This article was downloaded by: [Duke University Medical Center]On: 11 October 2014, At: 14:24Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

Journal of the Air & Waste Management AssociationPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/uawm20

Preliminary Assessment of ElectrokineticRemediation of Soil and Sludge Contaminated withMixed WasteKrishna R. Reddy a , Matthew Donahue b , Richard E. Saichek a & Robin Sasaoka ca Department of Civil and Materials Engineering , University of Illinois at Chicago ,Chicago , Illinois , USAb Geosyntec Associates , Huntington Beach , California , USAc U.S. Army Corps of Engineers , Chicago , Illinois , USAPublished online: 27 Dec 2011.

To cite this article: Krishna R. Reddy , Matthew Donahue , Richard E. Saichek & Robin Sasaoka (1999) PreliminaryAssessment of Electrokinetic Remediation of Soil and Sludge Contaminated with Mixed Waste, Journal of the Air &Waste Management Association, 49:7, 823-830, DOI: 10.1080/10473289.1999.10463849

To link to this article: http://dx.doi.org/10.1080/10473289.1999.10463849

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purposeof the Content. Any opinions and views expressed in this publication are the opinions and views of theauthors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content shouldnot be relied upon and should be independently verified with primary sources of information. Taylorand Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses,damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connectionwith, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Reddy et al.

Volume 49 July 1999 Journal of the Air & Waste Management Association 823

ISSN 1047-3289 J. Air & Waste Manage. Assoc. 49:823-830

Copyright 1999 Air & Waste Management Association

TECHNICAL PAPER

Preliminary Assessment of Electrokinetic Remediation of Soiland Sludge Contaminated with Mixed Waste

Krishna R. ReddyDepartment of Civil and Materials Engineering, University of Illinois at Chicago, Chicago, Illinois

Matthew DonahueGeosyntec Associates, Huntington Beach, California

Richard E. SaichekDepartment of Civil and Materials Engineering, University of Illinois at Chicago, Chicago, Illinois

Robin SasaokaU.S. Army Corps of Engineers, Chicago, Illinois

ABSTRACTIn order to avoid the effects of complex soil composition orcontaminant interaction, previous bench-scale electrokineticexperiments were generally performed using simplified con-ditions. An ideal soil such as kaolinite was often used, andtesting was frequently restricted to an individual contami-nant or a small group of contaminants. However, actual wastesites consist of soils that are usually quite different from ka-olinite, and many sites are polluted by a large number ofmixed contaminants. Therefore, this preliminary study wasundertaken to assess electrokinetic performance on a site-specific field soil and a simulated sludge mixture containingmixed wastes in the form of metals, organic compounds,and radionuclides. Bench-scale experiments showed that thefield soil had a high buffering capacity that resulted in highpH conditions throughout the soil, whereas the simulatedsludge had a low buffering capacity that resulted in low pH

IMPLICATIONSElectrokinetics is an innovative, versatile, and economicaltechnology for remediating contaminated waste sites. Itmay be used to remediate soils, sludges, and groundwa-ter that have been contaminated with metals, organic com-pounds, radionuclides, or a combination of these. Previ-ous investigations involving bench-scale tests have shownpromising results, but they were often deficient in repre-senting actual field conditions. In this study, bench-scaleelectrokinetic experiments were performed using a site-specific field soil and a simulated sludge mixture contain-ing various mixed wastes. The soil, sludge, contaminants,and their concentrations were based upon the actual con-ditions existing at a U.S. Department of Energy waste site.

conditions except near the cathode. The high pH conditionsin the soil allowed the migration of anionic metallic con-taminants, such as hexavalent chromium, but inhibited themigration of cationic metallic contaminants, such as cad-mium. The low pH conditions in the sludge allowed simul-taneous migration of both anionic and cationic contami-nants in opposite directions, respectively, but the synergisticeffects of co-contaminants retarded contaminant removal.The removal of organic compounds and radionuclides fromboth the soil and the sludge were achieved. However, addi-tional research is warranted to systematically investigate thesynergistic effects and the fate of different contaminants aswell as to develop electrode-conditioning systems that en-hance contaminant migration.

