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Keywords: Cd, electrokinetic soil processing, kaolinite, Pb, removal efficiency, soil pH ... ing abandoned mining wastes, improper treatment of industrial wastes, ... for waste remediation has resulted in several studies (Putnam, 1988; Khan et al.,.

REMOVAL OF HEAVY METALS FROM SOILS USING ENHANCED ELECTROKINETIC SOIL PROCESSING SOON-OH KIM, SEUNG-HYEON MOON and KYOUNG-WOONG KIM∗ Department of Environmental Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712, Korea (∗ author for correspondence, e-mail: [email protected]

(Received 3 August 1999; accepted 31 January 2000)

Abstract. In order to remediate contaminated land, a new process of electrokinetic purging of heavy metals from saturated soil is examined by laboratory experiments. Electrokinetic soil remediation is one of the most promising soil decontamination processes as it has high removal efficiency and time-effectiveness in low permeability soils such as clay. Being combined with several mechanismselectromigration, electroosmosis, diffusion and electrolysis of water, electrokinetic soil processing can remove non-polar organics as well as ionic contaminants. This study suggests that the removal efficiencies for Pb and Cd are significantly influenced by applied voltage and current, type of purging solutions, soil pH, permeability and zeta potential of soil. The removal efficiencies for Pb and Cd were 75–85% for the kaolinite soil and 50–70% for the tailing soil over the duration of 4 days. For heavy metals, their adsorption capacities on the soil surface and mobilities in soil have significant effects on the removal efficiency. Keywords: Cd, electrokinetic soil processing, kaolinite, Pb, removal efficiency, soil pH, tailing-soil

1. Introduction Soils can be contaminated with heavy metals derived from various sources including abandoned mining wastes, improper treatment of industrial wastes, incomplete collection of used batteries, leakage of landfill leachate, accidental spills and military activities (Adriano, 1986). The contamination often affects a large volume of soil underlying several acres of the surface area. There are also various types of contaminated lands such as paddy fields, farms, factory sites, mine fields and residential districts. Contaminants migrating from these sources threaten the human health in the local area and the ground-water supply. However, technologies for decontaminating these sites have not been well developed. In addition, it has been recently reported that soil contamination is increasing in various sites such as residential areas near industrial complexes and reservoirs of drinking water. Decontamination of hazardous waste sites is one of the most important technological challenges, and newly developed techniques for the remediation of soils can be generally classified into two groups. The first is biological remediation which has been mainly used to detoxify organic contaminants. The other is physicochemical decontamination that has been usually applied to remove inorganic conWater, Air, and Soil Pollution 125: 259–272, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.



taminants (heavy metals). This includes excavation, soil washing and flushing, solidification and stabilization, electrokinetic soil processing (Chambers et al., 1991). Although many techniques have been proposed for removing contaminants from waste sites, most suffer from several technical or economic disadvantages (Probstein and Renaud, 1986; Shapiro, 1990). Electrokinetic soil processing is also called electrokinetic remediation, electroreclamation, electrochemical decontamination. It needs low-level direct current in the order of mA cm−2 of cross-sectional area between the electrodes to remove contaminants from soils (Acar and Alshawabkeh, 1993). The low-level direct current results in physico-chemical and hydrological changes in the soil mass, leading to species transport by coupled mechanisms. Electrolysis of water produces hydrogen ions in the anode compartment and the acid front migrates across the soil cell and makes contaminants desorbed from soil surfaces, resulting in an initiation of electromigration, i.e. the transport of ions and polar molecules in electric field. Electric potential also leads to electroosmosis, i.e. the flow of an ionic liquid under the action of an applied electric field relative to a charged surface (Reuss, 1809; Acar et al., 1995). Electromigration and electroosmosis are the important mechanisms in the electrokinetic soil processing which remove contaminants from soil. Electrokinetic soil processing is an effective technology for removal of contaminants in low-permeability soils ranging from clay to clayey sand. The advantages of this technology are its low cost of operation and its potential applicability to a wide range of contaminant types (Pamukcu and Wittle, 1994). Electrokinetic soil processing is envisioned for the removal/separation of organic and inorganic contaminants and radionuclides. The potential of the technique for waste remediation has resulted in several studies (Putnam, 1988; Khan et al., 1989; Thompson, 1989; Mitchell and Yeung, 1991). Electrokinetic remediation technology has recently made significant strides and been tested for commercial application in the United States and Netherlands. The company named Geokinetics (The Netherlands) has successfully completed several field studies (Lageman, 1993); Electrokinetics Inc. (Baton Rouge, LA) has completed several large-scale pilot studies using 2–4 ton soil specimens (Acar and Alshawabkeh, 1993). The objectives of this study are; (1) to explore the feasibility of electrokinetic soil processing on the removal of heavy metals from soils, (2) to find out the optimum condition for the most efficient removal, (3) to investigate the applicability to the tailing-soils in abandoned mining area, (4) to examine the effect of soil pH and conductivity on the transport phenomena of elements in the soil, and (5) to relate the removal efficiencies to the different behaviours of contaminant species in soils.



