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Remediation and Control Technologies

Enhanced Perfluorooctanoic Acid Degradation by Electrochemical Activation of Sulfate Solution on B/N Codoped Diamond Yanming Liu, Xinfei Fan, Xie Quan, Yaofang Fan, Shuo Chen, and Xueyang Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06130 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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Environmental Science & Technology

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Enhanced Perfluorooctanoic Acid Degradation by Electrochemical

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Activation of Sulfate Solution on B/N Codoped Diamond Yanming Liu,† Xinfei Fan,‡ Xie Quan,*,† Yaofang Fan,† Shuo Chen,† and Xueyang Zhao†

3 4

†Key

5

School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China.

6

‡ College

Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China),

of Environmental Science and Engineering, Dalian Maritime University, Dalian 116024, China.

1

ACS Paragon Plus Environment


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Abstract: Electrochemical oxidation based on SO4•‒ and •OH generated from sulfate electrolyte is a cost-

8

effective method for degradation of persistent organic pollutants (POPs). However, sulfate activation

9

remains a great challenge due to lack of active and robust electrodes. Herein, B/N codoped diamond (BND)

10

electrode is designed for electrochemical degradation of POPs via sulfate activation. It is efficient and stable

11

for perfluorooctanoic acid (PFOA) oxidation with first-order kinetic constants of 2.4 h-1 and total organic

12

carbon removal efficiency of 77.4% (3 h) at relatively low current density of 4 mA cm-2. The good activity

13

of BND is mainly originated from B and N codoping effect. PFOA oxidation rate at sulfate electrolyte is

14

significantly enhanced (2.3-3.4 times) compared with those at nitrate and perchlorate electrolytes. At sulfate,

15

PFOA oxidation rate decreases slightly in the presence of •OH quencher while it declines significantly with

16

SO4•‒ and •OH quenchers, indicate both SO4•‒ and •OH contribute to PFOA oxidation but SO4•‒ contribution

17

is more significant. Based on intermediates analysis, proposed mechanism for PFOA degradation is that

18

PFOA is oxidized to shorter chain perfluorocarboxylic acids gradually by SO4•‒ and •OH until it is

19

mineralized.

20

TOC: C7F15COOH

BND anode

SO42‒ e-

e-

SO4• ‒ •OH

CO2

F‒

H2O 21

2


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Introduction

23

Persistent organic pollutants (POPs) are globally distributed in environment with increasing amount.

24

They are refractory, highly toxic and bioaccumulative, which have caused environmental and human health

25

concerns. POPs such as perfluorooctanoic acid (PFOA) are difficult to be degraded by most conventional

26

methods and frequently detected in environment. Developing efficient methods to degrade these

27

contaminants is urgently needed. Electrochemical oxidation has attracted increasing interest in water

28

treatment due to its versatility, capability for various contaminants degradation, mild and reagent free

29

conditions.1,2 It can be powered by solar electricity, which makes it promising and sustainable.

30

Indirect electrochemical oxidation through reactive specials (•OH, SO4•‒, O3, ClO‒ etc.) generated on

31

anode is a preferable pathway for POPs degradation as it is usually more efficient than direct oxidation.2-6

32

SO4•‒ and •OH radicals have strong oxidation ability towards a broad spectrum of refractory organic

33

pollutants (Eº(SO4•‒/SO42‒) = 2.5-3.1 V, Eº(•OH/H2O) = 1.8-2.7 V).7,8 SO4•‒ is considered to be more

34

effective than •OH for organic contaminants oxidation due to its electrophilic property9,10 and longer

35

lifetime (SO4•‒ = 30-40 μs, •OH < 1 μs).11,12 It can be generated by peroxymonosulfate,13 persulfate14-16 or

36

sulfate activation,17,18 where sulfate activation is more attractive given that sulfate commonly exists in

37

natural water and wastewater. During SO4•‒ generation via electrochemical activation of sulfate solution,

38

•OH

39

POPs degradation. It is well known the activity for sulfate activation and POPs degradation mainly depends

40

on properties of electrode. Many electrodes (PbO2, B doped diamond (BDD), SnO2 etc.) are found to be

41

active for electrochemical oxidation of pollutants via reactive specials generated in situ,20-23 but only BDD

42

can electrochemically activate sulfate to produce SO4•‒. Despite the progress has been made, searching for

43

highly active electrodes which can enhance kinetics and decrease energy consumption for electrochemical

44

activation of sulfate solution to generate SO4•‒ and •OH has been actively pursued.

