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Oxidation kinetics of bromophenols by nonradical activation of peroxydisulfate in the presence of carbon nanotube and formation of brominated polymeric products Chao-ting Guan, Jin Jiang, Cong-wei Luo, Jun Ma, Su-yan Pang, Yang Zhou, and Yi Yang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02271 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Environmental Science & Technology

Oxidation kinetics of bromophenols by nonradical activation of peroxydisulfate in the presence of carbon nanotube and formation of brominated polymeric products

1 2 3 4 5 6

Chaoting Guana, Suyan Pangb,*, Congwei Luoa, Jun Ma a, Yang Zhoua, Yi Yanga, Jin

7

Jianga,*

8 9

a

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

10

Technology, Harbin, 150090, China.

11

b

12

and Technology, Harbin 150040, China.

College of Chemical and Environmental Engineering, Harbin University of Science

13 14 15

*Corresponding Authors:

16

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

17 18

451 86283010 (J.J.) *E-mail: [email protected]; tel: +86 451 86392714; fax: +86 451 86392714 (S. P.)

19 20

1

ACS Paragon Plus Environment


21

Abstract

22

This work demonstrated that bromophenols (BrPs) could be readily oxidized by

23

peroxydisulfate (PDS) activated by a commercial carbon nanotube (CNT), while

24

furfuryl alcohol (a chemical probe for singlet oxygen (1O2)) was quite refractory.

25

Results obtained by radical quenching experiments, electron paramagnetic resonance

26

spectroscopy, and Fourier transform infrared spectroscopy further confirmed the

27

involvement of nonradical PDS-CNT complexes rather than 1O2. Bicarbonate and

28

chloride ion exhibited negligible impacts on BrPs degradation by the PDS/CNT

29

system, while a significant inhibitory effect was observed for natural organic matter.

30

The oxidation of BrPs was influenced by solution pH with maximum rates occurring

31

at neutral pH. Linear free energy relationships (LFERs) were established between the

32

observed pseudo-first-order oxidation rates of various substituted phenols and the

33

classical descriptor variables (i.e., Hammett constant σ+, and half-wave oxidation

34

potential E1/2). Products analyses by liquid chromatography tandem mass

35

spectrometry clearly showed the formation of hydroxylated polybrominated diphenyl

36

ethers and hydroxylated polybrominated biphenyls on CNT surface. Their formation

37

pathway possibly involved the generation of bromophenoxyl radicals from BrPs

38

one-electron oxidation and their subsequent coupling reactions. These results suggest

39

that the novel nonradical PDS/CNT oxidation technology is a good alternative for

40

selectively eliminating BrPs with alleviating toxic byproducts in treated water effluent.

2


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41

TOC ART

42

3



43

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Introduction

44

Bromophenols (BrPs) are an important class of phenolic contaminants, and they

45

have been frequently detected in aquatic environments, typically at the level of ng/L ~

46

µg/L ranges.1, 2 The extensive production and application of BrPs in some industries

47

(e.g., used as polymer intermediates, flame retardant intermediates, and wood

48

preservatives) result in their release to freshwater environments through various waste

49

streams.1 BrPs are also widely distributed in marine environments due to their natural

50

biosynthesis by some aquatic organisms such as algae, polychaetes, and

51

hemichordates.1-3 Toxicological studies indicate that BrPs have potential toxicity and

52

harmful effects on aquatic organisms and human bodies.1, 4-6 In addition, BrPs are

53

off-flavor causing compounds responsible for medicinal taste and odor episodes in

54

drinking water.7, 8 Therefore, many investigations have been conducted to examine the

55

treatment of BrPs by various water oxidants, including permanganate (Mn(VII)),9

56

ferrate (Fe(VI)),10 chlorine dioxide (ClO2),11 and manganese dioxide.12

57

On the oxidative treatment of BrPs, the formation of brominated polymeric

58

products, such as hydroxylated polybrominated diphenyl ethers (OH-PBDEs) and

59

hydroxylated polybrominated biphenyls (OH-PBBs), becomes an important issue.

60

Several studies have reported that OH-PBDEs and/or OH-PBBs can be appreciably

61

generated from precursor BrPs when treated by Mn(VII),9 naturally occurring

62

manganese oxides,12 photolysis,13,

63

brominated polymeric products have altered or enhanced toxicological effects (e.g.,

64

disruption to thyroid hormone homeostasis, disruption to sex hormone steroidogenesis,

65

and neurotoxicity) in comparison to parent compounds,17-19 and hence, their

66

occurrence in natural environments and engineered processes has raised great

67

concerns.

14

or enzymes-mediated oxidation.15,

16

These

68

Recently, a novel oxidation technology based on nonradical activation of

69

peroxydisulfate (PDS) by carbon nanotubes (CNTs) has received increasing attention,

70

which shows selective reactivity toward some phenolic contaminants (e.g., phenol,

71

bisphenol A, and 2,4,6-trichlorophenol) and several pharmaceuticals (e.g., propranolol, 4


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72

sulfamethoxazole, and acetaminophen), and the degradation kinetics are not

73

influenced by radical scavengers (e.g., methanol and dimethyl sulfoxide).20-22 Lee et al.

