Transformation of Iodide by Carbon Nanotube Activated

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Transformation of iodide by carbon nanotube activated peroxydisulfate and formation of iodoorganic compounds in the presence of natural organic matter Chao-ting Guan, Jin Jiang, Cong-wei Luo, Jun Ma, Su-yan Pang, Chengchun Jiang, Yi-xin Jin, and Juan Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04158 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Environmental Science & Technology

Transformation of iodide by carbon nanotube activated peroxydisulfate and formation of iodoorganic compounds in the presence of natural organic matter

1 2 3 4 5 6

Chaoting Guana, Jin Jianga,*, Congwei Luoa, Jun Ma a,*, Suyan Pangb,

7

Chengchun Jiang c, Yixin Jina, Juan Lia

8

a

9

Technology, Harbin, 150090, China.

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of

10

b

11

Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin

12

University of Science and Technology, Harbin 150040, China.

13

c

14

518055, China.

Key Laboratory of Green Chemical Engineering and Technology of College of

School of Civil and Environmental Engineering, Shenzhen Polytechnic, Shenzhen

15 16

*Corresponding Authors:

17

*E-mail: [email protected], [email protected]; tel: +86 451 86283010; fax:

18

+86-451 86283010.

19

*E-mail: [email protected], [email protected]; tel: +86 451 86283010; fax:

20

+86-451 86283010.

-1ACS Paragon Plus Environment


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Abstract

22

In this study, we interestingly found that peroxydisulfate (PDS) could be

23

activated by a commercial multi-walled carbon nanotube (CNT) material via a

24

nonradical pathway. Iodide (I-) was quickly and almost completely oxidized to

25

hypoiodous acid (HOI) in the PDS/CNT system over the pH range of 5-9, but the

26

further transformation to iodate (IO3-) was negligible. A kinetic model was proposed,

27

which involved the formation of reactive PDS-CNT complexes, and then their

28

decomposition into sulfate anion (SO42-) via inner electron transfer within the

29

complexes or by competitively reacting with I-. Several influencing factors (e.g., PDS

30

and CNT dosages, and solution pH) on I- oxidation kinetics by this system were

31

evaluated. Humic acid (HA) decreased the oxidation kinetics of I-, probably resulting

32

from its inhibitory effect on the interaction between PDS and CNT to form the

33

reactive complexes. Moreover, adsordable organic iodine compounds (AOI) as well as

34

specific iodoform and iodoacetic acid were appreciably produced in the PDS/CNT/I-

35

system with HA. These results demonstrate the potential risk of producing toxic

36

iodinated organic compounds in the novel PDS/CNT oxidation process developed very

37

recently, which should be taken into consideration before its practical application in

38

water treatment.


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Page 3 of 28


39

TOC ART

40



41

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Introduction

42

Iodine is ubiquitous in natural waters in the form of iodide (I-), iodate (IO3-) and

43

iodoorganic compounds. These iodine species can be transformed into each other,

44

which is affected by the environmental conditions such as microbial activities

45

chemical redox reactions

46

iodide-containing waters can lead to the formation of iodinated disinfection

47

by-products (I-DBPs) 6-8. In the past decades, I-DBPs have drawn increasing concerns

48

due to their much higher toxicity than their brominated and chlorinated analogues 9-13.

49

In addition, some I-DBPs such as iodoform (CHI3) and dichloroiodomethane can also

50

cause taste and odor problems in drinking waters 14-16.

51

3-5

1, 2

or

. It has been reported that oxidative treatment of the

The oxidation of I- and the formation of I-DBPs have been extensively 6, 17, 18

52

investigated in the cases of common selective oxidants including ozone (O3)

,

53

chlorine (Cl2)

54

permanganate (KMnO4) 22-24, manganese oxide

55

treatment processes, hypoiodous acid (HOI) is well known to be the main reactive

56

iodine species from I- oxidation by oxidants/disinfectants

57

transformation of HOI involves three different pathways: (i) its further oxidation to IO3-,

58

(ii) its reaction with natural organic matter (NOM) to form I-DBPs, and (iii) the

59

disproportionation of itself 6. The reaction (iii) is quite a slow process and thus is

60

neglected under typical water treatment conditions

61

reactions (i) and (ii) is vital for the product distribution between IO3- and I-DBPs 6. For

62

instance, in ozonation processes, HOI is quickly oxidized by O3 to IO3-, and thus the

63

formation of I-DBPs is rather unlikely (i.e., reaction (i) dominates) 18. In the case of

64

relatively weak NH2Cl, the formation of I-DBPs is very favorable because it can just

65

oxidize I- to HOI but the continued oxidation to IO3- is extremely weak (i.e., reaction (i)

66

is negligible) 18, 29. For some oxidants that can moderately oxidize HOI, the formation

67

of I-DBPs greatly depends on reaction conditions, such as oxidant doses, solution pH,

68

and the characteristics of raw waters. For instance, in chlorination processes, the

69

conditions of low pH values and low chlorine concentrations are found to cause a

, chloramine (NH2Cl) 6, 18, chlorine dioxide (ClO2) 8, 21, potassium

6, 18-20

7, 25-28

, and lead oxide 5. During water

18, 21

. The subsequent

7, 24

. So the competition between


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70

stronger formation of I-DBPs 6, 18, 30. Gallard et al. demonstrated that iodide-containing

71

waters in contact with manganese oxides resulted in the formation of undesired I-DBPs,

72

and the yields of such compounds maximized in the pH range of 5.0-7.0 due to the high

73

HOI exposures. When solution pH7.5, I- can not be oxidized to a significant extent 7, 25.

