir NOM (1R108N, NRNOM) were purchased

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from the International Humic Substances Society (IHSS). Aldrich humic acid (AHA) was

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obtained from Sigma-Aldrich (Lot # BCBC9785V). AHA solution was used after filtration

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through a 0.45 µm filter at pH 3.0 to remove undissolved fraction. Dissolved organic carbon

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(DOC) concentrations in stock solutions of SRFA, SRNOM, NRNOM and AHA results were

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152.1, 104.8, 138.2 and 112.8 mg L-1, respectively. DOC concentrations of DOM solutions were

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measured with a Shimadzu TOC-V CPN instrument. Stock solution of resorcinol was prepared in

5


24, 29

. Accordingly, the


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DI water with a 356 mg L-1 concentration. All stock solutions were stored at ca. 4 oC in a

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refrigerator.

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Electrochemical Measurements. EC experiments were performed using a 125 mL five-port cell

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with a gold RRDE working electrode affixed to a Pine AFMSRX rotator, a platinum counter

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electrode (AFCTR5), and a Ag/AgCl FastFil reference electrode. The disk and ring electrodes

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were controlled by a Pine AFCBP1 bipotentiostat (Pine Instrument Co., Lawrence, KS). The

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diameter of the disk electrode was 5 mm, the inner and outer diameters of the ring electrode were

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6.5 mm and 7.5 mm, respectively, the ring−disk gap was 0.75 mm. For this electrode geometry,

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the highest efficiency of collecting disk- generated intermediates on the ring is 0.256

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counter electrode was separated from the working electrode by a glass frit. EC potentials were

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measured and quoted vs the Ag/AgCl reference electrode (+0.197 V vs the standard hydrogen

113

electrode, SHE).

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All EC measurements were carried out in the presence of 0.1 mol L-1 Na2SO4 and 0.01 mol L-1

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phosphate buffer at 25±2 °C. Prior to each experiment, the surface of the RRDE was polished

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with 0.05 µm alumina slurry (Allied High Tech Products, Inc. Lot# 030513-1/E14DD) and

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rinsed with methanol and DI water for 30 seconds, respectively. All solutions were purged with

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N2 for 10 min to remove dissolved oxygen, and N2 gas line was taken out of solution and

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positioned in EC cell slightly above the air-solution interface to ensure that the ingress of oxygen

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to the solution was prevented by the continuing flow of nitrogen.

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Before each EC experiment, the disk electrode was placed in the background electrolyte and

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cycled ten times from – 1000 mV to 1000 mV using a 100 mV s-1 scan rate. The same procedure

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was used for the ring electrode. Following the cycling, a cathodic scan in the background 6


30

. The

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electrolyte was performed to check the reproducibility of the RRDE performance. These showed

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that increases of the RRDE rotation rate caused no appreciable changes of the ring and disk

126

currents in the deoxygenated background electrolyte (Figure S1 the Supporting Information, SI

127

section).

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In the case of EC reduction of MIAA and CHI3, cathodic scans of the disk electrode potential

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were carried out from 200 mV to -1000 mV using a 50 mV s-1 scan rate. The ring potential was

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kept during these scans at 800 mV. The disk potential was kept at 200 mV for 20 second before

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each scan.

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Results and Discussion

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RRDE measurements on the oxidation of iodide. Prior to the examination of the

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electrochemical reduction of MIAA, a series of RRDE experiments was carried to examine the

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oxidation of iodide. As the EC behavior of this anion has been well studied in prior research

136

25

137

were carried out at pH 7 using 50 to 100 mmol L-1 iodide concentrations (6.35 to 12.7 mg L-1).

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The data presented in Figure S2 demonstrate that the oxidation of iodide at the disk electrode

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resulted in the appearance of a pronounced increase of the anodic current at potentials exceeding

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ca. +550 mV. The anodic current tended to plateau for disk potentials from ca. +600 to +900 mV

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although some increase of the current was observed in this range.

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The ring currents measured when the ring potential was kept at 0 mV during the anodic scans of

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the disk potentials indicated the occurrence of the EC reduction. The EC reduction currents

144

measured using the ring electrode behaved largely similarly to the oxidation currents measured at

24,

, our experiments were designed to test the performance of the RRDE system. The experiments

7



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the disk. Calculations of the absolute values of the ratios of ring to disk currents, or the ring

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collection efficiency, showed that for range of potentials from +650 to 900 mV and a wide range

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of RRDE rotation rates, the collection efficiency was nearly constant and close to 0.254±0.002.

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This value is very close to the theoretical collection efficiency (0.256) of the RRDE with

149

dimensions used in this study 25, 30.

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The iodide oxidation current measured at the disk electrode increased linearly as a function of

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the square root of the electrode rotation rate. This indicated the prevalence of the diffusion

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controls of the EC oxidation of iodide. This allowed estimating the diffusion coefficient for

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iodide based on the Levich equation:

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I D = 0.62nFAD2 / 3v −1/ 6ω1/ 2C

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In the above equation, ID is the diffusion-limited EC current, n is the number of electrons

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transferred in the EC reaction (n=1), F is the Faraday constant (F=9649 C mol-1), A is the

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electrode area (A=0.1963 cm2), D is the diffusion coefficient, ω is the angular rotation rate of the

158

electrode, v is the kinematic viscosity (v=0.01 cm2 s-1), and C is the analyte concentration.