74

proposed that PDS might bind onto CNT surface to form a tentative reactive complex

75

responsible for enhanced oxidation of contaminants,20 which was also confirmed in

76

our recent study on inorganic iodide transformation by the PDS/CNT system.23 The

77

mechanistic reactions could be described by reactions (1)-(3):

78

PDS + CNT

[P-C]

(1)

[P-C] + S

Sox + 2SO42- + CNT

(2)

2SO42- + CNTox

(3)

[P-C]

79

where [P-C] represented the reactive PDS-CNT complexes, S represented the target

80

substrate, Sox was the corresponding oxidation product, and CNTox was the oxidation

81

state of CNT. As shown, the reactive complexes were firstly formed between PDS

82

with CNT active sites (reaction (1)), and then they underwent slow decomposition to

83

generate sulfate ions (SO42-) by reacting with target substrate (reaction (2)) or via

84

competitive inner electron transfer within the complexes (reaction (3)). Similarly, this

85

nonradical mechanism involving the formation of reactive complexes was also

86

reported in activation of PDS and/or peroxymonosulfate (PMS) by other carbon

87

materials (e.g., nanodiamond and reduced graphene oxide).24-28 However, the

88

oxidative treatment of BrPs by the PDS/CNT system has not been investigated so far,

89

and it is unknown whether undesirable brominated polymeric products (i.e.,

90

OH-PBDEs and OH-PBBs) may be generated or not.

91

The main objective of this study was to evaluate the oxidation kinetics of BrPs

92

by the PDS/CNT system and potential formation of brominated polymeric products.

93

Firstly, the transformation of BrPs (i.e., 2-, 3-, and 4-BrP) by the PDS/CNT system

94

was investigated in synthetic waters, and the nonradical mechanism involving the

95

formation of reactive complexes was confirmed. Secondly, the effects of water

96

matrices (i.e., bicarbonate (HCO3-), chloride ion (Cl-) and natural organic matter

97

(NOM)) and solution pH, as well as PDS and CNT dosages on the degradation of

98

BrPs were evaluated. Then, linear free energy relationships (LFERs) were established

99

to assess the effect of substituents on the reaction kinetics of various phenolic 5



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100

compounds with the nonradical PDS/CNT system. Finally, the formation of

101

brominated polymeric products from BrPs treated by the PDS/CNT system were

102

explored by high pressure liquid chromatography with electrospray ionization-triple

103

quadrupole mass spectrometry (HPLC/ESI-QqQMS) using a powerful precursor ion

104

scan (PIS) approach.

105

Materials and methods

106

Chemicals.

107

(KHSO5·0.5KHSO4·0.5K2SO4)), phenol (99%), 2-BrP (98%), 3-BrP (98%), 4-BrP

108

(99%), 2-chlorophenol (2-ClP, 99%), 3-chlorophenol (3-ClP, 98%), 4-chlorophenol

109

(4-ClP, 99%), 2-methylphenol (2-MP, 99%), 3-methylphenol (3-MP, 99%),

110

4-methylphenol (4-MP, 99%), 2-nitrophenol (2-NP, 98%), 3-nitrophenol (3-NP, 99%),

111

4-nitrophenol (4-NP, 99%), FFA (98%), acetaminophen (98%), and methanol (MeOH)

112

were

113

2,2,6,6-Tetramethyl-4-piperidinol (TMP, 99%) and cobalt sulfate (CoSO4, 98%) were

114

purchased from J&K Scientific Ltd. A commercial multiwalled CNT (>97% purity)

115

with length of 5−15 µm and outer diameter of 10−20 nm was obtained from Shenzhen

116

Nanotech Port Co., Ltd, which was also used in our recent study.23 Suwannee River

117

Humic Acid (SRHA) as a model NOM was obtained from International Humic

118

Substances Society. All other chemicals were of analytical grade or better and used

119

without further purification. All solutions were prepared using deionized (DI) water

120

(18.2 MΩ/cm) from a Milli-Q purification system (Millipore, Billerica, MA). Stock

121

solutions of PDS were freshly prepared by dissolving weighed amounts of PDS in DI

122

water and standardized by an iodometric method.29

123

Experimental Procedures. Kinetic experiments were conducted in 250 mL

124

glass bottles in 20 °C water bath under magnetic stirring. Solution pH (5, 7, and 9)

125

was controlled using 2 mM phosphate buffer, and the change of pH value was

126

relatively low (±0.3) during the kinetic runs. Reactions were initiated by

127

simultaneously adding PDS and a target compound (i.e., BrPs, FFA or other

128

substituted phenol) into pH buffered solutions containing CNT with/without a

all

(≥99.0%),

PDS

purchased

from

PMS

(available

Sigma-Aldrich

6


Chemical

as

Oxone

Co.

Ltd.

Page 7 of 34


129

constituent of interest (i.e., HCO3-, Cl-, or NOM) at desirable concentrations. Samples

130

were periodically collected and quickly filtered through 0.2 µm glass fiber filters, and

131

then they were quenched with excess ascorbic acid before analyses with HPLC and/or

132

ion chromatography (IC). All kinetic experiments were conducted in duplicates or

133

triplicates, and the averaged data and their standard deviations were presented.

134

For the analyses of BrPs oxidation products, the reaction solutions containing

135

CNT (50 mg/L) were prepared at pH 7 (2 mM phosphate buffer), and then PDS (500

136

µM) and one selected BrP (10 µM) were simultaneously added to initiate the reaction

137

(similar to the procedure of kinetic experiments). They were allowed to react at a

138

specific time (typically 40min) and quenched with excess ascorbic acid. The

139

suspensions were transferred to 50-mL Teflon centrifuge tubes, and then were

140

centrifuged at 2000 rpm for 30 min to achieve the solid/water separation. The

141

obtained aqueous supernatants were directly analyzed by the HPLC/ESI-QqQMS. On

142

the other hand, the separated solid CNT was collected and then extracted by organic

143

solvent methanol with an assistant by ultra-sonification in an ice-water mixture to

144

avoid possible heat-induced degradation of products.30 Subsequently, the resulting

145

suspensions were centrifuged, and the solvent supernatants were collected for product

146

analyses as well.