INTRODUCTIONThe U.S. Department of Energy (DOE) has estimated that it isresponsible for over 3,700 buried waste sites located through-out the United States.1 At a majority of these sites, the soilsand/or groundwater have been contaminated due to the pres-ence of leaking underground storage tanks (LUSTs). Becausethe LUSTs often are very large in size, even a small, slow leakmay lead to a significant amount of contaminant introduc-tion into the subsurface. The bulk of the waste material re-mains inside the tank and commonly exists in the form of asolid and liquid mixture, or sludge mixture. Due to the widerange of toxic contaminants and the high concentrations in-volved, most DOE waste sites require remediation to elimi-nate the risks to public health and/or the environment. Thesesites vary in size from a few acres to several hundred acres,and the specific site conditions are often extremely diverse.Frequently, the sites are located in fine-grained soils, such as

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4

Reddy et al.

824 Journal of the Air & Waste Management Association Volume 49 July 1999

silt or clay, and the resulting contaminationmay consist of mixed wastes such as radio-nuclides, metals, and organic compounds.Furthermore, since most remediation tech-nologies are ineffective or inefficient in treat-ing fine-grained soils contaminated withmixed wastes, DOE sites are often highlychallenging to remediate.

Electrokinetic remediation offers greatpotential for treating fine-grained soils andother contaminated media such as sludge.In addition, it has the potential to be ef-fective in removing a number of differentcontaminants, including metals, organiccompounds, radionuclides, or a combina-tion of these. Electrokinetic remediationmay be performed as either an in situ tech-nique, eliminating excavation costs andgreatly reducing the risk to onsite person-nel, or as an ex situ technique, in whichexcavated soils and/or pumped-out sludgecan be treated in onsite reactors. Electro-kinetic remediation requires a conductingpore fluid within the soil mass which can be either water ora selected liquid introduced at the electrodes. When select-ing the pore fluid, the transport processes and the chemi-cal reactions that occur within the contaminated mediamust be considered so that the remedial efficiency of theprocess is improved.

Since electrokinetics involves complex, coupled electro-chemical and geochemical processes that occur under an in-duced electric potential, previous investigations involvingbench-scale testing have often been simplified in order todevelop a fundamental understanding of the process. Themajority of these investigations used ideal soils, such as ka-olinite, and a single contaminant. Investigations using ac-tual field soils or soils containing multiple contaminants aregenerally very limited. In this study, preliminary bench-scaleexperiments were performed to determine the potential useof electrokinetics to remediate a contaminated soil and acontaminated sludge encountered at a DOE waste site. Thebrown silty loam/silty clay loam soil used for this study wasobtained from an uncontaminated area near an actual wastesite. Several LUSTs holding a sludge mixture were also lo-cated at the site. The sludge contained contaminants similarto those found in the soil, but in different concentrations.Since a sample of the actual sludge mixture could not beobtained, a sludge mixture was prepared using kaolinite anddeionized water to simulate actual sludge conditions. Thesite-specific soil and simulated sludge mixture were then bothindividually spiked in the laboratory with mixed contami-nants. These spiked contaminants and their concentrationswere selected based on the conditions known to exist at the

waste site. Table 1 lists these contaminants and their respec-tive concentrations in the soil or sludge. The experimentalresults were then assessed to determine the potential of us-ing electrokinetic remediation at the DOE waste site.

ELECTROKINETICS: AN OVERVIEWA comprehensive review of the basic electrokinetic prin-ciples has been reported by several researchers.2-8 Funda-mentally, electrokinetic remediation involves applying alow DC current or low voltage gradient across electrodesthat encompass the contaminated media, as shown in Fig-ure 1. The induced low voltage gradient causes electroly-sis of the water molecules at the electrodes. At the anode,hydrogen (H+) ions and oxygen gas are generated, so thepH decreases and acidic conditions exist. At the cathode,hydroxyl (OH-) ions and hydrogen gas are generated, sothe pH increases and basic, or alkaline, conditions exist.As electrolysis occurs, the generated gases are usually al-lowed to escape into the atmosphere, and the H+ and OH-

ions migrate toward the oppositely charged electrode. TheH+ ions are smaller and have greater mobility than theOH- ions, and, as a result, they migrate much faster. Theextent of acid front migration from the anode toward thecathode, and the extent of base front migration from thecathode toward the anode, depend on the soil’s bufferingcapacity. When the two fronts meet, water molecules areagain formed.

