Photochemical Anti-Fouling Approach for Electrochemical Pollutant

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Photochemical Anti-Fouling Approach for Electrochemical Pollutant Degradation on Facet-Tailored TiO2 Single Crystals Chang Liu, Ai-Yong Zhang, Yang Si, Dan-Ni Pei, and Han-Qing Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04105 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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Environmental Science & Technology

Photochemical Anti-Fouling Approach for Electrochemical Pollutant Degradation on Facet-Tailored TiO2 Single Crystals

Chang Liu1, Ai-Yong Zhang1,2,*, Yang Si1, Dan-Ni Pei1, Han-Qing Yu1,* 1

CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China

2

Department of Municipal Engineering, Hefei University of Technology, Hefei, 230009, China

*Corresponding authors: Dr. Ai-Yong Zhang, E-mail: [email protected]; Prof. Han-Qing Yu, E-mail: [email protected].

1

ACS Paragon Plus Environment


1

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ABSTRACT

2

Electrochemical degradation of refractory pollutants at low bias before oxygen

3

evolution exhibits high current efficiency and low energy consumption, but its

4

severe electrode fouling largely limits practical applications. In this work, a new

5

anti-fouling strategy was developed and validated for electrochemical pollutant

6

degradation by photochemical oxidation on facet-tailored {001}-exposed TiO2

7

single crystals. Electrode fouling from anodic polymers at a low bias was greatly

8

relieved by the free ·OH-mediated photocatalysis under UV irradiation, thus

9

efficient and stable degradation of bisphenol A, a typical environmental endocrine

10

disrupter, and treatment of landfill leachate were accomplished without remarkable

11

oxygen

12

Electrochemical and spectroscopic measurements indicated a clean electrode

13

surface during cyclic pollutant degradation. Such a photochemical anti-fouling

14

strategy for low-bias anodic pollutants degradation was mainly attributed to the

15

improved electric conductivity and excellent electrochemical and photochemical

16

activities of tailored TiO2 anodic material, whose unique properties originated from

17

the favorable surface atomic and electronic structures of high-energy {001} polar

18

facet and single-crystalline structure. Our work opens up a brand new approach to

19

develop catalytic systems for efficient degradation of refractory contaminants in

20

water and wastewater.

evolution

in

synergistic

photo-assisted

2


electrochemical

system.

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21

Table of Contents (TOC) Art

22

23

3



24

INTRODUCTION

25 26

The direct electrochemical oxidation (EC) at low bias before oxygen evolution has

27

several merits for water treatment, such as high current efficiency and low energy

28

consumption.1 Pollutants are oxidized at a low rate after interfacial adsorption,

29

without involvement of any substrate other than electron. One of its main bottlenecks

30

is anode fouling and activity deterioration from intermediates surface deposition.1

31

Photocatalysis is an efficient and exhaustive technology for advanced water

32

treatment,2 and is demonstrated to be an effective strategy to remove anodic

33

polymer.3-7. When PC and EC are combined into photo-assisted electrochemical

34

oxidation (PEC), the bias is sufficiently higher than anodic oxidation potentials of

35

both pollutants and water.8,9 In this case, synergistic effects are anticipated: the

36

improved PC activity due to anodic bias and oxygen evolution in EC and the

37

enhanced EC activity owning to active oxygen species in PC. Compared to those

38

combining indirect EC with indirect PC at high bias after oxygen evolution in

39

literatures,8-16 the combination of direct EC with indirect PC at low bias before

40

oxygen evolution is more attractive for water treatment due to its high current

41

efficiency, low energy consumption and relieved electrode fouling. Thus, EC bias

42

should be well controlled to be higher than pollutant oxidation potential but lower

43

than water oxidation potential. However, no study is available.

44

In PEC, electrode material should have both high PC and EC activities. However,

45

metal oxides usually lack either sufficient EC or PC activity due to their crystal and

4


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46

electronic structures,17 thus can't be directly used in PEC. To resolve this problem,

47

combining EC catalyst with PC catalyst onto one single side or two different sides of

48

electrode is an effective way to prepare PEC anodic material. Thus, the photochemical

49

properties of one substrate (e.g., TiO2 and γ-Bi2MoO6) and the electrochemical

50

properties of another substrate (e.g., boron-doped diamond, Pt and transition metal

51

oxides, such as RuO2, SnO2, PbO2, IrO2 and Ta2O5) are combined onto one single

52

electrode.8-16 Up to now, most of PEC systems are constructed on these mixed

53

electrodes by a complicated procedure, which might suffer from limited surface

54

reactive sites due to the reduced interfacial domains.18 Recently, several

55

single-component PEC electrodes, i.e., ZnWO4, Bi2WO6 and γ-Bi2MoO6, have been

56

developed,19-21 and their pollutant degradation is superior in synergistic PEC over the

57

sum of PC and EC. To develop simple, efficient and stable anodic material is of

58

considerable interest for PEC applications in water treatment.

59

TiO2 is widely used as a good PC material for water treatment, but usually not

60

recognized as an efficient EC material due to low conductivity and poor reactivity.22

61

However, with tailored structural modifications by self and/or guest doping, the

62

electrochemical properties of TiO2 can be largely improved.23 Very recently, we

63

improved the electrochemical capacity of TiO2 by finely tuning the geometrical and

64

electronic structures of surface localized constituent atoms.24-27 The shape-tailored

65

TiO2 single crystals (SCs) with dominant high-energy {001} polar facet (0.90 J m-2)

66

has been demonstrated as an efficient anodic material, and exhibits a great activity for

67

pollutants degradation via direct anodic pathway at low bias before oxygen

5



68

evolution.24-26 In addition to the excellent PC activity of TiO2 SCs,28-30 their emerging

69

EC activity is even more important for practical treatment of wastewaters, which are

70

often characterized of high concentration, heavy colority and turbidity. Considering

71

the dual functions of tailored TiO2 SCs, it might be an excellent PEC anodic material

72

to construct the photochemical anti-fouling model for water treatment.

