Enhanced Photoelectrocatalytic Decomplexation of CuâEDTA and Cu

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Enhanced Photoelectrocatalytic Decomplexation of Cu-EDTA and Cu Recovery by Persulfate Activated by UV and Cathodic Reduction Huabin Zeng, Shanshan Liu, Buyu Chai, Di Cao, Yan Wang, and Xu Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00632 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 29, 2016

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Environmental Science & Technology

1

Enhanced Photoelectrocatalytic Decomplexation of Cu-EDTA and Cu Recovery

2

by Persulfate Activated by UV and Cathodic Reduction

3 4

Huabin Zeng a, b, Shanshan Liu a, b, Buyu Chaia, Di Cao a, b, Yan Wanga, Xu Zhao a, b*

5 6

a

7

Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

Key Laboratory of Drinking Water Science and Technology, Research Center for

8 9

b

University of Chinese Academy of Sciences, Beijing 100085, China

10 11 12

*Corresponding author: e-mail address: [email protected]; Tel: +86-10-62849667;

13

Fax: +86-10-62849667; Add: P.O. Box 2871, 18 Shuangqing Road, Haidian District,

14

Beijing, 10085, P.R. China.

15 16

1 ACS Paragon Plus Environment


17

Abstract

18

In order to enhance Cu-EDTA decomplexation and copper cathodic recovery via the

19

photoelectrocatalytic (PEC) process, S2O82- was introduced into the PEC system with

20

a TiO2/Ti photoanode. At a current density of 0.2 mA/cm2 and initial solution pH of

21

3.0, the decomplexation ratio of Cu complexes was increased from 47.5% in the PEC

22

process to 98.4% with 5 mM S2O82- addition into the PEC process (PEC/S2O82-).

23

Correspondently, recovery percentage of Cu was increased to 47.4% from 98.3%

24

within 60 min. It was observed that nearly no copper recovery occurred within the

25

initial reaction period of 10 min. Combined with the analysis of ESR and

26

electrochemical LSV curves, it was concluded that activation of S2O82- into SO4•-

27

radicals by cathodic reduction occurred, which was prior to the reduction of liberated

28

Cu2+ ions. UV irradiation of S2O82- also led to the production of SO4•-. The generated

29

SO4•- radicals enhanced the oxidation of Cu-EDTA. After the consumption of S2O82-,

30

the Cu recovery via cathodic reduction proceeded quickly. Acidification induced by

31

the transformation of SO4•- to OH• favored the copper cathodic recovery. The

32

combined PEC/S2O82- process was also efficient for the TOC removal from a real

33

electroplating wastewater with the Cu recovery efficiency higher than 80%.

34

35


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Table of Contents/Abstract Art

37 38 39



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40

Introduction

41

The Cu ions and chelating agents, such as EDTA and CN-, widely coexisted in the

42

effluents from the electroplating, mining and photographic industries.1 The stable

43

Cu-complexes formed from chelating agents and copper ions were difficult to be

44

removed by conventional processes, such as coagulation, adsorption, precipitation and

45

biological process.2, 3 To achieve an effective treatment of Cu complexes, not only the

46

destruction of the chelating agents but also the copper recovery was desirable.

47

Photoelectrocatalytic (PEC) process can destroy the Cu-EDTA, Ni-EDTA and

48

Cu(CN)32- with the simultaneous recovery of the liberated Cu/Ni ions reported in our

49

previous work.4-6 Under the light irradiation, photogenerated holes and induced

50

hydroxyl radicals (OH•) from the semiconductor photoanode can destroy

51

metal-complexes, and the liberated metal ions were deposited onto the cathode by

52

electrochemical reduction, which was convenient for the recovery.6 Unfortunately, the

53

PEC oxidation of metal-complexes mainly occurred around the photoanode with a

54

low oxidation efficiency and long reaction time, which limited its application.

55

In recent years, persulfate (S2O82-) has received an increasing interest in both

56

scientific research and application for the groundwater remediation and wastewater

57

treatment.7-9 When being activated, they can produce strong sulfate radicals (SO4•-),

58

which can react with a wide range of organic and inorganic contaminants,10-12 due to

59

its high redox potential, even compared with that of OH•.13,

60

activation to SO4•- can be initiated under the thermal, photochemical and metal ions

61

catalysis conditions.15-18 In the application of S2O82− for in situ chemical oxidation,


14

Generally, S2O82-

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the low reactivity of S2O82− under low temperature or the natural minerals conditions

63

ensured the S2O82− reaching the contaminated zone.9, 19 However, in order to employ

64

the S2O82− in the wastewater treatment, acceleration of the S2O82− activation was

65

desired. It has been exhibited that 4-chloro-3-methylphenol was abated within 3~59 s

66

in pressurized hot water and in supercritical water with the S2O82- addition activated

67

by high temperature.20 To some extent, the limitation in such homogeneous processes

68

was the energy consumption, as well as the thermal pollution to the environment.18

69

The coupling of S2O82- with metals such as Fe2+ and Co2+ led to the formation of SO4•-