The pH changes that occur in the soil or sludge maysignificantly affect the electrokinetic remediation process.For instance, many heavy metal contaminants desorb,

Table 1. Contaminant concentrations in soil and sludge.

Contaminant Group Contaminant Concentration in Concentration in

Soil (mg/kg) Sludge (mg/kg)

Metals Cadmium 253 121

Calcium 35,300 35,300

Chromium (hexavalent) 475 398

Lead 343 282

Magnesium 29 6,290

Volatile organic Benzene 5 5

compounds 1,1-Dichloroethane 5 5

Radionuclides Cerium 156 237.5

Cesium 850 875

Cobalt 411 515

Iron (metallic) 12.5 19

Lanthanum 68.6 104.5

Neodymium 56.2 85.5

Praseodymium 18.7 28.5

Strontium 591 781

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4

Reddy et al.

Volume 49 July 1999 Journal of the Air & Waste Management Association 825

solubilize, and migrate when acidic conditions exist, butwhen they enter into basic conditions, the contaminantsadsorb to soil particles or precipitate. Consequently, soilsor sludges with a high acid buffering capacity can neutral-ize acidic conditions and severely limit contaminant mi-gration. Additionally, some metallic contaminants, suchas chromium, can exist in different valence states and maypossess quite different characteristics under different pHconditions. For example, chromium in hexavalent formexists as oxyanions and, therefore, it exists in a solubleform over a wide pH range. Alternatively, chromium intrivalent form exists as hydroxides that adsorb to the soilsignificantly as the soil pH increases. Therefore, redox re-actions and speciation of contaminants are important con-siderations. In summary, the remediation of most metal-lic contaminants is accomplished by desorption and solu-bilization, and these processes depend on the pH changescaused by the electrolytic acid front and redox chemistry.The specific site conditions, such as the type of contami-nants present and the composition of the contaminatedmedia, significantly affect the electrokinetic process.

The principal contaminant transport mechanismsthat occur during electrokinetics are electroosmosis,electromigration, electrophoresis, and diffusion. Briefly,electroosmosis refers to the bulk movement of pore wa-ter that causes the advective transport of contaminants.The negative electrical charge of the soil particles attractsexcess cations, and when an electric potential is applied,the cations produce an electroosmotic flow as they movetoward the cathode. Electromigration is the movementof charged, solubilized contaminants or chemical spe-cies, and it is the primary mechanism responsible forionic chemical species migration. Electrophoresis is themigration of charged particles, aggregates, or colloids inrelation to the pore water. Finally, diffusion involves themovement of contaminants from areas of high concen-tration to areas of low concentration.

Although electrokinetic remediation is relatively easyto implement, its performance is difficult to predict becauseof inadequate understanding of the effects of electrochemi-cal and geochemical processes on the contaminant trans-port mechanisms. The composition of the contaminatedmedia, soil or sludge, is often complex due to the presenceof a variety of minerals and different percentages of solidsand liquids. Furthermore, the presence of mixed contami-nants may also lead to complex synergistic effects. Thus, mostprevious investigations involving bench-scale testing con-sidered simplified conditions, such as homogeneous soil anda single contaminant, so that the electrokinetic process couldbe better understood. Obviously, in order to completely com-prehend the electrokinetic process, this technology must beassessed under actual site conditions. Therefore, this prelimi-nary study is unique in that the bench-scale tests employeda site-specific soil and a sludge that were spiked with variouscontaminants in different concentrations that existed at anactual DOE waste site.

