Enhanced Photoelectrocatalytic Decomposition of Copper Cyanide

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Enhanced Photoelectrocatalytic Decomposition of Copper Cyanide Complexes and Simultaneous Recovery of Copper with Bi2MoO6 Electrode under Visible Light by EDTA/K4P2O7 Xu Zhao, Juanjuan Zhang, Meng Qiao, Huijuan Liu, and Jiuhui Qu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5062374 • Publication Date (Web): 13 Mar 2015 Downloaded from http://pubs.acs.org on March 20, 2015

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Environmental Science & Technology

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Enhanced Photoelectrocatalytic Decomposition of Copper Cyanide Complexes

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and Simultaneous Recovery of Copper with Bi2MoO6 Electrode under Visible

3

Light by EDTA/K4P2O7

4 5

Xu Zhao, Juanjuan Zhang, Meng Qiao, Huijuan Liu, Jiuhui Qu*

6 7

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

8

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

9 10

*Corresponding author: e-mail address: [email protected]; Tel.: 86-010-62849151;

11

Fax: 86-010-62849160; Add: P.O. Box 2871, 18 Shuangqing Road, Haidian District,

12

Beijing, 10085, P.R. China.

13

1

ACS Paragon Plus Environment


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ABSTRACT

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Simultaneous photoelectrocatalytic (PEC) oxidation of cyanides and recovery of

16

copper in a PEC reactor with a Bi2MoO6 photoanode was investigated at alkaline

17

conditions under visible light irradiation. The surface variation of the Bi2MoO6

18

photoanode and titanium cathode was characterized. The Cu mass distribution onto

19

the anode, in the solution, onto the cathode was fully investigated. In the individual

20

PEC oxidation of copper cyanides, the formation of a black copper oxide on the anode

21

occurred. By keeping the initial cyanide concentration at 0.01 mM, the effect of

22

EDTA/K4P2O7 was examined at different molar ratios of EDTA/K4P2O7 to cyanide. It

23

was indicated that the oxidation of cyanides was increased and simultaneous copper

24

electrodeposition with zero value onto the cathode was feasible at pH 11. Under the

25

optimal conditions, the total cyanide concentration was lowered from 250 to 5.0 mg/L

26

and Cu recovery efficiency deposited onto the cathode was higher than 90%. Cyanate

27

was the only product. The role of photogenerated hole in the oxidation of cyanide ions

28

was confirmed.

29

Keywords: Photoelectrocatalysis; Copper cyanides; Electrodeposition; Heavy metal

30

recovery

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2


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Table of contents/Abstract art

32 33 34

Cu(CN)32CuO Cu O 2

Cu(CN)32- ＋ P2O74-

Cu(CN)32-

＋

EDTA

CuOH

Cu(CN)32-

CNO- Cu(CN)32- CNOCu2+/Cu+ Cu2+/Cu+

Cu-EDTA

Intermediates

Cu(CN)32- CNO-

＋

Cu2+ EDTA

P2O74Cu2P2O7 Cu4P2O7

Cu(CN)32Cu0

Cu0

0 Cu0 Cu

Cu2+

Cu0

3


0 Cu0 Cu


35

Introduction

36

Cyanides are widely used in electroplating, mining and photographic processes due

37

to their unique properties for complexing metals such as copper, silver, gold, or zinc.

38

As a consequence, wastewaters coming out from these activities usually contain large

39

amounts of metal cyanide complexes.1 Therefore, it is important to treat the effluents

40

containing these cyanocomplexes previously to their discharge into the environment.

41

Various treatment procedures, such as adsorption, complexation, and oxidation are

42

known for treating cyanides.2,

3

43

highly concentrated products in which toxic cyanide still exist. The ozone or

44

peroxides oxidation do not achieve the complete removal of metal cyanide complexes.

45

The most common method to treat cyanide is alkaline chlorination. However,

46

improper chlorination of cyanide would give evolution of toxic cyanogens chloride.

47

Ferrate as a green chemical oxidant can address some of the concerns of chlorination

48

in the treatment of cyanides.4 However, the iron cyanide complexes are still needed to

49

be treated carefully.

