ion and ionic current sinks for electrodeionization of simulated cadmium plating rinse waters

Ion and Ionic Current Sinks forElectrodeionization ofSimulated CadmiumPlating Rinse WatersKonstantinos Dermentzis,a Achilleas Christoforidis,b Despoina Papadopoulou,a and Anthimos Davidisaa Department of Science, Laboratory of Chemical Technology and Electrochemistry, Technological Education Institute TEI ofKavala, 65404 Agios Loucas, Kavala, Greece; [email protected] (for correspondence)b Department of Petroleum Technology, Laboratory of Environmental Protection, Technological Education Institute TEI ofKavala, Agios Loucas, Kavala, Greece

Published online 15 April 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.10438

In this study, electrostatic shielding zones made ofelectrode graphite powder were constructed and usedas a new type of ionic and electronic current sinks.Because of the local elimination of the applied electricfield, voltage, and current within these zones, ionsare led inside them and accumulate there. The cur-rent sinks were implemented in electrodeionization ofsimulated cadmium plating rinse waters containing50 mg/L of Cd21 ions with simultaneous electrochem-ical regeneration of the ion exchange resin beds.Pure water was obtained with a Cd21 ion concentra-tion of less than 0.1 mg/L at a flow rate of 3.27 31024 L/s diluate stream, a current density of 2 mA/cm2, and a current efficiency of 28% for Cd21 ion re-moval. � 2010 American Institute of Chemical EngineersEnviron Prog, 30: 37–43, 2011Keywords: cadmium removal, membrane-less elec-

trodialysis, Faraday cage, electrostatic shielding,water reuse

INTRODUCTION

Industrial effluents originating mainly from electro-plating industries contain high amounts of heavy-metal ions (Cr, Ni, Cu, Cd, and Zn). These heavy-metal bearing wastewaters are of considerable con-cern because they are nonbiodegradable, highly

toxic, and probably carcinogen. Only 30–40% of allmetals used in plating processes are effectively uti-lized, which is plated on the articles. The rest of themetal contaminates the rinse waters during the plat-ing process when the plated objects are rinsed uponremoval from the plating bath.

Cadmium is considered highly toxic, and itsallowed concentration in water resources is limited toonly 5–10 ppb. In recent years, cadmium has becomea major heavy-metal pollutant of the environmentoriginating from various agriculture, mining, andindustrial activities. Cadmium plating rinse watersmay contain up to 500 ppm Cd that, according toenvironmental regulations worldwide, must be con-trolled to an acceptable level before being dischargedto the environment.

Several treatment processes have been suggestedfor the removal of cadmium from aqueous wastestreams, such as adsorption on activated charcoal [1],biosorption on marine algae [2], ion exchange [3], ca-thodic reduction [4, 5], chemical precipitation [6],electrodialysis [7], and electrodeionization [8–11].

Chemical hydroxide precipitation is the most eco-nomic and the most commonly utilized procedure forthe treatment of cadmium-bearing industrial effluents.However, after this treatment the wastewater streamcan still contain up to 5 ppm cadmium, which is anunacceptable concentration for discharge to the envi-ronment [6]. To remove cadmium down to the ppb� 2010 American Institute of Chemical Engineers

Environmental Progress & Sustainable Energy (Vol.30, No.1) DOI 10.1002/ep April 2011 37

concentration level, the wastewater stream must befurther treated using a second sulfide precipitation asa polishing step or a series of ion exchange columns.The precipitated sludge containing the concentratedCd(OH)2 is an extremely hazardous waste and mustbe disposed of using special facilities at greatexpense to industry.

From the viewpoint of environmental protectionand resource saving, effective recycling and reusingof the heavy-metal wastewater is strongly expected.Closed-recycle system or so-called effluent-free tech-nology should be developed.

Electrodeionization is the removal of ions and ion-izable species from water or organic liquids. It useselectrically active media and an electrical potential tocause ion transport and may be operated batch wise,or continuously.