75

Interestingly, several recent studies have reported a novel and nonradical

76

oxidation process based on peroxydisulfate (PDS) activation by carbon nanomaterials

77

(e.g., carbon nanotube (CNT) and graphited nanodiamond (G-ND)), which exhibits a

78

notable substrate-specific reactivity for oxidizing organic contaminants in water

79

For instance, Lee et al. demonstrated that single- or multi-walled CNT effectively

80

enhanced the degradation of some phenolic compounds (e.g., 4-chlorophenol and

81

2,4,6-trichlorophenol) and pharmaceuticals (e.g., sulfamethoxazole and propranolol)

82

by PDS, while benzoic acid and nitrobenzene (i.e., probes for sulfate and hydroxyl

83

radicals) were resistant to degradation in this system

84

have proposed that the formation of reactive complexes between PDS and G-ND

85

might be responsible for phenol oxidation by means of various characterizations of

86

the carbon catalyst (e.g., linear sweep voltammetry, thermogravimetric analysis, and

87

fourier transform infrared spectroscopy) 33. Nevertheless, no kinetic models have been

88

established to describe these nonradical processes so far. Considering that (i) the

89

nonradical pathway provides an advantage of little influence by background

90

constituents, and (ii) the metal-free nature of carbon catalysts avoids the problems of

91

toxic metal ion leaching, these novel technologies by combining PDS with carbon

92

nanomaterials show a great promise for effective elimination of contaminants in water

93

treatment. However, the transformation of I- and/or HOI in these nonradical oxidation

94

processes has not been investigated so far, and it is unknown whether the undesired

95

I-DBPs can be formed or not.

31-33

.

31

. Very recently, these authors

96

In this study, we interestingly found that a commercial pristine multi-walled CNT

97

could effectively activate PDS via a nonradical pathway. The transformation of I- in

98

the PDS/CNT system was investigated in details, and the influences of several critical

99

factors (i.e., radical scavengers, PDS and CNT dosages, solution pH, and NOM) were -5ACS Paragon Plus Environment


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evaluated. A kinetic model was developed to describe the oxidation kinetics of I- in

101

the PDS/CNT system and the concomitant formation of sulfate anion (SO42-) from

102

PDS decomposition. Finally, the potential of producing I-DBPs in the PDS/CNT/I-

103

system was assessed by monitoring the formation of adsordable organic iodine

104

compounds (AOI) and specific iodinated organic compounds (i.e., CHI3 and

105

iodoacetic acid (IAA)) in the presence of NOM.

106

Material and Methods

107

Chemicals. Potassium iodide, potassium iodate, and sodium hypochlorite (NaOCl)

108

were of analytical-reagent grade and purchased from Sinopharm Chemical Reagent Co.,

109

Ltd., China. PDS, phenol, methanol (MeOH) were of ACS reagent grade and obtained

110

from Sigma-Aldrich Chemical Co. Ltd. Standards of CHI3 (99% purity) and IAA

111

(97% purity) were purchased from J&K Scientific Ltd., China. A commercial

112

multi-walled CNT (≥95% purity) with a length of 5-15 µm, a diameter of 10-20 nm and

113

specific surface areas of 100~160 m2/g was purchased from the Shenzhen Nanotech

114

Port Co., Ltd (Shenzhen, China). All other chemicals were of analytical grade or better

115

and were used without further purification. All solutions were prepared using deionized

116

(DI) water (18.2 MΩ/cm) that was produced from a Millipore Milli-Q purification

117

system. Stock solutions of NaOCl were prepared by diluting a commercial solution

118

(4.7% active chlorine) and standardized spectrophotometrically by an iodometric

119

method

120

prior to experiments

121

weighed amounts of PDS in DI water and standardized spectrophotometrically by the

122

iodometric method 36. A commercial humic acid (Sigma-Aldrich) was employed as a

123

model NOM, and its purification followed the procedure described in the literature 37.

124

Experimental procedures. Batch experiments were conducted in 250 mL glass

125

bottles in water bath at 20°C under magnetic stirring. Typically, the reactions were

126

initiated by simultaneous addition of predetermined volumes of I- and PDS stock

127

solutions into pH buffered solutions containing CNT and/or a constituent of interest

128

(e.g., MeOH or HA). At given time intervals, samples (1mL) were withdrawn and

. HOI was freshly prepared by the stoichiometric oxidation of I- by NaOCl

34, 35

24

. Stock solutions of PDS were freshly prepared by dissolving


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129

quickly filtered through 0.2 µm glass fiber filters, and then injected into LC (liquid

130

chromatography) vials containing excess phenol for scavenging HOI. Then, I-, IO3-,

131

and HOI were quickly analyzed. A similar experimental procedure was used for AOI

132

measurements except that samples of a large volume (50 mL) were withdrawn and

133

quenched with excess ascorbic acid. Solution pH (5, 7, and 9) was controlled using 2

134

mM phosphate buffer, and sodium perchlorate was used to maintain ionic strength (10

135

mM). The change of solution pH was low (±0.3) during the kinetic runs. All

136

experiments were run in duplicates or triplicates, and the average data with their

137

standard deviations were presented.

138

Analytical methods. Measurements of I- and SO42- were conducted with ion

139

chromatography and conductometric detection after chemical ion suppression (Dionex

140

AS3000). IO3- was analyzed with ion chromatography with UV/vis detection after

141

postcolumn reaction according to the method described by Bichsel and von Gunten 38.

142

A Dionex AS19 column (internal diameter, 4 mm; length, 250 mm) and a Dionex

143

AG19 guard column (internal diameter, 4 mm; length, 50 mm) were used.