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Calculations using Eq. (1) yielded the diffusion coefficient for iodide of (2.00±0.07) .10-5 cm2 s-1.

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This is very close to the reported values of the iodide diffusion coefficient (1.98.10-5 cm2 s-1 in

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Friedman et al. 31 and 2.09.10-5 cm2 s-1 in Hawlicka et al. 32) thus confirming the acceptable level

162

of the performance of our RRDE system.

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RRDE measurements of the electrochemical reduction of monoiodoacetic acid and iodoform.

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Results of the measurements of the ring and disk currents in the system containing MIAA and

165

the background electrolyte are shown in Figure 1. They demonstrate that the presence of MIAA

(1)

8


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results in a prominent rise of the reduction current at the disk potentials < ca. -400 mV. The

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reduction currents increase with the rotation rate but they tend to plateau for the ring potentials

168

from -650 to -850 mV. The reduction currents increase at the disk potentials < -900 mV, likely

169

due to the contribution of the cathodic reduction of water to hydrogen. The oxidation currents on

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the ring behave somewhat similarly exhibiting a rapid increase for the disk potentials < -400 mV

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and a plateau for the range of the disk potentials from -650 to -850 mV.

172

Figure 1

173

The dependence of the MIAA reduction current (measured at -800 mV) vs. ω0.5 was linear

174

(Figure 2) and not affected by the presence of DOM or model compound resorcinol. The data

175

presented in Figure 1 and Figure 2 demonstrate that the EC reduction of MIAA appears to be

176

diffusion-controlled 30, 33. Accordingly, the diffusion coefficient for MIAA was calculated using

177

the data presented in Figure 2 and results generated in experiments with varying MIAA and

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DOM concentrations. The interpretation of these results based on Eq. (1) yielded the value of the

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diffusion coefficient of MIAA of (1.86±0.24)·10-5 cm2 s-1.

180

Figure 2

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RRDE data for the EC reduction of iodoform (Figure S3) were similar to those for MIAA

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although the reproducibility of the EC measurements in the former case was lower, due to the

183

volatility of CHI3 and difficulties of its control. In addition, the degradation of CHI3 may be more

184

complicated due to the possible formation of iso-iodoform and other reaction products observed

185

in the case of photochemical degradation of iodoform 27, 34.

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Effects of dissolved organic matter and resorcinol on the EC reduction of MIAA and iodoform. 9



187

While the EC reduction of MIAA or CHI3 was not affected by DOM or resorcinol, the ring

188

currents associated with the oxidation of the iodide formed upon the reduction of MIAA was

189

strongly affected by resorcinol (Figure 1(c) and Figure 2) and DOM, as shown in Figure 3 for

190

SRNOM and SRFA, and Figure S4 for NRNOM and AHA. Data for the EC reduction of CHI3 in

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the presence of resorcinol and SRNOM are shown in Figure S5. These figures demonstrate that

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the ring currents did not increase in proportion to the square root of the rotation rate but in some

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cases decreased when DOM or resorcinol were present. An example of this behavior is shown in

194

Figure 1(c).

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Figure 3

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Calculations of the ring collection efficiency in the experiments on the EC reduction of MIAA

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and CHI3 in the presence of varying concentrations of DOM or resorcinol showed that even in

198

the absence of the organic species the ring collection efficiency was somewhat lower than the

199

theoretically expected value of ca. 0.256. The collection efficiency decreased slightly as the

200

rotation rate increased.

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Effects of resorcinol on the ring collection efficiency were prominent (Figure 2 and Figure S6).

202

As the concentration of resorcinol increased, the ring collection efficiency decreased rapidly.

203

Similar effect was observed for DOM, although the decrease of collection efficiency in the latter

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case was not as prominent as that in the case of resorcinol.

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The observed effects were interpreted based on a hypothesis assuming that as opposed to the

206

established pathway of the EC reduction of aliphatic chlorine-containing organic species XR

207

where the first electron transfer results in the formation of X- anion and a radical •R (Reaction 2

10


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and 3) 19, the EC reduction of MIAA results in the generation of iodine atom •I and a molecule

209

of acetic acid, as shown in Reaction 4 below.

210

CH 2 ICOOH + e → I − + •CH 2COOH

(2)

211

• CH 2COOH + e + H + → CH 3COOH

(3)

212

CH 2 ICOOH + e + H + → • I + CH 3COOH

(4)

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2 • I → I2

(5)

214

I 2 + H 2O → I − + HOI + H +

(6)

215

In the case of the CHI3, the first step of the EC reduction of this species can also potentially

216

proceed via the classical pathway (Reaction 7 and 8) or CHI3 can be first reduced to CH2I2 and

217

iodine atom •I (Reaction 9):

218

CHI 3 + e → I - + •CHI 2

(7)

219

• CHI2 + e + H + → CH 2 I 2

(8)

220

CHI3 + e + H + → • I + CH 2 I 2

(9)

