electrokinetic remediation of metal contaminated glacial tills

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Geotechnical and Geological Engineering, 1997, 15 3-29 Electrokinetic remediation of metal contaminated glacial tills K.R. REDDY and A.B. SHIRANI Department of Civil and Materials Engineering, University of Illinois at Chicago, 2095 Engineering Research Facility, 842 West Taylor Street, Chicago, Illinois 60607, USA Received 1 January 1996 Accepted 29 November 1996 Summary This paper presents the results of an experimental investigation which studied the feasibility of using the electrokinetic process to remediate contaminated clays of glacial origin, otherwise known as glacial tills. An overview of the electrokinetic phenomena, as well as previously performed laboratory and field investigations, is first presented. The methodology of the electrokinetic experiments which were conducted to investigate the removal of metals from a glacial till is then described. A total of 16 experiments were conducted using glacial till samples obtained from a project site near Chicago. Sodium and calcium were used as the surrogate cationic metallic contaminants. These experiments demonstrated that ion transport during the electrokinetic process occurs due to both electro-osmosis and electromigration, but that due to electromigration is significantly higher than that due to electro-osmosis. Unlike other clays such as kaolinite, the glacial till used for this investigation possessed high buffering capacity because of its high carbonate content which prevented the acid front migration from the anode to the cathode during the electrokinetic process. The ion removal efficiency of the electrokinetic process was found to increase when: (1) the voltage gradient applied to the soil was increased, (2) the initial concentration of the contaminants was increased, and (3) the duration of the treatment process was increased. The ion removal efficiency was also greater for smaller ions which possess less ionic charge and when the ions existed independently in the soil as compared to when they coexisted. This investigation suggests that the electrokinetic process has significant potential for remediating glacial tills contaminated with metals. However, the properties of Na and Ca are not representative of contaminants, such as heavy metals, so further investigations are needed. Keywords: Soil, glacial till, contamination, metals, electrokinetics, remediation Introduction In recent years, the contamination of subsurface soils and groundwater from landfills, industrial activities and other sources has generated enormous public concern and has created an urgent need to find feasible solutions to the problem. Soil and groundwater contamination has been one of the most expensive and time-consuming issues faced by environmental professionals. Due to the low hydraulic conductivity of fine-grained soils, the remediation of these contaminated soils by conventional methods such as in-situ bioremediation and in-situ chemical treatment has been found to be very costly and mostly 0960-3182 © 1997 Chapman & Hall

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Page 1: Electrokinetic remediation of metal contaminated glacial tills

Geotechnical and Geological Engineering, 1997, 15 3-29

Electrokinetic remediation of metal contaminated glacial tills K.R. REDDY and A.B. SHIRANI

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

Received 1 January 1996 Accepted 29 November 1996

Summary

This paper presents the results of an experimental investigation which studied the feasibility of using the electrokinetic process to remediate contaminated clays of glacial origin, otherwise known as glacial tills. An overview of the electrokinetic phenomena, as well as previously performed laboratory and field investigations, is first presented. The methodology of the electrokinetic experiments which were conducted to investigate the removal of metals from a glacial till is then described. A total of 16 experiments were conducted using glacial till samples obtained from a project site near Chicago. Sodium and calcium were used as the surrogate cationic metallic contaminants. These experiments demonstrated that ion transport during the electrokinetic process occurs due to both electro-osmosis and electromigration, but that due to electromigration is significantly higher than that due to electro-osmosis. Unlike other clays such as kaolinite, the glacial till used for this investigation possessed high buffering capacity because of its high carbonate content which prevented the acid front migration from the anode to the cathode during the electrokinetic process. The ion removal efficiency of the electrokinetic process was found to increase when: (1) the voltage gradient applied to the soil was increased, (2) the initial concentration of the contaminants was increased, and (3) the duration of the treatment process was increased. The ion removal efficiency was also greater for smaller ions which possess less ionic charge and when the ions existed independently in the soil as compared to when they coexisted. This investigation suggests that the electrokinetic process has significant potential for remediating glacial tills contaminated with metals. However, the properties of Na and Ca are not representative of contaminants, such as heavy metals, so further investigations are needed.

Keywords: Soil, glacial till, contamination, metals, electrokinetics, remediation

Introduction

In recent years, the contamination of subsurface soils and groundwater from landfills, industrial activities and other sources has generated enormous public concern and has created an urgent need t o find feasible solutions to the problem. Soil and groundwater contamination has been one of the most expensive and time-consuming issues faced by environmental professionals. Due to the low hydraulic conductivity of fine-grained soils, the remediation of these contaminated soils by conventional methods such as in-situ bioremediation and in-situ chemical treatment has been found to be very costly and mostly

0960-3182 © 1997 Chapman & Hall

Page 2: Electrokinetic remediation of metal contaminated glacial tills

4 Reddy and Shirani

ineffective. Electrokinetic remediation is one of the developing techniques that has significant potential for in-situ remediation of these fine-grained soils. Electrokinetic remediation involves applying a low direct current (in the order of milliamps per square centimetre of the cross-sectional area of the electrodes) or a low potential gradient (in the order of a few volts per centimeter of distance between the electrodes) to electrodes that are inserted into the ground. A typical implementation of in-situ electrokinetic remediation is shown in Fig. 1. As a result, the contaminants are transported to either the cathode or anode where they are extracted by one or more of the following methods: electroplating, adsorption onto the electrode, precipitation or co-precipitation at the electrode, pumping water near the electrode, or complexing with ion-exchange resins (Acar and Alshawabkeh, 1993; Lindgren et al., 1994).

Most of the previous electrokinetic remediation studies were performed in the laboratory using commercial clays, such as kaolinite and Na-montmorillonite, and individual chemicals, such as lead, zinc or cadmium to represent the metallic contaminants (Hamed et al., 1991; Acar et al., 1992a; Pamukcu and Wittle, 1992; Probstein and Hicks, 1993; Acar et al., 1994). The results of these laboratory studies demonstrated excellent contaminant removal efficiencies by the use of the electrokinetic process. However, recent field applications of the electrokinetic technology have shown anomalous results (Guzman et al., 1990; Lageman, 1993; US Environmental Protection Agency (USEPA), 1993), which have been attributed mainly to the interaction of the contaminants with naturally occurring electrolytes, humic substances, and mixed wastes which are present in the subsurface (Lageman, 1993; Hicks and Tondorf, 1994). In order to use electrokinetic remediation in the field successfully, the different geochemical interactions that occur in the field soils under induced electricity must first be accurately determined.