144

P- and o-iodophenols formed from the reaction of HOI with scavenger phenol

145

were analyzed by high performance liquid chromatography (HPLC, Waters 2695) 25.

146

The HPLC separation was achieved on a symmetry C18 column (4.6×150 mm, 5 µm

147

particle size, Waters) and a photodiode array detector (PDA, Waters 2998). The mobile

148

phase consisted of water (containing 1‰ (v/v) acetic acid) and MeOH at a ratio of

149

40:60 (v/v), and the wavelengths were set at 280 nm for o-iodophenol and 231 nm for

150

p-iodophenol, respectively 6.

151

AOI was determined by an AOX analyzer multi X 2500 (Jena, Germany)

152

following the method by Nie39 and Xie

40

153

provided in the SI Text S1. CHI3 and IAA were determined by gas chromatography

154

(GC) (model 6890, Agilent, Santa Clara, USA) coupled with a flame ionization

155

detector (FID) and a HP5 column (30 m × 0.25 mm, ID × 0.32 µm) 7. The preparation

156

of samples and GC operational parameters were described in SI Text S2.

. Details of analytical procedures were



157

Results and Discussion

158

Transformation of I- by PDS in the presence of CNT.

159

(i) Evolution of iodine species.

160

Preliminary experiments were conducted to examine the oxidation of halogen

161

ions X- (including chloride (Cl-), bromide (Br-), and I-) by PDS in the presence of

162

CNT at pH 7. Both Cl- and Br- showed no obvious concentration changes in the

163

PDS/CNT system (Figure S1), while their transformations were extensively reported

164

in conventional sulfate radical (SO4•−) based advanced oxidation processes (AOPs) . Comparatively, a noticeable abatement of I- was observed, exhibiting

165

41-45

166

pseudo-first-order kinetics (Figure 1a and the inset), and control experiments ruled out

167

the possibility of I- adsorption on CNT (Figure S2). The discrepancy on the observed

168

loss rates of I- vs Br-/Cl- in the PDS/CNT system indicated the possible occurrence of

169

a nonradical catalytic mechanism. Further, this nonradical mechanism was confirmed

170

by comparatively examining the degradation of phenol and benzoic acid (BA) in this

171

system, similar to the protocol used by Lee et al 31, 33. As shown in Figure S3, phenol

172

was appreciably transformed but BA was inert in the PDS/CNT system, and MeOH

173

(as radical scavenger

174

These results were contrasted to the findings reported in SO4•− based AOPs, where

175

both phenol

176

competitively react with SO4•− and thus greatly inhibited their transformation.

49, 50

46-48

) in great excess had no inhibitory effect on phenol loss.

and BA

51

could be effectively oxidized and MeOH could

177

Further, the formation of inorganic iodine species (i.e., HOI and IO3-) was

178

followed along with the loss of I- in this process. The almost complete transformation

179

of I- to HOI was achieved (Figure 1a), while the concentration levels of IO3- were

180

extremely low. It seems likely that the PDS/CNT system is unable to further transform

181

HOI to IO3-. To confirm this, experiments were performed to examine the oxidation of

182

HOI prepared ex situ in the PDS/CNT system. As expected, the abatement of HOI in

183

the PDS/CNT system was negligible (Figure S4). Interestingly, we found that CNT

184

could also reduce HOI back to I- 52, 53, but the rate was much slower than that of I-

185

oxidation by the PDS/CNT system (Figure S4). So, it was not difficult to understand


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186

that nearly 100% transformation from I- to HOI was achieved in the PDS/CNT system

187

(Figure 1a). For simplicity, the reduction of HOI by CNT was not considered in the

188

following discussion.

Figure 1

189 190

(ii) Formation of SO42-

191

The formation of SO42- from PDS decomposition was also monitored during the

192

kinetic runs. It was found that (i) the formation of SO42- did occur in the PDS/CNT

193

system in the absence of I-, and (ii) I- enhanced the generation of SO42- (Figure 1b).

194

This result further confirmed that SO4•− was not involved in the oxidation of I- by the

195

PDS/CNT system. If SO4•− was otherwise produced in this system, the formation of

196

SO42- would not be affected by the presence of I-, given that reactive SO4•− could be

197

immediately transformed into SO42- through accepting electrons from the

198

electron-rich CNT surface. The enhanced SO42- formation by I- in the PDS/CNT

199

system was also consistent with two recent studies, in which phenol could accelerate

200

the decomposition of PDS by carbon nanomaterials as well 31, 33.

201

Further, the formation of SO42- from PDS decomposition by CNT in control

202

experiments was investigated at various PDS dosages at pH 7. Figure 2 showed that at

203

each PDS dosage, SO42- gradually accumulated within the investigated time scales.

204

Furthermore, the initial formation rate of SO42- (V(SO42-))

205

dosage increasing from 20 to 500 µM at a fixed CNT dosage (50 mg/L), and then

206

remained constant as the dosage further increased to 800 µM, exhibiting the saturation

207

kinetics (Figure 2a inset).

increased with PDS

Figure 2

208 209

54, 55

Kinetic model.

210

Very recently, Lee et al. proposed that the interaction between electrons in π bonds

211

of CNT and the electrophilic oxygen of PDS might generate reactive complexes

212

responsible for the effective degradation of organic compounds

213

reactions (1) and (2):

214

k1  →[ P − C ] PDS + CNT ← k −1


31

, as described by

(1)


215

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k2 [ P − C ] + S  → S ox + 2 SO4 2 − + CNT

(2)

216

where k1 and k-1 were the reaction rates for the formation and dissociation of reactive

217

PDS-CNT complexes (written as [P-C]), respectively, and k2 was the reaction rate for

218

[P-C] oxidizing target substrates (written as S). In this work, S represented I- and the

219

corresponding oxidation product (written as Sox) represented HOI.

