Efficient Reductive Decomposition of Perfluorooctanesulfonate in a

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Efficient Reductive Decomposition of Perfluorooctane Sulfonate in a High Photon Flux UV/Sulfite System Yurong Gu, Wenyi Dong, Cheng Luo, and Tongzhou LIU Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03261 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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Environmental Science & Technology

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Efficient Reductive Decomposition of Perfluorooctane Sulfonate in a High

2

Photon Flux UV/Sulfite System

3

Yurong GU1, Wenyi DONG1,*, Cheng LUO1, Tongzhou LIU1,**

4 5 6

1. Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, P. R. China.

7

* Corresponding author, tel & fax: +86-755-2603 2718, email: [email protected]

8

**

9

[email protected]

Corresponding

author,

tel

&

fax:

+86-755-2603

2718,

email:

10

Abstract

11

Hydrated electron (eaq-) induced reduction techniques are promising for decomposing

12

recalcitrant organic pollutants. However, its vigorous reactivity with co-present

13

scavenging species and the difficulty in minimizing the competitive reactions make

14

the proportion of eaq- participating in pollutant decomposition low, reflecting by slow

15

decomposition kinetics. In this study, a high photon flux UV/sulfite system was

16

employed to promote eaq- production. Its feasibility in enhancing a notorious

17

recalcitrant pollutant, PFOS, decomposition was investigated. The effective photon

18

flux utilized for producing eaq- was 9.93×10−8 einstein/cm2·s. At initial solution pH

19

9.2, with DO about 5 mg/L, and at around 25 oC, 98% PFOS was decomposed within

20

30 min from its initial concentration of 32 µM. The kobs of PFOS decomposition was

21

0.118 min-1 (7.08 h-1), and about 8-400 folds faster than those obtained in other

22

reductive approaches. In this system, PFOS decomposition showed can tolerate

23

co-present 7 mg N/L of NO3-. Suggested by molecular orbitals and thermodynamic

24

analyses, the mechanisms responsible for PFOS decomposition involve defluorination,

25

desulfonation, and centermost C-C bond scission. By demonstrating a more practical 1

ACS Paragon Plus Environment


26

relevant treatment process, the outcomes of this study would be helpful for facilitating

27

future applications of eaq- induced reduction techniques for efficient recalcitrant

28

pollutants decomposition.

29 30

Abstract art

31

2


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Introduction

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Perfluorinated compounds (PFCs) have been incorporated into industrial applications

34

since 1950s due to their excellent thermal and chemical stability and effective

35

surfactant

36

bioaccumulative and toxic.2 Perfluorooctane sulfonate (PFOS) and perfluorooctanoate

37

(PFOA), the two predominant PFCs, receive most environmental attention, and their

38

production have been restricted in developed countries.3 However, it’s rather difficult

39

to achieve a total restriction of PFOX (X stands for S or A) in globe, because to meet

40

the still strong demand of PFCs related products, PFOX involved production and

41

manufacturing have moved to and been keeping rising in some developing countries

42

(e.g. China) since 2000s.4,5 The manufacturers in the developing countries won’t

43

voluntarily replace PFOX by more environmental friendly but costly substitutes.6 As a

44

consequence, their concentrations are observed increasing in the environmental media

45

adjacent to these PFOX involved industrial activities.7 Investigations have revealed

46

that wastewater discharge is the principal source of PFOX releasing into the

47

environment.8,9 Hence, efficient approaches for onsite decomposing PFOX present in

48

wastewater are of great importance for minimizing their release, besides the attempts

49

of limiting their industrial use.

properties,1

but

are

notorious

by

environmentally

persistent,

50

Due to their inherent recalcitrance to microbiological and ordinary chemical

51

treatment, PFOX decomposition needs advanced treatment techniques, such as

52

photochemical,10 sonochemical,11 electrochemical,12 subcritical Fe0 reductive,13 and

53

mechanochemical destruction.14 Amongst them, photochemical decomposition 3



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process is the most favored one because of its operational simplicity. Photochemical

55

decomposition for PFOA has been investigated in many studies,10,15-17 whereas much

56

fewer study focused on PFOS decomposition. In the very limited literatures, PFOS

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decomposition displayed apparent less effectiveness than PFOA. The reported PFOS

58

decomposition kinetics by photochemical approaches, including direct UV

59

photolysis,18,19 UV/alkaline 2-Propanol,19 UV/Fe3+ and UV/KI systems,20,21 were slow.

60

In these systems half-life time (t1/2) of PFOS decomposition ranged in 87 to 7700 min.

61

It’s difficult to incorporate these techniques into onsite treatment of PFOS bearing

62

wastewater which requires short retention time.