BENCH-SCALE TESTING PROCEDUREPreparation of Contaminated Soil

The soil used was a brown silty loam/silty clay loam com-posed of approximately 75% silt and clay and 25% sand.The initial pH of the soil was 7.8, and the percentage ofcarbonates as CaCO3 equivalents was approximately 5.8%.The contaminants were individually dissolved in deion-ized water and then added to one kilogram of oven-driedsoil for each test to obtain the contaminant concentra-tion levels shown in Table 1. The amount of deionizedwater added to the contaminated soil provided an initialsoil moisture content of approximately 30%. As the con-taminants and water were added, the soil was thoroughlymixed with a stainless steel spoon in a plastic containerto achieve a uniform distribution of the contaminants.After mixing, the contaminated soil was allowed to equili-brate for 24 hr.

(a)

(b)

Figure 1. Schematic of electrokinetic remediation: (a) in situimplementation and (b) ex situ implementation.

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4

Reddy et al.

826 Journal of the Air & Waste Management Association Volume 49 July 1999

Preparation of Contaminated SludgeThe simulated sludge mixture was prepared based on thecharacteristics of the actual sludge present in LUSTs at a DOEwaste site. The sludge mixture contained 50% liquid and50% sludge, and the sludge portion was further composedof 50% liquid and 50% solids. In the simulated sludge por-tion, the solids were composed of the solid contaminantsand kaolinite, and the liquids were composed of the liquidcontaminants and deionized water. The final simulated sludgemixture used deionized water for the remaining liquid por-tion and featured the contaminants and concentrations listedin Table 1. These ingredients were thoroughly mixed in aglass container with a stainless steel spoon and allowed toequilibrate for 24 hr.

Testing of the Contaminated SoilThe electrokinetic testing was performed using an electroki-netic reactor developed by Reddy and co-investigators.8-10 Aschematic diagram of this reactor is shown in Figure 2. First,the contaminated soil was compacted into the cylindricalPlexiglas cell. Then a piece of filter paper, a porous stone, andan electrode were placed at each end of the cell. Plexiglas capswere used to seal the ends. Next, deionized water was added tothe anode and cathode reservoirs, and the water levels in thereservoirs were maintained at the same elevation to prevent ahydraulic head from forming across the sample. Using a DCsource, the soil sample was subjected to an electric potential of1.31 volts/cm for 92 hr. After the testing period, the soil was

removed and sectioned from 0 to 4 cm, 4 to 8 cm, 8 to 12 cm,12 to 15.5 cm and 15.5 to 19.1 cm from the cathode end of thecell, respectively. In addition, liquid samples were collectedfrom the anode and cathode compartments and reservoirs.

Testing of the Contaminated SludgeFirst, a piece of filter paper, a porous stone, and an elec-trode were placed at one end of the electrokinetic cell andsubsequently sealed with a Plexiglas cap. The contami-nated, simulated sludge mixture was poured through theopen end into the cell and sealed using a filter paper, aporous stone, an electrode, and a Plexiglas cap. Next,deionized water was added to the anode and cathode res-ervoirs, and the water levels were maintained at the sameelevation to prevent a hydraulic head from forming acrossthe sample. Then, using a DC source, the sludge samplewas subjected to an electric potential of 1.05 volts/cm for48 hr. After the testing period, the sludge was separatedinto four parts measured from 0 to 5 cm, 5 to 10 cm, 10 to15 cm, and 15 to 19.1 cm from the cathode end of thecell, respectively. In addition, liquid samples were collectedfrom the anode and cathode compartments and reservoirs.

Analytical TestingAnalytical testing was performed on both the soil and sludgesamples to determine the contaminant concentration dis-tribution present after electrokinetic treatment. The aque-ous samples from the anode and cathode compartmentsand reservoirs were also analyzed to determine the con-taminant concentrations present. To determine the totalmetal concentrations in both the soil and the sludge, aportion of each sample section was acid-digested, using U.S.Environmental Protection Agency (EPA) method 3080.11 Thesamples were then centrifuged and filtered, and the super-natant was analyzed for metal concentrations using atomicabsorption spectrophotometry (AAS). The aqueous samplesfrom the electrode reservoirs were directly analyzed for metalconcentrations using AAS. To determine the organic con-taminant concentrations, the soil and sludge samples, aswell as the aqueous samples, were tested using gas chroma-tography. The amount of radionuclides present in both thesoil and the sludge samples was determined by TeledyneTesting Company in Northbrook, IL.

