simultaneous silver recovery and cyanide removal from electroplating wastewater by pulse current...

Simultaneous Silver Recovery and Cyanide Removal fromElectroplating Wastewater by Pulse Current Electrolysis Using StaticCylinder ElectrodesYixian Gao,†,‡,§ Yao Zhou,†,‡,§ Haitao Wang,†,‡,§ Wenshuang Lin,†,‡,§ Yuanpeng Wang,*,† Daohua Sun,†

Jinqing Hong,† and Qingbiao Li*,†,‡,§

†Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, ‡National EngineeringLaboratory for Green Chemical Productions of Alcohols, Ethers and Esters, and §Key Lab for Chemical Biology of Fujian Province,Xiamen University, Xiamen, 361005, People’s Republic of China

ABSTRACT: An effective electrochemical approach for simultaneous silver recovery and cyanide removal from electroplatingwastewater was presented. Accordingly, pulse current (PC) electrolysis with parameters including voltage (4.0 V), frequency(800 Hz), and duty cycle (50%) were settled using static cylinder electrodes. Then the influences of technological conditions onthe electroplating wastewater treatment process were investigated, which manifested that the concentration of silver ions inelectroplating wastewater could be reduced from 221 to 0.4 mg L−1 and cyanide could be simultaneously removed from 157 to4.9 mg L−1 after 3.0 h of PC electrolysis at pH 9.5 ± 0.5, aeration rate of 100 L h−1, and stirring speed of 1000 rpm with NaCladdition of 0.05 mol L−1 at room temperature. The results of XRD and EDX analysis showed that the silver deposits on thecathode were crystalline in face centered cubic structure and had a high purity.

1. INTRODUCTION

Electroplated silver, owing to its well-known electrical andthermal conductivities and corrosion resistance, has importantapplications in the electronic industry as components of printedcircuit systems including switches, connectors, and the like.1

Cyanide-free baths using phosphate, thiosulfate, 5-sulfosalicylicacid dehydrate, and ammonium salts as complexing agentscould be applied in the plating industry. However, compared tocyanide baths, the costs of silver salts required for cyanide-freebaths are very high. Therefore, for the stability in achievingexcellent surface appearance and adhesion, silver cyanide(AgCN) and dicyanoargentate (KAg(CN)2) solutions are stillbroadly used in silver plating baths on a large scale.2,3 Forexample, around 4 billion tons of electroplating wastewatercontaining AgCN and KAg(CN)2 result from metal surfacecleaning, rinsing, acid pickling, and silver stripping processesper year in China. Both cyanide and silver are toxic to animalsand plants. Exposure to waterborne silver ions results in severedisturbance of branchial Na+ and Cl− regulation. For humanhealth protection, free cyanide values must be <10 μg L−1 indrinking water.4−6 Also, the recovery of silver from industrialwastewater is of practical importance due to the consumptionand the economic value of natural silver resources. Up to now,various technologies including chemical oxidation7 andreduction,8,9 physicochemical methods such as ion-exchangeresins,10 membrane separate processes,11,12 activated carbonadsorption,13,14 and biochemical treatment15,16 have beendeveloped for electroplating wastewater treatment. Thesemethods are mainly employed for the recovery of silver;however, results of research targeted at simultaneous silverrecovery and cyanide removal from wastewater are much lessfrequently reported, owing to the difficulty in degradation ofAgCN and KAg(CN)2.

Electrochemical methods including electrodialysis,17 electro-coagulation,18−20 electrooxidation,21−23 and electroreduc-tion24−28 have drawn more and more attention. Gonzalez etal. confirmed the potential of electrochemical methods intreating electroplating wastewater through a series ofstudies.29−36 Compared with other methods, the advantagesof electrochemical methods include lower operating costs andless usage of extra chemical reagents and simultaneousachievements of fairly pure metals and removal of organicpollutants. The efficiency of the electrolysis process inrecovering metals depends on a variety of factors such as thehydrodynamic and mass transport characteristics, features ofelectrodes, and power supply. For batch electroplating, themass transport at the solid−liquid interface is an importantfactor affecting the plating efficiency. However, the features ofthe power supply directly affect the driving force of theelectrochemical reaction and the electrodes define where thereaction occurs. Therefore, they are of primary importance indetermining the performance of the electroplating system andwill be the focus of the present work. Direct current and planarstatic electrodes are employed in conventional electrochemicaltreatment. However, the direct current must be carefully tunedto avoid producing dendritic and spongy silver deposits.37