This paper presents an experimental investigation of the applicability of using the electrokinetic process for removing metallic contaminants from glacial tills. Glacial tills are the most common types of soils encountered in the Midwest of the United States. The paper first presents an overview of the fundamental contaminant transport mechanisms and the different physicochemical processes that affect these transport mechanisms during the electrokinetic remediation of soils. Then, previous laboratory and field investigations are presented. Finally, details on laboratory electrokinetic experiments conducted at the University of Illinois at Chicago to study the removal of metallic contaminants from glacial till using the electrokinetic process are presented.

Background

Electrokinetic phenomena

Electrokinetic phenomena represents the combined effects of hydraulic, chemical and electrical conditions on soil solids, pore fluid, and contaminant behaviour. Under induced electrical potential, electrolysis reactions generate H + ions and oxygen gas at the anode and OH- ions and hydrogen gas at the cathode. The H ÷ ions create an acidic environment, pH = 2-3, at the anode and the OH- ions create an alkaline environment, pH = 10-12, at the cathode (Acar and Alshawabkeh, 1993). The migration of H+ ions from the anode towards the cathode produces an acid front which flushes across the soil. The contaminants in the soil are then transported towards either the cathode or the anode depending on their

Page 3: Electrokinetic remediation of metal contaminated glacial tills

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charge. This contaminant transport occurs primarily because of the following four mechanisms: electro-osmosis, electromigration, diffusion, and electrophoresis (Fig. 1). These four mechanisms are briefly described in the following paragraphs.

Electro-osmosis is the movement of the pore fluid which contains dissolved ionic and non-ionic species, relative to the stationary soil mass, toward the cathode due to the application of a low direct current or voltage gradient to the electrodes (Pamukcu and Wittle, 1992; Shapiro and Probstein, 1993). Electromigration is the movement of the dissolved ionic species which are present in the pore fluid, including H + and OH- ions which are produced by water electrolysis, toward the opposite electrode (Acar and Alshawabkeh, 1993). For inorganic contaminants such as heavy metals, electromigration is considered the dominant transport mechanism at high concentrations of ionic species (Acar and Alshawabkeh, 1993), while electro-osmosis is dominant at lower concentrations (Gray and Mitchell, 1967). For organic compounds such as benzene, toluene, xylene, phenolic compounds and chlorinated solvents, electro-osmosis is considered to be the dominant process in electrokinetic remediation (Acar et al., 1992a; Bruell et al., 1992).

Diffusion refers to the ionic and molecular constituent forms of the contaminants moving from areas of higher to areas of lower concentration because of the concentration gradient or chemical kinetic activity (Shackleford and Daniel, 1991), and during electrokinetic remediation is generally insignificant. Electrophoresis is the transport of charged particles due to the application of a low direct current or voltage gradient relative to the stationary pore fluid. It is dominant in remediating slurries or if surfactants are present in pore fluid (Acar and Alshawabkeh, 1993), but in compact systems such as glacial till, it is insignificant because solid particles do not move (Pamukcu and Wittle, 1992; Probstein and Hicks, 1993).

The mass flux transported during the electrokinetic process depends on the transient geochemistry that takes place under the influence of an induced electrical field. Specifically, the sorption-desorption, precipitation-dissolution, and oxidation-reduction behaviour of the contaminants during the electrokinetic process significantly affect the remediation efficiencies. Sorption refers to the partitioning of the contaminants from the solution or pore fluid to the solid phase or soil surface. Sorption includes adsorption and ion exchange and it is dependent on (1) the type of contaminant, (2) the type of soil, and (3) the pore fluid characteristics. Desorption is the reverse process and is responsible for the release of contaminants from the soil surface. Both sorption and desorption are affected by soil pH changes caused by the migration of H + and OH- ions, which are produced by the electrolysis reactions (Acar and Alshawabkeh, 1993). The pH dependent sorption- desorption behaviour is generally determined by performing batch experiments using the soil and contaminant of particular interest.

The precipitation and dissolution of the contaminant species during the electrokinetic process can significantly influence the removal efficiency of the process (Acar and Alshawabkeh, 1993). The soil decontamination process is affected by the hydrogen ions generated at the anode migrating across the contaminated soil and neutralizing the hydroxyl ions at the cathode. However, in some types of soils, the migration of the hydrogen ions will be hindered due to the relatively high buffering capacity of the soil. The presence of the hydroxyl ions at the cathode will increase the pH value (pH = 10-12). In a high pH environment, heavy metals will precipitate, and the movement of the contaminants will be impeded.

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Electrokinetic remediation of metal contaminated glacial tills 7

The high pH and the low heavy metals concentration condition at the cathode may also lead to the formation of a negatively charged complex species at the cathode compartment. The movement of these negatively charged complex species towards the anode and of the heavy metals towards the cathode relies upon the relative mobility of the hydrogen and hydroxyl ions. In other words, species migration ceases at a region closer to the cathode where the pH varies substantially because this is most likely to be where heavy metals accumulate and eventually precipitate, clogging soil pores and hindering the remediation process. For efficient contaminant removal, it is essential to prevent precipitation and to have the contaminants in dissolved form during the electrokinetic process.

Oxidation and reduction reactions are important when dealing with metallic con- taminants such as chromium. Chromium exists most commonly in two valence states: trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Cr(III) exists in the form of cationic hydroxides such as Cr(OH) 2+ and it will migrate towards the cathode during electrokinetic remediation. However, Cr(VI) exists in the form of oxyanions such as CrO4 2- , which migrate towards the anode. The valence state depends on the soil com- position, especially the presence of reducing agents such as organic matter and Fe(II) and/ or oxidizing agents such as Mn(IV), so it is important to know the valence state of metals and their possible redox chemistry.

Electrode conditioning procedures are sometimes necessary to induce favourable geochemistry and, as a result promote greater remediation efficiency. Desorption may be enhanced by using surfactants. Precipitation of the contaminants may be prevented by different methods, such as depolarizing the anode reaction by fluid conditioning such as calcium hydroxide and/or depolarizing the cathode reaction by an acid solution such as acetic acid. Oxidation may be enhanced by introducing oxidants such as hydrogen peroxide. In addition to the economic considerations, any of the selected procedures for conditioning the electrokinetic process must satisfy the following criteria: (1) prevent the precipitation and adsorption of contaminants, (2) prevent the production of hydrogen ions in a relatively short period of time that will lead to the reduction of electro-osmosis flow and cationic contaminants removal, (3) prevent any reaction with contaminants that causes precipitation, and (4) prevent toxic effects on the soil.

