Kinetics and Mechanism of Ultrasonic Activation of Persulfate: An in

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Kinetics and Mechanism of Ultrasonic Activation of Persulfate: An In-Situ EPR Spin Trapping Study Zongsu Wei, Frederick A. Villamena, and Linda K. Weavers Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05392 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017

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Environmental Science & Technology

1

2

Kinetics and Mechanism of Ultrasonic

3

Activation of Persulfate: An In-Situ EPR Spin

4

Trapping Study

5 6 Zongsu Wei†, Frederick A. Villamena‡, and Linda K. Weavers†*

7 8 9 10

†Department of Civil, Environmental and Geodetic Engineering, The Ohio State

11

University, Columbus, Ohio, U.S.A 43210

12 13

‡

Department of Biological Chemistry and Pharmacology and The Davis Heart and Lung Research Institute, The Ohio State University, Columbus, Ohio, U.S.A 43210

14 15 16

*To whom correspondence should be addressed. Phone: (614) 292-4061; fax: (614) 292-

17

3780; e-mail address: [email protected]. 1

ACS Paragon Plus Environment


18

ABSTRACT

19

Ultrasound (US) was shown to activate persulfate (PS) providing an alternative

20

activation method to base or heat as an in-situ chemical oxidation (ISCO) method. The

21

kinetics and mechanism of ultrasonic activation of PS were examined in aqueous solution

22

using an in-situ electron paramagnetic resonance (EPR) spin trapping technique and

23

radical trapping with probe compounds. Using the spin trap, 5,5-dimethyl-1-pyrroline-N-

24

oxide (DMPO), hydroxyl radical (•OH) and sulfate radical anion (SO4•‒) were measured

25

from ultrasonic activation of persulfate (US-PS). The yield of •OH was up to one order of

26

magnitude greater than that of SO4•‒. The comparatively high •OH yield was attributed to

27

the hydrolysis of SO4•‒ in the warm interfacial region of cavitation bubbles formed from

28

US. Using steady-state approximations, the dissociation rate of PS in cavitating bubble

29

systems was determined to be three orders of magnitude greater than control experiments

30

without sonication at ambient temperature. From calculations of the interfacial volume

31

surrounding cavitation bubbles and using the Arrhenius equation, an effective mean

32

temperature of 340 K at the bubble-water interface was estimated. Comparative studies

33

using the probe compounds tert-Butyl alcohol and nitrobenzene verified the bubble-water

34

interface as the location for PS activation by high temperature with •OH contributing a

35

minor role in activating PS to SO4•‒. The mechanisms unveiled in this study provide a

36

basis for optimizing US-PS as an ISCO technology.

37 38

Keywords: ultrasound, persulfate, EPR, DMPO, bubble-water interface

39 2


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TOC

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42 43 44 45 46 47 48 49 50 51 52

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53

INTRODUCTION

54

Activated persulfate (PS) has recently gained use as an in-situ chemical oxidation

55

(ISCO) technology.1 PS has a long half-life (600 days at 25 °C) allowing for transport of

56

the reactant from an injection point to locations of contamination.2 Within the

57

remediation region, the activation of PS to sulfate radical anion (SO4•‒), a strong oxidant,

58

is achieved through thermal treatment,3, 4 base injection,5, 6 or catalyst addition (e.g., Fe2+,

59

Fe2+-EDTA, and Co2+).7, 8 PS activation has also been achieved by reaction with natural

60

organic matter (NOM)9, 10 and naturally occurring minerals,2 as well as by electrokinetic

61

reaction11 and UV irradiation.12, 13 In limited studies, ultrasound (US) has been shown to

62

activate PS.14-20 Ultrasound has the potential for use in ISCO settings by flooding a

63

contaminated region with PS and driving ultrasonic units into the subsurface using direct

64

push technology.21, 22

65

Activation by US is thought to occur in two possible ways. In both mechanisms,

66

the activation begins by the sonication of aqueous solution inducing cavitation bubbles.

67

Collapse of those bubbles generates localized “hot spots” where temperature and pressure

68

are as high as 5000 K and 1000 atm, respectively.23 This high localized temperature has

69

been reported as one mechanism for PS dissociation in US systems. In the second

70

mechanism, hydroxyl radical (•OH) generated from dissociation of water molecules in

71

cavitation bubbles attacks PS ions forming SO4•‒, bisulfate ion, and oxygen.17 However,

72

these proposed mechanisms of ultrasonic activation of PS have not been confirmed

73

experimentally.

4


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To characterize radical formation from PS dissociation, SO4•‒ is usually estimated

75

through an indirect measurement of a decrease in PS concentration.2, 4, 24 This indirect

76

measurement may over-estimate the activation process because PS may undergo

77

reactions other than the assumed cleavage of the peroxide bond to form SO4•‒. Electron

78

paramagnetic resonance (EPR) has been widely used as a method to directly detect free

79

radicals through use of a spin trap such as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO).10,

80

25

81

compared to radical production in the ultrasound-persulfate (US-PS) system. Thus, the

82

detection of DMPO adducts in EPR is an almost simultaneous reflection of radical

83

formation.

The addition of •OH (2.8 × 109 M-1 s-1)26 or SO4•‒ (this study) to the spin trap is rapid as

84

The objective of the current study was to investigate the activation mechanisms of

85

PS in a cavitating bubble system. In order to elucidate reaction pathways in the US-PS

86

system, an ultrasonic reactor coupled to an EPR spectrometer through a flow cell was

87

designed to detect and quantify radical formation. We hypothesized that PS ions undergo

88

thermal dissociation and •OH activation into SO4•‒ at the high temperature (~1900 K)27, 28

89

of the bubble-water interface. To test this hypothesis, batch sequential experiments and

90

kinetic modeling were conducted. Using the spin trap DMPO, •OH and SO4•‒ production

91

were first determined in the US and US-PS systems, respectively. Next, since it was

92

challenging to directly probe PS activation in the interfacial region, a kinetic model was

93

developed to fit the PS dissociation rate at the interface. Finally, to gain insight into the

94

role of •OH in PS activation, two probe compounds, tert-Butyl alcohol (TBA) and

95

nitrobenzene (NB), were employed to manipulate and verify the reaction pathway 5



96

involved in PS activation. The experimental and modeling results provide a more

97

transparent picture of the kinetics, reactivity, and distribution of PS activation at the

98

bubble-water interface in the US system.

