6660293 electrokinetic soil remediation review

.The Science of the Total Environment 289 2002 97121

Electrokinetic soil remediation critical overview

Jurate Virkutytea,, Mika Sillanpaaa, Petri LatostenmaabaUniersity of Oulu, Water Resources and Enironmental Engineering Laboratory, Tutkijantie 1 F 2, 90570 Oulu, Finland

b Finnish Chemicals Oy, P.O. Box 7, FIN-32741 Aetsa, Finland

Received 28 May 2001; accepted 31 August 2001

Abstract

In recent years, there has been increasing interest in finding new and innovative solutions for the efficient removalof contaminants from soils to solve groundwater, as well as soil, pollution. The objective of this review is to examineseveral alternative soil-remediating technologies, with respect to heavy metal remediation, pointing out theirstrengths and drawbacks and placing an emphasis on electrokinetic soil remediation technology. In addition, thereview presents detailed theoretical aspects, design and operational considerations of electrokinetic soil-remediationvariables, which are most important in efficient process application, as well as the advantages over other technologiesand obstacles to overcome. The review discusses possibilities of removing selected heavy metal contaminants fromclay and sandy soils, both saturated and unsaturated. It also gives selected efficiency rates for heavy metal removal,the dependence of these rates on soil variables, and operational conditions, as well as a costbenefit analysis. Finally,several emerging in situ electrokinetic soil remediation technologies, such as LasagnaTM, Elektro-KleanTM, elec-trobioremediation, etc., are reviewed, and their advantages, disadvantages and possibilities in full-scale commercialapplications are examined. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Electrokinetic soil remediation; Heavy metals

1. Introduction

Every year, millions of tonnes of hazardouswaste are generated in the world. Due to ineffi-cient waste handling techniques and hazardouswaste leakage in the past, thousands of sites werecontaminated by heavy metals, organic com-

Corresponding author.

pounds and other hazardous materials, whichmade an enormous impact on the quality ofgroundwater, soil and associated ecosystems. Dur-ing the past decades, several new and innovativesolutions for efficient contaminant removal fromsoils have been investigated and it is stronglybelieved that they will help to solve groundwaterand soil pollution. Despite numerous promisinglaboratory experiments, there are not many suc-cessfully implemented in situ soil-treatment tech-

0048-969702$ - see front matter 2002 Elsevier Science B.V. All rights reserved. .PII: S 0 0 4 8 - 9 6 9 7 0 1 0 1 0 2 7 - 0

( )J. Virkutyte et al.The Science of the Total Enironment 289 2002 9712198

niques yet. Because of uncertainty, lack of ap-propriate methodology and proven results, manyin situ projects are currently under way. It islikely that there will not be a single universal insitu soil-treatment technology. Instead, quite alarge variety of technologies and their combina-tions suitable for different soil remediation situa-tions will be developed and implemented.

Although the successful and environmentallyfriendly soil treatment technologies have not beencompletely investigated and implemented, thereare several techniques which have attracted in-creased interest among scientists and industryofficials. These are:

Bioremediation despite a demonstratedability to remove halogenated and non-halogenated volatiles and semi-volatiles, aswell as pesticides, this technique has failed toshow efficient results in removing heavy met-als from contaminated soils.

Thermal desorption this treats halogenatedand non-halogenated volatiles and semi-vola-tiles, as well as fuel hydrocarbons and pesti-cides. It has failed to demonstrate an ability toremove heavy metals from contaminated soils.

Soil vapour extraction there are severalpromising results in reducing the volume oftreated heavy metals. Nevertheless, this tech-nique cannot reduce their toxicity.

Soil washing this technique has demon-strated potential effectiveness in treatingheavy metals in the soil matrix.

Soil flushing according to laboratory-scaleexperiments, this is efficient in removing heavymetals from soils, despite the fact that it can-not reduce their toxicity.

Electrokinetic soil remediation.

As none of the other in situ soil remediationtechniques has demonstrated the efficient re-moval of heavy metals, there was a necessity todevelop other methods to remediate soil contami-nated by heavy metals.

Electrokinetic soil remediation is an emergingtechnology that has attracted increased interestamong scientists and governmental officials in the

last decade, due to several promising laboratoryand pilot-scale studies and experiments. Thismethod aims to remove heavy metal contami-nants from low permeability contaminated soilsunder the influence of an applied direct current.However, regardless of promising results, thismethod has its own drawbacks. First of all, thewhole electrokinetic remediation process is highlydependant on acidic conditions during the appli-cation, which favours the release of the heavymetal contaminants into the solution phase. How-ever, achieving these acidic conditions might bedifficult when the soil buffering capacity is high.In addition, acidification of soils may not be anenvironmentally acceptable method. Second, theremediation process is a very time-consuming ap-plication; the overall application time may varyfrom several days to even a few years. There aresome other limitations of the proposed techniquethat need to be overcome: i.e. the solubility of thecontaminant and its desorption from the soil ma-trix; low target ion concentration and high non-target ion concentration; requirement of a con-ducting pore fluid to mobilise contaminants; andheterogeneity or anomalies found at sites, such aslarge quantities of iron or iron oxides, large rocks

.or gravel, etc. Sogorka et al., 1998 .According to the experiments and pilot-scale

studies conducted, metals such as lead, chromium,cadmium, copper, uranium, mercury and zinc, aswell as polychlorinated biphenyls, phenols,chlorophenols, toluene, trichlorethane and aceticacid, are suitable for electrokinetic remediationand recovery.

2. Theoretical, design and operationalconsiderations

2.1. Theoretical aspects

The first electrokinetic phenomenon wasobserved at the beginning of the 19th Century,when Reuss applied a direct current to a

claywater mixture Acar and Alshawabkeh,.1993 . However, Helmholtz and Smoluchowski

were the first scientists to propose a theory deal-ing with the electroosmotic velocity of a fluid and

( )J. Virkutyte et al.The Science of the Total Enironment 289 2002 97121 99

the zeta potential under an imposed electric gra- . .dient Acar and Alshawabkeh, 1993 . Sibel

Pamukcu and her research group have derivedthe following HelmholtzSmoluchowski equation:

.u 1EO x

where u is the electroosmotic velocity, is theEOdielectric constant of the pore fluid, is theviscosity of the fluid and x is the electric

.gradient Pamukcu and Wittle, 1992 .When DC electric fields are applied to con-

taminated soil via electrodes placed into theground, migration of charged ions occurs. Positiveions are attracted to the negatively charged cath-ode, and negative ions move to the positivelycharged anode. It has been experimentally provedthat non-ionic species are transported along withthe electroosmosis-induced water flow. The direc-tion and quantity of contaminant movement isinfluenced by the contaminant concentration, soiltype and structure, and the mobility of contami-nant ions, as well as the interfacial chemistry andthe conductivity of the soil pore water. Electroki-netic remediation is possible in both saturatedand unsaturated soils.

Electrokinetic soil treatment relies on severalinteracting mechanisms, including advection,which is generated by electroosmotic flow andexternally applied hydraulic gradients, diffusionof the acid front to the cathode, and the migra-tion of cations and anions towards the respective

.electrode Zelina and Rusling, 1999 . The domi-nant and most important electron transfer reac-tions that occur at electrodes during the elec-trokinetic process is the electrolysis of water:

. H O2H 12 O g 2e2 2

. .2H O2e 2OH H g 22 2

The acid front is carried towards the cathodeby electrical migration, diffusion and advection.The hydrogen ions produced decrease the pHnear the anode. At the same time, an increase inthe hydroxide ion concentration causes an in-crease in the pH near the cathode. In order to

solubilise the metal hydroxides and carbonatesformed, or different species adsorbed onto soilsparticles, as well as protonate organic functionalgroups, there is a necessity to introduce acid intothe soil. However, this acid addition has somemajor drawbacks, which greatly influence the ef-ficiency of the treatment process. The addition ofacid leads to heavy acidification of the contami-nated soil, and there is no well-established methodfor determining the time required for the systemto regain equilibrium.

The main goal of electrokinetic remediation isto effect the migration of subsurface contami-nants in an imposed electric field via electro-osmosis, electromigration and electrophoresis.These three phenomena can be summarised asfollows:

Electroosmosis is the movement of soil mois-ture or groundwater from the anode to thecathode of an electrolytic cell.

Electromigration is the transport of ions andion complexes to the electrode of oppositecharge.

Electrophoresis is the transport of chargedparticles or colloids under the influence of anelectric field; contaminants bound to mobileparticulate matter can be transported in thismanner.

The phenomena occur when the soil is chargedwith low-voltage direct current. The process mightbe enhanced through the use of surfactants orreagents to increase the contaminant removalrates at the electrodes. Upon their migration tothe electrodes, the contaminants may be removedby electroplating, precipitationco-precipitation,pumping near the electrode, or complexing withion exchange resins.