The adsorption and complexation processes give

50

The electrochemical oxidation of cyanide and copper cyanides has been largely

51

investigated.5, 6 The results indicated that rapid oxidation of cyanide to cyanate was

52

observed with the formation of a black copper oxide film at the anode. It was

53

recognized that copper oxide film is found to be catalytic, capable of electro-oxidizing

54

hydroxide to oxygen and cyanate to nitrate.7 However, the overdose copper oxide film

55

will limit the oxidation reaction of cyanide or copper cyanide. Meantime, the Cu

56

recovery onto cathode with zero value was desired. Thus, it was desired to oxidize the 4


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cyanide and recovery of liberated Cu ions with zero value simultaneously. Organic

58

ligands such as EDTA or K4P2O7 forms strong complexes with heavy metal ions are

59

commonly used in the industrial cyanide plating processes. Their effect on the cyanide

60

oxidation was little studied.

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Photocatalytic oxidation of cyanide was also extensively investigated. When using

62

TiO2 in the slurry form or deposited on a support, the removal of cyanides was shown

63

to be possible. 8, 9, 10, 11 In most cases, cyanate is found to be the oxidation product

64

which could be further oxidized simultaneously on the surface of TiO2 to produce

65

nitrate and carbonate. The photocatalytic oxidation of cyanide in aqueous TiO2

66

suspensions with and without EDTA present in the reaction mixture was investigated

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by Osathaphan et al. 12 The results indicated that the presence of EDTA reduces the

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photocatalytic oxidation rate of cyanide due to occupy of the active sites.

69

Unfortunately, TiO2 when applied as a suspension or slurry has to be filtered, making

70

this approach uneconomical and preventing the industrial application of this process

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for the cyanides elimination.

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To achieve an effective treatment of copper cyanides not only the oxidation of the

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cyanide (CN-) groups but also the recovery of metal would be also desirable in order

74

to accomplish a double objective: to avoid cyanide and metal pollution and to reuse

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copper in new processes. Recently, combined electro-oxidation and photocatalysis

76

using a semiconductor film electrode has shown great potential for destruction of

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organic

78

electro-oxidation and photocatalysis.13, 14, 15 In our previous work, TiO2 film electrode

contaminants

and

bacteria

inactivation

combined

5


with

individual


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was used as anode and photoelectrocatalytic (PEC) treatment of Cu-EDTA complexes

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with the simultaneous recovery of Cu onto the stainless cathode was achieved at

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neutral conditions. 16 Generally, the copper cyanides exhibited different behavior with

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Cu-EDTA complexes. It was known that oxidation of cyanide should be in the pH

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value higher than 10.0 in order to avoid the generation of HCN toxic gas. Thus, the

84

hydrolysis and precipitation of the liberated metal ions occur easily, which will inhibit

85

the metal ions deposition onto the cathode. Recently, Bi-based ternary oxide

86

photocatalysts responsive to visible light have shown great potential in applications

87

such as splitting water, solar cells, and environmental purification.17,

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previous work, the Bi2MoO6 film photoanode was prepared and exhibited high PEC

89

activities towards the degradation of organic dyes under visible light irradiation.20

18, 19

In our

90

Herein, the PEC degradation of copper cyanides using the Bi2MoO6 photoanode in

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the absence and presence of EDTA or K4P2O7 was investigated under visible light

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irradiation. The results indicated that the copper oxide deposition on the anode with

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the simultaneous cyanide destruction without EDTA or K4P2O7, leading to the

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inhibition of PEC decomposition of CN- ions using Bi2MoO6 photoanode under

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visible light irradiation. In the presence of EDTA or K4P2O7, copper cyanides were

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efficiently destroyed and copper was efficiently recovered by electrochemical

97

reduction deposition at the cathode. The enhanced mechanism was also proposed.

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Experimental section

99

Photoelectrochemical experiments

100

Degradation experiments were performed in a rectangular (50 mm × 50 mm × 100 6


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mm) reactor which is made from quartz glass. The reactor, which contained a 70 mL

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sample solution allowing 6 cm of the supported film electrode to be immersed into the

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solution, was placed 3 cm in front of a 150 W Xe lamp purchased from the German

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Osram. A UV cutoff filter ( λ > 420 nm) filter purchased from Shanghai Seagull

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Colored Optical Glass Co., Ltd. was used to remove light with wavelengths below

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420 nm. The PEC reaction employed a basic electrochemical system (CHI 660b,

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Shanghai Chenhua Instruments Co., Ltd) connected with a working electrode (the

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Bi2MoO6 film deposited onto the ITO electrode, active area of 25 cm2), a

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counter-electrode (titanium plate with the same active area), and a reference electrode

