enhanced electrokinetic remediation of contaminated manufactured gas plant soil

(2006) 132–146www.elsevier.com/locate/enggeo

Engineering Geology 85

Enhanced electrokinetic remediation of contaminatedmanufactured gas plant soil

Krishna R. Reddy a,⁎, Prasanth R. Ala a, Saurabh Sharma a, Surendra N. Kumar b

a University of Illinois at Chicago, Department of Civil and Materials Engineering, 2095 Engineering Research Facility, 842 West Taylor Street,Chicago, Illinois 60607, USA

b STAT Analysis Corporation, 2201 West Campbell Park Drive, Chicago, Illinois 60612, USA

Accepted 15 September 2005Available online 17 April 2006

Abstract

This paper evaluates different flushing agents to enhance the efficiency of electrokinetic remediation of a manufactured gasplant (MGP) soil contaminated with polycyclic aromatic hydrocarbons (PAHs) and heavy metals. Because of high concentrations,PAHs were of environmental concern and required to be removed to acceptable levels. Four flushing agents, which included twosurfactants (3% Tween 80, and 5% Igepal CA-720), one cosolvent (20% n-Butylamine) and one cyclodextrin (10% hydroxypropyl-β-cyclodextrin or HPCD), were examined to enhance the solubilization of PAHs in the soil. Four electrokinetic experiments wereconducted at 2.0 VDC/cm voltage gradient and 1.4 hydraulic gradient in order to assess the effectiveness of these flushing solutionsfor the removal of PAHs. Variables measured during the application of electric potential were electric current, electroosmotic flow,and contaminant removal from the soil. After the completion of each test, the soil was further examined for moisture content, pH,redox potential, electrical conductivity, and residual contaminant distribution. It is found that cosolvent increased the soil pH, whilethe surfactants and HPCD did not induce substantial change in the soil pH. The current densities fluctuated with time for all testsand remained less than 1 mA/cm2. The current density for the test conducted with cosolvent was higher as compared to the testsconducted with surfactants and HPCD. Electroosmotic flow was the maximum with the cosolvent, while the lowest flow wasobserved with Tween 80 surfactant. Overall, Igepal CA-720 surfactant yielded the highest removal efficiency due to partialsolubilization of PAHs, causing some PAHs to migrate towards the cathode. Heavy metals are found to be strongly adsorbed/precipitated and showed negligible migration behavior in all the tests. Based on the contaminant mass remaining in the soil, it isapparent that further optimization of the electrokinetic system is required to improve PAH removal efficiency for the MGP soil.© 2006 Elsevier B.V. All rights reserved.

Keywords: Electrokinetics; Remediation; Soils; Polycyclic aromatic hydrocarbons; Heavy metals; Surfactants; Cosolvents; Cyclodextrins

1. Introduction

There are over 3000 to 5000 former manufacturedgas plant (MGP) sites across the United States (USEPA,

⁎ Corresponding author. Tel.: +1 312 996 4755; fax: +1 312 9962426.

E-mail address: [email protected] (K.R. Reddy).

0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2005.09.043

2000). The contaminants found at these sites includemainly polycyclic aromatic hydrocarbons (PAHs), butsmall amounts of heavy metals are also often encoun-tered. PAHs are compounds composed of two or morefused aromatic rings, and PAHs with higher molecularweights are proven carcinogenic and mutagenic.Because of their low volatility, low solubility, and lowbiodegradability, PAHs are difficult to treat (Williamson

mailto:[email protected]

http://dx.doi.org/10.1016/j.enggeo.2005.09.043

Table 1Properties of the contaminated manufactured gas plant soil

Property Test method Value

Specific gravity ASTM D854 2.54Grain size distribution ASTM D422 % gravel=1.8–15.4

% sand=50.1–65.6% fines=32.6–34.5

Atterberg limits ASTM D4318 Non-plasticHydraulic conductivity ASTM D2434 2.1×10−4 cm/spH ASTM D4972 6.9Organic content ASTM D2974 2.69–3.75%USCS classification ASTM D2488 SM

133K.R. Reddy et al. / Engineering Geology 85 (2006) 132–146

et al., 1998; Hatheway, 2002). Conventional ex situremediation methods, such as excavation, incineration,thermal desorption, soil washing, and bioremediation,are found to be either expensive and/or ineffective atfield scale application (Shosky, 1996). Therefore, in situremediation of soils is preferred due to simplicity, lesssite disturbance, and minimal public exposure. As aresult, a variety of in situ technologies have beendesigned and developed, but they are found to be lesseffective and costly for treatment of low permeabilityand heterogeneous soils (McGowan et al., 1996;Chowdiah et al., 1998; Lee et al., 2001).