63

Decomposition techniques employing hydrated electron (eaq-) are seen as an

64

efficient

approach

for

halogenated

organic

pollutants

65

detoxification.22-24 eaq- is one of the most reactive species (Eo = -2.9 V). It can act as a

66

nucleophile when reacts with organic compounds. The reactivity can be greatly

67

enhanced when organic molecules contain halogen atoms, leading to C-X bond

68

cleavage and halide ion release.25 eaq- can be produced by pulse radiolysis of pure

69

water or photolytical methods including direct photolysis and inorganic anions

70

mediated photolysis, i.e. ferrocyanide mediated laser flash photolysis,26 and KI or

71

sulfite mediated UV photolysis.16,27 Inorganic anions mediated photolysis showed

72

higher quantum yield of eaq- than direct photolysis.28 However, ferrocyanide and KI

73

mediated photolysis suffer drawbacks of limited practical application of laser flash

74

photolysis and potential detrimental effects on human induced by purposefully added

75

iodides. A recent comprehensive investigation carried out by Li et al. using a model 4


decomposition

and

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compound (monochloroacetic acid, MCAA) exhibited that eaq- production through

77

sulfite mediated UV photolysis (eqs 1-3) is more practical relevant and environmental

78

friendly.28 The work of Song et al. further demonstrated the feasibility of using the

79

UV/sulfite system to decompose recalcitrant pollutant (PFOA). 17

80

SO32- + hν → SO3·-+ eaq-

(1)

81

SO3·- + SO3·- → S2O62-

(2)

82

SO3·- + SO3·- + H2O → SO42- + H+ + HSO3-

(3)

83

Nevertheless, due to its high standard reduction potential, besides target

84

pollutants, eaq- reacts rapidly with many co-present species (such as H+, NO2-, NO3-,

85

DO, and N2O) in water matrix those have more positive reduction potentials.25 It

86

causes eaq- scavenging effect and hence imposes important challenges in further

87

practical applications of eaq- induced reductive decomposition treatment.28 For

88

example, a relatively high solution pH (pH 10.3) accompanied by N2 purging were

89

used to attenuate the eaq- quenching reactions by H+ and DO, respectively, in a

90

previous study on PFOA decomposition by a UV/sulfite system.17 Yet, to what extent

91

the co-presence of NO3-, a common constituent in wastewater, can be tolerated in the

92

UV/sulfite system during PFOX decomposition hasn’t been studied. It seems

93

unachievable to minimize eaq- scavenging effect by excluding its competing species

94

from water matrix, since they are always abundant over the target pollutants. Thus, a

95

plausible way to increase the amount of eaq- available for pollutants decomposition is

96

promoting eaq- production through an efficient manner. When using radiolytic

97

generation methods, increasing reaction temperature up to 300 oC29 or lowering 5



98

energy deposition density in the tracks of heavy ions were reported can enhance eaq-

99

production.30 Previous photolytic studies showed that eaq- quantum yield can be

100

promoted by applying higher photon flux in photo-excitation.31 Because of its higher

101

emission intensity and broader emission spectrum, compared to low- and medium-

102

pressure UV lamps, high-pressure UV lamp is considered promising to promote eaq-

103

generation, and subsequently achieve more efficient pollutants decomposition (Text

104

S1 and Table S1). To our best knowledge, systematic investigation about the

105

enhancement of recalcitrant pollutants decomposition through photolytic promoting

106

eaq- production remains sparse.

107

In the presented study, a UV/sulfite system configured with a high-pressure UV

108

lamp as the irradiation source was utilized for promoting eaq- production and applied

109

for decomposing a notorious recalcitrant pollutant, PFOS. The feasibility of achieving

110

efficient PFOS decomposition through enhanced eaq- production was investigated

111

without controlling DO. The promoted eaq- production and the associated improved

112

tolerance to co-present NO3- were examined. Additionally, PFOS decomposition in

113

the studied UV/sulfite system was compared with UV/persulfate and UV/KI treatment

114

using the same high-pressure UV lamp. The mechanisms responsible for PFOS

115

decomposition were discussed with the aid of molecular orbitals and thermodynamic

116

analyses. By demonstrating a more practical relevant treatment process, the outcomes

117

of this study would be helpful for facilitating future applications of eaq- induced

118

reduction techniques for efficient recalcitrant pollutants decomposition.

6


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Experimental Section

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Chemicals. All chemicals, including PFOA (96%), PFOS (98%), perfluorobutyric

121

acid (PFBA, 98%), perfluoropentanoic acid (PFPeA, 97%), perfluorohexanoic acid

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(PFHxA, 98%), perfluoroheptanoic acid (PFHpA, 99%), potassium persulfate

123

(K2S2O8, > 99%), potassium iodide (KI, > 99%), Na2SO3 (98%), NaNO3 (> 99%),

124

NaNO2 (> 99%), monochloroacetic acid (MCAA, 98%), K3[Fe(C2O4)3], NaOH and

125

H2SO4 were obtained from commercial sources (Sigma-Aldrich and Aladdin) and

126

used as received in this study. Ultrapure water (Milipore Milli-Q) was used for

127

solution preparation.