In contrast to direct current, pulse current (PC) generates aconstant current density during the pulse on time (Ton),followed by a pause during the pulse off time (Toff). The majoradvantages of PC lie in the Toff, which provides spares time forcomplete deposition. It was first used as a novel means ofcontrolling membrane ion transport selectivity in electrodialysis

Received: June 30, 2012Revised: March 14, 2013Accepted: April 5, 2013Published: April 5, 2013

Article

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© 2013 American Chemical Society 5871 dx.doi.org/10.1021/ie301731g | Ind. Eng. Chem. Res. 2013, 52, 5871−5879

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of Na+ and Ca2+ ions, where the desalination can beenhanced.37 It was reported that PC had the effect ofreplenishing the diffusion layer and selectively dissolved theprotrusions of the metal surface, which hence ensured auniform deposit and prevented the passivation of electrodematerials.37 Meanwhile, the cylinder electrode has beensuccessfully applied to metal removal with high-purity metalsurface finishing.38 The uniform equipotential lines of thecylinder electrode allow obtaining high current efficiencies andreaction selectivity.29,30

In our previous work,39 a method using PC together withstatic cylinder electrodes (SCE) was successfully adopted torecover 99% of silver from wastewater, which confirmed theadvantage of PC in the silver electrodeposition process.However, the removal of cyanide, an issue of equal importancein electroplating wastewater, was ignored. Herein we reportsimultaneous recovery of silver and removal of cyanide throughthis electrochemical method using PC and SCE with furtherinvestigation. In this paper, parameters of PC electrolysis werefirst investigated in terms of their influences on treatingwastewater. Then the effects of temperature, pH, stirring speed,aeration rate, and concentration of NaCl addition on therecovery of silver and removal of cyanide in the PC electrolysisprocess were investigated.

2. MATERIALS AND METHODS

2.1. Wastewater and Reagents. Silver electroplating rinsewastewater (containing 200 ± 10.0 mg L−1 silver ions and 150± 10.0 mg L−1 cyanide at pH 10.0 ± 0.5) from a local industrialplating company was treated. The reagents used in thedetermination of cyanide concentration included potassiumdihydrogen phosphate (KH2PO4), disodium hydrogen phos-phate (Na2HPO4), chloramine-T (C7H7ClNNaO2S·3H2O),isonicotinic acid (C10H10ON2), 3-methyl-1-phenyl-5-pyrazo-lone (C6H6NO2), N,N-dimethylformamide (HCON(CH3)2),potassium cyanide (KCN), and sodium hydroxide (NaOH).They were all of analytical grade and were purchased fromSinopharm Chemical Reagent Co., Ltd. All solutions wereprepared with deionized water.

2.2. Apparatus and Electrodes. The experimental setup isshown in Figure 1. A glass beaker of 1 L was employed as theelectrolysis cell. Besides the voltage (U), the characteristicparameters of PC electrolysis are the frequency ( f) and dutycycle (r). Herein, a high-frequency switching power supply(GGMF-100/12-A, Beijing New Technology Co., Ltd., China)was used to offer a regular PC (U = 0−12 V, f = 50−2000 Hz, r= 10−100%), and the electrolytic current was monitored by anammeter (C31-A, Shanghai Electric Meter Factory Co., Ltd.,China). A magnetic heating stirrer (79-2, Shenzhen WorldwideIndustrial Co., Ltd., China) with a stir bar whose stirring speedcould be tuned within 0−2400 rpm, was used to generatevortices and turbulent convection. The SCE in this researchincluded a stainless steel cylinder cathode (outer diameter =88.4 mm, inner diameter = 82.2 mm, height = 80 mm) and aporous graphite anode (diameter = 30 mm, height = 80 mm).