70

radicals (reaction 1~2), which were responsible for the rapid removal of

71

microcystin-LR and bisphenol A.15, 21

72

S2O82- + Fe2+ → SO4•- + SO42- + Fe3+

(1)

73

S2O82- + Co2+ → SO4•- + SO42- + Co3+

(2)

74

In these processes, the subsequent recovery of metal ions had to be performed for the

75

purpose of catalysts reuse and prevention from secondary contamination.15 The

76

copper and iron oxides were employed as heterogeneous catalysts for S2O82-

77

activation and convenient to reuse, the metal leaching at acid condition cannot be

78

completely avoided and the rate of heterogeneous activation remained low.22-24 With

79

respect to the UV activation, large amount of SO4•- were produced in the bulk solution,

80

which led to the efficient degradation of 2, 4-dichlorophenol.25 And secondary

81

pollution was greatly minimized with sulfate ions as the only end-product.12

82

Furthermore, the photolysis activation can work over a broad pH range.12, 17, 24

83

In light of the UV photolysis activation, S2O82- ions were added into the PEC system



84

consisting of TiO2 photoanode and stainless steel cathode with the UV light

85

irradiation. The results indicated that, in comparison with the individual PEC process,

86

the Cu-EDTA destruction and the recovery of the liberated Cu2+ ions by a cathodic

87

deposition were largely improved with the addition of S2O82-. SO4•- generated by the

88

UV activation of S2O82- was confirmed. Based on the ESR and XPS analysis, it was

89

found that, at the initial reaction phase, a cathodic activation of S2O82- ions occurred,

90

leading to the production of SO4•- radicals, which can also enhance the destruction of

91

Cu-EDTA. After the consumption of S2O82- ions, the electrodeposition of the liberated

92

Cu2+ ions occurred quickly. Furthermore, the enhanced mechanism induced by S2O82-

93

was proposed.

94

Experimental Section

95

Chemicals

96

Copper sulfate (CuSO4), sodium persulfate (Na2S2O8) and sodium sulfite (Na2SO3)

97

were purchased from Acros. Sodium sulfate (Na2SO4) and ethylenediaminetetraacetic

98

acid disodium salt (Na2EDTA) were purchased from Sinopharm chemical reagent Co.,

99

Ltd. China.

100

The simulated wastewater containing Cu-EDTA was prepared by mixing a solution of

101

CuSO4 and Na2EDTA to obtain a 1:1 molar ratio of Cu2+/EDTA. Solution pH

102

regulation was performed using either 100 mM NaOH or 50 mM H2SO4. Deionized

103

water was used for the preparation and dilution of the solutions. A sample of a real

104

electroplating wastewater was taken from Quanzhou City, China. The wastewater

105

contained Cu2+ ions and organic chelating agent including EDTA. The concentration


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of Cu was determined to be 65 mg/L; Total organic carbon (TOC) was measured to be

107

244 mg/L; Solution pH was measured to be 2.6; the qualities of the real wastewater

108

were shown in the Table SI-1.

109

Setup and Experiment

110

The PEC experiments with a batch mode were performed in an undivided cell with a

111

volume of 500 mL. The tubular TiO2/Ti mesh electrode (50 mm diameter) was used as

112

anode, and the TiO2 film was deposited onto the Ti plate via a dip-coating method as

113

described by Shang et al..26 A tubular mesh stainless steel (66 mm) was used as

114

cathode. The gap between the anode and cathode was 8 mm. A UV lamp (main

115

wavelength 254 nm; 9 W) in a quartz tube was put in the center of the tubular TiO2/Ti

116

mesh electrode.

117

Analytical Method

118

The concentration of total Cu complexes was measured by a high-performance liquid

119

chromatography (HPLC, 1260, Agilent Technology) with a C18 column and

120

ultraviolet detector. The elution was comprised of 92% oxalic acid (concentration, 15

121

mM; pH, 3.0)/8% acetonitrile (V/V) at a temperature of 25 oC. The flow rate was set

122

as 1 mL/min and the detection wavelength was set as 254 nm. The detection method

123

of total Cu complexes was attached in the supporting information (Text SI-1).

124

Concentration of total Cu2+ ions was measured using 700 series inductively coupled

125

plasma-optical emission spectrometry (ICP-OES, Agilent Technology). Total organic

126

carbon (TOC) was measured using a Shimadzu TOC analyzer (TOC-VCPH,

127

Shimadzu, Japan). The procedure of the copper mass balance was illustrated in the



128

Text SI-2 (supporting information) in details. The pH was measured using a 9165 BN

129

pH electrode connected to an Orion-828 pH analyzer (Orion Research, Inc., Beverly,

130

MA). The linear sweep voltammetry (LSV) was conducted in a single-compartment,

131

three electrode cell on a CHIN800D instrument (CH Instrument Co.). S2O82-

132

concentration was determined colorimetrically by a similar method to H2O2 on a

133

spectrophotometer, which was illustrated in the supporting information (Text SI-3). 27

