Activation of Peroxymonosulfate by Benzoquinone: A Novel

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Activation of Peroxymonosulfate by Benzoquinone: A Novel Non-Radical Oxidation Process Yang Zhou, Jin Jiang, Yuan Gao, Jun Ma, Su-yan Pang, Juan Li, Xue-Ting Lu, and Li-Peng Yuan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03595 • Publication Date (Web): 09 Oct 2015 Downloaded from http://pubs.acs.org on October 14, 2015

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Environmental Science & Technology

Activation of Peroxymonosulfate by Benzoquinone: A Novel Non-Radical Oxidation Process Yang Zhou†, Jin Jiang*,†, Yuan Gao†, Jun Ma*,†, Su-Yan Pang‡, Juan Li†, Xue-Ting Lu‡, Li-Peng Yuan‡ †

State Key Laboratory of Urban Water Resource and Environment, School of Municipal and

Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China ‡

Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang

Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China

*Corresponding Authors: Prof. Jin Jiang and Prof. Jun Ma (J.J.) Phone: 86−451−86283010; fax: 86 − 451−86283010; E-mail: [email protected]. (J.M.) Phone: 86 −451− 86283010; fax: 86−451− 86283010; E-mail: [email protected].

1

ACS Paragon Plus Environment


1

Abstract

2

The reactions between peroxymonosulfate (PMS) and quinones were investigated for the

3

first time in this work, where benzoquinone (BQ) was selected as a model quinone. It was

4

demonstrated that BQ could efficiently activate PMS for the degradation of sulfamethoxazole

5

(SMX; a frequently detected antibiotic in the environments), and the degradation rate

6

increased with solution pH from 7 to 10. Interestingly, quenching studies suggested that

7

neither hydroxyl radical (•OH) nor sulfate radical (SO4•-) was produced therein. Instead, the

8

generation of singlet oxygen (1O2) was proved by using two chemical probes (i.e.,

9

2,2,6,6-tetramethyl-4-piperidinol and 9,10-diphenylanthracene) with the appearance of 1O2

10

indicative products detected by electron paramagnetic resonance spectrometry and liquid

11

chromatography mass spectrometry, respectively. A catalytic mechanism was proposed

12

involving the formation of a dioxirane intermediate between PMS and BQ and the subsequent

13

decomposition of this intermediate into 1O2. Accordingly, a kinetic model was developed,

14

and it well described the experimental observation that the pH-dependent decomposition rate

15

of PMS was first order with respect to BQ. These findings have important implications for

16

the development of novel non-radical oxidation processes based on PMS, because 1O2 as a

17

moderately reactive electrophile may suffer less interference from background organic

18

matters compared with non-selective •OH and SO4•-.

19

2


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Page 3 of 28


20

Introduction

21

Peroxomonosulfate (PMS), hydrogen peroxide (H2O2), and peroxydisulfate (PDS) are

22

considered as inexpensive oxidants for the remediation of contaminated water or soil.1-3

23

Advanced oxidation processes (AOPs) based on these three common peroxides have received

24

much attention because of high standard redox potentials of sulfate radical (SO4•-, 2.5~3.1V) 4

25

and hydroxyl radical (•OH, 1.9~2.7V)

26

contaminants, such as pharmaceuticals, odor-causing compounds, and pesticides.6-8

27

Transition metal oxides, energy (e.g. heat, ultraviolet, and ultrasound), and base are

28

commonly used to activate PMS, H2O2, and PDS to generate SO4•- and •OH, and the

29

associated catalytic mechanisms have been well studied.9-13

5

. These radicals can destruct many organic

In addition to these catalytic methods, it has been reported that PDS and H2O2 can also

30

activated

by

organic

be

32

2,4,4'-trichlorobiphenyl (PCB28) could be efficiently degraded by PDS in the presence of

33

1,4-benzoquinone

34

2-chloro-1,4-benzoquinone (CBQ). This process can be described as a semiquinone

35

radical-dependent Fenton-like reaction (taking BQ for example, as shown in reactions 1 and

36

2): the comproportionation between BQ and its self-condensation or decomposition product

37

hydroquinone (HQ) can generate benzosemiquinone (BSQ), leading to the decomposition of

38

PDS into SO4•- .

(BQ)

quinones.

as

well

O

For

as

OH HQ

et

and

OH BSQ O +

+

+

(2)

O

OH BSQ

Zhu

that

(1)

+

Interestingly,

(MBQ)

2

O

40

reported

O

+ O BQ

Fang

2-methyl-1,4-benzoquinone

OH

39

41

instance,

et al.14

31

BQ

al.15

found

that

halogenated

3


quinones

(e.g.,


Page 4 of 28

42

tetrachloro-1,4-benzoquinone (TCBQ), tetrabromo-1,4-benzoquinone (TBBQ), and tetra

43

fluoro-1,4-benzoquinone (TFBQ)) could activate H2O2 to produce •OH but those

44

nonhalogenated

45

tetramethyl-1,4-benzoquinone (TMBQ)) couldn’t. Further, these authors observed that TCBQ