147

In addition, the evolution of BrPs oxidation products in the PDS/CNT system

148

was monitored. A series of conical flasks containing one selected BrP, PDS, and CNT

149

at desirable concentrations were prepared, and they were sacrificed individually at a

150

specific time. Following the procedure as described above, aqueous supernatant and

151

solvent supernatant samples were obtained and then subjected to analysis by

152

HPLC/ESI-QqQMS at multiple reaction monitoring (MRM) mode.

153

Analytical Methods. A Waters 2695 HPLC equipped with a Waters Symmetry

154

C18 column (4.6 × 150 mm, 5 µm particle size), a Waters 717 autosampler, and a

155

Waters 2487 dual λ UV−vis detector was used for analysis of substituted phenols and

156

FFA. The mobile phase consisted of methanol (A) and deionized water containing 0.1%

157

(v/v) acetate acid (B) at a flow rate of 1 mL/min. SO42− was determined by IC

158

(Dionex AS3000) equipped with a conductivity detector. The separation was carried 7



159

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

160

Dionex AG19 guard column (internal diameter, 4 mm; length, 50 mm) with 20 mM

161

NaOH eluent. The electron paramagnetic resonance (EPR) spectra were obtained on a

162

Bruker A200 spectrometer with a microwave frequency of 9.833 GHz, a microwave

163

power of 2.2 mW, a modulation frequency of 100 kHz, and sweep width 100.0 G. The

164

Fourier transform infrared (FT-IR) spectra were recorded in the spectral range of

165

400-4000 cm–1 using a PerkinElmer Spectrum One spectrometer.

166

An ABSciex QTrap 5500 MS with an ESI source was coupled with an Agilent

167

1260 HPLC for the HPLC/ESI-QqQMS analysis. A Waters XBridge C18 column (2.5

168

mm particle size, 3.0 × 100 mm) was used for separation. The gradient mobile phase

169

consisted of acetonitrile/water (A/B) at a flow rate of 0.2 mL/min, and the sample

170

injection volume was 10 µL. The MS instrumental parameters were optimized and set

171

as follows: ion spray voltage, -4500 V; collision energy (CE), -20 ~ -100 V; collision

172

cell exit potential (CXP), -9 ~ -17 V; declustering potential (DP), -40 ~ -120 V;

173

entrance potential (EP), -10 V; ion source gas I and II, 50 arbitrary units; curtain gas,

174

35 arbitrary units; source temperature, 500 °C. The same chromatographic separation

175

conditions and MS instrumental parameters were used for both PIS and MRM

176

analysis. The scan range (m/z) of 100 ~ 800 amu was set for PIS, and bromide ion

177

(Br−) was used as characteristic product ion for MRM.

178

Results and discussion

179

Oxidation kinetics of BrPs by the PDS/CNT system and nonradical

180

mechanism.

181

Experiments were conducted to examine the oxidation of BrPs (i.e., 2-, 3-, and

182

4-BrP) by the PDS/CNT system. As shown in Figure 1a, these three selected BrPs (10

183

µM) were effectively degraded by PDS (500 µM) in the presence of CNT (50 mg/L)

184

at pH 7, and their loss exhibited the pseudo-first-order kinetics (Figure S1 in

185

Supporting Information (SI)). Control experiments suggested that the abatements of

186

BrPs by PDS or CNT alone were negligible (data not shown). Meanwhile, the

187

formation of SO42− from PDS decomposition by CNT was appreciably enhanced in 8


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Page 9 of 34


188

the presence of BrPs (Figure 1b), as described by reactions (2) and (3). MeOH as a

189

scavenger for both sulfate radical (SO4 −) and hydroxyl radicals (•OH)20, 31 exhibited a

190

negligible effect on BrPs degradation by the PDS/CNT system (Figure S2). In

191

addition, it was found that the pretreatment of CNT by concentrated hydrochloric acid

192

(HCl) to remove residual metal impurities (e.g., Fe, Mn, and Cu)20 did not influence

193

the oxidation kinetics of BrPs (Figures S3). Furthermore, compared to the case of

194

CNT alone, the FT-IR spectrum of PDS/CNT showed increased absorption at around

195

1171 and 1074 cm−1 (Figure S4), which could be attributed to the symmetric and

196

asymmetric vibrations of S=O=S of sulfonate groups, respectively.28, 32 These findings,

197

in good agreement with Lee’s studies,20, 28 clearly confirmed the adsorptive interaction

198

of PDS on CNT surface.

•

Figure 1

199 200

Alternatively, Cheng et al. have recently suggested that a nonradical reactive

201

oxygen species singlet oxygen (1O2) is responsible for the effective degradation of

202

2,4-dichlorophenol (2,4-DCP) by the PDS/CNT system, mainly based on the fact that

203

furfuryl alcohol (FFA, a chemical probe for 1O233, 34) in great excess (16 mM) had a

204

significant inhibitory effect.35 In their work, CNT has been considered as a ketone to

205

react with PDS to form a dioxirane intermediate, which can undergo subsequent

206

decomposition to generate

207

mechanism for the formation of 1O2 via ketone catalysis is well known in the case of

208

PMS34, 36-38 but is rather reported for PDS.