128

Experimental setup. PFOS photolysis decomposition experiments were carried out

129

in an opened glass reactor (50 mm in diameter and 60 mm in length) without

130

controlling DO in the testing solution. A high-pressure mercury UV lamp was used as

131

the irradiation source assembled with a reflector, a shutter, and a timer in a closed

132

box.32 UV photon flux entering the reactor can be adjusted by transmission filters with

133

different density, and were determined varied in 1.98 × 10−7 to 6.6 × 10−7

134

einstein/cm2·s by ferrioxalate actinometry method.33

135

25 mL of testing solutions were used in the experiments, and the respective

136

initial concentrations of PFOS and Na2SO3 were 32 µM and 10 mM. The initial PFOS

137

concentration falls in a typical concentration range present in the untreated

138

perfluorinated organic pollutants bearing industrial wastewater.34 Besides UV/sulfite

139

treatment, two blank experiments with sole UV irradiation at 100% I0 (I0 = 6.6×10−7

140

einstein/cm2·s) and sole sulfite addition in the solution without UV irradiation were 7



141

also carried out. Initial solution pH was 9.2 and not adjusted unless specified. In cases

142

investigating the influences of pH, initial solution pH was adjusted by dropwise

143

addition of diluted NaOH or H2SO4 solution and varied from pH 10.2 to 7.0. For

144

comparing the effectiveness of the studied UV/sulfite reductive PFOS decomposition

145

with other photo-oxidative or -reductive treatments, two experiments employing

146

persulfate (S2O82-) as the photochemical oxidant and iodide (I-) as the eaq-

147

photo-exciting agent, respectively, were conducted using the same high-pressure UV

148

lamp. The detailed solution conditions were described in Text S2. PFOS photolysis

149

decomposition experiment was started by putting the reactor 5 cm beneath the already

150

warmed UV lamp. Experimental time was varied up to 30 min. A water cooling jacket

151

was applied to maintain the solution temperature at 25 ± 3 oC throughout the

152

experiment. At the end of the experiment, the reactor was moved away from the UV

153

lamp and the testing solution was withdrawn for immediate chemical analyses. All

154

experiments were carried out in duplicate.

155

In eaq- scavenging experiments, sufficient NO3- and NO2- (10 mM) were used as

156

the scavengers to examine the dominant species responsible for PFOS decomposition.

157

PFOS decomposition experiments with lower concentration of NO3- at 0.5, 1, and 2

158

mM were conducted to investigate the NO3- tolerance of the studied system.

159

Meanwhile, NO3- degradation and sulfite concentration remained in the solution after

160

total NO3- degradation were monitored. Due to its good photo-stability and low

161

reactivity with many reductive species (such as ·H and SO3·-), MCAA is often chosen

162

as a model compound to assess the reductive capacity of active species in a specific 8


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treatment system.25,35 In this study, MCAA was used to indirectly examine the

164

promoted eaq- production in the reductive pollutant decomposition process. Detailed

165

experimental condition of MCAA decomposition is presented in Text S2.

166

Analytical methods. The concentrations of PFOS and its decomposition

167

intermediates were determined using an UPLC-MS-MS analyzer (Waters). The UPLC

168

system was equipped with a BEH C18 column (2.1×50 mm, 1.7 µm). Methanol

169

(solvent A) and 2 mM CH3COONH4 (solvent B) were used as the mobile phase. The

170

injection volume was 1 µL, and column temperature was set at 40 oC. The elution

171

flow rate was maintained at 0.3 mL/min. ESI mass spectrometry in negative mode

172

was used to identify perfluorinated compounds. The source temperature and

173

desolvation temperature were 120 oC and 400 oC, respectively, and desolvation gas

174

flow was 800 L/h. Multiple reaction monitoring (MRM) mode was used for

175

identifying intermediately generated perfluorocarboxylic acids and PFOS, and the

176

elution was started with 5% methanol followed by a linear increase to 95% methanol

177

at 7 min, and reversed to original conditions at 10 min.

178

Fluoride concentration was measured by a fluoride ion selective electrode

179

(INESA & Scientific instrument CO., Ltd., China) with the detection limit of 0.02

180

mg/L. Solution pH was measured using a pH meter (Sartorius). UV-vis absorbance

181

spectrum of the 10 mM Na2SO3 solution was determined by a UV spectrometer

182

(Shimadzu 2450). NO3- concentration was measured using colorimetric method. DO

183

in the solution was determined using an YSI 550A DO meter. Concentrations of

184

MCAA and sulfite were analyzed by ion chromatography (Dionex, ICS-5000, USA) 9



185

(see Text S3 for detailed measurement procedures).