Recently, attention has focused on developing in situelectrokinetic technique for the treatment of lowpermeable soils contaminated with heavy metals, radio-nuclides, and selected organic pollutants. This techniqueinvolves applying a low-level DC electric potentialthrough electrodes, which are placed into the contami-nated soil. If the contaminants are ionic compounds, theycan be transported to the oppositely charged electrode byelectromigration. In addition, electroosmotic flow (EOflow) provides a driving force for the movement ofcontaminants. Therefore, soluble contaminants may be

Fig. 1. Grain size distribution of contami

removed by EO flow. However, it is difficult to apply theelectrokinetic remediation method to remove hydropho-bic and strongly adsorbed contaminants especially fromthe low permeability clayey soils. The use of solubilizingagents, such as surfactants, cosolvent and cyclodextrinsis considered to enhance the efficiency of removing thesehydrophobic pollutants from the soils (Maturi, 2004).

The purpose of the present study is to develop aneffective electrokinetic remediation system for theremoval of hydrophobic PAHs from the field soilobtained from actual MGP site. In particular, severalflushing solutions, specifically two different surfactants(5% Igepal CA-720 and 3% Tween 80), a cosolvent(20% n-Butylamine), and a cyclodextrin (10% HP-β-CD), were examined for their potential use in theremoval of hydrophobic PAHs from the field soil. Aseries of bench-scale electrokinetic experiments wereconducted using these different flushing solutions toassess the extent of contaminant migration and removal.

2. Experimental methodology

2.1. Soil characterization

Contaminated soil sample, selected for this study, wasobtained from a former manufactured gas plant (MGP)site in Chicago, Illinois, USA. The received soil samplewas thoroughly homogenized. The homogenized samplewas analyzed for different physical properties accordingto the respective ASTM standard testing procedures andthe results are presented in Table 1. Fig. 1 shows the grainsize distribution of the field soil. The homogenized soilsample was also analyzed by standard EPA method SW6020 for metals (SW 7471A for mercury), EPA methodSW 8260B for volatile organic compounds (e.g., BTEX),and EPA method SW 8270C (Selective Ionic Mode) for

nated manufactured gas plant soil.

Table 2Contaminants found in the manufactured gas plant soil

(a) Total metals (USEPAmethod SW6020/SW7471A)

(b) Polycyclic aromatic hydrocarbons(USEPA method SW8270C(SIM))

Chemical Concentration(mg/kg)

Chemical Concentration(mg/kg)

Aluminum 3800 2-Methylnaphthalene 230Arsenic 11 Acenaphthene 25–40Barium 38 Acenaphthylene 84–120Calcium 38000 Anthracene 69–92Chromium 8.3 Benz(a)anthracene 66–82Cobalt 4.9 Benzo(a)pyrene 59–62Copper 13 Benzo(b)

fluoranthene31–33

Iron 15000 Benzo(g,h,i)perylene

4.8–33

Lead 25 Benzo(k)fluoranthene

23–30

Magnesium 15000 Chrysene 39–75Manganese 440 Dibenz(a,h)

anthracene9.1

Nickel 14 Dibenzofuran 7.7Potassium 700 Fluoranthene 92–130Sodium 88 Fluorene 92Thallium 1.8 Indeno(1,2,3-cd)

pyrene12–21

Vanadium 12 Naphthalene 600Zinc 66 Phenanthrene 260–350

Pyrene 130–210

Chemicals for which measured concentrations were below detectionlimits are not listed.

134 K.R. Reddy et al. / Engineering Geology 85 (2006) 132–146

PAHs (USEPA, 1986), and their respective concentrationsfound in the MGP soil are presented in Table 2.