1

O2 (SI Scheme S1). However, we note that this

209

In this work, it was found that the PDS/CNT system could not effectively

210

transform FFA at a low concentration (10 µM) under the identical condition to that in

211

the case of BrPs (Figure 1a), suggesting that 1O2 might not be the main reactive

212

species responsible for BrPs oxidation. The result of EPR experiments by using TMP

213

as the spin-trapping agent further excluded the involvement of 1O234, 39-41 (see Figure

214

S5 and Text S1 for the detail). Interestingly, it was found that the presence of a high

215

concentration of FFA (16 mM) effectively inhibited the degradation of 2-BrP by the

216

PDS/CNT system (similar to Cheng’s observation35) and simultaneously promoted the

217

formation of SO42− (Figure S6a and b), while FFA at a low concentration of 10 µM 9



218

had no effect (data not shown). Control experiments revealed that the adsorption of

219

FFA on CNT surface was rather negligible (Figure S7). In these regards, the

220

confounding effect of FFA can be reasonably explained by that FFA at high

221

concentrations (e.g., 16 mM) competitively consumed the reactive PDS-CNT species,

222

thus leading to the inhibition of organic substrates (i.e., 2-BrP and 2,4-DCP) oxidation

223

as well as the enhancement of SO42− formation, while this competition effect became

224

negligible when FFA was present at a low concentration (e.g., 10 µM). Nevertheless,

225

it is unknown whether 1O2 can be produced from PDS activation by other CNTs with

226

different properties (e.g., surface functional groups, and sp2 carbon network), which

227

warrants further investigations.

228

Effects of water matrices.

229

The influence of several water matrix constituents including HCO3- and Cl-, and

230

NOM on the degradation of BrPs by the PDS/CNT system was investigated. Taking

231

2-BrP as an example, the presence of inorganic anions (i.e., HCO3- and Cl-) had a

232

negligible effect on 2-BrP oxidation (Figure 2a), while the abatement of 2-BrP was

233

significantly suppressed in the presence of NOM (Figure 2b). The observed

234

pseudo-first-order rate (kobs) for 2-BrP oxidation gradually decreased from 0.1870

235

min-1 to 0.0236 min-1 with NOM concentration increasing from 0 to 5 mgC/L. Similar

236

inhibitory effect of NOM was also observed during the oxidation of phenol20 or

237

iodide23 by the PDS/CNT system. On the one hand, NOM was very likely to hinder

238

the interaction between PDS with CNT to form the reactive complexes due to its

239

competitive adsorption on CNT surface active sites as well as the steric and

240

electrostatic effects, and thus inhibited the formation of reactive complexes.23 On the

241

other hand, the electron-rich NOM with multi-benzene rings and oxygen

242

functionalities might compete with 2-BrP for the reactive species, hereby slowing

243

down the reaction rate. It was found that SO42− formation from PDS decomposition by

244

CNT was obviously suppressed due to the presence of NOM (Figure S8). This

245

suggested that the hindering effect of NOM on the formation of reactive PDS-CNT

246

complexes should be a dominant factor, in contrast to the case of FFA at high

247

concentrations. 10


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Page 11 of 34


Figure 2

248 249

Effect of pH.

250

Further, experiments were conducted to investigate the effect of solution pH (5, 7,

251

and 9) on oxidation kinetics of BrPs by the PDS/CNT system. For all selected BrPs,

252

the degradation rates observed at pH 7 were appreciably higher than those observed at

253

pH 5 and pH 9 (Figure 3). For instance, the kobs values for 2-BrP oxidation by the

254

PDS/CNT system at pH 5, 7, and 9 were 0.0620 min-1, 0.1870 min-1, and 0.1129 min-1,

255

respectively. Figure 3

256 257

It is well-known that dissociated phenols have higher electron densities and thus

258

are more susceptible to oxidation than non-dissociated ones. However, the increase of

259

solution pH resulting in the dissociation of phenols did not accelerate the oxidation of

260

BrPs by the PDS/CNT system. The plausible explanations involved that (i) solution

261

pH impacted the interaction of PDS with CNT active sites, thus influencing the

262

formation of reactive PDS-CNT complexes; and/or (ii) the reactive complexes had

263

their own pKa and the change of pH would lead to the change of their

264

protonation/deprotonation state. It should be noted that the generation of other

265

reactive species (e.g., 1O2, SO4 −, or •OH) did not occur in the PDS/CNT system at

266

pH 5 or 9 either, based on the observations of negligible FFA degradation (Figure S9)

267

as well as no effects of excess MeOH (Figure S10).

268

Effects of PDS and CNT dosages.

•

269

Effects of PDS and CNT dosages on BrPs oxidation by the PDS/CNT system

270

were examined at pH 7, and Figures 4a and b exemplified the case of 2-BrP. Under

271

the typical CNT dosage condition (50 mg/L), the rate for 2-BrP oxidation by the

272

PDS/CNT system increased with increasing PDS dosage from 50 to 400 µM, and then

273

a kinetic plateau was reached as PDS dosage further increased to 600 µM (Figure 4a

274

and the inset). The saturation kinetics with respect to PDS dosage were also observed

275

in the case of iodide oxidation by this system,23 suggesting that the active sites on

276

CNT surface were rather limited. Meanwhile, it was observed that the degradation of

277

2-BrP was gradually accelerated with increasing CNT dosage from 25 to 200 mg/L at 11



278

a fixed PDS dosage (500 µM) (Figure 4b), which could be attributed to the increase of

279

CNT surface active sites. Figure 4

280 281

LFERs

282

Further, the oxidation of other ten substituted phenols (i.e., phenol, 3 ClPs, 3

283

MPs, and 3 NPs) by the PDS/CNT system at pH 7 was examined under the same

284

experimental conditions (i.e., [phenols] = 10 µM, [PDS] = 500 µM, and [CNT] = 50

285

mg/L). The pseudo-first-order kinetics were also found for their abatements, and the