Figure 1. Schematic diagram of experimental apparatus.

2. Materials and Methods 2.1. E XPERIMENTAL


A schematic diagram of the experimental apparatus used in this study is shown in Figure 1. The experimental apparatus consists of four principle parts; soil cell, electrode compartments, electrolyte solution reservoirs and power supply. The acryl soil cell measures 9 × 9 × 15 cm with a volume of 1215 cm3 . Each end of the soil cell had 81 holes (diameter; 0.5 cm) to enhance uniform electroosmotic flow. At both sides of the soil cell, two sheets of GF/B filter paper were inserted to prevent clay particles from flowing into the electrode compartments. Platinum wire was placed in the both ends of the soil cell to measure the overall voltage drop of soil cell and electrode compartments. Six stainless steel electrodes were inserted into the soil cell to measure the voltage drop between the sampling locations. Platinum wire that was plated like a net of 9 × 9 cm with the interval of 1 cm was used as the anode to prevent electrode-electrolysis reaction, and titanium plate, 11 × 11 cm, was used as the cathode. The electrode compartments contained 400 mL of electrolyte solution ensuring that sufficient volume was present to avoid sudden variations of electrolyte solution. The compartments also have a role to reject gas from the electrode, and to provide water for electroosmosis. Two mass cylinders (2 L volume) were used as electrolyte solution reservoirs to measure the water volume transported. The electrolyte solutions were recirculated in both electrode compartments by peristaltic pumps (Masterflex, 1 ∼ 100 rpm, 3 heads), and



TABLE I Physical properties of kaolinite soils used in this study Physical property

Measured value

Group symbol according to USCS Liquid limit (%) Plastic limit (%) Specific gravity pH of soil at 50% water content Permeability (cm sec−1 ) Initial water content (%)

CL 78 32 2.64 4.93–5.20 1 × 10−7 50–54

a BIORAD DC power supply (model PowerPac 200, 5–200 V, 0.01–2 A, 200 W) was used. 2.2. P HYSICAL


Soils used in this experiment were commercial kaolinite soils contaminated with Pb and Cd artificially and tailing-soils taken from the abandoned Gubong mining area which is located in the middle of South Korea. Tailing-soils were contaminated with Pb, Cd, Zn and Cu. The kaolinite soils were passed through an ASTM No. 200 sieve of mesh size 75 µm. The physical properties of kaolinite soils used in this study are summarized in Table I. In order to be valid for the analysis, tailing-soils were pretreated by No. 80 sieve of mesh size 180 µm. Two soil samples were prepared for the removal experiments of heavy metals in soils. The first was the kaolinite soil which was artificially contaminated with Pb(NO3 )2 and Cd(NO3 )2 solutions. One liter of 1000 mg L−1 Pb(II) and Cd(II) solutions were prepared by dissolving 1.615 g Pb(NO3 )2 and 2.801 g Cd(NO3 )2 respectively in 1 L of distilled water. Two kilogram samples of air-dried kaolinite soils were mixed with the prepared solution of 1000 mg L−1 Pb(II) and Cd(II) at 50% water content. The other was the tailing soil taken from the abandoned Gubong mining area which were contaminated with Pb, Cd, Zn and Cu. Tailing-soil samples were analyzed by ICP-AES to determine the initial concentrations of Pb, Cd and other elements (Cu, Zn). Four kilogram of tailing-soils were mixed with 1 L of distilled water to give 25% water content. The slurries of kaolinite and tailingsoils were mixed mechanically for 1 hr with a electric stirrer, and these mixtures were allowed to settle down for more than 3 days to attain the uniform distribution of contaminants and to complete adsorption in the soil samples. Five kilogram of three samples was then taken from the prepared soils for the determination of initial concentration of contaminants.