RESULTS AND DISCUSSIONpH Distribution

The final pH values at the completion of the field soil andsimulated sludge bench-scale electrokinetic experiments areshown in parts (a) and (b) of Figure 3, respectively. In addi-tion to the pH of soil and sludge sections, these figures alsoexhibit the final pH values of the liquid remaining in theanode and cathode compartments. As expected from pre-vious bench-scale testing, the electrolytic reactions increased

(a)

(b)

Figure 2. Schematic of laboratory electrokinetic reactor: (a) overallsetup and (b) reactor details.

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4

Reddy et al.

Volume 49 July 1999 Journal of the Air & Waste Management Association 827

pH at the cathode to approximately 12 and decreased pHat the anode to approximately 2.0.2-7

Figure 3(a) shows that the final pH values of the fivefield soil sections were approximately equal to the initialsoil pH. The initial pH of the spiked contaminated soilwas 7.5, and, after testing, the pH in all five sections re-mained in the range of 7.0 to 7.5. The insignificant changein the soil pH is attributed to the acid buffering capacityof the soil. Although the electrolysis reactions decreasedthe pH of the liquid in the anode compartment to a valueof approximately 2.0, the soil immediately adjacent tothe compartment had a value of over 7.5. Such a differ-ence in pH between the anode compartment liquid andthe adjacent soil section was also observed in previousstudies using glacial till that had a high acid bufferingcapacity.8 Therefore, due to the acid buffering reactionsin the soil, the acidic front, or H+ ions, generated duringelectrolysis at the anode were quickly neutralized. Figure3(b), however, exhibits quite a different pH distributionin the sludge. After being spiked with contaminants, the

simulated sludge had an initial pH of 2.9. After the elec-trokinetic treatment, the pH was significantly lower than2.9 for sludge sections two through four, indicating thatthe acidic front migrated from the anode compartmentto within the vicinity of the cathode compartment. Thepoint where the pH dramatically increases indicates theapproximate location where the acid front was neutral-ized by the base front.

Titration analyses were performed to assess the acidbuffering capacities of the soil and the sludge. Both thecontaminant-spiked field soil and the clean field soil wereanalyzed for comparison purposes. For titration analysis,20 g of dry soil or dry sludge were placed in a glass beakerthat contained 300 mL of deionized water with an initialpH of 6.1. The soil or sludge and water were mixed for 30min using a magnetic stirrer, and then the pH of the mix-ture was measured. One mL of 12.1N hydrochloric acidwas added to the soil or sludge slurry, it was mixed for anadditional 30 min, and then the pH of the slurry was mea-sured again. This procedure of adding 1-mL increments ofHCl, mixing the slurry for 30 min, and then measuring thepH, was repeated. The results of these analyses for the cleanand spiked field soils and spiked sludge are shown in Fig-ure 4. The figure shows that the clean field soil had a fairlyhigh acid buffering capacity that neutralized approximately4 mL of HCl before the pH stabilized at a value of about2.0. These results are reasonable, since the clean soil con-tained about 5.8% carbonates as CaCO3 equivalents. Thecontaminant spiked field soil had a slightly higher buffer-ing capacity because some of the contaminants were car-bonate chemical species that were able to neutralize addi-tional acid. The sludge had an initial pH of 2.9 and theaddition of 1 mL of HCl caused the pH to drop to 1.7. ThepH of the sludge slightly decreased with additional 1-mLincrements of HCl. These results clearly showed that thesludge possesses a significantly lower acid buffering capac-ity as compared with that of the field soil.

(a)

(b)

Figure 3. pH profile in (a) soil and (b) sludge. Figure 4. Buffering capacity test results.

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4

Reddy et al.