2.3. Electrolysis Process. All experiments were carried outin a batchwise mode at room temperature unless otherwisespecified. Variations of parameters including electric current,temperature, and pH were monitored. The concentrations ofsilver ions and cyanide were detected by analyzing samplesfrom the electrolysis cell at preestablished intervals. To obtainsilver deposits for analysis, small copper sheets (10 mm × 2mm) were pasted using conductive adhesive tape on thecathode. After the electrolysis experiment, the silver depositswere scraped off from the cathode, and then the electrodeswere cleaned with 1.0 mol L−1 H2SO4, rinsed for 10 min withdeionized water, and dried at room temperature.The experimental design is shown in Table 1. The effects of

parameters of PC electrolysis and technological conditionsincluding temperature, pH, stirring speed, aeration rate, andconcentration of NaCl addition were investigated in terms oftheir influences on the recovery of silver and removal ofcyanide. The PC power supply determines the intrinsicelectrochemical efficiency, and these technological conditionsare considered to be important in the treatment of industrialelectroplating wastewater.

2.4. Analysis Methods. The concentration of silver ionswas determined by the atomic absorption spectrophotometricmethod (National Standard of China, GB11907-89) using an

Figure 1. Schematic diagram of electrochemistry experimental setup: (1) switching power supply; (2) ammeter; (3) magnetic heating stirrer; (4)electrolysis cell; (5) anode; (6) cathode; (7) stir bar.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie301731g | Ind. Eng. Chem. Res. 2013, 52, 5871−58795872

atomic absorption spectrophotometer (TAS-986, BeijingPurkinje General Instrument Co., Ltd., China). Total cyanide(free as well as coordinated cyanide) concentration wasobtained by the isonicotinic acid−pyrazolone colorimetricmethod (National Standard of China, GB 7486-87) using aUV−vis spectrophotometer (UV310, Thermo Scientific Inc.,America). This method involves the oxidation of cyanide tocyanogen chloride with chloramine-T. The cyanogen chloride isthen reacted with isonicotinic acid containing pyrazolones, andthe blue color produced is measured at 638 nm. It is asuccessful colorimetric method for the determination of lowconcentrations of cyanide.40

The silver recovery ratio (RAg, %) and cyanide removal ratio(RCN, %) are defined by eqs 1 and 2, respectively:

=−

·RC C

C100%

tAg

Ag,0 Ag,

Ag,0 (1)

=−

·RC C

C100%t

CNCN,0 CN,

CN,0 (2)

where CAg (mg L−1) is the silver ion concentration and CCN(mg L−1) is the cyanide concentration; the subscripts 0 and tindicate the initial and the sampling time, respectively.Surface morphology and elemental composition of the silver

deposits were analyzed through scanning electron microscopy(SEM) with energy dispersive X-ray spectroscopy (EDX; XL30,Philips-FEI, Netherlands). The crystalline structure of depositswas determined through X-ray diffection (XRD; X’Pert Pro,PANalytical B.V., Netherlands) with a Cu Kα radiation sourceunder the conditions of λ = 0.154 18 nm, 40 kV, and 30 mA at20−80°.

3. RESULTS AND DISCUSSION3.1. Investigation of Parameters of PC Power Supply.

The major parameters of the electrolysis cell system includingthe voltage, frequency, and duty cycle of PC electrolysis wereinvestigated. Figure 2 displays the effect of voltage on RAg. Theresults show that it takes more than 4 h to achieve maximumRAg at U ≤ 3.0 V, whereas RAg reaches 90.0% within 2.0 h at U≥ 4.0 V, indicating that the recovery rate increased significantlywith increasing voltage. However, at U = 5.0 V the silverdeposits became black and porous easily; they were the so-called burnt products and are also a common problem in theplating industry. This might be caused by the overcurrentdensity at high voltage. At such an electrolysis condition, thesilver recovery rate was so fast that the concentrationpolarization became influential, leading to a low silver ionconcentration in the vicinity of the cathode interface.

