Enhanced Photoelectrocatalytic Decomposition of Copper Cyanide

Mar 13, 2015 - Abstract Image. Simultaneous photoelectrocatalytic (PEC) oxidation of cyanides and recovery of copper in a PEC reactor with a Bi2MoO6 ...
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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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Enhanced Photoelectrocatalytic Decomposition of Copper Cyanide Complexes

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and Simultaneous Recovery of Copper with Bi2MoO6 Electrode under Visible

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Light by EDTA/K4P2O7

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

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

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

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

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

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Beijing, 10085, P.R. China.

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ABSTRACT

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Simultaneous photoelectrocatalytic (PEC) oxidation of cyanides and recovery of

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

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

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

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

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

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

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

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

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

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

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

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

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was confirmed.

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Keywords: Photoelectrocatalysis; Copper cyanides; Electrodeposition; Heavy metal

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recovery

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Table of contents/Abstract art

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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

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0 Cu0 Cu

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Introduction

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

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

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

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

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containing these cyanocomplexes previously to their discharge into the environment.

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

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known for treating cyanides.2,

3

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

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

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

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

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

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

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be treated carefully.

The adsorption and complexation processes give

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The electrochemical oxidation of cyanide and copper cyanides has been largely

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

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

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

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

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

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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

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

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

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oxidation was little studied.

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Photocatalytic oxidation of cyanide was also extensively investigated. When using

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

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

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

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

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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.

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

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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

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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

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

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organic

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

contaminants

and

bacteria

inactivation

combined

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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

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

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

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photocatalysts responsive to visible light have shown great potential in applications

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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

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

18, 19

In our

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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

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

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Experimental section

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Photoelectrochemical experiments

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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

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3.0.21

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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

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

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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,

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Thermo Fisher, USA).

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The Bi2MoO6 film anode and titanium cathode under various processes were

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

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

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

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

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

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

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

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solution under various conditions was measured using an Inductively coupled plasma

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

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Results and discussion

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Photoelectrocatalytic treatment of Copper cyanides

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

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

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

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RCu (%) = 100 ×

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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

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

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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

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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

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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

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

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

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

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

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

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

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

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

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N-containing products of the reaction is also noted.

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Photoelectrocatalytic treatment of copper cyanides in the presence of EDTA

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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EDTA with high concentration was limited.

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

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

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

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

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

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

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

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

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

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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.

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Photoelectrocatalytic treatment of copper cyanides in the presence of K4P2O7

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

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

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

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

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

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

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

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

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

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

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

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

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Involved active species

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

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

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

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

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

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

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

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

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

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

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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

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hydrolyzes to result in cyanate and cyanide ions.12

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

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

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

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

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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

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

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

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

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

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

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

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Bi2MoO6 film anode.

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Analysis of the surface variation of the electrodes

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

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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

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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

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

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cathode, respectively.

be seen from

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

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

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

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

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

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

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

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

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

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

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the cathode.

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

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

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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

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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

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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,

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

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

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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.

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A proposed reaction mechanism

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

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electrochemical oxidation of copper cyanide complex solutions under alkaline

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

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

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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)

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The mechanism of photocatalytic oxidation of cyanide using TiO2 has been

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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

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cyanide photocatalytic oxidation is the formation of cyanide radical, which

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

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

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

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

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CN- + h+/OH·→ CN·

(5)

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2CN·→(CN)2

(6)

282

(CN)2 + 2OH- → CN- + CNO- + H2O

(7)

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

(8)

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

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

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

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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

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

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

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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

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

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of copper onto the cathode. 15

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

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

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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

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

301

feasible.

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Acknowledgments

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

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51222802, 51438011).

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References

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

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Environmental contaminants Encyclopedia: Cyanide, National Park Service,

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Water Resources Division, Fort Collins, Colorado, 1997.

309 310

(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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modeling with explicit radiation adsorption effects of the photocatalytic oxidation

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of cyanide with TiO2 and silica-supported TiO2 suspensions. Appl. Catal., B:

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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

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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

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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

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120

Recovery ratio of Cu (%)

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Concentration of total cyanide (Ct/C0)

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Concentration of N species (mM)

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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

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120

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+

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

Recovery ratio of Cu (%)

Concentration of total cyanide (Ct/C0)

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

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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)

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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

1.4 Concentration of NH4 (mM)

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

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Recovery ratio of Cu (%)

0.8

Blank K4P2O7=0.1mM

-

1.0

Page 26 of 31

Concentration of CNO (mM)

Concentration of total cyanide (Ct/C0)

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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

1.4 Concentration of NH4 (mM)

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

100 Proportion of Cu (%)

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

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Recovery ratio of Cu (%)

0.8

Blank C4H10O=0.1mM

-

Concentration of CN (CT/CO)

1.0

Concentration of CNO (mM)

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

100 Proportion of Cu (%)

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

Recovery ratio of Cu (%)

0.8

Blank N2

N2

Figure 6

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O2

-

1.0

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Concentration of CNO (mM)

Concentration of total cyanide (Ct/C0)

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anode-(a)

cathode-(a)

anode-(b)

cathode-(b)

anode-(c)

cathode-(c)

anode-(d)

cathode-(d)

Figure 7

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(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

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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

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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

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930