828 Journal of the Air & Waste Management Association Volume 49 July 1999

Contaminant Migration/RemovalAfter the completion of the bench-scale testing, chemicalconcentrations in each soil and sludge section were mea-sured. The normalized concentration or concentration ra-tio (final/initial) of three metallic contaminants—cadmium,calcium, and chromium—were plotted versus their sectionlocation within the electrokinetic cell. This is depicted inparts (a) and (b) of Figure 5 for the field soil and the simu-lated sludge, respectively. Generally, under low pH condi-tions, cadmium and calcium exist in ionic form in solutionand migrate toward the cathode due to their positive charge.However, the chromium was initially introduced inhexavalent form (Cr VI), and, since it commonly exists asoxyanions such as CrO2

2- at a high pH and HCrO4- at a low

pH, these ions are expected to migrate toward the anodedue to their negative charge.

The concentration profiles shown in Figure 5(a) indi-cate that cadmium and calcium did not migrate signifi-cantly in the soil. The cadmium ratio approached or ex-ceeded 1.0 in the first, third, fourth, and fifth sections of

the electrokinetic cell, and it only decreased significantlyin the second section. The calcium concentration ratiofollowed a similar trend, but its ratio was lower than cad-mium in all the sections, especially in fourth section. Dueto the high acid buffering capacity of the field soil, andsince the pH did not drop below 7.0 (Figure 3a), cadmiumand calcium desorption and solubilization did not likelyoccur. On the other hand, the chromium concentrationratios, as shown in Figure 5(a), were much lower than 1.0throughout the cell; therefore, they indicate that signifi-cant chromium was removed from the soil. This higherchromium removal is attributed to the fact that chromiumwas introduced initially as hexavalent chromium, whichexists mostly as aqueous CrO2

2- at high pH conditions.The concentration ratio is higher in the first, third, andfifth sections and lower in the second and fourth sections,indicating that chromium migration occurred readily fromthe anode regions of the soil into the anode compart-ment, and the chromium from the cathode regions at-tempted to migrate toward the anode. Overall, Figure 5(a)shows that significant cationic metallic contaminant mi-gration did not occur in the field soil due to high soil pHas a result of the soil’s high acid buffering capacity.

Figure 5(b) shows that the cadmium concentrationsincreased significantly from the anode to the cathode, and,thus, indicates a significant migration of cadmium towardthe cathode in the sludge. On the other hand, Figure 5(b)shows that chromium concentrations increased signifi-cantly from the cathode to the anode, indicating that chro-mium migration occurred toward the anode. Very low con-centrations of chromium in the first two sections near thecathode indicate that most of the chromium from thesesections had moved toward the anode regions. As shownin Figure 5(b), the calcium concentration ratio did not dis-play a noticeable migration pattern, and it remained fairlyclose to 1.0. Overall, Figure 5(b) shows that simultaneousmigration of cadmium and chromium occurred in oppo-site directions in the simulated sludge; however, calciummigration did not occur.

In order to assess the total electrokinetic removal effi-ciencies for cadmium, calcium, and chromium, all sectionsof the field soil were combined, mixed thoroughly, and thenanalyzed for those specific contaminant concentrations. Thisprocedure was performed to eliminate the anomalies in as-suming a homogeneous contaminant dispersement in dif-ferent sections. The same procedure was followed for thesimulated sludge. The contaminant removal efficiency wascalculated by dividing the contaminant mass removed fromthe soil or sludge by the corresponding initial contaminantmass. A summary of these removal efficiency values for thefield soil and the simulated sludge is shown in Figure 6. Theremoval efficiencies ranged from a low of approximately 5%for calcium to a high of about 70% for hexavalent chromium.

(a)

(b)

Figure 5. Inorganic contaminant concentration profiles in (a) soiland (b) sludge.

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4

Reddy et al.

Volume 49 July 1999 Journal of the Air & Waste Management Association 829

Figure 6. Comparison of inorganic contaminant removalefficiencies.

Figure 7. Variation of water content in (a) soil and (b) sludge.

(a)

(b)

Both of these results were observed in the simulated sludgeexperiment. Of the contaminants in the field soil test, cal-cium had the highest removal efficiency, with a value of ap-proximately 35%.