99 100

MATERIALS AND METHODS

101

Materials

102

The spin trap reagent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO; > 99.0%),

103

purchased from Dojindo Molecular Technologies and the radical standard, 2,2,6,6-

104

tetramethylpiperidine (TEMPO; 98.0%), purchased from Fisher Scientific were used

105

without further purification. Sodium persulfate (Na2S2O8; > 98.0%), tert-Butyl alcohol (>

106

99.9%), and nitrobenzene (99.5%) purchased from Fisher Scientific were used as

107

received. The trityl was prepared following the method by Kutala et al.29 All stock

108

solutions were prepared using Milli-Q deionized (DI) water buffered with sodium

109

dihydrogen phosphate (NaH2PO4) and disodium hydrogen phosphate (Na2HPO4) at pH =

110

3.5, 7.4, and 9.4. The phosphate buffer was selected due to its relatively slow reaction

111

rate with radicals30, 31 and inert reactivity toward the spin trap DMPO.25, 32, 33

112

Experimental Setup

113

The experimental setup is shown in Figure 1. A horn-type ultrasonic system

114

(Sonic Dismembrator 550, Fisher Scientific) with a tip area of 1.2 cm2 was used to

115

activate PS in a water-jacketed glass rosette reactor. A cooling bath (1006S, Fisher 6


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Scientific) was used to maintain the solution temperature at 20°C in the reactor. The

117

sonicated solution was pumped into an Aqua-X multi-bore sample tube with closely

118

spaced capillaries (Bruker) by a peristaltic pump (8 Roll Masterflex Pumphead, Cole-

119

Parmer). The sample tube was placed in the cavity of a Bruker EMX Plus EPR

120

spectrometer equipped with a high sensitivity resonator. Finally, the solution was

121

circulated back to the reactor. The set-up was run as a closed system. The in-situ setup

122

offers advantages such as simultaneous measurements of DMPO adduct formation

123

without delay, the ability to maintain a closed system, and reliable monitoring of adduct

124

decay in the US-PS system.

125

Sonication and EPR Experiments

126

Sequential batch experiments were conducted at different PS (0.1 ‒ 100.0 mM)

127

and DMPO (1.0 ‒ 80.0 mM) concentrations. Prior to each experiment, a reactant solution

128

(25 mL) was transferred into the reactor and purged with argon (99.9999%) for 5 min.

129

Using a micro DO probe (Lazar Research Laboratories, Inc.), 99.4% of dissolved oxygen

130

(DO) was removed in 5 min of argon purging (Figure S1 in the supporting information,

131

SI). Oxygen removal minimizes the formation of superoxide radical (O2•‒) and excludes

132

its interference in experiments. The argon gas flow was stopped at the start of an

133

experiment to avoid entrainment of gas bubbles in the Aqua-X tube. Control experiments

134

with an O2 sensitive probe compound, trityl,34 indicate there is no O2 intrusion into the

135

closed system experimental set-up after argon purging was turned off (Figure S2). The

136

ultrasonic power density (PD) was 1.5 W mL−1 with a sonication period of 10 ‒ 30 min. 7



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The radical adduct signals were acquired and analyzed using WINEPR software.35 All

138

scans were carried out at room temperature with the following EPR instrument settings:

139

sweep width, 120 G; power, 10.02 mW; modulation amplitude, 1.00 G; time constant,

140

81.92 ms; conversion time, 40 ms; sweep time, 40.96 s; resolution in X, 1024; and

141

resolution in Y, 20 ‒ 40. The spin trap of DMPO reacts with the short-lived radicals (R•,

142

•

143

longer, measureable life-times (Eq. 1):

OH or SO4•‒) that are generated in the US-PS system, thus forming spin adducts with

+

N

O

+

•

R

R

N

(1)

• O

-

144

A typical EPR spectrum obtained containing DMPO•−OH and DMPO•−SO4− is shown in

145

Figure 2. Hyperfine coupling constants (hfcc) for nitrogen ( a N ) and hydrogen ( a H )

146

atoms were obtained by fitting the spectrum using WinSim. To quantify spin adducts in

147

the US-PS system, TEMPO was used as a stable nitroxide radical standard. A 7-point

148

calibration curve of TEMPO was prepared in buffer. The spin adduct concentrations in

149

samples were then calculated by comparing integrated areas of all detected peaks with the

150

best-fit linear regression fit of TEMPO peak areas. Select experiments were performed in

151

triplicate; error between experiments was near or below 5%.

152 153

RESULTS AND DISCUSSION

154

Radical Production in US-PS System 8


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Formation of radical species during sonication at different DMPO and PS

156

concentrations was determined first. DMPO-radical adduct formation with respect to

157

DMPO concentration is shown in Figure S3. DMPO•−OH and DMPO•−SO4− formation

158

reached plateaus at 10 and 40 mM, respectively. The occurrence of the plateau at a higher

159

DMPO concentration for SO4•‒ over •OH indicates that the trapping rate of SO4•‒ by

160

DMPO may not be as fast as •OH. To ensure that DMPO adequately trapped both

161

radicals, 40 mM of DMPO was utilized in this study unless noted otherwise.

162

Adduct formation with sonication time in the absence and presence of PS is

163

illustrated in Figure 3. DMPO•−OH and DMPO•−SO4− concentrations reached plateaus

164

between 6 and 9 min of sonication. The plateaus indicate that formation and

165

decomposition of adducts were in equilibrium. Overall, the DMPO•−OH yield in Figure 3

166

was higher than the corresponding DMPO•−SO4− yield in this US-PS system consistent

167

with Figure S3. Additionally, the DMPO•−OH yield in the presence of PS is nearly 3

168

times that in the US system, suggesting a source of •OH production other than

169

dissociation of water molecules in the gas phase of cavitation bubbles. Even though a

170

higher concentration of trapping reagent was required to trap SO4•−, DMPO•−SO4− (~ 6

171

min) adduct formation reached steady-state faster than DMPO•−OH (~ 9 min), as shown

172

in Figure 3. In control experiments, obvious decay of spin adducts was observed in the

173

US-PS system (see Text S1 and Figure S4). Decay kinetics were quantified and

174

incorporated into our kinetic analysis.