Electromigration takes place when highly solu-ble ionised inorganic species, including metalcations, chlorides, nitrates and phosphates, arepresent in moist soil environments. Electrokineticremediation of soils is a unique method, becauseit can remediate even low-permeability soils.

Other mechanisms that greatly affect the elec-trochemical remediation process are electroosmo-sis, coupled with sorption, precipitation and disso-


.lution reactions van Cauwenberghe, 1997 . Thisis the reason why all the appropriate processesshould be taken into consideration and investi-gated before implementation of the techniquecan take place.

Once the remediation process is over, extrac-tion and removal of heavy metal contaminantsare accomplished by electroplating at the elec-trode, precipitation or co-precipitation at theelectrode, pumping water near the electrode, orcomplexing with ion exchange resins. Adsorptiononto the electrode may also be feasible, as someionic species will change their valency near the

.electrode depending on the soil pH , makingthem more likely to adsorb van Cauwenberghe,

.1997 .Prediction of THE decontamination time is of

great importance in order to estimate possiblepower consumption and to avoid the occurrenceof reverse electroosmotic flow, i.e. from the cath-

ode to the anode, during the process Baraud et.al., 1997, 1998 . The phenomenon of reverse elec-

troosmotic flow is not well understood and shouldbe further investigated.

Decontamination velocity depends on two .parameters Baraud et al., 1997, 1998 :

Contaminant concentration in the soil solu-tion, which is related to the various possible

solidliquid interactions adsorptiondesorp-tion, complexation, precipitation, dissolution,

.etc. and to the speciation of the target species. Velocity in the pore solution when species are

in the soil solution and not engaged in anyreactions or interactions. The velocity depends

on different driving forces electric potentialgradient, hydraulic head differences and con-

.centration gradient and is not closely relatedto soil properties, except for the electroosmo-sis phenomenon.

The success of electrochemical remediation de-pends on the specific conditions encountered inthe field, including the types and amount of con-taminant present, soil type, pH and organic con-

.tent Acar and Alshawabkeh, 1993 .For in situ conditions, the contaminated site

itself and the immersed electrodes form a type of

electrolytic cell. Usually, the electrokinetic celldesign in laboratory experiments consists of anopen-flow arrangement at the electrodes, whichpermits injection of the processing fluid into theporous medium, with later removal of the con-

taminated fluid Sogorka et al., 1998; Reddy andChinthamreddy, 1999; Reddy et al., 1997, 1999;

.Zelina and Rusling, 1999 .It seems that there is a controversy as to where

electrodes should be placed to obtain the mostreliable and efficient results. It is obvious thatimposition of an electrical gradient by havinginert electrodes results in electroosmotic flow tothe cathode. Many authors propose that position-ing of the electrodes directly into the wet soil

mass produces the most desirable effect Sims,1990; Acar and Alshawabkeh, 1993; Reddy et al.,

.1999; Sogorka et al., 1998 . Through seeking im-provements in experiments, some researchers tendto place the electrodes not directly into the wetsoil mass, but into an electrolyte solution, at-tached to the contaminated soil, or else to use

different membranes and other materials vanCauwenberghe, 1997; Baraud et al., 1998; Bena-

.zon, 1999 . In order to maintain appropriateprocess conditions, a cleaning agent or clean wa-ter may be injected continuously at the anode.Thus, contaminated water can be removed at thecathode. Contaminants at the cathode may beremoved by electrodeposition, precipitation or ionexchange.

Electrodes that are inert to anodic dissolutionshould be used during the remediation process.The most suitable electrodes used for researchpurposes include graphite, platinum, gold and sil-ver. However, for pilot studies, it is more ap-propriate to use much cheaper, although reliable,titanium, stainless steel, or even plastic elec-trodes. Using inert electrodes, the electrode reac-tions will produce H ions and oxygen gas at theanode and OH ions and hydrogen gas at thecathode, which means that if pH is not controlled,an acid front will be propagated into the soilpores from the anode and a base front will moveout from the cathode.

It has been proved by experiments that whenheavy metals enter into basic conditions, theyadsorb to soil particles or precipitate as hydrox-


ides, oxyhydroxides, etc., and in acidic conditions,those ions desorb, solubilise and migrate.

Another important parameter in the electroki-netic soil-remediation technique is the conductiv-ity, since this, together with soil and pore fluid,affects the electroosmotic flow rate.

The conductivity of soil depends on the concen-tration and the mobility of the ions present, i.e.contaminant removal efficiencies decrease with a

reduction in contaminant concentration Reddyet al., 1997, 1999; Reddy and Chinthamreddy,

.1999; Zelina and Rusling, 1999 . This is due tohydrogen ion exchange with cationic contami-nants on the soil surface, with release of thecontaminants. As the contaminant is removed,the hydrogen ion concentration in the pore fluidincreases, resulting in an increasing fraction ofthe current being carried by the hydrogen ionsrather than by the cationic contaminants.

It is possible to conclude that the variableswhich have impact on the efficiency of removingcontaminants from soils are:

Chemical processes at the electrodes; Water content of the soil; Soil type and structure; Saturation of the soil; pH and pH gradients;

Type and concentration of chemicals in thesoil;

Applied current density; and Sample conditioning.

In addition, insoluble organics, such as heavyhydrocarbons, are essentially not ionised, and thesoils in contact with them are not charged. Theremoval of insoluble organics by electric field islimited to their movement out of the soil byelectroosmotic purging of the liquid, either withwater and surfactant to solubilise the compounds,or by pushing the compounds ahead of a water

.front Probstein and Hicks, 1993 .Ionic migration is the movement of ions sub-

jected to an applied DC electric field. Electromi-gration rates in the subsurface depend upon van

.Cauwenberghe, 1997 :

Soil porewater current density; Grain size; Ionic mobility; Contaminant concentration; and Total ionic concentration.

The process efficiency is not as dependent onthe fluid permeability of soil as it is on the pore-water electrical conductivity and path length

.Fig. 1. Electroosmosis and electromigration of ions adapted from Acar et al., 1994, 1996; Acar and Alshawabkeh, 1996 .


through the soil, both of which are a function ofthe soil moisture content. As electromigrationdoes not depend on the pore size, it is equally

applicable to coarse and fine-grained soils van.Cauwenberghe, 1997 .

Electroosmosis in water-saturated soil is themovement of water relative to the soil under theinfluence of an imposed electric gradient. Whenthere is direct current applied across the porousmedia filled with liquid, the liquid moves relativeto the stationary charged solid surface. When thesurface is negatively charged, liquid flows to the

.cathode. Acar et al. 1994, 1996 have conductednumerous experiments and found that this process

.works well in wet i.e. water-saturated fine-grained soils and can be used to remove solublepollutants, even if they are not ionic. The dis-solved neutral molecules simply go with the flow.Fig. 1 shows a schematic representation of thisprocess.

An excess negative surface charge exists in allkinds of soil. For example, many clays are anionic,colloidal poly-electrolytes. The surface chargedensity increases in the following order: sandsilt kaolinite illitemontmorillonite. Injec-tion of clean fluid, or simply clean water, at theanode can improve the efficiency of pollutantremoval. For example, such a flushing techniqueusing electroosmosis has been developed for theremoval of benzene, toluene, trichlorethane andm-xylene from saturated clay.

According to that stated above, the main fac-tors affecting the electroosmotic transport of con-taminants in the soil system are as follows:

Mobility and hydration of the ions and chargedparticles within the soil moisture;

Ion concentration; Dielectric constant, depending on the amount

of organic and inorganic particles in the poresolution; and

Temperature.

Most soil particle surfaces are negativelycharged as a result of isomorphous substitution

and the presence of broken bonds Yeung et al.,.1997 .

Experiments have determined the dependenceof the zeta potential of most charged particles onsolution pH, ionic strength, types of ionic species,

temperature and type of clay minerals Vane and.Zang, 1997 . For water-saturated silts and clays,

the zeta potential is typically negative, with valuesmeasured in the 10100-mV range.

However, if ions produced in the electrolysis ofwater are not removed or neutralised, they lowerthe pH at the anode and increase it at the cath-ode, accompanied by the propagation of an acidfront into the soil pores from the anode and abase front from the cathode. This process can

significantly effect the soil zeta potential drop in.zeta potential , as well as the solubility, ionic state

and charge, level of adsorption of the contami- .nant, etc. Yeung et al., 1997 .

In addition, different initial metal concentra-tions and sorption capacity of the soil may pro-duce soil surfaces that are less negative, which atthe same time may become positive at a pH ofapproximately the original zero-point charge .Yeung et al., 1997 . Similarly, chemisorption ofanions makes the surface more negative.

