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Treatment of cadmium and nickel electroplating rinsewater by electrocoagulationM. Kobya a , E. Demirbas b , N.U. Parlak a & S. Yigit aa Department of Environmental Engineering , Gebze Institute of Technology , 41400 Gebze,Turkeyb Department of Chemistry , Gebze Institute of Technology , 41400 Gebze, TurkeyPublished online: 05 Nov 2010.

To cite this article: M. Kobya , E. Demirbas , N.U. Parlak & S. Yigit (2010) Treatment of cadmium and nickel electroplatingrinse water by electrocoagulation, Environmental Technology, 31:13, 1471-1481, DOI: 10.1080/09593331003713693

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Environmental Technology

Vol. 31, No. 13, 1 December 2010, 1471–1481

ISSN 0959-3330 print/ISSN 1479-487X online© 2010 Taylor & FrancisDOI: 10.1080/09593331003713693http://www.informaworld.com

Treatment of cadmium and nickel electroplating rinse water by electrocoagulation

M. Kobya

a,

*, E. Demirbas

b

, N.U. Parlak

a

and S. Yigit

a

a

Department of Environmental Engineering, Gebze Institute of Technology, 41400 Gebze, Turkey;

b

Department of Chemistry, Gebze Institute of Technology, 41400 Gebze, Turkey

Taylor and Francis

(

Received 14 December 2009; Accepted 18 February 2010

)

Environmental Technology

Treatments of cadmium-cyanide and nickel-cyanide electroplating rinse water were investigated in anelectrochemical reactor equipped with iron plate electrodes in a batch mode by electrocoagulation (EC). Effectsof the process variables such as pH, current density, and operating time were explored with respect to removalefficiencies of cadmium, nickel and cyanide in electroplating rinse water and operating costs as well. Removalefficiencies and operating costs under the optimum conditions (30 A/m

2

, 30 min and pH 8–10 for cadmium;60 A/m

2

, 80 min and pH 8–10 for nickel) for the EC process in electroplating rinse water were determined as99.4% and 1.05 /m

3

for cadmium, 99.1% and 2.45 /m

3

for nickel and >99.7% for cyanide, respectively. Theresults indicated that EC was very effective treatment for the removals of cadmium, nickel, and cyanide ionsfrom the electroplating rinse water.

Keywords:

electrocoagulation; electroplating wastewater; operating cost; cyanide; nickel; cadmium

1. Introduction

Hazardous wastes containing free and metal cyanidesare generated in large volumes during various industrialactivities such as electroplating processes, metalfinishing and mining [1–2]. The release of industrialwastewaters containing cadmium and cyanide to theenvironment is strictly controlled due to the toxic natureof these substances. To reduce the environmentalimpacts it is necessary to remove these substances fromwastewater before its discharge in the environment. Inspent baths and rinse waters of metal electroplatingoperations are a major portion of the waste generated bythe metal plating/finishing industry because of the highvolume and heavy metal content from these sources.The metal finishing industry typically uses highly alka-line cyanide plating baths, which consist mainly ofheavy metal cyanide complexes (Cu, Ag, Cd, Ni, Au,Zn, etc.) and free cyanide salts due to brightness of themetal deposits and their good adherence [3–5]. In thepresence of metallic compounds in the alkaline bathsolutions, cyanide ion readily combines with the metalion, Me

z+

, to form a stable cyanide complex ion of theform, [Me(CN)

y

]

(y-z)-

[6]. In this case the wastewatergenerated during rinsing of worked pieces bears themetal cyanide complexes. The treatment of wastewaterin the electroplating industry generates a strong concernrelated to environmental impacts due to containing

toxic metal ions such as Cd, Cu, Au, Pb, Ni, Ag, and Znions, as well as acids, alkalis, and cyanide compounds[1]. For complete treatment of wastewater containingmetal-cyanide complexes, strategies for effectiveremoval of both metals and cyanide must be specified.Therefore, the toxic and hazardous effluents thatemanate from the metal plating factories must be prop-erly treated so that they do not cause more damage tothe environment.