Previous laboratory investigations

Several laboratory investigations have been performed to evaluate the feasibility of using the electrokinetic process for removing various contaminants from soils. A summary of these previous investigations and related parameters is provided in Table 1. Lageman et al. (1990) used the electrokinetic technique for removing different inorganic contaminants from fine sand and fiver slush. The contaminants removed in this study included cadmium, chromium, nickel, lead, mercury, copper, zinc and arsenic. Hamed et al. (1991) and Acar et al. (1994) reported on electrokinetic experiments conducted on kaolinite which was contaminated with Pb(II) and Cd(II), respectively. Pamukcu and Wittle (1992) performed laboratory experiments to investigate the feasibility of electrokinetic removal of Cd, Co, Ni, and Sr from clays and a sand-clay mixture using different pore fluid solutions. Probstein and Hicks (1993) and Eykholt and Daniel (1994) investigated the removal of zinc and copper from kaolin, respectively. Lindgren et al. (1991) studied the electrokinetic process for remediating both saturated and unsaturated sand which had been contaminated with large anionic dyes that are similar to chromate ions. These laboratory investigations

Page 6: Electrokinetic remediation of metal contaminated glacial tills

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proved that electrokinetic remediation is a feasible technology for removing inorganic contaminants from the soils tested.

The removal of organic contaminants, particularly phenol, from kaolin by electro- kinetics was studied by Shapiro et al. (1989) and Acar et al. (1992a). Bruell et al. (1992) investigated the removal of gasoline hydrocarbons (benzene, toluene, hexane, isooctane, m-xylene) and trichloroethylene (TCE) from kaolin clay. These studies showed that the electrokinetic remediation can be used for the removal of organic contaminants from clays. The use of the electrokinetic process for the removal of radioactive contaminants from soils has also been investigated by Acar et aI. (1992b) and Ugaz et al. (1994), who determined that the electrokinetic process is feasible for removing uranyl, thorium, and radium from kaolinite.

Most of these laboratory investigations were performed on commercial kaolinite, and investigations on actual field soils are limited. The composition and geochemistry of field soils can be significantly different from those of commercial kaolinite, so these research studies on commercial clays must be carefully examined when using them to determine the feasibility of the electrokinetic process for remediating field soils.

Previous field investigations

Field scale demonstrations of the electrokinetic technique are extremely important for identifying and evaluating operational parameters, determining remedial efficiencies for different types of contaminants, and studying the interactions which occur among different contaminants under field conditions. However, actual applications of the electrokinetic remediation technique in the field are very limited. Lageman (1993) reported five field applications which used the electrokinetic remediation technique in the Netherlands. A summary of these applications is provided below:

1. Electrokinetic technology was used in remediating a sediment which was dredged from a water-bearing ditch at a former paint factory site. The sediment was distributed 20-50 cm deep over an area of 70 m × 3 m and contained lead and copper at concentrations of up to 5000 ppm and 1000 ppm, respectively. A series of vertical anodes, 2 m apart, were installed along a length of 70 m, and a horizontal cathode was laid along the length of the site. The remediation efficiency was 70% for lead and 80% for copper after 430 h.

2. Electrokinetic remediation was used at a galvanizing plant which contained sandy clay containing zinc to a depth of 40 cm. The contaminant concentration was not uniform and varied from 500 ppm to 3000 ppm. Two cathode drains were installed at a depth of 50 cm below the ground surface and 33 anodes in three rows were installed vertically to a depth of 1 m in a test area of 15 m × 6 m. The current density was 8 A/m 2 and the potential gradient was 40 V/m. The removal efficiency was poor, due to the presence of ammonia and ammonium chloride in the soil.

3. Electrokinetic remediation was used at a former wood-treating facility for clayey soil which was contaminated with arsenic to depths of 2 m and 1 m in areas of 10 m × 10 m and 10 m × 5 m, respectively. The highest concentration of arsenic was 500 ppm. Cathode drains were installed at a depth of 0.5 m and 1.5 m below the surface at a mutual distance of 3 m. Thirty-six anodes were installed at a separation distance of 1.5 m between the cathodes. At the beginning o f the treatment, the

Page 8: Electrokinetic remediation of metal contaminated glacial tills

10 Reddy and Shirani

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resistivity of the clay and the soil temperature were 10 ohm-meters and 7 ° C, at a depth of 0.5 m. After 4 weeks, the resistivity had changed to 5 ohm-meters, and the temperature had risen to 50 ° C. The potential gradient was also changed from 40 V/m to 20 V/m to keep a constant current of 4 A/m 2. The cleanup goal of 30 ppm was achieved after 3 months for over 75% of the site. Electrokinetic remediation was used at a landfill site. The contaminated region, which measured 70 m × 40 m × 2.6 m, contained soil and sludge which had been contaminated with cadmium in concentrations up to 3400 ppm. Cathodes were installed horizontally and anodes vertically. In spite of the presence of concretions of cadmium sulphide in the soil in concentrations up to 5000 ppm, the electrokinetic process reduced cadmium concentrations to less than 40 ppm in 2 years. Electrokinetic remediation was also reportedly used at an air-base where a region measuring 90 m × 20 m × 2.5 m was contaminated with Cd, Pb, Cu, Ni, Cr and Zn. The details of the cathode and anode configurations and the removal efficiencies were not reporte&

In the United States, field applications of the electrokinetic technology have recently been initiated. US Environmental Protection Agency (1993) reported a pilot-scale test at a site where soils were contaminated with lead in concentrations of up to 100 000 ppm. This test was performed with a current of up to 0.8 mA/cm 2 across electrodes placed at a spacing of 2 m to 4 m. The presence of calcium in concentrations up to 90 000 ppm was cited as being responsible for the low efficiency of the electrokinetic process, but in areas where calcium concentrations were low, higher removal efficiencies were obtained.

The low electrokinetic removal efficiencies in these field applications were attributed mainly to complex geochemistry, especially interactions with the naturally occurring electrolytes and compositional differences between the soils tested in the laboratory and those which were actually encountered in the field, and mixed contaminants.

Experimental methodology

To date, the feasibility of using the electrokinetic technique for remediating glacial tills has not been investigated. Glacial tills possess complex mineralogical and chemical character- istics, and their physicochemical behaviour during electrokinetic remediation may differ from that of commercial clays. A comprehensive laboratory investigation was initiated in 1993 at the University of Illinois at Chicago to study the electrokinetic process for remediating metal-contaminated glacial tills. This paper presents the first series of experiments. A description of the glacial till, the development of the electrokinetic test setup, and the test variables and procedures is provided in this section.