Electroosmotic flow from the anode to thecathode promotes the development of a low-pHenvironment in the soil. This low-pH environmentinhibits most metallic contaminants from beingsorbed onto soil particle surfaces and favours theformation of soluble compounds. Thus, electro-osmotic flow from the anode to cathode, resultingfrom the existence of a negative zeta potential,enables the removal of heavy metal contaminantsby the electrokinetic remediation process.

The pH of the soil should be maintained lowenough to keep all contaminants in the dissolvedphase. Nevertheless, when the pH becomes toolow, the polarity of the zeta potential changes and

reversed electroosmotic flow i.e. from the cath-.ode to the anode may occur. In order to achieve

efficient results in removing contaminants fromsoils, it is necessary to maintain a pH low enoughpH to keep metal contaminants in the dissolvedphase and high enough to maintain a negative

.zeta potential Yeung et al., 1997 . Despite thisapparently easily implemented theory, simultane-ous maintenance of a negative zeta potential and


dissolved metal contaminants remains the great-est obstacle in the successful implementation ofthe electrokinetic soil remediation process.

2.2. Design considerations

In order to obtain efficient and reliable results,electrokinetic remediation of soil should be im-plemented under steady-state conditions. It isobvious that during the remediation process, otherreactions, such as transport and sorption, andprecipitation and dissolution reactions, occur andaffect the remediation process.

There have been numerous indications of theimportance of heat and gas generation at elec-trodes, the sorption of contaminants onto soilparticle surfaces and the precipitation of contami-nants in the electrokinetic remediation processAcar and Alshawabkeh, 1993; Lageman, 1993;

.Zelina and Rusling, 1999 . These processes shouldbe further investigated, because it is believed thatthey may weaken the removal efficiency for heavymetal contaminants. It is reported that differentphysicochemical properties of the soil may influ-ence the removal rates of heavy metal contami-nants, due to changed pH values, hydrolysis, andoxidation and reduction reaction patterns.

In order to enhance the electrokinetic remedia-tion process, several authors recommend the useof a multiple anode system, which is shown in Fig.2.

2.3. Operational considerations

As there are several experimental techniquesto remediate coarse-grained soils, in situ elec-trokinetic treatment has been developed for con-taminants in low-permeability soils. Electrokinet-ics is applicable in zones of low hydraulic conduc-tivity, particularly with a high clay content.

Contaminants affected by electrokineticprocesses include:

Heavy metals; Radioactive species Cs , Sr , Co , ura-137 90 60

.nium ; . Toxic anions nitrates and sulfates ;

. Dense, non-aqueous-phase liquids DNAPLs ;

Fig. 2. Multiple anodes system US EPA, 1998.

Cyanides; Petroleum hydrocarbons diesel fuel, gasoline,

.kerosene and lubricating oils ; Explosives; Mixed organicionic contaminants; Halogenated hydrocarbons; Non-halogenated pollutants; and Polynuclear aromatic hydrocarbons.

Heavy metal interactions in the soil solutionare governed by several processes, such as Sims,

.1990 :

Inorganicorganic complexation; Acidbase reactions; Redox reactions; Precipitationdissolution reactions; and Interfacial reactions.

The choice of appropriate soil for electroki-netic remediation process should be made withextreme caution and possible soil pre-treatmentexperiments should be carried out.

Soils that may be used for the electrokinetic .remediation process should have Sims, 1990 :

Low hydraulic conductivity; Water-soluble contaminants if there are any

poorly soluble contaminants, it may be essen-.tial to add solubility-enhancing reagents ; and


Relatively low concentrations of ionic materi-als in the water.

It is reported that with applied electric fields,the most suitable soils for heavy metal remedia-

.tion are kaolinite, clay and sand Sims, 1990 . Asrecommended, clay has low hydraulic conductiv-ity, reducing redox potential, slightly alkaline pHwhich is suitable for the remediation of several

.heavy metal contaminants , high cation exchangecapacity and high plasticity. Under normal condi-tions, migration of ions is very slow, but is en-hanced by electrical fields or hydraulic pressure.

The highest degree of removal of heavy metals .over 90% of the initial contaminant has beenachieved for clayey, low-permeability soils,whereas for porous, high-permeability soils, suchas peat, the degree of removal was only 65% .Chilingar et al., 1997 . Laboratory results showedthat electrokinetic purging of acetate and phenolfrom saturated kaoline clay resulted in greaterthan 94% removal of the initial contaminants.However, this methodology needs to be furtherinvestigated, because phenol has been reported tobe toxic to humans and the environment.

3. Removal of metals

If heavy metal contaminants in the soil are inionic forms, they are attracted by the static elec-trical force of negatively charged soil colloids.The attraction of metal ions to the soil colloidsprimarily depends on the soil electronegativity

and the dissociation energy of ions Sah and.Chen, 1998 . If there are appropriate pH condi-

tions, heavy metals are likely to be adsorbed ontothe negatively charged soil particles. The mainsorption mechanisms include adsorption andorion exchange. Desorption of cationic species fromclay surfaces is essential in extraction of speciesfrom fine-grained deposits with high cation-exchange capacity.

As Acar and his research group have indicatedAcar and Alshawabkeh, 1993, 1996; Acar et al.,

.1994, 1996 , the sorption mechanisms depend onthe surface charge density of the clay mineral, thecharacteristics and concentration of the cationic

species, and the presence of organic matter andcarbonates in the soil. The mechanism is alsosignificantly dependent on the pore fluid pH. Thehigher the content of carbonates and organicmaterial in soils, the lower the heavy metal re-moval efficiency, which is why the former shouldbe further investigated and taken into the con-sideration.

During numerous experiments, a decrease incurrent density was observed Acar and Al-

shawabkeh, 1993, 1996; Acar et al., 1994, 1996;.Sah and Chen, 1998 . The possible reasons might

be as follows:

Activation polarisation: during the electroki-netic remediation process, gaseous bubbles O2

.and H cover the electrodes. These bubbles2are good insulators and reduce the electricalconductivity, subsequently reducing the cur-rent.Resistance polarisation: after the electrokineticremediation process, a white layer was observedon the cathode surface. This layer may be theinsoluble salt and other impurities that werenot only attracted to the cathode, but alsoinhibited the conductivity, with a subsequentdecrease in current.Concentration polarisation: the H ions gener-ated at the anode are attracted to the cathodeand the OH ions generated at the cathodeare attracted to the anode. If acid and alkalineconditions are not neutralised, the current alsodrops.

It is possible to conclude that soil containingheavy metal contaminants influences the conduc-tivity.

Interaction of the pollutants with the soil alsoaffects the remediation process. In order to in-crease the solubility of complexes formed, or toimprove electromigration characteristics of speci-fic heavy metal contaminants, an enhancementsolution may be added to the soil matrix.

Sometimes electroosmotic flow rates are toolow, and it may be necessary to flush the elec-trodes with a cleaning agent, or simply clean tap

.water Probstein and Hicks, 1993 . In addition,the electrode may be surrounded by ion-exchange


material to trap the contaminant and prevent itsprecipitation. It is essential to know the bufferingcapacity of the soil in order to alter the pH withsuitable solutions or clean water. Many ground-waters contain high concentrations of bicarbon-ates, which consume added hydrogen ions to formcarbonic acid, or hydroxyl ions to form carbonateions. It is vital to draw attention to the limitedsolubility of metal carbonates, as well as the needfor evaluation of sulfide, sulfate, chlor-ide and ammonia effects, which may occur whenthese compounds are introduced into the soil

system during the remediation process Probstein.and Hicks, 1993 .

New alternatives have been suggested for theremediation of heavy metals from soils without

having low pH conditions Probstein and Hicks,.1993 . When the metal enters the region of high

pH near the cathode, it may adsorb onto the soil,precipitate, or form hydroxy complexes. At higherpH values, the solubility increases because of theincreasing stability of soluble hydroxy complexes.Despite favourable soluble complexes, the disso-lution process may be time-consuming and tooslow to be successfully implemented.

Concerning the process of transport of con-taminants and their derivatives, two major pheno-

.mena were indicated Chilingar et al., 1997 :

1. The flow of contaminant solution through asolid matrix due to Darcys law and electroki-netics; and

2. Spatial redistribution of dissolved substanceswith respect to the moving liquid due to thediffusion and migration of charged particles.

The total movement of the matter of the con-taminant solution in the DC electric field can beexpressed as the sum of four components .Chilingar et al., 1997 :

The hydrodynamic flow of liquids driven bythe pressure gradient;

The electrokinetic flow of fluids due to inter-action of the double layer with the DC field;

The diffusion of components dissolved in theflowing solution; and

The migration of ions inside moving fluids due

to the attraction of charged particles to theelectrodes.