The lower removal efficiencies of cadmium in the soilare attributed to its precipitation due to high soil pH.Chromium migration in the soil was clearly evident be-cause of the presence of aqueous Cr(VI) under high pHconditions. The initial concentration of calcium was sig-nificantly greater than the other contaminants, and theform in which it exists and its synergistic effects need tobe further investigated in order to explain low migrationbehavior. The low pH conditions within the sludge, asseen in Figure 3(b), allowed cationic metallic contami-nants to exist in aqueous form and contributed to highermigration. Overall, both bench-scale tests demonstratedat least some contaminant mobilization and migration.

The two volatile organic compounds (VOCs), benzeneand 1,1-dichloroethane, were initially present in very lowconcentrations (5 mg/kg) in both the field soil and the simu-lated sludge experiments. After the electrokinetic treatment,the concentrations of these VOCs were measured in boththe electrode aqueous solutions and the individual sectionsof the soil and the sludge. The measured concentrationswere found to be below the detection limits. These con-taminants may have volatilized during sample preparationand/or analytical testing. Thus, the migration behaviorsand the removal efficiencies of these organic compoundswere inconclusive. However, note that electroosmosis isthe major mechanism responsible for the migration andremoval of organic compounds during the electrokineticprocess. Outflow measurements were made in each bench-scale test to assess the rates of electroosmotic flow in thesoil and the sludge. The measured average electroosmoticflow rates were approximately 1.6 mL/hr for the field soiland approximately 12.1 mL/hr for the simulated sludge.

Additionally, the water content in each section was mea-sured at the completion of each experiment, and thesevalues are shown in parts (a) and (b) of Figure 7 for thefield soil and simulated sludge, respectively. These resultsindicate that the water content of the soil remained fairlyconstant, but a slight reduction in the water content in thesludge occurred. Overall, the electroosmotic flow rates in-dicate that migration and removal of organic compoundswill be possible due to advective transport as a result ofelectroosmotic flow.

The concentrations of different radionuclides weremeasured in the composite samples of the soil and thesludge and are summarized in Table 2. As seen in the table,the radionuclide concentrations were below the detectionlimits, except for the Cs-137 in the soil, which had a verylow detectable concentration. Unfortunately, the liquidsfrom the electrode reservoirs were not analyzed in orderto perform a mass balance. Although not conclusive, elec-trokinetics appears to have removed the radionuclidesfrom the soil and the sludge. Additional testing is requiredin order to determine the fate of these contaminants.

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4

Reddy et al.

830 Journal of the Air & Waste Management Association Volume 49 July 1999

With the exception of the hexavalent chromium in thesimulated sludge experiment, the removal efficiencies ofmetallic contaminants were relatively low. However, thesebench-scale tests were performed to provide preliminary in-formation on the transport of the multiple contaminants inthe field soil and the simulated sludge. Several electrode con-ditioning systems used to enhance the migration and re-moval of metallic contaminants have been reported in theliterature.2,3 These systems utilize weak acids, surfactants, orchealating agents in the electrode compartments. In addi-tion, the use of remediation programs greater in duration, orthe employment of higher voltage gradients, may also in-crease the removal efficiency. Additional research is warrantedto determine the optimal approach to achieve high removalefficiency. Note that the soil and sludge were spiked withthe contaminants, but they may not reflect the aged fieldcontamination. Therefore, a systematic, comprehensive test-ing program involving actual field contaminated soil orsludge will be essential in order to demonstrate that electro-kinetic remediation is potentially applicable for situationswhen mixed contaminants exist.

CONCLUSIONPreliminary, laboratory bench-scale tests were performed ona field soil and a simulated sludge that were spiked with sev-eral different types of contaminants including metals, or-ganics, and radionuclides. These experiments were conductedto assess the applicability of electrokinetics for theremediation of complex contaminated waste conditions thatexist at a DOE waste site. Both experiments employed a DCvoltage source; the field soil experiment was performed witha voltage gradient of 1.31 V/cm for approximately 92 hr,and the simulated sludge experiment was performed with avoltage gradient of 1.05 V/cm for 48 hr.