45

can be produced simultaneously.17,19 The coupling effects of SO4•‒ and •OH oxidation may facilitate

B and N codoped diamond (BND) is an attractive electrode material with high oxygen evolution potential, 3



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outstanding electrochemical stability and chemical inertness.24 Previous works show the synergistic effect

47

of B and N codoping into diamond can enhance its electrocatalytic performance.25,26 B and/or N doping can

48

tune the electronic structure of carbon materials and polarize the C atoms adjacent to B and N, resulting in

49

enhanced activity for electrocatalysis.27-29 The polarized carbon atoms, strain and defect sites introduced by

50

B and N doping may act as active sites for electrocatalysis.30,31 Therefore, BND is expected to enhance the

51

performance for electrochemical activation of sulfate solution and POPs degradation. However, BND

52

hasn’t been explored for electrochemical oxidation of pollutants before.

53

Herein, BND is explored for electrochemical degradation of POPs via sulfate solution activation.

54

perfluorooctanoic acid (PFOA) were selected as the targets to evaluate its performance for POPs oxidation.

55

The B and N codoping effect on electrochemical oxidation of PFOA via sulfate solution activation was

56

investigated as well as the reactive specials responsible for PFOA oxidation. The mechanism for PFOA

57

mineralization was proposed based on intermediate analysis.

58

Materials and Methods

59

Chemicals and Materials. All chemicals with high purity (analytical and HPLC grade) were used as

60

received without further purification. Perfluorocarboxylic acids (C2-C8, 96-98%) were purchased from

61

J&K Scientific Ltd. Acetonitrile and methanol were obtained from Sigma-Aldrich. The other reagents

62

including sodium sulfate, sodium nitrate, sodium perchlorate, ammonium acetate and tert-butanol were

63

supplied by Sinopharm Chemical Reagent Co., Ltd. Nanodiamond powder (50 nm) was purchased from

64

Beijing Grish Hitech Co. Ti substrates (≥99.9%) were purchased from Central Iron & Steel Research

65

Institute. Milli-Q water was used for all the experiments.

66

Preparation of BND, BDD and NDD electrodes. BND, BDD and NDD films were deposited on Ti

67

substrates by hot filament chemical vapor deposition (HFCVD). The Ti sheet was polished by sandpaper,

68

and ultrasonically washed by ethanol and ultrapure water for 10 min, respectively. To further polish its

69

surface and facilitate diamond nucleation, Ti sheet was ultrasonically pretreated for 20 min in nanodiamond 4


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suspension with acetone as solvent. BND film deposition was performed on the pretreated Ti for 12 h with

71

a gas mixture of CH4 (2.5%)/H2 (85%)/N2 (2.5%)/B2H6 (10%) at 600 ºC (substrate temperature) and 600

72

Pa. The total gas flow rate is 100 sccm and temperature ramping rate is about 10 ºC/min. As comparison,

73

BDD and NDD electrodes were prepared under the same conditions without N2 and B2H6, respectively

74

(The cost and scalability for BND preparation were discussed in supporting information).

75

Electrochemical Experiments. Electrochemical degradation of PFOA was performed in a three-electrode

76

system with home-made H cell and batch mode. Doped diamond was used as working electrode, Pt foil and

77

Ag/AgCl (saturated KCl) as the counter electrode and reference electrode, respectively. The anodic

78

chamber and cathodic chamber were separated by Nafion 117 membrane. 0.05 M Na2SO4 was used as

79

electrolyte. 30.0 mL Na2SO4 electrolyte was filled on each chamber. The distance between working

80

electrode and counter electrode was 2.5 cm. The effective area of working electrode was 10.5 cm2.