The very questionable concept that removal ofheavy metals in the direct current field is effectivewas also expressed, because electromigration ofions is rapid and does not depend on the zetapotential. In order to prove or disapprove this,further investigations of this concept should becarried out. Despite some disagreements, it wasagreed that in order to obtain efficient and reli-able results and control the remediation process,there is a need to provide continuous control of

the pH in the vicinity of the electrodes Acar andAlshawabkeh, 1993, 1996; Acar et al., 1994, 1996;

.Chilingar et al., 1997 . One possible way to achievethis is periodic rinsing of the cathode with freshwater.

Experiments have proved that electrical fieldapplication in situ leads to an increase in temper-ature, which in turn reduces the viscosity of hy-

drocarbon-containing fluids Chilingar et al.,.1997 . The reduction in fluid viscosity leads to an

increase in the total flow rate. .It is reported Chilingar et al., 1997 that in

order to accelerate the fluid transport in situ,electrical properties of soils, such as electricalresistivity and the ionisation rate of the flowingfluids that can affect the total rate flow, shouldconsider. In an applied DC field, some soil typesshowed an increase in their hydraulic permeabil-ity, which allows us to conclude that direct cur-rent may accelerate fluid transport. However, thismethod is not applicable to some clays, becauseunder the DC field, those clays become amor-phous. It is possible to avoid such a transforma-tion if interlayer clay water is trapped and is notable to leave the system.

From the numerous laboratory and field experi-ments and studies conducted, it is possible toconclude that migration rates of heavy metal ions .i.e. removal efficiencies are highly dependent onsoil moisture content, soil grain size, ionic mobil-ity, pore water amount, current density and con-

taminant concentration Acar and Alshawabkeh,1993, 1996; Acar et al., 1994, 1996; Chilingar et

.al., 1997; Sah and Chen, 1998 . Also, in order toassure the efficient and successful heavy metal


removal from soils, one of the main drawbacks ofthis process must be solved, which is prematureprecipitation of metal species close to the cathodecompartment.

3.1. Limitations of the technique

The removal of heavy metals from soils usingelectrokinetic remediation has some limitations,which have been widely discussed among manyscientists and researchers. For example, the sur-face of the electrode attracts the gas generatedfrom the electrolytic dissociation process and in-creases the resistance, which significantly slows

down the remediation process Sah and Chen,.1998 . It is obvious that soil resistance is lower in

the earlier stages of the electrokinetic process,and therefore a lower input voltage is required.When the electrokinetic process continues, gasbubbles from electrolytic dissociation cover thewhole cathode surface and the resistance in-creases. To continue the soil remediation process,the input voltage must be increased to maintainthe same current, which also increases the voltagegradient. OH ion that are formed react withcations and form a sediment, which plugs thespacing between soil particles, subsequently hin-dering the electrical current and decreasing thediffusive flow over time when the voltage is ap-

.plied Sah and Chen, 1998 .

3.2. Enhancement and conditioning

To overcome the premature precipitation ofionic species, Acar and his research group haverecommended using different enhancement tech-niques to remove or to avoid these precipitates inthe cathode compartment. Efficient techniquesshould have the following characteristics:

The precipitate should be solubilised andorprecipitation should be avoided.

Ionic conductivity across the specimen shouldnot increase excessively in a short period of

time to avoid a premature decrease in theelectroosmotic transport.

The cathode reaction should possibly be de-polarised to avoid the generation of hydroxideand its transport into the specimen.

Depolarisation will decrease the electrical po-tential difference across the electrodes, whichwould result in lower energy consumption.

If any chemical is used, the precipitate of themetal with the new chemical should be per-fectly soluble within the pH range attained.

Any special chemicals introduced should notresult in any increase in toxic residue in thesoil mass.

The cost efficiency of the process should bemaintained when the cost of enhancement isincluded.

It is obvious that an enhancement fluid in-creases the efficiency of contaminated soil treat-ment; however, there is a lack of data whichwould clarify further soil and contaminant inter-actions in the presence of this fluid.

.As a depolariser i.e. enhancement fluid in thecathode compartment, it is possible to use a low

concentration of hydrochloric or acetic acid Acarand Alshawabkeh, 1993, 1996; Acar et al., 1994,

.1996 . The main concern with hydrochloric acidas the depolariser is that due to electrolysis, thechlorine gas formed may reach the anode, as wellas groundwater, and increase its contamination.Acetic acid is environmentally safe and it doesnot fully dissociate. In addition, most acetate saltsare soluble, and therefore acetic acid is preferredin the process.

The anode reaction should also be depolarised,because of the dissolution and release of silica,alumina and heavy metals associated with the claymineral sheets over long exposure to protonsAcar and Alshawabkeh, 1993, 1996; Acar et al.,

.1994, 1996 .In order to accomplish both tasks successfully,

it is better to use calcium hydroxide as the en-hancement fluid to depolarise the anode reaction,and hydrochloric acid as the enhancement fluid todepolarise the cathode reaction.

The use of an enhancement fluid should be


examined with extreme care to prevent Yeung et.al., 1997 :

The introduction of a secondary contaminantinto the subsurface;

The generation of waste products or by-prod-ucts as a result of electrochemical reactions;and

The injection of an inappropriate enhance-ment fluid that will aggravate the existing con-tamination problem.

4. Electrokinetic soil remediation processes

4.1. Remoal of heay metals using cation-selectiemembrane

In alkaline medium, heavy metals are likely tobe adsorbed onto the soil particles and forminsoluble precipitates. The high pH region in clos-est proximity to the cathode is the main obstacle

to heavy metal removal Acar and Alshawabkeh,

1993, 1996; Acar et al., 1994, 1996; Li et al., 1997;Li and Neretnieks, 1998; Li and Li, 2000; Yeung

.et al., 1997 . However, the latest experimentalstudies show that it is possible to deal with the

pH impact Li et al., 1997; Li and Neretnieks,.1998; Li and Li, 2000 . A conductive solution,

which simulates the groundwater conditions, wasplaced between the cathode and the soil to betreated. However, the length of conductive solu-tion must be at least twice the length of thetreated soil, which may be impossible to imple-ment at a site. In addition, the solution has to beplaced in a special container, which would sig-nificantly increase the costs of the overall remedi-ation process. The pH buffer capacity, cationexchange capacity of the medium, and interac-tions of the solution with the soil may affect thespeed of the advancement of the acidic and the

basic fronts and the location of the pH jump Li.and Li, 2000 . In order to overcome these obsta-

cles, a new method was proposed which shouldsignificantly improve the overall remediationprocess. To reduce the relative length or volumeof the water in the system, a cation-selective

.Fig. 3. Electrokinetic cell with cation-selective membrane adapted from Li and Neretnieks, 1998; Li et al., 1997; Li and Li, 2000 .


membrane is placed in front of the cathode Li etal., 1997; Li and Neretnieks, 1998; Li and Li,

. .2000 Fig. 3 .Due to an applied electric current, ions move

to the electrodes, according to their charges. Thecation-selective membrane, placed between thesoil and cathode, allows cations and very fewanions to pass through it. This is why almost allthe hydroxyl ions produced at the cathode remainon the cathodic side of the membrane. The hy-drogen ions generated at the anode move throughthe soil and into the membrane. The basic frontcannot pass through the membrane, where itmeets the acidic front. The main pH changes

occur near the membrane Li et al., 1997; Li and.Neretnieks, 1998; Li and Li, 2000 . It is possible

that the membrane determines the pH jump andmay control the cathode solution volume. Acation-selective membrane maintains the low soilpH during the remediation process and signifi-cantly reduces the length of the conductive solu-tion required. Hence, the proposed electrokineticcell consist of the treated soil, a conductive solu-tion, which is placed between the soil and themembrane, and the cathode compartment withelectrolyte solution, which is between the mem-brane and cathode. After numerous experiments,it has been observed that the smaller the volumeof conductive solution, the higher the pH will beand the larger will be the leakage of the anions

through it Li et al., 1997; Li and Neretnieks,.1998; Li and Li, 2000 .

However, a small amount of anions passingthrough the membrane may be favourable for theremediation process. Precipitation decreases theremediation time, because this reduces the con-

centration of heavy metals in the liquid phase Li.and Li, 2000 . At the same time, back-diffusion of

heavy metals is greatly reduced, since the concen-tration of heavy metals near the membrane doesnot exceed the solubility of the metals. It hasbeen proved by experiments that precipitationdecreases the electrical energy consumption, be-cause the potential drop between the electrodesand the remediation time are proportional to the

distance between the electrodes Li et al., 1997;.Li and Neretnieks, 1998; Li and Li, 2000 .