Various techniques have been employed for thetreatment of metal electroplating wastewaters contain-ing heavy metals and cyanides, including alkaline-chlorination-oxidation, electrocoagulation, chemicalprecipitation, adsorption, ion-exchange, membraneprocesses, reverse osmosis and biological treatment [7–13]. Cyanide wastes usually have been destroyed bychlorination using chlorine gas or hypochlorite or bythermal hydrolysis methods which require high costs tooperate [14]. The chlorination process in particular,which has been used widely, cannot treat cyanide-metalcomplexes and produces secondary sludge waste [15].The electrochemical process is another treatmentmethod in which cyanide is destroyed while recoveringmetal [16]. Reverse osmosis, membrane processes,adsorption and ion-exchange can effectively reducemetal ions, but their use were limited due to a numberof disadvantages such as material and operational high

*Corresponding author. Email: kobya@gyte.edu.tr

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cost in addition to the limited pH range and regenerationfor the ion-exchange resin and adsorbent. Therefore, theconventional methods have limitations and are expen-sive [17]. This necessitated the industries to look for analternative treatment method for complete removalwithout generation of secondary pollutant(s). The ECinvolves the generation of coagulant

in situ

by electro-oxidation of a sacrificial anode, generally made ofaluminum or iron. EC is characterized by simple andeasy operated equipment, short operation time, none ora negligible amount of chemicals and stable and lesssludge production [18,19]. The electrocoagulation hasbeen successfully used to treat different industrialwastewaters such as textile wastewater [20–24], yeastwastewater [25], potato chips wastewater [26], slaugh-terhouse wastewater [27], dairy wastewater [28], pastaand cookie process wastewater [29], alcohol distillerywastewater [30], olive mill wastewaters [31], corkprocess wastewater [32], chemical mechanical polishingwastewater [33], waste metal cutting fluids [34], metalsin leachate from sewage sludge [35] and phosphorusremoval [36]. In addition, a great deal of workperformed in the last decades has proved that electroco-agulation is an effective technology for the treatment ofheavy metal containing industrial wastewaters [37–46].

In spite of the considerable success of electrocoagu-lation for treatment of various types of wastewater, itsapplication as a possible technique for treatment ofcadmium and nickel electroplating rinse wastewaterusing iron electrodes is very rare in the literature. There-fore, the main objective of the present study is to deter-mine the effect of initial pH, current density and operatingtime on the removal efficiencies of metal plating waste-water and the removal mechanism. The operating costsat the optimum operational conditions are also calculated.

2. Treatment mechanism of wastewater by EC process

In an EC process, the coagulating ions are produced

insitu

involving three successive stages: (1) formation ofcoagulants by electrolytic oxidation of sacrificial elec-trode such as iron or aluminum, (2) destabilization ofthe contaminants, particulate suspension and breakingof emulsions, (3) aggregation of the destabilized phasesto form flocs. Fe/Al gets dissolved from the anodegenerating corresponding metal ions, which almostimmediately hydrolyze to polymeric iron or aluminumoxyhydroxides. In the EC, the anodic reaction involvesthe dissolution of metal, and the cathodic reactioninvolves the formation of hydrogen gas and hydroxideions [18,19]. The gelatinous charged hydroxo cationiciron ion complexes can effectively remove pollutants(metal ions) by: (1) adsorption, as the hydroxide floc is

relatively large and of less density, so it can be easilyfloated and separated, (2) co-precipitation, as thehydroxide ions formed at the cathode increase the pH ofwastewater there by inducing precipitation of metal ionsas corresponding hydroxides and co-precipitation withiron hydroxides, (3) electrostatic attraction followed bycoagulation.