The presence of calcium carbonates or othercompounds such as magnesium carbonates or sodiumcarbonates causes high buffering capacity of the soil.Buffering capacity of soil refers to the capability of soilto neutralize acid. Buffering capacity of the MGP soil

Fig. 2. Acid buffering capacity of contam

was determined by titration analysis using 2 M nitricacid as titrant solution. A soil slurry sample wasprepared by mixing 20 g of soil in 200 mL of water.The acid was added incrementally to the slurry while itwas being mixed with a magnetic stirrer. A deionizedwater sample was used as a control sample. Theequilibrium pH of the slurry was measured with a pHmeter (Thermo Orion model 720 A). The results showedthat the buffering capacity of the aqueous MGP soilslurry with a solids concentration of 8.5% is 3.7 eq/kg(dry soil) at the inflection point of the titration curve (pH6.2) (see Fig. 2). This indicates that the MGP soilpossesses high acid buffering capacity.

2.2. Electrokinetic test setup

Fig. 3 shows the schematic of the electrokinetic testsetup used for this study and has been described indetail by Reddy and Parupudi (1997) and Reddy andChinthamreddy (2003). The test setup mainly consistof an electrokinetic cell, two electrode compartments,two electrode reservoirs, a power source, and amultimeter. Plexiglas cell having inside diameter of6.3 cm and a total length of 19.1 cm was used aselectrokinetic cell. Each electrode compartment includ-ed a valve to control the flow into the cell, a slottedgraphite electrode, and a porous stone. Small holes inthe electrode compartment contained the electrodepins, and filter paper was placed between the soilsample and the electrode. The electrode reservoirs weremade of 3.2 cm inner diameter. Plexiglas reservoirswere connected to the electrode compartments usingTygon tubing. Exit ports were created in the electrodecompartments, and the tubing was attached to theseports to allow the gases generated due to theelectrolysis of water to escape. The other end of

inated manufactured gas plant soil.

Fig. 3. Electrokinetic test setup.

Table 3Electrokinetic testing program

Testnumber

Voltage gradient(VDC/cm)

Hydraulicgradient

Flushingsolution

1 2.0 1.4 5% Igepal CA–7202 2.0 1.4 3% Tween 803 2.0 1.4 20% n-Butylamine4 2.0 1.4 10% HP-β-CD


these gas tubes was connected to the reservoirs tocollect any liquid that was removed along with thegases. A power source was used to apply a constantvoltage to the electrodes, and a multimeter was used tomonitor the voltage and measure the current valuethrough the soil sample during the test.

2.3. Test variables

Table 3 shows the details of the four experimentsconducted for this study. All of the experiments wereconducted at a constant voltage gradient of 2.0 VDC/cm. The hydraulic gradient that existed under theseexperimental conditions was approximately 1.4, and isnot significant enough to generate substantial hydraulicflow because of the characteristic low permeability ofthe soil. Flushing solutions examined to enhancesolubilization of PAHs were: two different surfactants(5% Igepal CA-720 and 3% Tween 80), a cosolvent

(20% n-Butylamine), and a cyclodextrin (10% HP-β-CD) and these particular types of flushing solutions andtheir concentrations were selected on the basis of resultsfrom several series of previous batch and electrokineticexperiments (Saichek and Reddy, 2004; Maturi, 2004).

2.4. Testing procedure

The contaminated field soil was placed in theelectrokinetic cell in layers and compacted uniformly

Fig. 4. Measured current densities.


using a hand compactor. The electrode compartmentswere then connected to the electrokinetic cell. In eachelectrode compartment, filter papers were insertedbetween the electrode and the porous stone as well asbetween the porous stone and the soil. The electrodecompartments were connected to the anode and cathodereservoirs using Tygon tubing. The anode reservoir wasfilled with a selected flushing solution and the cathodereservoir was filled with deionized water. The water levelin both reservoirs was monitored and adjusted carefullythroughout the tests in order to maintain a constanthydraulic gradient across the specimen. The electroki-netic cell was then connected to the power supply and aconstant voltage gradient of 2.0 VDC/cm was applied tothe soil sample. The flushing solution was circulatedusing peristaltic pump in the anode reservoir and theelectroosmotic flow from the cathode reservoir wascollected periodically. Each test was terminated when thecurrent value, flow rate, or contaminant concentrationsin effluent was significantly reduced.

