ELECTROKINETIC REMEDIATION OF HEAVY METAL … · 2015-04-29 · 103 Int. J. Struct. & Civil Engg. Res. 2014 Sruthy O A and S Jayalekshmi, 2014 ELECTROKINETIC REMEDIATION OF HEAVY

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Int. J. Struct. & Civil Engg. Res. 2014 Sruthy O A and S Jayalekshmi, 2014

ELECTROKINETIC REMEDIATION OF HEAVY

METAL CONTAMINATED SOIL

Sruthy O A1* and S Jayalekshmi1

Heavy metal contamination is a major social issue in the era of urbanization and industrialization.Industries are practicing open dumping since it is an economic means of waste disposal. Wasteswhich contain heavy metal will contaminate soil as well as ground water. So soil remediation isnowadays gaining importance. Not only industries, but also households and vehicles causeheavy metal contamination. A number of methods are available for soil contamination. But 100%removal is not possible. But electrokinetic remediation method removes heavy metals effectivelybecause en external energy is applied in the form of current. This method does not cause anysecondary pollution. Various enhancement techniques are available to increase the removalefficiency. This article provides a brief description of different remediation techniques availableand various enhancement techniques practiced in electrokinetic remediation.

1 Department of Civil Engineering, National Institute of Technology, Tiruchirappalli-620015, Tamil Nadu, India.

*Corresponding author:Sruthy O A � [email protected]

ISSN 2319 – 6009 www.ijscer.com

Vol. 3, No. 1, February 2014

© 2014 IJSCER. All Rights Reserved

Int. J. Struct. & Civil Engg. Res. 2014

Review Article

Keywords: Heavy metal, Contaminated soil, Remediation Techniques, ElectrokineticRemediation, Enhancement techniques

INTRODUCTION

Due to rapid industrialization and urbanization,soils are increasingly getting polluted. Manyindustries are generating wastes which containheavy metals. Nowadays, industries arepracticing open dumping, since it is aneconomical means of waste disposal. Waste,which contains heavy metal, either in liquid orsolid form, will pollute soil and ground water,when it gets disposed off to the soil. It hasbecome a major social issue. So removal ofheavy metal from soil or decontamination ofsoil is now gaining importance.

Heavy metals consist of group of metals andmetalloids. Their atomic density is more than5 times that of water. Heavy metalcontamination of soil is a common occurrencein industrial areas and in areas with high trafficvolume. Heavy metal contamination of soiloccurs during mining, manufacturing, and theuse of synthetic products such as pesticides,paints, batteries, industrial waste, and landapplication of industrial or domestic sludge(USDA NRCS, 2000).

Old landfills with industrial wastes dumpedget contaminated. Fields and orchards

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sprayed with insecticides with arsenic, pastapplications of waste water or municipalsludge, areas in or around mining waste pilesand tailings, industrial areas with chemicalsmay have been dumped on the ground, or inareas downwind from industrial sites (USDANRCS, 2000).

The density of heavy metal is more than 5g/cm3. Most common problem causing heavymetals are Lead, Mercury, Cadmium, Arsenic,etc. Other problems associated with heavymetals is, it is non-biodegradable. Unlikecarbon based molecules, it remains in the soilfor decades (Giannis et al., 2009). So it isimportant to have an effective and economicalsoil remediation technique.

REMEDIATION TECHNIQUES

A number of technologies have beendeveloped in response to address increasingenvironmental problems. A particularcontaminated site requires a combination ofdifferent technologies (Khan et al., 2004). Soilremediation techniques can broadly bedivided into physical, chemical and biologicalremediation (Zhitong Yao et al., 2012). It canalso be divided into in-situ and ex-situtechniques. In many site physical, chemical andbiological methods are used in conjunctionwith one another to reduce the contaminantlevel (Khan et al., 2004).