134

The morphology of the cathode was characterized using JSM 6301 scanning electron

135

microscopy (SEM). X-ray photoelectron spectra (XPS) were used to analyze the

136

surface variation of the electrode using a PHI Quantera SXM (PHIEnvironmental

137

5300/ESCA, ULVAC-PHI, Inc.). Prior to the measurement, the electrode was washed

138

with water to remove electrolyte. Formation of active radicals was identified with

139

electron spin resonance (ESR) on a Bruker ESR 300E with a microwave bridge

140

(receiver gain, 1×105; modulation amplitude, 2 G; microwave power, 10 mW;

141

modulation frequency, 100 kHz). Since the HO2•-/O2•- in water are very unstable and

142

underwent facile disproportionation rather than slow reaction with DMPO, 28 OH• and

143

SO4•- were identified with ESR in the experiment. The samples (100 µL) were

144

collected from the reaction solution at different times. For OH• and SO4•-

145

measurement, the sample mixed immediately with 20 µL DMPO (0.2 M) to form

146

DMPO-OH adduct and DMPO-SO4 adduct.

147

Results and Discussion

148

Enhanced performance of S2O82- addition on the PEC process


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The performance of the PEC process for the Cu-EDTA destruction and Cu recovery

150

with the addition of the given amount of S2O82- into the PEC process was examined

151

firstly. The recovery percentage of Cu2+ ions was defined as following:

152

RCu = 100% × (initial amount of Cu2+ ions – remained amount of Cu2+ ions) in

153

the reaction cell/ initial amount of Cu2+ ions in the reaction cell.

154

As shown in Figure 1(a), decomplexation ratio of Cu complexes are 47.5% in the PEC

155

process with the current density of 0.2 mA/cm2 at 60 min. By contrast, in the PEC

156

process with the S2O82- addition (PEC/S2O82), the decomplexation ratio is increased to

157

98.4%. Meantime, with the addition of the same amount of S2O82- ions, Cu complexes

158

decomplexation and copper recovery by the UV irradiation (UV/S2O82-) process and

159

the electro-oxidation (EC/S2O82-) process were also tested as a reference. In the

160

UV/S2O82- process, nearly no decomplexation of Cu complexes occur (Figure 1(a)).

161

With the Cu-EDTA destruction, complexation occurred between the Cu2+ ions with

162

the

163

nitrilotriacetic acid (NTA) and iminodiacetic acid (IDA), which was illustrated in the

164

Text SI-4. As a result, nearly no concentration decrease of total Cu complexes is

165

observed. In the EC/S2O82- process, decomplexation ratio of Cu complexes are 21.6%.

166

Similar results can be obtained for the Cu recovery (Figure 1(a)). The above results

167

indicated that not only efficient decomplexation of Cu complexes was obtained but

168

also Cu recovery was largely improved when a given amount of 5 mM S2O82- was

169

added into the PEC process.

170

In the individual PEC process, the oxidation destruction of Cu complexes was mainly

generated

intermediates

including

ethylendiamine


diacetate

(EDDA),


171

induced by the active radicals, particularly OH•, generated from the TiO2 photoanode

172

under UV light irradiation.4 Due to the limitation of the diffusion mass transfer and

173

low concentration of generated OH•, the destruction efficiency of Cu complexes was

174

low. It has been reported that the UV photolysis of one molar of S 2O82- produced two

175

molar of SO4•- according to the reaction 3.29

176

S2O82- + hν → 2SO4•-

177

The addition of S2O82- into the PEC system may lead to an efficient production of

178

SO4•- in the bulk solution. Therefore, it can be concluded that the efficient

179

decomplexation of Cu complexes was induced by the oxidation of SO4•- radicals in

180

the bulk solution and active OH• generated via the PEC process. With the improved

181

destruction of Cu-complexes, liberated copper ions were efficiently recovered by the

182

cathodic reduction deposition. TOC variation under the various processes was

183

furthermore compared. As shown in Figure 1(b), TOC removal efficiencies increase

184

with the reaction time for all of the processes. At 60 min, TOC removal efficiencies

185

are 20.5%, 13.3%, 9.35% and 52.1% for the UV/S2O82- process, PEC process,

186

EC/S2O82- process and PEC/S2O82- process. Obviously, the addition of S2O82- largely

187

enhanced the TOC removal in the PEC process.

188

To optimize the conditions for PEC/S2O82- process, the effect of S2O82- concentration

189

and current density on the decomplexation of Cu complexes and Cu recovery was

190

investigated. Figure 2(a) displays the residual ratio of Cu complexes and recovery

191

percentage of Cu as a function of time during the PEC/S2O82- process with different

192

S2O82- concentrations. The decomplexation ratios of Cu complexes at the initial S2O82-

(3)


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concentration of 0 mM, 5 mM, 20 mM and 50 mM are 27.4%, 98.4%, 88.1% and

194

57.7%, respectively. The best performance in the decomplexaiton process of Cu

195

complexes was obtained at the initial S2O82- concentration of 5 mM. A similar trend

196

for the Cu recovery is observed. Figure 2(b) investigated the effect of current density

197

on the performance of the PEC/S2O82- process. Corresponding to the current density

198

of 0.1 mA/cm2, 0.2 mA/cm2, 0.5 mA/cm2 and 1.0 mA/cm2, the decomplexation ratios

199

of Cu complexes are 19.6%, 62.2%, 93.1% and 100%, and the recovery percentages

200

of Cu are 17.5%, 60.0%, 92.7% and 100%. It can be seen that the increase of the

201

current density significantly enhanced the decomplexaiton of Cu complexes and Cu

202

recovery.