81

Chronopotentiometric electrolysis of PFOA were conducted at current density of 0.5~4.0 mA cm-2 on CHI

82

660 potentiostat. To investigate the effect of sulfate activation on PFOA degradation, NaNO3 (79.4 mM)

83

or NaClO4 (81.8 mM) with the same conductivity as 0.05 M Na2SO4 (8.4 mS cm-1) were also used as

84

electrolytes for electrochemical oxidation of PFOA. The initial solution pH for the mixture of electrolyte

85

and PFOA was 4.8 without pH adjustment, which was not controlled during PFOA degradation and

86

decreased to 4.2 after PFOA degradation for 2 h at 4.0 mA cm-2. For selected PFOA degradation

87

experiments, tert-butanol and methanol were added into 0.05 M Na2SO4 electrolyte as radical scavengers

88

with final concentration of 1.0 M and 1.5 M.

89

Analytical Methods. PFOA concentration was measured by Waters 2695 HPLC equipped with a C18

90

column (4.6 mm × 250 mm × 5.0 μm) and UV detector. Isocratic elution was used with acetonitrile and 20

91

mM sodium dihydrogen phosphate (v/v=55/45) as mobile phase at flow rate of 1.0 mL min-1. The

92

intermediates of PFOA degradation were analyzed by liquid chromatography-Triple Quadrupole mass

93

spectrometer (Agilent 1100-6410) with C18 column (2.1 mm × 100 mm × 3.5 μm) and a mixture of 5



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acetonitrile (A)/10 mM ammonium acetate (B) as mobile phase. The mobile phase increased linearly from

95

40% A (initial) to 90% A in 9 min and kept at 90% for 3min, followed by returning to 40% A in 6.5 min

96

and held at 40% for 2.0 min. Its flow rate was 0.25 mL min-1. A negative ESI multiple reaction monitoring

97

(MRM) mode was adopted for intermediates identification. The capillary potential was 4 kV and gas

98

temperature was 350 ºC. Total organic carbon (TOC) was tested by a TOC analyzer (multi N/C 2100,

99

Analytik Jena, Germany).

100

Results and Discussion

101

BND electrode characterization. The morphology of synthesized BND electrode was observed from

102

scanning electron microscope (SEM). As shown in Figure 1a, a continuous and crack free BND film is

103

covered on Ti substrate. The BND film consists of pyramidal particles with edges and corners. The grain

104

size of BND is about 50-200 nm. In the X-ray diffraction (XRD) spectrum (Figure 1b), diffraction peaks at

105

43.9º and 75.3º appear on BND, which arises from (111) facet and (220) facet of cubic diamond. The sharp

106

peaks indicate BND has good crystallinity. The peaks located at 35.2º, 63.1º and 70.7º are attributed to Ti,

107

while the other peaks ascribe to TiC. These peaks usually can be observed when carbon films are deposited

108

on Ti substrate.32,33 X-ray photoelectron spectroscopy (XPS) analysis shows peaks around 186.9 eV and

109

400.5 eV arise from B-C and N-C bonds (Figure S1), revealing B and N atoms have been doped into

110

diamond lattice. The B doped into BND is 1.8 at%, while its N content is 0.8 at%. (b)

(a) Intensity (a.u.)

Ti TiC

Ti

TiC diamond (111) TiC

diamond (220) Ti

500 nm

TiC

20

30

111 112

40

50 60 2theta (deg.)