73

In this work, we developed a new photochemical anti-fouling strategy for direct

74

anodic oxidation of refractory pollutants at low bias before oxygen evolution, by

75

using facet-tailored TiO2 SCs in PEC and bisphenol A (BPA), a typical environmental

76

endocrine disrupter, as target pollutant.31 PEC performance was comprehensively

77

explored in terms of degradation efficiency, anti-fouling performance and energy

78

consumption, with EC as reference. Furthermore, the application of such a PEC for

79

treatment of landfill leachate, a complex and refractory wastewater, was also

80

examined. In this way, the feasibilities of photochemical anti-fouling strategy and

81

TiO2-based PEC system for water treatment were demonstrated. The facet-tailored

82

TiO2 SCs exhibited high PC and EC activities, and were simple, abundant,

83

cost-effective, safe and easy to be prepared compared to the reported PEC

84

materials.8-21 Moreover, a unique EC-dominant PEC catalytic mechanism and

85

pollutant degradation pathway occurred on the TiO2 SCs electrode, in comparison

86

with the PC-dominant catalytic process in other TiO2-based PEC systems reported in

87

literature.32-34

88 89

MATERIALS AND METHODS

6


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90 91

Electrode Preparation and Characterization. Facet-tailored TiO2 SCs were

92

prepared with a solvothermal method (Figures S1-S3, please see Supporting

93

Information, SI, for details). Degussa P25, polycrystalline TiO2 particles dominantly

94

exposed by low-energy {101} facets (mean particle size of ~25 nm, anatase/rutile =

95

80:20, BET surface area of ~50 m2/g, exposed percentage of high-energy {001} facets

96

less than 5%), were purchased for reference (Degussa Co, Germany) (Figures S4-S7).

97

The well-defined Sb-SnO2 and Ti0.7Ru0.3O2 electrodes were also prepared by the

98

high-temperature decomposition and oxidation of chloride precursors, TiCl4, RuCl3

99

and SnCl2, respectively, onto the degreased and etched Ti substrate (Figures S8-S13).

100

TiO2 SCs were characterized in terms of morphology and structure (Figures S1,

101

S2, S5 and S6), their PC activity was evaluated by diffuse reflectance spectra (DRS)

102

and transient photocurrent response, EC activity was characterized by cyclic

103

voltammetry (CV) and differential pulse anodic stripping voltammetry (DPASV) in

104

three-electrode system, PEC activity was examined by electrochemical impedance

105

spectroscopy (EIS) and DPASV, either in dark or under UV irradiation (please see SI

106

for details). The anti-fouling properties of electrodes were evaluated by DPASV, CV,

107

DRS, Fourier transformation infrared spectroscopy (FTIR) and Raman analyses.

108

BPA Degradation Test. BPA degradation test was carried out in a cylindrical

109

three-electrode single-compartment cell at ca. 20 oC (Scheme S1). The anode had an

110

effective area of 6.0 cm2 with a TiO2 SCs loading of approximately 0.05 mg/cm2

111

(totally 0.30 mg dosage), and a Ti sheet with the same area was used as the cathode.

7



112

The gap between electrodes was 1.0 cm. A dosage of 80 mL 0.1 M Na2SO4 aqueous

113

solution containing 5~100 mg L-1 BPA was electrolyzed, and the applied bias (versus

114

SCE) was controlled in a range of 0.5 ~ 2.0 V with an electrochemical workstation

115

(CHI 760d, Chenhua Co., China). For UV illumination, a high-pressure 500 W Xe arc

116

lamp (PLS-SXE500, Trusttech Co., China) equipped with a 10 cm IR water filter was

117

used. The UV light intensity was ~ 2.1 mW cm-2 in a range of 300 ~ 400 nm, which

118

was measured at 3-cm distance from the central lamp by a commercial radiometer

119

(Model FZ-A, Photoelectric Instrument Plant of Beijing Normal University, China).

120

No additional external resistance or iR compensation was used in the PEC system.

121

Landfill Leachtate Treatment Test. The landfill leachate was collected from a

122

local municipal landfill site of Hefei City in Anhui, China during a period from May

123

2016 to August 2016, and then stored in a refrigerator (4 oC). The raw leachate was

124

initially treated on site by aerobic lagoon, denitrification and activated sludge process

125

to remove the biodegradable organic compounds and ammonia. The physicochemical

126

characteristics of the pretreated leachate are shown in Table S1. In UV-vis spectra of

127

the raw leachate, no characteristics in 250~700 nm were observed. Thus, 300 nm was

128

chosen to evaluate the color reduction.24

129

Analysis. UV-visible absorption and fluorescence spectra were recorded on a

130

spectrophotometer (UV-2401PC, Shimadzu Co., Japan) and spectrofluorophotometer

131

(RF-5301PC, Shimadzu Co., Japan) respectively. BPA was determined by

132

high-performance liquid chromatography (HPLC-1100, Agilent Inc., USA) with a

133

Hypersil-ODS reversed-phase column and detected at 254 nm using a VWD detector.