186

The relative energies of dissociation fragments with respect to eaq- attached PFOS

187

radical anion (C8F17SO3·2-) (△E) was calculated at the B3LYP/6-311++G** level

188

with the ZPVE correction. The molecular orbitals of PFOS are generated by using the

189

GAUSSIAN09 program,36 and the lowest unoccupied molecular orbital (LUMO) was

190

visualized by means of the Multiwfn software Version 3.3.8 with the isovalue being

191

0.050.37

192

Results and Discussion

193

PFOS decomposition kinetics. Technique PFOS has been reported18,19 and was

194

verified contains linear and branched isomers (Figure S1a). At 3 min of the

195

decomposition experiment, the peak of branched PFOS disappeared, whereas that of

196

linear PFOS decreased slightly (Figure S1b). The branched isomer decomposed much

197

faster than the linear one. This observation is consistent with previous studies.18,19 It

198

shall because the tertiary C-F bonds being possessed in the branched PFOS have

199

much higher electron affinity, and is hence more vulnerable to reductive

200

decomposition than their straight chain analogues.38 In the presented study, only the

201

decomposition of linear PFOS was discussed in detail, and its defluorination

202

efficiency was calculated assuming all the fluoride produced within the first 3 min

203

was attributable to branched PFOS (Text S4).

204

As shown in Figure 1, neither sole high photon flux UV irradiation nor sole

205

sulfite addition in the solution achieved observable PFOS decomposition in the 30

206

min experiment. In contrast, a fast PFOS decomposition kinetics was observed in the 10


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high photon flux UV/sulfite system. Almost all PFOS (98%) was decomposed within

208

30 min from its initial concentration of 32 µM. The decomposition kinetics can be

209

well described by a pseudo first order model and the observed reaction kinetics rate

210

constant (kobs) of PFOS decomposition was 0.118 min-1 (Table 1). It’s nearly 8-400

211

folds faster than PFOS decomposition by other reported reductive approaches, and the

212

time span needed for reaching 50% defluorination efficiency was decreased from

213

hours to minutes (Table 2). Meanwhile, the energy required for decomposing PFOS

214

using the high photon flux UV/sulfite system showed comparable with other reductive

215

techniques (Table 2). By applying the same high photon flux UV, both persulfate

216

induced photo-oxidation and iodide induced photo-reduction hadn’t observable

217

decomposition for linear PFOS within 30 min (Figure S2). It indicated the studied

218

high photon flux UV/sulfite system is much more competitive than previously studied

219

photo-oxidative or -reductive approaches in regard of PFOS decomposition.

220

In a supplementary experiment where NO2- and NO3- were used as the eaq-

221

scavengers,25 the co-presence of 10 mM NO2- or NO3- totally suppressed PFOS

222

decomposition (Figure S3). These results confirmed that eaq- was the predominant

223

reductive species responsible for the observed fast PFOS decomposition. Other

224

species those were possibly generated in the UV/sulfite system, such as ·H and SO3·-,

225

shall have negligible effect on PFOS decomposition. The ·H scavenging capacity of

226

NO2- is about 500 folds higher than that of NO3-,25 but they caused equal inhibition on

227

PFOS decomposition at the same molar concentration. It indicated ·H played a minor

228

role in PFOS decomposition. As for SO3·-, it has been demonstrated had negligible 11



229

contribution to MCAA decomposition,28 let alone more recalcitrant pollutants like

230

PFOS. In the experiments with varying initial solution pH, PFOS decomposition

231

kinetics and defluorination efficiencies showed faster and higher at initial solution pH >

232

9.2, became apparently reduced when decreased to pH 8.0, and were almost totally

233

suppressed at initial solution pH 7.0 (Table 1 and Figure S4). The strong dependence

234

on initial solution pH indirectly indicated the role of eaq- in the reductive

235

decomposition process. SO32- is the strongest UV adsorption sulfite species which

236

intrinsically governs UV photolytic eaq- generation,27 and dominates at pH above 7

237

(Figure S5) (pKa2 of sulfite is 7.2).39 eaq- generation is positively correlated to SO32-

238

concentration in UV/sulfite system.17 More SO32- in solution at higher pH (pH>8.0)

239

would lead to more eaq- generation, which was reflected by the observed faster PFOS

240

decomposition kinetics and higher defluorination efficiencies. On the other hand, at a

241

lower pH, the effect of eaq- quenching by H+ to produce ·H cannot be omitted,40 which

242

in turn suppressed the generated eaq- participating in decomposition reactions.

243

One point that weakens the engineering application of eaq- induced reduction

244

process is the scavenging of eaq- by NO3-,28 which is commonly present in wastewater

245

stream with a typical concentration range of 0~20 mg N/L.41 PFOS decomposition in

246

the co-presence of 0.5, 1, and 2 mM NO3- was monitored. In the presence of 0.5 mM

247

NO3- (7 mg N/L, representing a moderate level in wastewater), efficient PFOS

248

decomposition (94.1%) was still experienced (Figure S6). This result demonstrated

249

much improved tolerance to the co-present NO3- in the high photon flux UV/sulfite

250

system. In comparison, NO3- concentration at 1.3 mg N/L was observed already 12


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inhibited MCAA dechlorination by eaq- generated in a low-pressure UV/sulfite

252

system.28 Fast NO3- degradation occurred in the high photon flux UV/sulfite system,

253

especially in the first 6 min, and all NO3- was degraded from its initial concentration

254

of 0.5 mM at 15 min of the experiment (Figure S7). The obvious suppression on

255

PFOS decomposition in the first 6 min (Figure S6) echoed the strong competition of

256

eaq- by NO3-. After the total NO3- degradation, sulfite concentration remained in the

257

solution was determined as 0.57 mM. When the concentration of co-present NO3-

258

increased to 1 and 2 mM, respectively, remarkable and even total suppression on

259

PFOS decomposition was recorded in the 30 min experiment (Figure S6).