In order to compare the removal efficiencies of different contaminants and soil types under the same conditions, the equivalent anode purging and cathode electrolyte solutions were used. Anode purging solutions of 0.005 N H2 SO4 solution (2 L) in kaolinite soil experiments, and 0.005 N H2 SO4 solution (4 L) in tailing-soil experiments were used over a four day period. Hydroxides were precipitated by hydroxide ions generated by electrolysis of water in the cathode compartment. These precipitates prevented the removal of contaminants from the soil cell, and cathode electrolyte solutions may buffer the concentration of hydroxide ions. Therefore, 1 L of 0.5 N H2 SO4 solution was used as the cathode electrolyte solution in both kaolinite and tailing-soil experiments. In order to enhance the effectiveness of the process in removing heavy metals from soils, concentrations and volumes of anode purging and cathode electrolyte solutions were determined by preliminary experiments. Three tests were conducted for the removal of heavy metals from soils by electrokinetic soil processing. Constant-current was used in all tests to keep the net rates of the eletrolysis reactions constant and to minimize complicated currentboundary conditions during the experiment. All experiments were carried out under the same conditions of applied current, area and length of soil cell, and duration of treatment. Anode purging and cathode electrolyte solutions were compared for Pb and Cd depending on the difference of the removal efficiencies. The parameters measured in each experiment are summarized in Table II. 2.4. M EASUREMENT,


The prepared soil samples were packed into the soil cell and the acryl cover was set on the packed soil cell. The cover was then sealed tightly by silica bond in order to prevent the leakage of porewater. The anode and cathode electrolyte solutions were pumped into the electrode compartments for 30 min. without electric currents to equalize the electrolyte solutions. During the experiments, the overall voltage drops of the soil cell and electrode compartments; pH and conductivity variations of anode purging and cathode electrolyte solutions; soil pH variation and transported porewater volume by electroosmotic flow were measured every 4 hr. Five samples were obtained from the soil bed using a stainless steel sampler (diameter; 1.2 cm) at every 3 cm to analyze the concentrations of contaminants and soil pH. The duration of all tests was 96 hr. Heavy metals in the samples taken during the experiments were extracted with 0.1 N HCl; Wet soil samples were dried at 110 ◦ C. 50 mL of 0.1 N HCl solution was added into 5 g of each dry soil sample (dilution factor; 10) and agitated (100 rpm, 30 ◦ C and 1 hr). The solutions were then analyzed by ICP-AES (Thermo Jarrel Ash) and AAS (Perkin Elmer-PC5100).



TABLE II Summary of testing program and measured parameters for the removal experiment of heavy metals Parameters

Test 1

Test 2

Test 3

Soil specimen Contaminants Initial concentration of contaminants (µg g−1 ) Applied current (A) Area of soil cell (cm2 ) Length of soil cell (cm) Duration (hr) Anode purging solution Cathode electrolyte solution

Kaolinite Lead (Pb(NO3 )2 ) 391

Kaolinite Cadmium (Cd(NO3 )2 ) 367

0.1 81 15 96 0.005 N H2 SO4 Solution 2 L−1 0.5 N H2 SO4 Solution 1 L−1

0.1 81 15 96 0.005 N H2 SO4 Solution 2 L−1 0.5 N H2 SO4 Solution 1 L−1

Tailing Pb, Cd 1438 (Pb) 22 (Cd) 0.1 81 15 96 0.005 N H2 SO4 Solution 2 L−1 0.5 N H2 SO4 Solution 1 L−1

3. Results and Discussion 3.1. VARIATION


Variations of pH in the soil cell are shown in Figure 2. Electrolysis of water in the anode compartment generated hydrogen ions and those hydrogen ions migrated from anode to cathode. Over the period of the experiment, the overall soil pH decreased but the decrease was less marked toward the cathode compartment. It was this migration of the acid front that made the soil pH decrease. As shown in Figures 2a and b, corresponding to tests 1 and 2, one kind of anode purging solution and kaolinite soil was used, resulting in a similar pH pattern. However, test 3 was conducted in tailing-soil which had large pH buffering capacity and the overall soil pH in test 3 was higher than those in tests 1 and 2 (Figure 2c). This large pH buffering capacity of tailing-soil decreased dissolution and desorption of adsorbed species on the soil surface in test 3. If the soil pH data are combined with the removal efficiency at different locations in the soil cell, the influence of the migration of the acidic front from anode to cathode should be taken into great account due to the effect of dissolution and desorption on heavy metal ions. This result is consistent with other previous researches (Putnam, 1988; Acar et al., 1989; Acar and Alshawabkeh, 1993). Figure 3 shows the voltage profiles of the three tests. The voltage drop in the soil cell is related to the migration of ions in the soil cell and power consumption. Since all tests were conducted under the constant current, the voltage profile reflects



Figure 2. Variation of pH at sampling points in the soil cell during the removal process.

the conductivity and resistance of the soil bed directly. Voltage profiles for tests 1 and 2 showed similar patterns. The reason was that two tests were carried out under the equivalent current (0.1 A) and in kaolinite soils. In the initial stage of the treatments, the desorption mechanism was much more dominant than the migration of the species and the resistance and voltage drop increased in the soil bed. In the later stages of the tests, however, desorbed species migrated toward the cathode compartment, and the resistance and voltage drop gradually decreased across the soil cell. The voltage profile of test 3 was quite different from those of tests 1 and



Figure 3. Voltage profile in the soil cell during the removal process.

