Enhanced Electrokinetic Remediation Of Contaminated Manufactured Gas Plant Soil

Engineering Geology 85 (2006) 132 – 146 www.elsevier.com/locate/enggeo

Enhanced electrokinetic remediation of contaminated manufactured 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 2005 Available online 17 April 2006

Abstract This paper evaluates different flushing agents to enhance the efficiency of electrokinetic remediation of a manufactured gas plant (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 two surfactants (3% Tween 80, and 5% Igepal CA-720), one cosolvent (20% n-Butylamine) and one cyclodextrin (10% hydroxypropylβ-cyclodextrin or HPCD), were examined to enhance the solubilization of PAHs in the soil. Four electrokinetic experiments were conducted at 2.0 VDC/cm voltage gradient and 1.4 hydraulic gradient in order to assess the effectiveness of these flushing solutions for 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, while the surfactants and HPCD did not induce substantial change in the soil pH. The current densities fluctuated with time for all tests and remained less than 1 mA/cm2. The current density for the test conducted with cosolvent was higher as compared to the tests conducted with surfactants and HPCD. Electroosmotic flow was the maximum with the cosolvent, while the lowest flow was observed with Tween 80 surfactant. Overall, Igepal CA-720 surfactant yielded the highest removal efficiency due to partial solubilization 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 is apparent 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 manufactured gas plant (MGP) sites across the United States (USEPA, ⁎ Corresponding author. Tel.: +1 312 996 4755; fax: +1 312 996 2426. 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 include mainly polycyclic aromatic hydrocarbons (PAHs), but small amounts of heavy metals are also often encountered. PAHs are compounds composed of two or more fused aromatic rings, and PAHs with higher molecular weights are proven carcinogenic and mutagenic. Because of their low volatility, low solubility, and low biodegradability, PAHs are difficult to treat (Williamson

K.R. Reddy et al. / Engineering Geology 85 (2006) 132–146 Table 1 Properties of the contaminated manufactured gas plant soil Property

Test method

Value

Specific gravity Grain size distribution

ASTM D854 ASTM D422

Atterberg limits Hydraulic conductivity pH Organic content USCS classification

ASTM D4318 ASTM D2434 ASTM D4972 ASTM D2974 ASTM D2488

2.54 % gravel = 1.8–15.4 % sand = 50.1–65.6 % fines = 32.6–34.5 Non-plastic 2.1 × 10− 4 cm/s 6.9 2.69–3.75% SM

et al., 1998; Hatheway, 2002). Conventional ex situ remediation methods, such as excavation, incineration, thermal desorption, soil washing, and bioremediation, are found to be either expensive and/or ineffective at field scale application (Shosky, 1996). Therefore, in situ remediation of soils is preferred due to simplicity, less site disturbance, and minimal public exposure. As a result, a variety of in situ technologies have been designed and developed, but they are found to be less effective and costly for treatment of low permeability and heterogeneous soils (McGowan et al., 1996; Chowdiah et al., 1998; Lee et al., 2001). Recently, attention has focused on developing in situ electrokinetic technique for the treatment of low permeable soils contaminated with heavy metals, radionuclides, and selected organic pollutants. This technique involves applying a low-level DC electric potential through electrodes, which are placed into the contaminated soil. If the contaminants are ionic compounds, they can be transported to the oppositely charged electrode by electromigration. In addition, electroosmotic flow (EO flow) provides a driving force for the movement of contaminants. Therefore, soluble contaminants may be