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(a saturated calomel electrode (SCE)). The Bi2MoO6 film deposited onto the ITO

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electrode was prepared according to the procedure described in our previous work.20

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The ratio of [CN-]: [Cu] remained much higher than four in the experiment and it is

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therefore likely that the predominant species was Cu(CN)32- according to our previous

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analysis of cuprous cyanide species distribution with the program Visual MINTEO

115

3.0.21

116

Analytical methods

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All experiments were performed in duplicate, and the analysis of each parameter

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was done in triplicate for each run. Total cyanide concentration (free cyanide plus

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weak acid dissociable cyanocomplexes) was determined through a standard

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colorimetric method using a pyridine-barbituric acid reagent to form a highly colored

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complex with a maximum absorbance at 575 nm.22 The detection limit of determining

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total cyanide concentration was 0.0045 mg/L. Free cyanide concentration was 7



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potentiometricaly determined by using a CN--selective electrode in an expandable ion

124

analyzer (Orion 720A). UV-Vis analysis was performed using an UV-Vis

125

spectrophotometer (U-3010, HITACHI). The detection limit of determining cyanide

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concentration was 0.5000 mg/L. pH values were measured using a pH meter (310P-02,

127

Thermo Fisher, USA).

128

The Bi2MoO6 film anode and titanium cathode under various processes were

129

analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and

130

scanning electron microscopy (SEM)- Energy dispersive x-ray (EDX). Prior to the

131

measurement, the electrode was washed with water to remove electrolyte. XPS

132

analysis was obtained using a PHI Quantera SXM (PHI-5300/ESCA, ULVAC-PHI,

133

INC). SEM micrographs were obtained by using JEOL JSM-6301 equipment with

134

electron beam energy of 20 keV. XRD was recorded on a Scintag-XDS-2000

135

diffractometer with Cu Kα radiation. The concentration of copper ions in the reaction

136

solution under various conditions was measured using an Inductively coupled plasma

137

optical emission spectrometer (OPTIMA 200, Perking Elmer, U.S.A).

138

Results and discussion

139

Photoelectrocatalytic treatment of Copper cyanides

140

Firstly, the PEC treatment of Cu(CN)32-s under various bias potentials was

141

investigated. Meantime, the recovery ratio of Cu was also given. Herein, the recovery

142

ratio of Cu2+ ions is defined as RCu(%) shown in the following equations.

143

RCu (%) = 100 ×

144

As shown in Figure 1, a slight removal of total cyanide is observed in the individual

(Initial amount of Cu ions - remained amount of Cu ions) in the reaction cell Initial amount of Cu ions in the reaction cell

8


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photocatalysis process and nearly no Cu recovery is observed. With the applied bias

146

potential of 1.0 V, nearly no change is observed. By contrast, an obvious increase of

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total cyanide removal and the recovery ratio of Cu are observed with the applied bias

148

potentials of 1.5, 2.0, and 2.5 V (vs. SCE). Among them, the total cyanide removal

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and the Cu recovery achieve the largest value at the bias potential of 2.0 V, which are

150

determined to be 68.4% and 64.1% at 120 min, respectively.

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Meantime, the conversion byproducts of CN- ions were investigated. As shown in

152

Figure 2, the intermediate of cyanate (CNO-) is identified and its concentration

153

increases with the decrease of total cyanide concentration. It has been recognized that

154

cyanide ion was believed to react with O3, ·OH and H2O2, yielding intermediate

155

compounds (OCN-, NO2-, HO-C·=N, O=C·-NH2, ·OOC(O)NH2) and final

156

by-products (NO3-, NO2-, NH4+, N2, HCO3-, CO32-, saponified compounds).23 The

157

by-products observed in solution during the photoctalytic degradation of cyanide were

158

identified as cyanates and nitrites.24 Herein, no intermediates such as NH4+ and NO3-

159

are identified in the process. The almost stoichiometric nitrogen balance, given by the

160

N-containing products of the reaction is also noted.

