Desalination 227 (2008) 241–252

Transport of zinc complexes through ananion exchange membrane

M.A.S. Rodriguesa,b, F.D.R. Amadob,c, M.R. Bischoffa, C.A. Ferreirac,A.M. Bernardesb, J.Z. Ferreirab

aInstituto de Ciência Exatas e Tecnológicas, Centro Universitário Feevale, Novo Hamburgo/RS, Brazilemail: marcor@feevale.br

bLaboratório de Corrosão, Proteção e Reciclagem de Materiais – LACOR;cLaboratório de Materiais Poliméricos – LAPOL, Universidade Federal do Rio Grande do Sul, UFRGS,

Av. Bento Gonçalves, 9500, Setor 4, Prédio 74, Porto Alegre/RS. Brazil

Received 17 February 2006; Accepted 17 July 2007

Abstract

The transport of zinc complexes ions through an anion exchange membrane was evaluated by the treatment ofsolutions with and without cyanide. The ionic transport was analyzed as a function of the applied current density andof the cyanide and hydroxyl concentration. Experimental results showed that the ionic transfer in electrodialysis wasmainly affected by concentration. There is an ideal molar relationship among the concentration of the cyanide ions,hydroxyl and zinc ions, in solution, in which the zinc transport is maximized. For values above or below this, thetransport decreases. Zinc extraction from solution containing CN! was more effective when the current density of29 mA.cm!2 was applied. The current–voltage curves (CVC) of the anion exchange membrane show that theelectrical resistance of the AMV membrane increases with the presence of the zinc–cyanide complex in the solution.

Keywords: Transport; Zinc; Cyanide; Electrodialysis; Membrane

1. Introduction

The treatment of industrial effluents contain-ing heavy metals represents a major environ-mental issue affecting every industrializedcountry in the world. These toxic effluents are

*Corresponding author.

generated by a number of industrial processessuch as steel treatment, metal refining andchemical production. In most cases, considerablevolumes of acid must be treated.

The traditional treatment method of metalfinishing industrial wastewater is based on the

0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.07.018

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transfer of the liquid phase for the solid phase,involving stages such as reduction/oxidation,precipitation-coprecipitation and filtration. Thistreatment presents some deficiencies, mainlyconcerned to the wastewater treatment thatcontains complexed metallic ions. In this case, themetal cannot be separated by the conventionalprocess. The cyanide is capable of formingcomplexes, even in low concentrations, with themajority of the heavy metals. The investigationsof these complexation reactions are important,because they change fundamental properties ofthe metallic ions in solution such as; charge, sizeand even the solubility of the ions [1].

The most employed method for the waste-water treatment containing cyanide is its destruc-tion by oxidation to cyanate through an alkalinechlorination. However, this treatment can presentsome disadvantages, such as the low efficiency inthe wastewater treatment that contains very stablemetal-cyanide complexes, as gold and mercury.Another great inconvenient of this treatment isthe possibility of the organo-halogenatedformation (AOX) that are highly toxic substances[2]. The solid residue generated in this treatmentis classified as hazardous and therefore it shouldbe stored at an appropriate place, which repre-sents a high cost.

Among the possible alternatives of treatment,membrane technology offers many advantages,which correspond to the general tendencies ofresources conservations and energy managementin the world. Because of their modularity,membrane techniques in general, and electro-membrane techniques in particular, are very welladapted to pollution treatment at its source. Theemployment of electrochemical methods to theindustrial wastewater treatment has beenincreasing in the last years [3]. Among thesemethods, the electrodialysis stands out, becausebesides not forming AOX, it has the greatadvantage of allowing the recovery and thereutilization of the cyanide, the metallic com-

pound and the water used in the industrialprocess.

Electrodialysis (ED) is a membrane separationprocess based on the selective migration ofaqueous ions through ion-exchange membrane asa result of an electrical driving force. The trans-port for each ion depends on its charge, mobility,solution conductivity, relative concentrations,applied voltage, etc. Ion separation is closelyrelated to characteristics of the ion-exchangemembrane, especially its permselectivity.