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removed by EO flow. However, it is difficult to apply the electrokinetic remediation method to remove hydrophobic and strongly adsorbed contaminants especially from the low permeability clayey soils. The use of solubilizing agents, such as surfactants, cosolvent and cyclodextrins is considered to enhance the efficiency of removing these hydrophobic pollutants from the soils (Maturi, 2004). The purpose of the present study is to develop an effective electrokinetic remediation system for the removal of hydrophobic PAHs from the field soil obtained from actual MGP site. In particular, several flushing solutions, specifically two different surfactants (5% Igepal CA-720 and 3% Tween 80), a cosolvent (20% n-Butylamine), and a cyclodextrin (10% HP-βCD), were examined for their potential use in the removal of hydrophobic PAHs from the field soil. A series of bench-scale electrokinetic experiments were conducted using these different flushing solutions to assess the extent of contaminant migration and removal. 2. Experimental methodology 2.1. Soil characterization Contaminated soil sample, selected for this study, was obtained from a former manufactured gas plant (MGP) site in Chicago, Illinois, USA. The received soil sample was thoroughly homogenized. The homogenized sample was analyzed for different physical properties according to the respective ASTM standard testing procedures and the results are presented in Table 1. Fig. 1 shows the grain size distribution of the field soil. The homogenized soil sample was also analyzed by standard EPA method SW 6020 for metals (SW 7471A for mercury), EPA method SW 8260B for volatile organic compounds (e.g., BTEX), and EPA method SW 8270C (Selective Ionic Mode) for

Fig. 1. Grain size distribution of contaminated manufactured gas plant soil.

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Table 2 Contaminants found in the manufactured gas plant soil (a) Total metals (USEPA method SW6020/SW7471A)

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

Chemical

Concentration (mg/kg)

Chemical

Concentration (mg/kg)

Aluminum Arsenic Barium Calcium Chromium Cobalt Copper

3800 11 38 38000 8.3 4.9 13

230 25–40 84–120 69–92 66–82 59–62 31–33

Iron

15000

2-Methylnaphthalene Acenaphthene Acenaphthylene Anthracene Benz(a)anthracene Benzo(a)pyrene Benzo(b) fluoranthene Benzo(g,h,i) perylene Benzo(k) fluoranthene Chrysene Dibenz(a,h) anthracene Dibenzofuran Fluoranthene Fluorene Indeno(1,2,3-cd) pyrene Naphthalene Phenanthrene Pyrene

Lead Magnesium Manganese

25 15000 440

Nickel Potassium Sodium Thallium

14 700 88 1.8

Vanadium Zinc

12 66

4.8–33 23–30 39–75 9.1 7.7 92–130 92 12–21 600 260–350 130–210

Chemicals for which measured concentrations were below detection limits are not listed.

PAHs (USEPA, 1986), and their respective concentrations found in the MGP soil are presented in Table 2. The presence of calcium carbonates or other compounds such as magnesium carbonates or sodium carbonates causes high buffering capacity of the soil. Buffering capacity of soil refers to the capability of soil to neutralize acid. Buffering capacity of the MGP soil

was determined by titration analysis using 2 M nitric acid as titrant solution. A soil slurry sample was prepared by mixing 20 g of soil in 200 mL of water. The acid was added incrementally to the slurry while it was being mixed with a magnetic stirrer. A deionized water sample was used as a control sample. The equilibrium pH of the slurry was measured with a pH meter (Thermo Orion model 720 A). The results showed that the buffering capacity of the aqueous MGP soil slurry with a solids concentration of 8.5% is 3.7 eq/kg (dry soil) at the inflection point of the titration curve (pH 6.2) (see Fig. 2). This indicates that the MGP soil possesses high acid buffering capacity. 2.2. Electrokinetic test setup Fig. 3 shows the schematic of the electrokinetic test setup used for this study and has been described in detail by Reddy and Parupudi (1997) and Reddy and Chinthamreddy (2003). The test setup mainly consist of an electrokinetic cell, two electrode compartments, two electrode reservoirs, a power source, and a multimeter. Plexiglas cell having inside diameter of 6.3 cm and a total length of 19.1 cm was used as electrokinetic cell. Each electrode compartment included a valve to control the flow into the cell, a slotted graphite electrode, and a porous stone. Small holes in the electrode compartment contained the electrode pins, and filter paper was placed between the soil sample and the electrode. The electrode reservoirs were made of 3.2 cm inner diameter. Plexiglas reservoirs were connected to the electrode compartments using Tygon tubing. Exit ports were created in the electrode compartments, and the tubing was attached to these ports to allow the gases generated due to the electrolysis of water to escape. The other end of

Fig. 2. Acid buffering capacity of contaminated manufactured gas plant soil.