Glacial till characterization

Glacial till soil obtained from a project site near Chicago was used for this study. Various physical and chemical tests were performed to characterize it. The different test methods used, as well as a summary of all test results, is provided in Table 2 and the particle size distribution is shown in Fig.2. In accordance with the Unified Soil Classification System

Page 9: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills

Table 2. Physical and chemical characterization of glacial till

11

Soil Property Value Test method

% Sand 16 ASTM D422 % Silt 47 % Clay 37 Liquid limit (%) 31 ASTM D2487 Plastic limit (%) 16 Specific gravity 2.65 ASTM D854 Maximum dry unit weight (kN/m ~) 18.2 ASTM D698 Optimum moisture content (%) 14.9 Coefficient of permeability (cm/s) 8.77 × 10 -8 ASTM D2434 Porosity (%) 44.8 Calculated Activity 0.41 Calculated Organic content (%) 2.8 ASTM D2974 pH 7.7 ASTM D4972 Cation exchange capacity (meq/100 g) 13 USEPS CEC-calcium (txg/g) 200 SW-846 Method CEC-magnesium (txg/g) 28 9080 CEC-sodium (p.g/g) 15 CEC-potassium (t~g/g) 8.5

(USCS), ASTM (1994) D 2487, the soil is classified as silty clay with USCS symbol CL. The mineralogy of the soil, based on X-ray diffraction and other analyses, was found to be 31% quartz, 13% feldspar, 35% carbonates, 15% illite, 4-6% chlorite, 0.5% vermiculite and trace smectite.

Electrokinetic test setup

Schematic diagrams of the test setup is shown in Fig. 3. This test setup was designed to simulate the one-dimensional contaminant migration which occurs under the combined influences of electrical, hydraulic, and chemical gradients. Either voltage gradient or current may be used as the power source for this cell. This investigation was focused solely on the contaminant migration due to voltage gradient. Three identical test setups were fabricated in order to perform multiple tests simultaneously.

The electrokinetic cell used for the tests was made of lucite and measured 62 mm in diameter and 191 mm long. The cell was positioned horizontally on a stand. Slotted graphite electrodes were placed in contact with the reservoirs. The electrodes were located at each end of the test cell so that the soil was only in contact with one face of each electrode. At each end of the soil sample a filter paper was used to retain particulate matter. The electrode was placed next to the filter paper. A porous stone was then placed next to the electrode to separate the current and voltage electrodes as shown in Fig. 3. A DC power supply, HP Model 6205B dual DC, was used to apply the desired voltage gradient. Constant water elevations were maintained in the reservoirs to eliminate any hydraulic gradient across the cell. Gas vents were provided in the electrode compartments to allow for the escape of gases which resulted from the electrolysis reactions.

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12 Reddy and Shirani

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Testing programme

Sodium (Na) and calcium (Ca) were selected as the surrogate metallic contaminants. Both the single and the synergistic effects of these contaminants were evaluated. It should be mentioned here that Na and Ca were selected for this study to obtain first-hand information on ionic migration in glacial tills. They are, of course, common ionic species present in soils. The properties of these ions will not necessarily be representative of contaminants, such as heavy metals. An additional series of tests were conducted to determine the migration of toxic metallic contaminants such as chromium, nickel and cadmium (Parupudi and Reddy, 1996; Reddy et al., 1996; Reddy and Parupudi, 1996). A total of 16 tests were performed for this study, and the variables used for each test are shown in Table 3. The voltage gradient, initial ion concentration, and test duration were varied to study the effects of these variables on the removal efficiency.

Contaminated soil preparation

Approximately 1100 g of soil were used for each test. Sodium chloride and calcium chloride solutions were added to the soil to produce predetermined concentrations of the ions. Deionized water was then added to the soil to achieve 30% water content. The contaminated soil sample was homogeneously mixed with a plastic spatula and allowed to equilibrate. From this mixture, soil samples were collected for measuring water content, pH, and initial ion concentration.

Test procedure

The contaminated soil was placed in the cell in uniform layers and compacted to provide the desired density. The cell was then connected to the cathode and anode compartments

Table 3. Summary of electrokinetic testing programme

Initial Na conc. Initial Ca conc. Voltage gradient Duration Test No. (ppm) (ppm) (Vdc/cm) (days)

1 505 0 0.785 4 2 568 0 1.05 4 3 534 0 1.3 4 4 461 0 1.57 4 5 492 0 2.09 4 6 1022 0 1.57 4 7 1536 0 1.57 4 8 523 0 0.785 8 9 574 0 0.785 15

10 514 0 0.785 21 11 0 540 1.3 4 12 0 515 1.3 10 13 0 556 1.3 14 14 513 518 1.3 4 15 538 1057 1.3 4 16 1068 558 1.3 4

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14 Reddy and Shirani

and reservoirs as shown in Fig. 3. A preselected constant DC voltage was applied across the electrodes for the required time. After the test, the soil sample was extruded using a mechanical extruder and sectioned into five parts. From each of these five parts, soil samples were collected for measuring water content, pH and ion concentration. Deionized

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Page 13: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills 15

water was used to extract the ions from the soil sample and the extract was then analysed using an atomic absorption spectrophotometer to measure the Na and Ca ion concentrations.

Results and discussion

The electrokinetic experiments performed in this study allowed investigation of the effect of varying voltage gradient, test duration, initial ion concentration, and type of ion on the electrokinetic removal efficiency in glacial tills. For each experiment, the following test data was obtained:

1. The moisture content, pH and ion concentration in the soil prior to the electrokinetic treatment;

2. The moisture content, pH and ion concentration distribution within the soil after the electrokinetic treatment; and

3. The pH and ion concentration of the aqueous solutions in both the anode and cathode reservoirs after the electrokinetic treatment.

A mass balance analysis was performed for each test. The results showed a mass balance discrepancy ranging from 5% to 15% of the initial ion loaded onto the specimen. This mass balance discrepancy is attributed to the electroplating and/or trapping of ions in both the electrode and porous stone in the cathode compartment.

To present the test results in nondimensional form, the distances to the centre of each sectioned sample length from the anode were divided by the total sample length, and the ion concentrations were divided by the initial ion concentration. These normalized distances from the anode versus the ion concentration ratios were plotted. The pH and moisture content versus the normalized distances from the anode were also plotted. All of the plots for each test are given by Shirani and Reddy (1995). An analysis of these test results is provided below.