46

itself rather than its corresponding semiquinone radical was essential for •OH production.16

47

So, the authors suggested a mechanism as following: the nucleophilic attack of TCBQ by

48

H2O2 formed a trichlorohydroperoxyl-1,4-benzoquinone (TrCBQ-OOH) intermediate, which

49

decomposed

50

trichlorohydroxy-1,4-benzoquinone (TrCBQ-OH).16 However, little is known about the

51

reaction between PMS and quinones so far.

quinones (e.g., BQ,

homolytically

to

2,6-dimethyl-1,4-benzoquinone

produce

OH

and

a

(DMBQ),

major

and

product

52

Quinones are ubiquitous in water, soil, and atmosphere,17-20 and they are potent redox

53

active compounds. Many studies have reported that quinones can participate in various

54

chemical and biochemical processes.21-23 For instance, Chen et al.24 found that quinone

55

intermediates could enhance Fenton oxidation, where BQ as an electron-transfer catalysts

56

greatly accelerated the conversion of Fe(III) to Fe(II). Jiang et al.25 found that semiquinone

57

radical produced during microbial or chemical reduction of a humic substance model quinone

58

(AQDS, 9,10-anthraquinone-2,6-disulfonic acid) could oxidize arsenite to arsenate, thus

59

decreasing arsenite toxicity and mobility.

60

In this work, the reactions between PMS and quinones were investigated for the first

61

time, where BQ was chosen as a model quinone. First, the feasibility of BQ activating PMS

62

to degrade a sulfonamide antibacterial, sulfamethoxazole (SMX), which has been frequently

63

detected in the environments, under various experimental conditions was examined. Then,

64

primary oxidizing species produced in such reactions were identified by chemical quenching

65

and trapping methods. Further, the involved mechanisms were tentatively proposed and the

66

kinetic model was developed accordingly.26

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67 68

Experimental Section Materials.

PMS

(available

Oxone®

as

(KHSO5·0.5KHSO4·0.5K2SO4)),

69

1,4-benzoquinone (BQ, 98%), sulfamethoxazole (SMX, 99%), atrazine (ATZ, 99%), benzoic

70

acid (BA, 99.5%), 2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid diammonium

71

(ABTS, 99%), sodium azide (NaN3, 99.5%), and furfuryl alcohol (FFA, 98%) were

72

purchased from

73

9,10-diphenylanthracene (DPA, 99%) were purchased from J&K Scientific Ltd and their

74

chemical structures were shown in SI Table S1. Methanol, ethanol, tert-butanol, acetonitrile,

75

and acetone of HPLC grade were purchased from Tedia and Ficher. A purified commercial

76

soil-humic acid which had been characterized previously was used in this study.27 Other

77

chemicals of analytical grade or better were purchased from Sinopharm Chemical Reagent

78

Co., Ltd. Stock solutions were always prepared in ultrapure water produced by a Milli-Q

79

Biocel ultrapure water system. Due to the limited aqueous solubility, TMP stock solutions

80

were made in acetonitrile and DPA stock solutions were made in acetonitrile:chloroform

81

mixture (1:1, v:v).28

Sigma-Aldrich.

2,2,6,6-tetramethyl-4-piperidinol

(TMP,

99%)

and

82

Experimental Procedure. All experiments were conducted in brown triangular flask on

83

a reciprocating shaker at 25±1 oC in the dark. Reactions were initiated by simultaneously

84

adding BQ (1-300 µM) and PMS (0.44 mM) into pH-buffered solutions (20mM sodium

85

borate; pH 7-10) containing a target compound [e.g., SMX (8 µM), ATZ (1 µM) or BA (8

86

µM)] with or without a quenching reagent [e.g., methanol (0.22 M), ethanol (0.22 M),

87

tert-butanol (0.22 M), NaN3 (30-400 µM), or FFA (2-4 mM)]. The exact experimental

88

conditions were also clearly shown in the figure captions. ATZ and BA were selected as

89

probe compounds for •OH and SO4•- in this work. Samples were periodically withdrawn and

90

quenched with sodium thiosulfate before analyzed by high performance liquid

91

chromatography (HPLC) and UV detection.29 It was demonstrated that sodium thiosulfate

5



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92

had no interference with the analysis of SMX by HPLC/UV in the preliminary study. The

93

concentration of PMS was measured by an ABTS colorimetric method.30 All the kinetic

94

experiments were conducted in duplicates or triplicates. The averaged data and standard

95

deviation were presented.

96

Chemical Detection of Singlet Oxygen. In order to verify the generation of 1O2 in the

97

reaction between PMS and BQ, TMP was chosen as a spin-trapping reagent for 1O2.31 The

98

pH-buffered solutions (pH 10) containing PMS (0.44 mM), BQ (25 µM), and TMP (1 mM)

99

were allowed to react for 60 min during which 1O2 formed could oxidize TMP to

100

2,2,6,6-tetramethyl-4-piperidinol-N-oxyl radical (TMPN). Then, the resulting solutions were

101

subjected to the detection by electron paramagnetic resonance (EPR) spectrometry.