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Fig. 3. Electrokinetic test setup.

these gas tubes was connected to the reservoirs to collect any liquid that was removed along with the gases. A power source was used to apply a constant voltage to the electrodes, and a multimeter was used to monitor the voltage and measure the current value through the soil sample during the test.

(20% n-Butylamine), and a cyclodextrin (10% HP-βCD) and these particular types of flushing solutions and their concentrations were selected on the basis of results from several series of previous batch and electrokinetic experiments (Saichek and Reddy, 2004; Maturi, 2004). 2.4. Testing procedure

2.3. Test variables Table 3 shows the details of the four experiments conducted for this study. All of the experiments were conducted at a constant voltage gradient of 2.0 VDC/ cm. The hydraulic gradient that existed under these experimental conditions was approximately 1.4, and is not significant enough to generate substantial hydraulic flow because of the characteristic low permeability of the soil. Flushing solutions examined to enhance solubilization of PAHs were: two different surfactants (5% Igepal CA-720 and 3% Tween 80), a cosolvent

The contaminated field soil was placed in the electrokinetic cell in layers and compacted uniformly Table 3 Electrokinetic testing program Test number

Voltage gradient (VDC/cm)

Hydraulic gradient

Flushing solution

1 2 3 4

2.0 2.0 2.0 2.0

1.4 1.4 1.4 1.4

5% Igepal CA–720 3% Tween 80 20% n-Butylamine 10% HP-β-CD

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Fig. 4. Measured current densities.

using a hand compactor. The electrode compartments were then connected to the electrokinetic cell. In each electrode compartment, filter papers were inserted between the electrode and the porous stone as well as between the porous stone and the soil. The electrode compartments were connected to the anode and cathode reservoirs using Tygon tubing. The anode reservoir was filled with a selected flushing solution and the cathode reservoir was filled with deionized water. The water level in both reservoirs was monitored and adjusted carefully throughout the tests in order to maintain a constant hydraulic gradient across the specimen. The electrokinetic cell was then connected to the power supply and a constant voltage gradient of 2.0 VDC/cm was applied to the soil sample. The flushing solution was circulated using peristaltic pump in the anode reservoir and the electroosmotic flow from the cathode reservoir was collected periodically. Each test was terminated when the current value, flow rate, or contaminant concentrations in effluent was significantly reduced.

After the completion of each test, aqueous solutions from the anode and cathode reservoirs and the electrode assemblies were collected and the volumes were measured. Then, the reservoirs and the electrode assemblies were disconnected, and the soil specimen was extruded from the cell using a mechanical extruder. Each of the extruded soil specimen was sectioned into three or five equal parts to determine the final distribution of pH values across the soil specimen. Each soil section was weighed and preserved in a glass bottle. From each soil section, 10 g of soil was taken and mixed with 10 mL of a 0.01 M CaCl2 solution in a glass vial. The slurry was shaken thoroughly by hand for several minutes and the solids were allowed to settle for an hour. This slurry was then used for measuring the soil pH, redox potential and electrical conductivity. The pH, redox potential and electrical conductivity of the aqueous solutions from the electrodes were also measured. The moisture content of each soil section was also

Fig. 5. Measured electroosmotic flow.

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determined in accordance with ASTM D2216 (ASTM, 2004). 2.5. Chemical analyses Representative samples of reservoir solutions, soil sections, and the initial soil for each test were analyzed for total metals and PAHs using the standard USEPA

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methods (USEPA, 1986). The total metals in soil and liquid samples were analyzed using the USEPA Method SW6020 and the mercury was analyzed using SW7471A for soil and 7470A for liquid samples. The PAHs were analyzed using the USEPA Method SW8270C (Selective Ionic Mode). The chemical analyses were conducted with a stringent quality control by the STAT Analysis Corporation, Chicago,

Fig. 6. (a) Removal of total metals from the soil. (b) Removal of toxic metals from the soil. (c). Removal of polycyclic aromatic hydrocarbon from the soil.