286

obtained kobs values were exhibited in Table S1. Among these substituted phenols,

287

2-MP exhibited the maximum degradation rate by the PDS/CNT system and the kobs

288

value was 0.2751 min-1, while 4-NP underwent the slowest oxidation with the rate of

289

0.0199 min-1. In order to quantitatively describe the effect of substituents on oxidation

290

rates of phenols by the PDS/CNT system, these kobs values were expressed as relative

291

rates (krel) using 4-ClP as the reference compound for normalization, as described by

292

eq (4):

= log

(4)

293

where k4-ClP was the observed pseudo-first-order rate for 4-ClP oxidation (i.e., 0.1046

294

min-1). This normalization approach was also used to describe the oxidation of

295

substituted phenols and anilines by manganese dioxide,42, 43 and it was considered as a

296

reasonable and effective means to compare kinetic data obtained in different ways or

297

under different experimental conditions. The resulting logkrel values and substituent

298

descriptor variables (i.e., Hammett constants σ, σ+ and σ−, and E1/2) were summarized

299

in Table S1.

300

For correlation analysis of phenols oxidation rates, Hammett constants (i.e., σ, σ+

301

and σ−) are usually employed as the substituent descriptor variables, which

302

quantitatively express the electron-donating (large negative value) or withdrawing

303

(large positive value) properties of substituents.44 Of the three σ scales, σ+ gave the

304

best correlation for phenols oxidation by the PDS/CNT system (Figure 5a), and eq (5)

305

represented the corresponding linear regression: 12


Page 12 of 34

Page 13 of 34


= 0.17(±0.11) − 1.03(±0.04) ∙ ! " = 0.885 (5) 306

Rates of these substituted phenols oxidation generally decreased as the Hammett

307

constant of ring substituents became more positive, and the obtained negative slope

308

was typical for electrophilic reactions.44 Figure 5

309 310

Alternatively, E1/2 values reflect the potential for the first one-electron oxidation

311

step, and this is supported by good agreement with standard one-electron potentials

312

for phenol and 4-methoxyphenol determined by pulse radiolysis.45 The values of E1/2

313

for substituted phenols or anilines have been largely measured and/or estimated by

314

Suatoni et al.,46 and they have been frequently used to establish QSARs in previous

315

studies.47-50 In this work, a satisfactory correlation of log krel values to E1/2 values was

316

also achieved, as shown in Figure 5b and eq (6).

= 1.96(±0.21) − 2.89(±0.15) ∙ ()/" ! " = 0.947 (6) 317

The strong correlation based on E1/2 values suggested that one-electron oxidation

318

might be the rate-determining step during phenols oxidation by the PDS/CNT

319

system.42, 43, 51

320

These relationships were very useful in predicting oxidation rates for phenolic

321

pollutants by the PDS/CNT system. For acetaminophen, a widely used analgesic and

322

antipyretic drug, the predicted reaction rate was 0.6420 min-1 by the Hammett

323

correlation (i.e., eq (5)) using the σ+ value of -0.60,44 which was very close to the

324

experimentally obtained value (0.6682 min-1) (Figure S11). The prediction by the E1/2

325

based correlation was not conducted due to the lack of E1/2 value of acetaminophen.

326

Moreover, the LFERs for substituted phenols oxidation by Co(II)/PMS process

327

(i.e., a typical SO4 − generating system52-54) were examined for comparison. As shown

328

in Figure S12, there were poor correlations between the pseudo-first-order oxidation

329

rates of these phenols and the used substituent descriptors (i.e., Hammett constants,

330

and E1/2), in contrast to the case of the nonradical PDS/CNT system (Figure 5).

331

Recently, Luo et al. have explored the influence of electron donating/withdrawing

332

effects of substituents on reaction kinetics of aromatic contaminants by SO4 −, and

•

•

13



333

poor correlations were also obtained.55 These findings could be explained by the fact

334

that SO4 − based oxidation involved different reaction pathways, such as electron

335

transfer, radical addition, and hydrogen abstraction, and the dominant channel was

336

possibly dependent on the structures of diverse compounds.

337

Formation of brominated polymeric products.

•

338

The potential formation of toxic brominated polymeric products (e.g.,

339

OH-PBDEs and OH-PBBs) from BrPs treated by the PDS/CNT system was examined

340

by HPLC/ESI-QqQMS. In this work, a novel and powerful PIS approach was used,

341

which could selectively pick out polar (electrospray-ionizable) bromine-containing

342

compounds from complex background matrices by setting PIS at m/z of 79 or 81. The

343

working principle for the PIS approach had been well described by Zhang et al.,56 and

344

it was briefly presented in the SI Text S2. Interestingly, for aqueous supernatant

345

samples, there were no discernible peaks in HPLC/ESI−QqQMS PIS chromatograms

346

other than parent compounds, and the chromatograms at m/z 79 of parent BrPs were

347

also presented for comparison (Figures S13-S15). Howbeit, the chromatograms of

348

solvent supernatant samples obtained by methanol extracting spent CNT exhibited

349

clear product peaks.

350

(i) 2-BrP. Figure 6 exemplified the HPLC/ESI-QqQMS PIS chromatogram at

351

m/z 79 of the solvent supernatant for 2-BrP, and the corresponding chromatogram at

352

m/z 81 was presented in Figure S16. As could be seen, the peaks in the

353

chromatograms obtained by the PIS of m/z 79 and 81 had the same chromatographic

354

retention times and intensities, consistent with the natural isotope abundance ratio of

355

79

356

bromine-containing products.