Fig. 5. Measured elect

After the completion of each test, aqueous solutionsfrom the anode and cathode reservoirs and theelectrode assemblies were collected and the volumeswere measured. Then, the reservoirs and the electrodeassemblies were disconnected, and the soil specimenwas extruded from the cell using a mechanicalextruder. Each of the extruded soil specimen wassectioned into three or five equal parts to determinethe final distribution of pH values across the soilspecimen. Each soil section was weighed andpreserved in a glass bottle. From each soil section,10 g of soil was taken and mixed with 10 mL of a0.01 M CaCl2 solution in a glass vial. The slurry wasshaken thoroughly by hand for several minutes andthe solids were allowed to settle for an hour. Thisslurry was then used for measuring the soil pH, redoxpotential and electrical conductivity. The pH, redoxpotential and electrical conductivity of the aqueoussolutions from the electrodes were also measured. Themoisture content of each soil section was also

roosmotic flow.


determined in accordance with ASTM D2216 (ASTM,2004).

2.5. Chemical analyses

Representative samples of reservoir solutions, soilsections, and the initial soil for each test were analyzedfor total metals and PAHs using the standard USEPA

Fig. 6. (a) Removal of total metals from the soil. (b) Removal of toxic metals fsoil.

methods (USEPA, 1986). The total metals in soil andliquid samples were analyzed using the USEPA MethodSW6020 and the mercury was analyzed usingSW7471A for soil and 7470A for liquid samples. ThePAHs were analyzed using the USEPA MethodSW8270C (Selective Ionic Mode). The chemicalanalyses were conducted with a stringent qualitycontrol by the STAT Analysis Corporation, Chicago,

rom the soil. (c). Removal of polycyclic aromatic hydrocarbon from the

Fig. 7. (a) Moisture distribution in the soil. (b). Soil pH variation in the soil. (c). Redox potential variation in the soil. (d) Conductivity variation in thesoil.


Fig. 7 (continued).


Illinois, a certified laboratory. To ensure accuracy of thetest results, new electrodes, porous stones, and tubingwere used for each experiment, and the electrokineticcell and compartments were washed thoroughly andthen rinsed first with tap water and finally withdeionized water to avoid cross contamination betweenthe experiments.

3. Results and analysis

The results of the electrokinetic experiments wereanalyzed to assess the electric current, electroosmoticflow, and contaminant removal during the electricpotential application as well as the moisture content,pH, redox potential, electrical conductivity, and residualcontaminant distribution in the soil after the experimentswere terminated.

3.1. Electric current density

Themeasured electric current densities for all the testsare plotted against elapsed time in Fig. 4. The currentdensities for each test were obtained by dividing currentvalues measured during the testing by the cross-sectionalarea of the EK cell. The results showed that the currentdensity values fluctuated with time for all testsconducted with surfactants, cosolvent and HPCD andremain less than 1mA/cm2. However, the current densityfor the test conducted with 20% n-Butylamine washigher as compared to the other tests using surfactantsand HPCD as flushing solutions. This behavior can beexplained by considering that when the flushing fluidspass through the soil, the solubilization of the contam-