161

Photoelectrocatalytic treatment of copper cyanides in the presence of EDTA

162

The above results indicated that the efficiencies of total cyanide removal and Cu

163

recovery in the PEC process were desired to be increased. It was known that EDTA

164

used as chelant usually coexisted with the cyanide pollutants. Therefore, the total

165

cyanide removal and Cu recovery with the addition of various EDTA amounts were

166

investigated under the bias potential of 2.0 V with the same initial concentration of

167

Cu(CN)32-. As shown in Figure 3 (a), with the increase of ratio of [EDTA] to [CN-], 9



168

the efficiencies of total cyanide removal and Cu recoveries increase obviously. At the

169

[EDTA]: [CN-] ratio of 5:1, the removal efficiency of the total cyanide is determined

170

to be 99.8% and the Cu recovery ratio is 91.0% at 120 min. It is clear that the addition

171

of EDTA largely increases the efficiencies of cyanide oxidation and Cu recovery. With

172

respect to the intermediates, it is interesting to observe from Figure 3(b) that the

173

amount of CNO- decreases with the increase of NH4+ when the ratio of [EDTA] to

174

[CN-] increases. The NH4+ ions were generated from the PEC oxidation of EDTA, as

175

indicated in our previous results.16 Meantime, total organic carbon (TOC) variation in

176

the reaction processes with various ratio of [EDTA]: [CN-] were analyzed. The results

177

indicated that nearly no TOC decrease in the reaction process (Figure SI-1). On one

178

way, EDTA was destroyed into small molecular acids; on another way, destruction of

179

EDTA with high concentration was limited.

180

Furthermore, the distribution of Cu species was analyzed at various EDTA amounts.

181

As shown in Figure 3(c), without the EDTA addition, the percentages of Cu (by wt%)

182

deposited onto the anode, in the solution, and deposited onto the cathode are 31.3%,

183

37.7%, and 31.0%, respectively. With the increase of EDTA amount, the Cu amount

184

deposited onto the anode and in the solution decrease and the Cu amount deposited

185

onto the cathode increase gradually. At the [EDTA] to [CN-] ratio of 5:1, the Cu mass

186

percentage deposited onto the anode and the cathode are determined to be 10.7% and

187

86.6%, respectively. Thus, it is obvious that the addition of EDTA to the solution

188

increases the Cu amount deposited onto the cathode. The Cu deposition onto the

189

anode is inhibited, which is favorable to the PEC oxidation of Cu-cyanides. The 10


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enhanced process will be discussed subsequently.

191

Photoelectrocatalytic treatment of copper cyanides in the presence of K4P2O7

192

Generally, K4P2O7 coexisted with the cyanide in the electroplating wastewater.

193

Therefore, the effect of K4P2O7 on the total cyanide removal and copper recovery in

194

the PEC degradation of Cu(CN)32- was investigated at the 2.0 V bias potential with

195

the same initial concentration of Cu(CN)32- (Figure 4(a)). Meantime, the intermediates

196

and Cu mass distribution were also given in Figure 4(b) and Figure 4(c), respectively.

197

It can be seen from Figure 4(a) that the efficiencies of total cyanide removal and Cu

198

recovery increase with the amount of K4P2O7. With respect to the intermediates, the

199

intermediate of CNO- increase with the increased amount of K4P2O7; nearly no NH4+

200

generation is observed (Figure 4(b)). With respect to the Cu mass distribution, it is

201

observed from Figure 4(c) that the addition of K4P2O7 does not only lead to the

202

efficient removal of Cu from the solution but also increases the deposited amount of

203

Cu onto the cathode. The enhanced mechanism will be also discussed subsequently.

204

Involved active species

205

The photocatalytic oxidation of cyanide over TiO2 particles has been reported to

206

occur via both direct electron transfer to the holes and homogeneous hydroxyl radicals

207

attack in the bulk solution.8, 9 Tertiary butanol (t-BuOH) which is known as ·OH

208

radicals scavengers, were used to test the role of ·OH radicals. As shown in Figure

209

5(a), the efficiencies of total cyanide removal and Cu recovery remain nearly constant

210

with the addition of various t-BuOH concentrations in the presence of K4P2O7. Nearly

211

no effect of t-BuOH addition on the generation of intermediates and Cu mass 11



212

distribution is observed in Figure 5 (b) and (c), respectively. Therefore, it was

213

concluded that the role of active ·OH radicals in the decomposition of total cyanide

214

can be ignored. Maybe, the photogenerated hole was responsible for the total cyanide

215

decomposition.25 It was reported that cyanide is oxidized by photogenerated hvb+ to

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form CN. radicals. The dimerization of CN. forms cyanogen, which subsequently

217

hydrolyzes to result in cyanate and cyanide ions.12

218

Effect of aeration on the PEC treatment of Cu(CN)32- was furthermore investigated.