357

Br/81Br (i.e., 1:1).9,

56-59

This suggested that these peaks in pairs should be

Figure 6

358

There were five product peaks (I-V) at retention times of 28.45, 31.58, 33.97,

359

27.78, and 29.81 min in both chromatograms of m/z 79 and 81, respectively. Products

360

I-III had the same molecular ions of m/z 341/343 in the PIS of m/z 79, so they might

361

be isomeric dimers of 2-BrP, which had a molecular ion of m/z 171 in the PIS of m/z

362

79 (Figure S13). These products were likely to be formed by C−C and C−O coupling 14


Page 14 of 34

Page 15 of 34


363

between bromophenoxyl radicals generated from one-electron oxidation of 2-BrP

364

precursor by the PDS/CNT system. Due to the occurrence of phenoxyl radicals in

365

several resonance forms caused by the delocalization of the unpaired electron, diverse

366

products might appear during coupling processes.9, 50, 60-65 In theory, there were nine

367

possible combinations for four radicals of 2-BrP, assuming that all these radicals could

368

participate in C−O and C−C coupling reactions (Figure S17). Among them, seven

369

combinations could lead to the formation of dimers with molecular ions of m/z

370

341/343 in the PIS of m/z 79 (i.e., dimers containing 2 Br without the loss of Br). The

371

observed products I-III could be assigned to three of them.

372

Combinations involving 2-BrP radicals that Br was attached to the carbon with

373

the unpaired electron should produce dimers with the release of 1 or 2 Br. Indeed, one

374

monobrominated dimeric product (IV) appeared at 27.78 min with molecular ions of

375

m/z 263 in the PIS of m/z 79, and it was eluted faster than products I-III as expected.

376

Since dimeric product without Br of molecular ion of m/z 185 (i.e., two Br atoms

377

release; pathway (4) in Figure S17) could not be detected by the PIS approach even if

378

it was formed, a selective ion monitoring (SIM) mode at m/z 185 was performed. No

379

discernible peak was observed, indicating negligible formation of fully debrominated

380

dimers.

381

Product V at 29.81 min had the molecular ions of 340/342 in the PIS of m/z 79,

382

and the even number suggested that it should be a quinone-like compound possibly

383

formed from further oxidation of those dimeric products. Previous studies have

384

reported the possible formation pathways for the unique mass spectra of

385

halobenzoquinones (HBQs) shown in negative ESI-QqQMS, involving the reduction

386

of HBQs by accepting electron and/or proton under negative ESI.9, 58, 59, 66-71 It is

387

noteworthy that HBQs has been identified as a new class of disinfection byproducts in

388

drinking water and predicted to be highly cytotoxic and genotoxic by recent

389

studies.67-73

390

(ii) 3-BrP and 4-BrP. The formation of brominated dimeric products was also

391

observed from 3-BrP and 4-BrP in the PDS/CNT system, and their typical

392

HPLC/ESI-QqQMS PIS chromatograms at m/z 79 were shown in Figures 7 and 8, 15



393

respectively (the corresponding chromatograms at m/z 81 were shown in Figures S18

394

and S19). In the case of 3-BrP, four dimeric products (VI-IX) were detected at

395

retention time of 29.80, 32.34, 34.54, and 35.78 min, respectively, and all of them had

396

the molecular ions of m/z 341/343 in the PIS of m/z 79. Similar to 2-BrP, there were

397

four different resonance forms for 3-BrP radical in theory, and the C−O and C−C

398

coupling reactions of these radicals involved nine possible combinations (Figure S20).

399

Because the bromine atom occupied the meta position where the unpaired electron did

400

not occur, no Br release took place during 3-BrP radical coupling, consistent with the

401

observation by the HPLC/ESI-QqQMS PIS.

402

Figure 7

403

Figure 8

404

Comparatively, 4-BrP only possessed three free radicals, whose coupling

405

reactions would lead to five possible combinations (Figure S21). Two of them could

406

produce dimers without the loss of Br, and three could produce dimers with the

407

release of 1 or 2 Br. The HPLC/ESI−QqQMS PIS chromatograms showed that three

408

brominated dimeric products (X-XII) were produced from 4-BrP oxidation by the

409

PDS/CNT system (Figure 8). Products X and XI had the same molecular ions of m/z

410

341/343 in the PIS of m/z 79, and they were eluted at 32.86 and 35.70 min,

411

respectively. Based on the fact that the elution of dihydroxyl biphenyls was much

412

faster than their isomeric phenoxyphenols,9 product X was assigned as dibrominated

413

dihydroxyl biphenyl formed by ortho−ortho C−C couplings between two 4-BrP

414

radicals (i.e., pathway (3) in Figure S21), while product XI was suggested to be

415

dibrominated phenoxyphenol possibly formed by ortho C−O couplings of 4-BrP

416

radicals (i.e., pathway (1) in Figure S21). Product XII with the molecular ions of m/z

417

263 in the PIS of m/z 79 should be one monobrominated dimer, and it was eluted

418

faster (28.24 min) than dibrominated products (i.e., X and XI) as expected. The

419

HPLC/ESI-QqQMS SIM approach did not find out the fully debrominated product

420

(i.e., products with the molecular ions of m/z 185) either. In addition, it was noticed

421

that the number of experimentally detected products was always less than the

422

theoretical one for each BrP. Such results were also observed in other oxidative 16


Page 16 of 34

Page 17 of 34


423

transformation processes of BrPs, including the abiotic oxidation induced by synthetic

424

birnessite12 and Mn(VII),9 and several enzyme-catalyzed oxidation processes.15,

425

This discrepancy probably resulted from the difference in the rates of coupling

426

between various radicals of BrPs. For instance, the rate of coupling for the radical