203

Additionally, it is notable from Figure 2 that the decomposition of Cu complexes and

204

Cu recovery can be divided into two phases. In the initial phase, the Cu complexes

205

decomplexaiton and Cu recovery proceeded at a low rate. After that, the rate rapidly

206

increased. The transition point varied with the S2O82- concentration and current

207

density. It can be seen from the Figure 2(a) that the transition points between these

208

two stages appear at 5 min, 20 min and 45 min, when the initial concentrations of

209

S2O82- are 5 mM, 20 mM and 50 mM, respectively. With higher concentration of

210

S2O82-, the period of Cu complexes decomplexaiton with a low rate by this system

211

remained longer. It should be point out here that the reaction rate stays relatively

212

stable in the PEC process without S2O82-. In Figure 2(b), the rate increased at the point

213

of 60 min, 30 min, 10 min and 10 min with the current density of 0.1 mA/cm2, 0.2

214

mA/cm2, 0.5 mA/cm2 and 1.0 mA/cm2. It was clear that the reaction time of the initial



215

stage with a slow reaction rate had a positive correlation with concentration of S2O82-,

216

which was negative to the current density. The differences will be explained

217

subsequently.

218

Acidification induced by S2O82- oxidation

219

Effect of initial pH on the decomplexation of Cu complexes and Cu recovery in the

220

PEC process and PEC/S2O82- process was investigated. In the individual PEC process,

221

Cu recovery obviously decreases under the alkaline and neutral condition (Figure

222

SI-11(a)), which can be explained by the different adsorption ability of Cu2+ and

223

EDTA on the anode and the variation of Cu-EDTA species with solution pH.4 By

224

contrast, the variation of initial solution pH has slight influence on the PEC/S2O82-

225

process (Figure SI-11(b)). It is obviously that the addition of S2O82- benefitted the

226

high efficiencies of Cu complexes decomplexation and Cu recovery under a wide pH

227

range.

228

Furthermore, the distribution of Cu species was analyzed at various initial solution pH.

229

With respect to the individual PEC process, as shown in Figure 3(a), under the initial

230

solution pH of 3.0, 22.9% of Cu is removed from the solution and 21.1% is deposited

231

on the cathode. It is noticed that the liberated Cu2+ ions prefer to deposit on the anode

232

rather than to deposit on the cathode with the increase of the initial solution pH.

233

Under the alkaline condition, the formation of the copper (hydro) oxides from the

234

liberated Cu2+ ions became easier,6 which would deposit onto the TiO2 photoanode

235

surface once the Cu complexes were completely destroyed. The copper (hydro)oxides

236

precipitated on the anode will occupy active sites or cause adverse effects on the


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generation of OH•, which inhibited the decomplexation of Cu complexes and Cu

238

recovery.30 By contrast, a different behavior was exhibited in the PEC/S2O82- system

239

with the variation of initial solution pH. While recovery percentages of Cu are 61.9%,

240

59.8% and 53.2%, percentages of Cu precipitated on the anode are detected as 1.71%,

241

1.18% and 0.511% respectively, corresponding to the initial solution pH of 3.0, 7.0

242

and 11.0 (Figure 3(a)). The liberated Cu ions were mainly deposited onto the cathode

243

under all initial solution pH conditions. As shown in Figure 3(b), the solution pH

244

values remain to be constant in the individual PEC process. By contrast, the solution

245

pH rapidly decreases to about 2.1 in the PEC/S2O82- process at various initial pH.

246

The redox potential of SO4•- was confirmed as 2.5~3.1 V,14 while the potential of OH•

247

was detected as 2.7 V in acid solution and 1.8 V in neutral solution. 13 Due to the

248

narrow gap of redox potential between SO4•- and OH•, the inter-conversion from SO4•-

249

to OH• occurred by the following reaction 4 and reaction 5.31

250

SO4•- + OH- → OH• + SO42-

(4)

251

SO4•- + H2O → OH• + SO42- + H+

(5)

252

According to the radicals quenching study, OH• played a dominant role in the

253

PEC/S2O82- process even in the acidic condition (Text SI-5). It was indicated that a

254

large portion of SO4•- was transferred into OH• with the consumption of the OH- or

255

the production of H+. As a result, the solution pH decreased at the initial phase. It

256

should be pointed out here that the acidic condition was beneficial to the

257

decomplexation process and Cu recovery process than the alkaline condition.4



258

It can be concluded that the S2O82- ions played a double role in the PEC/S2O82-

259

process. On one hand, SO4•- radicals generated from the S2O82- ions contributed to the

260

enhanced destruction of Cu complexes. On the other hand, the rapid decrease of the

261

system pH benefitted the further duestruction of Cu complexes and the subsequent

262

cathodic reduction of liberated Cu2+ ions onto the cathode.