70

Figure 1. (a) SEM image and (b) XRD spectrum of BND electrode. 6


80

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Electrochemical oxidation of PFOA by BND electrode. The performance of BND electrode for POPs

114

degradation was evaluated by using PFOA as a representative pollutant. Figure 2a shows PFOA is rapidly

115

degraded on BND electrode with 0.05 M Na2SO4 as electrolyte. Its removal efficiency enhances gradually

116

as current density increases from 0.5 to 4.0 mA cm-2 (potentials in Figure S2). A high PFOA removal

117

efficiency of 99.3% is achieved within 1.5 h at current density of 4.0 mA cm-2. All the PFOA oxidation

118

processes follow pseudo-first-order kinetics (Figure S3). The kinetic constant for PFOA oxidation is 2.40

119

h-1 at current density of 4.0 mA cm-2. It increases fast (1.14 h-1 to 1.93 h-1) when current density increases

120

from 2.0 mA cm-2 to 3.0 mA cm-2. Although the rate constant further increases to 2.40 h-1 at 4.0 mA cm-2,

121

the rate increase becomes less pronounced compared with that from 2.0 mA cm-2 to 3.0 mA cm-2. Therefore,

122

current density of 4.0 mA cm-2 was selected for the subsequent PFOA degradation experiments.

123

Interestingly, BND exhibits significantly enhanced PFOA degradation rate (2.40 h-1 at 4.0 mA cm-2,

124

corresponding to ~2.5 V vs SCE) compared with other electrodes reported (0.52-2.22 h-1),34-36 even though

125

these electrodes operated under much higher current density (20.0 mA cm-2) or potential (3.37 vs SCE). 0.5 mA cm-2 1.0 mA cm-2

100

2.0 mA cm-2 3.0 mA cm-2 4.0 mA cm-2

40 30 20 10 0

126

(a)

50

TOC removal efficiency (%)

PFOA concentration (mg L-1)

113

0

20

40 60 80 Time (min)

100

(b) 80 60 40 20 0

120

2 mA cm-2

3 mA cm-2

4 mA cm-2

127

Figure 2. (a) PFOA oxidation on BND electrode at 0.05 M Na2SO4 electrolyte with current density of 0.5-

128

4.0 mA cm-2, (b) corresponding TOC removal efficiency after 3 h.

129

To further investigate the performance of BND electrode for PFOA mineralization, total organic carbon

130

(TOC) removal efficiency were determined during PFOA degradation at 2.0-4.0 mA cm-2 (0.05 M Na2SO4).

131

Figure 2b shows TOC removal improves noticeably at higher current density. After electrochemical 7



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oxidation of PFOA for 3 h, TOC removal efficiency is 77.4% at 4.0 mA cm-2 (PFOA concentration and

133

TOC removal efficiency versus charge are shown in Figure S4). These results suggest BND electrode is

134

efficient for decomposing PFOA to CO2 at low current density.

135

The impact of B and N codoping on PFOA oxidation were probed by comparing the performance of

136

BND electrode with B doped diamond (BDD) and N doped diamond (NDD) electrodes. SEM images

137

(Figure S5) show both BDD and NDD films are composed of pyramidal particles, which cover the Ti

138

substrates without noticeable crack. XRD patterns (Figure S6) illustrate BDD and NDD are cubic diamond

139

with (111) facet and (220) facets (diffraction peaks related to Ti and TiC also can be observed on BDD and

140

NDD electrodes, the same as BND electrode). CV curves reveal BND, BDD and NDD electrodes have

141

similar oxygen evolution potential (Figure S7). Thus, BND, BDD and NDD electrodes have similar

142

morphology, oxygen evolution potential and identical crystal structure. XPS analysis reveal the B content

143

of BDD (1.8 at%) and N content of NDD (0.9 at%) are similar to those of BND (1.8 at% B and 0.8 at% N,

144

Table S1). During PFOA oxidation (Figure 3a), BDD presents slightly higher PFOA removal efficiency

145

(92.8% in 2 h) than NDD (86.4% in 2 h). As expected, both of their PFOA removal efficiencies are much

146

lower than that of BND (99.3% in 1.5 h) under the same conditions (0.05 M Na2SO4, 4.0 mA cm-2). The

147

kinetic constant for PFOA oxidation is 1.2 h-1 on BDD and 1.0 h-1 on NDD (Figure S8a). Note that PFOA