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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 the soil.

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Fig. 7 (continued).

Illinois, a certified laboratory. To ensure accuracy of the test results, new electrodes, porous stones, and tubing were used for each experiment, and the electrokinetic cell and compartments were washed thoroughly and then rinsed first with tap water and finally with deionized water to avoid cross contamination between the experiments. 3. Results and analysis The results of the electrokinetic experiments were analyzed to assess the electric current, electroosmotic flow, and contaminant removal during the electric potential application as well as the moisture content, pH, redox potential, electrical conductivity, and residual contaminant distribution in the soil after the experiments were terminated. 3.1. Electric current density The measured electric current densities for all the tests are plotted against elapsed time in Fig. 4. The current densities for each test were obtained by dividing current values measured during the testing by the cross-sectional area of the EK cell. The results showed that the current density values fluctuated with time for all tests conducted with surfactants, cosolvent and HPCD and remain less than 1 mA/cm2. However, the current density for the test conducted with 20% n-Butylamine was higher as compared to the other tests using surfactants and HPCD as flushing solutions. This behavior can be explained by considering that when the flushing fluids pass through the soil, the solubilization of the contam-

inant occurs and the ionic strength of pore fluid is increased. Thus, when the voltage gradient is applied, initially the current is low because it takes time for the solution to migrate into the soil from the electrode reservoirs and for the soil constituents/minerals and/or contaminants to dissolve from the soil surface. After some time (few hours), the initial current reaches its peak value due to the strong ionic concentration of the pore fluid and also due to the electromigration of contaminants towards their respective electrode. Then, current value gradually decreases because of decrease in the electromigration of the cations and anions in the pore fluid. In addition, the products of the electrolysis reactions or other chemical species may reduce the current by neutralizing the migrating ions. For instance, H + ions migrating towards the cathode could be neutralized by OH− ions migrating towards the anode, thereby forming water and diluting the number of ions in solution. Change in soil pH due to electrolysis reactions could also affect the current by causing changes such as mineral dissolution, or chemical precipitation/dissolution. Unless flushing solutions, which introduce additional, non-reactive, ions as charge carriers, are used, the current usually diminishes over time (Dzenitis, 1997). Thus difference in the current data among the four tests can be explained on the basis of their affinity towards the hydrophobic contaminants. 20% n-Butylamine was found to be more effective in solubilizing the contaminants from the field soil as compared to surfactants and cyclodextrin (as discussed in Section 3.3). The trend of current values of 5% Igepal 720 and 3% Tween 80 enhanced system was quite similar but slightly higher current values were recorded with 5% Igepal enhanced