In this study, iron electrode material was used as ananode produced iron hydroxide, Fe(OH)

n

where

n

= 2 or3. During electrolysis, the wastewater solution becamegreen and bubbles of gas seen at cathode during the ECprocess. The effluent became clear and a green andyellow sludge were formed. The green and yellowcolours were attributed to Fe

2+

and Fe

3+

hydroxides[20,38]. The following shows the major reactions takingplace in the EC reactor:

Anodic reactions:

Cathodic reactions:

The metal cyano-complex formed due to the presenceof cadmium and nickel in metal rinse water was reducedas in Equation (2b):

Several competitive reactions occurred simultaneouslyeither at the cathode (metals deposition on thecathode electrodes or in solution (precipitation andco-precipitation of metals with ferrous hydroxides),Ferrous hydroxide particles were produced up to a suffi-cient concentration to initiate polymerization orcondensation reactions illustrated by reaction (2d) [45]:

The appearance of polymeric complexes [Fe

2

(O)(OH)

2

]allowed removing metallic pollutant from rinse water,mainly by adsorption mechanism [45,46]. The formediron flocks in the solution by co-precipitation and/oradsorption mechanism were also expressed as:

Fe s Fe e( ) ( )→ ++ −2 2 1a

Fe Fe e2 3 1→ ++ −. ( )b

2 2 2 22 2H O e H OHg+ → +− −( ) . ( )a

Me e Me Me Cd or Ni2 2 2+ + → ( : ) ( )b

Me CN e Me CN( ) . ( )42 2 4 2− −+ ⇔ + c

Fe OH Fe OH OH Fe O

Fe OH H O

( ) ( ) ( )

( ) . ( )2 2

2 2

+ → −− + d

Fe OH Fe OH s2

22 3+ −+ → ( ) ( )( ) a

Fe OH Fe OH s3

33 3+ −+ → ( ) ( )( ) b

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When pH>10, Me

2+

exists in the solution as and . When 5<pH<10, Me

2+

and cyanide aremainly in the solution as (i = 0, 1, 2, 3, 4) [17].

Stability constants for CdCN

+

, Cd(CN)

2

, , NiCN

+

and are 6.0, 11.1, 15.7,17.9, 7.3 and 30.2, respectively. In addition, free metalions (Cd and Ni) can be recovered in soluble forms ashydroxides according to Equation (4e) [45].

Equation (4f) is quite similar to that proposed by theauthors in the works given below [41,46], whiledescribing the mechanism of pollutant removal fromsolution during electrocoagulation. Cd(OH)

2

andNi(OH)

2

were formed at pH>9.5 and can be removedfrom the rinse water by adsorption on the electrodes ascharged colloidal particles. Constants of solubilityproducts of metal hydroxides (pK

s

) are Fe(OH)

2

(15.4),Fe(OH)

3

(37.4), Cd(OH)

2

(13.6), Ni(OH)

2

(14.7),respectively. Fe-CN complexes might be existed insolution in the form of Equations (4g) and (4h):

During electrocoagulation, Equation (5) illustrates thesurface complexation [45] in which the pollutant can actas a ligand (L: CN

−

or ) to bind a hydrousiron moiety and can be removed by surface complex-ation or electrostatic attraction.

3. Materials and method

3.1 The cadmium and nickel electroplating rinse water

Two separate amounts of cadmium and nickelelectroplating rinse water was obtained from a local

electroplating factory located in Istanbul, Turkey. Thesamples were analyzed promptly after collection usingstandard analytical methods [48]. The characteristics ofelectroplating rinse water were shown in Table 1.

3.2 Apparatus and instruments

The experimental set-up containing the specificationsof electrode, reactor and power supply is given else-where [25]. The EC apparatus equipped with a ther-mostat for the temperature control consists of anelectrolytic cell which is a 1 L Plexiglas reactor. Castiron (Fe) plates chosen as the anode/cathode pair weresituated 1 cm apart from each other and connected inmonopolar parallel mode. The electrodes were dippedin the cadmium or nickel electroplating rinse water toa depth of 80 mm, yielding a total effective electrodesurface area of 159 cm

2

. The electrodes wereconnected to a digital dc power supply (Agilent6675A model; 30 V, 6A) operated at galvanostaticmode.