175

Two other radicals, •H and O2•‒/HO2•, were tested for their presence in the system.

176

Negligible levels of •H in the presence of PS were detected due to their lack of formation, 9



177

insufficient trapping by DMPO, or instability of the DMPO•−H adduct.36 Additionally,

178

the relatively high PS concentration may compete with DMPO for consumption of •H

179

through S2O82‒ + •H → H+ + SO42‒ + SO4•‒ (k = 2.5×107 M-1 s-1), resulting in a very small

180

amount of DMPO•−H formation. Likewise, DMPO•−O2‒ was not detected in the US-PS

181

system. At pH 7.4 used in these experiments, HO2• deprotonates to O2•‒ (pKa = 4.8).37 In

182

the argon-purged system, production of •HO2 through •H + O2 → HO2• does not occur.

183

Therefore, O2•‒ formation was primarily from reactions of •OH and/or SO4•‒ with H2O2

184

through R10 and R20. At the high [DMPO] used, •OH and SO4•‒ were both sufficiently

185

trapped by DMPO (Figure S3); thus, within our expectations, features in the EPR spectra

186

consistent with DMPO•−O2‒ were not observed (Figure 2).

187

Tables 1 and 2 summarize principal reactions in the US-PS system (R# indicates

188

the assigned reaction number). According to these reactions and consistent with our

189

observations, •OH and SO4•‒ are the major radical species formed. The higher •OH yield

190

in the presence of PS as compared to the US-only system is attributed to the hydrolysis of

191

SO4•‒ (R3), even though the ambient temperature rate constant (460 s‒1) is reported to be

192

comparatively low.38 The dissolved PS ions do not partition into the high temperature gas

193

phase of cavitation bubbles. However, when cavitation bubbles collapse producing

194

temperatures on the order of 2000 ‒ 5000 K,39 the temperature from the collapse heats the

195

liquid sheath surrounding the bubble, resulting in a higher temperature, pressure, and

196

possible supercritical water zone in the interfacial region.40 Inside the supercritical water

197

layer, the diffusivity and reactivity of PS ions is enhanced resulting in reduced activation

198

energy for PS dissociation.41 Based on the Arrhenius equation, the reaction rate constant 10


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for R3 is elevated in the high temperature region surrounding these cavitation bubbles

200

leading to the higher •OH yield.

201

Generally, for any activation method, increasing the dose of PS results in

202

improved radical production and consequently contaminant degradation but no relative

203

change in the ratio of formation of radicals.8,

204

methods, our results in Figure S5 demonstrate that both DMPO•−OH and DMPO•−SO4−

205

concentrations increase slightly as PS concentration increases. Yet, the ratio of radicals

206

formed is similar (i.e., around 8) over the [PS] range tested. A consistent ratio of radical

207

formation over a range of [PS] agrees well with other activation methods. Thus, it seems

208

[PS] is not a determining factor for the radical distribution in US-PS systems.

209

pH Effect on Radical Distribution in US-PS System

42, 43

Consistent with other activation

210

The effect of solution pH on the ratio of •OH to SO4•‒ formed varies by activation

211

method; thus, we explored the role of pH on the formation of •OH and SO4•‒. Radical

212

production at pH 3.5 (acidic), 7.4 (neutral) and 9.4 (basic) in both US and US-PS systems

213

is shown in Figure 4. Sulfate radical formation is similar at pH values tested and is lower

214

than •OH formation. The •OH yield under US only is 50% or less than that in the US-PS

215

system ([PS] = 1.0 mM), suggesting a significant amount of •OH was formed through

216

hydrolysis of SO4•‒ in addition to sonochemically generated •OH. As described above,

217

[DMPO] affects radical trapping (Figure S3). These experiments were performed at a

218

lower DMPO concentration ([DMPO] = 5 mM) compared to changing [PS] experiments

219

([DMPO] = 40 mM). As a result, less adduct formation, and particularly less

220

DMPO•−OH formation was observed leading to a lower radical ratio. 11



221

Clearly, •OH is the predominant radical species formed over the pH range tested.

222

As shown in Figure 4, the ratio of •OH to SO4•‒ (ca. 5 ‒ 5.5) is similar at acidic and

223

neutral pH, then doubled (ca. 10) at pH 9.4. The higher overall radical production at pH

224

9.4 suggests a possible synergism between US and base activation of PS. In base

225

activation of PS, the apparent PS activation to SO4•‒ by base is not obvious until pH >

226

12.42 Our results indicate that US may accelerate base activation of PS at lower pH values

227

(i.e., 9.4). Thus, US may reduce the amount of base used in the base PS ISCO

228

technology.

229

Although PS activation technologies are classified together, the technologies can

230

form •OH and SO4•‒ at much different ratios and as pH changes. Metal catalyzed

231

activation of PS usually requires acidic conditions so that metal ions are dissolved in

232

solution.8 •OH is preferentially formed with ratios of •OH to SO4•‒ as high as 6 in Fe(II)-

233

EDTA activation of PS.8 The preferential formation of •OH is comparable to our US-PS

234

results in which we observed a ratio of •OH to SO4•‒ between 5 and 8 at pH values

235

between 3.5 and 7.4. In thermal and base activation of PS, SO4•‒ is the dominant radical

236

species at pH values at and below 7. Due to base catalyzed hydrolysis of SO4•‒, the

237

amount of •OH formed increases with pH becoming the dominant radical formed at pH

238

values greater than 12.42, 43 In base activation studies, hydroxyl radical production at pH