148

oxidation rate on BND is 2.0-2.4 times greater than those on BDD and NDD. TOC removal efficiency on

149

BND is also much higher (Figure S8b). As BND, BDD and NDD electrodes have identical crystal structure,

150

similar morphology and dopant content, the enhanced performance of BND electrode could be attributed

151

to B and N codoping effect. B and N codoping improves the electron transfer rate of doped diamond

152

electrode (Figure S9). The electron-deficient B and electron-rich N can induce active sites for

153

electrocatalysis, which play important roles in different elementary reactions during electrocatalytic process,

154

and thereby result in lower potential and improved kinetics for electrochemical oxidation reaction via B/N

155

codoping.25-31 Thus, BND shows better performance than BDD and NDD for PFOA oxidation via 8


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electrochemically activate sulfate solution.

158

degradation experiments (batch mode) at the highest current density applied (4.0 mA cm-2). Its PFOA

159

removal efficiency exhibits no obvious change during 20 consecutive PFOA degradation experiments

160

(Figure 3b), implying BND electrode is stable and reusable for electrochemical oxidation of POPs. The

161

good durability of BND is originated from the chemical and mechanical stability of diamond,37 which is

162

important for cost-effective advanced oxidation techniques. 50

NDD BDD BND

40 30 20 10 0

163

(a)


The stability of BND electrode for electrochemical oxidation was examined by 20 consecutive PFOA

PFOA concentration (mg/L)

157

0

20

40 60 80 Time (min)

100

120

50

(b)

40 30 20 10 0

1st

5th

10th 15th Cycle

20th

164

Figure 3. (a) PFOA oxidation on BND, BDD and NDD electrodes and (b) 20 cycles of PFOA oxidation on

165

BND electrode (0.05 M Na2SO4, 4.0 mA cm-2).

166

Effects of electrolytes and specific radical scavengers on PFOA oxidation. The possible oxidative

167

species electrogenerated from sulfate solution activation include SO4•‒, •OH and S2O82-.1,17 To probe which

168

species contributed to the superior performance of BND for electrochemical oxidation, PFOA degradation

169

was also conducted in NaNO3 and NaClO4 electrolytes with the same conductivity as 0.05 M Na2SO4 (8.4

170

mS cm-1) given that both nitrate and perchlorate ions are usually inert during electrochemical oxidation

171

process. Linear sweep voltammograms of BND show its current density at Na2SO4 is larger than those at

172

NaClO4 and NaNO3 when potential is more positive than 2.2 V, which may be originated from SO42‒

173

oxidation (Figure S10). Figure 4a shows PFOA removal efficiency is 76.1% in NaNO3 (8.4 mS cm-1) and

174

87.1% in NaClO4 (8.4 mS cm-1) after 2 h electrolysis on BND electrode at 4.0 mA cm-2, while PFOA is 9



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already completely removed in Na2SO4 (8.4 mS cm-1) under the same conditions. As expected, PFOA

176

oxidation in Na2SO4 electrolyte is much faster. The kinetic constant for PFOA oxidation is 2.4-3.9 times

177

higher in Na2SO4 electrolyte than those in NaNO3 and NaClO4 electrolytes (Figure S11a). Meanwhile, TOC

178

removal efficiency in Na2SO4 is significantly enhanced relative to those in NaNO3 and NaClO4 (Figure

179

S11b). Since •OH can be produced from all the three electrolytes, the enhanced performance in Na2SO4

180

may be attributed to active species electrogenerated from sulfate activation. Both SO4•‒ and S2O82- species

181

can be produced from sulfate activation (After electrolysis on BND for 1 h and 2 h at 0.05 M Na2SO4 and

182

4.0 mA cm-2, the S2O82- produced is 6.8 mg L-1 and 13.0 mg L-1 (Figure S12) and H2O2 is undetectable,

183

details in supporting information), but S2O82- is unlikely to oxidize PFOA.38 Therefore, the enhanced

184

performance of BND electrode in Na2SO4 electrolyte relative to NaNO3 and NaClO4 electrolytes is

185

probably originated from SO4•‒ oxidation.