inant occurs and the ionic strength of pore fluid isincreased. Thus, when the voltage gradient is applied,initially the current is low because it takes time for thesolution to migrate into the soil from the electrodereservoirs and for the soil constituents/minerals and/orcontaminants to dissolve from the soil surface. Aftersome time (few hours), the initial current reaches its peakvalue due to the strong ionic concentration of the porefluid and also due to the electromigration of contami-nants towards their respective electrode. Then, currentvalue gradually decreases because of decrease in theelectromigration of the cations and anions in the porefluid. In addition, the products of the electrolysisreactions or other chemical species may reduce thecurrent by neutralizing the migrating ions. For instance,H+ ions migrating towards the cathode could beneutralized by OH− ions migrating towards the anode,thereby forming water and diluting the number of ions insolution. Change in soil pH due to electrolysis reactionscould also affect the current by causing changes such asmineral dissolution, or chemical precipitation/dissolu-tion. Unless flushing solutions, which introduce addi-tional, non-reactive, ions as charge carriers, are used, thecurrent usually diminishes over time (Dzenitis, 1997).Thus difference in the current data among the four testscan be explained on the basis of their affinity towards thehydrophobic contaminants. 20% n-Butylamine wasfound to be more effective in solubilizing the con-taminants from the field soil as compared to surfactantsand cyclodextrin (as discussed in Section 3.3). The trendof current values of 5% Igepal 720 and 3% Tween 80enhanced system was quite similar but slightly highercurrent values were recorded with 5% Igepal enhanced


system. This is due to the formation of more stablemicelles due to the higher concentration of the Igepal.The lowest current values were recorded for HPCDenhanced system. The higher initial values of electriccurrent are obviously due to the higher electrolyteconcentration in the pore water but soon it decreased andshowed a fluctuation in the later stages of operation. Thisis due to the constant change in polarization of soilparticles due to the change in the double layer by theaction of HPCD solution.

3.2. Electroosmotic flow

Fig. 5 represents the electroosmotic flow data for alltests. In all the tests, the electroosmotic flow at thecathode increased with an increase in the operatingduration, i.e. elapsed time. It can be seen that electro-osmotic flow behavior was dependent on the type offlushing solutions. Maximum electroosmotic low wasobserved within the cosolvent system, while the lowestflow was observed with Tween 80 surfactant system. Atotal of 7.2, 2.1, 10.7 and 7.5 pore volumes of flow weremeasured in tests with 5% Igepal CA-720, 3% Tween 80,20% n-Butylamine, and HPCD tests, respectively. Theelectroosmotic flow variation is found to be consistentwith their respective trend as observed for the variationin current densities in all the tested systems.

As seen in Fig. 4, the electric current variessignificantly with elapsed time, and was attributed tothe physico-chemical processes, such as the electromi-gration of ionic species and the electrolysis reactions.These processes affect the surface charge of the soilparticles (zeta potential) and the pore fluid properties,such as dielectric constant and viscosity, with time, andhence influence the electroosmotic flow. Initially, duringthe beginning of the test, when the current is high(electromigration is high), the transfer of momentum tothe surrounding fluid molecules may be substantial. Thisoften corresponds to a significant volume of electroos-motic flow. A high ionic strength of the pore fluid canalso be detrimental for electroosmotic flow, because itreduces the thickness of the diffuse double layer and,thereby, constricts the electroosmotic flow. The chargeon the soil surface must also be considered, becausewhen the pH is below its ZPC, the soil particle surfacespossess a positive zeta potential and the electroosmoticflow occurs towards the anode, and when the pH is abovethe ZPC, the soil particles have a negative zeta potentialand the electroosmotic flow occurs towards the cathode.After few days, it was observed that the electroosmoticflow sharply decreased with time. The reason for this isthat electroosmotic flow was inhibited by a decrease of

zeta potential of soil particles by excess H+ and heavymetal precipitation by excess OH−.

3.3. Contaminant removal

Effluent samples collected at different time intervalsfor all the tested systems were analyzed for metals andPAHs. Fig. 6(a) shows the cumulative metal removal forall of the metals shown in Table 2. These plots revealedthat 20% n-Butylamine has pronounced affinity ascompared to other flushing solutions for the cumulativeremoval of metals from the soil under the constantvoltage gradient. Since toxic metals (all metals exceptAl, Ca, Fe, Mg, K and Na in Table 2) are of primeconcern, the cumulative toxic metal removal is depictedin Fig. 6(b). The removal of total PAHs with number ofpore volumes is shown in Fig. 6(c). These results showthat 20% n-Butylamine cosolvent has a maximumaffinity towards the removal of metals and PAHs ascompared to surfactants and HPCD. The affinity of allthe tested systems decreases in the following order 20%n-ButylamineN5% Igepal CA-720N10% HPCDN3%Tween 80 system. It is pertinent to mention here that avery low concentration of the surfactants and HPCDwere employed during these investigations as comparedto n-Butylamine system. Therefore, the performance ofthese systems may be increased using higher concentra-tions of flushing solutions.