219

It can be seen from Figure 6 (a) that the removal efficiency of the total cyanide in the

220

presence of O2 is higher than in the presence of N2 and blank conditions; a similar

221

trend is observed with the Cu recovery. Meantime, more cyanide is transferred into

222

the CNO- in the presence of more dissolved O2 (Figure 6 (b)). It is proposed that O2

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supports the oxidation of the liberated Cu+/ Cu2+ ions and acts as reagent in the

224

oxidation of released cyanide to cyanate species. The Cu mass distributions in the

225

various processes were also investigated. As shown in Figure 6 (c), most of Cu is

226

deposited onto the anode with the Cu oxides species in the presence of O2. By contrast,

227

the amount of Cu deposited onto the cathode is increased in the presence of N2. In the

228

PEC oxidation of Cu(CN)32-, the oxidation of Cu1+/Cu2+ ions in the presence of more

229

dissolved O2 is enhanced, leading to the more deposition of Cu oxides onto the

230

Bi2MoO6 film anode.

231

Analysis of the surface variation of the electrodes

232

The surface variation of the Bi2MoO6 film anode and titanium cathode under

233

various processes was characterized by SEM-EDX, XRD, and XPS analysis. In 12


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comparison with that in the absence of Cu(CN)32- (Figure 7(a)), it can

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Figure 7(b) and Figure SI-2(a) that the Bi2MoO6 film anode and the cathode was

236

covered by a layer of copper oxides in the PEC treatment of Cu(CN)32-. By contrast,

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as shown in Figure 7(c), Figure SI-2(b) Figure 7(d), and Figure SI-2(c), the formation

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of copper oxides film on the anode was efficiently inhibited in the presence of EDTA

239

or K4P2O7, respectively. And, the liberated Cu ions were efficiently deposited onto the

240

cathode, respectively.

be seen from

241

The chemical nature of the surface layers deposited onto the anode and cathode was

242

further investigated by XPS analysis and XRD analysis. For the chemical

243

identification of the copper surface species, analysis of both Cu 2p and Cu LMM

244

Auger lines was performed. It can be seen from Figure 8 (a) that (b) that the Cu

245

appears on the Bi2MoO6 film anode and the titanium cathode in the presence of

246

Cu(CN)32-. By contrast, the peak intensity of Cu decreases at the anode with the

247

increase of the Cu amount at the cathode in the presence of EDTA and K4P2O7.

248

Particularly, the peak intensity of Cu on the anode nearly disappears in the presence of

249

K4P2O7. The SEM-EDX and XPS results furthermore confirmed the results shown in

250

Figure 3 (c) and 4 (c) on the Cu mass distribution at the anode, in the solution, and at

251

the cathode.

252

For the chemical identification of the copper surface species, XPS spectrum in

253

Cu2p3/2 region for the Bi2MoO6 film anode and titanium cathode under various

254

conditions were given in Figure 8 (c) and (d), respectively. The presence of Cu2+ is

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characterized with the specific binding energies of 934.5±0.3 eV in the presence of 13



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Cu(CN)32- complexes after the PEC reaction for 120 min, which confirm the

257

formation of Cu oxides in the PEC process (Figure 8 (c)). By contrast, the peak at the

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binding energies of 932.5±0.3 eV appear in the presence of EDTA which indicate

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that the presence of Cu1+ on the anode.26 Particularly, no peak was observed for Cu in

260

the presence of K4P2O7. In the case of titanium cathode (Figure 8 (d)), the peaks at

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binding energies of 935 eV and 937 eV confirm the presence of Cu2+ and Cu1+/ Cu0,

262

respectively. The Cu2+ and Cu1+ will be electrochemically reduced to Cu0. Combined

263

with the XRD results (shown in Figure SI-3), it was concluded that the addition of

264

EDTA /K4P2O7 efficiently inhibited the deposition of Cu oxides onto the anode and

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promoted the electrochemical reduction of the liberated Cu ions onto the cathode.

266

A proposed reaction mechanism

267

It has been well recognized that the formation of Cu oxides on the anode during the

268

electrochemical oxidation of copper cyanide complex solutions under alkaline

269

conditions, which can be expressed in terms of the following reactions (Eqs.