186

To further elucidate the contribution of SO4•‒ and •OH radicals for PFOA oxidation in Na2SO4 electrolyte,

187

PFOA electrolysis on BND electrode was performed in the presence of radical scavengers, methanol and

188

tert-butanol (TBA). Methanol has similar reaction rate towards SO4•‒ and •OH (kSO4•‒=1.0107 M-1 s-1,

189

k•OH=9.7108 M-1 s-1), whereas TBA lacking α-hydrogen has higher reactivity for •OH than SO4•‒ with 3

190

orders of magnitude greater reaction rate constant (kSO4•‒=4.0-9.1105 M-1 s-1, k•OH=3.8-7.6108 M-1 s-1).7,39

191

Figure 4b shows the inhibition effects of methanol and tert-butanol on PFOA oxidation by BND at 0.05 M

192

Na2SO4 and 4.0 mA cm-2. In the presence of TBA, PFOA oxidation is suppressed relative to that without

193

alcohol. Its removal efficiency is 95.4% (2 h) with the addition of 1.0 M TBA, and it further decreases to

194

91.8% (2 h) at 1.5 M TBA, while PFOA is almost degraded (99.3%) at 1.5 h without alcohol. The declined

195

performance induced by TBA indicates •OH contributes to PFOA oxidation. The increase of TBA

196

concentration results in slightly decreased PFOA removal efficiency, implying 1.5 M TBA is almost

197

sufficient to scavenge •OH. However, PFOA removal efficiency decreases dramatically to 73.9% (2 h) in

198

the presence of 1.0 M methanol, which further decreases to 56.9% (2 h) at 1.5 M methanol (PFOA removal 10


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efficiency under SO4•‒ and •OH radicals completely quenched is difficult to obtain because methanol is easy

200

to be electrochemically oxidized17 on BND, which can be revealed by the low PFOA removal efficiency of

201

4.4% in the first 0.33 h and gradually increased PFOA oxidation rate from 0.33 h to 2 h with 1.5 M

202

methanol). At the first 20 min, only 4.4% of PFOA is removed in 1.5 M methanol while 49.0% of PFOA

203

is removed without methanol, indicating PFOA is primarily degraded via indirect oxidation. The

204

significantly declined performance in methanol relative to TBA implies SO4•‒ is generated via sulfate

205

activation, and it play a major role on PFOA oxidation.

207

oxidation on BND electrode, and the contribution of SO4•‒ is more significant. During electrochemical

208

oxidation process, the in situ electrogenerated SO4•‒ (SO42‒ ‒ e‒ → SO4•‒) reacts with PFOA, resulting in

209

the reduction of SO4•‒ to SO42‒. SO42‒ is attracted to the positively charged BND anode, which can be

210

electrochemically oxidized to SO4•‒ again on BND. The continuous regeneration of SO4•‒ may promote

211

PFOA oxidation by SO4•‒. Considering SO4•‒ oxidation via electron-transfer and •OH oxidation via

212

hydrogen abstraction and addition to unsaturated bonds, SO4•‒ oxidation combined with •OH oxidation may

213

promote contaminants oxidation kinetics.

214

50

(a)


The effects of electrolyte and radical scavengers verify that both SO4•‒ and •OH are responsible for PFOA


206

NaNO3 NaClO4 Na2SO4

40 30 20 10 0

0

20

40 60 80 Time (min)

100

120

1.5 M methanol 1.0 M methanol 1.5 M TBA 1.0 M TBA without alcohol

(b)

50 40 30 20 10 0

0

20

40

60 80 Time (min)

100 120

215

Figure 4. PFOA oxidation on BND electrode (a) at 0.05 M Na2SO4 (8.4 mS cm-1), NaNO3 (8.4 mS cm-1),

216

NaClO4 (8.4 mS cm-1) electrolytes and (b) at 0.05 M Na2SO4 with the presence of methanol or tert-butanol,