102

In addition, DPA was also used as a chemical trapping reagent to confirm the generation

103

of 1O2. This approach was based on the fact that the rapid and specific reaction between DPA

104

and 1O2 (kr=1.3×106 M-1s-1) forms a stable DPA endoperoxide (DPAO2).

105

pH-buffered solutions (pH 10) containing DPA (24 µM) were treated by PMS with varying

106

doses (300-900 µM) in the absence or presence of BQ (25 µM) for 60min. The resulting

107

solutions

108

chromatography/atmospheric

109

spectrometry (HPLC/APCI−QqQMS) at multiple reaction monitoring (MRM) mode.

were

analyzed

for

DPA

pressure

and

DPAO2

chemical

by

high

ionization-triple

28, 32, 33

A series of

performance

liquid

quadrupole

mass

110

Analytical Methods. A Waters 1525 HPLC equipped with a Waters Symmetry C18

111

column (4.6×150 mm, 5µm particle size), a Waters 717 autosampler, and a Waters 2487 dual

112

λ UV-vis detector was used for the HPLC/UV analysis. A Varian Carry 300 UV-Vis

113

spectrometer was used for ABTS developed color measurements (i.e., ABTS•+). Oxygen

114

generation was measured in an airtight triangular flask by a portable hand-held dissolved

115

oxygen (DO) meter (HACH, HQ30D). A Bruker A200 spectrometer was used for EPR

6


Page 7 of 28


116

analysis under the following condition: temperature=293 K, microwave frequency=9.833

117

GHz, microwave power=2.2 mW, and modulation amplitude=0.1 mT.

118

An Agilent 1260 HPLC was directly coupled to an AB SCIEX QTrap 5500 MS with an

119

atmospheric pressure chemical ionization (APCI) source in the positive ion mode for the

120

HPLC/APCI−QqQMS analysis. A Poroshell 120 EC-C18 column (3.0×50 mm, 2.7 µm

121

particle size) was used for separation. The isocratic mobile phase consisted of

122

acetonitrile/water (v/v, 80/20) at a flow rate of 0.5 mL/min. To avoid the possible

123

contamination of mass spectrometer, a switching valve was used to divert the HPLC fluid to

124

the waste in a first few minutes as well as in a last few minutes.34 The MS parameters were

125

optimized and set as follows: ionspray voltage, +5500 V; source temperature, 450 °C; ion

126

source gas 1 and 2, 50 arbitrary units; curtain gas, 35 arbitrary units; declustering potential

127

(DP), 90 V; entrance potential (EP), 10 V; collision energy (CE), 24 V; collision cell exit

128

potential (CXP), 18 V; MRM ion pair, 363/330 28.

129

Results and Discussion

130

Degradation Efficiency of SMX in PMS/BQ Process. The oxidation kinetics of SMX

131

by PMS with and without BQ over a wide pH range of 7-10 were shown in Figure 1. As can

132

be seen, in the absence of BQ, the degradation of SMX by PMS was negligible within the

133

time scale investigated. Comparatively, SMX could be appreciably degraded by PMS in the

134

presence of BQ, and the degradation rate increased with the increase of BQ concentrations.

135

Also, the degradation rate showed a pH dependency and increased gradually from pH 7 to 10.

136

For instance, when BQ was 10 µM, the degradation of SMX in three minute increased from

137

5% to 86% with pH from 7 to 10. As for pH 10, with the increase of BQ concentration from 2

138

to 10 µM, degradation of SMX in three minute increased from 40% to 86%. Additionally, the

139

first-order rate constants derived from Figure 1 were listed in SI Table S2. These rate

140

constants also suggest that BQ has a significant effect on the degradation of SMX by PMS.

7



141

For instance, when BQ was 10µM, with increasing pH from 8 to 10, the degradation rate

142

increased from 0.0434 to 0.6786 min-1. As for pH 10, SMX degradation rate increased from

143

0.1507 to 0.6786 min-1 with the addition of 2 to 10 µM BQ. These results above suggest that

144

BQ can significantly enhance the degradation of SMX by PMS and this process is dependent

145

on pH (see the following sections for details about discussion on this pH dependency).

146

In control experiments with BQ alone, the loss of SMX was always negligible (data

147

were not shown). This suggested that (i) the relatively strong oxidant BSQ, which appeared in

148

aqueous BQ solutions, contributed negligibly to SMX degradation, and (ii) the nucleophilic

149

addition reactions between sulfonamide antibiotics and quinone moieties widely reported in

150

the literatures were insignificant in this work due to the slow rate and limited time scale

151

investigated (the nucleophilic addition reactions usually needs several weeks to months). 25, 35,

152 153

36

In addition, no difference between carbonate buffer and borate buffer on SMX degradation

as well as on PMS decomposition under similar conditions was observed.