221

The iodine atoms produced upon the EC reduction of MIAA and CHI3 can rapidly dimerize to

222

form I2 (Reaction 5) that readily undergoes disproportionation form HOI and I- ion detected on

223

the ring (Reaction 6). Alternatively, the •I produced via the EC reduction of MIAA or CHI3 on

224

the disk can react with organic species, for instance resorcinol or DOM. These reactions will 11



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34

225

consume the EC-produced •I thus resulting in the decrease of the iodide production

and

226

formation of I-containing reaction products. Further studies need to be carried out to establish

227

their identities, in view of potentially extremely high toxicity of I-DBPs and related compounds

228

35-37

229

with the oxidation of free iodide on the ring, indicates that as a result of the absence of free

230

iodide at the initiation of the examined reactions and the low concentrations of this anion and

231

molecular iodine formed via the EC reduction of the examined I-DBPs, the formation of triiodide

232

ion is likely to be largely precluded, given the relatively low equilibrium constant of triiodide

233

formation (K=729) 38.

234

To model the observed effects, we introduced a phenomenological formula that interprets the

235

changes of the collection efficiency as a function of the concentration of the organic substrate

236

(Eq. 10). In agreement with the RRDE theory 30, 33, this formula contains a ωn term that reflects

237

the increasing mass transfer of the reactants to the electrode surface at increased rotation rates:

238

N = exp − (k1 + k 2C )ω m N0

239

In the above formula, N0 stands for the maximum collection efficiency, N is the collection

240

efficiency measured for any particular concentration of the organic substrate and electrode

241

rotation speed (denoted as ω), m is the coefficient reflecting effects of diffusion on reactions in

242

the RRDE system (theoretically, m is 0.5), k1 is the apparent rate of the loss of the EC produced

243

iodide due to limited rates of reactions (5) and (6) that affect the formation of I- during the transit

244

of the solution from the disk to the ring, and k2 is the rate of reactions that cause the loss of the

245

EC produced iodide due to the consumption of •I in reactions with the organic substrate.

. Examination of possible formation of triiodide ion that can decrease currents associated

[

]

(10)

12


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Eq. (10) is phenomenological and designed to fit the experimental data shown in Figure 2, Figure

247

S6 and Figure S7. A more detailed analytical examination of combined effects of diffusion to the

248

electrode surface and chemical reactions involving the reactants produced on the disk in the

249

framework of general RRDE theory is complex 33, 39, 40 and goes beyond the scope of this study.

250

Figure S8 demonstrates that in accord with Reaction (7), the logarithms of N0/N ratios increase

251

linearly with the square root of the electrode rotation rate while the slopes of these correlations

252

increase with the concentration of the organic substrate. This indicates increasing suppression of

253

the production of iodide at increasing concentrations of organic species that consume •I that is

254

hypothesized to be formed as an intermediate prior to the generation of the iodide.

255

Modeling of the data presented in Figure S7 and optimization of the rate constants k1 and k2 in

256

Eq. (10) was done using PIKAIA algorithm

257

for the entire dataset (Figure S8). While the method used in this study does not allow intrinsic

258

rates of reactions of •I with the organic species, the conditional rate of interactions of •I with

259

resorcinol was estimated at ca. 700 L mol-1 s-0.5 (for rotation rates measured in radian/second

260

units).

261

Similarly defined conditional reaction rates for SRFA, SRNOM, NRNOM and AHA calculated

262

per molar concentrations of organic carbon were estimated at 2.4, 1.6, 1.1 and 2.3 L mol-1 s-0.5,

263

respectively. Given that the aromatic groups in DOM are likely to dominate its interactions with

264

•I, the aromaticity of the solutions of SRFA, SRNOM, NRNOM and AHA was estimated based

265

on their SUVA254 values and the correlation reported

266

recalculated per molar concentrations of DOM aromatic groups. This resulted in conditional

41

. This allowed achieving a reasonably good fitting

42

. Following this, the reaction rates were

13



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reaction rates for the aromatic groups in DOM of 57, 28, 29 and 24 L mol-1 s-0.5 for SRFA,

268

SRNOM, NRNOM and AHA, respectively.

269

Comparison of the conditional rates determined for the aromatic groups in DOM and resorcinol

270

indicates that the latter compound reacts with the active iodine species ca. 10 to 30 times faster

271

than DOM. This can be partially attributable to a faster diffusion of resorcinol to the electrode

272

surface due to its lower molecular weight and thus higher diffusion coefficient. While we were

273

unable to find a reported value of the diffusion coefficient for resorcinol, it can be estimated

274

based on the relationships between the apparent molecular weight of DOM and their diffusion

275

coefficients

276

and resorcinol estimated as (MWDOM/MWres)0.42 is likely to be in the range from ca. 2.4 to 3. Thus

277

the different reactivity of the aromatic groups in DOM and resorcinol determined based on the

278

RRDE data cannot be attributed solely to the higher diffusivity of resorcinol but also to its

279

intrinsically higher reactivity. While per se the latter conclusion is not surprising

280

presented discussion indicates that the RRDE method may be used to evaluate the kinetics and

281

intrinsic mechanisms of DOM reactions with halogen species.