263

Involved active radicals

264

Firstly, the active radicals in the UV/S2O82-, PEC and PEC/S2O82- processes were

265

analyzed using the ESR technique with the DMPO addition. The hyperfine splitting

266

constants of DMPO radicals adducts of DMPO-OH and DMPO-SO4 were

267

representative of OH• and SO4•- radicals, respectively.32, 33 As shown in Figure 4(a),

268

the signals of DMPO-SO4 and DMPO-OH in the PEC/S2O82- and UV/S2O82- processes

269

are observed, indicating the generation of SO4•- and OH• radicals. Furthermore, the

270

signals intensities of DMPO-OH and DMPO-SO4 in the PEC/S2O82- process are

271

stronger than that in the PEC process and that in UV/S2O82- process, respectively.

272

With respect to the EC/S2O82- process, nearly no enhanced signal of DMPO-SO4 and

273

DMPO-OH in the bulk solution was observed (Figure 4(b)); it is interesting to

274

observe that the obvious signals of DMPO-SO4 around the cathode appear (Figure

275

4(b)). It was suggested that the S2O82- can be activated into SO4•- by the cathodic

276

reduction. To support this conclusion, the variation of radicals’ production rate around

277

cathode with increase of current density in the EC/S2O82- process was investigated at

278

the solution pH of 10.0, where the ESR signals of DMPO-OH can be regard as

279

representative of total radicals due to nearly complete transformation of SO4•- to


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OH•.31 As shown in Figure 4(c), high current density has a positive effect on the

281

intensity of DMPO-OH signals, indicating that increasing current density can promote

282

the cathodic activation of S2O82- and was beneficial to the production of active

283

radicals.

284

Activation of S2O82- to SO4•- by UV irradiation has been largely investigated.15, 17 In

285

our case, SO4•- in the bulk solution may be mainly induced by the UV activation

286

process. It also has been recognized that SO4•- can be formed as a result of the

287

electron transfer from the reductive materials, such as Fe2+, Fe and other reducing

288

organics to S2O82-.15, 34-39 In the EC system, the cathode also had the same ability to

289

donate electron. Thus, the SO4•- may be generated from S2O82- via a cathodic

290

activation (reaction 6).

291

S2O82- + e- → SO4•- + SO42-

292

It has been reported that photogenerated electron of C3N4-photocatalysis can activate

293

S2O82- under visible light irradiation.39 In the PEC/S2O82- process, activation of S2O82-

294

by photogenerated electrons transferred to the cathode by bias potential may occur. In

295

general, in the PEC/S2O82- process, the cathodic activation and UV photolysis

296

activation accelerated the production of active radicals, which were responsible for

297

the enhanced performance of Cu complexes oxidation and Cu recovery.

298

A proposed S2O82- enhanced reaction process

299

The electrochemical behaviors of Cu2+, Cu-EDTA and S2O82- in solution of 0.1 M

300

Na2SO4 were investigated by linear sweep voltammetry. It can be seen from Figure

301

5(a) that the current intensity increases with the concentration of S2O82- at the

(6)



302

potential lower than 0.20 V, which indicates that the cathodic reduction of S 2O82-

303

begins at the potential of 0.20 V vs. Ag/AgCl. By contrast, the cathodic reduction of

304

Cu2+ and Cu-EDTA begins at the potential of -0.02 V (Figure 5(b)) and -0.28 V

305

(Figure 5(c)), respectively. The lower reduction potential of both Cu2+ and Cu-EDTA

306

than S2O82- indicated that S2O82- was prior to be activated through reaction 6 on the

307

cathode. Then, the cathodic reduction of the liberated Cu2+ ions occurred.

308

The residual ratio of S2O82- and Cu recovery percentage in the PEC/S2O82- process

309

were investigated at a S2O82- concentration of 5 mM and a Cu-EDTA concentration of

310

0.5 mM. The S2O82- concentration decreases rapidly with a low rate of Cu recovery at

311

the beginning of 10 min (Figure SI-12). After the S2O82- concentration decreases to 1

312

mM, the Cu ions are removed rapidly.

313

Surface variation of the titanium cathode at the different reaction time in the

314

PEC/S2O82- process was characterized by SEM and EDS analysis. In comparison with

315

that before the reaction (Figure SI-13 (a-I)), it can be seem from Figure SI-13 (a-II)

316

that the nearly no copper can be observed on the surface of cathode at 10 min. By

317

contrast, as shown in Figure SM-13 (a-III), the cathode at 45 min is occupied with

318

granular deposition. It can be seen from Figure SI-13 (b-I), (b-II) and (b-III) that the

319

atomic percentage of Cu from the EDS analysis on the cathode is increased from

320

0.838% to 0.979% after 10 mins’ reaction. By contrast, it is increased to 14.41% at 45

321

min. The XPS analysis further confirmed the above results (shown in Figure 6(a)).