154

(Figure 1)

155

After the reactions, the residual contents of PMS (relative to the initial ones) were

156

determined and were shown in SI Figure S1. As can be seen, the decomposition of PMS

157

without BQ was negligible. However, with increasing concentrations of BQ, the

158

decomposition of PMS enhanced gradually. The pH dependent decomposition of PMS was

159

also observed, which was consistent with the trend of SMX degradation (Figure 1). For

160

instance, when BQ was 10 µM, the decomposition of PMS increased from 3% to 35% with

161

increasing pH from 7 to 10. As for pH 10, with increasing the concentration of BQ from 2 to

162

10 µM, PMS decomposition increased from 10% to 35%.

163

Identification of Oxidizing Species by Specific Quenchers. The results above suggest

164

that reactive oxidizing species is produced in the reaction between PMS and BQ, leading to

165

the enhanced degradation of SMX. Generally, •OH or SO4•- is considered to be the oxidizing

8


Page 8 of 28

Page 9 of 28


166

species in oxidation processes involving PMS. Both •OH and SO4•- show high reactivity

167

towards SMX, and the rate constants are 7.89×109 M-1s-1 and 1.17×1010 M-1s-1, respectively.6

168

For the convenience, experiments to identify reactive oxidizing species were conducted at pH

169

10 due to their relatively fast production therein.

170

To verify the generation of •OH or SO4•-, the effects of radical quenchers (i.e., methanol,

171

ethanol, and tert-butanol) were investigated. The second-order rate constants for these radical

172

quenchers with •OH and SO4•- were shown in SI Table S3. If •OH or SO4•- was the primary

173

oxidizing species, alcohol scavengers in great excess (0.22 M) would completely

174

out-compete SMX (8 µM) and thus significantly inhibit SMX degradation. Surprisingly, they

175

had no effects on the degradation of SMX (Figure 2a), suggesting that neither •OH nor SO4•-

176

was produced in the reaction. To further confirm this, ATZ and BA, the widely used probe

177

compounds for •OH and SO4•- were tested. As can be seen (SI Figure S2), the combination of

178

PMS and BQ could not degrade ATZ or BA either even when the reaction time was

179

prolonged from 12 min to 120 min (Figure 1 vs. Figure S2), where PMS was fully

180

decomposed. This provides another supporting evidence that neither •OH nor SO4•- was

181

produced in the reaction between PMS and BQ.

182

(Figure 2)

183

It is well known that the self-decomposition of PMS can slowly generate 1O2 (reaction

184

3), and the rate constant k1 of this reaction is about 0.2 M-1s-1. 37,38 k1 HSO5− + SO 52 −  → HSO 4− + SO 24− + 1 O 2

185 186 187

(3)

The presence of BQ can greatly accelerate the decomposition of PMS. So, it is likely that 1

O2 is produced in the reaction between PMS and BQ and thus results in the enhanced

188

degradation of SMX. 1O2 as a selective oxidizing species shows high reactivity towards

189

electron-rich compounds (e.g., phenols, sulfides, and amines) but negligible reactivity

190

towards saturated alcohols (e.g. methanol, ethanol, and tert-butanol).39, 40 NaN3 and FFA are 9



Page 10 of 28

191

reported to be efficient quenchers for 1O2 with the rate constants of 1×109 and 1.2×108 M-ls-l,

192

respectively.41 So their effects were examined to verify the generation of 1O2.

193

As shown in Figure 2b and SI Figure S3, the degradation rate of SMX markedly slowed

194

down by the addition of NaN3 or FFA as expected. For instance, SMX was completely

195

degraded in 30 min without NaN3, while in the presence of 30 and 100 µM NaN3, only 75%

196

and 32% was degraded (shown in Figure 2b). It should be noted that NaN3 and FFA are also

197

efficient scavengers for SO4•- and •OH (rate constants were shown in SI Table S3). For

198

instance, the scavenging capacities (i.e., kc value) for SO4•- and •OH of NaN3 at 100 µM are

199

calculated to be 2.52×105 and 1.2×106 s-1, respectively, and they are about one or two orders

200

of magnitude lower than those of 0.22 M methanol (about 5.5×106 and 2.13×108 s-1

201

respectively). If SO4•- or •OH is the dominant oxidizing species, a more pronounced

202

inhibitory effect of methanol than NaN3 would be noted. However, a contrasting effect was

203

observed (Figure 2a vs. Figure 2b). The comparison of the inhibitory effects of methanol vs.

204

NaN3 further confirms that neither SO4•- nor •OH is generated while 1O2 is likely produced

205

in the reaction between PMS and BQ.

206

Chemical Detection of 1O2. To further confirm the generation of 1O2 in BQ/PMS system,

207

EPR spectroscopy was used with TMP as a spin trap agent. TMP is generally considered as

208

a good probe for 1O2, because it can readily react with 1O2 to form a stable radical TMPN.42,

209

43

TMPN shows a typical three-line EPR spectrum with equal intensities (aN=16.9 G,

210

g=2.0054).44 By using this approach, the production of

211

suspensions of derivatized C60 has been demonstrated by Lee et al.