220

Meanwhile,

our

findings

suggested

that

[P-C]

could

also

undergo

221

self-decomposition to form SO42- (i.e., inner electron transfer from CNT to PDS

222

within the complexes), as described by reaction (3): k3 [ P − C ]  → 2 SO4 2 − + CNTox

223

(3)

224

where k3 was the reaction rate for [P-C] self-decomposition, and CNTox represented

225

the oxidation state of CNT.

226

In order to verify the competitive effect of [P-C] self-decomposition (i.e.,

227

reaction (3)) on I- oxidation (i.e., reaction (2)), the influence of CNT pretreated by

228

PDS on the oxidation of I- was examined (i.e., PDS and CNT were pre-mixed for a

229

certain time before the addition of I-). As shown in Figure 3, the rate for I- oxidation

230

decreased with increasing contact time between PDS and CNT, indicating the

231

depletion of CNT active sites after [P-C] self-decomposition.

232 233 234

235

236

Figure 3 According to reactions (1)-(3), the rates for I- abatement and SO42- formation in the PDS/CNT/I- system could be respectively described as eqs (4) and (5):

−

d[I − ] = k 2 [ P − C ][ I − ] dt

(4)

d [ SO4 2 − ] = 2 k 2 [ P − C ][ I − ] + 2 k 3 [P − C] dt

(5) 56

237

By making use of the steady-state assumption regarding the reactive complexes [P-C] , 238

they could be rewritten as following: 239

d[I − ] − = K[I − ] dt

(6)

- 10 ACS Paragon Plus Environment

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240 241

d [ SO4 2 − ] = 2K[I − ] + 2B dt

(7)

where K was the pseudo-first-order rate for I- oxidation (i.e., K=k2[P-C]) that could be

242

experimentally acquired, and B was the decomposition rate of [P-C] (i.e., B=k3[P-C]). 243

Through the numerical integration, eq (8) was obtained: 244

[ SO4 2 − ] = 2[ I − ]0 ⋅ {1 − exp( − Kt )} + 2 Bt

245 246 247

(8)

Then, the formation of SO42- as a function of time was simulated using Matlab software to obtain the B values. Consequently, the k3/k2 value (i.e., the relative importance of reaction (3) vs (2)) could be determined:

248

B k 3 [ P − C ] k3 = = K k2 [ P − C ] k2

(9)

249

Influencing factors on I- oxidation.

250 251

(i) Effect of PDS dosage The oxidation of I- in the PDS/CNT system was examined at various PDS

252

dosages at pH 7 (Figure 4a). In the absence of CNT, there was no noticeable I253

abatement over the time scale investigated under all PDS concentration conditions 254

(Figure S5). The presence of CNT caused significant oxidation of I- by PDS, and the 255

abatement of I- followed the pseudo-first-order kinetics at each PDS dosage (the 256

resulting K values were presented in Table S1). Under the typical CNT dosage 257

condition (50 mg/L), the rate of I- abatement (i.e., K value) increased with increasing 258

PDS dosage, and then reached a plateau (Figure 4a and the inset). The saturation 259

kinetics with respect to PDS dosage was consistent with that obtained in the absence 260

of I- (Figure 2), suggesting that the active sites on CNT surface were limited in 261

relative to PDS in the formation of reactive complexes. 262

Figure 4 - 11 ACS Paragon Plus Environment


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In parallel, the formation of SO42- at various PDS dosages was examined (Figure 264

4b), which was simulated by eq (8) to obtain the B values. Then, the k3/k2 values were 265

calculated and listed in Table S1. A similar k3/k2 value of 2.6 (±0.3) was achieved at 266

different PDS dosages. 267 268

(ii) Effect of CNT dosage Furthermore, effect of CNT dosage on I- oxidation in the PDS/CNT system was

269

examined at pH 7. As shown in Figure 5a and the inset, the rate for I- oxidation 270

increased linearly with increasing CNT dosage from 25 to 200 mg/L at a fixed PDS 271

dosage (500 µM). The formation rate of SO42- followed a similar trend (Figure 5b). 272

These results were not difficult to understand because the increase of CNT dosage 273

provided more active sites for PDS activation. Moreover, a similar k3/k2 value of 2.6 274

(±0.3) was obtained at different CNT dosages, consistent with that achieved in the 275

case of different PDS dosages as well (Table S2).

Figure 5

276 277

(iii) Effect of pH.

278

Experiments were conducted to examine the effect of pH (5, 7, and 9) on the

279

abatement of I- in the PDS/CNT system (Figure 6a). As shown, solution pH had a

280

slight impact on I- abatement, and the rates followed an order of pH 7 > pH 9 > pH 5.

281

Comparatively, the formation rate of SO42- showed a slightly different trend with the

282

order of pH 9 > pH 7 > pH 5 (Figure 6b). This trend was consistent with that observed

283

in control experiments without I- (Figure S6). Further, the k3/k2 values were calculated

284

to be 2.7 (±0.2) for pH 5 and 3.8 (±0.3) for pH 9 (Figure 6b inset) with the latter one

285

being slightly higher than that for pH 7 (i.e., 2.6).