322

XPS spectra in the Cu 2p3/2 region for the titanium cathode for the 45 min reaction are

323

given in Figure 6(b). The signals are presented at 932.8 and 952.4 eV, which are


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324

assignable to the emissions from the Cu 2p3/2 and Cu 2p1/2 levels and respondent to

325

the binding energies of Cu0.40 It is notable to point out that there is no signal at 934 eV,

326

which is directly related to the presence of CuO species.

327

Summarily, as shown in Figure 7, at the beginning of the PEC/S2O82- process, besides

328

the PEC oxidation of Cu-EDTA, the SO4•- was greatly generated through the cathodic

329

activation and photolysis activation to enhance the decomplexation process of

330

Cu-EDTA. As a result, the efficient decomplexation was achieved. The reduction of

331

S2O82- was prior to that of the liberated Cu2+ ions at the cathode. Once the S2O82- ions

332

were consumed, the cathodic reduction of liberated Cu2+ ions dominated the cathodic

333

reaction. Meantime, the conversion of SO4•- to OH• rapidly contributed to the pH

334

decrease, which was beneficial for the Cu2+ recovery. The liberated Cu2+ ions were

335

efficiently deposited onto the cathode followed by the Cu0 electroreduction.

336

Environmental implications for wastewater treatments

337

This paper exhibited that activation of S2O82- to SO4•- can be achieved by cathodic

338

reduction and UV irradiation jointly in the PEC system, which largely increased the

339

oxidation efficiency of the PEC process towards the destruction of Cu-EDTA and

340

cathodic recovery of Cu ions across a wide range of pH conditions. In order to check

341

the efficiency of PEC/S2O82- process, a real electroplating effluent was treated by the

342

PEC/S2O82- process. The initial Cu2+ and TOC content were determined to be 65 mg/L

343

and 244 mg/L, respectively. Due to many kinds of organics chelating agents in the

344

effluent, the variation of TOC removal was used as a probe of the organics abatement

345

level. As shown in Figure 8(a), with the increase of S2O82- concentration from 0 mM



346

to 10 mM, the TOC removal was improved from 30.29 mg/L to 64.00 mg/L,

347

indicating that the organics in the electroplating wastewater were efficiently abated in

348

the PEC/S2O82- process. At the same time, the percentage of Cu recovery in 90 min is

349

as high as 82.8% with the addition of 5 mM S2O82- (Figure 8(b)). This result

350

furthermore demonstrated that the S2O82- enhanced PEC process can achieved a fast

351

destruction of Cu complexes with simultaneous Cu cathodic reduction.

352

Generally, penetrability of UV light due to the block of turbidity and chromaticity, as

353

well as the NO3- of high concentration (Text SI-6), and the design of the PEC system

354

should be considered carefully in the application of PEC/S2O82- process. The impact

355

of some ions in the real electroplating wastewater, such as Cl- and SO42-, on the

356

PEC/S2O82- process also was investigated. The SO42- exhibited a limited effect on

357

TOC removal and Cu recovery (Text SI-7). By contrast, an obvious negative influence

358

was observed with the Cl- addition into the PEC/S2O82- process (Text SI-8). Besides

359

the ions in the wastewater, the dissolved oxygen may also impede the S2O82-

360

activation and Cu recovery on the cathode due to its competition between O2 with

361

S2O82- or Cu2+ on the cathode (Text SI-9). A pilot-scale experiment of the application

362

of this PEC/S2O82- process in treating the real electroplating wastewater containing

363

heavy metal complexes with the simultaneous metal ions recovery will be performed

364

to further optimize the reaction parameters and confirm its adaptation. And, a

365

large-scale testing and engineering cost comparisons are necessary before PEC/S 2O82-

366

process can be recommended for full scale applications.

367

Supporting Information Available


Page 18 of 35

Page 19 of 35


368

Additional information about the detection methods of total Cu complexes and S2O82-,

369

procedure of Cu mass balance, proposed degradation pathway of Cu-EDTA, radicals

370

quenching study, effect of NO3-, SO42- , Cl- and O2 on the PEC/S2O82- process, effect of

371

effect of initial solution pH on the PEC process and PEC/S2O82- process, the variation

372

of S2O82- concentration and Cu2+, characteristics of the cathode at different reaction

373

time and qualities variation of the real electroplating wastewater. This information is

374

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

375

Acknowledgements

376

This work was supported by National Natural Science Foundation of China (No.

377

21377148, 51438011, 51222802).

378

References

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by EDTA/K4P2O7. Environ. Sci. Technol. 2015, 49, 4567-4574.

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Zhang, R. C.; Sun, P; Boyer, T. H.; Zhao L.; Huang C. H. Degradation of

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Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical-review

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Chovelon, J. M. Optimization of imazalil removal in the system

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Hanna, K.; Vione, D. Activation of persulfate by irradiated magnetite:

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Zhang, T.; Chen, Y.; Wang, Y.; Le Roux, J.; Yang, Y.; Croué, J. Efficient

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peroxydisulfate activation process not relying on sulfate radical generation for

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water pollutant degradation. Environ. Sci. Technol. 2014, 48, 5868-5875.