8


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134

Mobile phase was a mixture of water and methanol (30:70) delivered at a flow rate of

135

1 mL min-1. Mineralization was estimated from total organic carbon (TOC) (Vario

136

TOC cube, Elementar Co., Germany). Intermediates were identified by gas

137

chromatography mass spectrometry (GC/MS, GCT Premier, Waters Inc., USA) and

138

liquid chromatography mass spectrometer (LC-MS, LCMS-2010A, Shimadzu Co.,

139

Japan). COD was determined using the Standard Methods.24

140 141

RESULTS AND DISCUSSION

142 143

Morphology, Structure and PEC Properties of TiO2 SCs. The prepared

144

bipyramidal TiO2 SCs exhibited sheet shape and were enclosed by eight equivalent

145

{101} facets and two equivalent {001} facets (Figure S1a and b). The clear crystalline

146

lattice fringes and the indexed {001} zone axis diffraction further confirm their

147

single-crystalline structure (Figure S1c-e).28,29 The X-ray diffraction results highly

148

matched anatase TiO2 (JCPDS No. 21-1272), and the diffraction peak broadening was

149

attributed to their small crystal size. Deposition of TiO2 SCs onto carbon paper to

150

fabricate the PEC electrode was performed using dip-coating technology, and the

151

TiO2 SCs were spread uniformly and tightly with a loading of ca. 0.05 mg/cm2

152

(Figure S1f), suggesting that the carbonaceous substrate interacted with the TiO2 SCs

153

through physical adsorption, electrostatic binding or charge transfer interaction.

154

An efficient PEC material should simultaneously possess excellent PC and EC

155

activities on one single anode. Superior PC (Figure 1a and b), EC (Figure 1c and d)

9



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156

and PEC (Figure 1e-h) activities were observed for the TiO2 SCs electrode in terms of

157

optical absorption, electron transfer, photocurrent generation, electrochemical

158

response,

159

degradation (Figures S14-S19), compared to typical P25 and Ti0.7Ru0.3O2 electrodes

160

under the identical conditions. The reduced △Ep of the TiO2 SCs electrode indicates

161

its reduced kinetic barrier of electron transfer and/or improved active surface area for

162

interfacial reaction (Figure 1c), both of which are essential for the enhanced EC

163

activity from polar-facet engineering and single-crystalline structure.35 The high EC

164

activity of the TiO2 SCs electrode was further confirmed by its large peak current for

165

the BPA anodic oxidation (Figure 1d).36 These electrochemical superiorities could be

166

attributed mainly to the single crystal structure and exposed high-energy polar {001}

167

facets.24-27 Moreover, by a combination of PC with EC, the electron transfer resistance

168

was further decreased, and the BPA oxidation and ·OH yield were substantially

169

improved for the PEC electrode (Figure 1e-h). All of these properties endowed the

170

{001}-TiO2 SCs as an excellent PEC material for pollutant degradation.

·OH-mediated

photoluminescence

(PL)

generation

and

pollutant

171

BPA Degradation Performance of PEC. The greatest BPA degradation rate was

172

obtained in the PEC (k = 26.92 × 10-3 min-1), which was much higher than the sum of

173

the PC (k = 0.41 × 10-3 min-1) and EC (k = 19.10 × 10-3 min-1) (Figure 2a). BPA of 30

174

mg/L-1 was completely degraded in 2.0 h in the PEC, rather than 4.0 h in EC. The

175

substantially enhanced BPA degradation efficiency in the PEC indicates a possible

176

synergistic effect when the EC and PC were combined on the TiO2 SCs electrode. It

177

should be mentioned that the observed low BPA removal efficiency in the PC did not

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178

indicate a weak photo-driven activity of the electrode, but should be ascribed to the

179

excessive BPA solution (80 ml) and insufficient catalyst dosage (0.3 mg). The

180

photocatalyst/solution weight/volume ratio, 0.00375 mg/ml, was only ca. 1/300 of the

181

typical value, 1 mg/ml, widely used in previous PC-related studies.2 Also, although a

182

small amount of water molecules could be oxidized at the selected +1.3 V/SCE on the

183

TiO2 SCs, this side reaction was limited due to the large ohmic drop and overpotential,

184

as indicated by the LSV results and the digital photos of the PEC cells with the TiO2

185

SCs electrode (Figures S20 and S21), in which no obvious gas bubbles were

186

generated at the solution surface.

187

Both high EC and PEC activities were simultaneously observed for the

188

facet-tailored TiO2 SCs electrode in terms of both BPA degradation and

189

mineralization, compared to those of the typical P25 and Ti0.7Ru0.3O2 electrodes under

190

the identical conditions (Figures 2b-f and S22-S24). Since the amount of BPA

191

removed in unit time has an upper limit, the degradation rate constant continuously

192

decreased with the increasing substrate concentration. These results indicate that TiO2,

193

a well-known semiconductor photocatalyst, can actually become a superior

194

electrocatalyst when its crystal shape and exposed facet are finely tuned (Figure S1).

195

The superior electro-catalytic superiority of the TiO2 SCs should be mainly attributed

196

to their single-crystalline structure and exposed polar {001} facet.24 By virtue of both

197

high EC and PC activities on one single TiO2 SCs electrode, a novel EC-dominant

198

PEC system was effectively established for efficient pollutant degradation at a low

199

potential bias (Figure 2), in comparison with the widely reported PC-dominant PEC

11



200

systems on other TiO2-based electrodes.32-34 In addition, the bifunctional TiO2 SCs

201

electrode also exhibited a much higher activity than the carbon and Sb-doped SnO2/Ti

202

electrodes in both EC and PEC systems (Figure S25).