260

All the PFOS photolysis decomposition experiments were carried out in an

261

opened reactor. Initial DO in the solution was around 5 mg/L, and not controlled

262

throughout the experiment. Regardless of DO presence, the observed fast PFOS

263

decomposition and the improved tolerance to the co-present 0.5 mM NO3-

264

demonstrated the powerful treatability of the studied high photon flux UV/sulfite

265

system, and indicated its flexibility in practical application where excluding DO is

266

difficult. Detailed investigation on the effects of DO is important for further

267

optimizing this process, and is under way.

268

Efficient PFOS decomposition attributable to high photon flux UV. The fast PFOS

269

decomposition and the improved tolerance to the co-present NO3- indicated the

270

abundance of eaq- generated in the high photon flux UV/sulfite system. PFOS

271

decomposition was further investigated by varying UV irradiation intensity, where

272

initial solution pH was kept at pH 9.2. PFOS decomposition followed a pseudo first 13



273

order model well (Figure 2a), but kobs decreased from 0.118 min-1 to 0.020 min-1 when

274

UV intensity was adjusted from 100% I0 to 30% I0 (Table 1). PFOS defluorination

275

efficiencies were also observed decreased correspondingly (Figure 2b). In a

276

supplementary experiment (Figure S8), co-present 0.5 mM NO3- completely

277

suppressed PFOS decomposition under 30% I0, whereas it almost had no influence on

278

the final PFOS decomposition efficiency under 100% I0. It further attested that

279

applying high photon flux is essential for generating abundant eaq- in the studied

280

system.

281

eaq- is a very active reductant. In the studied system, H+, H2O, S2O62-, SO3-,

282

HSO3-, O2 and eaq- itself can be the species that react rapidly with eaq-.25, 26,42-45 They

283

exhibited 2~3 order faster reaction kinetics with eaq- than PFOA and PFOS (Table S2).

284

Because of these competing reactions, the proportion of the generated eaq- available

285

for PFOS decomposition was very low. It was reportedly about 0.1% by Park et al.21

286

and was calculated as 0.01% in the presented study (Text S5 and Figure S9). Since the

287

strong eaq- competing species are always present and abundant in the treated water, it

288

is very difficult to increase eaq- availability for PFOS decomposition by minimizing

289

the competitive reactions. Hence, increasing eaq- availability through significant

290

promotion on its production becomes a plausible way. The UV system used in this

291

study showed great promotion on eaq- production. By employing a high-pressure

292

mercury lamp and configuring a quartz reflector,32 the UV irradiation source has high

293

luminous efficiency. The photon flux in the studied UV system was determined as 6.6

294

×10−7 einstein/cm2·s. Given the effective wavelengths (200-260 nm) for producing 14


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eaq- through exciting SO32- account for 15% of the total photon flux (Text S6 and

296

Figure S10 & S11), the calculated effective photon flux being utilized for producing

297

eaq- is 9.93×10−8 einstein/cm2·s. When using MCAA as the model compound to

298

assess the reductive capacity of the active species in the studied system, the kobs of

299

MCAA decomposition obtained was found nearly 1250-2150 folds higher than those

300

values reported by other reductive approaches (Table S3). Given eaq- has been

301

identified as the predominant reductive species, this result indicated the plenty of eaq-

302

available for reductive pollutant decomposition. Besides its higher irradiation intensity,

303

comparing to low UV pressure lamp that mainly emits at 254 nm, the broader

304

emission spectrum (200-400 nm) (Figure S11) of the high-pressure UV lamp can also

305

benefit the eaq- induced reductive PFOS decomposition. Because sulfite can absorb

306

UV wavelength up to 260 nm, and absorbs strongly around 220 nm in alkaline

307

solution (Figure S10), a broader emission spectrum would enhance photo-excitation

308

of sulfite and then promote eaq- generation.

309

PFOS decomposition mechanisms. When being attacked by eaq-, complicated

310

reaction processes including defluorination, desulfonation, and centermost C-C bond

311

scission may involve in PFOS decomposition.18,19,21,46 Reaction of initial PFOS anion

312

with eaq- yields PFOS·2- (C8F17SO3·2-) (eq 4). It further dissociates and produces

313

different fragments. Based on the calculated relative energies of dissociation

314

fragments with respect to C8F17SO3·2- (△E) (Table 3), C8F17- is the most likely

315

produced species with the lowest △E. It shall be due to the lower bond energy of C-S

316

(272 kJ/mol) compared with that of C-C (346 kJ/mol). The cleavage of C-S bond of 15



317

PFOS has been confirmed in a previous study with observed increasing of sulfate ion

318

concentration in a catalyst-free UV system.46

319

The dissociated C8F17- fragment was likely to transform to PFOA through

320

hydrolysis reactions (eqs 5-8). PFOA concentration showed increased and then

321

decreased quickly in the first 15 min of the experiment (Figure 3), indicating its

322

generation and the following fast decomposition. PFOA was observed decomposed

323

fast within 10 min under the same conditions of PFOS decomposition (Figure S12).