239

12 is up to 6 times that observed between pH 2 to 7.42 Similar to thermal activation, we

240

observed the ratio of •OH to SO4•‒ from US activation to increase as pH increased,

241

primarily due to an increase in •OH formation. However, the high •OH production at

242

acidic and neutral pH indicates that the US-PS system will have different reactivity than 12


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thermal activation that produces primarily SO4•‒ at acidic and neutral pH values. Our

244

results also indicate that US activation will be effective for contaminants prone to •OH

245

attack rather than the more selective SO4•‒.44

246 247

Thermal Activation at the Bubble-Water Interface

248

Collapsing cavitation bubbles are extremely hot and dynamic micro-reactors with

249

mean gaseous temperatures estimated to be 3400 K at collapse in an ambient temperature

250

bulk aqueous solution.45 Due to this high localized temperature at collapse, spatially and

251

temporally heterogeneous radical formation occurs. Due to temporal and spatial

252

heterogeneity, large cooling rates and gradients in heat and radicals are present.23 Since

253

PS is non-volatile and PS dissociation is negligible in ambient temperature bulk solution,3

254

the observed formation of SO4•‒ should occur in the •OH rich and elevated temperature

255

regions at or near the bubble-water interface with heterogeneous temperature and radical

256

distribution.

257

The compound, TBA, was used to probe the activation mechanism of PS. TBA

258

reacts slowly with SO4•‒ (k = 4.0 × 105 M-1 s-1; R24)31 but is an efficient scavenger of •OH

259

(k = 6.0 × 108 M-1 s-1; R25).30 Thus, at a high concentration of TBA, the reaction of •OH

260

with PS forming SO4•‒ is quenched, leaving only heat activation (R2) as the source of

261

SO4•‒. Although a portion of TBA added into the sonicated solution will enter into the gas

262

phase of the bubble and lower the gaseous collapse temperature (to 2800 K in the

263

presence of 100 mM TBA),45, 46 the corresponding reduced temperature at the interface is 13



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still far beyond that needed for PS dissociation. As shown in Figure S6, we observed no

265

change in SO4•‒ production due to TBA addition. The insensitivity in SO4•‒ production to

266

the presence of TBA supports the hypothesis that thermal dissociation of PS (R2) is the

267

dominant reaction pathway forming SO4•‒ at the bubble-water interface.

268

The thermal dissociation rate constant of PS ( k 2 ) cannot be determined directly

269

since EPR measures the DMPO•−SO4‒ adduct in aqueous solution. In order to quantify

270

thermal dissociation of PS due to cavitation, a kinetic model was developed to probe the

271

activation reaction and fit the dissociation rate in aqueous solution. For this analysis, the

272

40 mM DMPO concentration used assured that the free radicals in the US-PS system

273

were fully trapped. As detailed in Text S2, a linear relationship between the observed rate

274

constant for DMPO•−SO4− formation, k obs, DMPO-SO4- , and [S2O82− ] was formulated using a

275

steady-state approach:

276

k obs, DMPO-SO4- =

 k13 k14  k1k 4   × [S2 O 82− ] + k17 + k 4 [DMPO]  k 21 k 22 

 k k [DMPO]  +  2 4 − k hyd, DMPO-SO4-   k17 + k 4 [DMPO] 

277 278

(2a)

Or, k obs, DMPO-SO4- = M × [S 2 O 82− ] + N

279

M=

(2b)

 k13 k14  k1k 4   , + k17 + k 4 [DMPO]  k 21 k 22 

N=

k 2 k 4 [DMPO] − k hyd, DMPO-SO4- , k17 + k 4 [DMPO]

280

where

281

k hyd, DMPO-SO4- = k5 [DMPO • − SO 4− ] . By altering the PS concentration, we obtained

282

concentration dependent zero-order rate constants, k obs, DMPO -SO4 - , as shown in Figure 5a. 14


and

Page 15 of 35


283

Given parameter constants M and N, the first-order reaction rate constant for PS

284

dissociation was calculated and reported in Table 1 as k 2, obs . In addition, Eq. (2) was

285

used to estimate the equilibrium adduct concentration at different PS loadings if assuming

286

the same equilibrium time for the batch run as observed in Figure 3. The comparable

287

adduct concentrations between estimated and observed results in Figure 3 confirmed the

288

validity of the developed kinetic model. Since we did not monitor the adduct formation at

289

PS concentrations from 10 to 100 mM, the extrapolation of Eq. (2) is not reliable in this

290

concentration range.

291 292

In the same way, we obtained the following expression for the observed DMPO•−OH adduct formation rate constant ( k obs, DMPO-OH ): k obs, DMPO-OH = M ' ×[S 2 O 82− ] + N'

293

M' =

(3) k1k 3 k 4 [DMPO] 2

 k13 k14    + k k  21 22 

294

where

295

  k2 N ' =  + k1 + k hyd, DMPO -SO4- − k hyd, DMPO -OH   k 4 [DMPO] 

296

k hyd, DMPO -OH = k 6 [DMPO • − OH] . Eq. (3) also indicates a linear relationship between

297

k obs, DMPO - OH and [S2 O8 ] , as confirmed in Figure 5b. Using Eq. (3), k 3, obs was

298

determined. As shown in Table 1, k 3, obs is two orders of magnitude larger than k 5, obs ,

299

suggesting that DMPO•−OH formation is dominated by hydrolysis of SO4•‒ forming •OH.

300

Therefore, the potential artifact of DMPO•−SO4− hydrolysis contributing to DMPO•−OH

301

production does not appear to be significant in our US-PS system and thus, our results

,

,

and

2−

15



302

indicate that •OH is the dominant radical. Additionally, the rate constant, k 4, obs , in our

303

system was determined using kinetic modeling and fitting. No experimentally reported

304

values exist for k 4, obs . The k 4, obs in Table 1 confirms that the reaction of DMPO with

305

SO4•‒ is slower than with •OH as Figures 3 and S3 indicate. With this kinetic analysis,

306

we quantitatively determined the thermal activation rate of PS in ultrasonically irradiated

307

solution and kinetic rate constants of key reactions that are not reported in the literature.