212

three-line EPR spectrum supporting the appearance of TMPN was observed in BQ/PMS

213

system as shown in Figure 3a. Comparatively, a weak signal of TMPN was detected in PMS

214

alone, and this might be due to the generation of 1O2 from the self-decomposition of PMS

215

(reaction 3).

10


1

O2 in irradiated aqueous 45

In this work, a

Page 11 of 28


(Figure 3)

216 217

In addition, 1O2 was detected by an alternative HPLC/APCI−QqQMS technique, which

218

was based on the fact that 1O2 could react with a chemical probe DPA to generate the

219

indicative endoperoxide (DPAO2) (reaction 4). By using this method, Miyamoto et al.28, 46

220

successfully verified the generation of 1O2 in the reaction of lipid hydroperoxides with ceric

221

ion as well as in the reaction of linoleic acid hydroperoxide with peroxynitrite. Ph

Ph +

222

O

1O 2

O

(4)

Ph

Ph DPA

DPAO2

223

As shown in Figure 3b, the chromatographic peak of DPAO2 appeared in the reaction

224

between PMS and BQ, and the intensity of DPAO2 gradually increased with increasing the

225

concentrations of PMS (from 300 to 900 µM) in the presence of 25 µM BQ. The peak of

226

DPAO2 was also observed in blank experiment with DPA alone, and this may be due to the

227

impurity of the commercial DPA chemical. In the presence of 900 µM PMS, the peak

228

intensity of DPAO2 was slightly higher than that in DPA blank experiment. This may be

229

attributed to the slow production of 1O2 from the self-decomposition of PMS, which is

230

consistent with the result obtained by EPR. Also, the effect of SMX on the DPAO2 signals

231

was examined. It was found that SMX could greatly decreased the DPAO2 signals by

232

competing for 1O2 formed in the reaction between PMS and BQ. For instance, in the presence

233

of SMX (0.3 mM) the intensity of DPAO2 in the reaction of PMS (300 µM) with BQ (25 µM)

234

was similar to that in DPA blank experiment (data were not shown for clarity). These results

235

further confirm the involvement of 1O2 in the degradation of SMX by PMS with BQ (Figure

236

1).

237

Stoichiometric Evolution of O2. These results suggest that 1O2 is produced in the reaction

238

between PMS and BQ. Once formed, 1O2 will decay rapidly to triplet oxygen (3O2). So, the

11



Page 12 of 28

239

increase of solution DO level will be an indirect evidence for the generation of 1O2. The yield

240

of O2 ( YO ) in this reaction can be described as eq 5 2

YO =

241

2

[O2 ]t -[O2 ]0 [PMS]0 -[PMS]t

(5)

242

where [O2]0 is the initial concentration of O2, [O2]t is the concentration of O2 at time t,

243

[PMS]0 is the initial concentration of PMS, and [PMS]t is the concentration of PMS at time t.

244

As shown in SI Figure S4, the plot of [O2 ]t -[O2 ]0 vs. [PMS]0 -[PMS]t at pH 8, 9, and 10

245

was found to be linear, and the slope was 0.50. This suggests that the formation of one mole

246

of O2 requires two moles of PMS in its reaction with BQ.

247

Proposed Mechanism of PMS Activation by BQ for 1O2 Production. It is well known

248

that PMS can be catalyzed by ketones to produce 1O2. In a pioneer work, Montgomery47

249

found that cyclohexanone significantly enhanced the decomposition of PMS to 1O2 in

250

alkaline solutions, where the involvement of a dioxirane intermediate was proposed. Edwards

251

et al.48 further confirmed the generation of the dioxirane intermediates in the reaction

252

between PMS and ketones (e.g., acetone) by

253

and Brauer

254

ketone-catalyzed decomposition of PMS by direct measurement of monomol light emission

255

in the near-infrared region (λ=1270 nm).

49

18

O-labeling and kinetic studies. Later, Lange

provided spectroscopic evidence for the generation of

1

O2 from

256

In this work, BQ can be considered as a ketone containing two carbonyl groups, thus a

257

similar pathway for PMS activation may occur (Scheme 1). The first step is the nucleophilic

258

addition of PMS (i.e., HSO5-) to the carbonyl group of BQ, i.e., two molecules of HSO5-

259

attack the carbonyl carbon atoms to form a peroxide adduct intermediate I (reaction 7). The

260

conjugate base of I (i.e., intermediate II) further decomposes to a dioxirane intermediate III

261

with the release of the sulfate moiety (reaction 9) via intramolecular nucleophilic

262

displacement of alkoxide oxygen at the O-O bond.48 According to the study of Edwards et

12


Page 13 of 28


263

al.,50 the intramolecular nucleophilic displacement reaction of intermediate II (reaction 9) is

264

rate-limiting step. Then intermediate III will experience nucleophilic attack by two molecules

265

of ionized PMS ions ( SO 52− ) to produce 1O2 and reform BQ (reaction 10). In the proposed

266

mechanism, the formation of one molecule 1O2 requires one molecule of HSO5- as well as one

267

molecule of SO52-, i.e., the yield of O2 is expected to be 0.5. This is in good agreement with

268

the experimentally obtained value (shown in Figure S4).