286

Figure 6

287

Interestingly, we found that the experimentally obtained k3/k2 values always

288

remained constant. This suggests that the relative importance of reaction (3) vs - 12 ACS Paragon Plus Environment

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289

reaction (2) is mainly controlled by the nature of the selected CNT, which warrants

290

further studies by employing various carbon materials. Solution pH plays a

291

comprehensive role, and its impacts may involve the effects on the interaction of PDS

292

with CNT to form reactive complexes as well as on their protonation/deprotonation

293

state.

294

Transformation of I- in the presence of NOM.

295

(i) I- oxidation kinetics

296

The formation of SO42- in the PDS/CNT system without I- was investigated in

297

the presence of a commercial HA used as a representative NOM. As shown in Figure

298

S7a-c, SO42- formation was obviously suppressed by the addition of HA at each pH (5,

299

7, and 9), while the adsorption of HA by CNT was relatively low (~ 0.048 mgC/m2,

300

Figure S8). Many studies reported that the interactions between NOM molecules and

301

CNT surfaces involved hydrophobic, π-π and hydrogen-bond interactions, among

302

which the π-π bonds formed between the aromatic moiety of NOM and the bulk π

303

system of CNT was generally recognized as the main driving force

304

adsorbed HA was likely to competitively occupy the sites for PDS activation. In

305

addition, the adsorbed HA could also indirectly limit the approach of PDS towards

306

CNT surface by enhancing the steric and electrostatic repulsion between them.

307

Similar inhibitory effects of HA at comparable coverage (~ 0.04 mgC/m2) were also

308

reported in the oxidation of I- by manganese dioxide 7.

57-60

. So, the

309

The effect of HA on the oxidation of I- in the PDS/CNT/I- system was examined

310

at pH 7. As shown in Figure 7a, the abatement rate of I- gradually slowed down with

311

increasing HA concentration from 0 to 5 mgC/L. The inhibition of HA on I- oxidation

312

could be well explained by its inhibitory effects on the formation of reactive

313

PDS-CNT complexes, as above mentioned. Additionally, it has been reported that the

314

reaction of HOI with HA involves not only the way of substitution reactions (iodination)

315

to form iodinated organic compounds but also the way of HOI reduction to I-

316

Control experiment confirmed the consumption of HOI by HA (2 mgC/L) alone at pH

317

7, but the reaction rate was much slower than that of I- oxidation by the PDS/CNT


6, 7, 18

.


318

system (Figure S9). So, the HA-induced reduction of HOI back to I- would not be a

319

main reason for its inhibitory effect on I- oxidation in the PDS/CNT system.

Figure 7

320 321

The effect of HA on I- oxidation kinetics in the PDS/CNT system was further

322

investigated under various pH conditions (5, 7, and 9). Compared to the case without

323

HA, I- oxidation was significantly suppressed due to the presence of HA (2 mgC/L) at

324

each pH, and the inhibitory effect enhanced as solution pH increased (Figure 7b). For

325

instance, the inhibitory degree of I- oxidation rate (i.e., [K - K’]/K× 100%, where K

326

and K’ were the pseudo-first-order rates for I- oxidation by the PDS/CNT system in

327

the absence and presence of HA, respectively) was calculated as 67% at pH 5, while it

328

increased to 82% at pH 9 (Figure S10). This was also consistent with the observation

329

that the suppression of HA for SO42- formation in the PDS/CNT system without I- was

330

a little stronger at higher pH (Figure S7d). So, it seems likely that HA limits the

331

approach of PDS towards CNT surface active sites more effectively under alkaline pH

332

conditions.

333

(ii) The formation of iodinated organic compounds.

334

Along with I- abatement as shown in Figure 7, the formation of HOI in the

335

PDS/CNT/I- system with HA was also examined, and then total inorganic iodine

336

species (TIS) was analyzed. Figure S11a showed that the formation rate of HOI

337

decreased with increasing HA concentration, as expected. There was a degree of TIS

338

loss when HA was present, and the loss progressively became larger with increasing

339

HA concentration (Figure S11b). Solution pH had a minor impact on HOI formation

340

as well as TIS loss (Figure S11c and d). These results suggested the transfer of

341

inorganic iodine into organic species.

342

Indeed, appreciable amounts of AOI were detected, and the yields of AOI

343

increased with the increase of HA concentration (Figure 8a) and were slightly

344

influenced by solution pH (Figure 8b). By summing up organic (i.e., AOI) and

345

inorganic iodine species (i.e., I- and HOI), a good iodine mass balance of ≥90% was

346

achieved at each condition. Control experiments showed that yields of AOI were

347

negligible in PDS solution containing I- and HA but without CNT (data not shown), - 14 ACS Paragon Plus Environment

Page 14 of 28

Page 15 of 28


348 349

which was also consistent with the fact that I- oxidation by PDS alone was quite slow 61-63

.

Figure 8

350 351

Also, specific iodinated organic compounds (i.e., CHI3 and IAA) were measured

352

in the PDS/CNT/I- system with HA, and their formation followed a similar trend to

353

AOI (Figure 8 and the inset). The maximum concentrations of CHI3 and IAA were

354

21.5 µg/L and 7.4 µg/L in the presence of 5mg C/L HA at pH 7, respectively.