427

having the bromine atom next to the unpaired electron should be much lower than that

428

for the radical with the unsubstituted carbon due to steric hindrance.60-62

16

429

Figure 9 exemplified the evolution of 4-BrP oxidation products (i.e., products

430

X-XII) in the PDS/CNT system. As shown, the concentrations of these polymeric

431

products rapidly reached to their maxima (at about 15 min) and then declined

432

gradually (until 90 min). This result suggested that brominated polymeric products

433

existing on CNT surface could be further oxidized by the reactive PDS-CNT

434

complexes. Figure 9

435

•

−

436

Transformation products of BrPs in SO4

based oxidation

437

processes: Comparison to the nonradical PDS/CNT system. •

438

The transformation products of BrPs by SO4 − were also explored by using the

439

Co(II)/PMS process, and the HPLC/ESI-QqQMS PIS chromatograms at m/z 79 of

440

samples containing 2-, 3-, or 4-BrP treated by SO4

441

S22-S24, respectively. The formation of monohydroxylated products (i.e., products S1,

442

S3, S4, and S7) were observed during the oxidation of these three BrPs by SO4 −, and

443

they had the same molecular ions of m/z 187/189 in the PIS of m/z 79 and were eluted

444

earlier than the parent compounds. Meanwhile, one brominated dimeric product that

445

had the molecular ions of m/z 341/343 in the PIS of m/z 79 was detected in the case

446

of 2-BrP or 4-BrP (products S2 and S8, respectively), and for 3-BrP, two brominated

447

dimeric products with the same molecular ions of m/z 341/343 in the PIS of m/z 79

448

(products S5 and S6) were found. These results further confirm that the oxidation

449

pathways of BrPs by SO4•− are different from those involved in the PDS/CNT system

450

(Figures 6-8).

451

Implications.

•−

were presented in Figures

•

17



Page 18 of 34

452

Previous studies have widely reported the oxidative transformation of BrPs to

453

produce brominated polymeric products (i.e., OH-PBDEs and OH-PBBs) in natural

454

environments and engineered processes,9, 12-16 which attracts considerable concern due

455

to their much higher toxicity to ecosystem and human health. This work provides

456

another case for the formation of these polymeric products from BrPs oxidation by the

457

PDS/CNT system. Their formation pathway possibly involves the generation of BrPs

458

radicals and their subsequent coupling. Fortunately, these undesired polymeric

459

products do not appear in aqueous solution, while they are found on CNT surface and

460

can undergo further transformation. It seems likely that these polymeric products are

461

directly generated on CNT surface where PDS is activated or they are formed in

462

solution followed by adsorption onto CNT surface. Given the powerful oxidizing

463

capacity of SO4 −, the conventional SO4 − generating systems may be efficient for

464

elimination/mineralization of BrPs as well as their transformation products, eventually

465

leading to the complete release of inorganic Br−.53 However, the conversion of Br− to

466

carcinogenic bromate by SO4 − has raised serious concerns in recent years.54,

467

Comparatively, our recent work has demonstrated that Br− cannot be oxidized by the

468

nonradical PDS/CNT system.23 So, this novel PDS/CNT oxidation technology shows

469

a great promise for oxidative treatment of BrPs with alleviation of toxic byproducts in

470

treated water.

471

Acknowledgments

472

•

•

•

74-76

This study was supported by the National Natural Science Foundation of China

473

(51578203),

474

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

475

Heilongjiang Province Postdoctoral Science Foundation (LBH-Q15057), and the

476

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

477

2016DX13).

478

Supporting Information

479 480

the

National

Key

Research

and

Development

Program

The additional texts, figures, and tables addressing supporting data. This material is available free of charge via the Internet at http://pubs.acs.org. 18


Page 19 of 34


481

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Environ. Sci. Technol. 2014, 48 (1), 615-623.

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(59) Gao, Y.; Pang, S.; Jiang, J.; Ma, J.; Zhou, Y.; Li, J.; Wang, L.; Lu, X.; Yuan, L. Transformation of

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flame retardant tetrabromobisphenol a by aqueous chlorine and the effect of humic acid. Environ. Sci.

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Technol. 2016, 50 (17), 9608-9618.

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(60) Dec, J.; Haider, K.; Bollag, J. M. Release of substituents from phenolic compounds during oxidative

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coupling reactions. Chemosphere 2003, 52 (3), 549-56.

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(61) Dec, J., Bollag, J. M. Dehalogenation of chlorinated phenols during oxidative coupling. Environ.

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Sci. Technol. 1994, 28 (3), 484-490.

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(62) Dec, J., Bollag, J. M. Effect of various factors on dehalogenation of chlorinated phenols and

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anilines during oxidative coupling. Environ. Sci. Technol. 1995, 29 (3), 657-663.

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(63) Huguet, M.; Deborde, M.; Papot, S.; Gallard, H. Oxidative decarboxylation of diclofenac by

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manganese oxide bed filter. Water Res. 2013, 47 (14), 5400-5408.

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(64) Jiang, J.; Pang, S.; Ma, J. Dechlorination of chlorophenols mediated by carbon nanotubes in the

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presence of oxygen. Carbon 2009, 47 (8), 2115-2117.

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(65) Jiang, J.; Pang, S.; Ma, J. Comment on “adsorption of hydroxyl- and amino-substituted aromatics to

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carbon nanotubes”. Environ. Sci. Technol. 2009, 43 (9), 3398-3399.

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dioxide in the oxidation of phenolic compounds by aqueous permanganate. Environ. Sci. Technol. 2015,

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49 (1), 520-528.