Therefore, as shown in Table 2, the electrolysis of waterwould be significant. This resulted in the recovery of H2, which

impacted on the crystallization of silver.23 Taking the aboveresult as well as the energy consumption into consideration, thevoltage of 4.0 V was selected in the following experiments.Frequency ( f) and duty cycle (r) are the characteristic

parameters of PC. They are defined by eqs 3 and 4,respectively:

= =+

fT T T1 1

on off (3)

= =+

·rTT

TT T

100%on on

on off (4)

where Ton is the electrolysis working time and Toff is theelectrolysis interval time, which were determined by f and raccording to eqs 3 and 4, and are important in determining therecovery of silver.As seen in Figure 3, the silver recovery rate increased with

increasing frequency from 400 to 800 Hz. After 1.0 h ofelectrolysis, when the frequency increased from 400 to 800 Hz,RAg increased from 58.4 to 70.2%. However, further increase ofthe frequency to 1200 Hz led to adverse impacts on RAg. RAgreduced to 67.6% at the frequency of 1200 Hz and furtherdecreased to 65.5% at 1400 Hz.A similar effect upon RAg was displayed for the case of the

duty cycle. As shown in Figure 4, at low pulse duty cycle (r <50%), RAg increases with an increasing duty cycle, butincreasing the duty cycle to 70% led to a decrease in RAg. Forexample, after 1.0 h of electrolysis, when the duty cycle

Table 1. Experimental Parameters and Their Levels

parameter level 1 level 2 level 3 level 4

PC power supplyvoltage (V) 1.5 3.0 4.0 5.0frequency (Hz) 400 800 1200 1400duty cycle (%) 30 40 50 70

technological conditionstemperature (°C) 30 40 50 60pH value 7 8 9 10aeration (L h−1) 0 100 150 200stirring speed (rpm) 0 500 1000 2000NaCl addition (mol L−1) 0 0.01 0.03 0.05

Figure 2. Silver recovery ratio (RAg) for treatment of electroplatingwastewater at different voltages with 800 Hz frequency and 50% dutycycle.

Table 2. Main Reactions and Standard Electrode Potential inElectrolysis Process of Electroplating Wastewater at 25 °C

cathode reaction E° (V) anode reaction E° (V)

Ag+ + e = Ag +0.799 2CN− + 2O2 − 2e = N2 +2CO2

−5.873

AgCN + e = Ag +CN−

−0.017 4CN− + 6H2O + O2 − 4e =4NH3 + 4CO2

−2.356

O2 + H2O + 2e =OH− + HO2

−−0.065 CN− + 2OH− − 2e = CNO−

+ H2O−0.996

[Ag(CN)2]− + e = Ag

+ 2CN−−0.310 2CNO− + 4OH− − 6e =

2CO2 + N2 + 2H2O−0.757

2H2O + 2e = H2 +2OH−

−0.828 4OH− − 4e = O2 + 2H2O +0.402



increased from 30 to 40%, RAg increased from 38.3 to 66.6%and RAg was highest (70.3%) at a 50% duty cycle. However,when the duty cycle was increased to 70%, RAg dropped to60.4%.

The effects of the influence of f and r upon the electrolysiscould be explained by looking at the relationship among theelectrolysis working time Ton, the frequency, and duty cycle,37

which is eq 5:

=Trfon

(5)

Increasing the electrolysis working time Ton accelerates theformation of nuclei, and thus silver deposition is enhanced.However, at a too-long electrolysis working time Ton, thesmoothness and the morphology of the deposits would beaffected for ineffective transfer mass during the electrolysisinterval time.37 That is, there is a suitable combination offrequency and duty cycle based on the results, and the resultsrevealed that it is 800 Hz and 50%.