In the past years, several works have beenpresented referring to the electrodialysis appli-cation to the solutions treatment containingmetallic ions in absence of complexing agents[4–8]. However, few works refer to the electro-dialysis application in the presence of thesecompounds [9–11]. In this ED application, twolimiting effects must be taken into consideration:the competition in the membrane transportbetween the organic (complexing agent) andinorganic ions and on the other hand, the poison-ing of the membrane material by charged com-plexing agent. In a general way the ionic speciesflow present in the solution are the complex ionsformed by the metal and the complexing agent,the free ions from the complexing agent and themetallic ions not complexed [9].

Although the electrodialysis techniquebecomes more and more efficient in the treatmentof different types of industrial wastewater, theinsufficient perfomance of certain membranes incertain applications, limits the viability of usingthis technique [12,13]. In order to design newelectrodialysis membranes for the treatment ofthese effluents or to develop and adapt electro-membrane processes, it is necessary to know thetransport properties of commercial membrane incontact with solutions containing complex ions.Than the knowledge of peculiarities of thetransport through the membrane in complexedions solutions is of great importance for theoptimization of conditions for the electrodialysis

M.A.S. Rodrigues et al. / Desalination 227 (2008) 241–252 243

process. The present work, therefore, deals withthe study of zinc transport through the AMVanion exchange membrane in contact withsolutions containing zinc-complexes.

2. Experimental

Electrodialysis was carried out in a pilotsystem TI-1624 (Tecnoimpianti). The stack usedin this work had three pairs of ion-selectivemembranes, Selemion® CMT and AMV. Themembrane dimension used was 16×24 cm with aneffective area of 1.72 dm2/membrane. The dis-tance between the membranes was 0.75 mm.Platinized-titanium and steel inox 316 were usedas the anode and cathode respectively. Current,potential, conductivity, volume and pH weremonitored during the experiments. The solutionscirculated through pumps with a flow of 50 L/h.At the beginning, the alkaline metal solution(with or without cyanide) was placed in thediluted compartment, and it was used the samesolutions in the concentrated compartment, how-ever, without the metal presence. The sodiumhydroxide solution (0.1 M) was used in the elec-trode compartments. The solution volume used inall the compartments was 1.5 L. The time of theexperiment was 3 h. The membranes were equili-brated in their respective solutions for 24 h.During electrodialysis, samples were collectedfrom the diluted compartment for the determi-nation of ionic species concentrations. Electro-dialysis experiments were evaluated in terms ofpercent extraction and transport number [13]. Theexperiments were carried out in duplicate.

Transport numbers were determined by theequation [14]:

. . .1000i ii

z J Ftj

=

where ti is the transport number of the i species,

Fig. 1. General scheme of the electrodialysis pilot systemmodel TI-1624 –Tecnoimpianti. 1, 2, 3: concentrated,electrode and diluted compartments respectively, 4: stack,5: pump.

Fig. 2. Stack configuration. Scheme of the membranesand separators disposition. MA, anionic membrane;MC, cationic membrane; CC, cathodic compartment;CA, anodic compartment; D, diluted compartment;C, concentrated compartment.

z is the valence of the i species, Ji is the flux ofthe i species (mol/cm2.s), F is the Faraday con-stant and j is the current density (mA/cm2).

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Flux Ji can be calculated by the expression:

( )0

.

Si i

i S

V C CJ

A T−

=

where V is the cathodic compartment volume (L),Ci

s is the concentration of the i species in theperiod Ts (mol/L), Ci

0 is the initial concentrationof the i species (mol/L), A is the apparent area ofthe membrane (cm2) and Ts is the sampling time(s).

Fig. 1 presents a general scheme of the pilotsystem used in ED and Fig. 2 presents the stackconfiguration used in the experiments.