Effect of voltage gradient

Figure 4 compares the results of Tests 1 to 5 which had approximately the same initial Na concentration (ranging from 461 to 568 ppm) and which were conducted for 4 days under applied voltage gradients of 0.785, 1.05, 1.3, 1.57 and 2.09 Vdc/cm, respectively. Figure 4(a) shows the Na concentration variation in the soil from the anode to the cathode, and Figs 4(b) and 4(c) show the pH and moisture content variations, respectively, in the soil from the anode to the cathode. These test results demonstrate the effect of voltage gradient on electrokinetic remediation.

The results shown in Fig. 4(a) indicate that the Na ion transport occurred from the anode to the cathode and the amount of Na ions transported increased with an increase in the voltage gradient. Using these test results, the remedial efficiency in each test was calculated based on the ratio of the total Na mass removed by the process to the total Na mass initially added to the soil prior to the electrokinetic treatment. These calculated remedial efficiencies for Tests 1 to 5 are shown in Fig. 5 and demonstrate the effect of voltage gradient on the overall ion removal from the soil. As seen in Fig. 5, the ion removal efficiency increased with an increase in voltage gradient up to 1.3 Vdc/cm at

Page 14: Electrokinetic remediation of metal contaminated glacial tills

16 Reddy and Shirani

E 2.5

2.0

1.5

--41-- .79 V D C / C M

- -m- - 1.05 V DC/CM ---,t,-- 1.31 V DC/CM

i.57 V DC/CM " " 0 - - 2.09 V DC/CM

1.0

0.5

0.0 Z 0.0

TEST DURATION = 4 DAYS

INITIAL C O N C E N T R A T I O N ~ 500 P P M

0.2 0.4 0.6 0.8

NORMALIZED DISTANCE FROM ANODE

1.0

m

12

lo -

82

6- 42 22

0 0.0

! w _ r

0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

z @

@

40

30

20

10

0 0.0 0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

Fig. 4. Electrokinetic tests with different voltage gradients: (a) Na concentration, (b) pH and (c) moisture content

Page 15: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills 17

65

60

;~ 55 r,.) Z

50

~ 45

@

~ 4O

35

30 0.5

INITIAL Na CONCENTRATION = 461-568 PPM

I I I

1.5 2.5

VOLTAGE GRADIENT (V DC/CM)

Fig. 5. Effect of voltage gradient on Na removal efficiency

which the removal efficiency peaked. Beyond 1.3 Vdc/cm, the removal efficiency decreased with a further increase of voltage gradient. The initial increase in removal efficiency with an increase in voltage gradient is attributed to the increase in both electro- osmotic and electromigration rates which are linearly proportional to the applied electrical gradient. However, at higher voltage gradients, higher current densities are generated which increase the rate of electrolysis reactions at the electrodes. These electrolysis reactions generate H + and OH- at a faster rate than the rate at which soil can allow them to migrate; as a result, the removal efficiency does not increase proportionally.

Figure 4(b) shows the pH distribution in the soil for the different voltage gradient conditions after the testing was completed. It was observed that the pH values of solutions in the anode and cathode compartments and reservoirs were 1-2 and 10-13, respectively. These indicate that electrolysis reactions occurred at the electrodes. The pH values within the soil ranged from approximately 7 near the anode to approximately 12 at the cathode. The high pH values were attributed to the high buffering capacity of the soil. The glacial till used in this study contained about 35% carbonates which act as good buffers. This pH variation differs from that observed for commercial kaolinite by several previous investigators. In kaolinite, a distinct pH gradient is created ranging from 2 near the anode to over 12 near the cathode (Hamed et al., 1991). The high pH environment in glacial till influences the adsorption-desorption and precipitation-dissolution of the ions and, consequently, affects the ion removal. Because of the distinct differences in pH in glacial till as compared to kaolinite during electrokinetics, the published research results using

Page 16: Electrokinetic remediation of metal contaminated glacial tills

18 Reddy and Shirani

kaolinite must be carefully evaluated before applying them to field applications where glacial tills are present.

After the electrokinetic treatment, the water content of the glacial till decreased along the length of the specimen as shown in Fig. 4(c). This decrease was the general trend in all of the tests conducted. Higher water contents near the anode were observed since the soil specimen was directly connected to the water reservoir at the anode. Initially, the soil specimen was not fully saturated. The flux of pore fluid due to electrical gradients in the cathode region and the insufficient supply of the pore fluid from the anode compartment due to the low permeability of the soil causes suction to develop across the specimen which initiates the consolidation process. When the soil is fully saturated, this con- solidation process diminishes. Consolidation of the soil near the anode was observed in all the tests conducted in this study.

Electro-osmotic flow was not observed during the first 6 to 9 h of all the tests. This behaviour may be due to the initial partially saturated condition of the soil. After this period, the flow rate increased substantially within 48 h and then decreased with time. This flow behaviour may be due to the decrease in the number of cations in the diffusive double layer of the soil (migration of which is responsible for electro-osmotic flow) with time. In general, the electro-osmotic flow was relatively low for all the tests and electromigration appeared to be the major mechanism responsible for ion removal. The concentration of ions in the effluent was indicative of the removal efficiency across the specimen in such a way that for tests which demonstrated higher ion removal, the effluent in the outflow reservoir was more concentrated.

Effect of initial contaminant concentration

Figure 6 compares the results from Tests 4, 6 and 7 which were performed with initial Na concentrations of 461 ppm, 1022 ppm and 1536 ppm, respectively, under a constant voltage gradient of 1.57 Vdc/cm for a 4 day period. Figure 6(a) shows the contaminant concentration profiles, while Figs 6(b) and (c) show the pH and moisture content profiles, respectively. These results show that the ion transport from the anode towards the cathode is higher in tests which were conducted using a higher initial ion concentration in the soil. This effect of the initial ion concentration is clearly evident from Fig. 7 which shows the initial ion concentration versus the final removal efficiency. As the initial concentration increases, the removal efficiency also increases. The increase in removal efficiency is higher at low concentration ranges as compared to the high initial concentration ranges. The increase in removal efficiency for high initial ion concentrations is attributed to the high amounts of the Na ions which are present in the dissolved phase. At low ion concentrations, the ions are adsorbed onto the clay surfaces and only low amounts of ions are present in the dissolved phase. The greater the amount of Na ions in the dissolved phase, the easier the ions are transported by the electrokinetic process. As shown in Figs 6(b) and (c), the pH and water content distributions in the soil at the end of the experiment did not change due to the differences in the initial ion concentrations.