269 270

Scheme 1. Proposed Mechanism for the Generation of 1O2 from PMS Activation by BQ

271

Ka O

(6)

OH O O SO3-

272 O O SO3OH

O

(7)

I O-

OH O O SO3-

273

O O SO3-

O O SO3OH

O-

I

II OO O SO3-

k4

O O SO3-

O

O

O

O

slow

274 O-

(9)

O O SO3-

II O

(8)

III O

O

k5

fast

275

(10) O

O

O

III 276

Kinetics for BQ-catalyzed Decomposition of PMS. By assuming that reactions 7, 8,

277

and 10 are fast while reaction 9 is the rate-determining step,49 the rate law of PMS

278

decomposition can be described as eq 11 by using the steady-state approach regarding

279

intermediates I, II, and III (see SI Text S1 for details).

13



280

Page 14 of 28

K 2d [ 1 O 2 ] d [PMS]T [BQ][PMS]T =− = 4 K 2 K 3k4 + w [H ] + K a dt dt

(11)

281

where [PMS]T (=[HSO5-]t + [SO52-]t) is the total concentration of PMS at time t, [BQ] is the

282

concentration of BQ, K2 (M-1) and K3 (M-1) are the equilibrium constants for eqs 7 and 8,

283

respectively, k4 (s-1) is the rate constant for rate-determining step (reaction 9), Kw (=10-14)

284

represents the ionic product of water (i.e., [H+][OH-]), and Ka (=3.98×10-10)

285

ionization constant of PMS. When defining F =

286

s-1) for the decomposition of PMS could be described by eq 12

is the

Kw , the apparent rate constant (kobs, [H ] + K a +

kobs = 4 K 2 K 3k4 F [BQ]

287

38

(12)

288

According to the study of Lange and Brauer,49 K2K3k4 (i.e., M-2s-1) was the rate constant (kDI)

289

for the formation of dioxirane intermediate III.

290 291

To further confirm the proposed mechanism described above, the effects of BQ concentration and pH on the decomposition of PMS by BQ were evaluated.

292

1) BQ concentration. Figure 4a showed the decomposition of PMS in the presence of

293

different concentrations of BQ at pH 10. As can be seen, the rate increased with the increase

294

of BQ concentration. However, it should be noted that the loss of PMS slightly deviated from

295

the first-order kinetics and decreased as the reaction progressed. Such kinetics were also

296

reported in cyclohexanone-catalyzed decomposition of PMS.47-49 This phenomenon may be

297

attributed to side reactions occurred in PMS/BQ system. For instance, intermediate I that

298

formed in the reaction between PMS and ketone can undergo Baeyer-Villiger oxidation to

299

produce esters (reaction 13).

300

can be destroyed by 1O2 (reaction 14).51-53 In addition, quinones are susceptible to hydrolysis

301

especially in alkaline solution (reaction 15).54-56

302

48, 49

Also, quinones (e.g., BQ, tocopherols, and hydroquinones)

BQ + 2HSO5−

k2 k-2

BV I  → product

14


(13)

Page 15 of 28


BQ + 1 O 2 → product

303

hydrolysis BQ  → product

304

(14) (15)

305

The initial rate was used to determine the rate constant kobs (i.e., dashed lines in Figure 4a).

306

As shown in Figure 4b, the plot of kobs vs. [BQ] resulted in a straight line, demonstrating that

307

the decomposition rate of PMS was first-order with respect to BQ. Similar finding was also

308

observed at pH 9 (data were not shown). These results are in good agreement with the

309

proposed kinetics (eq 12).

310

(Figure 4)

311

2) Effects of pH. The reactions of PMS with BQ were conducted at various pH with a

312

constant concentration of BQ, and apparent rate constant kobs for PMS decomposition and

313

factor F at each pH were calculated. According to eq 11, it can be clearly seen that with

314

increasing pH (i.e., decreasing the concentration of H+), the decomposition rate of PMS will

315

increase. This is consistent with the experimentally observed trend of pH-affected

316

decomposition of PMS (SI Figure S5a). The plot of kobs vs. F resulted in a straight line (SI

317

Figure S5b), suggesting that the decomposition rate of PMS was first order with respect to F

318

as predicted by eq 12. The pH-dependence of BQ-catalyzed decomposition of PMS (i.e.,

319

generation of 1O2 according to eq 11) well explained the pH-dependence of SMX degradation

320

shown in Figure 1. So, pH adjustment may be a good option to enhance reaction rates if

321

necessary.