217

current density of 4.0 mA cm-2. 11



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PFOA oxidation mechanism. The organic intermediates produced during electrochemical oxidation of

219

PFOA on BND electrode (0.05 M Na2SO4, 4.0 mA cm-2) were determined by LC-MS-MS. The

220

intermediates identified are short-chain perfluorinated carboxylic acids (PFCAs). Figure 5a shows the time

221

dependent concentration of these short-chain PFCAs. Their concentrations first increase gradually with

222

time, and then decrease after reaching the maximum values. At the initial 1 h of PFOA degradation,

223

perfluoroheptanoic acid (C6F13COOH), perfluorohexanoic acid (C5F11COOH) and perfluoropentanoic acid

224

(C4F9COOH) are the main intermediates. The concentration of C6F13COOH increases during the first 0.67

225

h, and then declines over time after achieving maximum at 0.67 h, while concentrations of C5F11COOH

226

and C4F9COOH starts to decrease after reaching maximum at 1 h. Further electrolysis results in increased

227

concentrations of perfluorobutanoic acid (C3F7COOH), perfluoropropionic acid (C2F5COOH) and

228

trifluoroacetic acid (CF3COOH). Their concentrations decline after 2 h electrolysis. These results can be

229

explained by that the produced PFCAs intermediate is further oxidized to shorter chain PFCAs, followed

230

by mineralization to CO2, which can be revealed by the high TOC removal efficiency of 93.7% at 4 h

231

(Figure S13). The low concentrations of all PFCAs intermediates after 2 h electrolysis suggest PFOA can

232

be effectively mineralized by BND electrode. C6F13COOH C5F11COOH C4F9COOH

6 3

0

30

C3F7COOH C2F5COOH CF3COOH

100

60 90 120 150 180 Time (min)

Short-chain PFCAs TOC PFOA

(b)

80 60 40 20 0

100 F mass balance (%)

9

0 233

(a)

C mass balance (%)

Concentration (mg L-1)

12

0

20 60 120 Time (min)

180

(c)

Short-chain PFCAs FPFOA

80 60 40 20 0

0

20 60 120 Time (min)

180

234

Figure 5. (a) Concentrations of short-chain perfluorinated carboxylic acids, (b) C and (c) F mass balance

235

during PFOA oxidation on BND electrode at 0.05 M Na2SO4 and 4.0 mA cm-2.

236

The C mass balance during PFOA oxidation process was investigated to examine whether the main 12


Page 13 of 19


237

organic intermediates have been identified. Figure 5b shows the C mass balance calculated from (nC of

238

undegraded PFOA

239

are obtained during 3 h of PFOA oxidation on BND electrode (4.0 mA cm-2, 0.05 M Na2SO4), suggesting

240

the main organic intermediates have been detected. The F mass balance is calculated from (nF of undegraded

241

PFOA

242

(Figure 5c). The defluorination efficiency is 76.8% after 3 h.

+ nTOC removed + nC of PFCAs intermediates)/nC of initial PFOA. A high C recovery rates of 90.7%-96.2%

+ nF of PFCAs intermediates + nF ion)/nF of initial PFOA, which is 90.3%-93.6% during 3 h of PFOA oxidation

243

The possible mechanism for PFOA degradation (Figure 6) is proposed based on the PFCAs intermediates

244

analysis and the reported pathways for PFOA oxidation by SO4•‒ and •OH radicals,14,35,36,38 as radical

245

quenching experiments reveal they are mainly responsible for PFOA oxidation on BND. For SO4•‒

246

oxidation, direct electron transfer occurs between SO4•‒ and carboxylic group of PFOA (C7F15COOH) due

247

to its electrophilic property, leading to the formation of C7F15COO• and subsequent Kolbe decarboxylation

248

reaction which transfers C7F15COO• to C7F15•. The produced C7F15• reacts with oxygen and proton in

249

solution and transfers to C7F15OH (C6F13CF2OH). As C6F13CF2OH is unstable, it will convert to C6F13COF

250

by intramolecular rearrangement, which will further undergo hydrolysis and then generate C6F13COOH.