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system. This is due to the formation of more stable micelles due to the higher concentration of the Igepal. The lowest current values were recorded for HPCD enhanced system. The higher initial values of electric current are obviously due to the higher electrolyte concentration in the pore water but soon it decreased and showed a fluctuation in the later stages of operation. This is due to the constant change in polarization of soil particles due to the change in the double layer by the action of HPCD solution. 3.2. Electroosmotic flow Fig. 5 represents the electroosmotic flow data for all tests. In all the tests, the electroosmotic flow at the cathode increased with an increase in the operating duration, i.e. elapsed time. It can be seen that electroosmotic flow behavior was dependent on the type of flushing solutions. Maximum electroosmotic low was observed within the cosolvent system, while the lowest flow was observed with Tween 80 surfactant system. A total of 7.2, 2.1, 10.7 and 7.5 pore volumes of flow were measured in tests with 5% Igepal CA-720, 3% Tween 80, 20% n-Butylamine, and HPCD tests, respectively. The electroosmotic flow variation is found to be consistent with their respective trend as observed for the variation in current densities in all the tested systems. As seen in Fig. 4, the electric current varies significantly with elapsed time, and was attributed to the physico-chemical processes, such as the electromigration of ionic species and the electrolysis reactions. These processes affect the surface charge of the soil particles (zeta potential) and the pore fluid properties, such as dielectric constant and viscosity, with time, and hence influence the electroosmotic flow. Initially, during the beginning of the test, when the current is high (electromigration is high), the transfer of momentum to the surrounding fluid molecules may be substantial. This often corresponds to a significant volume of electroosmotic flow. A high ionic strength of the pore fluid can also be detrimental for electroosmotic flow, because it reduces the thickness of the diffuse double layer and, thereby, constricts the electroosmotic flow. The charge on the soil surface must also be considered, because when the pH is below its ZPC, the soil particle surfaces possess a positive zeta potential and the electroosmotic flow occurs towards the anode, and when the pH is above the ZPC, the soil particles have a negative zeta potential and the electroosmotic flow occurs towards the cathode. After few days, it was observed that the electroosmotic flow sharply decreased with time. The reason for this is that electroosmotic flow was inhibited by a decrease of

zeta potential of soil particles by excess H+ and heavy metal precipitation by excess OH−. 3.3. Contaminant removal Effluent samples collected at different time intervals for all the tested systems were analyzed for metals and PAHs. Fig. 6(a) shows the cumulative metal removal for all of the metals shown in Table 2. These plots revealed that 20% n-Butylamine has pronounced affinity as compared to other flushing solutions for the cumulative removal of metals from the soil under the constant voltage gradient. Since toxic metals (all metals except Al, Ca, Fe, Mg, K and Na in Table 2) are of prime concern, the cumulative toxic metal removal is depicted in Fig. 6(b). The removal of total PAHs with number of pore volumes is shown in Fig. 6(c). These results show that 20% n-Butylamine cosolvent has a maximum affinity towards the removal of metals and PAHs as compared to surfactants and HPCD. The affinity of all the tested systems decreases in the following order 20% n-Butylamine N 5% Igepal CA-720 N 10% HPCD N 3% Tween 80 system. It is pertinent to mention here that a very low concentration of the surfactants and HPCD were employed during these investigations as compared to n-Butylamine system. Therefore, the performance of these systems may be increased using higher concentrations of flushing solutions. 3.4. Moisture content, pH, redox potential and electrical conductivity The initial moisture content of the MGP soil was 15%, and the variation of moisture content with normalized distance from the anode after the electrokinetic treatment is shown in Fig. 7(a). The normalized distance is defined as the distance to the specific location from the anode divided by the total distance from the anode to the cathode. In general, moisture content of the soil near the anode increased slightly, while the moisture content near the cathode decreased slightly. This behavior can be seen in HPCD system, where the moisture content clearly increases at anode and decreases at cathode. In 20% n-Butylamine system moisture content was high in the first three soil specimens but then sharply decreases and again increases at cathode end. This may be due to the enhanced electroosmotic flow behavior of this system. However interesting results were obtained for the surfactant enhanced systems. It is pertinent to mention here again that the concentration of the surfactants employed in these investigations were low. It has been

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observed that in 5% Igepal-system, the moisture content of all the soil specimens was found to be higher than the other tested systems. The moisture content decreases from anode to cathode. But in 3% Tween 80-system the moisture content at anode was found to be low at anode end and was found to be just 15% (initial soil condition) in the other soil specimens. Overall it is concluded that the electrokinetic process in all the tested systems does not significantly alter the moisture content. Slight changes in moisture contents are evident which can be attributed to the variations in the electroosmotic flow that occurred as a result of the changes in parameters such as the ionic strength, conductivity, and/or electrical gradient. These results suggest that the electroosmotic flow might not be uniform and there might be changes in pore pressures (Eykholt, 1997). Nevertheless, it appears