203

Various intermediates, such as 4-isopropenylphenol (Figure S26a), hydroquinone

204

(Figure S26b), benzoquinone (Figure S26c), citric acid (Figure S26d), maleic acid

205

(Figure S26e) and acetic acid (Figure S26f), were detected in BPA degradation. The

206

maximum accumulation concentrations of hydroquinone and maleic acid were higher

207

and the time to peak was shorter in the PEC than those in the EC (Figure S26b and e),

208

indicating a faster BPA degradation rate in the PEC. Furthermore, the maximum

209

accumulation concentrations of the other four intermediates were higher and the time

210

to peak was longer in the EC than in the PEC (Figure S26a, c, d and f), suggesting a

211

slower BPA mineralization rate in the EC. These results are highly consistent with the

212

superior BPA degradation performance in PEC with TiO2 SCs electrode (Figure 2a).

213

In the PEC, the BPA removal efficiency was drastically improved, and the

214

current efficiency was much higher than that in the EC (Table S2). It should be noted

215

that the energy consumption in the PC was not provided as its TOC removal was too

216

low to be determined accurately. The energy consumption in the PEC was calculated

217

from the combined electrochemical process only, because the used Xe arc lamp had a

218

both high energy input and low UV output (Figure S27). In this case, the calculated

219

energy consumption represented only a minor part of the total energy consumption for

220

the PEC, as the energy consumption in UV irradiation was not taken into account

221

(Table S2). The electro-assisted adsorption also contributed to the BPA removal by

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222

establishing the synergistic adsorption-promoted degradation on the TiO2 SCs

223

electrode (Figure S28).37 The BPA degradation and mineralization decreased slightly

224

after 20 cycles in the PEC, indicating their good stability for long-term applications

225

(Figure 3a, c and d). In the EC, it began to continuously decrease after the 7th run,

226

demonstrating the progressive surface fouling on the TiO2 electrode by the increased

227

accumulation of the polymeric products at a low bias (Figure 3b).

228

Anti-Fouling Properties of PEC Electrode. The well-defined oxidation peak

229

toward BPA at ca. 0.60 V in the first cycle was substantially reduced and even

230

completely disappeared in subsequent cycles in EC (Figure 4a), suggesting the

231

formation and progressive accumulation of anodic polymeric film, which deactivated

232

TiO2 electrode.1,31 In comparison, such a deactivation was considerably decreased in

233

PEC due to its enhanced catalytic capacity and anti-fouling properties (Figure 4b).

234

Both FTIR and DRS were used to characterize the samples extracted from the

235

used electrodes with methanol as solvent. Compared to BPA monomer, in the

236

characteristic spectra of polymerized BPA from EC (Figure 4c), the broad band of

237

O-H stretch in hydroxyl groups, C=C ring stretch, and out-of-plane C-H bend, at

238

3220-, 1600-, and 830- cm-1, disappeared, but two strong and sharp bands at 2924-

239

and 2854 cm-1, which are the characteristic of sp3 C-H stretching modes, and the band

240

for typically non-conjugated C=O bonds at 1750 cm-1, were observed.31 In DRS

241

spectra, B band at 270 nm associated with π-π electronic transition of phenol

242

disappeared and only E2 band was observed at 227 nm (Figure 4d). These results

243

indicate that the polymeric film on EC electrode was mainly composed of aliphatic

13



244

hydrocarbons including carbonyl groups.31 In comparison, no obvious characteristic

245

spectra of both FTIR and DRS were measured for samples extracted from PEC,

246

implying the complete degradation of polymeric film and full regeneration of active

247

sites on TiO2 SCs electrode. Thus, a much higher electrochemical activity and

248

stability were obtained.

249

The electrochemical stability of the TiO2 SCs electrode was also evaluated by

250

examining the CV response of ferrocyanide ion (10 mM) as a redox marker (Figure

251

4e). The oxidative current on the used EC electrode (3.92 mA, scanning rate of 10

252

mV/s) was substantially lower than that of the raw one (7.11 mA), suggesting the

253

anodic deactivation by the non-exhaustive incineration of polymerized BPA due to

254

the limited EC activity. In contrast, a much less decrease in oxidative current was

255

observed for the PEC electrode after the BPA degradation (6.14 mA), implying that

256

no massive organics remained because of the sufficiently high PEC activity. These

257

results agree well with the FTIR and DRS results. In addition, Raman analysis was

258

used to explore the changes in crystal structure and electrode stability after cyclic

259

operation. Raman peaks at around 142, 394, 512 and 634 cm-1 could be identified

260

(Figure 4f), neither broadening nor shift occurred after 5 cyclic degradation in both

261

EC and PEC, excluding the possible structural destruction of the tailored TiO2 SCs.

262

Therefore, the TiO2 SCs electrode deactivation in the EC should be attributed

263

mainly to the residual polymerized BPA film adsorbed onto the active and inactive

264

sites because of the limited electrochemical capacity at 1.3 V, rather than the

265

structural and crystal destruction of the anodic material. This was further validated by

14


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266

DRS and FTIR measurements. In comparison, no any deactivation and structural

267

destruction occurred in PEC because of the synergistic effects between the EC and PC

268

on the bifunctional TiO2 SCs anode.

269

PEC Mechanism and BPA Degradation Pathway on the TiO2 SCs Electrode.