3.2. Investigation of Technological Conditions forTreatment of Industrial Electroplating Water. 3.2.1. Effectof Temperature. Technological conditions including temper-ature, pH value, stirring speed, aeration rate, and concentrationof NaCl addition are important factors affecting treatment ofindustrial electroplating wastewaters, and they were investigatedherein. When electrolysis was conducted at room temperaturewith no control over the temperature, as depicted in Figure 5a,the temperature of the solution would increase steadily withextension of reaction time, which was mainly owing to theexcess heat generated by the exothermic electrolysis reactionsshown in Table 2. To investigate the effect of temperature uponon the recovery ratio of silver RAg and the removal ratio of totalcyanide RCN, the electrolysis temperature was controlled at 40,50, and 60 °C, respectively, by a heating magnetic stirrer. Asshown in Figure 5b, when temperature increased from 30 to 60°C, RCN increased from 57.5 to 91.0% within 4.0 h; however, noevident influence on the silver recovery was observed. Theincrease in temperature enhanced mass transport of ions in thesolution and decreased the complexation of cyanide. Nonethe-less, increasing the temperature would lead to low solubility ofHCN and quick evaporation of treated wastewater which couldbe recycled to the electroplating bath; also, it would increasethe energy cost. Hence, room temperature was a recommendedchoice.

3.2.2. Effect of pH Value. According to the reactions shownin Table 2, OH− is a necessary reactant consumed in theelectrolysis process. Accordingly, as shown in Figure 6a, the pH

Figure 3. Silver recovery ratio (RAg) for treatment of electroplatingwastewater at different frequencies with 50% duty cycle and 4.0 Vvoltage.

Figure 4. Silver recovery ratio (RAg) for treatment of electroplatingwastewater at different duty cycles with 800 Hz frequency and 4.0 Vvoltage.

Figure 5. (a) Changes of temperature of reaction solution versus electrolysis time and (b) influence of reaction temperature on RAg and RCN underPC electrolysis (U = 4.0 V, f = 800 Hz, and r = 50%) using SCE. Note: 30 °C in (b) was actually the average temperature for the experimentconducted at room temperature.



decreased from 10.3 to 9.0 after electrolysis for 3.0 h. Thecyanide oxidation process is very sensitive to the pH of thesolution.2 Hence, the pH was maintained at 7.0, 8.0, 9.0, and10.0 using NaOH or HNO3 to investigate its effect on RAg andRCN. The results from Figure 6b showed that, when the pHdecreased from 10.0 to 7.0, RCN after 4.0 h of electrolysis

increased evidently from 58.1 to 92.0%. The reason might bethat a low pH value would inhibit the complexation behaviorsof free CN− ions and thus lead to a higher concentration of freeCN− ions. Nevertheless, no evident change in RAg after 4.0 h ofelectrolysis was observed when the pH increased from 7.0 to10.0 (Figure 6b). Moreover, at pH ≤9.2 cyanide mainly existed

Figure 6. (a) Changes of pH value of the reaction solution versus electrolysis time and (b) influence of pH value on RAg and RCN under PCelectrolysis (U = 4.0 V, f = 800 Hz, and r = 50%) using SCE.

Figure 7. (a) Influence of aeration on RAg and RCN and (b) RAg as a function of reaction time under PC electrolysis (U = 4.0 V, f = 800 Hz, and r =50%) using SCE.

Figure 8. (a) Influence of stirring speed on RAg and RCN and (b) RAg as a function of reaction time under PC electrolysis (U = 4.0 V, f = 800 Hz, andr = 50%) using SCE.



in the form of HCN. It was very dangerous if the toxic andvolatile gas HCN was formed. Although HCN is miscible withwater, its solubility decreases with increasing temperature.2 Asmentioned previously, the temperature of the reaction solutionwould increase (Figure 5a) and its pH value would decrease(Figure 6a) along with the electrolysis process. Therefore, theinitial pH value of the solution was controlled at 9.5 ± 0.5 usingNaOH to ensure the safety of the electrolysis process.3.2.3. Effect of Aeration. As shown in Table 2, oxidation of