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that the soil moisture content remained fairly consistent and comparable to the initial moisture content. It is possible that regions where the electroosmotic flow was high, a pressure gradient was created so that the solution was pulled from regions where the electroosmotic flow was lower. Since the solution was continuously transported through the soil, the moisture content did not substantially deviate from the initial moisture content. Fig. 7(b) shows the pH distribution across the soil after the completion of the experiments. Considering that the MGP soil had a pH of 6.9 before the experiments, the soil pH after the experiment was analyzed for each soil specimen of all the tests. Generally, the electrolysis of water results in the formation of H+ ions (low pH solution) at the anode and OH− ions (high pH solution) at the cathode, and, primarily due to

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

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electromigration, these ions tend to migrate towards the oppositely charged electrode(s). Because of high acid buffering capacity of the MGP soil, the H+ ions are neutralized and are not migrated through the soil. However, OH− migrate through the soil towards the anode. Thus, Fig. 7(b) illustrates that a weak acidic front of solution was generated by the electrolysis reaction at the anode and pH slightly decreased in the first section near the anode in the surfactant and HPCD enhanced systems. In the second section the difference in the pH behavior becomes more pronounced as it increases for surfactant enhanced systems while remains approximately the same as initial pH in the HPCD enhanced systems. The third section, which is closest to the cathode, high soil pH was observed in the surfactant enhanced systems while remain constant to the initial

pH in the HPCD system. In the 20% n-Butylamine system, the soil pH remains maximum for all the soil section specimens. This cosolvent is highly alkaline in nature and its migration into the soil by electroosmosis increased pH throughout the soil. The transport of OH− into the soil from the cathode also contributed to increase in soil pH. The redox potentials of the soil specimens for all the tested systems are shown in Fig. 7(c) and reflect the opposite trend to that observed for pH. Redox potentials were low for the cosolvent test, while they were high for the HPCD test. Electrical conductivity values, as shown in Fig. 7(d), reveal that the test with cosolvent had higher electrical conductivity and it decreased significantly from the anode to the cathode. On the other hand, the tests conducted with surfactants and HPCD had

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

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lower electrical conductivity and the values increase slightly from the anode to the cathode. The lowest electrical conductivity was observed in the Tween 80 enhanced system. 3.5. Residual contaminant distribution After the completion of experiments, the soil samples 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-Butylamine enhanced system was sectioned into five equal parts: S1 (near anode), S2, S3 (middle), S4, and S5 (near cathode). The contaminant concentrations determined for each of these sections are plotted together in order to elucidate the migration behavior of the contaminants through the soil. Fig. 8(a) and (b) show the residual distribution of total metals and PAHs concentrations respectively, in

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the MGP soil treated with 5% Igepal CA-720. As seen from Fig. 8(a) all the metals are found to be evenly distributed throughout the soil sample even after the completion of the test. Only cadmium concentration was found to be higher at cathode i.e. section S3, while copper 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 complexes migration towards cathode and anode. This shows that soluble metals are negligible in the field soil. This also reflects that Igepal CA-720 is not suitable for the removal of metals from the MGP soil under the investigated concentration range. In contrast, Fig. 8(b) showed that Igepal CA-720 has strong affinity to remove a wide array of PAHs from the MGP soil. This plot also indicates that all the PAHs from the MGP soil were significantly removed near anode i.e. section S1 and middle section i.e. section S2. Comparatively higher concentration of PAHs is found to be