Solidification/Stabilization

Solidification or stabilization technique is usedto remediate soil as well as sludge. It can alsobe used to treat hazardous residues of anyindustrial activity. By this method the metalmobility is decreased by physically restrictingthe contact between the contaminant and the

ground water or the chemical properties of thecontaminants are altered to restrict the contact.Thus waste will be converted to a less mobileform (Evanko and Dzombak, 1997). Thoughthese immobilization methods cannot removemetal from sediment, due to their low cost andfast remediation effect, they are still popularlyapplied (Peng et al., 2009).

Soil Washing

Soil washing is relatively a simple methodwhich is adopted to decontaminate soil.Washing water is added to remove thecontaminant from the soil (Peng et al., 2009).Commonly used washing agents are,surfactant, chelating agents, acids, etc.(Zhitong et al., 2012). Inorganic acids such assulphuric acid and hydrochloric acid chelatingagents such as ethylene diaminetetra aceticacid (EDTA) and nitrilotriacetate (NTA) organicacids such as acetic acid, citric acid etc., areexamples of washing agents used. Thismethod alone is sometimes used for removalof heavy metals. But more often it is combinedwith other technologies as a pretreatment orpost treatment technique (Jankaite andVasarevicius, 2005).

Vitrification

This is also an immobilization technique. Astrong energy source is applied to melt soil orother hard material (600-1200oC). By thismethod, organic contaminants are destroyedby pyrolysis and inorganic contaminants areimmobilized (Acar and Alshawabkeh, 1993).The contaminants/metals may get volatilizeddue to high temperature and that is collectedfor treatment or disposal. Though in-situprocesses are preferred due to lower energyconsumption, it can be performed both ex-situ

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and in-situ. Typical stages in ex situ vitrificationprocesses may include excavation,pretreatment, mixing, feeding, melting andvitrification, off-gas collection and treatment,and forming or casting of the melted product(Evanko and Dzombak, 1997). Energy forheating can be applied by means of fossil fuelburning or using electrodes, through arc,plasma and microwave energy in case of ex-situ remediation. Electrodes inserted in the soilprovide the required heat in case of in-situprocess (Zhitong et al., 2012).

Flotation

This is a separation method for hetero phasesystem. Gas bubbles are attached todispersed phase to form aggregates. Thenthese aggregates are floated, and thusseparated from the dispersing medium. Thismethod is mainly used in mining industry toseparate valuable metals (Peng et al., 2009).

Isolation

This is an in-situ remediation technology toprevent the movement of contaminant to theground water where other methods are notapplicable. The contaminated site is isolatedfrom ground water by containing thecontaminant in a designated area (Evanko etal., 1997).

Electrokinetic Remediation

This method can be used either in-situ or ex-situ. This is one of the most promisingtechnologies to remove the heavy metals fromthe contaminated soils. Remediation ispossible by the application of low voltageelectric field across the soil. An acid front isgenerated in the positively charged anodeelectrode and a base front is generated in the

negatively charged cathode electrode. Theacid and base fronts are formed because ofthe electrolysis reaction and are explained bygiven equations (Orcino et al., 1998).

2H2O – 4e– → O2 + 2OH

–

2H2O + 2e– → O2 + 2OH

–

Capping

The surface water infiltration into contaminantis restricted to prevent further release ofcontaminant into ground water. This barrier alsocontrols gas and odor emission, improvesaesthetics and provides a stable structure.Dermal contact of contaminant is alsoeliminated by this method (Evanko et al., 1997).

Biological Remediation

This method involves the utilization ofmicroorganisms or plants. Organisms or plantswere used to decontaminate soil, water, airand sludge. Pollutants are accumulated asbiomass. Traditional methods available for soilremediation are expensive. That leads to theintroduction of bioremediation. Broadlyspeaking, bioremediation involves all theprocesses necessary for the transformation ofa contaminated environment to its originalform.