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chromatography–negative electrospray ionization mass spectrometry determination of twelve

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halobenzoquinones at ng/L levels in drinking water. Anal. Chem. 2013, 85 (9), 4520-4529.

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25



(a)

2-BrP 3-BrP 4-BrP Blank

40

8 6 4

2-

2-BrP 3-BrP 4-BrP FFA

30 20 10

2

0

0 0

678

(b)

50

[SO4 ] (µM)

[Organic substrate] (µM)

10

Page 26 of 34

10

20

30

40

0

10

Time (min)

20

30

40

Time (min)

679

Figure 1. Degradation of BrPs and FFA by the PDS/CNT system (a) and formation of SO42− from

680

PDS decomposition by CNT in the absence vs presence of BrPs (b). Experimental condition:

681

[PDS]0 = 500 µM, [CNT]0 = 50 mg/L, [2-BrP]0 = [3-BrP]0 = [4-BrP]0 = [FFA]0 = 10 µM, and pH 7.

26


Page 27 of 34


(a)

10

6

[2-BrP] (µM)

[2-BrP] (µM)

Blank NOM=1mgC/L NOM=2mgC/L NOM=5mgC/L

8

8

Blank -

With HCO3

4

-

With Cl 2

6 4 2 0

0 0

682

(b)

10

10

20

30

40

0

10

Time (min)

20

30

40

Time (min)

683

Figure 2. Effects of inorganic anions (i.e., 1 mM HCO3- or 1mM Cl-) (a) and NOM (0-5 mgC/L) (b)

684

on 2-BrP degradation by the PDS/CNT system. Experimental condition: [PDS]0 = 500 µM,

685

[CNT]0 = 50 mg/L, [2-BrP]0 = 10 µM, and pH 7.

27



pH=5 pH=7 pH=9

0.20

-1

kobs (min )

0.16 0.12 0.08 0.04 0.00 686

2-BrP

3-BrP

4-BrP

687

Figure 3. Effect of solution pH on BrPs oxidation rates by the PDS/CNT system. Experimental

688

condition: [PDS]0 = 500 µM, [CNT]0 = 50 mg/L, and [2-BrP]0 = [3-BrP]0 = [4-BrP]0 = 10 µM.

28


Page 28 of 34


(a) PDS=50µM PDS=200µM PDS=500µM

[2-BrP] (µM)

-1

6

0.15 0.10 0.05 0.00

4

0

200

400

600

PDS (µM)

2

CNT=50mg/L CNT=150mg/L

0.6

6 4

0.4 0.2 0.0 0

50

100

150

CNT (mg/L)

2

0

0 0

689

CNT=25mg/L CNT=100mg/L

8

0.20

kobs (min )

[2-BrP] (µM)

8

(b)

10

PDS=100µM PDS=400µM PDS=600µM

-1

10

kobs (min )

Page 29 of 34

10

20

30

40

0

10

Time (min)

20

30

40

Time (min)

690

Figure 4. Effects of PDS or CNT dosages on 2-BrP (10µΜ) degradation by the PDS/CNT system

691

at pH 7: (a) various PDS dosages (50-600 µM) with a fixed CNT dosage (50 mg/L); and (b)

692

various CNT dosages (25-150 mg/L) with a fixed PDS dosage (500 µM). Insets indicated the

693

observed pseudo-first-order rates for 2-BrP oxidation at different PDS or CNT dosages.

29



0.5

(a)

2 8

5

log krel

3 0.0

(b)

2 4

10

5 10

3

1

log krel

4

0.5

9

7

6

Page 30 of 34

1

0.0

9

7 6

12 -0.5

11

-0.5

11

13

13 -0.5

694

0.0

δ

+

0.5

12

1.0

0.4

0.6

E1/2 (volt)

0.8

1.0

695

Figure 5. Correlations of log krel for substituted phenols oxidation by the PDS/CNT system to the

696

Hammett σ+ constant (a) and to E1/2 (b). Experimental condition: [PDS]0 = 500 µM, [CNT]0 = 50

697

mg/L, [substituted phenol]0 = 10 µM, and pH 7.

30


Page 31 of 34


698 699

Figure 6. The HPLC/ESI−QqQMS PIS chromatogram at m/z 79 of a solvent supernatant sample

700

obtained in the 2-BrP/PDS/CNT system (a), and corresponding molecular ion mass spectra of the

701

chromatographic peaks (b-d). Experimental condition: [2-BrP]0 = 10 µM, [PDS]0 = 500 µM,

702

[CNT]0 = 50 mg/L, pH 7, and reaction time of 40 min.

31



703 704


705


706

chromatographic peaks (b and c). Experimental condition: [3-BrP]0 = 10 µM, [PDS]0 = 500 µM,

707


32


Page 32 of 34

Page 33 of 34


708 709


710


711

chromatographic peaks (b-d). Experimental condition: [4-BrP]0 = 10 µM, [PDS]0 = 500 µM,

712


33



5

Page 34 of 34

6

5x10

6x10

product X product XI product XII

5

4x10

6

5x10

6

Peak area

4x10 5

3x10

6

3x10 5

2x10

6

2x10 5

1x10

6

1x10

0

0 0

713

20

40

60

80

100

Time (min)

714

Figure 9. Evolution of 4-BrP oxidation products in the PDS/CNT system. Experimental condition:

715

[4-BrP]0 = 10 µM, [PDS]0 = 500 µM, [CNT]0 = 50 mg/L, and pH 7.

34


Oxidation Kinetics of Bromophenols by Nonradical ... - ACS Publications

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