4.2. Remoal of heay metals using surfactant-coatedceramic casings

For many years, the main emphasis of elec-trokinetic soil remediation was on saturated,fine-grained soils and clays, which led to the mis-conception that electrokinetics was not suitablefor unsaturated, sandy soils. Laboratory experi-ments proved that with appropriate technologyand well-designed methods, it is possible to reme-diate heavy metals from unsaturated and sandy

.soils Mattson and Lindgren, 1995 . The treat-ment of unsaturated soils has several limitations.The electrical conductivity of soil depends on the

.moisture content Mattson and Lindgren, 1995 .During electroosmotic migration through the soil,the water content near the anode is reduced. Asthe moisture content decreases, the soil conduc-tivity becomes too low for the electrokinetic re-mediation application. In order to control thehydraulic flux of water in the treated soil, the useof porous ceramic castings has been proposed.During the application, it should be rememberedthat the direction of electroosmotic flow in porousceramic media has a strong influence on theamount of water being added to the soil from theceramic castings. Anode ceramic casting would besuitable for long-term electrokinetic remediationprocesses if it was ensured that electroosmoticflow occurred from the surrounding soil towards

the interior of the anode casting Mattson and.Lindgren, 1995 . As efficient electrokinetic reme-

diation in unsaturated soils depends on the wateramount at the anode, there is a necessity tocontinuously inject water during the whole reme-diation process. Despite the addition of water, itis important to maintain unsaturated conditionsin the soil, because excess water may cause satu-rated conditions and contaminants will be able tomigrate into the deeper layers of the soil.

A number of experiments with an anode cer-amic casting were conducted and it was provedthat it is possible to remove heavy metal contami-nants from unsaturated, sandy soils using the

electrokinetic remediation technique Mattson.and Lindgren, 1995 .

First of all a laboratory cell was designed andconstructed, which consisted of a plastic con-


tainer filled with buffering solution. The polyvinylchloride plate glued to the bottom of the con-tainer, the porous ceramic castings, woven wirecathode and graphite anode are shown in Fig. 4.The most suitable buffering solution for this ex-periment is a phosphate solution with a pH of 6 .Mattson and Lindgren, 1995 . To overcome thehydraulic counterflow, the experiment should onlybe conducted until the fluid level differencebetween the inner and outer reservoirs becomes

.1 cm Mattson and Lindgren, 1995 .After laboratory experiments, a number of field

studies were conducted and the initial resultsobtained are very promising. It is possible to statethat the use of anode ceramic casting may sig-nificantly improve the application of electroki-netic remediation in unsaturated soil media.

4.3. LasagnaT M process

In 1995, a novel integrated method for in situelectrokinetic remediation of soils, calledLasagnaTM, was developed and implemented atthe Paducah site, in Kentucky, USA. This tech-nology is useful for removing heavy metal con-taminants from heterogeneous, low-permeability

.soils Ho et al., 1997, 1999 .In brief, the LasagnaTM process contains the

following concepts:

The creation of several permeable treatment

zones in close proximity through the wholesoil matrix by adding sorbents, catalyticreagents, buffering solutions, oxidising agents,etc.

Application of an electric current in order totransport contaminants into the treatmentzones created.

The LasagnaTM process has several advantagesin comparison to other techniques. First, it ispossible to recycle the cathode effluent by aimingit back to the anode compartment, which wouldfavour neutralising of the pH and simplify watermanagement. In addition, the fluid flow may be

reversed by simply switching the polarity Ho et.al., 1999 . The switching of polarity promotes

multiple contaminant passes through the treat-ment zones and helps to diminish the possibilityof non-uniform potential and pH jumps in the soilsystem.

Two schematic LasagnaTM model configura- .tions were suggested: horizontal Fig. 5 and verti-

.cal Ho et al., 1999 .The process was called Lasagna due to the

layering of treatment zones between the elec-trodes. The formation of horizontal fractures inover-consolidating clays due to the horizontalelectrodes and vertical pressuring system makethis method especially effective in removing con-

taminants from deeper layers of the soil Ho et

.Fig. 4. Electrokinetic cell with ceramic castings Mattson and Lindgren, 1995 .


TM .Fig. 5. Horizontal Lasagna configuration adapted from Ho et al., 1999 .

.al., 1999 . In addition, for shallow contaminationwhich does not exceed 15 m and in not over-con-solidated soils, the vertical treatment configura-

. .tion is more appropriate Ho et al., 1997 Fig. 6 .According to laboratory experiments and

promising pilot-scale studies at the Paducah sitein Kentucky, LasagnaTM technology may becomeone of the most widely used electrokinetic reme-diation technologies for removing heavy metalcontaminants from various soils. Nevertheless,there are several technological and other limita-tions, which should be improved for future stud-ies. It is obvious that LasagnaTM technology ispotentially capable of treating multiple contami-

TM Fig. 6. Vertical Lasagna configuration adapted from Ho et.al., 1997 .

nants in clay and laden soils, but additional exper-iments and studies should be conducted in orderto assure that the treatment process is compatiblefor individual contaminants. In addition, one ofthe biggest technology drawbacks is the entrap-ment of gases formed by electrolysis and theassurance of good electrical contact to the elec-trodes. To increase the LasagnaTM process effi-ciency, there were attempts to implement biore-mediation in treatment zones. It is believed thatbioremediation together with electrokinetic reme-diation may significantly increase the overall re-moval of heavy metals, as well as other contami-nants, from clays and other soils.

4.4. Electro-KleanT M electrical separation

Electro-KleanTM technology is applied in situ,as well as ex situ, in Louisiana, USA. This is anew methodology, which is used to remove heavymetals, radionuclides and specific volatile organiccontaminants from saturated and unsaturatedsands, silts, fine-grained clays and sediments. Thistechnology uses two electrodes to apply DC di-

rectly into the contaminated soil mass van.Cauwenberghe, 1997 . In order to improve the

remediation efficiency, enhancement fluids,mostly acids, are added into the soil. The mainlimitation of this technique is the high buffering


capacity of the soils and different coexistingchemicals and their concentrations.

4.5. Electrokinetic bioremediation

Electrokinetic bioremediation technology is de-signed to activate microbes and other micro-organisms present in soils by the use of selectednutrients to promote the growth, reproductionand metabolism of micro-organisms capable of

transforming organic contaminants in soil van.Cauwenberghe, 1997 . Nutrients reach the or-

ganic contaminants by specially applied bioelec-tric technology. It is believed that this technologymay be very successful in the future, because itdoes not require an external microbial populationto be added into the soil system. In addition,nutrients may be uniformly dispersed over thecontaminated soil or directed to a specific loca-

.tion van Cauwenberghe, 1997 and the methodavoids the problems associated with transport of

micro-organisms through fine-grained soils Fig..7 .Despite promising results, this technology has

some major limitations. Sometimes the concen-tration of organic pollutant exceeds the toxic limitfor the microbial population and micro-organismsdie. Simultaneous bioremediation of various or-ganic contaminants may produce by-products,which are highly toxic to micro-organisms. Those

by-products may significantly inhibit the bioreme-diation rates.

4.6. Electrochemical geooxidation

Electrochemical geooxidation is used in Ger-many to remediate soil and water contaminated

with organic and inorganic compounds van.Cauwenberghe, 1997 . The in situ process in-

volves the application of an electrical current toprobes driven into the ground. The applied cur-rent creates favourable conditions for oxidationreduction reactions, which lead to the immobilisa-tion of inorganic contaminants in the soil orgroundwater matrix between the electrode loca-tions. The main advantage of this technology isthat there is no need to use catalysts for theoxidationreduction reactions, because in almostall soils, natural catalysts, such as iron, magne-sium, titanium and elemental carbon, are present.The limitations of this technology are the verylong remediation time and the lack of provenresults.

4.7. Electrochemical ion exchange

This technology employs a series of electrodes,placed in porous castings, which are supplied withcirculating electrolytes. During the remediationprocess, ion contaminants are captured in these

.Fig. 7. Electrokinetic bioremediation according to Thevanayagam and Rishindran, 1998 .


electrolytes and pumped to the surface, wherethey are passed through an electrochemical ion

.exchanger van Cauwenberghe, 1997 . Thismethod is used to remove heavy metals, halidesand specific organic species from different typesof soils. The most important limitation of thistechnology is that it is a very expensive procedurefor cleaning effluents containing low levels ofcontaminants.

4.8. ElectrosorbT M

ElectrosorbTM technology is mostly used inLouisiana, USA, and uses cylindrical electrodescoated with a specially designed polymer mate-rial. This polymer is impregnated with pH-regu-lating chemicals in order to prevent pH jumps .Reddy and Chinthamreddy, 1999 . During theremediation process, electrodes are placed inboreholes in the soil and direct current is applied.Ions move through the pore water to the elec-trode, where they are trapped in the electrodepolymer matrix. Although there are no indica-tions of the limitations of the technique proposed,it is believed that in order to be commerciallyavailable, it should be further investigated.