Continuous processes such as electrodialysis [7]and filled cell electrodialysis or otherwise called con-tinuous electrodeionization [8–11], comprise alternat-ing permselective cation exchange membranes andanion exchange membranes. These membranesunder the influence of the electric field allow onlycations or only anions, respectively, to permeate theirmass and simultaneously retain cations so that diluateand concentrate compartments are created and deion-ization occurs.

Batch processes such as capacitive deionization[12] are collection/discharge processes that rely onthe formation of double-layer supercapacitor at thesolution/electrode interface and need electrodes withlarge specific areas such as nanostructured activatedcarbon aerogels.

This article offers a new alternative way of a mem-brane-less electrodeionization process of water andindustrial effluents by means of electrostatic shieldingzones (ESZs) and ionic current sinks (ICSs).

The proposed new electrodeionization process dif-fers from classical electrodialysis-continuous electro-deionization processes in that it does not use anypermselective ion exchange membranes. Therefore, itdoes not exhibit the membrane associated limitationssuch as concentration polarization and water dissocia-tion. It also differs from classical batch-wise operatedcapacitive deionization in that it is a continuous pro-cess. Diluate and concentrate are received from sepa-rate and unchanged compartments without any re-moval of diluate and concentrate or any down timefor electrode saturation, regeneration, and rinsingsteps.

EXPERIMENTAL PROCEDURES

ElectrodesWe used platinized titanium grids (TiTaN, Titanium

Tantalum Products Limited, India) as end-electrodesin our experiments.

The intermediate electrodes ICSs must be electron-ically and ionically conducting. They are either twoparallel short-circuited platinized titanium grids(Figures 1a–1c), or bipolar packed beds of graphitepowder (Merck, particle size < 50 lm, electrical con-ductivity 2 3 104 S/m2) or electrode graphite powder

Figure 1. (a) Electrolytic shell without ESZ-ICS-ECS.The intermediate electrodes B and C aredisconnected, ions move in field direction throughoutthe cell, (b) the same cell without electron transferreactions at the short-circuited electrodes B and C.Ions accumulate inside the ESZ-ICS-ECS, (c) ionspartially accumulate inside the ESZ-ICS and arepartially discharged at B and C.

38 April 2011 Environmental Progress & Sustainable Energy (Vol.30, No.1) DOI 10.1002/ep

(Figure 2) which is used as anode by the electrolyticproduction of aluminium (Aluminium of Greece, par-ticle size < 1 mm, electrical conductivity 3.3 3 104 S/m). Anode graphite is preferable because of its betterelectrical and electrocatalytic properties.

CellsFigure 1 shows a self-made electrolysis cell with a

suitable electrode arrangement to demonstrate theeffect of electrostatic shielding and Faraday cage andthe formation of ESZ, ICS, and ECS. All platinized tita-nium grid electrodes A, B, C, and D have the dimen-sions 5 cm 3 5 cm 3 0.1 cm and are placed in paral-lel. The distance between two successive electrodesis 10 mm. The electrodes are housed in a plastic elec-trolytic cell, and the area of each electrode is equalto the vertical cross section of the electrolytic cell. A0.001 M of CdSO4 solution with a conductivity of 252lS/cm was used as electrolyte.

Figure 2 shows the self-made electrodeionizationcell consisted of seven separate, in parallel placedcompartments: anode compartment, left ICS, cationexchange resin (CR) loaded compartment, centralconcentrate compartment (ICS), anion exchange resin(AR) loaded compartment, right ICS, and cathodecompartment. The compartments are separated fromeach other by the six ion conducting porous polypro-pylene separators S (Celgard 3704) and are pressedtogether to form an assembly using spacers andscrews. Each compartment is 8 cm in length and 8cm in width. The area of each ICS is equal to the ver-tical cross section of the electrolytic cell with aneffective area of 48 cm2. The central ICS and the twoouter ICSs have a thickness of 5 and 10 mm, respec-tively. The rest of the four compartments are 10 mmthick each.