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

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accumulated at cathode end i.e. section S3. This reflects the migration behavior of PAHs from anode towards cathode. The results also confirm that hydrophobic character of the PAHs increases with the number of rings, as the concentration of higher ringed PAHs was found high in S2 section, indicating that these PAHs were strongly attached to the soil under the investigated test conditions. Figs. 9(a), 10(a) and 11(a) show the residual metal concentrations in different sections for tests conducted with 3% Tween 80, 20% n-Butylamine, and 10% HPCD, respectively. These findings suggest that only 3% Tween 80 system influenced the migration of mercury from the soil samples. Though the removal was not so significant, but it was found comparably suitable

for the removal of mercury complexes from the soils towards anode. Based on these results, it can be formulated that there are no significant changes in the metal concentrations of different sections for all these tested systems. This implies that the metals are not migrated towards the electrodes under the influence of flushing solutions used. This indicates that flushing solutions used were not effective for desorption and/or dissolution of metals in the soils (Maturi, 2004). This may be due to significant amount of organic matter that strongly adsorbed metals. In addition, the high buffering capacity of the soil may have caused metals to exist as precipitates. Thus strong adsorption and precipitation of metals results into low migration and for this reason metals did not exist in pore water.

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

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The residual concentrations of PAHs in different sections are plotted in Figs. 9(b), 10(b), and 11(b), for the tests conducted with Tween 80, n-Butylamine, and HPCD, respectively. Fig. 9(b) shows low concentration of PAHs at anode indicating the migration of PAHs occurs from cathode to anode. Further the concentration of all the PAHs was found to be higher in the third section i.e. S3 section. This reflects that the flushing solution Tween 80 is also suitable to solubilize appreciable amount of PAHs from the soil sample. It is pertinent to mention here that the concentration of Tween 80 employed in this system was just 3% and was comparatively low with respect to the other flushing solutions including Igepal CA-720 system. Fig. 10(b) shows very low migration trend of PAHs in 20% n-Butylamine test, while Fig. 11(b) shows some migration of selected PAHs towards the cathode (i.e., from section S1 towards sections S2 and S3) in HPCD test. In general the concentration of PAHs increased from section S-1 to section S-3 in all the studied systems. This shows that PAHs migrated towards the cathode. One conclusion that can be drawn regarding this observation is that the use of appropriate flushing solution enhances desorption/solubilization of PAHs in soils. It is also observed that surfactant enhanced removal of PAHs depends upon the micelle formation at appreciable CMC. The differences between the efficiency of 5% Igepal 720 and 3% Tween 80 most likely resulted from competitive behavior among various PAH compounds for partitioning into the stable micelles as well as competitive sorption of PAH compounds and surfactant to the soil organic matter and soil particles. This study also indicates that 10% HPCD system had contributed partial solubilization of the PAHs resulting in their migration towards the cathode. HPCD enhanced system was found to be more effective for the solubilization of low polarity PAHs. This partial solubilization of low-polarity PAHs is attributed to the formation of inclusion complexes within the relatively non-polar cavity of the HPCD. The higher electroosmotic flow in 20% n-Butylamine test resulted in higher contaminant removal as compared to other tests; however, very low migration trend of PAHs in this test show that 20% n-Butylamine did not effectively solubilize/desorb PAHs in the soil. The variation in the concentrations of PAHs in their respective sections may also be contributed by heterogeneous distribution of the contaminants in the soil. These results show that Igepal CA-720, Tween 80 and HPCD systems are effective for solubilization of the PAHs from the MGP soil (under investigated conditions). These studies also elucidate that high buffering capacity of the soil also impede the efficiency of the