324

Its kobs was recorded 0.452 min-1, nearly 4 folds faster than that of PFOS. Besides

325

PFOA, concentration variation of other short chain PFCAs, including PFHpA,

326

PFHxA, PFPeA and PFBA, during the experiment were determined (Figure 3),

327

indicating the occurrence of stepwise defluorination (eqs 9-14).16,17 In an experiment

328

increasing the reaction time from 30 min (within which almost all PFOS (98%) was

329

decomposed, Figure 1) to 60 min, fluoride concentration in the testing solution was

330

observed kept increasing. The final defluorination efficiency was near 70%. It

331

evidenced that other F-containing intermediates still underwent defluorination

332

reactions after the total PFOS decomposition.

333

C8F17SO3- + eaq- → C8F17SO3·2-

(4)

334

C8F17SO3·2- → C8F17- + SO3·-

(5)

335

C8F17- + H3O+→ C8F17OH + ·H

(6)

336

C8F17OH → C7F15COF + HF

(7)

337

C7F15COF + H2O → C7F15COOH + HF

(8)

338

C7F15COOH + eaq- →·C7F14COOH + F-

(9) 16


Page 16 of 31

Page 17 of 31


339

·C7F14COOH + H2O → C7F14HCOOH + ·OH

(10)

340

C7F14HCOOH + eaq- →·C7F13HCOOH + F-

(11)

341

·C7F13HCOOH + H2O → C7F13H2COOH + ·OH

(12)

342

C7F13H2COOH →·C6F13 + ·CH2 + ·COOH

(13)

343

·C6F13 + ·COOH → C6F13COOH

(14)

344

Notably, the concentration of PFBA was extraordinary higher than all other

345

PFCAs, suggesting its production might involve other mechanisms. LUMO of PFOS

346

is mainly located on the moiety of C4, C5, C6, C7 and C8 atoms in the perfluoroalkyl

347

chain with clear sigma anti-bonding nature (Figure 4). Once an external electron is

348

attached to PFOS anion, it would tend to localize in the region over C4, C5, C6, C7

349

and C8 atoms, consequently introduce reaction driving energy there, and weaken the

350

corresponding C-C sigma bonds. Moreover, as shown in Table 3, C6F13-, C5F11-, C4F9-

351

and C3F7- showed lower △E with respect to C8F17SO3·2- than other dissociation

352

fragments (i.e. C7F15-, C2F5-, CF3-), indicating their relatively thermodynamically

353

favorable formation. Based the molecular orbitals and thermodynamic analyses, the

354

formation of C3F7-, C4F9-, and C5F11- shall be the most favorable in C8F17SO3·2-

355

dissociation. It has been revealed that the external attached electrons prefer to locate

356

on the centermost C-F bond of linear perfluoroalkane,38 and C3F7- is likely to be

357

generated through reductive C-C bond scission involving the dianion produced from

358

C8F17SO3·2- dissociation from the α-positon (eq 15-17).21 The generated C3F7- would

359

further recombine with ·COOH that is produced in PFOA stepwise defluorination and

360

result in the formation of PFBA (eq 18). The carbanion produced in eq 16 retaining 17



Page 18 of 31

361

sulfonate terminal group will be protonated in presence of water and undergo further

362

H/F exchange by reacting with eaq- (eq 19).21

363

C8F17SO3·2-→C8F16SO3·- + F-

(15)

364

C8F16SO3·- + eaq-→C8F16SO32-

(16)

365

C8F16SO32- (CF3(CF2)3CF-(CF2)3SO3-) → CF3CF2 CF2-+CF2=CF(CF2)3SO3-

(17)

366

CF3CF2CF2- + ·COOH + ·OH→CF3CF2CF2COOH + OH-

(18)

367

C8F16SO32- + H2O→C8F16HSO3- + OH-

(19)

368

Technical implication. As one of the most reactive species, eaq- is promising for

369

reductively decomposing recalcitrant organic pollutants, such as PFOS. However, due

370

to its vigorous reactivity with co-present scavenging species (e.g. H+, H2O, DO, and

371

NO3-) and the difficulty in minimizing the competitive reactions, the proportion of eaq-

372

available for decomposing the target pollutant is very low, leading to slow

373

decomposition kinetics. This study demonstrated the feasibility of enhancing PFOS

374

decomposition through significantly promoting eaq- production by using a high photon

375

flux UV/sulfite system.