308

Role of Free Radicals in Activation

309

Radical recombination reactions, shown in Table 2, may also alter SO4•‒

310

production and subsequent contaminant oxidation. For example, excess •OH may reduce

311

SO4•‒ formation due to the reaction of the two radical species (R18). To evaluate the

312

effects of •OH on SO4•‒ formation (R13) and decay (R18), the probe compound of NB

313

was used. Based on its high hydrophobicity (log KOW = 1.86), we assume that NB

314

accumulates on the bubble-water interface and scavenges •OH (R27) there.47 NB is not

315

expected to scavenge SO4•‒ due to its low reaction rate constant with SO4•‒ (R26). Thus,

316

the presence of NB in solution interrupts R13 and R18 and ultimately alters

317

DMPO•−SO4− formation. Additionally, at high [DMPO], radical concentrations are very

318

low and R18 will be negligible compared with R13. Thus, [DMPO] = 1 mM, which fell

319

in the linear region of Figure S3, was used to determine if both R13 and R18 play roles in

320

SO4•‒ production.

321

Comparative adduct formation in the absence and presence of NB was used to

322

explore radical activation of PS. As shown in Figure 6, an obvious reduction was 16


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323

observed for both adducts with NB present. The decrease in DMPO•−OH concentration

324

was attributed to scavenging of •OH by NB. This decrease in •OH lowered its reaction

325

with PS (R13) resulting in a reduced DMPO•−SO4− concentration. The decrease in

326

DMPO•−SO4− concentration at two initial NB concentrations was not significantly

327

different confirming that NB did not react with SO4•‒. Even under a low concentration of

328

spin trap reagent, the concentrations of •OH and SO4•‒ were relatively low compared to

329

PS, as evidenced by the fact that the adduct concentration was less than 1% of the DMPO

330

concentration. Therefore, R13 has a higher reaction rate and out-competes R18 in spite of

331

a higher reaction rate constant for the latter reaction. This comparative test verified that

332

•

333

and •OH (R18). Additionally, the difference in DMPO•−SO4− concentration in the

334

presence and absence of NB suggests that •OH activation of PS accounts for at most 30%

335

of the SO4•‒ production.

336

Assessing the Thickness and Effective Temperature of the Bubble-Water Interface

337

for PS Activation

OH activation of PS through R13 is prevalent over radical combinations between SO4•‒

338

The reaction site for both thermal and radical activation of PS is the bubble-water

339

interface. The observed rate constants ( k obs ) from EPR measurements reported in Table 1

340

are those measured in the total reactor solution volume. Thus, a conversion from

341

observed reaction rates to the rate constants in the reactive volume (i.e., k int at the

342

interface) is needed. Using a simple mass balance approach, the following expression

343

relates k int to k obs : 17



344

k int =

V total k obs Vint

Page 18 of 35

(4)

345

where Vtotal is the total solution volume (25 mL) and Vint is the volume of interfacial

346

shell surrounding cavitation bubbles.

347

To assess the total volume of the interfacial region, we estimated the total number

348

of active cavitation bubbles in our reactor and the thickness of the interface of the

349

cavitation bubbles. We assumed: (1) all cavitation bubbles are spherical with bubble sizes

350

as in Colussi et al.;48 (2) heat transfer is instantaneous across the interface;23 (3) the

351

thickness of the interfacial shell is 10% of the radius of cavitating bubbles;49 (4) bubble-

352

bubble interactions are negligible; and (5) all reactions occurred at the interface and

353

formed adducts were immediately released from the compact interfacial region to bulk

354

solution following collapse of the cavitation bubbles.

355

As described in Text S3 of the SI, the total number of bubbles present in the

356

reactor was estimated using an acoustic emission method based on hydrophone

357

measurements.50 The number of cavitating bubbles was estimated from the

358

sonochemiluminescence volume and an analysis as described in Text S4 in the SI.48 For

359

our operational conditions, the number density ( ρ N ) of cavitation bubbles was calculated

360

to be 1.73 × 106 mL-1, which falls within the predicted range of 106 ‒ 108 mL-1 for a horn

361

type ultrasonic system driven at 20 kHz.51,

362

R0 = 2.0 µm and the interface thickness ( wint ) is 10% of the radius of the bubble,

363

wint = 0.2 µm and Vint =

[

52

Assuming a cavitating bubble radius

]

4π 3 3 × (R0 + wint ) − R0 × ρ N × Vcloud = 2.36 × 10 −5 mL for all 3

18


Page 19 of 35


364

cavitation bubbles in the cloud ( Vcloud = 1.23mL ) below horn tip. Therefore, the rate

365

constants at the interface, k int , were calculated to be 6 orders of magnitude

366

( 25mL / 2.36 × 10 −5 mL ≈ 10 6 ) greater than the observed values in aqueous solution (Table

367

1). Substituting these interfacial k values into Eqs. (S4b and 6b), the concentrations of

368

•

369

water interface, respectively. At the hot bubble-water interface, the hydrolysis of SO4•‒

370

(R3) was much faster compared to reported values at ambient conditions (R17) resulting

371

in higher •OH yields in our system (Figures S3 and 3).

OH and SO4•‒ were calculated to be 1.20 × 10‒10 M and 1.10 × 10‒10 M at the bubble-

372

Using k 2, int and the Arrhenius equation, the effective mean temperature of the

373

interfacial region was estimated. Johnson et al. determined Arrhenius parameters for the

374

thermal dissociation of PS:3

375

lnk = lnA − E a / RT

(5)

376

where lnA = 33.6, E a = 134 kJ M-1, R = 8.314 J K-1 M-1, and T is the temperature (K).

377

Using Eq. (5) and k 2, int , the corresponding temperature was calculated to be 340 K.

378

Although the interfacial zone will have a steep temperature gradient between the hot

379

cavitation bubble and the ambient temperature of the bulk solution, we consider 340 K to

380

be the mean effective temperature of the interfacial region with a thickness of 10% of the

381

bubble radius for PS dissociation.