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Lei, Y.; Chen, C. S.; Tu, Y. J.; Huang, Y. H.; Zhang, H. Heterogeneous

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degradation of organic pollutants by persulfate activated by CuO-Fe3O4:

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mechanism, stability, and effects of pH and bicarbonate ions. Environ. Sci.

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Ma, W. H.; Huang, Y. P.; Li, J.; Cheng, M. M.; Song, W. J.; Zhao, J. C. An

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efficient approach for the photodegradation of organic pollutants by

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immobilized iron ions at neutral pHs. Chem. Commun. 2003, 9, 1582-1583.

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Yang, Y.; Pignatello, J. J.; Ma, J.; Mitch, W. A. Comparison of halide impacts

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radical-based advanced oxidation processes (AOPs). Environ. Sci. Technol.

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Zhao, X.; Guo, L. B.; Qu, J. H. Photoelectrocatalytic oxidation of Cu-EDTA

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complex and electrodeposition recovery of Cu in a continuous tubular

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photoelectrochemical reactor. Chem. Eng. J. 2014, 239, 53-59.

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Liang, C.; Su, H. W. Identification of sulfate and hydroxyl radicals in thermally activated persulfate. Ind. Eng. Chem. Res. 2009, 48, 5558-5562.



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organic pollutant degradation system: Evidence for H2O2, HO center dot, and

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the homogeneous activation of O-2 by Fe(II)EDTA. Ind. Eng. Chem. Res.

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2008, 47, 6502-6508.

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Danilczuk, M.; Schlick, S.; Coms, F. D. Cerium(III) as a stabilizer of

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perfluorinated membranes used in fuel cells: In situ detection of early events

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in the ESR resonator. Macromolecules 2009, 42, 8943-8949.

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Liang, C. J.; Bruell, C. J.; Marley, M. C.; Sperry, K. L. Persulfate oxidation

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for in situ remediation of TCE. II. Activated by chelated ferrous ion.

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Chemosphere 2004, 55, 1225-1233.

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Anipsitakis, G. P.; Dionysiou, D. D. Radical generation by the interaction of

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transition metals with common oxidants. Environ. Sci. Technol. 2004, 38,

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Fang, G. D.; Gao, J.; Dionysiou, D. D.; Liu, C.; Zhou, D. M. Activation of

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persulfate by quinones: Free radical reactions and implication for the

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degradation of PCBs. Environ. Sci. Technol. 2013, 47, 4605-4611.

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Drzewicz, P.; Perez-Estrada, L.; Alpatova, A.; Martin, J. W.; El-Din, M. G.

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Impact of peroxydisulfate in the presence of zero valent iron on the oxidation

492

of cyclohexanoic acid and naphthenic acids from oil sands process-affected

493

Water. Environ. Sci. Technol. 2012, 46, 8984-8991.


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Rodriguez, S.; Vasquez, L.; Costa, D.; Romero, A.; Santos, A. Oxidation of

495

orange G by persulfate activated by Fe(II), Fe(III) and zero valent iron (ZVI).

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Chemosphere 2014, 101, 86-92.

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Tao, Y.; Ni, Q.; Wei, M.; Xia, D.; Li, X.; Xu, A. Metal-free activation of

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peroxymonosulfate by g-C3N4 under visible light irradiation for the

499

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Abdulla-Al-Mamun, M.; Kusumoto, Y. New simple synthesis of Cu-TiO2

501

nanocomposite: Highly enhanced photocatalytic killing of epithelia carcinoma

502

(heLa) cells. Chem. Lett. 2009, 38, 826-827.

503



504

The captions of Figures and Tables

505

Figure 1. (a) Variation of residual Cu-complexes ratio and recovery percentage of Cu

506

and (b) TOC removal ratio in the UV/S2O82- process, PEC process, EC/S2O82- process

507

and PEC/S2O82- process. ([Cu-EDTA], 0.2 mM; [Na2S2O8], 5 mM; initial solution pH,

508

3.0; current density, 0.2 mA/cm2)

509

Figure 2. (a) Effect of S2O82- concentration on the PEC process (current density, 0.2

510

mA/cm2); (b) Effect of current density on the PEC/S2O82- process ([Na2S2O8], 10 mM)

511

([Cu-EDTA], 0.5 mM; initial solution pH, 3.0; current density, 0.2 mA/cm2)

512

Figure 3. (a) The copper mass balance and (b) variation of the solution pH in the PEC

513

process and the PEC/S2O82- process under various initial solution pH. ([Cu-EDTA],

514

1.0 mM; [Na2S2O8], 50 mM; current density, 0.2 mA/cm2)

515

Figure 4. (a) DMPO spin-trapping ESR spectra recorded at ambient temperature in

516

the PEC process, UV/S2O82- process and PEC/S2O82- process; (b) DMPO

517

spin-trapping ESR spectra recorded at ambient temperature in the S2O82- oxidation

518

process and in the bulk solution and around the cathode of the EC/S2O82- process; (c)

519

Effect of the current density on the ESR signals of DMPO-OH at the solution pH of

520

10.0.