2.1. Current–voltage curves

Current–voltage curves (CVC) were obtainedin galvanostatic mode using a classical two-compartment cell [15]. This cell was composed oftwo symmetrical 200 cm3 half-cells. Thesecompartments were separated by gaskets, whichclamp the membrane. In the geometrical center ofgaskets, there was a cylindrical hole. The work-ing area of the AMV membrane was 9.98 cm2.Two Ag/AgCl electrodes, immersed intoLuggin’s capillaries, allowed the measurement ofthe potential difference between the two sides ofthe membrane. Mechanical stirrers were placed ineach compartment. The solutions used in bothsides of the membrane are the same. The elec-trical current was supplied with two platinumelectrodes (Fig. 3). The electric current wasapplied using a dc power source for 120 s. Thecurves were obtained by potential measurementsthrough the membrane corresponding to theapplied current. A pH electrode was used to mea-sure the pH variations during the experiments.

2.2. Solutions

Table 1 shows the chemical composition in thesolutions:

Fig. 3. Scheme of the cell used in the polarization curves;M: Anionic membrane AMV; anode and cathode of thePt; 1: mechanical stirring; 2: pH electrodes; 3: Ag./AgClelectrodes with the help of Luggin capillaries.

Table 1Chemical composition of the solutions

Composition Solutions

ZnO 0.18 M + NaOH 2.25 MZnO 0.18 M + NaOH 1.75 M

+ NaCN 0.30 MZnO 0.18 M + NaCN 0.61 MZnO 0.18 M + NaCN 0.92 MZnO 0.18 M + NaCN 1.63 MZnO 0.18 M + NaOH 3.50 MNaCN 0.61 MNaCN 0.92 MNaCN 1.63 MNaOH 0.61 MNaOH 0.75 MNaOH 0.92 MNaOH 1.00 MNaOH 1.75 MNaOH 2.25 M

AB

CDEFGHIJLMNOP

All chemicals were reagent grade and wereused without further purification. The zinc ioncomplex solutions were prepared mixing zincoxide with sodium hydroxide with the minimalquantity of deionised water and with intenseagitation until the dissolution of zinc oxide wasobserved [16]. After the dissolution, the volumewas completed with water. The same procedurewas adopted to prepare solutions with zinccomplexes and cyanide. It was added sodium

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cyanide with sodium hydroxide to these solu-tions, which favoured the zinc oxide dissolution.The zinc concentration was determined by com-plexometry and cyanide concentration wasaccomplished by argentimetric methods [17,18].

2.3. Morphology

The surface morphology of the AMV andCMT membranes was studied by scanning elec-tron microscopy (SEM), using a Phillips XL 20electron microscope. A thin layer of gold wassputtered on the sample surface prior to the SEMmeasurements.

3. Results and discussion

The behavior of zinc complexes in presence ofCN! and OH! ions strongly depends on the con-stant formation for different complexes, whichcan be formed between Zn+2, CN! and OH! ions.Using the Hidra Medusa program [19], it is pos-sible to see some compounds of zinc formed insolution C. These forms are presented in Fig. 4.

As it can be observed in Fig. 4, the zinc itselfforms complexes with the cyanide ions as withthe hydroxyls ions, existing equilibrium betweenthe two compounds, according to the equationshown below:

Zn(CN)n!(n!2) + 4 OH! ] Zn(OH)n!(n!2)

+ 4 CN!

Based on this equilibrium between the com-pounds, it can be stated that the increase in thehydroxyls ions concentration moves the balanceto the zinc-hydroxyl formation side, and theincrease in the ions cyanide concentration, movesthe balance to the contrary side, favoring thezinc-cyano formation [20].

Fig. 5a–c presents the zinc complex extractionthat permeate through the AMV anionic mem-brane as a function of different applied currentdensity and different times durations in the A, Band C solutions. It is possible to observe in Fig. 5that the extraction of zinc complex ions throughthe membrane AMV is related to both parameters(time and current density). The results show that

Fig. 4. Zinc complex species present in the investigated solutions.

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(a) (b)

(c)

Fig. 5. Zinc percent extraction as a function of timeand current density applied on experiments using theA, B, C solutions in (a), (b) and (c) respectively.

an increase on the applied current density ionspercent extractions enhances through the mem-brane. Nevertheless, it can be seen in Fig. 5b thatthere is an increase in extraction with currentdensities in the range of 5.2 to 29 mA.cm!2, butthere is a decrease in extraction with the currentdensity of 40.7 mA.cm-2. We relate this pheno-menon to the precipitate of zinc hydroxideformed on the membrane surface that could notbe dissolved and the electric resistance of the

membrane increased. This phenomenon increasesthe membrane electrical resistance and decreaseszinc extraction efficiency.