Effect of treatment duration

Tests 1, 8, 9 and 10 were performed using the same applied voltage gradient and approximately the same initial Na concentrations, ranging from 505 to 574 ppm; however,

Page 17: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills

.1 ,e 3.0

19

• 2.5 t ~ 461ppm TEST DURATION f 4 DAYS I 1022 ppm ~ 1536 ppm VOLTAGE GRADIENT = 1.57 V DC/CM

2.0

1.5

1.0

0.5

0.0 ea

z 0.0 0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

e ~

12

10

8

6

4

2

0 0.0

, I , I , I , I ,

0.2 0.4 0.6 0.8

NORMALIZED DISTANCE FROM ANODE

(b) .0

40

[..

[=. r.~

30

20

10

(c) 0 , I i I , I , I ,

0.0 0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

Fig. 6. Electrokinetic tests with different initial ion concentration: (a) Na concentration, (b) pH and (c) moisture content

Page 18: Electrokinetic remediation of metal contaminated glacial tills

Reddy and Shirani

V O L T A G E ORAD~NT = 1.57 V DC/CM 48

46

L~ 44

~ 42

38

36 Z

34

20

50

32 , I ~ I ~ I J l i I r

400 600 800 1000 1200 1400 1600

INITIAL CONCENTRATION (PPM)

Fig. 7. Effect of initial ion concentration on Na removal efficiency

different test durations were used for all four of these tests. These results from Tests 1, 8, 9 and 10 are shown in Fig. 8. Figure 8(a) shows the concentration variations and Figs 8(b) and (c) show the pH and moisture content variations, respectively, for these tests. The concentration distributions show that the ion transport from the anode to the cathode is increased as the test duration is increased. The effect of the test duration on the overall ion removal is shown in Fig. 9. As seen in Fig. 9, an increase in the duration of the test resulted in an increased removal efficiency. Initially, the ion transport due to electro- osmosis and electromigration is low due to the partially saturated condition of the soil. When the soil is completely saturated, a higher concentration of ions are desorbed into the pore fluid where they are easily transported by electro-osmosis and electromigration. At first, electromigration is predominant and, later, with an increase in electro-osmosis, the ions are removed faster. Figures 8(b) and (c) show that the pH and water content distributions did not significantly change with changes in the test duration.

Effect of contaminant type

The test results for calcium (Ca) removal from the glacial till based on Tests 11, 12 and 13 are shown in Fig. 10. Figure 10(a) shows the variation in Ca concentration from the anode to the cathode, and Figs 10(b) and (c) show the pH and water content variation, respectively, from the anode to the cathode. For the Ca experiments, only the test duration was varied. The removal efficiencies are shown in Fig. 11. From Fig. 11, it can be seen that

Page 19: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills 21

3.0

E z 2.5

2.0

"" 1.5

~ 1.0

0.5 z

0.0 ~q

l i d - 4 ~ 4 DAYS INITIAL CONCENTRATION = 500 ppm / / ---4-- 8 DAYS / --&-- 15 DAYS VOLTAGE GRADIENT = .79 V DC/CM

21 DAYS

_ / ~ ' ~ ' ~

• (al I I I [ I

0.0 0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

12

10

8

6

4

2

0 0.0 . / / /

j J

(b) I , I I I , I ,

0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

40

[..

[.. r ~

30

20

10

- I' ~ -

(c) 0 , I , I , I , I ,

0.0 0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

i

Fig. 8. Electrokinetic tests with different treatment duration: (a) Na concentration, (b) pH and (c) moisture content

Page 20: Electrokinetic remediation of metal contaminated glacial tills

22 Reddy and Shirani

80

75

g ~" 70

Z 65

60

so

45

INITIAL Na CONCENTRATION = 505-574 PPM

VOLTAGE GRADIENT = 0.785 V DC/CM j

4 0 t I , I , 1 , I r

0 5 10 15 20 25

TEST DURATION (DAYS)

Fig. 9. Effect of treatment duration on Na removal efficiency

the removal efficiencies for Ca are generally lower than those obtained for Na under identical test conditions. The low Ca removal efficiencies are attributed to the higher Ca exchange capacity of the glacial till as shown in Table 2. The smaller ion size and monovalent nature of Na is responsible for its more efficient removal by the electrokinetic process. These findings are consistent with other investigations which proved that ions of heavy metals with smaller ionic radii such as Na can be removed with a greater efficiency than ions with larger ionic radii such as Ca.

The test results for Tests 14, 15 and 16 in which the glacial till contained both Na and Ca in various proportions are shown in Figs 12, 13 and 14, respectively. Test 14 had an initial concentrations of 513 ppm Na and 518 Ca, Test 15 had initial concentrations of 538 ppm Na and 1057 ppm Ca, and Test 16 had initial concentrations of 1068 ppm Na and 558 ppm Ca. All of these tests were performed with a constant voltage gradient of 1.3 Vdc/cm and for 4 days.

A removal efficiency comparison based on Tests 3, 11, 14, 15 and 16 is shown in Fig. 15. Table 4 also summarizes these removal efficiencies based on Tests 3, 11, 14, 15, and 16. From these results, it can be concluded that the ion removal efficiencies are higher when the ions exist individually than when they coexist. The presence of mixed ions, especially in high concentrations, will increase the chemical interactions such as competitive adsorption and ion pairing, which could retard the migration of the ions.

Page 21: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills

.,~ 3.5

23

Z

Z

Z; 0

3.0 " I 2.5

2.0

1.5

1.0

0.5

0.0 0.0

i0DDAA)SS INIT1ALCONCENTRATION=500ppm y

0.2 0.4 0.6 0.8

NORMALIZED DISTANCE FROM ANODE

1.0

12

=

10

8

6

4

2

0 0.0

, i , i , i , i , ( b )

0 . 2 0 . 4 0 . 6 0 . 8 1 . 0

NORMALIZED DISTANCE FROM ANODE

40

30

Z: 20

10 r ~

0 , 1 , I , I , I , ¢ / . c

0.0 0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

Fig. 10. Electrokinetic tests with calcium: (a) Ca concentration, (b) pH and (c) moisture content

Page 22: Electrokinetic remediation of metal contaminated glacial tills

2 4 Reddy and Shirani

55

50

N 45

i4o

~ 35

INITAL Ca CONCENTRATION = 515-556 PPM

VOLTAGE GRADIENT = 1.3 V DC/CM

30 , f , i , ~ ,

4 6 8 10

T E S T D U R A T I O N ( D A Y S )

Fig. 11. Effec t o f t rea tment dura t ion on Ca r emova l eff iciency

[ I

12 14

2.5

2.0

~-" t.5 o

Z 1.0 0

0.5

Z O

0.0 0.0

pH Na CONC.

----II-- Ca CONC.