The CR and AR resins were pretreated andimmersed in a 0.1-M of CdSO4 and 0.1-M of Na2SO4

solutions, respectively, for 3 days, then packed intothe corresponding CR-loaded and AR-loaded com-

partment and rinsed with deionized water. The anodeand the cathode compartments were filled with 0.05M H2SO4 1 0.05 M Na2SO4 and 0.05 M NaOH 10.05M Na2SO4 solutions, respectively.

The graphite powder was activated by treating itin a 1-M of NaOH solution at 908C for 3 h. To com-plete physical absorption of CdSO4 in graphite, theICSs were rinsed with a 0.1 M of CdSO4 solution untilsaturation and then with deionized water.

The separators S can be omitted in case of porousconductive sheets, such as conductive ceramics, po-rous carbon paper, or carbon aerogel sheets, andother porous conducting composite electrodes areused. Such porous conducting sheets can functionboth as intermediate electrodes and separators aswell.

ApparatusAtomic absorption spectroscopy (AAS, Perkin

Elmer 5100 PC) was used to determine the Cd21 ionconcentrations in water. Power supply apparatuses(STELL TRAFO/POWER SUPPLY, PHYWE SystemeGmbH, KG, Germany) were used to maintain con-stant DC voltage or constant DC current. Voltage andcurrent were measured by a multimeter (PHYWE Sys-teme GmbH, KG, Germany). Conductivities weremeasured by means of a conductometer (inoLabCond Level 1, WTW). The temperature was held at298 6 3 K during all experiments.

ChemicalsCdSO4�8/3H2O, H2SO4, NaOH, and Na2SO4 were

of analytical grade (Merck). The ion exchange resinsused were strongly cation exchanger H1-form(Amberlite IR-120, Merck,) with an ion exchangecapacity of 1.7 mmol/mL and strongly basic anionexchanger OH2-form (Merck) with an ion exchangecapacity of 1.45 mmol/mL.

RESULTS AND DISCUSSION

Electrostatically Shielded Ion ConcentratingCompartments—Faraday cages—ESZs,ICSs, and ECSs

It is known from electric field theory and Faradaycage [13] and previous works [14–16] that when aconductor is placed inside an electric field, an oppo-site field is formed so that the original electric field inthe interior of the conductor is canceled. The field in-tensity inside the conductor is zero, and its wholespace is electrostatically shielded independent on theexternal field intensity. Therefore, an abrupt potentialjump is formed between the inside and the outside ofthe conductor. Since in an electrolytic or an electro-dialytic cell, ion migration is caused by the appliedelectric field, it will stop within an electronically andionically conducting ICS of zero field, interposedbetween the anode and cathode.

Current sinks and sources are local currents from alocation where they can be detected into a locationthey cannot be detected (current sink) or vice versa(current source). Current sinks and sources have par-

Figure 2. Electrodeionization cell for removal of Cd21

ions with simultaneous regeneration of the separatedion exchange resin beds CR and AR. Feed waterpermeates first the CR and then the AR resin to bedeionized. The accumulated CdSO4 concentrate isreceived from the bottom of the central ionconcentrating compartment ICS.


ticular relevance in current across biological mem-branes (neurobiology) and have proved to be valua-ble in the study of brain function [17]. Furthermore,current sinks are used in several electronic applica-tions.

However, all known current sinks are related toelectronic current sinks (ECSs). As far as we areaware, no other article has appeared in literaturedealing with cadmium removal by means of ICSs-based electrodeionization. The ICSs were createdthrough electrostatic shielding using electronicallyand ionically conducting media, e.g. graphite powder,and were used as ion concentrating compartments inform of ESZs.