382

Others have reported much larger temperatures such as 1900 K in interfacial

383

regions of cavitation bubbles.27, 28 Our estimated effective temperature of 340 K is far

384

below the reported interfacial temperature. It is likely that the interfacial region for PS 19



385

dissociation extends a much larger distance into bulk solution compared to previous

386

reports24, 25 due to the thermal instability of PS even at relatively low temperatures (e.g.,

387

303 K).3 The assumption that the interfacial shell is 10% of the radius of cavitating

388

bubbles may not fully reflect the PS activating system, but it allows a starting point for

389

interpreting the reactivity of PS at the bubble-water interface. In any case, it is evident

390

that an interfacial region, or reactive region, varies depending on the specific reaction: a

391

reaction which requires a high temperature and is diffusion limited necessitates a

392

relatively small and higher temperature interfacial region, or vice versa. Therefore, more

393

complete modeling of the temperature and reactivity distribution in the interfacial region

394

surrounding a cavitation bubble is recommended.

395 396

ENVIRONMENTAL APPLICATIONS

397

This systematic investigation of ultrasonic activation of PS is of fundamental

398

importance to understand the activation mechanisms in cavitating bubble systems.

399

Cavitation-induced heating decomposes PS to SO4•‒, while •OH reaction with PS plays a

400

minor role in SO4•‒ formation. At the bubble-water interface (T ≈ 340 K), the hydrolysis

401

of SO4•‒ is significant leading to a high •OH yield, which is unique as compared to heat-,

402

base- or catalyst-activated systems. To promote the application of US-PS in ISCO, the

403

potential effects of subsurface solids (e.g., iron oxide and clay) on PS activation should

404

be examined. In contrast with the drawbacks of attenuated ultrasonic intensity, presence

405

of mineral solids may serve as catalysts resulting in synergism for enhanced radical 20


Page 20 of 35

Page 21 of 35


406

production and subsequent contaminant degradation.53 Finally, the concept of ISCO using

407

US-PS may be implemented with ultrasonic units via a direct push type technology and

408

PS flooding at the contamination site.

409 410 411 412

ACKNOWLEDGEMENTS Funding from Ohio Sea Grant College Program (R/PS-050) is gratefully acknowledged.

413 414

SUPPORTING INFORMATION

415

Description of DO depletion during argon purging, EPR spectrum of trityl, spin

416

adduct decay with time, kinetic calculations of •OH and SO4•‒ formation using steady-

417

state approach, estimation of number for total bubbles using acoustic emission method,

418

estimation of number for cavitation bubbles, adduct formation at different DMPO and PS

419

concentrations, and DMPO•−SO4− adduct formation in the absence and presence of TBA

420

are provided in the supporting information.

421 422 423 424

REFERENCES

21



425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468

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511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554

33. Lee, H.; Lee, H. J.; Jeong, J.; Lee, J.; Park, N. B.; Lee, C., Activation of persulfates by carbon nanotubes: Oxidation of organic compounds by nonradical mechanism. Chem. Eng. J. 2015, 266, 28-33. 34. Bobko, A. A.; Dhimitruka, I.; Eubank, T. D.; Marsh, C. B.; Zweier, J. L.; Khramtsov, V. V., Trityl-based EPR probe with enhanced sensitivity to oxygen. Free Radical Bio. Med. 2009, 47 (5), 654-658. 35. Duling, D. R., Simulation of multiple isotropic spin-trap EPR spectra. J. Magn. Reson. Ser. B 1994, 104 (2), 105-110. 36. Gutierrez, M.; Henglein, A.; Dohrmann, J. K., H atom reactlons in the sonolysis of aqueous solutions. J. Phys. Chem. 1987, 91, 6687-6690. 37. Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B., Reactivity of HO2/O2radicals in aqueous solution. J. Phys. Chem. Ref .Data 1985, 14 (4), 1041-1100. 38. Yu, X. Y.; Bao, Z. C.; Barker, J. R., Free radical reactions involving Cl•, Cl2•, and SO4−• in the 248 nm photolysis of aqueous solutions containing S2O82- and Cl-. J. Phys. Chem. A 2004, 108 (2), 295-308. 39. Ashokkumar, M.; Grieser, F., A comparison between multibubble sonoluminescence intensity and the temperature within cavitation bubbles. J. Am. Chem. Soc. 2005, 127 (15), 5326-5327. 40. Hua, I.; Hochemer, R. H.; Hoffmann, M. R., Sonolytic hydrolysis of p-nitrophenyl acetate - the role of supercritical water. J. Phys. Chem. 1995, 99, (8), 2335-2342. 41. Kritzer, P.; Dinjus, E., An assessment of supercritical water oxidation (SCWO) Existing problems, possible solutions and new reactor concepts. Chem. Eng. J. 2001, 83 (3), 207-214. 42. Furman, O. S.; Teel, A. L.; Ahmad, M.; Merker, M. C.; Watts, R. J., Effect of basicity on persulfate reactivity. J. Environ. Eng. ASCE 2011, 137 (4), 241-247. 43. Liang, C. J.; Su, H. W., Identification of sulfate and hydroxyl radicals in thermally activated persulfate. Ind. Eng. Chem. Res. 2009, 48 (11), 5558-5562. 44. Xiao, R.; Ye, T.; Wei, Z.; Luo, S.; Yang, Z.; Spinney, R., Quantitative structureactivity relationship (QSAR) for the oxidation of trace organic contaminants by sulfate radical. Environ. Sci. Technol. 2015, 49, 13394-13402. 45. Ciawi, E.; Rae, J.; Ashokkumar, M.; Grieser, F., Determination of temperatures within acoustically generated bubbles in aqueous solutions at different ultrasound frequencies. J. Phys. Chem. B 2006, 110 (27), 13656-13660. 46. Tauber, A.; Mark, G.; Schuchmann, H. P.; von Sonntag, C., Sonolysis of tert-butyl alcohol in aqueous solution. J. Chem. Soc. Perkin Trans. 2 1999, 6, 1129-1135. 47. Sivasankar, T.; Moholkar, V. S., Physical insights into the sonochemical degradation of recalcitrant organic pollutants with cavitation bubble dynamics. Ultrason. Sonochem. 2009, 16, 769-781. 48. Colussi, A. J.; Weavers, L. K.; Hoffmann, M. R., Chemical bubble dynamics and quantitative sonochemistry. J. Phys. Chem. A 1998, 102 (35), 6927-6934. 49. Kamath, V.; Prosperetti, A.; Egolfopoulos, F. N., A theoretical study of sonoluminescence. J. Acoust. Soc. Am. 1993, 94 (1), 248-260. 50. Pandit, A. B.; Varley, J.; Thorpe, R. B.; Davidson, J. F., Measurement of bubble size distribution - An acoustic technique. Chem. Eng. Sci. 1992, 47 (5), 1079-1089. 24