The indigenous microorganisms will bestimulated for this. Microorganisms can bringchanges in valency of the heavy metal (e.g.,hexavalent Cr to trivalent Cr). Thus they canconvert it to a less toxic form. Also they canbio accumulate heavy metals on their surfaces.Some bacteria like sulphate reducing bacteriaconvert sulphate to hydrogen sulphide and thishydrogen sulphide precipitates heavy metalsas sulphides (e.g., Zn, Cd). In some casesbacteria may volatilize the heavy metal and

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remove the heavy metal from matrix. But onlya few heavy metals can be removed byvolatilization (e.g., Hg). But the heavy metalsare not actually removed from the site. Thoughthey are converted into less toxic form, theypersist in the soil. In this context,phytoremediation is practiced to extract theheavy metal from soil (Garbisu and Alkorta,2003).

Phytoremediation is a novel technologywithout the application of any chemical.Sunflower, Indian Mustard, etc., are commonlyused for phytoremediation of soil (Prasad,2007 and Falciglia and Vagliasindi, 2013).Sunflower (Helianthus annuus L.) is capableto extract heavy metals from soil as itsphytoextraction coefficient is very high. So itis now commonly used for environment studiesand remediation. Uranium contaminated soilwas remediated using Sunflower and theUranium was concentrated in roots of the plant(Prasad, 2007). But this method can’t beconsidered as an ultimate cleaner technologyas its efficacy depends on the contaminantconcentration and availability of contaminantto the root (Falciglia and Vagliasindi, 2013).But this method is very cheaper as the cost ofgrowing plants in the field is lesser than soilremoval and replacement. The main energysource for this method is sun, which alsoreduces the cost (Marques et al., 2009).

ELECTROKINETIC

REMEDIATION

Electrokinetic Remediation (EKR) is a greenremediation technology which has thefollowing advantages; (1) decreases on spotpollution; (2) reduces the remediation time; (3)lowers the cost; and (4) capable of treating low

permeability soil. So this method is nowgaining importance. A large number of studieshave been conducted on electrokineticremediation (Huang et al., 2012). Reuss(1808) identified the movement of watertowards the cathode under the influence ofelectric field. Geotechnical engineers identifiedthe use of electrokinetics for soil stabilization,slope stabilization and drainage in the late1930’s. But the use of this technology for soilremediation has been identified in 1980’s only(Azzam and Oey, 2000).

EKR involves the application of a lowvoltage DC across the soil by means ofpositive and negative electrodes. Under theapplied electric field following processes takeplace; electroosmosis, electromigration,electrophoresis. Electroosmosis is themovement of pore fluid which contains ionstowards the cathode. Electroosmosis is themovement of water molecule in appliedelectric field. Electromigration is the movementof dissolved ionic species to the respectiveelectrodes. Ions produced by water electrolysisare also migrated (i.e., H+ and OH–).Electrophoresis is the movement of chargedsoil particles towards the respectiveelectrodes (Acar et al., 1995 and Hansen etal., 1996).

First attempt of removal of heavy metal byelectrokinetics was done in 1980’s (Hansenet al., 1996). Henceforth a number of studieshad been conducted in laboratory undercontrolled condition on synthetic spiked soil.But a few experiments had been done directlyon polluted site. The success of this methoddepends on the type of soil, carbonate content,cation exchange capacity, applied current/

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voltage, pH of soil, duration of the experiment,use of complexing agents and surfactants(Hansen et al., 1999 and Jensen et al., 2007).

Ions are produced at anode and cathodeby water electrolysis. H+ ions are generated atthe anode and OH– ions are generated at thecathode. H+ ions generate at the anodemigrate to negatively charged cathode andOH– ions migrate towards positively chargedcathode. Thus an acidic front moves towardscathode and basic front towards anode. H+

ions are smaller than OH– ions. So themovement of H+ ions will be faster than that ofOH– ions. Acidic front solubilizes the metal ionsand causes its movement towards cathode byelectroosmosis or electromigration. On theother hand the basic front forms precipitatewith metal ions. Metal removal will be impededby this reaction. In order to prevent thegeneration of basic front at cathode, cathodecan be acidified. Electrolyte conditioningimproves the removal efficiency (Orcino andBricka, 1998).