282

Quantum-Chemical Examination of the EC Reduction of MIAA, CHI3 and other DBPs.

283

Quantum-chemical (QC) calculations of the standard potentials of EC reduction of halogen-

284

containing DBPs were carried out within the CCSD method using Gaussian 09 program package.

285

Full details of these calculations summarized in the SI section are similar to those reported in

286

Kuznetsov et al.

287

chosen based on benchmarking calculations performed for three fundamentally important EC

288

reactions with well-known experimental standard potentials. These reactions are listed below:

43

. This shows that the ratio of the diffusion coefficients of the used DOM samples

44

26, 33

, the

. The atomic basis sets used in calculating the EC reduction potentials were

14


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289

X (aq ) + 2e → 2X -(aq )

290

HXO(aq ) + H +(aq ) + 2e → X -(aq ) + H O

291

XO - (aq ) + H O + 2e → X -(aq ) + 2OH

292

(X = Cl, Br, I)

293

Additional details of the calculations performed to determine the standard EC potentials of

294

reactions R1 to R3 are presented in the SI section. For chlorine, bromine, oxygen and hydrogen

295

atoms, these calculations utilized standard Pople’s split-valence 6-31G and 6-311G basis sets

296

with added polarization functions augmented with diffuse functions. For iodine, 6-311G(d)

297

TZP

298

data generated using these alternative sets were compared with the published standard EC

299

potentials 48.

300

QC calculations for free chlorine species (reactions R1, R2 and R3) resulted in an excellent

301

convergence between the experimental and predicted values of the standard EC potentials (Table

302

S1 in the SI section). Similar calculations performed for EC reactions involving bromine species

303

(Table S1) also showed a good correspondence between the experimental and simulation data.

304

Based on the results shown in Table S1, 6-311+G(df,p) set was selected for further calculations

305

for the chlorine and bromine systems at the CCSD level.

306

Calculations of the standard potentials for the iodine system were performed using three

307

alternative basis sets (Table S2). Comparison of the relevant results showed that the best

(R1)

2

(R2)

2

-

(R3)

2

46

and the electron-core pseudopotential cc-pwCVDZ-PP

47

15


45

,

basis sets were employed and


Page 16 of 35

308

agreement between the simulations and experimental data for the reactions R1, R2 and R3

309

involving iodine atom was achieved when QC simulations at the CCSD computational level

310

were performed using the TZP atomic basis set for iodine in combination with the 6-

311

311+G(2df,p) basis set for O and H atoms.

312

The selected basis sets combinations were used to determine the standard potentials of reactions

313

(2)-(4) and (7)-(9) hypothesized to take place in the case of the EC reduction of MIAA and CHI3,

314

respectively. The EC potentials of similar reactions that take place upon the EC reduction of

315

MBAA and MCAA, the Br-and Cl-containing analogues of MIAA were calculated as well. To

316

probe the behavior of other tri-, di- and mono halomethanes, the standard potentials of the EC

317

reduction of such species were determined.

318

Results of these calculations for the reductive dehalogenation of chlorinated, brominated and

319

iodinated monohaloacetic acids and halomethanes are presented in Table 1. It demonstrates that

320

the standard EC potential of reaction (4) that yields acetic acid and •I is -0.699 V while that of

321

reaction (2) that directly yields I- ion and •CH2COOH radical is -1.144 V. Similarly, the standard

322

potential of the reduction of CHI3 to diiodomethane and •I is estimated at -0.521 V while the

323

standard potential of reduction of the same species to iodide ion and •CHI2 radical -0.937 V. On

324

the other hand, the modeled standard EC potentials of the EC reduction of MBAA predict that

325

the reaction leading to the formation of Br- and •CH2COOH radical has the potential of -0.367 V

326

compared with -0.530 V for the reaction leading to the formation of •Br and acetic acid. This

327

indicates that in the case of MBAA, the classical pathway of the EC reduction of this compound

328

may be preferable as it is likely to occur at somewhat less reducing condition.

16


Page 17 of 35


329

Table 1

330

Figure 4

331

The QC-estimated EC potentials of the dehalogenation reactions that lead to either the formation

332

of X- anions or •X radicals are compared in Figure 4. It shows that the standard EC potentials

333

corresponding to the generation of •Cl radicals formed as a result of reduction of MCAA,

334

CHCl3, CH2Cl2 and CH3Cl are considerably more negative than those of the reactions leading to

335

the cleavage of Cl- ion. On the average, the reduction of these species to Cl- and corresponding

336

organic radicals is expected to occur at potentials ca. -0.46 V more positive than the reduction of

337

the same species to •Cl radicals. Largely the same trend is predicted to take place in the case of

338

MBAA and brominated methanes but the difference of the potentials corresponding to the

339

production of Br- and •Br radicals is considerably less (ca. -0.15 V on the average) than that in

340

the case of Cl-containing species. However, in the case of MIAA, CHI3 and several other I-

341

containing halomethanes, the production of •Ι takes place at potentials much less negative than

342

those corresponding to the generation of I- ions (Figure 4). The reduction of MIAA and I-

343

containing mono-, di- and trihalomethanes to •Ι is expected to precede that for the alternative

344

pathways of the release of I- ions by, on the average ca. 0.49 V.