376

It achieved almost complete PFOS decomposition within 30 min from its initial

377

concentration of 32 µM and can tolerate co-present 0.5 mM (7 mg N/L) NO3-, a

378

common constituent and typical eaq- scavenger in wastewater. This approach is more

379

practical relevant. It uses a modified UV lamp as the irradiation source. Unlike other

380

eaq- photo-exciting agents (e.g. KI and ferrocyanide), the product of sulfite after the

381

reaction is sulfate having much less adverse environment effects. The reaction

382

condition is mild. By applying an alkaline pH of 9.2, at an initial DO about 5 mg/L, 18


Page 19 of 31


383

and maintaining solution temperature at around 25 oC, a fast PFOS decomposition rate

384

constant of 0.118 min-1 (7.08 h-1) was achieved. Suggested by molecular orbitals and

385

thermodynamic analyses, the mechanisms responsible for PFOS decomposition

386

involve defluorination, desulfonation, and centermost C-C bond scission. Successful

387

application of the studied system is of course dependent on many influencing factors,

388

so further detailed investigations on the effects of wastewater matrix (e.g. DO and

389

natural organic matter), hydraulics, as well as, optimizing approaches are underway.

390

Furthermore, investigations concerning the toxicity and biodegradability of the

391

decomposition products shall be carried out.

392

Associated Content

393

Supporting Information

394

Texts S1−S6, Figures S1−S12, and Table S1-S3. This information is available free of

395

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

396

Author Information

397

Corresponding Authors

398

*(Wenyi.Dong.) Phone & fax: +86-755-2603 2718; e-mail: [email protected].

399

** (Tongzhou.Liu.) Phone & fax: +86-755-2603 2718;

400

e-mail: [email protected].

401

Acknowledgement

402

The authors thank professor Chaolin LI and Qian ZHANG for providing the high 19



Page 20 of 31

403

photon flux UV lamp used in this study. This research was financially supported by

404

the Major Science and Technology Program for Water Pollution Control and

405

Treatment of China (Grant No. 2015ZX07206-006-04).

406

References

407

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408

Compounds: past, present, and future. Environ. Sci. Technol. 2011, 45 (19),

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7954-7961.

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dechlorination of triclosan by hydrated electron reduction in aqueous solution.

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Kinetics and efficiency of the hydrated electroninduced dehalogenation by the

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review of physicochemical properties, levels and patterns in waters and

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free radicals with haloacetates in aqueous solution. Int. J. Chem. Kinetics. 1995,

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Comput. Chem. 2012, 33 (5), 580-592. (38) Paul, A.; Wannere, C.S.; Schaefer, H. F. Do linear-chain perfluoroalkanes bind an electron? J. Phys. Chem. A. 2004, 108, 9428-9434. (39) Tartar, H. V.; Garretson, H. H. The Thermodynamic ionization constants of sulfurous acid at 25 oC. J. Am. Chem. Soc. 1941, 63, 808-816.

518

(40) Qu, Y.; Zhang, C. J.; Chen, P.; Zhou, Q.; Zhang, W. X. Effect of initial solution

519

pH on photo-induced reductive decomposition of perfluorooctanoic acid.

520

Chemosphere. 2014, 107, 218-223. 23



521 522 523 524

(41) Tchobanoglous, G.; Burton, F. L.; Stensel, H. D., Eds. Wastewater engineering

treatment and reuse, 4th, ed.; Metcalf & Eddy. Inc: USA, 2003. (42) Dogliott, L.; Hayon, E. Flash photolysis study of sulfite, thiocyanate, and thiosulfate ions in solution. J. Phys. Chem. 1968, 72 (5), 1800-1807

525

(43) Hayon, E.; Treinin, A.; Wilf, J. Electronic spectra, photochemistry, and

526

autoxidationmechanism of the sulfite-bisulfite-pyrosulfite systems. The SO2-,

527

SO3-, SO4-, and SO5- radicals. J. Am. Chem. Soc. 1972, 94 (1), 47-57.

528 529 530 531

(44) Dester, U.; Warneck, P. Photooxidation of SO32- in aqueous solution. J. Phys.

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532

(46) Lyu, X. J.; Li, W. W.; Lam, P. S.; Yu, H. Q. Insights into perfluorooctane

533

sulfonate photodegradation in a catalyst-free aqueous solution. Sci. Rep. 2015, 5,

534

1-6.

24


Page 24 of 31

Page 25 of 31


535

Caption of Tables and Figures

536

Table 1.

537

UV/sulfite system at varied initial solution pH or varied UV emission intensity.

538

Table 2.

539

reported photo-reductive treatments

540

Table 3.

541

Figure 1.

542

irradiation, and with sole sulfite addition in the solution

543

Figure 2.

544

UV emission intensity. (I0 = 6.6×10−7 einstein/cm2·s).

545

Figure 3.

Intermediate products detected during PFOS decomposition

546

Figure 4.