270

(1)-(4)).27 Simultaneously, copper electrodeposition occurs on the cathode with the

271

liberation of CN- ions according to the results.28 Cu(CN)n(1-n)+ + 2H2O → CuOOH + 2H+ + nCN- + 2e-

(1)

CuOOH + H+ + e-→ Cu(OH)2

(2)

CuOOH + H+ + e- → CuO + H2O

(3)

Cu(CN)2- + e-→ Cu + 2CN-

(4)

272

The mechanism of photocatalytic oxidation of cyanide using TiO2 has been

273

documented.8, 9, 10 The oxidation of cyanide is possible via the reaction of CN- with 14


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surface hydroxyls or holes. According to Domenech and Peral, the initial step of

275

cyanide photocatalytic oxidation is the formation of cyanide radical, which

276

subsequently dimerize to form cyanogens.12 Finally, the cyanogens molecule

277

undergoes dismutation under alkali conditions to give cyanide and cyanate. The

278

produced cyanate is further oxidized to give NO3- and CO2. The corresponding

279

reactions are presented as follows (Eqs. (5)-(8)).

280

CN- + h+/OH·→ CN·

(5)

281

2CN·→(CN)2

(6)

282

(CN)2 + 2OH- → CN- + CNO- + H2O

(7)

283

CNO- + 8OH- + 8 h+ → NO3- + CO2 + 4H2O

(8)

284

Therefore, it was concluded that CN- ions were oxidized into CNO- by

285

photogenerated holes with the simultaneous oxidation of Cu+ to Cu2+ in the PEC

286

process of Cu(CN)32- at the Bi2MoO6 film anode. Meantime, CuO and CuOOH

287

particles were generated. As a result, the PEC reaction was inhibited. In the presence

288

of EDTA or K4P2O7, with the oxidation of CN- ions into CNO-, the complexation

289

reaction of liberated Cu+/Cu2+ with EDTA or P2O74- occurred immediately with

290

oxidation of Cu-EDTA complexes, which were electrochemically reduced at the

291

titanium cathode. A model for such process can be assumed in four-step reaction as

292

follows: adsorption of copper cyanide complexes onto the Bi2MoO6 surface, followed

293

by oxidation of CN- with the liberation of copper into the solution, complextion of Cu

294

with EDTA or P2O74- ions with oxidation of Cu-EDTA complexes, electrodeposition

295

of copper onto the cathode. 15



296

Summarily, the PEC oxidation of copper cyanides with the simultaneous recovery

297

of copper by electrochemical reduction at cathode was achieved at the strong alkaline

298

conditions in the presence of EDTA or K4P2O7. Cu can be furthermore recovered by

299

scraping the deposited Cu from the cathode. Following this strategy, the large-scale

300

continuous degradation of cyanide to below legal emission levels and recovery Cu is

301

feasible.

302

Acknowledgments

303

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

304

51222802, 51438011).

305

References

306

(1) Irwin, R. J.; Van Mouweric, M.; Stevens, L.; Seese, M. D.; Basham, W.;

307

Environmental contaminants Encyclopedia: Cyanide, National Park Service,

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Water Resources Division, Fort Collins, Colorado, 1997.

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(2) Guro, M. D.; Bremen, W. M. Kinetics and mechanism of ozonation of free cyanide species in water. Environ. Sci. Technol. 1985, 19, 804-809.

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(3) Young, C. A. Remediation of technologies for the management of aqueous

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cyanide species. In cyanide: social, industrial and economic aspects Young, C.,

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Ed.; TMS (The Mineral, Metals, and Materials Society) Press: Warrendale, PA,

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2001, 175-194.

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(4) Sharma, V. K.; Burnett, C. R.; Yngard, R. A.; Cabelli, D. E. Iron(VI) and Iron(V) oxidation of copper(I) cyanide. Environ. Sci. Technol. 2005, 39, 3849-3854. (5) Szpyrkowicz, L.; Zilio-Grandi, F.; Kaul, S.N.; Rigoni-Stern, S. Electrochemical 16


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treatment of copper cyanide wastewaters using stainless steel electrodes. Water Sci.

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(6) Szpyrkowica, L.; Kelsall, G. H.; Souto, R. M.; Ricci, F.; Kaul, S. N.

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Hydrodynamic effects on the performance of an electrochemical reactor for

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destruction of copper cyanide. Part 2-reactor kinetics and current efficiencies.

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Chem. Eng. J. 2005, 60, 535-543.

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(7) Cheng, S. C.; Gattrell, M.; Guena, T.; MacDougall, B. The electrochemical

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oxidation of alkaline copper cyanide solutions. Electrochim. Acta. 2002, 47,

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3245-3256.