The formation of electrostatically shielded Faradaycages, ESZs, ICSs, and ECSs, can be demonstratedwith the setup shown in Figures 1a–1c. Measurementsof voltages and currents inside and outside the ICSs,i.e. with and without shielding, are given in Table 1.As it is obvious, when a voltage is applied betweenthe cathode A and the anode D, a homogeneouselectric field with a constant field intensity developsthroughout the cell. Potential difference, current flow,and field intensity are measured between the electro-des A-D, B-C, A-B, and C-D (Figure 1a). If, by meansof a conducting wire and a switch (G), the electrodesB and C are electrically connected to each other (Fig-ures 1b and 1c), the whole space determined bythem is electrically shielded. It constitutes an ESZ andthe former potential, current and field intensity valuesalmost disappear insight it [14]. These values canreappear again only if the electrodes B and C areelectrically disconnected (Figure 1a). The ESZbecomes a kind of an electrochemical Faraday cage,which is electrically ‘‘shielded’’ though current flowsin the electrolytic cell.

Because of the homogeneous electric field (Figure1a), both Cd21 cations and SO4

22 anions permeate theporous or perforated intermediate electrodes B and Cand migrate in field direction throughout the cell.

As can be seen in Figures 1b and 1c, the electricfield is inhomogeneous. Ions migrate only in the left(anodic) and the right (cathodic) cell compartment.Cd21 cations and SO4

22 anions enter the middle com-partment ESZ, which is determined by the short-circuited electrodes B and C from its cathodically(left) and anodically (right) polarized side, respec-tively. Because of the local absence of the electricfield (driving force), their electromigration is stoppedthere. These ions cannot leave this compartment andaccumulate inside it. The ESZ becomes an ion con-centrating compartment, while the adjacent compart-ments become ion diluting compartments. The ESZacts here as a ‘‘sink’’ for ions and ionic currents (ICS).The ionic current (real direction) is eliminated at thecathodically polarized side of the ICS (current sink)and appears again at its anodically polarized side(current source). Cd21 and SO4

22 ions can moveinside the ICS but only because of diffusion and con-vection and not in field direction.

Figure 1b differs from Figure 1c in the value ofpotential difference between the electrodes B and Cbefore their short-circuiting, that is about 2 V. Thiscritical potential value is higher than the Nerst poten-tial for the electrochemical breakdown of water (1.23V) or of CdSO4 (1.63 V). However, intrinsic resistanceof the electrodes permits an overvoltage and doesnot generate hydrogen and oxygen bubbles due towater electrolysis at this voltage [18]. Neither cad-mium electrodeposition is observed. When the poten-tial difference between B and C exceeds the value of2 V, ions not only enter the middle ICS compartmentbut simultaneously they can also be discharged at theelectrodes B and C through electron transfer reac-tions. In this case, a Faraday current If between Band C due to electrolysis is observed (Figure 1b, Ta-ble 1). The thicker the ICS, the higher the potentialdifference and therefore the more ions are dischargedat its two polarized sides B and C. At lower potentialdifference ions cannot be discharged at the electrodes

Table 1. Voltage and current measurements.

EA-D (V) EB-C* (V) EB-C** (V) EA-B* (V) EA-B** (V) EC-D* (V) EC-D** (V) Is* (mA) Is** (mA) If (mA)

2 0.10 0 1.17 1.22 0.73 0.87 0.6 0.6 0.03 0.34 0 1.56 1.72 1.10 1.30 2.2 2.2 0.04 0.60 0 1.93 2.23 1.47 1.77 4.1 4.1 0.05 0.92 0 2.25 2.73 1.83 2.27 6.1 6.4 0.16 1.26 0 2.56 3.21 2.18 2.78 8.8 9.1 0.57 1.62 0 2,84 3.65 2.54 3.36 11.7 12.4 0.78 1.93 0 3.18 4.05 2.89 3.93 14.8 16.0 1.99 2.35 0 3.39 4.46 3.26 4.53 18.9 21.1 3.410 2.80 0 3.62 4.85 3.58 5.11 23.0 25.8 5.911 3.19 0 3.86 5.32 3.95 5.67 27.2 32.1 9.912 3.55 0 4.10 5.75 4.35 6.17 31.8 38.8 13.313 3.93 0 4.34 6.23 4.73 6.64 37.2 47.0 20.114 4.34 0 4.58 6.67 5.08 7.14 42.1 53.6 24.115 4.69 0 4.84 7.22 5.47 7.55 47.4 59.4 28.0

*Without electrostatic shielding.**With electrostatic shielding.