3.3 Procedure

All runs were performed with 650 mL of wastewatersolution, at constant temperature (25

°

C) and mixingspeed (250 rpm). Before each run, electrodes werewashed with acetone to remove surface grease, andthe impurities on the iron electrode surfaces wereremoved by dipping for 5 min in a solution freshlyprepared by mixing 250 mL of HCl solution (35%)and 500 mL of hexamethylenetetramine aqueous solu-tion (2.8%) [25], and dried and re-weighted. In eachrun, 800 mL of the wastewater solutions was placedinto the electrolytic cell. The current density wasadjusted to a desired value and the EC was started. Atthe end of EC, the solution was filtered by microporemembrane filter with the pore diameter of 0.45

µ

mand then was analyzed.

Fe OH Me CN Fe OH

Me CN

s i

i s

( ) ( ) [ ( )

* ( ) ] ( )

( )

( )

22

2

2 3

+ →−

−

i

i c

Fe OH Me CN Fe OH

Me CN

s i

i s

( ) ( ) [ ( )

* ( ) ] . ( )

( )

( )

32

3

2 3

+ →−

−

i

i d

Me CN( )42−

Me CN( )3−

Me CN i( )2−i

Cd CN( ) ,3−

Cd CN( )42− Ni CN( )4

2−

Me OH Me OH s2

22 4+ −+ → ( ) ( )( ) e

Fe OH Me Fe OH

O Me zH

zz

z s

( ) ( )

( ) . ( )

( )

( )

2 2

4

+ ⇔

+

+−

+ f

Fe CN Fe CN3636 4+ − −+ → ( ) ( )g

Fe CN Fe CN3646 4+ − −+ → ( ) . ( )h

Me CN i( )2−i

L H HO OFe L OFe H Oaq s s− + → − +( ) ( ) ( )( ) ( )2 5

Table 1. Characteristics of metal plating rinse water.

ParametersCd electroplating

rinse waterNi electroplating

rinse water

pH 8.6 8.0Conductivity (mS/cm) 1.0 4.8Cd or Ni concentration

(mg/L)102 175

Total CN concentration (mg/L)

120 261

Total suspended solids (mg/L)

175 185

COD (mg/L) 180 220TOC (mg/L) 60 95

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3.4 Analysis

The cyanide, nickel, cadmium, COD and TOC analysiswere conducted by the procedures described in StandardMethods [47]. The cyanide concentration in the samplewas determined by pyridine-barbutiric acid method usingUV spectrophotometer (PerkinElmer Lambda 35 UV/Vis spectrophotometer, USA), the cadmium and nickelconcentrations were measured in using atomic absorp-tion spectroscopy (PerkinElmer AAS 6000 model). CODwas measured using COD reactor and direct readingspectrophotometer (DR/2000, HACH, USA). The TOClevels were determined through combustion of thesamples at 680

°

C using a non-dispersive infra-red source(Apollo 9000; Tekmar-Dohrmann, USA). The accuracyof these measured values for cyanide, cadmium, nickel,COD and TOC was estimated around 2%. The pH wasmeasured using AZ 8601 model pH meter (ThermoTrading, Taiwan), and the conductivity was determinedwith Lutron CD-4303 model (Thermo Trading) conduc-tivity meter. All the inorganic chemicals used were anay-tical grade and all reagents were prepared in Milliporemilli-Q deionized water.

4. Results and discussion

4.1 Effect of current density

Current density is important parameter for controllingthe reaction rate in the EC process. Current density notonly determines the coagulant dosage rate but also thebubble production rate and flocs growth which can affecttreatment efficiency of the process since the currentdensity increases and the efficiency of ion production onthe anode and cathode increases according to Faraday’slaw (Equation (8)). Therefore, this study involves anincrease in flocs production in the solution and hence animprovement in the efficiency of cyanide, cadmium andnickel removals. Generally an increase in current densitycauses the anodic oxidation to take place more readilywhich in turn favours formation of amorphous ironhydroxides species adequately in the vicinity of the elec-trode as well as in the bulk solutions.