cyanide can produce cyanate (CNO−) which is less toxic thanHCN, and further oxidation of cyanate gives ammonia. Such aprocess occurred on the electrode surface through both a directway by charge transfer and an indirect way by hydroxyl radicalsin the electrooxidation process. Hence the influence of thedissolved oxygen concentration was nonignorable. As observedin Figure 7a, with the increase of the aeration rate, RCN after 3.0h of electrolysis was promoted from 53.9% to around 80.0%.This is because the increase of aeration could enhance the masstransfer coefficient and concentration of oxygen in the water,and thus increase the oxidation of cyanide. On the other hand,the presence of air bubbles has a significant impact on the cellpotential drop. Nevertheless, as observed in Figure 7b, theaeration rate has little effect on RAg. When the aeration rate wasgreater than 150 L h−1, the surface of the silver deposits wouldbe oxidized and become spongy and gray. Therefore, theaeration rate was controlled at 100 L h−1.3.2.4. Effect of Stirring Speed. As mentioned above, when

the voltage was higher than 4.0 V, burnt products weregenerated due to the concentration polarization phenomenon.Hence, it is very critical to homogenize the silver ions in thesolution, and stirring is an important method to enhance theconvection and mass transport.30 Herein, the stirring speed wascontrolled at 0, 500, 1000, and 2000 rpm to investigate its effecton the electrolysis process. As shown in Figure 8, the silverrecovery rate was influenced strongly by the stirring speed. RAgafter 2.0 h of electrolysis increased from 40 to 90% when thestirring speed increased from 0 to 1000 rpm.At low stirring speed, e.g., 500 rpm, the electrolysis rate was

controlled by the diffusion of reactants toward the surface ofthe electrodes. Therefore, increasing the stirring speed wouldgenerate vortices in the hub of the SCE, and lead to a fastertransport of ions from the bulk to the electrode surface, thuspromoting the reaction rate. For a stirred tank, the Reynoldsnumber41 is usually denoted by eq 6:

ρμ

=ReND2

(6)

where ρ is the density of water, μ is the viscosity of water, D isthe length of the stir bar (D = 25 mm), and N is the revolutionsof the stir bar per second. At room temperature (25 °C), thedensity and viscosity of water are ρ = 997.04 kg m−3 and μ =0.8937 × 10−3 Pa s. When the stirring speed N is 1000 rpm, theReynolds number Re is 1.16 × 104, which means that it is a fullyturbulent flow (Re > 104). The stirring speed should be at least860 rpm to generate the fully turbulent flow.However, the speed is not the higher the better. As shown in

Figure 8, the silver recovery rate decreased when the stirringspeed reached 2000 rpm. That might be due to the adverseeffects on silver deposits caused by the overly high stirringspeed. Moreover, no evident changes were observed for RCNwith increasing stirring speed, as shown in Figure 8a, indicatingthat the diffusion of reactants toward the anode surface was notthe rate-determining step for cyanide removal. Consistently,according to the results in the previous sections, the reactiontemperature, pH value of the solution, and aeration rate hadstrong influences on the cyanide removal rate. Such resultsindicated that the rate-determining step for the cyanide removalprocess was the chemical reaction on the surface of the anodes,i.e., the oxidation of cyanide species. Taking the abovediscussion into consideration, the best preferable stirringspeed was 1000 rpm.

3.2.5. Effect of NaCl Addition. As discussed above, theremoval of cyanide was controlled by the oxidation of thecyanide on the surface of the anode. Meanwhile, theconductivity of the solution is important in the electrochemicalprocess. NaCl could be a supporting electrolyte, and it couldalso promote cyanide removal.23 Hence the effect of NaCladdition on the electrolysis process was also investigated. Asshown in Figure 9, the addition of NaCl could enhance silverrecovery and cyanide removal rates. With the addition of 0.05mol L−1 NaCl, the time to achieve RAg ≥ 95.0% wasdramatically reduced, dropping from 2.5 to 1.0 h (Figure 9b).The increase of the silver recovery rate is due to the addition ofNa+ and Cl− ions which could enhance the conductivity of thesolution. For removal of cyanide, before addition of NaCl, RCNafter electrolysis for 3.0 h was 57.5% whereas it became largerthan 99.0% within 3.0 h when the NaCl concentration was 0.05mol L−1.