2 as shown in Figure 3. The tailing-soil in test 3 had a much larger void fraction than kaolinite soils used in tests 1 and 2. During the duration of test 3, most of the hydrogen ions did not participate in desorbing species, but migrated toward the cathode compartment in the pore fluid. In test 3, there were other contaminants in the tailing-soil besides Pb and Cd such as Zn (926 µg g−1 ) and Cu (156 µg g−1 ). Continuous migration of the hydrogen ions in pore fluids and the increase of desorbed species meant the resistance in the soil cell decreased, resulting in a gradually decreasing voltage drop in test 3. 3.2. VARIATION



Variations of the conductivity within the anode purging and cathode electrolyte solutions and the soil cell are shown in Figure 4. The conductivity in anode purging and cathode electrolyte solutions was directly measured using a conductivity meter, but the conductivity in the soil cell was calculated in Equation (1) by Acar et al. (1989): Ka = Is L/Vs A


where, Ka is apparent conductivity of soils (mS cm−1 ), Is is current (mA), L is the length of soil cell (cm), Vs is voltage drop in soils (V ) and A is cross-sectional area of soil cell (cm2 ). Since the current density (Is /A) and the length (L) were constant in this study, the conductivity was inversely proportional to the voltage drop in the soil cell. As hydrogen ions were produced by the electrolysis of water in the anode compartment, conductivity in the anode purging solution increased during



Figure 4. Variation of conductivity during the removal process.

the treatment. However, hydroxide ions produced in the cathode compartment were buffered by the cathode electrolyte solution of sulfuric acid, and the conductivity in the cathode electrolyte solution decreased gradually. The variations of conductivity within the soil cell were different between tests 1 and 2 and test 3 (Figure 4). In the initial stages of tests 1 and 2, desorption by hydrogen ions was dominant in the soil system, resulting in decreasing conduct-



Figure 5. Volume of transported water by electroomosis.

ivity within the soil cell. However, in the later stage of the treatments, transport of hydrogen ions and desorbed species were more important than desorption, and conductivity increased constantly. In test 3, hydrogen ions and other species in soils migrated continuously owing to the large void fraction. Therefore, the conductivity in the soil bed increased during the treatment. The variation trends of conductivity in tests can explain the resistance, power consumption and transport phenomena of the existing species in the soil system. 3.3. WATER


In electrokinetic soil processing, pore water is transported by electroosmosis. The volume of transported water by electroosmosis is shown in Figure 5. Electroosmosis is affected by soil pH, applied electric field intensity and soil permeability. Since the pH and conductivity of the anode purging solution determine the pH and conductivity of the soil bed, the anode purging solution may affect electroosmosis in soils. Electroosmotic velocity on a plane surface, U (m sec−1 ), is expressed as U = −ζ Ex /µ


where,  is permitivity of the medium (CV −1 m−1 ), ζ is zeta potential (V ), Ex is electric field (V m−1 ) parallel to a direction of electroosmotic flow and µ is viscosity of the medium (N sec m−2 ). This formula for the electoosmotic velocity on a plane charged surface is known as the Helmholtz-Smoluchowski equation (Shapiro, 1990; Mitchell, 1991; Pamukcu and Wittle, 1994; Probstein, 1994). According to the Helmholtz-Smoluchowski Equation (2), the electroosmotic velocity (U ) is proportional to zeta potential (ζ ). If the cationic concentrations in pore water



TABLE III Removal efficiency of heavy metals for each test to remove (unit: %) Time (day)

Test 1

Test 2

Test 3 (Pb)

Test 3 (Cd)