Higher dissolution of electrode material (Faraday’slaw) with high rate of formation of monomeric and/orpolymeric iron hydroxides result in significantimprovement in Cd, Ni and cyanide removals mainlydue to co-precipitation/adsorption like, Me(OH)

2(s)

,Fe(OH)

(2-z)

(O)

z

Me

(s)

, , andL-OFe

(s)

species. This effect was also observed by otherresearchers during EC with electroplating wastewatercontaining Cu

2+

, Zn

2+

and Cr(VI) [37–47].A series of experiments for cadmium-cyanide and

nickel-cyanide electroplating rinse water wereconducted with applied current density (CD) and timevarying from 5 to 60 A/m

2

and 0 to 80 min and constant

pH of 8.6 for cadmium-cyanide electroplating rinsewater and 8 for nickel-cyanide electroplating rinsewater, respectively. Cadmium-cyanide and nickel-cyanide electroplating rinse water contained 102 mg/Lof cadmium and 120 mg/L of cyanide; and 175 mg/L ofnickel and 261 mg/L of cyanide, respectively. Figures 1and 2 and Table 2 showed removal efficiencies ofcadmium-cyanide and nickel-cyanide as a function ofCD. The removal efficiencies at 5, 10, 30 and 60 A/m

2

increased from 97.8 to 99.6% for cyanide and from95.2% to 99.1% for cadmium, and from 33.1 to 99.2%for nickel and 38.5 to 99.8%for cyanide, respectively.Final pHs for cadmium-cyanide and nickel-cyaniderinse water were changed to 9.3, 10.4, 10.9, 11.2 at30 min of operating time and 9.6, 10.1, 10.8 and 11.5 at80 min of operating time, respectively. The optimumcurrent density and operating time for cadmium-cyanideand nickel-cyanide plating rinse water were selected as30 A/m

2

and 30 min; and 60 A/m

2

and 80 min, respec-tively since it meets sewage discharge standards forWater Municipal of Istanbul (Istanbul Water SewageAdministration: ISKI) for removal of these pollutants[48]. ISKI regulations for releasing cadmium, nickeland cyanide in water to the sewage is below 2, 5 and10 mg/L, respectively. The amount of sludge producedin the EC process (Table 2) was increased from 0.5to 2.9 kg/m

3

for cadmium-cyanide and from 0.46 to1.6 kg/m

3

for nickel-cyanide rinse water with increasingof current density from 5 to 60 A/m

2

.

Figure 1. Effect of CD for cadmium-cyanide electroplating rinse water.Figure 2. Effect of CD for nickel-cyanide electroplating rinse water.

In this preliminary economic investigation, the costof the treated rinse water (OC,

€

/m

3

) can be calculatedby considering three parameters as major cost items[49,50]: the amount of energy consumption, the amountof electrode material used and the chemicals consumedin the process:

where

C

energy

(kWh/m

3

),

C

electrode

(kg Fe electrode/m

3

)and

C

chemicals

(kg/m

3

) are consumption quantities fortreatment of the plating rinse water, which are obtainedexperimentally. Prices for electrical energy, electrodematerial and chemicals such as NaOH and H

2

SO

4

aregiven for the Turkish market in November 2009 as 0.07

€

/kWh, 0.85

€

/kg Fe, 0.73

€

/kg and 0.29

€

/kg, respec-tively. Costs for

C

energy

and for

C

electrode

calculated fromFaraday’s law are shown in the following equations:

[ ( ) * ( ) ]( )Fe OH Me CN i s22−i

OC C C

C

= +

+

0 072 0 85

0 025 6

. .

. ( )

x x

x

energy electrode

chemicals

CU I t

venergyECx x= ( )7

CM I t

z F vw EC

electrode = ( )8

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Environmental Technology 1475

where U is cell voltage (V), I is current (A), tEC is theoperating time (min) and v is the volume (m3) of waste-water, Mw is the molecular mass of iron (56.8 g/mol), zis the number of electrons transferred (z = 2) and F isFaraday’s constant (96487 C/mol).

The operating costs increased from 0.17 to2.25 €/m3 for cadmium-cyanide electroplating rinsewaterwater and from 0.189 to 2.45 €/m3 for nickel-cyanide electroplating rinse water with increasingcurrent density from 5 to 60 A/m2, respectively(Table 2). The optimum operating conditions based onCD experiments as operating costs were 1.05 €/m3

for cadmium-cyanide and 2.45 €/m3 for nickel-cyanide electroplating rinse water. Higher currentdensity caused high electrical energy consumptionwhich increased operating costs for the treatment.