INITIAL Na CONCENTRATION = 513 PPM

INITIAL Ca CONCENTRATION = 518 PPM

VOLTAGE GRADIENT = 1.3 V DC/CM

TEST DURATION = 4 DAYS

0.2 0.4 0.6 0.8

N O R M A L I Z E D D I S T A N C E F R O M A N O D E

Fig. 12. Electrokinet ic test wi th N a = 513 p p m and Ca = 518 p p m

12

10

- I 0 1.0

Page 23: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills

2.5

Z

0

Z 0 m~

0 Z 0 0

2.0

1.5

1.O

0.5

-V - "

~ - ~ O N C E N T R A T I O N = 538 PPM

S ~ C a G C Op~FP~A= 1 .~ 0:~1/;: PPM

TEST DURATION = 4 DAYS

0.0 i i r r 0 0.0 0.2 0.4 0.6 0.8 1.0

N O R M A L I Z E D D I S T A N C E F R O M A N O D E

Fig. 13. Electrokinetic test with Na = 538 ppm and Ca = 1057 ppm

12

10

25

3.0

[-, 2.5

.<

~" 2.0

O

1.5

Z O

1.0

Z 0.5

Z O L)

0.0 0.0

_._p. /f j" Na CONC: / /

~ ~ INITIAL Na CONCENTRATION = 1068 PPM

TEST DURATION = 4 DAYS I I I I

0.2 0.4 0.6 0.8

N O R M A L I Z E D D I S T A N C E F R O M A N O D E

Fig. 14. Electrokinetic test with Na = 1068 ppm and Ca = 558 ppm

12

0 1.0

10

4

Page 24: Electrokinetic remediation of metal contaminated glacial tills

70

60

10

;~ 50

Z

30 29

I l l I I I I l l

f l i I I J I l l f f i I l l i l J I l l I l J I I J f l l l l J I l l I l J I l l I I J I l l f i t f f i I I J I l l I l l I l l f l J I f J I I I I l l I l J f l l I l l f l l I I J I l l I f J

26 Reddy and Shirani

34

VOLTAGE GRADIENT = 1.3 V DC/CM

TEST DURATION = 4 DAYS

INITIAL CONCENTRATIONS

ONLY Na = 534 PPM OR Ca = 540 PPM

Na = 513 PPM & Ca = 518 PPM

Na = 538 PPM & Ca = 1057 PPM

I - - 7 Na = 1068 PPM & Ca = 558 PPM

35 I i i I l l f J J

I 1 1 28 f J J

e.~¢, 22

%%'%

Na Ca

Fig. 15. Synergistic effects of co-ions on electrokinetic removal efficiency

Table 4. Synergistic effects during electrokinetic process (applied voltage gradient = 1.3 Vdc/cm, test duration = 4 days)

Intial concentration Removal efficiency

Na Ca Na Ca Test number (ppm) (ppm) (%) (%)

3 534 0 63 n/a l l 0 540 rda 35 14 513 518 29 24 15 538 1057 28 28 16 1068 558 34 22

Implications of test results

The results of this preliminary study which used sodium and calcium as the surrogate metallic contaminants indicate that the electrokinetic technique has definite potential for the remediation of glacial tills. A detailed investigation of glacial tills which have been contaminated with toxic metallic contaminants is currently being performed (Parupudi and Reddy, 1996).

Page 25: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills 27

Summary and conclusions

Several laboratory studies have been performed in the past to assess the feasibility of using the electrokinetic process for removing inorganic, organic, and radionuclide contaminants from fine-grained soils, but these studies were generally limited to commercial clays such as kaolinite. Recent field investigations, however, have shown that the contaminant removal efficiency using the electrokinetic process depends significantly on the soil composition, and the geochemical reactions which take place between the contaminants and the naturally occurring electrolytes. In this present study, laboratory electrokinetic experiments were performed using a glacial till which was obtained from an actual project site in order to evaluate the effect of soil composition and to assess the feasibility of using the technique for remediating contaminated sites where soils are of glacial origin. Based on these experimental results, the following conclusions can be drawn:

1. The pH values at the anode and the cathode were in the ranges 1-2 and 10-13, respectively, indicating that electrolysis reactions occurred (Acar and Alshawabkeh, 1993). The pH distribution within the glacial till in all tests varied from approximately 7 near the anode to approximately 12 near the cathode. This pH variation indicates that alkaline conditions existed in the soil during the entire remediation period. The alkaline environment is a result of approximately 35% carbonates which were present in the soil and which increased the buffering capacity of the soil. The observed pH distribution in glacial till is quite different from those reported for other clays such as kaolinite (Hamed et al., 1991; Pamukcu and Wittle, 1992). For kaolinite, a distinct pH gradient within the soil which varied between 2-4 near the anode and 10-12 near the anode was observed by numerous investigators (Hamed et al., 1991; Acar et al., 1994). The observed alkaline conditions present in the glacial till during the electrokinetic process can have a profound effect on the geochemical reactions such precipitation-dissolution and adsorption-desorption of toxic contaminants during the electrokinetic process (Parupudi and Reddy, 1996).

2. The two major ion transport mechanisms which were observed in the glacial till were electro-osmosis and electromigration. The measured flow was low which indicates low electro-osmotic effects. Therefore, the predominant ion transport mechanism in the glacial till is electromigration. Acar and Alshawabkeh (1993) also observed that electromigration is the major mechanism which is responsible for the transport of charged metallic species under electric fields. Electromigration of contaminants was shown to be two to three orders of magnitude higher than the electro-osmotic mobility.

3. The effects of voltage gradient, initial contaminant concentration, and test duration on the contaminant removal efficiencies for glacial till were similar to those reported for other clays (Pamukcu and Wittle, 1992; Acar and Alshawabkeh, 1993; Probstein and Hicks, 1993). An increase in the voltage gradient results in an increase in the removal efficiency of the contaminant. However, at high voltage gradients, high current densities are induced which increase the rate at which H + and OH- ions are generated due to electrolysis reactions. The rate at which the soil can allow these ions to migrate through it is limited and hence the removal efficiency does not increase proportionally to the rate at which electrolysis reactions occur. A high initial concentration of the contaminant results in better removal efficiency. At high

Page 26: Electrokinetic remediation of metal contaminated glacial tills

28 Reddy and Shirani

.