About the AuthorsKrishna R. Reddy is an assistant professor of civil and environ-mental engineering in the Department of Civil and MaterialsEngineering at the University of Illinois at Chicago (UIC). Hisresearch expertise includes soil and groundwater remediation,waste containment systems, and reuse of waste and scrapmaterials in engineering applications. Matthew Donahue is acivil engineer with Geosyntec Associates in Huntington Beach,CA. Richard E. Saichek is a doctoral graduate student in theDepartment of Civil and Materials Engineering at UIC. RobinSasaoka is an environmental engineer with the U.S. Army Corpsof Engineers, Chicago District.

The contaminated field soil possessed a high acid bufferingcapacity that significantly inhibited the formation of an acidfront. As a result, desorption and solubilization of the metalliccontaminants did not occur. However, in the simulated sludgeexperiment, acid front migration was evident, and significantcontaminant migration and removal efficiencies were achieved.Each of the experiments resulted in the simultaneous transportof several types of contaminants, which was encouraging, butsystem enhancements are needed to increase contaminant re-moval efficiencies. Additional research is warranted to evaluatethe synergistic effects of multiple contaminants and to investi-gate enhancement procedures to increase contaminant removal.Overall, this study has shown that electrokinetics offers greatpotential for the remediation of mixed wastes in various media,including fine-grained soils and sludges. Additional research willprovide both fundamental and practical knowledge that willimprove the application of this remediation technique to actualcontaminated sites.

ACKNOWLEDGMENTSThis study was performed as part of the environmentaldesign contest organized by the Waste-Management Edu-cation and Research Consortium (WERC) in Las Cruces,NM. Financial support was received from WERC and theUniversity of Illinois at Chicago. The assistance of ShirleyChiu and Melanie Laud during the performance of the ex-periments is greatly appreciated.

REFERENCES1. Kelsh, D.J.; Parsons, M.W. J. of Hazardous Materials. 1997, 55, 109-116.2. Acar, Y.B.; Gale, R.J.; Alshawabkeh, A.N.; Marks, R.E.; Puppala, S.; Brica,

M.; Parker, R. J. Hazardous Materials. 1995, 40, 117-137.3. Acar, Y.B.; Alshawabkeh, A.N. Environ. Sci. Technol. 1993, 27, 2,638-2,647.4. Shapiro, A.P.; Probstein, R.F. Environ. Sci. Technol. 1993, 27, 283-291.5. Probstein, R.F.; Hicks, R.E. Science 1993, 260, 498-503.6. Pamukcu, S.; Wittle, J.K. Environ. Progress. 1992, 11, 241-249.7. Hamed, J.; Acar, Y.B.; Gale, R.J. J. Geotechnical Engineering 1991, 117, 241-271.8. Reddy, K.R.; Shirani, A.B. Geotechnical and Geological Engineering 1997,

15, 3-29.9. Reddy, K.R.; Parupudi, U.S. J. of Soil Contamination 1997, 6, 391-407.10. Reddy, K.R.; Parupudi, U.S.; Devulapalli, S.; Xu, C.Y. J. Hazardous

Materials 1997, 55, 135-158.11. U.S. Environmental Protection Agency. Test Methods for Evaluating

Solid Waste, Vol.1 A-1C: Laboratory Manual Physical/Chemical Meth-ods, SW-846, 1986.

Table 2. Results of radionuclide testing.

Contaminant Concentration (pCi/g dry)

Sludge Soil

Sr-89 <0.048 <0.035

Sr-90 <0.021 <0.016

Co-57 <0.068 <0.055

Co-58 <0.041 <0.040

Co-60 <0.024 <0.027

Cs-134 <0.046 <0.032

Cs-137 <0.13 0.049 ± 0.017

La-140 <0.14 <0.098

Ce-141 <0.091 <0.064

Ce-144 <0.40 <0.32

Nd/Pm-147 <0.80 <0.86

Note: The error given is the probable counting error at the 95% confidence level. Less

than (<) values are based on a 4.66 sigma counting error for the background sample.

Dow

nloa

ded

by [

Duk

e U

nive

rsity

Med

ical

Cen

ter]

at 1

4:24

11

Oct

ober

201

4