355

Implication

356

The transformation of halogen ions (e.g., Cl- and Br-) as well as the formation of

357

toxic halogenated disinfection by-products in AOPs based on SO4•− generated from

358

PDS activation have been widely reported in recent years

359

demonstrates that PDS catalyzed by a commercial multi-walled CNT via a nonradical

360

mechanism cannot oxidize Cl- and Br-. Comparatively, I- can be quickly and

361

completely transformed into HOI by the PDS/CNT system, but the transformation to

362

IO3- is negligible. A kinetic model is proposed, including the formation of reactive

363

PDS-CNT complexes, and then their decomposition into SO42- via inner electron

364

transfer within the complexes or by competitively reacting with I-. This model can

365

well describe the influences of several critical factors on the oxidation of I- and

366

concomitant formation of SO42- in the PDS/CNT system. NOM has a significant

367

inhibitory effect on I- oxidation, probably because it can hinder the access of PDS to

368

CNT active sites to form the reactive complexes by its competitive adsorption, steric

369

and electrostatic effects. Meanwhile, considerable AOI as well as CHI3 and IAA are

370

produced in the PDS/CNT/I- system in the presence of NOM. These results advance

371

mechanistic understanding of nonradical activation of PDS by CNT as well as have

372

important implications on the potential application of this novel PDS/CNT technology

373

in water treatment.

374

Acknowledgment

375

This work was financially supported by the National Natural Science Foundation of

376

China (51578203&51378316), the National Key Research and Development Program - 15 ACS Paragon Plus Environment

41-45

. This study


377

(2016YFC0401107), the Chinese Postdoctoral Science Foundation (2015T80366), the

378

Funds of the State Key Laboratory of Urban Water Resource and Environment (HIT,

379

2016DX13), the Foundation for the Author of National Excellent Doctoral

380

Dissertation of China (201346), and the Fundamental Research Funds for the Central

381

Universities of China (AUGA5710056314).

382

Supporting Information

383

The additional figures, and tables addressing supporting data. This material is available

384

free of charge via the Internet at http://pubs.acs.org.

385

References

386

(1) Wong, G. T., The marine geochemistry of iodine. Reviews in Aquatic Sciences 1991, 4 (1), 45-73.

387

(2) Itoh, N.; Tsujita, M.; Ando, T.; Hisatomi, G.; Higashi, T., Formation and emission of

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monohalomethanes from marine algae. Phytochemistry 1997, 45 (1), 67-73. (3) Keppler, F.; Eiden, R.; Niedan, V.; Pracht, J.; Schöler, H. F., Halocarbons produced by natural oxidation processes during degradation of organic matter. Nature 2000, 403 (6767), 298-301. (4) Keppler, F.; Borchers, R.; Elsner, P.; Fahimi, I.; Pracht, J.; Schöler, H. F., Formation of volatile iodinated alkanes in soil: Results from laboratory studies. Chemosphere 2003, 52 (2), 477-483. (5) Lin, Y.; Washburn, M. P.; Valentine, R. L., Reduction of lead oxide (PbO2) by iodide and formation of iodoform in the PbO2/I-/NOM system. Environ. Sci. Technol. 2008, 42 (8), 2919-2924. (6) Bichsel, Y.; von Gunten, U., Formation of iodo-trihalomethanes during disinfection and oxidation of iodide containing waters. Environ. Sci. Technol. 2000, 34 (13), 2784-2791.

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(7) Gallard, H.; Allard, S.; Nicolau, R.; von Gunten, U.; Croué, J. P., Formation of iodinated organic

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compounds by oxidation of iodide-containing waters with manganese dioxide. Environ. Sci. Technol.

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by-products during oxidation of iodide-containing waters with chlorine dioxide. Water Res. 2013, 47 (9),

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(9) Richardson, S. D.; Fasano, F.; Ellington, J. J.; Crumley, F. G.; Buettner, K. M.; Evans, J. J.; Blount,

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haloform medicinal odor. Proc. Am. Water Works Assoc. Los Angeles, CA, 1989.

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birnessite (delta-MnO2) in the pH range 4-8. Water Res. 2009, 43 (14), 3417-3426.

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study. Sci. Total Environ. 2013, 463-464, 169-175.

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oxide. Water Res. 2010, 44 (15), 4623-4629.

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mineral birnessite. Geochim. Cosmochim. Ac. 2009, 73 (10), 2850-2861.

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alternative disinfectants. Water Res. 2007, 41 (8), 1667-1678.

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organoiodinated compounds: The important role of ammonia. Environ. Sci. Technol. 1998, 32 (11),

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1680-1685.

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Oxidation of organic compounds by nonradical mechanism. Chem. Eng. J. 2015, 266, 28-33.


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non-radical mechanism using persulfate activated by Fe/S modified carbon nanotubes. J. Colloid Interf.

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persulfates by graphitized nanodiamonds for removal of organic compounds. Environ. Sci. Technol.

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an electron shuttle to enhance the oxidation kinetics of substituted phenols by aqueous permanganate.

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(35) Gallard, H.; von Gunten, U., Chlorination of phenols: kinetics and formation of chloroform.

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(36) Frigerio, N. A., An iodometric method for the macro-and microdetermination of peroxydisulfate.

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Anal. Chem. 1963, 35 (3), 412-413.

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(37) Hong, S.; Elimelech, M., Chemical and physical aspects of natural organic matter (NOM) fouling of

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nanofiltration membranes. J. Membr. Sci. 1997, 132 (2), 159-181.

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postcolumn reaction and UV/visible detection. Anal. Chem. 1999, 71 (1), 34-38.

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(39) Nie, S.; Liu, X.; Wu, Z.; Zhan, L.; Yin, G.; Yao, S.; Song, H.; Wang, S., Kinetics study of oxidation

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of the lignin model compounds by chlorine dioxide. Chem. Eng. J. 2014, 241, 410-417.