1 2 3 4

14.5 28.9 66.1 74.9

34.2 45.8 55.1 85.5

12.7 20.3 34.6 49.8

39.2 53.6 63.6 67.6

increase by increasing the cationic concentrations in the anode purging solution, the cation concentrations adsorbed to the surface of the negative clay particles should increase. The increased concentration of adsorbed cations results in reduction of the zeta potential. The electroosmotic velocity then decreases by increasing of the cationic concentration in pore water. Consequently, the decrease of the soil pH and the increase of the conductivity of pore water will make the electroosmotic velocity decrease. As shown in Figure 5, the volume of transported pore water decreased significantly from the middle stage of each test. Since the applied electric field intensity in test 1 was very high compared with other two tests, the amount of transported water was more than the other two tests. In test 3, soil pH was relatively high (>5) during the treatment, and it caused the increase of electroosmotic flow. In test 2, the small amount of transported water was due to the low soil pH and the low electric field intensity. 3.4. R EMOVAL


The variation in the amount of contaminants in the soil cell for each test is shown in Figure 6. In the electric field, the acid front produced by electrolysis of water in the anode compartment migrated toward the cathode by electromigration and electroosmosis with desorbing contaminants adsorbed on the soil surface. The adsorption of hydrogen ions and the desorption of adsorbed species on the soilsurface were repeated in the soil bed during the treatment, and the contaminant species appear to be gradually transported toward the cathode in the electric field. The migration of desorbed species by hydrogen ions is shown in Figure 6, and the overall amount of contaminants in the soil cell gradually decreased as time went on. Table III presents the removal efficiency for each test. When the removal efficiency of the test using kaolinite soils was compared with that of the test using tailing-soils, the former showed higher efficiency than the latter. This is because tailing-soils had a larger void fraction than kaolinite soils. Hydrogen ions, therefore, have less chance of contact with the soil surface in tailing-soils than in kaolinite soils, and the chances of desorption of adsorbed species on the soil-surface



Figure 6. Variation of heavy metal concentration in the soil cell during the removal process (♦ = 0 day;  = 1 day; 4 = 2 day; # = 3 day; × = 4 day).

decreased. In the tests on the removal of Pb(II), the lower amount of removed Pb(II) was caused by the greater adsorption capacity and the immobility of Pb(II) (Rose et al., 1979; Alloway, 1995), as shown in Figures 6a and c. In the case of the Cd(II) removal test, the amount of Cd(II) removed was higher than that of Pb(II) owing to the lower adsorption capacity and higher mobility of Cd(II). Table IV presents the energy consumed during each test. In tests 1 and 2 for kaolinite soil, adsorption mechanisms influenced the soil bed significantly, and more



TABLE IV Energy consumption in each test (unit: kWh ton−1 ) Time (day)

Test 1

Test 2

Test 3

1 2 3 4

106.4 284.6 412.4 501.5

66.2 169.4 265.6 328.7

86.9 129.9 167.2 197.3

energy was consumed than in test 3 for tailing-soils. Because of the strong bonding of Pb(II) on the soil-surface, energy consumption in test 1 on the removal of Pb(II) was higher than that in test 2 for the removal of Cd(II). As shown in Table IV, energy consumption in the initial stage of the treatment was higher than that in the later stage, resulting from adsorption and desorption mechanisms in the soil cell. After the desorbed species began migrating, the rate of energy consumption decreased significantly.

4. Conclusions From this study on the removal of heavy metals from soils by electrokinetic soil processing, the following conclusions can have been obtained: (1) Heavy metals were effectively removed from artificially contaminated kaolinite and tailing-soils taken from an abandoned mine by enhanced electrokinetic soil processing in which acidic solution was used as a solubilizing agent. (2) The removal efficiencies for Pb(II) and Cd(II) in tailing-soils were lower than those in artificially contaminated kaolinite. It may be explained by the facts that the dissolution and desorption of heavy metals on the soil surface decreased in tailing-soils due to the large pH buffering capacity and that there were other cationic contaminants (Cu and Zn) competing with target contaminants (Pb(II) and Cd(II)). (3) The removal efficiencies of Cd(II) in kaolinite and tailing-soils were 86 and 68%, but those of Pb(II) were 75 and 50%, respectively. The relatively lower removal efficiency of Pb(II) compared to Cd(II) was explained by the greater affinity or adsorption onto soil surface and immobility of Pb(II) in soils. (4) Energy consumption was mainly dependent on the conductivity of soil cell. As the adsorption and desorption mechanisms were predominant on removing heavy metals from soils, the species in soils could not migrate freely through the soil bed, and the conductivity of the soil cell decreased.



Acknowledgements This research was supported by the Korea Research Foundation (Project No. 1997001-E00576) and by the Korea Science and Engineering Foundation (KOSEF) through the Advanced Environmental Monitoring Research Center at Kwangju Institute of Science and Technology.

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