345

The presented results of QC calculations of the electrode potentials reflect their values for

346

standard concentrations of all participating species and reaction products. These values can be

347

corrected based on the Nernst equation to determine actual EC potentials of the reduction MIAA,

348

CHI3 and other halomethanes based on the pH of the system, concentrations of iodide and other

349

halogenide ions, and other parameters that reflect applicable equilibria in the system. Exploration

17



350

of these aspects of the thermodynamics of the EC behavior of I-DBPs goes beyond the scope of

351

this study. Nonetheless, the presented QC data confirm the prevalence of •Ι formation in the EC

352

reduction of MIAA and CHI3. This result appears to distinguish the behavior of these and

353

potentially other I-DBPs from that of other halogen-containing DBPs for which the reductive

354

processes are likely to involve pathways resulting in the generation of Br- or Cl- ions.

355

Acknowledgements

356

This study was partially supported by China NSF (grant 21277005). Gregory Korshin thanks the

357

Foreign Experts Program of China for supporting his work at Peking University. The views

358

represented in this publication do not necessarily represent those of the funding agencies.

359

Supporting Information

360

Quantum-chemical calculations of the EC potentials of reduction of DBP species, Figures S1–

361

S8, Tables S1–S2. This information is available free of charge via the Internet at

362

http://pubs.acs.org.

18


Page 18 of 35

Page 19 of 35


363

References

364

1. Bichsel, Y.; von Gunten, U., Formation of iodo-trihalomethanes during disinfection and

365

oxidation of iodide containing waters. Environ Sci Technol 2000, 34, (13), 2784-2791.

366

2. Krasner, S. W.; Weinberg, H. S.; Richardson, S. D.; Pastor, S. J.; Chinn, R.; Sclimenti, M. J.;

367

Onstad, G. D.; Thruston, A. D., Occurrence of a New Generation of Disinfection Byproducts.

368

Environ Sci Technol 2006, 40, (23), 7175-7185.

369

3. Richardson, S. D., Environmental mass spectrometry: Emerging contaminants and current

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issues. Anal Chem 2008, 80, (12), 4373-4402.

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4. Richardson, S. D.; Fasano, F.; Ellington, J. J.; Crumley, F. G.; Buettner, K. M.; Evans, J. J.;

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Blount, B. C.; Silva, L. K.; Waite, T. J.; Luther, G. W.; McKague, A. B.; Miltner, R. J.; Wagner,

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E. D.; Plewa, M. J., Occurrence and Mammalian Cell Toxicity of Iodinated Disinfection

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Byproducts in Drinking Water. Environ Sci Technol 2008, 42, (22), 8330-8338.

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5. Plewa, M. J.; Wagner, E. D.; Richardson, S. D.; Thruston, A. D.; Woo, Y. T.; McKague, A.

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6. Bichsel, Y.; von Gunten, U., Oxidation of iodide and hypoiodous acid in the disinfection of

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7. Hua, G.; Reckhow, D. A., Comparison of disinfection byproduct formation from chlorine and

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alternative disinfectants. Water Res 2007, 41, (8), 1667-78.

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8. Jones, D. B.; Saglam, A.; Triger, A.; Song, H.; Karanfil, T., I-THM formation and speciation:

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9. Cancho, B.; Ventura, F.; Galceran, M.; Diaz, A.; Ricart, S., Determination, synthesis and

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survey of iodinated trihalomethanes in water treatment processes. Water Res 2000, 34, (13),

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3380-3390.

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10. Allard, S.; Charrois, J. W. A.; Joll, C. A.; Heitz, A., Simultaneous analysis of 10

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chromatography mass-spectrometry. J Chromatogr A 2012, 1238, (0), 15-21.

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11. Xiao, Y.; Fan, R.; Zhang, L.; Yue, J.; Webster, R. D.; Lim, T.-T., Photodegradation of

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275-285.

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12. Hansen, K. M.; Zortea, R.; Piketty, A.; Vega, S. R.; Andersen, H. R., Photolytic removal of

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DBPs by medium pressure UV in swimming pool water. Sci Total Environ 2013, 443, 850-6.

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13. Xiao, Y.; Zhang, L.; Yue, J.; Webster, R. D.; Lim, T. T., Kinetic modeling and energy

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efficiency of UV/H(2)O(2) treatment of iodinated trihalomethanes. Water Res 2015, 75, 259-69.

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14. Tang, S.; Wang, X. M.; Yang, H. W.; Xie, Y. F., Haloacetic acid removal by sequential zero-

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15. Trang, V. N.; Dan, N. P.; Phuong, L. D.; Thanh, B. X., Pilot study on the removal of TOC,

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16. Marković, N. M.; Lucas, C. A.; Gasteiger, H. A.; Ross, P. N., Bromide adsorption on

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17. Wang, T. C.; Tan, C. K.; Liou, M. C., Degradation of bromoform and chlorodibromomethane

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18. Criddle, C. S.; McCarty, P. L., Electrolytic model system for reductive dehalogenation in

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aqueous environments. Environ Sci Technol 1991, 25, (5), 973-978.