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(40) Xie, Y.; Chen, L.; Liu, R., Oxidation of AOX and organic compounds in pharmaceutical wastewater

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in RSM-optimized-Fenton system. Chemosphere 2016, 155, 217-224.

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(41) Fang, J.; Shang, C., Bromate Formation from Bromide Oxidation by the UV/Persulfate Process.

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(42) Lutze, H. V.; Bakkour, R.; Kerlin, N.; von Sonntag, C.; Schmidt, T. C., Formation of bromate in

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sulfate radical based oxidation: Mechanistic aspects and suppression by dissolved organic matter. Water

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Res. 2014, 53, 370-377.

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(43) Lu, J.; Wu, J.; Ji, Y.; Kong, D., Transformation of bromide in thermo activated persulfate oxidation

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processes. Water Res. 2015, 78, 1-8.



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(44) Lutze, H. V.; Kerlin, N.; Schmidt, T. C., Sulfate radical-based water treatment in presence of

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chloride: Formation of chlorate, inter-conversion of sulfate radicals into hydroxyl radicals and influence

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of bicarbonate. Water Res. 2015, 72, 349-360.

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Perfluorooctanoic acid degradation using UV-persulfate process: Modeling of the degradation and

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chlorate formation. Environ. Sci. Technol. 2016, 50 (2), 772-781.

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electrodes for trichloroethylene degradation in groundwater. Environ. Sci. Technol. 2014, 48 (1),

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656-663.

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(47) Zhao, J.; Zhang, Y.; Quan, X.; Chen, S., Enhanced oxidation of 4-chlorophenol using sulfate

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radicals generated from zero-valent iron and peroxydisulfate at ambient temperature. Sep. Purif. Technol.

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2010, 71 (3), 302-307.

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(48) Anipsitakis, G. P.; Dionysiou, D. D., Degradation of organic contaminants in water with sulfate

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radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci. Technol. 2003, 37

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(49) Avetta, P.; Pensato, A.; Minella, M.; Malandrino, M.; Maurino, V.; Minero, C.; Hanna, K.;

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Vione, D., Activation of persulfate by irradiated magnetite: Implications for the degradation of

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phenol under heterogeneous Photo-Fenton-Like conditions. Environ. Sci. Technol. 2015, 49 (2),

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1043-1050.

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(50) Saputra, E.; Muhammad, S.; Sun, H.; Patel, A.; Shukla, P.; Zhu, Z. H.; Wang, S., α-MnO2

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activation of peroxymonosulfate for catalytic phenol degradation in aqueous solutions. Catal.

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hydroxyl radicals in the UV/Peroxymonosulfate system. Environ. Sci. Technol. 2011, 45 (21),

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9308-9314.

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(54) Nagaveni, K.; Sivalingam, G.; Hegde, M. S.; Madras, G., Photocatalytic Degradation of Organic

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Compounds over Combustion-Synthesized Nano-TiO2. Environ. Sci. Technol. 2004, 38 (5), 1600-1604.

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(56) Mcnaught, A. D.; Wilkinson, A., IUPAC compendium of chemical terminology. Encyclopedic

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Dictionary of Polymers 1997. Doi:10.1351/goldbook.S05962.

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(57) Ren, X.; Chen, C.; Nagatsu, M.; Wang, X., Carbon nanotubes as adsorbents in environmental

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pollution management: A review. Chem. Eng. J. 2011, 170 (2-3), 395-410.

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(58) Yang, K.; Xing, B., Adsorption of fulvic acid by carbon nanotubes from water. Environ. Pollut.

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2009, 157 (4), 1095-1100.

529

(59) Lin, D.; Xing, B., Tannic acid adsorption and its role for stabilizing carbon nanotube suspensions.

530


531

(60) Pan, B.; Xing, B., Adsorption Mechanisms of Organic Chemicals on Carbon Nanotubes. Environ.

532

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533

(61) Moews, P.C, Petrucci, R.H., The oxidation of iodide ion by persulfate ion. J. Chem. Educ. 1964,

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41 (10), 549-551.

535

(62) Jette, E., King, C.V., 1929. The oxidation of iodide ion by persulfate ion. I. The effect of tri-iodide

536

information on the reation velocity. J. Am. Chem. Soc. 1929, 51 (4), 1034-1047.

537

(63) Knudsen, O.M., King, C.V., The Oxidation of Iodide Ion by Persulfate Ion. V. The Rate at Low

538

Ionic Strength. J. Am. Chem. Soc. 1938, 60 (3), 687-691.



Page 22 of 28

(a) Time (min)

8

0

0

3

6

-1

6

9

12

15

y=-0.1843x R2=0.997

-ln([I-]/[I-]0)

Iodine Species (µ µM)

10

-2 -3 Time (min)

4

IHOI TIS

2 0 0

5

10

15

20

Time (min) (b)

PDS/CNT

50

-

PDS/CNT/I

SO42- (µ µM)

40 30 20 10 0 0

5

10

15

20

Time (min)

539 540

Figure 1. Evolution of iodine species (a) and formation of SO42- (b) as a function

541

of time in the PDS/CNT system. Inset indicated the pseudo-first-order oxidation

542

kinetics of I-. TIS was used as the abbreviation for total inorganic iodine species.

543

Experimental condition: [PDS]0 = 500 µM, [CNT]0 = 50 mg/L, [I-]0 = 10 µM, and

544

pH = 7.