B and C. No electrolysis and no Faraday current isdetected between them (If 5 0, Figure 1c). H1 orCd21 and OH2 ions are discharged at the cathodeand the anode, respectively. The anions (OH2 orSO4

22) left over in the cathodic compartment and thecations (H1 or Cd21) left over in the anodic compart-ment permeate B and C, respectively, and accumulateinside the ICS, which is determined by them (Figure1b). In this way electroneutrality is maintained in allcell compartments.

The ion concentrating compartment ICS is never sat-urated with the accumulated ions. Because of the elec-tric field, its two connected sides become polarizedand saturated with oppositely charged ions but at thesame time they are discharged and release the sameions because of shorting. As illustrated in Figure 1c, theionic current disappears inside the ESZ-ICS and is par-tially converted to electronic current If. In Figure 1b,both ionic and electronic current disappear completelyinside the ESZ-ICS-ECS, though the circuit is closedand current flows through the cell (If 5 0, Is > 0).

It can be obtained from Table 1 that the ICS (elec-tronic conductor) in practice does not need anypotential to carry the current through its mass. Byshort-circuiting the electrodes B and C of the ICS theinitial potential difference between them becomeszero, while the potential between the electrodes A-Band C-D and the current Is flowing inside the cellincrease (Table 1, Figures 1a–1c). The relatively toelectrolytic solutions high electronically conductingICS does not bring about any additional potentialdrop in the cell. On the contrary, it helps the currentIs that flows inside the cell to increase by lowering thetotal resistance of the cell. Its initial potential differencebetween B-C is balanced between A-B and C-D.

We can take advantage of these useful new find-ings and drive ions inside the ICSs, create ionconcentrating and ion depleting compartments andperform electrodeionization of water and industrialeffluents without using membranes.

Conventional membrane electrodeionization proc-esses exhibit the known limitations associated withmembranes, such as membrane fouling, scaling, andconcentration polarization. Furthermore, these proc-esses cannot avoid the precipitation of bivalent metalhydroxide occurring at the resins interface and theanion exchange membranes [9–11]. Improvementswith new configuration of the electrodeionizationmembrane stack to overcome the undesirable hy-droxide precipitation have been reported by Fenget al. [8]. Ion exchange membranes are troublesomeand expensive. For these reasons, the application ofelectrodeionization has been limited in wastewatertreatment. Eliminating use of membranes may help indrastically reducing the production and operationcost of the electrodeionization process.

Regeneration of Ion Exchange Resins—Electrodeionization of a CdSO4 Solution

High-purity deionized water can be produced withthe filled cell electrodeionization device (Figure 2), iffeed water permeates first through the CR and then

the AR resin, while a CdSO4 concentrate is formedinside the central ICS.

The intermediate bipolar packed bed electrodesserving as ICSs are not directly connected to the ter-minal electrodes but only indirectly through the elec-tric field. Their left sides facing the anode act as cath-odes while their right sides facing the cathode act asanodes. As pointed out in the preceding section, suchintermediate bipolar electrodes can function as ESZs,ICSs, and ECSs. The central bipolar bed is thinner sothat Cd21 and SO4

22 ions mostly accumulate inside it(ESZ, ICS, and ECS) without being discharged at itspolarized sides. The two outer beds are thicker,where partially ion accumulation and partially iondischarge can occur (ESZ and ICS).

A synthetic CdSO4 solution containing 50 mg/L ofCd 21 was used as feed water, and pure water with aCd21 ion concentration of less than 0.1 mg/L wasobtained in both modes, batch or continuous. Theresins are simultaneously regenerated without the useof any chemical reagents. The product water qualitystays constant over time, whereas in regenerable ionexchange with chemicals it degrades as the resinsapproach exhaustion.