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19. Matheson, L. J.; Tratnyek, P. G., Reductive dehalogenation of chlorinated methanes by iron

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metal. Environ Sci Technol 1994, 28, (12), 2045-2053.

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20. Sonoyama, N.; Sakata, T., Electrochemical continuous decomposition of chloroform and

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other volatile chlorinated hydrocarbons in water using a column type metal impregnated carbon

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21. Korshin, G. V.; Jensen, M. D., Electrochemical reduction of haloacetic acids and exploration

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of their removal by electrochemical treatment. Electrochim Acta 2001, 47, (5), 747-751.

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22. Radjenovic, J.; Farre, M. J.; Mu, Y.; Gernjak, W.; Keller, J., Reductive electrochemical

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remediation of emerging and regulated disinfection byproducts. Water Res 2012, 46, (6), 1705-

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1714.

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23. Chen, S.; Huang, W.; Niu, Z.; Li, Z., A new oscillatory mechanism for the electro-oxidation

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of iodide involving two phase transitions and a disproportional reaction. Chem Phys Lett 2006,

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421, (1-3), 161-165.

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24. Liao, X.; Tanno, K.; Kurosawa, F., Rotating-disc electrode studies of the anodic oxidation of

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Interfacial Electrochemistry 1988, 239, (1–2), 149-159.

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25. Cantrel, L.; Chaouche, R.; Chopin-Dumas, J., Diffusion coefficients of molecular iodine in

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26. Elving, P. J.; Rosenthal, I.; Kramer, M. K., Polarographic behavior of organic compounds. 9.

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Iodoacetic acid the bromoacetic acids. J Am Chem Soc 1951, 73, (4), 1717-1722.

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27. Lee, J. H.; Kim, J.; Cammarata, M.; Kong, Q.; Kim, K. H.; Choi, J.; Kim, T. K.; Wulff, M.;

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Ihee, H., Transient X-ray diffraction reveals global and major reaction pathways for the

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28. Tarnovsky, A. N.; Pascher, I.; Pascher, T., Reactivity of iso-diiodomethane and iso-iodoform,

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A 2007, 111, (46), 11814-11817.

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29. Liu, H.; Kuznetsov, A. M.; Masliy, A. N.; Ferguson, J. F.; Korshin, G. V., Formation of

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Pb(III) Intermediates in the Electrochemically Controlled Pb(II)/PbO2 System. Environ Sci

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Technol 2011, 46, (3), 1430-1438.

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30. Pleskov, Y. V.; Filinovskii, V. Y., The Rotating Disc Electrode. Consultants Bureau: New

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31. Friedman, A. M.; Kennedy, J. W., The self-diffusion coefficients of potassium, cesium,

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iodide and chloride ions in aqueous solutions. J Am Chem Soc 1955, 77, (17), 4499-4501.

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32. Hawlicka, E., Self-diffusion of sodium, chloride and iodide ions in acetonitrile-water

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mixtures. In Zeitschrift für Naturforschung A, 1987, 42, 1014.

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33. Galus, Z., Fundamentals of electrochemical methods. Ellis Horwood 1976.

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34. Jones, C. E.; Carpenter, L. J., Solar photolysis of CH2I2, CH2ICl, and CH2IBr in water,

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saltwater, and seawater. Environ Sci Technol 2005, 39, (16), 6130-6137.

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35. Ding, G. Y.; Zhang, X. R., A picture of polar iodinated disinfection byproducts in drinking

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water by (UPLC/)ESI-tqMS. Environ Sci Technol 2009, 43, (24), 9287-9293.

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36. Liu, J.; Zhang, X., Comparative toxicity of new halophenolic DBPs in chlorinated saline

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wastewater effluents against a marine alga: Halophenolic DBPs are generally more toxic than

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haloaliphatic ones. Water Res 2014, 65, 64-72.

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37. Yang, M.; Zhang, X., Comparative Developmental Toxicity of New Aromatic Halogenated

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DBPs in a Chlorinated Saline Sewage Effluent to the Marine Polychaete Platynereis dumerilii.

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Environ Sci Technol 2013, 47, (19), 10868-10876.

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38. Warner, J. A.; Casey, W. H.; Dahlgren, R. A., Interaction kinetics of I-2(aq) with substituted

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phenols and humic substances. Environ Sci Technol 2000, 34, (15), 3180-3185.

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39. Albery, W. J.; Bruckenstein, S., Ring-disc electrodes. Part 2.-Theoretical and experimental

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collection effciencies. Transactions of the Faraday Society 1966, 62, (0), 1920-1931.

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40. Bruckens.S; Feldman, G. A., Radial transport times at rotating ring-disk electrodes.

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Limitations on detection of electrode intermediates undergoing homogeneous chemical reactions.

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J Electroanal Chem 1965, 9, (5-6), 395-&.

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41. PIKAIA, http://www.hao.ucar.edu/Public/models/pikaia/pikaia.html.

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42. Peuravuori, J.; Pihlaja, K., Molecular size distribution and spectroscopic properties of aquatic

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humic substances. Anal Chim Acta 1997, 337, (2), 133-149.