322

Important Role of Dioxirane Intermediate III. The results obtained above suggest that

323

the reaction between PMS and BQ undergoes a dioxirane intermediate pathway rather than a

324

semiquinone radical-dependent Fenton-like mechanism. Dioxiranes are commonly used as

325

mild oxidants for organic synthesis.57,

326

substrates through oxygen transfer processes including epoxidations (e.g., alkene),

327

carbon-hydrogen bond insertions (e.g., aldhydes), and lone-pair oxidations (e.g., amines).59

58

They can appreciably oxidize many organic

15



Page 16 of 28

328

The involvement of dioxirane may reasonably explain the finding that 1O2 quenchers can’t

329

completely suppress the degradation of SMX (as shown in Figure 2b and SI Figure S3). For

330

example, no further inhibition was observed when the concentration of NaN3 increased from

331

100 to 400 µM (Figure 2b)

332

According to eq 12, the rate constant for the formation of dioxirane intermediate III (i.e., kobs ) for BQ was derived from Figure 4b. Then kDI values in the cases of 4 F [BQ]

333

kDI = K 2 K 3 k4 =

334

BQ as well as seven ketones (i.e., acetone, 2-acetylpyridine, di-2-pyridyl ketone,

335

fluoroacetone,

336

comparatively shown in Table 1. As can be seen, kDI for BQ (1.16×106 M-2s-1) was 1~3 orders

337

of magnitude greater than those for other ketones. This marked difference may be attributed

338

to the cyclic structure of BQ and its strong electrophilicity resulting from two C=C bonds and

339

two carbonyl groups. This is in good agreement with the findings of Lange and Brauer 49 that

340

(i) kDI increased with increasing the electrophilicity of the ketones, and (ii) kDI was strongly

341

ring-size dependent as cyclic ketones showed much higher rates.

342

Environmental implications

1,1,1-trifluoroacetone,

cycloheptanone,

and

cyclohexanone)49

are

343

This study has demonstrated for the first time that BQ can efficiently activate PMS for

344

the degradation of SMX via a novel non-radical mechanism, where reactive 1O2 was involved.

345

As a moderately reactive electrophile, 1O2 can effectively oxidize a variety of contaminants

346

even in the presence of background organic matters, where significant interference is

347

expected for non-selective •OH and SO4•-.7, 60 So far, the explicit pathway for 1O2 with SMX

348

is unclear. Further studies are needed to examine the transformation products of SMX by 1O2

349

and compare them with well documented •OH and/or SO4•--derived products.6

350

Our findings may have important implications for the development of heterogeneous

351

catalytic PMS oxidation processes by quinone-based materials (e.g., quinone loaded

352

carbons)61, 62 for selective contaminant remediation or bacterial inactivation,60, 63 as well as 16


Page 17 of 28


353

for the potential application of in situ chemical oxidation using PMS for the remediation of

354

soils and sediment, where quinone-like groups containing natural organic matters (NOM) are

355

ubiquitous.

356

in soils and groundwater,66 and the quinone group (C=O) content in HA is generally within

357

the range of 1-4 mmol/g HA.67,

358

environmental relevant concentration could appreciably enhance the degradation of SMX by

359

PMS at near neutral pH (as shown in Figure S6). Since the chemical structures as well as the

360

properties of HA are complex, the reactions of PMS with diverse sources of HA deserve

361

further studies.

362

Acknowledgments

363

This work was financially supported by the National Science & Technology Pillar Program,

364

China (2012BAC05B02), the National Natural Science Foundation of China (51178134 &

365

51378141), the Funds of the State Key Laboratory of Urban Water Resource and

366

Environment (HIT, 2013DX05), the Foundation for the Author of National Excellent

367

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

368

Central Universities of China (AUGA5710056314). The authors greatly thank Dr. Jimin Shen

369

for his help with EPR operation.

370

Supporting Information

371

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

372

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

373

Nomenclature

64, 65

The typical concentration of humic acid (HA) ranges from 1 to 50mg C L-1

68

PMS

peroxymonosulfate

BQ

benzoquinone

BSQ

benzosemiquinone

SMX

sulfamethoxazole

Preliminary experiments suggested that HA at

17



ATZ

atrazine

BA

benzoic acid

FFA

furfuryl alcohol

TMP

2,2,6,6-tetramethyl-4-piperidinol

DPA

9,10-diphenylanthracene

HA

humic acid

374 375

18


Page 18 of 28

Page 19 of 28


376

References:

377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418

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540

22


Page 22 of 28

Page 23 of 28

Figure and Table Captions (b) 1.0

0.8

0.8

0.6

0.6 C/C0

(a) 1.0

C/C0

541


BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ BQ=50 µΜ BQ=100 µΜ BQ=300 µΜ

0.4 0.2 0.0 0

2

4

BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ

0.2

pH=7

0.0 6

8

10

12

0

2

Reaction time(min)

542

6

8

10

12

(d) 1.0 pH=9

0.8

pH=10

0.8

BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ

0.6 C/C0

0.6 C/C0

4

pH=8

Reaction time(min)

(c) 1.0

0.4 BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ

0.2 0.0 0

543

0.4

2

4

0.4 0.2 0.0

6

8

10

12

0

2

Reaction time(min)

4

6

8

10

12

Reaction time(min)

544

Figure 1. Effect of BQ on SMX degradation by PMS. (a) pH 7; (b) pH 8; (c) pH 9; (d) pH 10.