Figure 9. (a) Influence of addition of NaCl on RAg and RCN and (b) RAg as a function of reaction time under PC electrolysis (U = 4.0 V, f = 800 Hz,and r = 50%) using SCE.



Such a result demonstrated that addition of NaCl couldenhance the oxidation of cyanide on the surface of the anode.As depicted in eqs 7−9, Cl− could be oxidized into Cl2 andthen transformed into HClO and ClO−.23

= + ° = +− E2Cl Cl 2e ( 1.36 V)2 (7)

+ = + ++ −Cl H O HClO H Cl2 2 (8)

+ = + +− − −Cl 2OH ClO Cl H O2 2 (9)

The formed ClO− was a main oxidant which turned cyanideinto CO2 and N2 through eqs 10−12.23 In fact, it is the alkalinechlorination−oxidation process that is commonly adopted intreatment of cyanide contaminated wastewater.15

+ + = +− − −ClO CN H O CNCl 2OH2 (10)

+ = + +− − −CNCl 2OH CNO Cl H O2 (11)

+ = + +− − −3ClO CNO CO N 3Cl2 2 (12)

All in all, the above investigations about the influences of thefactors showed that the efficiency of the electrolysis process interms of silver recovery ratio and cyanide removal ratio wasdetermined by the intrinsic kinetics of electrochemical reactionson the surface of the electrodes and the mass transfer betweenthe bulk solution and the solid−liquid interface. The temper-ature, pH value, and NaCl addition significantly influenced theintrinsic electrochemical reactions, and the mass transfer wasmainly determined by the stirring speed and the aeration.According to the results above, the appropriate technologicalconditions were 0.05 mol L−1 NaCl, 9.5 ± 0.5 pH, 100 L h−1

aeration rate, and 1000 rpm stirring speed at room temperature.3.3. Verification Experiments with Appropriate Tech-

nological Conditions. Under the appropriate conditions, fourverification experiments were carried out. As shown in Table 3,

within 3.0 h of PC electrolysis, silver was recovered with RAg =99.8% and cyanide was removed with RCN > 95.0%,simultaneously. The remaining silver was below 0.50 mg L−1,which reached the Chinese National Standard in EmissionStandard of Pollutants for Electroplating (GB 21900-2008).Removal of cyanide using electrochemical methods has beenreported previously. For instance, 93% cyanide could beremoved by the photoelectrocatalytic detoxification techni-que,22 or cyanide could be reduced from 1000 to 30 mg L−1 byanodic oxidation.23 It was also reported that cyanide could bereduced from 250 to 7.9 mg L−1 with simultaneous recovery ofcopper, but it required usage of the expensive Ti/Pt anode.25 Itcould be observed that our result was comparable to or evenbetter than those reported in those preexisting electrochemicalmethods.

3.4. Surface Morphology of Silver Deposits. To have aninsight into the quality of silver deposits, the morphology of thesilver deposits obtained at different stages during the treatmentof the electroplating wastewater was investigated. The XRDpatterns in Figure 10 indicate that silver deposits and powders

recovered from electroplating wastewater were crystalline inface centered cubic (fcc) crystal nature. The copper peaks inFigure 10a−d might be ascribed to copper sheets. Therefore, itshows that silver powders scraped from the cathode had a veryhigh purity without copper peaks (Figure 10e). The relativeintensity of silver and copper peaks reveals that silver depositsbecame denser within 3.0 h. The results could be confirmed bythe SEM observation of silver deposits.The SEM images in Figure 11 show that the surface of silver

deposits became smoother and their particle size grew largerand more uniform with increasing electrolysis time within 3.0 h.That was because PC could lessen the electrode passivation andconcentration polarization caused by the increasing electrolysistime. During the electrolysis interval time Toff, the silver ions inthe bulk solution could supplement silver ions near the cathode,which were diminished during the electrolysis working timeTon. However, as shown in Figure 11d, the surface of silverdeposits attained at 4.0 h was coarse and the purity reducedfrom 90.2% at 3.0 h to 72.4%. It can be seen from Figure 2 thatRAg increased very slightly when it was larger than 95.0%. Withlow silver ion concentration in the solution, there might beother subsidiary reactions such as hydrogen evolution or waterelectrolysis. Hence, to prevent excess electrical energyconsumption and damage of the formed silver deposits, theelectrolysis process should be terminated in an appropriatereaction time, which was 3.0 h in this study.