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43. Beckett, R.; Jue, Z.; Giddings, J. C., Determination of molecular-weight distributions of

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fulvic and humic acids using flow field-flow fractionation. Environ Sci Technol 1987, 21, (3),

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289-295.

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44. Kuznetsov, A. M.; Zueva, E. M.; Masliy, A. N.; Krishtalik, L. I., Redox potential of the

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Rieske iron-sulfur protein Quantum-chemical and electrostatic study. Biochimica Et Biophysica

472

Acta-Bioenergetics 2010, 1797, (3), 347-359.

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473

45. Glukhovtsev, M. N.; Pross, A.; McGrath, M. P.; Radom, L., Extension of Gaussian-2 (G2)

474

theory to bromine- and iodine-containing molecules: Use of effective core potentials (vol 103, pg

475

1878, 1995). J Chem Phys 1996, 104, (9), 3407-3407.

476

46. Campos, C. T.; Jorge, F. E., Triple zeta quality basis sets for atoms Rb through Xe:

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application in CCSD(T) atomic and molecular property calculations. Mol Phys 2013, 111, (2),

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165-171.

479

47. Peterson, K. A.; Yousaf, K. E., Molecular core-valence correlation effects involving the post-

480

d elements Ga-Rn: Benchmarks and new pseudopotential-based correlation consistent basis sets.

481

J Chem Phys 2010, 133, (17).

482

48. Line, D. E., Handbook of Chemistry and Physics, 71st Edition. CRC Press 1990.

483

484

24


Page 25 of 35


485

Captions of Table and Figures

486

Table 1 Results of quantum chemical calculations of the standard potentials of electrochemical

487

reduction of chlorinated, brominated and iodinated monohaloacetic acids and halomethanes.

488

Potentials are quoted in volts vs. the standard hydrogen electrode.

489

Figure 1 Effects of the disk potential and rotation speed on the disk (a) and ring (b) currents in

490

the presence of 20 mg L-1 MIAA concentration and absence of other organic species, and ring (c)

491

current in presence of 0.6 mg L-1 resorcinol. pH 7, background electrolyte 0.1 mol L-1 Na2SO4.

492

Ring potential +800 mV.

493

Figure 2 Effects of electrode rotation speed and resorcinol concentration on the disk (a) and ring

494

(b) currents and ring collection efficiency (c) in the case of the EC reduction of MIAA.

495

Resorcinol concentration in mg L-1 units. Ring potential +800mV. MIAA concentration 20 mg L-

496

1

497

Figure 3 Effects of the electrode rotation rate and concentrations of SRNOM (a) and SRFA (b)

498

on the ring current collection efficiency in the case of the EC reduction of MIAA. Disk potential

499

-800 mV, ring potential +800 mV. MIAA concentration 20 mg L-1, pH 7, background electrolyte

500

0.1 mol L-1 Na2SO4.

501

Figure 4 Comparison of the standard potentials of EC reduction of monohaloacetic acids and

502

halomethanes to the corresponding halogenide ion X- or halogen radical ·X. Standard EC

503

potentials were determined using QC simulations. (a) Standard EC potentials; (b) difference

504

between the potentials corresponding to the generation of X- and·X.

, pH 7, background electrolyte 0.1 mol L-1 Na2SO4.

25



Page 26 of 35

505

Table 1 Results of quantum chemical calculations of the standard potentials of electrochemical

506

reduction of chlorinated, brominated and iodinated monohaloacetic acids and halomethanes.

507

Potentials are quoted in volts vs. the standard hydrogen electrode.