V(SO42-) (µM• min-1)

Page 23 of 28

2.0

180

1.6 1.2

150

0.8 0.4 0.0

SO42- (µ µM)

120

0

200

400

600

800

PDS (µ µM)

90 60 30 0

20µM 200µM

0

50µM 500µM

100µM 800µM

180 360 540 720 900 1080 1260 1440

Time (min)

545 546

Figure 2. Effect of PDS dosage on the formation of SO42- from PDS

547

decomposition by CNT. Inset indicated the initial formation rate of SO42- (V(SO42-))

548

as a function of PDS dosage. Experimental condition: [PDS]0 = 20-800 µM,

549

[CNT]0 = 50 mg/L, and pH = 7.

550

10

0h 12h

6h 24h

I- (µ µ M)

8 6 4 2 0 0

551

5

10

Time (min)

15

20

552

Figure 3. Effect of pretreatment of CNT by PDS on I- oxidation in the PDS/CNT

553

system. Experimental condition: [PDS]0 = 500 µM, [CNT]0 = 50 mg/L, [I-]0 = 10

554

µM, pretreatment time = 0, 6, 12, 24h, and pH = 7.



Page 24 of 28

(a) 10 K (min-1)

0.20

I- (µ µM)

8

0.15 0.10 0.05 0.00 0

6

200 400 600 800

PDS (µ µM)

4 2 0 0

5

10

15

20

Time (min) PDS=20µM PDS=100µM PDS=500µM

50

PDS=50µM PDS=200µM PDS=800µM

(b)

SO42- (µ µM)

40 30 20 10 0 0

555

5

10

Time (min)

15

20

556

Figure 4. Effect of PDS dosage on I- oxidation (a) and corresponding SO42-

557

formation (b) in the PDS/CNT system. Inset showed the experimentally observed

558

rates for I- oxidation (K) at various PDS dosages. Symbols represent measured

559

data and dashed lines show the trend, while solid lines show the model fitting.

560

Experimental condition: [PDS]0 = 20-800 µM, [CNT]0 = 50 mg/L, [I-]0 = 10 µM,

561

and pH = 7.


Page 25 of 28


(a) 10 K (min-1)

1.0

I- (µ µM)

8

R2=0.97

0.8 0.6 0.4 0.2 0.0

6

0

50

100 150 200

CNT (mg/L)

4 2 0 0

5

10

15

20

Time (min) (b)

50

SO42- (µ µM)

40 30 20 CNT=25mg/L CNT=50mg/L CNT=100mg/L CNT=200mg/L

10 0 0

5

10

15

20

Time (min)

562 563

Figure 5. Effect of CNT dosage on I- oxidation (a) and corresponding SO42-

564

formation (b) in the PDS/CNT system. Inset showed the experimentally observed

565

rates for I- oxidation (K) as a function of CNT dosage. Symbols represent

566

measured data and dashed lines show the trend, while solid lines show the model

567

fitting. Experimental condition: [PDS]0 = 500 µM, [CNT]0 = 25-200 mg/L, [I-]0 =

568

10 µM, and pH = 7.



Page 26 of 28

(a)

10 5 4

k3/k2

8

3

I- (µ µM)

2 1

6

0

pH=5

pH=7

pH=9

4 2 0 0

5

10

15

20

Time (min) (b)

50

SO42- (µ µM)

40 30 20 pH=5 pH=7 pH=9

10 0 0

569

5

10

15

20

Time (min)

570

Figure 6. Effect of pH on I- oxidation (a) and corresponding SO42- formation (b)

571

in the PDS/CNT system. Inset indicated the k3/k2 values obtained under various

572

pH conditions. Symbols represent measured data and dashed lines show the trend,

573

while solid lines show the model fitting. Experimental condition: [PDS]0 = 500

574

µM, [CNT]0 = 50 mg/L, and [I-]0 = 10 µM.


Page 27 of 28


(a)

8

HA=0 mgC/L HA=0.5 mgC/L HA=1 mgC/L HA=2 mgC/L HA=5 mgC/L

µM) I-(µ

10

6 4 2 0 0

10

20

30

40

50

60

Time (min) (b)

10

pH=5 with HA pH=5 wtihout HA pH=7 with HA pH=7 without HA pH=9 with HA pH=9 without HA

I- (µ µM)

8 6 4 2 0 0

575

10

20

30

40

Time (min)

50

60

576

Figure 7. The abatement of I- in the PDS/CNT system with HA: (a) effect of HA

577

concentration (0-5 mgC/L) at pH 7, and (b) effect of solution pH (5, 7, and 9) in

578

the absence and presence of HA (2 mgC/L). Experimental condition: [PDS]0 =

579

500 µM, [CNT]0 = 50 mg/L, and [I-]0 = 10 µM.


AOI (µ µM)

10 8 6

Iodinated organics (µ µ g/L)


(a)

30

CHI3

25

IAA

20 15 10 5 0

HA=0.5 HA=1

HA=2

HA=5

4 2 0

10

AOI (µ µM)

8 6

Iodinated organics (µ µ g/L)

HA=0.5

HA=1

HA=2

HA=5 (b)

30

CHI3

25

IAA

20 15 10 5 0

pH=5

pH=7

pH=9

4 2 0

pH=5

pH=7

pH=9

580 581

Figure 8. Formation of AOI as well as CHI3 and IAA in the PDS/CNT/I-/HA

582

system: (a) effect of HA concentration (0.5-5 mgC/L) at pH 7, and (b) effect of

583

solution pH (5, 7, and 9) at a HA concentration of 2 mgC/L. Experimental

584

condition: [PDS]0 = 500 µM, [CNT]0 = 50 mg/L, [I-]0 = 10 µM, and reaction time of

585

60 min.


Page 28 of 28

Transformation of Iodide by Carbon Nanotube Activated

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