521

Figure 5. Linear sweep voltammetry of (a) S2O82-, (b) Cu2+ and (c) Cu-EDTA with

522

various concentrations at the glassy carbon electrode in 0.1 M Na2SO4 (Solution pH,

523

3.0).


Page 26 of 35

Page 27 of 35


524

Figure 6. XPS analysis of the titanium cathode at the different reaction time in the

525

PEC/S2O82- process: (a) XPS full scan of the Titanium cathode; (b) XPS analysis in

526

the Cu 2p region of the titanium cathode.

527

Figure 7. Schematic of the PEC/S2O82- process.

528

Figure 8. Effect of S2O82- concentration on the (a) TOC removal and (b) Cu recovery

529

in the PEC/S2O82- process to treat an actual wastewater containing Cu-EDTA. (Cu, 65

530

mg/L; initial TOC, 244 mg/L; initial solution pH, 2.6; current density, 0.4 mA/cm2)

531 532



28

Residual Ratio of Cu Complexes

UV/S2O

PEC

EC/S2O82-

PEC/S2O82-

PEC

2-

PEC/S2O82-

EC/S2O8

120

1.0

100

0.8

80

0.6

60

0.4

40

0.2

20

(a) 0.0

0 0

10

20

30

40

50

60

40

50

60

Time (min) 60

Removal Ratio of TOC (%)

UV/S2O82PEC EC/S2O82PEC/S2O82-

40

20

(b)

0 0

10

20

30

Time (min)

533 534

Figure 1.


Recovery Percentage of Cu (%)

UV/S2O82-

1.2

Page 28 of 35

Page 29 of 35


100

0.8

80 28

[S2O ] (mM) 0 5 20 50

0.6

0.4

0 5 20 50

60

40

(a)

0.2

20

0.0


Residual ratio of Cu complexes

1.0

0 0

10

20

30

40

50

60

1.0

100

0.8

80

j (mA/cm2) 0.1 0.2 0.5 1.0

0.6

0.4

60

0.1 0.2 0.5 1.0

40

(b)

0.2

20

0.0

0 0

10

20

30

40

535

Time (min)

536

Figure 2.

50

537


60


Residual ratio of Cu complexes

Time (min)


Solution

100

Anode

(a)

Cathode

Propertion (%)

80

60

40

20

0

3.0

7.0 PEC

3.0

11.0

11.0 7.0 PEC + S2O82-

Initial Solution pH (b)

12

Initial pH PEC+S2O82PEC 3.0 3.0 7.0 7.0 11.0 11.0

10

pH

8

6

4

2

0 0

10

20

30

40

50

60

70

80

Time (min)

538 539

Figure 2.


90

Page 30 of 35

Page 31 of 35




(a)

  





 

 OH  SO4-



Intensity (A.u.)



PEC/S2O82-

PEC

UV/S2O823480

3500

3520

3540

3560

Magnetic Field (G)



(b)

  





 

 OH  SO4-



Intensit (a.u.)



Cathode

Bulk Solution

Pure S2O823480

3500

3520

3540

3560

Magnetic Field (G)

(c)

Intensity (a.u.)

1.00 mA/cm2

0.50 mA/cm2

0.25 mA/cm2 0.10 mA/cm2 0.05 mA/cm2 0 mA/cm2 3480

540 541

3500

3520

Magnetic Field (G)

Figure 4.

542


3540


0

Page 32 of 35

(a)

Current (μA)

-10

-20

C(S2O82-) 1.0 mM 2.0 mM 5.0 mM 8.0 mM 10.0 mM

-30

-40

-50

-60 0

Current (μA)

-10

-20

-30

(b) C(Cu2+) 0.1 mM 0.2 mM 0.5 mM 0.8 mM 1.0 mM

-40

-50

-60

(c)

0

C(Cu-EDTA) 0.1 mM 0.2 mM 0.5 mM 0.8 mM 1.0 mM

Current (μA)

-10

-20

-30

-40

-50

-60 0.4

0.2

0.0

-0.2

-0.4

-0.6

Voltage (V)

543 544

Figure 5.

545


-0.8


800

600

Cu 3s Cu 3p

C 1s

Ti 2p

Intensity (A.u.)

1000

（a）

45 min 10 min 0 min

Cu LMM d Cu LMM b Cu LMM c Cu LMM a O 1s

Cu 2s

Cu 2pCu 2p

Page 33 of 35

400

200

0

Binding Energy (eV) （b）

Cu2p region Cu2+

Intensity (A.u.)

45 min 10 min 0 min

Cu

970

960

950

940

Binding Energy (eV)

546 547

Cu

Figure 6.


930


548 549

Figure 7.

550


Page 34 of 35

Page 35 of 35


100

TOC Removal (mg/L)

[S2O82-] 0 mM 5 mM 10 mM

80 60 40 20

(a) Recovery Percentage of Cu (%)

1000

[S2O82-] 80

0 mM 5 mM 10 mM

60 40 20

(b) 0 0

551 552

20

40

60

80

Time (min)

Figure 8.

553


Enhanced Photoelectrocatalytic Decomplexation of CuâEDTA and Cu

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