5. Remediation of specific heavy metalcontamination

As the heavy metal contaminants in a soil andsolution primarily exist in the form of salts andions, the potential of an electrokinetic remedia-tion technique depends on the quantity of thosecompounds.

5.1. Remoal of cadmium and lead

Under alkaline conditions, cadmium and leadin the soil may become sediments of hydroxides . . Cd OH , Pb OH and carbonates CdO ,2 2 3

.PbCO . Soil pH determines the concentrations3of hydroxide and carbonate in the soil solution,which play a crucial role in the formation of

heavy metal complexes in soil Sah and Chen,.1998 .

In order to understand the migration of Pb and

Cd between electrified vs. non-electrified soilsamples under different times, locations and solu-tion types, it is important to use heavy metal

.formal analysis Sah and Chen, 1998 . Also, dueto varying stability of different heavy metals inthe soil, there is a necessity to determine ap-propriate application times for electrokinetic re-mediation and the pH of the soil.

Experiments conducted show that Pb-con-taminated soil is usually quite difficult to remedi-ate. However, high removal rates for Pb, as wellas Cd, were obtained in experiments where HCl

solution was used Acar and Alshawabkeh, 1993;.Sah and Chen, 1998 .

If the environment near the cathode is basic, itmay favour the formation of the insoluble hydrox-

.ide Cd OH . However, this Cd species may not2be mobile under advective flow Acar and Al-

.shawabkeh, 1993, 1996; Acar et al., 1994, 1996 .In order to improve the removal rates of

cadmium and lead from soils, the following pro- .posals should be considered Sah and Chen, 1998 :

Experiments showed that soil could absorbmore Pb than Cd, which should be taken intoconsideration in further laboratory experi-ments, as well as pilot-scale studies.

Cd-spiked samples have revealed a higher cur-rent density than Pb-spiked samples duringthe remediation process. A thin, white oxidantfilm was found on the cathode, which reducedthe conductivity and removal efficiency ofmetals. Thus, an enhancement fluid should beadded at the electrodes, or the electrodesmust be cleaned regularly during the applica-tion.

The use of HCl acid increased the removalrates of lead and cadmium. In order to achieveoptimal removal results, acid solution has tobe added to the soil solution.

5.1.1. Lead migration in soilsCationic heavy metals, such as Pb, are most

soluble at a low pH. As the H produced at theanode moves across the soil sample, cationic met-als which were sorbed or precipitated onto thesoil particles are, in many cases, solubilised and


may be able to undergo transport by diffusion, aswell as via electrokinetic remediation processes,such as advection by electroosmotic flow andelectrolytic migration. Diffusion and electrolyticmigration of OH ions produced at the cathodeincrease the pH of the system near the cathode

and may precipitate desorbed ions Viadero et al.,.1998 . This is shown schematically in Fig. 8.

Experiments showed that at a pH above 44.5,lead was either adsorbed onto the soil andor

. .precipitated as Pb OH s , which reduced the2conductivity of the soil by removing cations from

.the liquid Viadero et al., 1998 . At high pH, mostof the lead is retained in hydroxide and carbonatephases.

5.1.2. Cadmium migration in soilsWhen the initial pH is low, the conductivity of

the medium is high, and very low electrical poten-tial gradients are initially generated across the

specimen Acar and Alshawabkeh, 1993, 1996;Acar et al., 1994, 1996; Probstein and Hicks,1993; Mattson and Lindgren, 1995; Sah and Chen,

.1998; Viadero et al., 1998 .Numerous experiments have been conducted to

remove cadmium from kaolin. In kaolin, withoutthe addition of a reducing agent and in the pres-ence of humic acid and ferrous iron, low pHconditions exists throughout most of the soil, ex-cept near the cathode. As low pH conditionsfavours the dissolution of Cd species, cadmium istransported to the cathode compartment .Pamukcu, 1997 . Low-concentration Cd speci-mens exhibit a larger influx of water than high Cdconcentration specimens for the same level of

.electricity Pamukcu, 1997 .

.H e 12 H 2

Cd22eCd0

. . 0 .Cd OH s 2e Cd 2OH 32

When the current density is greater than 5mAcm2, secondary temperature effects are re-ported to decrease the efficiency of electro-

.osmotic flow Hansen et al., 1997 .

5.2. Remoal of arsenic and chromium

The main substance used for desorbing cationicspecies is hydronium ions H O produced at the3anode during the electrolysis process. However,there are several major drawbacks of this process:it induces a dissolution of major soil components,

.such as carbonates, as well as oxides Fe, Mg .when strongly acidified Hecho et al., 1998 .

Anionic species are removed by the hydroxideions generated at the cathode. It is necessary toadd an anionic oxidising agent, which would mi-

grate to the anode through the soil matrix Hecho. .et al., 1998 . Chromium III can be oxidised into

.Cr VI as anionic species, which can be desorbedin alkaline medium. This method is useless witharsenic, because all soluble arsenic species are

.anionic above pH 9 and arsenic V is more .strongly sorbed that arsenic III .

In order to remove chromium from soils, it is . .necessary to oxidise Cr III first to chromium VI ,

.Fig. 8. Lead removal from soils according to 29 .


which is anionic. The removal of arsenic is not ascomplicated as that of chromium. The literature

.indicates that arsenic III is more soluble than .arsenic V , so the use of an oxidising agent does

not seem useful.Two alkaline reagents, i.e. sodium carbonate

and sodium hydroxide, are used to enhance theremediation process Reddy and Chinthamreddy,

.1999 . Earlier, two alternatives, i.e. hydrogen per-oxide and sodium hypochlorite, were used as oxi-dising agents. However, experiments proved thathydrogen peroxide tends to reduce very rapidly inthe soil, and only hypochlorite was used for fur-

ther laboratory and pilot studies Hansen et al.,.1997; Hecho et al., 1998 .

5.2.1. Chromium migrationChromium can exist in valence states ranging

from 2 to 6; however, 3 and 6 are theonly two valence states that prevail under subsur-

face conditions Reddy et al., 1997, 1999; Reddy.and Chinthamreddy, 1999 . Hexavalent

.chromium VI is highly mobile and toxic in com- . .parison to Cr III . Cr VI exists as anions, speci-

.fically hydrochromate HCrO , dichromate4 2. 2.Cr O and chromate CrO , and will mi-2 7 4grate towards the anode during the electrokinetic

.remediation process. On the other hand, Cr IIIexists as a cation Cr 3 and may form cationic,neutral and anionic hydroxy complexes, specifi-

.2 . . .cally Cr OH , Cr OH , Cr OH , Cr OH2 3 4 .2 .and Cr OH . Cr III may also exist as other5

cationic, neutral and anionic inorganic and or-ganic complexes, depending on the ligands pre-

.sent Reddy and Chinthamreddy, 1999 .In acidic regions and at relatively low redox

. 3potentials, Cr III exists as Cr and forms .2cationic complexes Cr OH . Being positively

.charged, Cr OH will migrate towards the cath-2ode during the electrokinetic remediation process.

. . Cr III precipitates as its hydroxide Cr OH 3between pH 6.8 and 11.3, while at higher pH

.values, Cr III may form anionic hydroxy com- . .2 plexes Cr OH and Cr OH Reddy et al.,4 5

.1997, 1999; Reddy and Chinthamreddy, 1999 .The removal of chromium from soils by elec-

trokinetic remediation is highly efficient if the . chromium exists as Cr VI Acar and Al-

shawabkeh, 1993, 1996; Acar et al., 1994, 1996;Reddy et al., 1997, 1999; Reddy and Chintham-

.reddy, 1999; Sah and Chen, 1998 . If reducingagents, such as organic matter, sulfides or ferrous

.iron, are present in natural soils, Cr VI is likely .to be reduced to Cr III , which may significantly

affect the electrokinetic migration of chromium,as well as the migration of co-existing metals

. . such, as Ni II and Cd II Reddy et al., 1997,.1999; Reddy and Chinthamreddy, 1999 .

As chromium species favour alkaline conditionsin soils, an alkaline reagent must be injected intothe soil system in order to neutralise H O ions.3

In order to enhance the electrokinetic remedia-tion application, an oxidising agent sodiumhypochlorite needs to be injected at the cath-

.ode compartment Reddy et al., 1999 . Hypochlo-rite ions can migrate towards the anode and oxi-dise trivalent chromium to hexavalent chromium,which in turn migrates towards the anode.