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51. Dubus, B.; Vanhille, C.; Campos-Pozuelo, C.; Granger, C., On the physical origin of conical bubble structure under an ultrasonic horn. Ultrason. Sonochem. 2010, 17 (5), 810-818. 52. Znidarcic, A.; Mettin, R.; Dular, M., Modeling cavitation in a rapidly changing pressure field – Application to a small ultrasonic horn. Ultrason. Sonochem. 2015, 22, 482-492. 53. Ahmad, M.; Teel, A. L.; Watts, R. J., Persulfate activation by subsurface minerals. J. Contam. Hydrol. 2010, 115 (1-4), 34-45. 54. Jiang, P. Y.; Katsumura, Y.; Nagaishi, R.; Domae, M.; Ishikawa, K.; Ishigure, K.; Yoshida, Y., Pulse radiolysis study of concentrated sulfuric acid solutions - Formation mechanism, yield and reactivity of sulfate radicals. J. Chem. Soc. Faraday Trans. 1992, 88 (12), 1653-1658.. 55. Klaning, U. K.; Sehested, K.; Appelman, E. H., Laser flash photolysis and pulse radiolysis of aqueous solutions of the fluoroxysulfate ion, SO4F-. Inorg. Chem. 1991, 30 (18), 3582-3584. 56. Li, B. Z.; Li, L.; Lin, K. F.; Zhang, W.; Lu, S. G.; Luo, Q. S., Removal of 1,1,1trichloroethane from aqueous solution by a sono-activated persulfate process. Ultrason. Sonochem. 2013, 20 (3), 855-863. 57. Finkelstein, E.; Rosen, G. M.; Rauckman, E. J., Spin trapping - Kinetics of the reaction of superoxide and hydroxyl radicals with nitrones. J. Am. Chem. Soc. 1980, 102 (15), 4994-4999. 58. Neta, P.; Madhavan, V.; Zemel, H.; Fessenden, R. W., Rate constants and mechanism of reaction of SO4.- with aromatic compounds. J. Am. Chem. Soc. 1977, 99 (1), 163-164.

25



Page 26 of 35

Table 1: Experimentally determined rate constants Rate constant a

R# Reaction )))

kobs

kint b

1

H2O ሱሮ •OH + •H

1.3×10-8 M s-1

1.4×10-2 M s-1

2

S2O82‒ ሱۛሮ 2 SO4•‒ SO4•‒ + H2O → H+ + SO42‒ + •OH DMPO + SO4•‒ → DMPO•−SO4− DMPO•−SO4−+ H2O → DMPO•−OH + H+ + SO42‒ DMPO•−OH → Product

୦ୣୟ୲

1.5×10-11 s-1

1.6×10-5 s-1

0.15 s-1 0.027 M-1 s-1 8.4×10-4 s-1 1.0×10-3 s-1

1.6×105 s-1 2.9×104 M-1 s-1 8.9×102 s-1 1.1×103 s-1

3 4 5 6

Note: a M s-1, s-1, and M-1 s-1 are units for zero-, first-, and second-order reaction rate constants, respectively; b k values are corrected for reactions at the bubble-water interface (effective temperature of ~340 K), whereas kobs values are EPR measured values in sonicated solution purged with argon (Temperature = 293 K and pH = 7.4).

26


Page 27 of 35


Table 2: Principal reactions in US-PS system Rate constant, M-1 s-1 7.0×109 5.5×109 7.8×109

R# 7 8 9

Reaction • OH + •H → H2O • OH + •OH → H2O2 • H + •H → H2

10

OH + H2O2 → H2O + H+ + O2•‒ • H + H2O2 → H2O + •OH • OH + H2 → H2O + •H S2O82‒ + •OH → H+ + SO42‒ + SO4•‒ + 1/2O2 S2O82‒ + •H → H+ + SO42‒ + SO4•‒ S2O82‒ + SO4•‒ → SO42‒ + S2O8•‒ 2 SO4•‒ → S2O82‒ SO4•‒ + H2O → H+ + SO42‒ + •OH SO4•‒ + •OH → HSO5‒ → H+ + SO42‒ + 1/2O2 SO4•‒ + •H → H+ + SO42‒

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

•

•‒ SO4•‒ + H2O2 → 2H+ + SO42‒ + O2 DMPO + •OH → DMPO•−OH DMPO + •H → DMPO•−H DMPO + •O2‒ → DMPO•−O2‒ (CH3)3COH + SO4•‒ → Product (CH3)3COH + •OH → Product (C6H5)NO2 + SO4•‒ → Product (C6H5)NO2 + •OH → Product

a

Ref. b 30 30 30

2.7×107

30

9.0×107 4.2×107 < 1×106 2.5×107 6.6×105 4.4×108 460 s-1 9.5×109 1.0×1010

30 30 30 54 54 55 38 55, 56 30, 55

1.2×107

56

2.8×109 3.8×109 10 4.0×105 6.0×108 < 106 3.9×109

26 30 57 31 30 58 30

Note: a units for zero- (M s-1) and first-order (s-1) reaction rate constants were noted otherwise; b k values are at ambient temperature unless noted otherwise.