4. CONCLUSIONSThe parameters of power supply and technological conditionsin treating silver electroplating wastewater through an electro-chemical process were determined. Specifically, SCE combininga stainless steel cylinder cathode and a porous graphite anodewith parameters of PC electrolysis determined as U = 4.0 V, f =800 Hz, and r = 50% were selected. The effects of technological

Table 3. Results of Verification Experimentsa

no.CAg,0

(mg L−1)CCN,0

(mg L−1)CAg,t

(mg L−1)CCN,t

(mg L−1)RAg(%)

RCN(%)

1 240 189 0.4 9.8 99.8 94.82 221 157 0.4 7.0 99. 8 95.63 219 158 0.5 1.6 99.8 99.04 213 143 0.5 4.7 99.8 96.8

aPower supply parameters: 4.0 V voltage, 800 Hz pulse frequency, and50% duty cycle. Technological conditions: 0.05 mol L−1NaCl, pH 9.5± 0.5, aeration rate 100 L h−1, and stirring speed 1000 rpm at roomtemperature. Electrolysis time: 3.0 h.

Figure 10. XRD patterns of silver deposits on copper sheet after PCelectrolysis (U = 4.0 V, f = 800 Hz, and r = 50%) using SCE under0.05 mol L−1 NaCl, pH 9.5 ± 0.5, 100 L h−1 aeration rate, and 1000rpm stirring speed at room temperature for (a) 1.0, (b) 2.0, (c) 3.0,and (d) 4.0 h, and of (e) silver powders scraped from cathode after PCelectrolysis for 4.0 h. Diffraction peaks were assigned to silver andcopper according to PDF File No. 040783 (Ag) and No. 040836 (Cu),respectively.



conditions (including pH, temperature, stirring speed, aerationrate, and NaCl addition) on the silver recovery and cyanideremoval rate were investigated. The results showed that theappropriate conditions were 0.05 mol L−1 NaCl, pH 9.5 ± 0.5,100 L h−1 aeration rate, and 1000 rpm stirring speed at roomtemperature. Under such conditions, high ratios of silverrecovery (99.8%) and cyanide removal (>95.0%) could beachieved simultaneously within 3.0 h, with the remaining silverof <0.5 mg L−1 and cyanide of <10.0 mg L−1. Thus treatedwastewater was expected to be reused, and the recovered silverwas found to have a high purity which therefore had financialbenefits.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: (+86) 592-2189595. Fax: (+86)592-2184822. E-mail:[email protected] (Y.W.); [email protected] (Q.L.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Basic ResearchProgram of China (2013CB733505), the National NaturalScience Foundation of China (41071302), the Developmentand Reform Commission of Fujian Province, China (2011-1598), the Science and Technology Program of Xiamen ofFujian Province, China (3502Z20126005), and the Ministry ofScience and Technology of the People’s Republic of China(2011BAE16B03). We thank the Xiamen YongHong Technol-ogy Co. Ltd. for their support, and we also thank the Analysis

and Testing Centre of Xiamen University for helping us withSEM and EDS analyses.

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Figure 11. SEM images of silver deposits on copper sheet after PC electrolysis (U = 4.0 V, f = 800 Hz, and r = 50%) using SCE under 0.05 mol L−1

NaCl, pH 9.5 ± 0.5, 100 L h−1 aeration rate, and 1000 rpm stirring speed at room temperature for (a) 1.0, (b) 2.0, (c) 3.0, and (d) 4.0 h.



mailto:[email protected]

mailto:[email protected]

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simultaneous silver recovery and cyanide removal from electroplating wastewater by pulse current...

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