270

The bifunctional TiO2 SCs could be simultaneously activated by potential and UV via

271

the non-band-gap and band-gap excitation mechanisms respectively (Scheme 1). At a

272

bias of +1.3 V/SCE (E < Eg = ~ 3.2 eV), electrons were excited from conductance

273

band (CB) to cathode to generate remaining holes in CB (Reaction 1), while electrons

274

were excited from valence band (VB) to CB to form reactive electron-hole pairs under

275

UV irradiation (λ = 300 ~ 400 nm) (Reaction 2). Then, carriers were spatially

276

separated by anodic bias, which provided a potential gradient within electrode to drive

277

electrons to counter electrode along conductive substrate (Reactions 3 and 4).38,39

278

Both electro- and photo-generated holes on TiO2 SCs could respectively form

279

surface-bound and free ·OH from water oxidation (Reactions 5-8).2 The

280

electro-generated holes and surface-bound ·OH under potential bias could oxidize

281

pollutants (≡C-OH) without exhaustive mineralization to generate various polymeric

282

products (Reactions 9 and 10), which could be effectively degraded and mineralized

283

by the photo-generated holes and free ·OH under UV irradiation (Reactions 11 and

284

12). When the potential bias was further increased, oxygen evolution from water

285

oxidation with surface-bound and free ·OH as the main intermediates would largely

286

occur (Reactions 13 and 14).1

287

TiO2 + Potential → TiO2(h+)CB + e-

(non-band-gap excitation)

15


(1)


288 289 290

TiO2 + UV → TiO2(h+)VB + TiO2(e-)CB

(band-gap excitation)

TiO2(h+)CB + e- + Potential → TiO2(h+)CB + Current TiO2(h+)VB + TiO2(e-)CB + Potential → TiO2(h+)VB + Current

Page 16 of 33

(2) (3) (4)

291

TiO2(h+)CB + OH- → TiO2(·OH)bound

(5)

292

TiO2(h+)CB + H2O → TiO2(·OH)bound + H+ + e-

(6)

293

TiO2(h+)VB + OH- → TiO2(·OH)free

(7)

294

TiO2(h+)VB + H2O → TiO2(·OH)free + H+ + e-

(8)

295

TiO2(h+)CB + ≡C-OH → ≡C-O·→ ≡C-O··O-C≡ → Polymeric products

(9)

296

TiO2(·OH)bound + ≡C-OH → ≡C-O·→ ≡C-O··O-C≡ → Polymeric products

(10)

297

TiO2(h+)VB + Polymeric products → Ring-opening intermediates → CO2 + H2O (11)

298

TiO2(·OH)free + Polymeric products → Ring-opening intermediates → CO2 +H2O (12)

299

TiO2(·OH)bound → 1/2O2 ↑ + H+ + e-

(13)

300

TiO2(·OH)free → 1/2O2 ↑ + H+ + e-

(14)

301

In this work, the BPA degradation in PEC was initiated primarily by EC and also

302

by PC with the TiO2 SCs anode (Scheme 1). BPA was mainly degraded under

303

potential bias via electrochemical direct oxidation, which was initiated by

304

electro-generated holes through direct electron transfer at anode surface (Pathway 1,

305

Scheme S2). The given anodic potential (+1.3 V/SCE) was markedly higher than

306

E0(BPA/BPA·-) (~ +0.7 V/SCE, Figure 1d and 1f), but substantially lower than

307

E0(H2O/·OH) (~ +2.3 V/SCE).1,31 Both the high catalytic activity (kEC/kPEC ratio in

308

BPA degradation) and the low PL signal (kEC/kPEC ratio in ·OH generation) in EC

309

system indicate a EC-dominant synergistic mechanism in PEC system with the main

16


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310

electro-generated hole-mediated BPA degradation pathway, while other three

311

oxidation pathways played minor roles only (Figures 2a, S29 and Scheme 1). Such an

312

EC-dominant PEC mechanism on the facet-tailored TiO2 SCs electrode was

313

substantially different from the widely reported PC-dominant PEC mechanism on

314

other TiO2-based electrodes (Scheme S3).32-34 At +1.3 V/SCE, although

315

electro-generated holes were unable to oxidize adsorbed H2O and OH- to generate free

316

•OH, the electrochemical surface-bound •OH-mediated indirect oxidation pathway

317

could not be ruled out in this case, as this potential bias is sufficient for water

318

oxidation initiated with surface-bound •OH (Figures S15-S17).1 In electrochemical

319

direct oxidation (Pathway 1, Scheme S2), the adsorbed BPA was initially transformed

320

to 4-isopropenylphenol (m/z = 133) and phenol respectively by electro-generated

321

holes. Then, the two aromatic intermediates were partially oxidized to some aliphatic

322

acids via aromatic ring clearage in the most direct and simple pathway. Finally, the

323

formed aliphatic acids were completely mineralized to CO2 and H2O.24 However, it is

324

well known that some polymeric products are substantially generated and

325

progressively accumulated in electrochemical direct oxidation of aromatic

326

compounds,1 which are highly resistant to further oxidation via direct anodic

327

mechanism.31 Furthermore, these polymeric products can be strongly adsorbed onto

328

anode surface and firmly block active sites from further reactions, leading to severe

329

electrode deactivation (Figure 4a and Scheme S2).1,31 Thus, only a partial, selective

330

and unstable BPA oxidation (i.e., conversion) occurred in EC (Figures 2, 3 and S26).