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contaminant removal. Metals are readily precipitated under the tested conditions due to the high buffering capacity of soil. PAHs were found to be efficiently solubilized by the flushing solutions under the experimental conditions. Substantial electroosmotic flow can be induced in the MGP soils using different flushing solutions resulting in the appreciable removal of contaminants from the MGP soil. It is also believed that longer durations and different applied voltage gradient and higher concentrations of the flushing solutions may also result in the better contaminant removal efficiency. 4. Conclusions Based on the experimental results, the following conclusions may be drawn: • The manufactured gas plant soil used in this study was contaminated with both heavy metals and polycyclic aromatic hydrocarbons. It is feasible to enhance extraction of PAHs using surfactants, cosolvents and cyclodextrin from this aged MGP field soil. However, no significant removal of heavy metals was observed in this study. This also indicates that heavy metals are mostly present as precipitates due to high pH and high acid buffering capacity of the soil. • Substantial electroosmotic flow can be induced in the soil using different flushing solutions. Maximum electroosmotic flow was observed in the 20% n-Butylamine enhanced system followed by HPCD enhanced system. Comparatively low flow was observed in surfactant enhanced systems. • PAHs were solubilized in the surfactant and HPCD enhanced systems more efficiently even at low concentration as compared to cosolvent system resulting in significant migration towards the cathode. The solubilization of PAHs using surfactants depends upon the stability and number of the micelles formed during the test. The mechanism of PAHs solubilization in HPCD enhanced system was found to be partial solubilization. The migration of PAHs in n-Butylamine enhanced system was attributed to desorption phenomenon. • The partially solubilized PAHs migrated from anode towards the cathode due to electroosmotic flow. The soil pH remains high due to its high pH buffering capacity and under such conditions heavy metals remain strongly adsorbed/precipitated. Therefore, the metals are not electromigrated and are not removed from the soil under all the tested systems.

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Acknowledgements The financial support for this project was received from a Technology Challenge Grant provided by the State of Illinois. The authors are grateful to Kranti Maturi and Craig Chawla for their assistance in this project. References American Society of Testing and Materials (ASTM), 2004. Annual Book of Standards, vol. 04.08. Soil and Rock, West Conshohocken, PA. Chowdiah, P., Misra, B.R., Kilbane II, J.J., Srivastava, V.J., Hayes, T.D., 1998. Foam propagation through soils for enhanced in situ remediation. J. Hazard. Mater. 62, 265–280. Dzenitis, J.M., 1997. Steady state and limiting current in electroremediation of soil. J. Electrochem. Soc. 144, 1317–1322. Eykholt, G.R., 1997. Development of pore pressures by nonuniform electroosmosis in clays. J. Hazard. Mater. 55, 171–186. Hatheway, A.W., 2002. Geoenvironmental protocol for site and waste characterization of former manufactured gas plants; worldwide remediation challenge in semi volatile organic wastes. Eng. Geol. 64, 317–338. Lee, P.H., Ong, S.K., Golchin, J., Nelson, G.L., 2001. Use of solvents to enhance PAH biodegradation of coal tar. Water Res. 35, 3941–3949.

Maturi, K., 2004. Enhanced electrokinetic remediation of soils contaminated with co-existing PAHs and heavy metals. M.S. Thesis, University of Illinois, Chicago, Illinois. McGowan, T.F., Greer, B.A., Lawless, M., 1996. Thermal and nonthermal technologies for remediation of manufactured gas plant sites. Waste Manage. 16, 691–698. Reddy, K.R., Chinthamreddy, S., 2003. Sequentially enhanced electrokinetic remediation of heavy metals in low buffering clayey soils. J. Geotech. Geoenviron. Eng., ASCE 129 (3), 263–277. Reddy, K.R., Parupudi, U.S., 1997. Removal of chromium, nickel, and cadmium from clays by in situ electrokinetic remediation. J. Soil Contam. 6, 391–407. Saichek, R.E., Reddy, K.R., 2004. Evaluation of surfactants/ cosolvents for desorption/solubilization of phenanthrene in clayey soils. Int. J. Environ. Stud. 61 (5), 587–604. Shosky, D.J., 1996. Emerging technologies for recycling MGP sites. Fuel Energy Abstr. 37, 56. United States Environmental Protection Agency (USEPA), 1986. Test Methods for Evaluating Solid Waste Third Edition. Laboratory Manual, Physical/Chemical Methods, SW-846, vol. 1A. Office of Solid Waste and Emergency Response, Washington, D.C. United States Environmental Protection Agency (USEPA), 2000. A Resource for MGP Site Characterization and Remediation. Office of Solid Waste and Emergency Response, Washington D.C. Williamson, D.G., Raymond, C.L., Kimura, Y.C., 1998. Release of chemicals from contaminated soils. J. Soil Contam. 7, 543–558.