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(8) Marugan, J.; van Grieken, R.; Cassano, A. E.; Alfano, O. M. Intrinsic kinetic

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Page 21 of 31


The captions of Figures and Tables Figure 1 Concentration variation of total cyanide and recovery ratio of copper in the photocatalysis and photoelectrocatalytic oxidation with various potentials ([Cu(CN)32-] = 0.5 mM; pH = 11) Figure 2 The generated intermediates and N mass balance in the photoelectrocatalytic oxidation of Cu(CN)32- at 2.0 V bias potential ([Cu(CN)32-] = 0.5 mM; pH = 11) Figure 3 (a) The total cyanide removal and copper recovery; (b) Generation of CNOand NH4+; (c) the copper mass balance in the photoelectrocatalytic process with various ratios of [EDTA] to [CN-] ([Cu(CN)32-] = 0.5 mM; pH=11; 2.0 V bias potentials) Figure 4 (a) Effect of K4P2O7 on total cyanide removal and copper recovery; (b) Effect of K4P2O7 on the concentration variation of CNO- and NH4+; (c) The copper mass balance in the photoelectrocatalytic process with various concentration of K4P2O7 ([Cu(CN)32-] = 0.5 mM; pH=11; 2.0 V bias potential) Figure 5 (a) The removal of total cyanide and copper recovery in the PEC process of Cu(CN)32-; (b) Concentration variation of CNO- and NH4+; (c) The copper mass balance in the photoelectrocatalytic process of Cu(CN)32- with various C4H10O concentration ([Cu(CN)32-] = 0.5 mM; pH = 11; [K4P2O7] = 5 mM ; 2.0 V bias potential; 120 min) Figure 6 The effect of N2 and O2 sparged electrolyte in the photoelectrocatalytic process: (a) The removal of total cyanide and copper recovery ratio; (b) The generation of cyanate and NH4+; (c) The copper distribution ([Cu(CN)32-] = 0.5 21



mmol/L; [Na2SO4] = 1.0 mmol/L; [NaOH] =1.0 g/L; pH = 11.0) Figure 7 SEM analysis of the Bi2MoO6 film anode and titanium cathode in the various processes: (a) blank; (b) in the presence of Cu(CN)3-; (c) in the presence of EDTA; (d) in the presence of K4P2O7 Figure 8 XPS analysis of the Bi2MoO6 anode and titanium cathode in the various processes: (a) XPS full scan of the Bi2MoO6 anode; (b) XPS full scan of the titanium cathode; (c) XPS spectrum in Cu2p region of the film for Bi2MoO6 anode; (d) XPS spectrum in Cu2p region of the film for the titanium cathode

22


Page 22 of 31

1.0

100

0.8

80

0.6

60 2.5V 2.0V 1.5V 1.0V PC

0.4

2.5V 2.0V 1.5V 1.0V PC

40

0.2

20

0.0

0 0

20

40

60 80 Time (min)

100

Figure 1

23


120

Recovery ratio of Cu (%)


Concentration of total cyanide (Ct/C0)

Page 23 of 31

Concentration of N species (mM)


1.4 1.2 1.0

-

CN CNO + NH4

0.8 0.6

-

NO3

0.4

N balance

0.2 0.0 0

15

30

60 Time (min)

90

Figure 2

24


120

Page 24 of 31


+

80 60

0.4

40

0.2

20

0.0

0 20

40

60 80 Time (min)

100

Blank EDTA:CN =0.5:1 EDTA:CN =1:1 EDTA:CN =2:1 EDTA:CN =5:1 Blank EDTA:CN =0.5:1 EDTA:CN =1:1 EDTA:CN =2:1 EDTA:CN =5:1

1.4 Concentration of NH4 (mM)

100

0.6

0

1.2 1.0 0.8 0.6

120

1.4

(b)

1.2 1.0 0.8 0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60 80 Time (min) Solution

100 Cu percentage ratio (%)

(a)

Anode

100

120 Cathode

(c) 80 60 40 20 0

Blank

0.5:1



0.8

Blank EDTA:CN =0.5:1 EDTA:CN =1:1 EDTA:CN =2:1 EDTA:CN =5:1

1:1 2:1 [EDTA]:[CN ]

Figure 3

25


5:1

-

Blank EDTA:CN =0.5:1 EDTA:CN =1:1 EDTA:CN =2:1 EDTA:CN =5:1

1.0

Concentration of CNO (mM)