331

BPA was also degraded under UV irradiation via photochemical direct oxidation

17



332

initiated by photo-generated holes (Pathway 1, Scheme S2) and photochemical

333

indirect oxidation initiated by free •OH (Pathway 2, Scheme S2) because of the

334

sufficiently high potential of photo-generated holes in band-gap excitation (EVB = ~

335

2.8 eV/NHE).2 The •OH-mediated oxidation has three representative steps (Pathway 2,

336

Scheme S2). First, some hydroxylated derivatives from BPA were rapidly generated

337

due to the selective attack of the free •OH on different carbon atoms of BPA

338

molecules, while these aromatic derivatives were unstable and readily decomposed

339

into one-ring aromatic compounds through isopropylidene bridge cleavage; second,

340

these aromatic compounds underwent further ring cleavage, leading to the formation

341

of short-chain aliphatic acids; last, further oxidation of these organic acids to CO2 and

342

H2O was undertook, accomplishing the mineralization of BPA.40

343

The progressive accumulation of the polymeric intermediates from the

344

electrochemical direct oxidation of BPA through electron transfer in Pathway 1 led to

345

the electrode fouling and thus a continuous deterioration in EC performance (Figures

346

3 and S26).1,31 When such an incomplete EC was combined with PC to construct a

347

synergistic PEC on the bifunctional TiO2 SCs, a complete, nonselective and stable

348

BPA degradation could readily occur (Figures 3 and S26). The main reason was that

349

these highly resistant and strongly adsorbed polymeric products formed in

350

electrochemical direct oxidation in Pathway 1 could also be oxidized to aliphatic acids

351

and finally mineralized to CO2 via photochemical indirect oxidation initiated by free

352

•OH (Pathway 3, Scheme S2).3-7 The well known EC-assisted PC and their PC-EC

353

synergism could favor such a photochemical anti-fouling capacity in the PEC (Figures

18


Page 18 of 33

Page 19 of 33


354

S15-S17),32-34 although the single PC exhibited a weak activity (Figure 2a). Thus, the

355

spatially occupied active sites on TiO2 SCs were rapidly released, and the EC

356

deactivated anodic material was promptly regenerated by •OHfree formed in PC

357

(Figure 4 and Scheme 1). In turn, these two processes could accelerate the overall

358

degradation, which was responsible for the excellent and stable BPA removal in PEC

359

(Table S2 and Figure S26).

360

Treatment of Landfill Leachate by PEC. To further examine the feasibility of

361

such a PEC for pollution control, it was also applied to treat landfill leachate, a typical

362

toxic and mixed wastewater (see SI for details).41 In 7-h treatment, although slight

363

difference was found for the discoloration efficiency between EC and PEC (ca. 70%

364

and 80%, respectively) (Figure 5a), a significant superiority was observed for the

365

COD removal in PEC (more than 75%) over EC (less than 40%) (Figure 5b). This

366

result indicated that the PEC with facet-tailored TiO2 SCs could be used as an

367

efficient technology for real complex wastewater treatment.

368

Environmental Implications. Oxygen evolution is usually an inevitable side

369

reaction to raise energy consumption in electrochemical water treatment. The direct

370

anodic oxidation pathway at low bias before oxygen evolution is a potential strategy

371

to this problem. However, how to overcome the serious electrode fouling and

372

continuous activity decrease still remains a great challenge. In this study, by virtue of

373

the excellent ·OHfree-mediated photochemical oxidation capacity of the bifunctional

374

TiO2 SCs with dominant high-energy {001} polar facets, electrochemical degradation

375

of refractory pollutants could be sustainably carried out at low bias with high energy

19



376

efficiency. The developed and validated photochemical anti-fouling strategy might be

377

of considerable interest for practical applications of anodic oxidation at low bias in

378

cost-effective removal of refractory pollutants in water and wastewater. Also, our

379

findings provide a promising catalytic material with good photochemical and

380

electrochemical activities, and opens up a brand new approach to develop more

381

practical transition metal-oxide-based catalysts for efficient water treatment. However,

382

considering the high energy consumption in UV irradiation, solar-light-driven

383

photochemical anti-fouling warrants further investigations for practical applications.

384 385

SUPPORTING INFORMATION

386

Characterizations of TiO2 SCs and P25 (Figures S1-S7), Sb-doped SnO2/Ti

387

(Figures S8-S10) and Ti0.7Ru0.3O2/Ti electrodes (Figures S11-S13), additional PC, EC

388

and PEC characterizations and BPA degradation tests (Figures S14-S25), evolution of

389

main BPA degradation intermediates (Figure S26), light spectrum of the utilized Xe

390

lamp (Figure S27), BPA adsorption tests (Figure S28), EC and PEC kinetic constant

391

ratios (Figure S29), diagram of the PEC cell (Scheme S1), BPA degradation pathway

392

(Scheme S2), the general PEC mechanism (Scheme S3), characteristics of landfill

393

leachate (Table S1) and BPA removal behaviors under various conditions (Table S2).

394

This material is available free of charge via the Internet at http://pubs.acs.org.

395 396 397

ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China

20


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398

(21261160489 and 51538011), the Anhui Provincial Natural Science Foundation

399

(1708085MB52) and the Collaborative Innovation Center of Suzhou Nano Science

400

and Technology of the Ministry of Education of China.