RESEARCH WORKS AND

DEVELOPMENTS

Electrodialytic remediation (EDR) is amodification of EKR in which the electrodecompartment is separated from the soil bymeans of ion selective membrane. Whencurrent is applied to the soil, ion selectivemembranes prevent the entry of ionsgenerated at the electrodes to the soilcompartment. Thus current wasted for carryinghighly mobile ions from anode to cathode canbe reduced and the competition betweenhighly mobile ions and metal ions also can beminimized (Hansen et al., 1996).

Jensen et al. (2007) studied the effect ofsoil type in EDR. They conducted the study onten industrially polluted lead contaminatedsoils. They observed that the insoluble organicmatter and stable compounds reduce theremoval efficiency. They used cylindricalplexiglass tank and platinum coated electrodesfor their tests. A constant current (0.2 mA/cm2)was maintained in the reactor. If the pollutingmetal species is very high, remediationproceeds very slowly. Soil which contains lessorganic matter can easily be remediated bythis method. In carbonate rich soil, dissolutionof carbonates takes place first. Thus thecarbonate content of pore fluid increases.

Precipitation and dissolution of metal ionsaffect the removal efficiency. Due to waterelectrolysis, H+ ions are generated at anodeand OH– ions are generated at cathode. If thebuffering capacity of the soil is high, themovement of hydrogen ions will be retarded.The presence OH– ions will increase the pH(10-12). Metal ions will get precipitated ashydroxide reducing the removal efficiency.Enhancing agents can be used to increase theremoval efficiency. Enhancing solutions canform complexes with metal and thesecomplexes can be removed easily. But someenhancing solutions change thephysicochemical properties (such as pH,redox potential) of soil and thus givingimproved removal efficiency (Hosseini et al.,2011). Hosseini et al. studied the effect ofchelating agents (EDTA, ammonium citrate)on the removal of Zn and Pb from the soil. Theycollected soil samples from a mine site.Electrokinetic experiments were conducted ina 40 × 10 × 10 cm plexiglass box with graphiteelectrodes (25 V DC). The soil they used was

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categorized as clay. Initial pH of the soil was7.6 pH changed considerably with water andEDTA as electrolytes. But pH didn’t vary muchwith citric acid. Use of complexing agentincreased the removal efficiency. EDTA formsnegatively charged complex with metal ionsand this complex can be collected at anode.EDTA is more effective in the removal of lead.Removal of zinc was more than lead in theirexperiment.

Giannis et al. (2009) also studied the effectof chelating agents in EKR. They carried outan extensive experimental program to removePb, Cd and Cu from contaminated soil. Theyused nitrilotriacetic acid (NTA),diethylenetriaminepentaacetic acid (DTPA)and ethyleneglycoltetraacetic acid (EGTA) aswashing solutions under different pHconditions and concentrations. They got theoptimum pH and concentration of washingagent from batch study results. They used a17 cm long soil cell with 10 cm × 10 cm crosssection. The working electrodes werecylindrical graphite electrodes. A neutral pHwas maintained in the cell. Results showedthat the extraction efficiency for Cd indecreasing order was NTA > EGTA >DTPA,while for Pb and Cu it was DTPA > NTA>EGTA. They observed that, higher washingagent didn’t necessarily increase the removalefficiency. The observed removal efficiency forCd was 65-95%, for Cu 15-60%, but for Pbwas less than 20%.

Solar energy can also be used for EKRexperiment (Yuan et al., 2009). Yuan et al.used solar cell for supplying energy requiredfor EKR. They conducted three set ofexperiments. One was conducted on a sunnyday and another on a cloudy day and the third

one with normal DC power supply. Theyconducted studies on Cd spiked soil. Theyused a reactor cell with an inner size of length11.1 cm × width 5.4 cm × height 5.0 cm.Graphite sheets were uses as anode andcathode. A solar cell of 30.6 cm length and 21.5cm width was used to supply power. The studyresults indicated that the current achieved bywas comparable with DC power. Out potentialof solar cell depended on daytime and weatherconditions.