4.2 Effect of initial pH on EC

The pH is an important operating factor that can influ-ence performance of the EC process [20–35]. A seriesof experiments were carried out to evaluate its effectusing solutions containing a sample with an initial pHin the range 5–13 at 30 A/m2 and 30 min forcadmium-cyanide electroplating rinse water and pH5–13 at 60 A/m2 and 80 min for nickel-cyanideelectroplating rinse water, respectively. Removal effi-ciencies of cadmium-cyanide and nickel-cyanide elec-troplating rinse water at constant operating conditionswith respect to pH are shown in Figure 3. Theremoval efficiencies of cyanide and cadmium in therinse water were 91.8–7.40% and 89.6–33.5% at pHvalues in the range 5.6–12.6. The maximum removal

Figure 1. Effect of CD for cadmium-cyanide electroplating rinse water.

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1476 M. Kobya et al.

Figure 2. Effect of CD for nickel-cyanide electroplating rinse water.

Table 2. Removal efficiencies and operating cost of cadmium-cyanide and nickel-cyanide electroplating rinse water with respectto current densities.

CD Re (%) Cenergy Celectrode Wsludge OC

Type of rinse water (A/m2) CN Cd or Ni (kWh/m3) (kg/m3) (kg/m3) (€/m3)

Cadmium-cyanide plating* 5 97.8 95.2 0.33 0.17 0.52 0.17110 96.5 98.7 1.01 0.31 0.76 0.34630 99.4 99.7 6.13 0.66 1.79 1.04560 99.6 99.1 11.65 1.65 2.85 2.252

Nickel-cyanide plating** 5 38.5 33.1 0.28 0.19 0.46 0.18910 98.5 43.6 0.48 0.25 0.79 0.24830 99.2 54.3 2.45 1.15 1.54 1.16260 99.8 99.2 11.94 1,85 1,62 2.445

*ISKI standards for removals of cadmium-cyanide is >92.0 % for cyanide− and >%98.0 for cadmium.** ISKI standards for removals of nickel-cyanide wastewater are 96.2% for cyanide and >%98.8 for nickel.

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Environmental Technology 1477

efficiencies of cyanide and cadmium were obtained atpH values 7.6–10.6 as 98.2–99.7% for cyanide and98.3–99.4% for cadmium. For nickel plating rinsewater in Figure 4, the cyanide and nickel removals atpH values range 6–13 were 15.7–56.24% and 85.71–93.1%. At pH values range 8–10, the maximumremoval efficiencies of cyanide and nickel wereobtained in the range 99.2–99.8% for cyanide and98.9–99.9% for nickel, respectively. The major ironcomplexes formed with respect to pH from the Pour-baix diagram are Fe(OH)2+ and , Fe(OH)2

and Fe(OH)3 [18]. At large cathodic potentials, thesolution can become alkaline as shown in Equations(2b) and (2c). Under these conditions, Cd(OH)2/Ni(OH)2 is formed and can be removed from the solu-tion by adsorption on the electrode as charged colloi-dal particle. It might be the low concentration ofelectrolyte causing a high concentration overpotentialthat prevents Cd/Ni ions from being reduced to metalform on the cathode [51]. Insoluble metal hydroxidesas Cd(OH)2 and Ni(OH)2 were precipitated as increasein solution pH. These results showed that pH was veryFe OH( )+

2

Figure 3. Effect of initial pH for cadmium-cyanide electroplating rinse water.

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1478 M. Kobya et al.

significant parameter which had an influence on theeconomic applicability of EC process.Figure 3. Effect of initial pH for cadmium-cyanide electroplating rinse water.Figure 4. Effect of initial pH for nickel-cyanide electroplating rinse water.