3.4. Moisture content, pH, redox potential andelectrical conductivity

The initial moisture content of the MGP soil was15%, and the variation of moisture content withnormalized distance from the anode after the electroki-netic treatment is shown in Fig. 7(a). The normalizeddistance is defined as the distance to the specificlocation from the anode divided by the total distancefrom the anode to the cathode. In general, moisturecontent of the soil near the anode increased slightly,while the moisture content near the cathode decreasedslightly. This behavior can be seen in HPCD system,where the moisture content clearly increases at anodeand decreases at cathode. In 20% n-Butylamine systemmoisture content was high in the first three soilspecimens but then sharply decreases and againincreases at cathode end. This may be due to theenhanced electroosmotic flow behavior of this system.However interesting results were obtained for thesurfactant enhanced systems. It is pertinent to mentionhere again that the concentration of the surfactantsemployed in these investigations were low. It has been


observed that in 5% Igepal-system, the moisture contentof all the soil specimens was found to be higher than theother tested systems. The moisture content decreasesfrom anode to cathode. But in 3% Tween 80-system themoisture content at anode was found to be low at anodeend and was found to be just 15% (initial soil condition)in the other soil specimens. Overall it is concluded thatthe electrokinetic process in all the tested systems doesnot significantly alter the moisture content. Slightchanges in moisture contents are evident which can beattributed to the variations in the electroosmotic flowthat occurred as a result of the changes in parameterssuch as the ionic strength, conductivity, and/or electricalgradient. These results suggest that the electroosmoticflow might not be uniform and there might be changes inpore pressures (Eykholt, 1997). Nevertheless, it appears

Fig. 8. (a) Distribution of metals in soils using 5% Igepal CA-720 surfactant.

that the soil moisture content remained fairly consistentand comparable to the initial moisture content. It ispossible that regions where the electroosmotic flow washigh, a pressure gradient was created so that the solutionwas pulled from regions where the electroosmotic flowwas lower. Since the solution was continuously trans-ported through the soil, the moisture content did notsubstantially deviate from the initial moisture content.

Fig. 7(b) shows the pH distribution across the soilafter the completion of the experiments. Consideringthat the MGP soil had a pH of 6.9 before the experi-ments, the soil pH after the experiment was analyzed foreach soil specimen of all the tests. Generally, the elec-trolysis of water results in the formation of H+ ions (lowpH solution) at the anode and OH− ions (high pHsolution) at the cathode, and, primarily due to

(b). Distribution of PAHs in soils using 5% Igepal CA-720 surfactant.


electromigration, these ions tend to migrate towards theoppositely charged electrode(s). Because of high acidbuffering capacity of the MGP soil, the H+ ions areneutralized and are not migrated through the soil.However, OH− migrate through the soil towards theanode. Thus, Fig. 7(b) illustrates that a weak acidic frontof solution was generated by the electrolysis reaction atthe anode and pH slightly decreased in the first sectionnear the anode in the surfactant and HPCD enhancedsystems. In the second section the difference in the pHbehavior becomes more pronounced as it increases forsurfactant enhanced systems while remains approxi-mately the same as initial pH in the HPCD enhancedsystems. The third section, which is closest to thecathode, high soil pH was observed in the surfactantenhanced systems while remain constant to the initial

Fig. 9. (a) Distribution of metals in soils using 3% Tween 80 surfactant

pH in the HPCD system. In the 20% n-Butylaminesystem, the soil pH remains maximum for all the soilsection specimens. This cosolvent is highly alkaline innature and its migration into the soil by electroosmosisincreased pH throughout the soil. The transport of OH−

into the soil from the cathode also contributed toincrease in soil pH.