Page 25 of 31

K4P2O7=1mM

K4P2O7=1mM

K4P2O7=5mM

K4P2O7=5mM

(a)

100 80

0.4

40

0.2

20

0.0

0

1.2

+

K4P2O7=0.5mM

60


K4P2O7=0.5mM

0.6

0

20

40

60 80 Time (min)

Blank K4P2O7 = 0.1 mM

Blank K4P2O7 = 0.1 mM

K4P2O7 = 0.5 mM

K4P2O7 = 0.5 mM

K4P2O7 = 1 mM

K4P2O7 = 1 mM

K4P2O7 = 5 mM

K4P2O7 = 5 mM

100

120 1.4

(b)

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60 80 Time (min)

Solution

100 Proportion of Cu (%)

Blank K4P2O7=0.1mM

Anode

100

120

Cathode

(c) 80 60 40 20 0

Blank

0.1mM 0.5mM 1mM 5mM Concentration of K4P2O7 (mM)

Figure 4

26



0.8

Blank K4P2O7=0.1mM

-

1.0

Page 26 of 31





C4H10O=0.5mM

C4H10O=0.5mM

C4H10O=1.0mM

C4H10O=1.0mM

(a)

100 80

0.4

40

0.2

20

0.0

0

1.2

+

C4H10O=0.2mM

60


C4H10O=0.2mM

0.6

0

20

40

60 80 Time (min)

Blank C4H10O=0.1mM

Blank C4H10O=0.1mM

C4H10O=0.2mM

C4H10O=0.2mM

C4H10O=0.5mM

C4H10O=0.5mM

C4H10O=1.0mM

C4H10O=1.0mM

100

120

(b)

1.4 1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60 80 Time (min)

Solution


Blank C4H10O=0.1mM

Anode

100

120

Cathode

(c) 80 60 40 20 0

Blank

0.1mM 0.2mM 0.5mM 1.0mM Concentration of C4H10O (mM)

Figure 5

27



0.8

Blank C4H10O=0.1mM

-

Concentration of CN (CT/CO)

1.0


Page 27 of 31

O2

60

0.4

40

0.2

20

0.0

0

1.4

20

40

60 80 Time (min)

Blank N2

Blank N2

O2

O2

100

120

1.6 1.4

1.2

1.2

1.0

1.0

0.8

0.8

(b)

0.6

0.6

0.4

0.4

0.2

0.2

0.0

0.0 0

20

40

60 80 Time (min)

Solution


80

(a)

1.6

+

O2

100

0.6

0

Concentration of NH4 (mM)

Blank N2

Anode

100

120

Cathode

(c)

80 60 40 20 0

Blank


0.8

Blank N2

N2

Figure 6

28


O2

-

1.0

Page 28 of 31




Page 29 of 31


anode-(a)

cathode-(a)

anode-(b)

cathode-(b)

anode-(c)

cathode-(c)

anode-(d)

cathode-(d)

Figure 7

29


(a)

Cu 3s Cu 3p O 2s

Mo 3pMo 3p

C 1s Mo 3d

O 1s

Bi 4p-

Cu 2s Cu 2pCu 2p

2-

Cu(CN)3 +K4P2O7

Intensity (a.u.)

Page 30 of 31

Bi 4f


2-

Cu(CN)3 +EDTA 2-

Cu(CN)3

Blank

Intensity (a.u.)

Cu 3s Cu 3p

2-

Cu(CN)3 +K4P2O7

0

(b) C 1s

Cu LMM d Cu LMM b Cu LMM c Cu LMM a O 1s Na KLL Bi 4dBi 4d N 1s

800 600 400 200 Binding Energy (eV)

O KLL Cu 2pCu 2p

Cu 2s

1000

2-

Cu(CN)3 +EDTA 2-

Cu(CN)3

Blank

1000

800 600 400 200 Binding Energy (eV)

30


0

Page 31 of 31


Cu2p3/2 (c) 2-

Intensity (a.u.)

Cu(CN)3 +K4P2O7

2-

Cu(CN)3 +EDTA

2-

Cu(CN)3

Blank

950

940 Binding Energy (eV) 2-

Cu2p3/2 (d)

Cu(CN)3 +K4P2O7

Intensity (a.u.)

930

2-

Cu(CN)3 +EDTA 2-

Cu(CN)3

Blank

950

940 Binding Energy (eV) Figure 8

31


930

Enhanced Photoelectrocatalytic Decomposition of Copper Cyanide

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