4.3 Effect of Me/CN ratio

Cadmium-cyanide and nickel-cyanide in differentproportions lead up to the formation of differentcadmium-cyanide and nickel-cyanide complexes in

alkaline media. and complex

might prevail which was formed with excess of cyanide.If ratio of Cd:CN and Ni:CN were 0.40 and 0.43, 99.9%

of cadmium and nickel were under the and

form [17,3].A series of experiments for cadmium-cyanide and

nickel-cyanide electroplating rinse water wereconducted with different metal-cyanide ratio (α) andconstant operating conditions such as pH 8.6, 30 A/m2

and 30 min for cadmium-cyanide and pH 8, 60 A/m2

and 80 min for nickel-cyanide electroplating rinsewater. For the experiments conducted with different Cd/CN− and Ni/CN− ratio (Figure 5), the highest removal

Cd CN( ) −

4

2Ni CN( )4

2−

Cd CN( ) −

4

2

Ni CN( )42−

Figure 4. Effect of initial pH for nickel-cyanide electroplating rinse water.

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efficiency was obtained at <

α

Cd/CN

= 0.40–0.85 and<

α

Ni/CN

= 0.43–0.67. At this range cadmium-cyanideand nickel-cyanide removal efficiencies were obtainedas 99.9–99.4% and 99.9–99.8% and operation costs forthe EC process were calculated as <0.80 and 1.72

€

/m

3

for the treated wastewater, respectively. The removalefficiencies were decreased for both Cd-CN and Ni-CNwhen Me/CN- ratio got bigger (Table 3) since thecomplexes became more stable at the beginning such as

, and other anionic complexes

arised as Cd(CN)

3

−

and Ni(CN)

3

−

along with the ratiosincreased in the solution.

Figure 5. Effects of metal/cyanide ratio on metal and cyanide removal efficiencies.

Conclusions

The results showed that EC system with Fe electrodecould successfully remove Ni

2+

, Cd

+2

and CN

−

ionsfrom the rinse water. Current density and pH wereidentified as major variables that significantly affectedthe removal efficiencies. Optimum operating condi-tions for the removal of cadmium-cyanide and nickel-cyanide electroplating rinse water were determined as30 A/m

2

and, and 60 A/m

2

and 80 min, respectively.Removal efficiencies and operating costs under theoptimum conditions for the EC process were 99.4%and 1.05

€

/m

3

for cadmium, 99.2% and 2.45

€

/m

3

fornickel and better than >99.7% for cyanide from bothelectroplating rinse water, respectively. The mecha-nism responsible for removing pollutants from rinsewater was co-precipitation and adsorption of metalswith ferrous hydroxides in solution. Ferrous hydroxideparticles were produced up to a sufficient concentrationto initiate polymerization. During electrocoagulation,the pollutants were removed by surface complexationas well.

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Cd CN( ) −

4

2Ni CN( )4

2−

Table 3. Effects of metal/cyanide ratio on metal and cyanide removal efficiencies.

Type ofrinse water

R

e

(%)

C

energy

C

electrode

W

sludge

OC

α

Me/CN

CN Cd or Ni (kWh/m

3

) (kg/m

3

) (kg/m

3

) (

€

/m

3

)

Cad

miu

mpl

atin

g

0.40 (48/120) 99.9 99.2 3.32 0.65 1.26 0.7930.70 (84/120) 99,6 99.5 6.36 0.63 1,02 0.9940.85 (102/120) 99.4 99.7 6.13 0.66 1.79 1.0451.43 (172/120) 98.2 91.5 12.30 0.56 0.91 1.3631.63 (196/120) 86.3 82.8 11.86 0.60 0.90 1.3651.83 (220/120) 72.1 65.6 6.18 0.37 0.47 0.761

Nic

kel

plat

ing

0.43 (112/261) 99.9 99.3 10.82 1.10 2.91 1.7150.67 (175/261) 99.8 99.2 11.94 1,85 1,62 2.4450.88 (230/261) 98.6 87.0 12.25 1.56 2.69 2.2111.34 (350/261)) 97.5 62.4 10.47 1.45 1.54 1.9871.60 (418/261) 93.8 48.3 10.59 1.55 1.62 2.082

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