Monochloroacetic acid (MCAA) СH2Cl-COOH •CH2-COOH СH2Cl-COOH •Cl

+e = +e + +e + +e =

+ Cl -

-0.578

=

CH3-COOH

1.722

=

CH3-COOH +

•CH2-COOH H

+

H

+

Cl

•Cl

-

-1.030 2.174

Trichloromethane СHCl3 •CHCl2 СHCl3

+e = +e + +e +

•CHCl2

+ Cl -

-0.444 1.603

H

+

=

CH2Cl2

H

+

=

CH2Cl2 +

•Cl

-1.016

Dichloromethane CH2Cl2 •CH2Cl CH2Cl2

+e = +e + +e +

•CH2Cl

+ Cl -

-0.602 1.702

H

+

=

CH3Cl

H

+

=

CH3Cl +

•Cl

-1.075

Monochloromethane CH3Cl •CH3 CH3Cl

+e = +e + +e +

•CH3

+ Cl -

-0.844 1.839

H

+

=

CH4

H

+

=

CH4 +

•Cl

-1.179

Monobromoacetic acid (MBAA) СH2Br-COOH •CH2-COOH СH2Br-COOH •Br

+e = +e + +e + +e =

+ Br -

-0.367

=

CH3-COOH

1.722

=

CH3-COOH +

•CH2-COOH H

+

H

+

Br

•Br

-

-0.530 1.885

Tribromomethane СHBr3 •CHBr2 СHBr3

+e = +e + +e +

•CHBr2

+ Br -

-0.158 1.634

H

+

=

CH2Br2

H

+

=

CH2Br2 +

•Br

-0.408

Dibromomethane CH2Br2

+e =

•CH2Br

+ Br -

-0.392 1.731

+

=

CH3Br

=

CH3Br +

•CH2Br

+e +

H

CH2Br2

+e +

H+

•Br

-0.545

Monobromomethane CH3Br

+e =

•CH3

+ Br

-

26


-0.669

Page 27 of 35


•CH3

+e +

H+

=

CH4

CH3Br

+e +

H+

=

CH4 +

1.839 •Br

-0.715

Dibromochloromethane -

CHBr2Cl

+e =

•CHBrCl

+ Br

-0.176

•CHBrCl

+e +

H+

=

CH2BrCl

CHBr2Cl

+e +

H+

=

CH2BrCl +

1.627 •Br

-0.433

Bromochloromethane CH2BrCl

+e =

•CH2Cl +

+ Br

-

-0.401 1.702

•CH2Cl

+e +

H

=

CH3Cl

CH2BrCl

+e +

H+

=

CH3Cl

+

•Br

-0.584

Bromodichloromethane -

CHBrCl2

+e =

•CHCl2

+ Br

•CHCl2

+e +

H+

=

CH2Cl2

CHBrCl2

+e +

H+

=

CH2Cl2

-0.193 1.603 +

•Br

-0.475

Monoiodoacetic acid (MIAA) СH2I-COOH

+e =

•CH2-COOH +

+I

-

-1.144

•CH2-COOH

+e +

H

=

CH3-COOH

СH2I-COOH

+e +

H+

=

CH3-COOH +

•I

+e =

I-

1.722 •I

-0.699 1.267

Triiodomethane +e =

•CHI2

•CHI2

+e +

H+

=

CH2I2

+e +

+

=

CH2I2 +

СHI3

H

+ I

-

СHI3

-0.937 1.682 •I

-0.521

Diiodomethane CH2I2

+e =

•CH2I

•CH2I

+e +

H+

CH2I2

+e +

H+

-

-1.099

=

CH3I

1.796

=

CH3I +

+ I

•I

-0.570

Monoiodomethane +e =

•CH3

•CH3

+e +

H+

=

CH4

+e +

+

=

CH4 +

CH3I

H

+ I

-

CH3I

Diiodochloromethane 27


-1.329 1.839 •I

-0.749


-

CHI2Cl

+e =

•CHICl

+ I

•CHICl

+e +

H+

=

CH2ICl

CHI2Cl

+e +

H+

=

CH2ICl +

Page 28 of 35

-0.924 1.664 •I

-0.526

Iodochloromethane -

CH2ICl

+e =

•CH2Cl

+ I

•CH2Cl

+e +

H+

=

CH3Cl

+e +

+

=

CH3Cl

CH2ICl

H

-1.065 1.702 +

•I

-0.623

Iododichloromethane -

CHICl2

+e =

•CHCl2

+ I

•CHCl2

+e +

H+

=

CH2Cl2

CHICl2

+e +

H+

=

CH2Cl2

-0.924 1.603 +

28


•I

-0.366

Page 29 of 35


(a)

508

(b)

509

(c)

510 29



Page 30 of 35

511

Figure 1 Effects of the disk potential and rotation speed on the disk (a) and ring (b) currents in

512

the presence of 20 mg L-1 MIAA concentration and absence of other organic species, and ring (c)

513

current in presence of 0.6 mg L-1 resorcinol. pH 7, background electrolyte 0.1 mol L-1 Na2SO4.

514

Ring potential +800 mV.

515

30


Page 31 of 35


(a)

516

(b)

517

(c)

518 31



Page 32 of 35

519

Figure 2 Effects of electrode rotation speed and resorcinol concentration on the disk (a) and ring

520

(b) currents and ring collection efficiency (c) in the case of the EC reduction of MIAA.

521

Resorcinol concentration in mg L-1 units. Ring potential +800mV. MIAA concentration 20 mg L-

522

1

, pH 7, background electrolyte 0.1 mol L-1 Na2SO4.

32


Page 33 of 35


(a)

523

(b)

524 525

Figure 3 Effects of the electrode rotation rate and concentrations of SRNOM (a) and SRFA (b)

526

on the ring current collection efficiency in the case of the EC reduction of MIAA. Disk potential

527

-800 mV, ring potential +800 mV. MIAA concentration 20 mg L-1, pH 7, background electrolyte

528

0.1 mol L-1 Na2SO4.

33



(a) 529

(b) 530 531

Figure 4 Comparison of the standard potentials of EC reduction of monohaloacetic acids and

532

halomethanes to the corresponding halogenide ion X- or halogen radical ·X. Standard EC

533

potentials were determined using QC simulations. (a) Standard EC potentials; (b) difference

534

between the potentials corresponding to the generation of X- and ·X. 34


Page 34 of 35

Page 35 of 35


I‐

e

Solution

Ring Electrode

Solution

Disk Electrode

Disk current

Solution

Ring Electrode e

Ring current

Solution

Rotating axis

·I + Organic matter I‐CH2COOH/CHI3 Iodinated organic compounds


Rotating Ring-Disk Electrode and Quantum-Chemical Study of the

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