401 402

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Page 27 of 33


Figure captions Figure 1. Properties of the TiO2 SCs in the PC (a, b), the EC (c, d) and the PEC (e-h). Measurement conditions: solution volume (80 mL), UV (500 W Xe arc lamp, λ < 420 nm), anode (6.0 or 0.196 cm2); supporting electrolyte (0.1 M Na2SO4), cathode (Ti sheet, 6.0 cm2), electrode gap (1.0 cm), reference electrode (saturated calomel electrode, SCE), temperature (~ 20 oC), photocurrent (bias: +0.30 and +0.45 V/SCE), EIS (voltage amplitude: 5 mV, frequency range: 105 ~ 10-2 Hz, bias: open-circuit potential) and photoluminescence (terephthalic acid: 3 mM, NaOH: 10 mM, bias: +1.3 V/SCE, pH: ~11.0, stirring rate: 500 rpm, excitation wavelength: 312 nm, emission wavelength: 426 nm). Figure 2. BPA removal behaviors in the PC, EC and PEC with the bifunctional TiO2 SCs, Degussa P25 and typical Ti0.7Ru0.3O2 electrodes: degradation (a-e) and mineralization (f). Reaction conditions: solution volume (80 mL), BPA concentration (30 mg L-1), Na2SO4 concentration (0.1 M), anode size (6.0 cm2), catalyst dosage (0.05 mg/cm2 × 6.0 cm2 = 0.3 mg), cathode size (6.0 cm2, Ti sheet), electrode gap (1.0 cm), bias (+1.3 V/SCE), UV (500 W Xe arc lamp, λ < 420 nm), pH (~5.0), temperature (~ 20 oC), stirring rate (500 rpm) and reaction time (6.0 h). Figure 3. Cyclic degradation of BPA in the EC and PEC: BPA degradation (a-c) and TOC removal (d). Reaction conditions: solution volume (80 mL), BPA concentration (30 mg L-1), Na2SO4 concentration (0.1 M), anode size (6.0 cm2), catalyst dosage (0.05 mg/cm2 × 6.0 cm2 = 0.3 mg), cathode size (6.0 cm2, Ti sheet), electrode gap (1.0 cm), bias (+1.3 V/SCE), UV (500 W Xe arc lamp, λ < 420 nm), pH (~5.0), temperature (~ 20 oC), stirring rate (500 rpm) and reaction time (6.0 h). Figure 4. Anti-fouling properties of the TiO2 SCs electrode in the EC and PEC: cyclic DPV curves (a, b), FTIR spectra (c), DRS spectra (d), CV curves (e) and Raman spectra (f). Measurement conditions: DPV (solution = 0.1 M Na2SO4 + 30 mg L-1 BPA, potential range = -0.3 ~ 1.0 V, scan rate = 0.1 V/s, UV = 500 W Xe arc lamp, λ < 420 nm and effective anode area = 6.0 cm2) and CV (solution = 0.1 M Na2SO4 + 5.0 mM [Fe(CN)6]3-/[Fe(CN)6]4-, pH = 5.5, potential range = -0.4 ~ 0.6 V, scan rate = 0.1 V/s and effective anode area = 6.0 cm2). Figure 5. Treatment of landfill leachate by the EC and PEC with the TiO2 SCs electrode: decolorization efficiency (a) and COD removal (b). Reaction conditions: solution volume (80 mL), ionic conductivity (13.7-20.0 mS), COD concentration (~1050 mg L-1), TKN (340 mg L-1), ammonium-N (220 mg L-1), sulfate (480 mg L-1), chlorides (140 mg L-1), Na2SO4 concentration (0.1 M), anode size (6.0 cm2), catalyst dosage (0.05 mg/cm2 × 6.0 cm2 = 0.3 mg), cathode size (6.0 cm2, Ti sheet), electrode gap (1.0 cm), bias (+1.3 V/SCE), UV (500 W Xe arc lamp, λ < 420 nm), pH (7.0 ~ 7.5), temperature (~ 20 oC), stirring rate (500 rpm) and reaction time (8.0 h).

27



0.8

45 30 15

0.6

0 1

2

3

0.4

TiO2 SC

0.2 0.0

4

5

6

7

Energy (eV) λEdge = ~ 380 nm

(b)

UV on UV off

2.0

1.0 0.5 0.0

300

400

500

600

700

800

0

200

Na2SO4: 0.1 M

0 -3 TiRu P25 TiO2 SC

-6 -9 -0.4

-0.2

0.0

0.2

0.4

12000 10000 8000

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

2

0.0

0.2

0.4

1.5

4

6

1.2

8

10

12

14

TiO2 SC

(e)

Na2SO4: 0.1 M

0.9

(f)

0.6

TiO2 SC

0.0

800 1600 2400 3200 4000 4800 5600

-0.2 900

30 min 25 min 20 min 15 min 10 min 5 min 0 min

600 450

0.0

0.2

0.4

0.6

300 150

0.8

1.0

Potential (V/SCE) 30 min 25 min 20 min 15 min 10 min 5 min 0 min

(h)

750

Intensity (a.u.)

(g)

PEC EC

pH: 5.0

0.3

750

Intensity (a.u.)

1.0

-1

Z' (ohm) 900

0.8

BPA: 30 mg L

1.2

4000

0

0.6

Potential (V/SCE)

6000

0

pH: 5.0 Na2SO4: 0.1 M

0.0

Z' (ohm)

2000

-1

BPA: 30 mg L

0.6

0.6

1.5 V, dark 1.5V, UV 2.0 V, dark 2.0V, UV

0

1000

0.3

Current (mA)

Z'' (ohm)

14000

Z'' (ohm)

0.0 V, dark 0.0 V, UV 0.5 V, dark 0.5 V, UV 1.0 V, dark 1.0V, UV 1.5 V, dark 1.5V, UV 2.0 V, dark 2.0V, UV

16000

800

(d)

0.9

Potential (V/SCE) 18000

600

TiRu P25 TiO2 SC

1.2

Current (µA)

Current (mA)

1.5

(c)

pH: 5.5

3

400

Irradiation time (sec)

K3FeC6N6: 5.0 mM

6

TiRu-0.45 V P25-0.45 V TiO2 SC-0.45 V

1.5

Wavelength (nm) 9

TiRu-0.30 V P25-0.30 V TiO2 SC-0.30 V

Eg = 1240/λ Edge = 1240/380 = 3.26 eV

(a) 200

Photocurrent (mA)