The H+ ions and OH– ions migrate towardscathode and anode respectively. During theirmigration they meet at a point were focusingof heavy metal takes place. At this point heavymetal accumulates and the removal efficiencywill be slackened. Use of complexing agents,electrolyte washing and use of ion selectivemembrane are practiced to overcome thisdifficulty. But these methods are deployed tochange the pH. Li et al. introduced anapproaching anode method to migrate thefocusing point to anode in a step by stepmanner. They used a rectangular tank reactor.The reactor was made of Plexiglass with aninner length of 43 cm, a width of 10 cm, and aheight of 10 cm. Graphite electrodes wereused as working electrodes. They conducted4 sets of experiment, 2 with fixed electrodesand 2 with variable electrodes. Acetic acid andcitric acid were used as electrolyte. Theyconducted tests on Chromium contaminatedsoil. Citric acid removed more Cr than aceticacid. Approaching cathode experimentsshowed higher removal efficiency and. Theaccumulation of chromium was alsodecreased.

Sun and Ottosen (2012) studied the effectsof pulse current on energy consumption and

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removal of heavy metals during electrodialyticsoil remediation. They conducted eightexperiments with constant and pulse currenton industrially polluted soils. Electrokinetic cellwith 3 compartments was used for tests. Ionselective membranes were also used.Platinum coated electrodes were used asworking electrodes. A constant current wasmaintained across the soil in constant currenttest. A power supply timer instrument was usedto provide pulse current. Electrolyte used wasNaNO3 adjusted to pH 2 with HNO3. Results ofthis study indicate that the power consumptiondecreased at high current density with pulsecurrent and the removal efficiency alsoincreased.

Chung and Kamon (2004) studied thecoupled effect of electrokinetics and ultrasonic(US) technique in cleanup of contaminatedsoil. Electrokinetic method removed heavymetal and ultrasonic technique removed theorganic matter. Natural clay contaminated withPb and phenanthrene was used as the testspecimen. Two types of tests were carried out.Tests included EKR and EKR+US. Graphiteelectrodes were used as anode and cathode.A constant current of 50 mA was applied. Thetest chamber is made of a plexiglass and ithas a 10 cm wide, a 10 cm height and a 20cm length. Inlet and outlet tubes were alsoinstalled at both the ends. Outflow wascollected and tested regularly. The water andcontaminant flowed under the effect of electricfield and acoustic flow by sound waves. Theaccumulated outflow and contaminant removalwere enhanced by sonication effect. Theremoval rates of Pb and phenanthrene areaverage 88% and 85% for electrokinetic testand average 91% and 90% for electrokinetic

and ultrasonic test, thus the removalefficiencies in electrokinetic and ultrasonicprocess are increased about 3.4% for Pb and5.9% for phenanthrene by the coupled effectsof electrokinetic and ultrasonic phenomenacomparing with simple electrokinetic process.

CONCLUSION

Electrokinetic remediation is an effectivemethod for the remediation of heavy metalcontaminated soil. This method mainlydepends on the soil type, soil pH, appliedcurrent, presence of carbonates, bufferingcapacity of soil. Generation of acid front in thecathode compartment impedes the removalefficiency. This situation can be alleviated bycertain enhancement technique. With thesupport of enhancement technique it ispossible to achieve comparatively highremoval efficiency. A brief description of someenhancement techniques is given in this paperand other available remediation techniquesare also explained.

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ELECTROKINETIC REMEDIATION OF HEAVY METAL … · 2015-04-29 · 103 Int. J. Struct. & Civil Engg. Res. 2014 Sruthy O A and S Jayalekshmi, 2014 ELECTROKINETIC REMEDIATION OF HEAVY