In order to evaluate the relation between zinctransport through the anionic membrane as afunction of cyanide ions concentration, some testswere made with solutions varying the concen-tration of cyanide. Table 2 presents the molarrelationship [CN]/[Zn] in C, D and E solutions. Itcan be seen that the extraction of cyanide and

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Table 2Cyanide/zinc molar relationship on Solutions C, D and E. Current Density: 20.3 mA.cm!2

Solution [Zn] [CN] Rate [CN]/[Zn] CN! extraction (%) Zn extraction (%)

C 0.18 0.61 3.58 18.4 16.1D 0.18 0.92 5.41 25.7 22.7E 0.18 1.63 9.6 16.2 8.9

zinc decreases in solution E when it is comparedto percent extraction of both ions in D solution.The excess of cyanide free ions, i.e. cyanide ionsthat are not forming zinc complexes, competeswith zinc complexes to the transport of electriccurrent. This causes a decrease on the extractionof zinc complexes through the membrane, sincethe ionic mobility of cyanide ions is higher thanthe mobility of cyanide complexes. By theseexperiments, it is possible to see that there is anoptimal relationship between the extraction ofzinc through the membrane and the cyanideconcentration in the solution.

In order to evaluate the relationship betweenthe zinc extraction through the AMV membraneand the cyanide and hydroxyls concentration inthe solution, some tests were made and the resultsare shown in Fig. 6. It is clear from the figure thatthe maximum zinc extraction occurred in the[CN!]/[OH!] concentration that was of 0.91/0.The minimal zinc extraction was in 0/3.5. Higherconcentration of hydroxyls ions decreased thezinc transport because of diffusion limitations ofthe zinc hydroxide complexes and the highermobility of OH! across the anion exchangemembrane. The concentration effect is apparentlyone of the main parameters which favours thezinc extraction. For an efficient separation of zincfrom solutions, the concentration of hydroxylsions must be lower. On the other hand, an idealmolar relationship exists among the concentrationof the cyanide, hydroxyl and zinc ions in solutionin which the zinc transport is maximized, and forvalues above or below this, the transportdecreases.

Fig. 6. Molar relationship between zinc extraction percentin treated solution and concentration of CN! and OH!

ions. Applied current density: 20.3 mA.cm!2.

Fig. 7 shows a cyanide concentration of0.61 M and electric current density of 20.3mA.cm!2. The transport for cyanide ions was0.97, but this decreased to 0.70 when the cyanideconcentration increased to 0.92 M. A decrease incyanide ions transfer with increasing cyanideconcentration is essentially due to the decrease ofthe membrane selectivity.

The transport numbers of hydroxyls ions arealso reported in comparison with the cyanide. Asshown in the figure, the transport number of cya-nide ions through the membrane AMV is higherthan the transports of hydroxyls ions.

Fig. 8 shows the superficial morphology of theAMV membrane analysed by SEM. Morpho-logical changes on AMV were evaluated whenthis membrane was in contact with solutionscontaining zinc complexes [21].

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Fig. 7. Hydroxyls and cyanide transport as a function ofapplied current density.

Fig. 8. Micrographs of the AMV membrane superficialmorphology new and after used in experiments.

An important change can be seen on the sur-face of the anion-exchange membrane, which

could indicate a modification on the structure.The membrane material lost its plasticity with theuse, turning to a breakable material. This pheno-menon was the decomposition in the concentratedalkali solutions by the Hofmann degradation reac-tion of the quaternary ammonium groups, whichare used as anion exchange groups.

In this work a membrane was defined as a newone when the total immersion time in zinc solu-tions was not longer than 72 h. A membrane wasconsidered used when it was in contact with zincsolutions for a minimal period of 6 months. Thiseffect could not be visualized in the CMT cati-onic membranes.