Lowest unoccupied molecular orbital (LOMO) of PFOS

PFOS decomposition kinetics rate constants in the high photon flux

Comparison of reductive PFOS decomposition in this study with other

Relative energies of dissociation fragments with respect to C8F17SO3·2PFOS decomposition in the UV/sulfite system, with sole UV

(a) PFOS decomposition, and (b) defluorination efficiency at varied

547 548

25



Page 26 of 31

549

Tables and Figures

550

Table 1. PFOS decomposition kinetics rate constants in the high photon flux

551

UV/sulfite system at varied initial solution pH or varied UV emission intensity. Initial solution pH a 10.2 9.2 8.0 7.0

kobs (min )

t1/2 (min)

R

0.126±0.002 0.118±0.004 0.026±0.001 n.a.

5.50 5.87 26.66 n.a.

0.9746 0.9702 0.8032 n.a.

-1

2

UV intensity (% I0) b

kobs (min-1)

t1/2 (min)

R2

100 75 50 30

0.118±0.004 0.059±0.001 0.036±0.001 0.020±0.002

5.87 11.75 19.25 34.66

0.9702 0.9896 0.9562 0.9658

Note -7 -2 -1 a. UV intensity was kept at 100% I0 = 6.6×10 einstein cm s . b. Initial solution pH was kept at 9.2.

26


Page 27 of 31


552

Table 2. Comparison of reductive PFOS decomposition in this study with other

553

reported photo-reductive treatments Method

Direct UV (catalystfree)

Direct UV

UV-isopro panol

UV-KI

UV/sulfite

Conditions [PFOS]=37.2 µM PBS:6.0 mM pH=11.8, 100 oC V=1000 mL MPMLd: 500 W [PFOS]=20 µM pH=12.5, 25 oC V=400 mL LPMLe: 23W (185nm) [PFOS]=40 µM [NaOH]=68 mM V=750 mL (isopropanol) 38–50 oC LPMLe: 32W [PFOS]=20 µM [KI]=10 mM V=30 mL ambient temperature LPMLe: 8W [PFOS]=32 µM [Na2SO3]=10 mM V=25 mL pH 9.2, 25 oC HPMLf: 250W

ka (h-1)

0.91

0.0175

Timeb (h)

-

> 48

Energyc (kJ /µmol)

Reference

73.7

25

-

17

0.039

-

137

18

0.18

> 2.5

370

20

7.08

0.5

220

This work

Note a. Pseudo-first-order rate constants b. Reaction time needed for achieving 50% defluorination efficiency c. Energy consumptions required to decompose PFOS to half of its initial concentration d. Medium-pressure mercury lamp with a low UVC luminous efficiency, which also acted as the heat source e. Low-pressure UV lamp mainly emits at 254 nm unless otherwise specified f. High-pressure UV lamp with a high UVC luminous efficiency mainly emits at 200-400 nm

27



554

Table 3. Relative energies of dissociation fragments with respect to C8F17SO3·2△E (kcal/mol)

Dissociation fragments C8F17 C7F15

-

-

SO3·

-

-30.99372

CF2SO3·

-

-11.85824 -16.92018

C6F13

-

C2F4SO3·

-

C5F11

-

C3F6SO3·

-

-16.66284

-

-

-16.50709 -15.42721

C4F9

C4F8SO3·

C3F7

-

C5F11SO3·

-

C2F5

-

C6F12SO3·

-

-11.37048

-

-

-2.866026

CF3 F

-

C7F14SO3· C8F16SO3·

-

-12.03953

28


Page 28 of 31

Page 29 of 31


1.0

80

0.8

C/C0

60 0.6 sole UV - C/C0

0.4

40

sole sulfite - C/C0 UV/sulfite - C/C0

20 UV/sulfite defluorination efficiency

0.2

0.0 0

5

10

15

20

25

Defluorination efficiency (%)

100

1.2

0 30

555

Time (min)

556

Figure 1. PFOS decomposition in the UV/sulfite system, with sole UV irradiation,

557

and with sole sulfite addition in the solution.

29



Page 30 of 31

558

(a) 1.0

0.8

C/C0

0.6

0.4 100% I0 75% I0 50% I0

0.2

30% I0

0.0 0

5

10

15

20

25

30

20

25

30

Time (min)

(b)

80

Dfluorination efficiency (%)

100% I0 75% I0 50% I0

60

30% I0

40

20

0 0

5

10

15

Time (min)

559 560

Figure 2. (a) PFOS decomposition, and (b) defluorination efficiency at varied UV

561

emission intensity. (I0 = 6.6×10−7 einstein/cm2·s).

562 30


Page 31 of 31


0.14 PFOA PFHpA PFHxA PFPeA PFBA

0.12

C (µM)

0.10

0.08

0.06

0.04

0.02

0.00 0

563 564

10

20

30

40

Time (min)

Figure 3. Intermediate products detected during PFOS decomposition.

565 566

Figure 4. Lowest unoccupied molecular orbital (LOMO) of PFOS.

567 568

31


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