27



Page 28 of 35

Figure Captions

Figure 1: Experimental setup for in-situ EPR measurements of sonicated solution. 1 — Transducer; 2 — Horn; 3 — Argon gas inlet; 4 — Reactor; 5 — •OH; 6 — SO4•−; 7 — DMPO; 8 — Peristaltic pump; 9 — EPR spectrometer; 10 — Aqua-X tube; 11 — Spectrum. Figure 2: EPR spectrum of sonicated persulfate. PS = 100 mM; DMPO = 40 mM; PD = 1.5 W mL-1; ● — DMPO•−OH,

= 15.01, aβ-H =14.72; ▲ —DMPO•−SO4−,

aN

=

13.67, aβ-H =10.24, aγ-H1 = 1.52, aγ-H2 = 0.79. The hyperfine coupling constants of

a

aN

were obtained by fitting spectra using WinSim. Figure 3: DMPO adduct formation with sonication time. ●— DMPO•−OH at 100 mM persulfate; ▲ — DMPO•−SO4− at 100 mM persulfate; ■ — DMPO•−OH at 1 mM persulfate; ▼— DMPO•−SO4− at 1 mM persulfate; ★ — DMPO•−OH under sonication only; DMPO = 40 mM; pH = 7.4; PD = 1.5 W mL−1. No measurable DMPO•−OH and DMPO•−SO4− were produced without sonication or trapping reagent of DMPO. Figure 4: pH effect on the adduct formation in US and US-PS systems. DMPO = 5 mM; PS = 1 mM; PD = 1.5 W mL−1; sonication time = 17 min. Figure 5: Linear relationship between initial zero-order formation rate constants and persulfate concentrations for DMPO•−SO4− (a) and DMPO•−OH (b). DMPO = 40 mM; pH = 7.4; PD = 1.5 W mL−1; sonication time = 17 min.

28


Page 29 of 35


Figure 6: Adduct formation in the absence and presence of nitrobenzene (NB). DMPO = 1 mM; PS = 1 mM; pH = 7.4; PD = 1.5 W mL−1; sonication time = 17 min; DMPO•−OH measurements are significantly different, while there is no significant difference for DMPO•−SO4− formation in the presence of 0.1 mM and 1.0 mM NB.

29



Figure 1: Experimental setup for in-situ EPR measurements of sonicated solution. 1 — Transducer; 2 — Horn; 3 — Argon gas inlet; 4 — Reactor; 5 — •OH; 6 — SO4•−; 7 — DMPO; 8 — Peristaltic pump; 9 — EPR spectrometer; 10 — Aqua-X tube; 11 — Spectrum.

30


Page 30 of 35

Page 31 of 35


6

8.0x10

6

6.0x10

6

Intensity

4.0x10

6

2.0x10

0.0 6

-2.0x10

6

-4.0x10

6

-6.0x10

3460

3480

3500

3520

3540

3560

Magnetic Field, G Figure 2: EPR spectrum of sonicated persulfate. PS = 100 mM; DMPO = 40 mM; PD = 1.5 W mL-1; ● — DMPO•−OH,

= 15.01, aβ-H =14.72; ▲ —DMPO•−SO4−,

aN

=

13.67, aβ-H =10.24, aγ-H1 = 1.52, aγ-H2 = 0.79. The hyperfine coupling constants of

a

aN

were obtained by fitting spectra using WinSim.

31



Page 32 of 35

DMPO-OH Concentration, µM

15

10

5

0 0

5

10

15

20

0

5

10

15

20

-

DMPO-SO4 Concentration, µM

5 4 3 2 1 0

Sonication Time, min Figure 3: DMPO adduct formation with sonication time. ●— DMPO•−OH at 100 mM persulfate; ▲— DMPO•−SO4− at 100 mM persulfate; ■ — DMPO•−OH at 1 mM persulfate; ▼— DMPO•−SO4− at 1 mM persulfate; ★ — DMPO•−OH under sonication only; DMPO = 40 mM; pH = 7.4; PD = 1.5 W mL-1. No measurable DMPO•−OH and DMPO•−SO4− were produced without sonication or trapping reagent of DMPO. 32


Page 33 of 35


30 25

DMPO-OH (US only) DMPO-OH DMPO-SO4

10 8

20

6

15 4

Adduct Ratio

Adduct Concentration, µM

35

10 2

5 0

0

pH 3.5

pH 7.4

pH 9.4

Figure 4: pH effect on the adduct formation in US and US-PS systems. DMPO = 5 mM; PS = 1 mM; PD = 1.5 W mL−1; sonication time = 17 min.

33



Page 34 of 35

-8

1.0x10

kobs, DMPO-SO4-, M s

-1

(a) -9

8.0x10

-9

6.0x10

-9

4.0x10

-7

-9

y = 6.31×10 x + 1.82×10 2 R = 0.9712

-9

2.0x10

0.0 0

2

4

6

8

10

-8

3.0x10

kobs, DMPO-OH, M s

-1

(b) -8

2.8x10

-8

2.6x10

-7

-8

y = 4.26×10 x + 2.40×10 2 R = 0.9238

-8

2.4x10

-8

2.2x10

0

2

4

6

8

10

Persulfate Concentration, mM Figure 5: Linear relationship between initial zero-order formation rate constants and persulfate concentrations for DMPO•−SO4− (a) and DMPO•−OH (b). DMPO = 40 mM; pH = 7.4; PD = 1.5 W mL-1; sonication time = 17 min.

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Page 35 of 35


Adduct Concentration, µM

8 DMPO-OH − DMPO-SO4

6

4

2

0 No NB

0.1mM NB

1.0 mM NB

Figure 6: Adduct formation in the absence and presence of nitrobenzene (NB). DMPO = 1 mM; PS = 1 mM; pH = 7.4; PD = 1.5 W mL-1; sonication time = 17 min; DMPO•−OH measurements are significantly different, while there is no significant difference for DMPO•−SO4− formation in the presence of 0.1 mM and 1.0 mM NB.

35


Kinetics and Mechanism of Ultrasonic Activation of Persulfate: An in

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