The redox potentials of the soil specimens for all thetested systems are shown in Fig. 7(c) and reflect theopposite trend to that observed for pH. Redox potentialswere low for the cosolvent test, while they were high forthe HPCD test. Electrical conductivity values, as shownin Fig. 7(d), reveal that the test with cosolvent hadhigher electrical conductivity and it decreased signifi-cantly from the anode to the cathode. On the other hand,the tests conducted with surfactants and HPCD had

. (b). Distribution of PAHs in soils using 3% Tween 80 surfactant.


lower electrical conductivity and the values increaseslightly from the anode to the cathode. The lowestelectrical conductivity was observed in the Tween 80enhanced system.

3.5. Residual contaminant distribution

After the completion of experiments, the soilsamples were sectioned into three equal parts: S-1(near anode), S-2 (middle), and S-3 (near cathode).However, the soil sample of 20% n-Butylamineenhanced system was sectioned into five equal parts:S1 (near anode), S2, S3 (middle), S4, and S5 (nearcathode). The contaminant concentrations determinedfor each of these sections are plotted together in order toelucidate the migration behavior of the contaminantsthrough the soil.

Fig. 8(a) and (b) show the residual distribution oftotal metals and PAHs concentrations respectively, in

Fig. 10. (a) Distribution of metals in soils using 20% n-Butylamine cosolvent

the MGP soil treated with 5% Igepal CA-720. As seenfrom Fig. 8(a) all the metals are found to be evenlydistributed throughout the soil sample even after thecompletion of the test. Only cadmium concentration wasfound to be higher at cathode i.e. section S3, whilecopper and magnesium were migrated toward anode(i.e. sections S1 and S2) from the cathode section S3.This may be due to their respective ions and complexesmigration towards cathode and anode. This shows thatsoluble metals are negligible in the field soil. This alsoreflects that Igepal CA-720 is not suitable for theremoval of metals from the MGP soil under theinvestigated concentration range. In contrast, Fig. 8(b)showed that Igepal CA-720 has strong affinity toremove a wide array of PAHs from the MGP soil.This plot also indicates that all the PAHs from the MGPsoil were significantly removed near anode i.e. sectionS1 and middle section i.e. section S2. Comparativelyhigher concentration of PAHs is found to be

. (b). Distribution of PAHs in soils using 20% n-Butylamine cosolvent.


accumulated at cathode end i.e. section S3. This reflectsthe migration behavior of PAHs from anode towardscathode. The results also confirm that hydrophobiccharacter of the PAHs increases with the number ofrings, as the concentration of higher ringed PAHs wasfound high in S2 section, indicating that these PAHswere strongly attached to the soil under the investigatedtest conditions.

Figs. 9(a), 10(a) and 11(a) show the residual metalconcentrations in different sections for tests conductedwith 3% Tween 80, 20% n-Butylamine, and 10%HPCD, respectively. These findings suggest that only3% Tween 80 system influenced the migration ofmercury from the soil samples. Though the removal wasnot so significant, but it was found comparably suitable

Fig. 11. (a) Distribution of metals in soils using 10% hydroxypropyle-β-cycloβ-cyclodextrin.

for the removal of mercury complexes from the soilstowards anode. Based on these results, it can beformulated that there are no significant changes in themetal concentrations of different sections for all thesetested systems. This implies that the metals are notmigrated towards the electrodes under the influence offlushing solutions used. This indicates that flushingsolutions used were not effective for desorption and/ordissolution of metals in the soils (Maturi, 2004). Thismay be due to significant amount of organic matter thatstrongly adsorbed metals. In addition, the high bufferingcapacity of the soil may have caused metals to exist asprecipitates. Thus strong adsorption and precipitation ofmetals results into low migration and for this reasonmetals did not exist in pore water.

dextrin. (b). Distribution of PAHs in soils using 10% hydroxypropyle-


The residual concentrations of PAHs in differentsections are plotted in Figs. 9(b), 10(b), and 11(b), for thetests conducted with Tween 80, n-Butylamine, andHPCD, respectively. Fig. 9(b) shows low concentrationof PAHs at anode indicating themigration of PAHs occursfrom cathode to anode. Further the concentration of all thePAHs was found to be higher in the third section i.e. S3section. This reflects that the flushing solution Tween 80is also suitable to solubilize appreciable amount of PAHsfrom the soil sample. It is pertinent to mention here thatthe concentration of Tween 80 employed in this systemwas just 3% and was comparatively low with respect tothe other flushing solutions including Igepal CA-720system. Fig. 10(b) shows very low migration trend ofPAHs in 20% n-Butylamine test, while Fig. 11(b) showssome migration of selected PAHs towards the cathode(i.e., from section S1 towards sections S2 and S3) inHPCD test.