545

Experimental conditions: [PMS]0=0.44 mM, [SMX]0=8 µM, [BQ]0=2-300 µM at pH 7 and

546

2-10 µM at pH 8-10, 20 mM borate buffer, and T =25 °C.

547

23



1.0

1.0

a

0.8

0.6 scavenger free control methanol=0.22 M ethanol=0.22 M ter-butanol=0.22 M

0.4 0.2

C/C0

C/C0

b

0.8

0.6

0.4 0.2

0.0

scavenger free control NaN3=30 µM NaN3=200 µM

0.0 0

548

Page 24 of 28

5

10

15

20

25

30

NaN3=100 µM

0

5

Reaction time (min)

10

NaN3=400 µM

15

20

25

30

Reaction time (min)

549

Figure 2. Effects of scavengers on SMX degradation in BQ/PMS system (a. for alcohols; b.

550

for NaN3). Experimental conditions: [PMS]0=0.44 mM, [SMX]0=8 µM, [BQ]0=2 µM, pH=10

551

(20 mM borate buffer), and T=25 °C, (a) [methanol]0=[ethanol]0=[tert-butanol]0=0.22 M, (b)

552

[NaN3]0=30-400 µM.

553 554

24


Page 25 of 28


555

a

Intensity(a.u.)

BQ/PMS/TMP

BQ/TMP

PMS/TMP

3480 556

3495 3510 Magneitc field(G)

100%

3525

b

DPAO2

Relative Intensity (%)

m/z 363→330

80% 60% DPA/BQ/PMS(900 µM) DPA/BQ/PMS(600 µM) DPA/BQ/PMS(300 µM) DPA/PMS(900 µM) DPA blank

40% 20% 0%

0

2

4

6 t (min)

557

8

10

558

Figure 3. EPR spectra of TMP-1O2 adduct (TMPN) formed in aqueous solution containing

559

PMS, TMP, and BQ (a), and HPLC/ESI−QqQMS chromatograms for the typical

560

endoperoxide (DPAO2) in MRM (b). Experimental conditions for (a): [TMP]0=1 mM,

561

[PMS]0=0.44 mM, [BQ]0=25 µM, pH=10 (20 mM borate buffer), T=25 °C, and reaction time

562

of 60 min; for (b): the mass transition from 363 to 330 m/z; [DPA]0=24 µM, [PMS]0=0.3-0.9

563

mM, [BQ]0=25 µM, pH=10 (20 mM borate buffer), T=25 °C, and reaction time of 60 min.

564

25



0.0

a

2

R =0.998

-1

kobs(min )

ln(C/C0)

-3

kobs=(5.56±0.10)×10 [BQ]

0.12 BQ=5 µM BQ=7 µM BQ=10 µM BQ=15 µM BQ=17 µM BQ=19 µM BQ=30 µM Fit Curve

-1.0 -1.5

565

b

0.16

-0.5

-2.0

Page 26 of 28

0

3

0.08 0.04 Fit Curve

0.00

6 9 12 Reaction time(min)

15

0

5

10 15 20 [BQ](µM)

25

30

566

Figure 4. Effect of BQ at varying concentrations on PMS decomposition (a), and plot of kobs

567

vs. [BQ] (b). The dashed lines (Figure 4a) represent the first-order model fit. Experimental

568

conditions: [PMS]0=0.44 mM, [BQ]0=5-30 µM, pH=10 (20 mM borate buffer), and T

569

=25 °C.

570

26


Page 27 of 28

571


Table 1. Rate Constants for the Formation of Dioxirane Intermediate III NO.

compounds

molecular

1

BQ

C6H4O2

2

acetone

CH3COCH3

3

2-acetylpyridine

C7H7NO

structure O

O

O H3C

CH3

N

kDI (M-2 s-1) a (1.16±0.02)×106 (5.7±0.1)×103

CH3

(1.4±0.2)×104

N

(1.0±0.1)×105

O

4

di-2-pyridyl ketone

C11H8N2O

N O

5

fluoroacetone

6

1,1,1-trifluoroacetone

CH3COCH2F CH3COCF3

O H3C

CH2F O

H3C

CF3

(1.1±0.1)×105 b (1.0±0.3)×105 b

O

7

cycloheptanone

(1.4±0.2)×103

C7H12O O

8 572 573

a

cyclohexanone

C6H10O

at pH=10 and 25 oC unless stated otherwise. b at pH=10 and 10 oC.

27


(5.6±0.1)×104


574 575

TOC Art

2HSO5O

OH O O SO3O-

O O SO3OH

O O SO3-

OO

O

O

O O SO3-

O

O

1O

2

2SO52H2N

576

O S NH O N O

CH3

2SO42products

Sulfamethoxazole

577

28


Page 28 of 28

Activation of Peroxymonosulfate by Benzoquinone: A Novel

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