The generated H1 ions in the anode compartmentand the anodically polarized side of the left ICS repeland replace the Cd21 cations from the CR resin, andthe generated OH2 ions in the cathode and the cath-odically polarized side of the right ICS repel andreplace the SO4

22 anions from the AR resin. Thereplaced cations and anions together with the electro-osmotically transported water molecules are trans-ferred to the central concentrate compartment (ICS).Because of electrostatic shielding, the electromigra-tion of ions is stopped and salt accumulation occursinside it. The formed CdSO4 concentrate is let todrain from its bottom and is collected in a containerplaced underneath, in which about 250 mL of deion-ized water were placed. Rinsing of the central ICSoccurs intermittently with the collected CdSO4 con-centrate solution itself.

Once 1 h, the collected CdSO4 concentrate was an-alyzed for its Cd21 ion concentration and pH deter-

Figure 3. Cd21 ion concentration variation with timein the central ion concentrating compartment ICS.


mination. The obtained experimental results concern-ing the Cd21 ion concentration and pH variation withtime are depicted in Figures 3 and 4, respectively.

The right and left ICSs do not at all contribute toelectrodeionization but are essential. We have foundthat electrostatic shielding and therefore the captureof the incoming ions inside the central ICS are not soeffective if the two outer ICSs are missing. Perhapsthe accumulated ions inside the sole ICS are strongerattracted by the adjacent anode and cathode, so theyare not sufficiently held inside it and migrate to theadjacent anodic and cathodic compartments respec-tively.

Precipitation of insoluble hydroxides (scaling)such as Cd(OH)2 inside the central ICS was notobserved because this ICS compartment becomesgradually acidic. Part of the generated H1 ions at theanodically polarized side of the left ICS migratethrough the CR resin bed into the central concentratecompartment (ICS). Similarly, part of the OH2 ionsgenerated at the cathodically polarized side of theright ICS migrate through the AR resin bed and resultin the same central concentrate compartment. It iswell known that the migration rate of the H1 ions isgreater than that of the OH2 ions. Therefore, pHinside the central ICS drops with time (Figure 4).

At higher current densities the removal of Cd21

ions and therefore the current efficiency can be fur-ther increased. However, this additional increase isattributed to electrodeposition of the pure metal atthe cathodically polarized side of the central ICSbecause of exceeding the electrochemical decomposi-tion potential of CdSO4 between the two polarizedsides of the ICS (Figure 1c).

The electrodeionization performance was eval-uated in terms of percent removal (pr), current effi-ciency (ce), and enrichment factor (ef), which werecalculated from the Eqs. 1–3, respectively:

pr ¼ Co � C

Co3 100 (1)

ce ¼ z 3 F 3 Q 3Co � C

I3 100 (2)

ef ¼ Cc

Co(3)

where Co and C are the inlet and outlet concentra-tions of Cd21 ions in the diluate compartments andCc in the concentrate compartment, respectively(moles/L), z is the ion charge, F is the Faraday con-stant (96,453 A/s), Q is the flow rate (L/s), and I isthe current (A).

In the continuous mode with a flow rate of 3.27 31024 L/s diluate stream and a current density of 2mA/cm2, the current efficiency for Cd21 ion removalis 28%.

The concentration of Cd21 ions in the central con-centrate compartment could be enriched from 50 to912 mg/L achieving the enrichment factor of 18.24.

CONCLUSIONS

The proposed new electrodeionization cell withelectrostatically shielded zones-ionic current sinksinstead of permeselective ion exchange membranescan find several electrochemical-technological appli-cations of antipollution. The proposed process seemspromising and advantageous over the existing classi-cal membrane processes as regards to concentrationpolarization and membrane fouling. It needs furtherinvestigation to be developed into an innovative andcompetitive electrochemical process for water electro-deionization and heavy-metal removal.

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Figure 4. pH variation with time in the central ionconcentrating compartment ICS.


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ion and ionic current sinks for electrodeionization of simulated cadmium plating rinse waters

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