1/2

60

[F(R&E)]

1.0

Absorbance

2.5

75

1.2

Page 28 of 33

600 450 300 150

TiO2 SC-EC

0 350

400

450

500

TiO2 SC-PEC

0

550

350

400

Wavelength (nm)

450

500

Wavelength (nm)

Figure 1

28


550

Page 29 of 33


1.0

1.0

0.8

-1

0.8

(b)

0.6

CH 3 C CH 3

OH

0.4

OH

PC EC PEC

0.2

(a) 0

Ct/C0

Ct/C0

TiO2 SC

0.0

0.6 0.4 0.2 0.0

1

2

3

4

5

6

0

1

2

1.0

0.6 0.4 0.2 0.0

0.6

-1

5 mg L -1 10 mg L -1 15 mg L -1 20 mg L -1 30 mg L -1 50 mg L -1 80 mg L -1 100 mg L

0.4

0.0 3

4

5

6

0

(d)

1

Reaction time (h)

2

3

4

5

6

Reaction time (h)

50

TiO2 SC

(e)

100

P25 TiRu

40

TiO2 SC

(f)

P25 TiRu

80

-1

k (× 10 min )

6

TiRu-PEC

0.2

2

5

0.8

Ct/C0

Ct/C0

(c)

1

4

1.0

-1


P25-PEC

0.8

3

Reaction time (h)

Reaction time (h)

0


TiO2 SC-PEC

-3

η (%)

30 20 10

60 40 20

0

0

0

20

40

60

80

100

0

20

-1

40

60

-1

BPA (mg L )

BPA (mg L )

Figure 2

29


80

100


1.0

0.6 0.4

(b)

0.8

th

21 run

Ct/C0

Ct/C0

1.0

(a)

0.8

st

1 run

0.2

th

21 run

0.6 0.4

st

1 run

0.2

PEC

0.0

0

0.0 1

2

3

4

5

6

EC 0

1

2

Reaction time (h)

4

1.2 0.8 0.4

EC PEC

100

TOC removal (%)

1.6 -1

3

5

6

Reaction time (h)

(c)

EC PEC

2.0

k (h )

Page 30 of 33

0.0

(d)

80 60 40 20 0

0

5

10

15

20

0

5

Run

10

Run

Figure 3

30


15

20


0.35

Current (mA)

0.30 0.25

(a)

1st 2nd 3rd 4th 5th

EC Rapid deactivation

0.20 0.15 0.10

1.4

(b)

0.05

1st 2nd 3rd 4th 5th

PEC Slow deactivation

1.2

Current (mA)

Page 31 of 33

1.0 0.8 0.6 0.4 0.2

0.00

0.0 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

-0.2

0.0

Potential (V, versus SCE)

0.2

0.4

0.6

0.8

1.0

3.5

EC PEC CH3OH

Absorbance (a.u.)

Intensity (a.u.)

3.0

CH3OH PEC EC 0

800

1600

(c) 2400

3200

2.5 2.0 1.5 1.0 0.5

(d)

0.0

4000

200

300

600

700

Fresh PEC EC

Na2SO4: 0.1 M

0.0 -2.5 -5.0 -7.5

(f)

(e)

-10.0 -0.45 -0.30 -0.15 0.00

800

Intensity (a.u.)

Current (mA)

2.5

500

Fresh PEC EC

pH: 5.5

5.0

400

Wavelength (nm)

Wavenumber (cm)

10.0 K FeC N : 5.0 mM 3 6 6 7.5

1.2


0.15 0.30 0.45 0.60

100


200

300

400

500

Wavenumber (cm)

Figure 4

31


600

700


1.0

Page 32 of 33

EC PEC

@ 300 nm

At/A0

0.8 0.6 0.4 0.2 0.0

(a) 0

1

2

3

4

5

6

7

8

Reaction time (h) 350

EC PEC

250

-1

COD (mg L )

300

200 150 100 50 0

(b) 0

1

2

3

4

5

6

7

Reaction time (h)

Figure 5

32


8

Page 33 of 33


BPA slow major

Electrochemical direct oxidation

electro-generated hole (non-band-gap excitation)

CO2 + H2O •BPA+ Polymeric Intermediates (≡C-OH)

(partial mineralization)

low energy consumption anode deactivation

BPA slow

Electrochemical

minor indirect oxidation

electro-generated •OH (surface-bound)

CO2 + H2O •BPA+

Novel PEC on TiO2 SCs electrode High PC activity, high EC activity

Polymeric Intermediates (≡C-OH)

(partial mineralization)

high energy consumption anode deactivation

Photo-assisted EC catalysis High bias, > 1.0 V/NHE BPA / Polymeric Intermediates (≡C-OH) fast minor

Photochemical direct oxidation

photo-generated hole (band-gap excitation)

high energy consumption anode regeneration

CO2 + H2O

•BPA+/ ≡C-O•

(complete mineralization)

BPA / Polymeric Intermediates (≡C-OH) fast

Photochemical minor indirect oxidation

photo-generated •OH (bulk-free)

BPA-OH • / ≡C-OH-OH•

CO2 + H2O

high energy consumption anode regeneration

(complete mineralization)

Scheme 1. Schematic diagram of the EC-dominant PEC synergistic mechanism (PC-assisted EC catalytic system) on the facet-tailored TiO2 SCs electrode for pollutant degradation.

33


Photochemical Anti-Fouling Approach for Electrochemical Pollutant

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