In general the concentration of PAHs increased fromsection S-1 to section S-3 in all the studied systems. Thisshows that PAHs migrated towards the cathode. Oneconclusion that can be drawn regarding this observationis that the use of appropriate flushing solution enhancesdesorption/solubilization of PAHs in soils. It is alsoobserved that surfactant enhanced removal of PAHsdepends upon the micelle formation at appreciableCMC. The differences between the efficiency of 5%Igepal 720 and 3% Tween 80 most likely resulted fromcompetitive behavior among various PAH compoundsfor partitioning into the stable micelles as well as com-petitive sorption of PAH compounds and surfactant tothe soil organic matter and soil particles. This study alsoindicates that 10% HPCD system had contributed partialsolubilization of the PAHs resulting in their migrationtowards the cathode. HPCD enhanced system was foundto be more effective for the solubilization of low polarityPAHs. This partial solubilization of low-polarity PAHsis attributed to the formation of inclusion complexeswithin the relatively non-polar cavity of the HPCD. Thehigher electroosmotic flow in 20% n-Butylamine testresulted in higher contaminant removal as compared toother tests; however, very low migration trend of PAHsin this test show that 20% n-Butylamine did not effec-tively solubilize/desorb PAHs in the soil.

The variation in the concentrations of PAHs in theirrespective sections may also be contributed by hetero-geneous distribution of the contaminants in the soil.These results show that Igepal CA-720, Tween 80 andHPCD systems are effective for solubilization of thePAHs from the MGP soil (under investigated condi-tions). These studies also elucidate that high bufferingcapacity of the soil also impede the efficiency of the

contaminant removal. Metals are readily precipitatedunder the tested conditions due to the high bufferingcapacity of soil. PAHs were found to be efficientlysolubilized by the flushing solutions under the exper-imental conditions. Substantial electroosmotic flow canbe induced in the MGP soils using different flushingsolutions resulting in the appreciable removal ofcontaminants from the MGP soil. It is also believedthat longer durations and different applied voltagegradient and higher concentrations of the flushingsolutions may also result in the better contaminantremoval efficiency.

4. Conclusions

Based on the experimental results, the followingconclusions may be drawn:

• The manufactured gas plant soil used in this studywas contaminated with both heavy metals andpolycyclic aromatic hydrocarbons. It is feasible toenhance extraction of PAHs using surfactants,cosolvents and cyclodextrin from this aged MGPfield soil. However, no significant removal of heavymetals was observed in this study. This also indicatesthat heavy metals are mostly present as precipitatesdue to high pH and high acid buffering capacity ofthe soil.

• Substantial electroosmotic flow can be induced inthe soil using different flushing solutions. Maxi-mum electroosmotic flow was observed in the 20%n-Butylamine enhanced system followed by HPCDenhanced system. Comparatively low flow wasobserved in surfactant enhanced systems.

• PAHs were solubilized in the surfactant and HPCDenhanced systems more efficiently even at lowconcentration as compared to cosolvent systemresulting in significant migration towards the cath-ode. The solubilization of PAHs using surfactantsdepends upon the stability and number of themicelles formed during the test. The mechanism ofPAHs solubilization in HPCD enhanced system wasfound to be partial solubilization. The migration ofPAHs in n-Butylamine enhanced system was attrib-uted to desorption phenomenon.

• The partially solubilized PAHs migrated from anodetowards the cathode due to electroosmotic flow. Thesoil pH remains high due to its high pH bufferingcapacity and under such conditions heavy metalsremain strongly adsorbed/precipitated. Therefore, themetals are not electromigrated and are not removedfrom the soil under all the tested systems.


Acknowledgements

The financial support for this project was receivedfrom a Technology Challenge Grant provided by theState of Illinois. The authors are grateful to KrantiMaturi and Craig Chawla for their assistance in thisproject.

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