After close investigation of the effects of reduc-ing agents on chromium species migration, it wasobserved that when the chromate front meets theanodic reaction product Fe2 in a region adjacentto the anode, it reacts to form Cr 3 and Fe3

species:

2 6 3 3 .Fe Cr Fe Cr 4

Thus, further migration of chromate is inhib-ited due to redox reactions with ferrous ions . .Haran et al., 1996 . Cr III is immobilised insand due to the formation of complex sulfates

.and hydroxides. When the pH is increased, Cr IIIis likely to be precipitated as chromic hydroxide:

3 . .Cr 3OH Cr OH 53

The reduction reaction is controlled by two im- .portant factors, the amount of Fe II in the sand

.and the soil pH Haran et al., 1996 :

2 .FeFe 2e 6

. Cr VI exists predominantly as HCrO at low pH42 and as CrO at high pH in solution Reddy et4

.al., 1997 :

.SOHSO H 7.1


.SOHH SOH 7.22

.SO M SOM 7.3

.SOH L SOH L 7.42 2

where SOH represents a typical surface functio-nal group, and M and L represent a cation andanion, respectively.

These complexation reactions are highly pH-dependent, because the extent of surface depro-

.tonation Sogorka et al., 1998 and protonation .reactions Acar and Alshawabkeh, 1993 is con-

.trolled by the solution pH Reddy et al., 1999 .

5.2.2. Chromium remoal from different soilsDifferent experiments were conducted to ob-

tain results for chromium removal efficiency fromseveral types of naturally occurring soils, such as

kaolin and glacial till Acar and Alshawabkeh,1993; Mattson and Lindgren, 1995; Reddy et al.,1997, 1999; Reddy and Chinthamreddy, 1999; Sah

.and Chen, 1998 .The presence of reducing agent in soils, such as

humic acid, did not retard the chromium migra-tion, either in kaolin or in glacial till; actually, itenhanced chromium migration towards the anode .Reddy and Chinthamreddy, 1999 . On the otherhand, ferrous iron, another reducing agent natu-rally present in soils, showed moderate retarda-tion of chromium migration. Finally, the presenceof sulfides showed the highest rate of retardationof chromium species migration towards the anode.It is possible to conclude that when a reducing

.agent was present, higher Cr III concentrationswere observed near the anode. On the other

.hand, the reduced Cr III tends to migrate to- .wards the cathode, resulting in high Cr III con-

.centrations in the section near the anode. Cr VIadsorption onto soil decreases with an increase in

.soil pH Reddy et al., 1997 .

5.2.2.1. Glacial till. Glacial till has high bufferingcapacity because of the presence of carbonates inthis soil. It is reported that there are no traces of

acid front formation in glacial till Reddy and.Chinthamreddy, 1999 . Carbonates have the abil-

ity to neutralise H ions generated, and blockdevelopment of an acidic pH environment nearthe anode. The adsorption of HCrO onto the4soils is significant, but the adsorption of CrO2 is4

.negligible Reddy et al., 1999 . It is obvious that .high pH in glacial till causes all Cr VI to exist as

CrO2, which therefore results in low adsorption4of species onto the soil. Soluble CrO2 ions are4transported to the anode by electromigration.

. .The possibility of Cr VI conversion to Cr III .was evaluated Reddy et al., 1997, 1999 . It was

proved that without reducing agents in the soil, . .significant Cr VI reduction to Cr III would not

occur.Iron deposits of hematite, pyrite and goethite

occur in abundance in natural soils. When thereare slightly alkaline conditions in glacial till,

. 2Cr VI exists predominantly in the form of CrO ,4and it is reported in the literature that CrO24adsorption onto Fe O is significant. In addition,2 3hematite may react with constituents of glacial

.till, which may favour further removal of Cr VI .in the pore water Reddy et al., 1997 .

5.2.2.2. Kaolin. A distinct pH gradient developed . 2in kaolin causes Cr VI to exist as both CrO4

and HCrO species Reddy and Chinthamreddy,4.1999 . In addition, alkaline conditions near the

.cathode favour the existence of Cr VI in theform of CrO2, which does not adsorb to the soil,4

.and therefore most Cr VI exists in solution andmigrates toward the anode. On the other hand,CrO2 ions enter an acidic region near the anode,4which favours the formation of HCrO ions. As4mentioned earlier, HCrO adsorbs significantly4

.to the soil, which retards Cr VI migration.

5.2.3. Arsenic migration and remoalIn alkaline conditions, arsenic species do not

demonstrate well-expressed adsorption, although .As V is usually more strongly adsorbed than .As III . It is indicated that alkaline conditions

favour arsenic electromigration, although it is veryslow and time-consuming Acar and Al-

shawabkeh, 1993; Acar et al., 1996; Mattson andLindgren, 1995; Haran et al., 1996; Sah and Chen,

.1998; Viadero et al., 1998 . In order to enhancethe electromigration process, sodium hypochlorite


is introduced into the process. To achieve theprocess efficiency desired and improve the systemperformance, it is necessary to inject an enhance-ment solution directly into the cathodic compart-

ment Reddy et al., 1997, 1999; Reddy and.Chinthamreddy, 1999 .

5.3. Remoal of mercury

Electrokinetic remediation of Hg-contaminatedsoils is very difficult because of the low solubilityof Hg in most natural soils. The predominant

.species of insoluble Hg in the soils are HgS, Hg I .and Hg Cl Cox et al., 1996 . Several years ago,2 2

a new method for Hg removal from soils wasintroduced. It uses an I I lixiviant solution to2solubilise Hg from contaminated solids. Oxidation

.of reduced insoluble Hg by I releases Hg II ,2which is complexed as soluble HgI2 and Hg ions4are ready to migrate through the soil towards the

.anode and be removed Cox et al., 1996 :

HgSI 2IHgI2S2 4 oxidised.

. 2Hg I I 2I HgI2 4

2 2 .HgO4I HgI O 84

Once solubilised, Hg is able to migrate throughthe soil and be removed.

It should be mentioned that iodide solutionand I crystals introduced near the cathode react2to form I complex. Reduced forms of insoluble3Hg can be oxidised by either I or I; however,2 3transport of oxidant through the soil is dependenton the electromigration of the I anion. The3HgI2 complex formed via reactions with lixiviant4solution is removed from the soil by electromigra-tion towards the anode.

A pH jump was observed during the electroki-netic remediation process. It is believed that thispH increase may be caused by the following reac-

tion, if an excess of Cl is present under aerobic .conditions Cox et al., 1996 :

O 2Hg8Cl2H O2HgCl24OH2 2 4 .9

Mercury removal may be more efficient if chlo-ride or another suitable component is added to

.the soil system Hansen et al., 1997 . Additionalchloride ions are able to mobilise the mercury,forming complex ions which are easily transportedout from the soil by electromigration. For in-stance, hypochlorite may be a suitable compound,which oxidises metallic mercury, forming HgCl2:4

2 .HOClHg3Cl HgCl OH 104

Although some promising results have beendemonstrated, this method has several majordrawbacks. First, in the presence of any organicmatter, hypochlorite may form toxic, halogenatedorganic compounds, which are dangerous for hu-mans and may severely harm the environment. Inaddition, if not removed before the electrokineticremediation process begins, metallic mercurywould inhibit the overall remediation process dueto its electric conductivity.

5.4. Remoal of zinc and copper

All calcium and magnesium should be removedbefore removal of zinc is initiated. The use ofenhancing solutions, such as sodium acetate, in-creases the removal efficiency for metal ions, aswell as reduces the process time. It is obvious thatthe cations with lower interaction energy will beremoved first and will be followed by cations withhigher interaction energy.

After the number of experiments, the sequenceof heavy metal removal from soils using sodium

acetate as enhancement fluid was proposed Cox. 2 2 2 2et al., 1996 : Ni Cd Ca CrZn

KMg2Cu2Pb2.Also, several experiments were conducted with

distilled water as the enhancement fluid and the .following results were observed Cox et al., 1996 .

Ca2, Mg2, Zn2, K and Pb2 percentageremoval efficiencies were low and sometimes close


.Fig. 9. Electrokinetic cell for copper removal from soils adapted from Cox et al., 1996 .

to zero. Only Ni2 and Cr had removal efficien-cies quantified as mediumhigh.

A schematic electrolytic cell for the removal ofcopper from contaminated soil was proposed by

. .Cox et al. 1996 Fig. 9 .The electrolytic cell is divided into three parts

and the contaminated soil and electrodes areseparated by anionic and cationic exchange mem-branes. The anode and cathode compartmentscontain electrolyte solution at constant pH 3.

A low pH value was maintained to keep copperdissolved in the soil, thus making migration to-wards the cathode and subsequent removal fromthe soil feasible. Despite the fact that almost allof the copper was found in the cathode compart-ment, a certain amount was found in the anionmembrane. It was also suggested that copperfound in the anionic membrane may be due to itscapability of forming complexes with different

.ligands present in soils Ribeiro et al., 1997 .

5.5. Other metals

Strontium remains as a divalent ion over alarge pH range. The cathode should not affect

strontium, since it will remain a divalent ion, even .at high pH Pamukcu, 1997 .