251

Meanwhile, PFOA can be oxidized by •OH, where PFOA loses one electron at anode as it is difficult to be

252

oxidized by •OH directly, followed by decarboxylation reaction, and then the formed C7F15• reacts with

253

•OH,

254

mentioned above. For both pathways, the formed C6F13COOH intermediate will undergo the same CF2

255

unzipping reactions as PFOA and transfer to C5F11COOH. The CF2 unzipping reactions will continue to

256

occur until PFCAs are oxidized to CO2.

resulting in C7F15OH formation, which is further converted to C6F13COOH by the same pathway

13



+ SO4•‒ C7F15COO• … ‒ CO2 C7F15•

CO2

+ O2

+ H+

258

‒ e‒ …

C6F13COO‒ + H2O C7F13OF

C7F15O•

257

C7F15COO‒ repeat cycle

‒ HF

Page 14 of 19

C7F15COO• ‒ CO2

CO2

C7F15• •OH

C7F15OH

Figure 6. The possible pathways for PFOA oxidation on BND electrode via sulfate solution activation.

259

Environmental Implications. This study demonstrates BND is an attractive alternative electrode

260

material for electrochemical mineralization of POPs based on SO4•‒ and •OH electrogenerated from sulfate

261

solution. The main challenges of advanced oxidation techniques for POPs degradation are to address the

262

problems associated with slow kinetics, high cost and secondary contamination. Here, both SO4•‒ and •OH

263

radicals are powerful oxidants. Given that mechanism for SO4•‒ (via electron-transfer) and •OH oxidation

264

(addition to unsaturated bonds and hydrogen abstraction) is different and SO4•‒ has longer lifetime and

265

wider working pH range than •OH, advanced oxidation techniques based on SO4•‒ and •OH radicals could

266

be more effective than that based on •OH radical. Since sulfate is abundant in natural aqueous environments,

267

SO4•‒ generation via sulfate activation is more economical and sustainable than persulfate or

268

peroxymonosulfate activation. For aquatic system with chloride, SO4•‒ mediated oxidation can be

269

hindered.40,41 Most importantly, BND electrode exhibits fast kinetics and low energy consumption for POPs

270

degradation via sulfate solution activation, as revealed by its high PFOA oxidation rate (2.4 h-1) at low

271

current/potential (4.0 mA cm-2 and ~2.5 V). Its current efficiencies for PFOA oxidation are 1.2-1.6% (4.0

272

mA cm-2 at 1.0-1.5 h), much higher than values reported.35,36 The future challenge will be further improving

273

its efficiency. Besides, the BND electrode we proposed for sulfate activation is composed of C, B and N,

274

which is reusable for a long time and can avoid the potential contamination caused by metals or

275

persulfate/peroxymonosulfate added. This study proposed a new electrocatalytic materials (BND) for 14


Page 15 of 19


276

simultaneously generating SO4•‒ and •OH via sulfate solution activation. It can significantly enhanced POPs

277

oxidation kinetics, decrease cost (energy consumption, electrolytes cost) and avoid secondary pollution,

278

which is appealing for water treatment.

279

Associated Content

280

Supporting Information. The Supporting Information is available free of charge on the ACS Publications

281

website at DOI: .

282

XPS spectrum of BND, PFOA removal kinetics of BND at sulfate, chronopotentiometric curve, SEM

283

images and XRD spectra of BDD and NDD, B/N contents of three electrodes, PFOA removal kinetics

284

of BDD and NDD at sulfate and BND at nitrate and perchlorate

285

Author Information

286

Corresponding Author

287

*E-mail: [email protected]

288

Notes

289

The authors declare no competing financial interest.

290

Acknowledgment

291

This work was supported by the National Natural Science Foundation of China (21707016 and 51708085),

292

China Postdoctoral Science Foundation (2016M600205) and open project of State Key Laboratory of Urban

293

Water Resource and Environment, Harbin Institute of Technology (HC201705).

294

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