.

concentrations, the amount of contaminants in the dissolved phase is higher and these contaminants can readily be transported by electromigration. By increasing the duration of the treatment process, ion removal is also increased. The electromigration of ions depends on the size and charge of the ionic species. Species which possess higher ionic mobilities are transported in higher rates compared to species possessing lower ionic mobilities. This conclusion is evident from the observations that Na was removed more efficiently than Ca. The removal of mixed ions depends on the type and concentration of species. Higher removal efficiencies were observed when the ions existed independently. Based on this study, the applicability of the electrokinetic technique for removing contaminants from glacial tills appears to be promising. However, the results are based on the use of Na and Ca ions, the properties of which are not typical of contaminants, such as heavy metals. Further research is currently being performed at UIC to understand the geochemical interactions which occur during the electrokinetic remediation of glacial tills contaminated with toxic metallic contaminants (Parupudi and Reddy, 1996).

Acknowledgements

The authors are grateful to Dr Art Anderson of Research Resource Center, for the analytical testing, advice, and general support of this project. The help of John Gramsas, Technician in the Department of Civil and Materials Engineering at UIC, in fabricating the test setup is highly appreciated. Thanks to Joanne Williams for reviewing the manuscript and providing valuable comments. This project was partially funded by the Campus Research Board at the University of Illinois at Chicago.

References

Acar, Y.B., Hamed, J.T., Alshawabkeh, A.N., and Gale, R.J. (1994) Removal of cadmium(II) from saturated kaolinite by the application of electrical current, Geotechnique, 44, 239-54.

Acar, Y.B. and Alshawabkeh, A.N. (1993) Principles of electrokinetic remediation, Environmental Science and Technology 27, 2638-47.

Acar, Y.B., Li, H. and Gale, R.J. (1992a) Phenol removal from kaolinite by electrokinetics, Journal of GeotechnicaI Engineering, ASCE, 118, 1837-52.

Acar, Y.B., Gale, R.J., Ugaz, A., Puppala, S. and Leonard, C. (1992b) Feasibility of removing uranyl, thorium and radium from kaolinite by electrochemical soil processing, Report EK-BR- 009-0292, Electrokinetics Inc., Baton Rouge, Louisiana.

ASTM Annual Book of Standards (1994) Vol 04.08, Soil and Rock, American Society of Testing and Materials, Philadelphia, PA.

Bruell, C.J., Segal, B.A. and Walsh, M.T. (1992) Electroosmosis removal of gasoline hydrocarbons and TCE from clay, Journal of Environmental Engineering, ASCE, 118, 68-83.

Eykholt, G.R. and Daniel, D.E. (1994) Impact of system chemistry on electrosmosis in contaminated soil, Journal of Geotechnical Engineering, ASCE, 120, 797-814.

Gray, D.H. and Mitchell, J.K. (1967) Fundamental aspects of electro-osmosis in soils, Journal of Soil Mechanics and Foundation Engineering, ASCE, 93, SM6, 209-36.

Guzman, D.C., Swartzbangh, J.T. and Weisman, A.W. (1990) The use of electrokinetics for hazardous waste site remediation, Journal of Air Waste Management Association 40, 1670-6.

Page 27: Electrokinetic remediation of metal contaminated glacial tills

Electrokinetic remediation of metal contaminated glacial tills 29

Hamed, J., Acar Y.B. and Gale R. (1991) Pb(II) removal from kaolinite by electrokinetics, Journal of Geotechnical Engineering, ASCE, 117, 241-71.

Hicks, R.E. and Tondorf, S. (1994) Electrorestoration of metal contaminated soils, Environmental Science and Technology 28, 2203-10.

Lageman, R., Pool, W. and Seffinga, G.A. (1990) Electroreclamation: state-of-the-art and future developments, Contaminated Soil '90, Arend, F., Hinsenveld, M. and van den Brink, W.J. (eds), Kluwer Academic Publishers, Karlshruhe, Germany, 1071-8.

Lageman, R. (1993) Electroreclamation: applications in The Netherlands, Environmental Science and Technology 27, 2648-50.

Lindgren, E.R., Kozak, M.W. and Mattson, E.D. (1991) Electrokinetic remediation of contaminated soils, in Proceedings of the ER'91 Conference, Washington, DC, 151-8.

Lindgren, E.R., Kozak, M.W. and Mattson, E.D. (1994) Electrokinetic remediation of unsaturated soils, in Symposium on Emerging Technologies in Hazardous Waste Management IV, American Chemical Society, Washington, DC, pp.33-50.

Mitchell, J.K. and Yeung, A.T. (1991) Electro-kinetic flow barriers in compacted clay, Transporta- tion Research Record 1288, Transportation Research Board, Washington, D.C., pp. 1-9.

Pamukcu, S. and Wittle, J.K. (t992) Electrokinetic removal of selected heavy metals from soil, Environmental Progress, 11, 241-50.

Parupudi, U.S. and Reddy, K.R. (1996) Geochemical processes affecting chromium removal from fine-grained soils by electrokinetics, Report Number UIC-GGEL-96-01, Department of Civil and Materials Engineering, University of Illinois at Chicago, IL.

Probstein, R.F. and Hicks, R.E. (1993) Removal of contaminants from soils by electric field, Science, 260, 498-503.

Reddy, K.R., Parupudi, U.S., Devulapalli, S.N. and Xu, C.Y.(1996) Effects of soil composition on removal of chromium by electrokinetics, Journal of Hazardous Materials (in press).

Reddy, K.R. and Parupudi, U.S. (1996) Removal of chromium, nickel and cadmium from clays by in-situ electrokinetic remediation, Journal of Soil Contamination (in press).

Schackleford, C.D. and Daniel, D.E. (1991) Diffusion in saturated soil. I. Background, Journal of Geotechnical Engineering, ASCE, 117 467-84.

Shapiro, A.P., Renaud, P.C. and Probstein, R.F. (1989) Preliminary studies on the removal of chemical species from saturated porous media by electroosmosis, Physico-Chemical Hydro- dynamics, 11, 785-802.

Shapiro, A.P. and Probstein, R.F. (1993) Removal of contaminants from saturated clay by electroosmosis, Environmental Science and Technology, 27, 283-91.

Shirani, A.B. and Reddy, K.R. (1995) Electrokinetic Remediation of Metal-Contaminated Glacial Till, Report No. UIC-GGEL-95-02, Department of Civil and Materials Engineering, University of Illinois at Chicago, Chicago.

Ugaz, A., Puppala, S., Gale, R.J. and Acar, Y.B. (1994) Complicating features of electrokinetic remediation of soils and slurries: saturation effects and the role of the cathode electrolysis, Communications in Chemical Engineering, 129, 183-200.

United States Environmental Protection Agency (1993) Approaches for the Remediation of Federal Facility Sites Contaminated with Explosive or Radioactive Wastes, EPA/625/R-93/013, Washington DC.