In alkaline solution, the predominant species ofCo2 are either positively charged ions or hydrox-

.ide Co OH salts. It is apparent that at high pH,2cobalt tends not to precipitate onto soil particles,and may therefore be removed.

According to the experiments, if Ca2 ions areremoved first, then Zn2 follow, and finally Cu2

2 and Pb ions are removed Hansen et al., 1997;.Hecho et al., 1998 . In order to mobilise contami-

nants, energy may be wasted in dissolving limeand carrying harmless Ca2 ions out of the soil. Itis obvious that further research concerning othersuitable soil pre-treatment methods to mobilisecontaminants need to be investigated and carried

out Hansen et al., 1997; Hecho et al., 1998;.Viadero et al., 1998 .

6. Heavy metal removal efficiency fromcontaminated soils

Electrokinetic remediation techniques have de-monstrated 8595% efficiency in removing ar-

Table 1Heavy metal removal efficiency from selective contaminated soils using the electrokinetic remediation technique adapted from

.Lageman, 1993; Sengupta, 1995

.Soil Metal removal efficiency %

Cd Cr Ni Pb Hg Cu Zn As Co Sr

Agrillaceous sandRiver mud 50 64 91 54 60 71 94 66 Kaolinite 94.6 93.1 88.4 69 26.5 54.6 54.7 92.2 97.8Kaolinite and 92.7 97.6 93.9 66.9 42.5 36.3 27.2 95.9 96humic substances

Montmorillonite 86.6 93.5 93.6 64.4 64.3 89.4 92.3Clayey sand 98 96.8 95.9 83 78.3 54.5 54.7 97.5 99


senic, cadmium, chromium, cobalt, mercury,nickel, manganese, molybdenum, zinc, antimonyand lead from low-permeability soils, i.e. clay,peat, kaolinite, high-purity fine quartz, Na andsandmontmorillonite mixtures, as well as from

.agrillaceous sand Yeung et al., 1997 . In addi-tion, highest removal efficiencies, i.e. more than90% of heavy metals, were obtained in kaolinite .Pamukcu and Wittle, 1992 . However, for porous,high-permeability soils, such as peat and riversediment, the removal efficiency was approxi-

.mately 65% Chilingar et al., 1997 .A low pH profile in fine-grained soils may

contribute to higher efficiency for metallic con-taminant removal. In addition, the low acidbasebuffering capacity of kaolinite also contributes tothe higher heavy-metal removal efficiency for this

type of soil Hamed et al., 1991; Hicks and Ton-.dorf, 1994 . Soils with a high content of humic

substances have higher cation exchange and buf-fering capacity, which is why electrokinetic reme-

.diation efficiencies may decrease Table 1 .It is very important to improve the removal

efficiency of heavy metals from high sorption-capacity clays, such as illitic mixture, i.e. synclays.Despite all the earlier accomplishments, elec-trokinetic remediation of such soils still requireshigher current density, remediation time, energyexpenditure and costs in comparison to kaolinite .Puppala et al., 1997 .

7. Costbenefit analysis

There are several factors that influence thecost of the electrokinetic remediation process.

.These are as follows van Cauwenberghe, 1997 :

Soil characteristics and moisture content; Contaminant concentrations; Concentrations of non-target ions and con-

ductivity of the pore water; Depth of the remediated soil; Site preparation requirements; and Electricity and labour costs.

During numerous laboratory experiments, it wasdetermined that if the distance between elec-

trodes was 11.5 m, the total removal of heavymetals from contaminated soil would require ap-proximately 500 kW hm3 of energy. Energy ex-penditure is directly proportional to the completeremoval of contaminants from soil, i.e. remedia-

.tion time van Cauwenberghe, 1997 . The totalenergy consumption can be lowered if appropri-ate cathodic polarisation techniques are used .Acar and Alshawabkeh, 1997; Li and Li, 2000 .The migration rate of contaminants through thesoil matrix is approximately 23 cmday. If thedistance between the electrodes is 23 m, thetime frame for successful remediation would be

.more than 100 days van Cauwenberghe, 1997 .However, the use of a cation-selective membranereduces the remediation period to 1020 days.

The situation with in situ experiments is slightlydifferent. The main parameters that influence theoverall process cost are as follows:

Soil properties; Depth of contamination; Cost of accommodating electrodes and placing

treatment zones; Clean-up time; and Cost of labour and electrical power.

In order to avoid soil overheating and shorten therequired time frame, the cost-optimised distancebetween electrodes needs to be maintained at

36 m for most soils Lageman, 1993; Ho et al.,.1997, 1999 . Electrode construction costs account

for up to 40% of the overall remediation costs. .Other expenses are Ho et al., 1997 :

1015% for electricity; 17% for labour; 17% for materials; and Up to 16% for licenses and other fixed costs.

The first in situ electrokinetic remediationtechnique implemented, the LasagnaTM process,has reduced the clean-up time and power inputrequired, as well as the total costs, by insertingtreatment zones between the electrodes. Treat-ment zones diminish the need for above-groundwaste handling and are cheaper to implement


Table 2Costbenefit analysis of selected techniques

Technique Costs Remarks

3TMLasagna 50120 $USDm Mandreltremie-tube method ofapproximately Emplacement will be used instead .over 3 years of earlier proposed steel plate

Electrodes with wick drainsand carbon-filled treatment zone

3Soil heatingvapour extraction 65123 US$ydtechnology

3Chemical oxidation 130200 US$m Technique was mostly usedwith potassium permanganate to remove DNAPLs in situ

.or hydrogen peroxide

. .than electrodes Ho et al., 1997 . Ho et al. 1997have presented a comparison of the costbenefitanalysis for selected techniques, which is shownin Table 2.

8. Conclusions

Electrokinetic soil remediation is an emergingin situ technology with demonstrated efficiency toremediate fine-grained soils, and especially to re-

.move heavy metals from the soil matrix Table 3 .According to that stated in the articles re-

viewed, it is possible to draw the following conclu-sions on the main advantages of this technique:

Electrokinetics is very targetable to any speci-fic location, because treatment of the soil oc-curs only between two electrodes.

Electrokinetics is able to treat contaminatedsoil without excavation being necessary.

Electrokinetics is most effective in clay, be-cause it has a negative surface charge, and insoils with low hydraulic conductivity.

Electrokinetics is potentially effective in bothsaturated and unsaturated soils.

Electrokinetics is able to treat both organicand inorganic contaminants, such as heavymetals, nitrates, etc.

Electrokinetics demonstrated the ability to re-move contaminants from heterogeneous natu-ral deposits.

Good cost effectiveness.

Despite all the advantages, this technique hassome limitations, which are:

The solubility of the contaminant is highlydependent on the soil pH conditions.

The necessity to apply enhancing solution. When higher voltage is applied to the soil, the

Table 3Conclusions on heavy metal removal from contaminated soils

Metal Remarks

Lead and Successful removal is obtained only under acidic conditionsCadmium High removal rates were achieved with the use of HCl solution

. .Chromium Significant part of Cr VI is reduced to Cr III if there are sulfides or otherreducing agents present in the soilLow chromium migration was observed in the soil in the presence of sulfidesand no retardation in the soil with humic acid

Arsenic Sufficient arsenic removal is achieved only in alkaline conditionsMigration of arsenic is accelerated by an oxidising agent

Mercury Efficient mercury removal is achieved using I I lixiviant solution2Higher removal efficiency is obtained using chloride or other suitablecomponent added to the soil


efficiency of the process decreases due to theincreased temperature.

Removal efficiency is significantly reduced ifsoil contains carbonates and hematite, as wellas large rocks or gravel.

In order to guarantee efficient electrokineticremediation of soil, among other variables, it isimportant to investigate physicochemical con-taminantsoil interactions and the impact of en-hancing agents on these interactions, the occur-rence of reverse electroosmotic flow and the in-fluence of organic substances present in the re-mediated soil.

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Electrokinetic soil remediation - critical overviewIntroductionTheoretical, design and operational considerationsTheoretical aspectsDesign considerationsOperational considerations

Removal of metalsLimitations of the techniqueEnhancement and conditioning

Electrokinetic soil remediation processesRemoval of heavy metals using cation-selective membraneRemoval of heavy metals using surfactant-coated ceramic casingsLasagnaTM processElectro-KleanTM electrical separationElectrokinetic bioremediation 4.6. Electrochemical geooxidationElectrochemical geooxidationElectrochemical ion exchangeElectrosorbTM

Remediation of specific heavy metal contaminationRemoval of cadmium and leadLead migration in soilsCadmium migration in soils

Removal of arsenic and chromiumChromium migrationChromium removal from different soilsGlacial till.Kaolin.

Arsenic migration and removal

Removal of mercuryRemoval of zinc and copperOther metals

Heavy metal removal efficiency from contaminated soilsCost-benefit analysisConclusionsReferences

6660293 electrokinetic soil remediation review

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