3.1. Current–voltage curves (CVC)Many papers have focused on the occurrence

of concentration polarization [22–24]. Accordingto the classical theory of concentration polariza-tion for ion-exchange membranes, the current–voltage response shows three regions. The shapeof current–voltage curves can be distinguished. Inthe first region, a linear relationship is obtainedbetween current and voltage drop, and thereforegenerally referred to as the ohmic region. In thesecond region the current varies very slightlywith voltage, denoting an almost unrelated cur-rent applied voltage (plateau), corresponding tothe so-called limiting current. In the over-limitingcurrent region, the current intensity increasesagain with the applied voltage (region III)[25–28].

The CVC of anion exchange membrane AMV,equilibrated with ZnO/NaCN/NaOH solutions isshown in Figs. 9–11. In all solutions, the CVCshapes are similar. For these systems, the CVChave only a linear relation related to region I. It ispossible that this phenomenon is associated thehigher concentration of ions in treated solutions.However does not present a limiting current inthe current range studied.

Fig. 9 presents CVC of the cyanide and zinc-cyanide complex solutions and it can be observed

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Fig. 9. Current–voltage curves for AMV membrane in contact with NaCN, zinc–cyanide complexes and the zinc–cyanide–hydroxide solutions where the representation x/y/z means the values of the ZnO/NaCN/NaOH, molar concentrationsrespectively.

Fig. 10. Current–voltage curves of NaOH solutions and zinc–hydroxide complexes where the representation x/y/z meansthe values of the ZnO/NaCN/NaOH, molar concentrations respectively.

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Fig. 11. Current–voltage curves of NaCN and NaOH solutions where the representation x/y/z means the values of theZnO/NaCN/NaOH, molar concentrations respectively.

that the increase in the cyanide concentrationcauses a decrease in the membrane potential.However, the presence of the zinc–cyanide com-plex in the solution causes a considerable increasein the membrane potential. It is possible that thisphenomenon is associated with the interactionsbetween the zinc ion complex and the membrane,also due to the difference in mobility, anddiffusion coefficient of the existing species inmembrane. Similar results were found by Gavachet al. [29] when the ionic transport of the zinc–chloride complex was investigated.

Fig. 10 presents the CVC of the NaOHsolutions and zinc–hydroxyl complexes and it canbe observed that the increase in the [OH!] con-centration causes a decrease in the membranepotential, due to decrease in the selectivity of themembrane.

Fig. 11 presents CVC of the NaOH and NaCNsolutions without the presence of the zinc ioncomplex. It is clearly observed that the NaCNsolutions present a higher membrane potentialthan the NaOH solutions. It is possible that this

phenomenon is associated to the interactions ofhydrophilic/hydrophobic character that occurwith larger intensity between the membrane andthe cyanide ions than with the hydroxyl ions.

In all experiments, the zinc transport was notobserved through the CMT membrane, whichindicates that all the present zinc in the solutionswas in the complex form of negative charge.Another important observation was that, duringthe experiments, pH variations were not ob-served. This phenomenon is associated with thehigher ionic concentration of the solutionsemployed in this study.

4. Conclusions

The results presented in this paper allowed usto identify and to relate important parameters thatinfluence the zinc ion complex transport throughthe AMV anionic membrane. The results alsoshow that zinc extraction by means of electro-dialysis is linked to the CN! and OH! concen-tration in the solutions. The transport in cyanidric

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media (NaCN) is larger than the transport inalkaline media (NaOH) and in the mixture ofthese two media (NaOH + NaCN). There is anideal molar relationship among the concentrationof the cyanide, hydroxyl and zinc ions, in thesolution. For values above or below this relation-ship the transport decreases. However, from CVCit can be observed that the solutions with NaCNpresent a membrane potential larger than thesolutions with NaOH. The presence of the zincions complex with cyanide increases considerablythe membrane potential when compared to themembrane potentials equilibrated without thiscomplex.

The results show that electrodialysis can be analternative to the recovery of the cyanide ions.Changes on the AMV membrane were observedby SEM. The membrane material lost its plas-ticity with the use, turning into a breakablematerial.

Acknowledgements

The authors wish to thank the CNPq (Con-selho Nacional de Pesquisa e DesenvolvimentoTecnológico) and Capes (Conselho de Aperfei-çoamento de Pessoal do Ensino Superior) forsupporting this work.

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