62 1

Soil Decontamination Using Electrokinetic Processing

R. J. Ga1e.a Heyi Li,a and Y . B. Acarb

Whemistry Department and bCivil Engineering Department, Louisiana State University, Baton Rouge, LA 70803, USA.

INTRODUCTION

In industrialized countries worldwide, the irresponsible or poorly engineered disposal of hazardous chemicals or chemicals that react to produce toxic byproducts in the environment has necessitated extensive (and expensive) clean-up efforts. Nowhere is this more true than in the United States, where the problems are being well documented. Hazardous waste, as defined by the U.S. Resource Conservation and Recovery Act (RCRA), is a solid waste material which may cause, or significantly contribute to, an increase in mortality, or serious irreversible, or incapacitating reversible illness: alternatively, it may pose a substantial present or potential hazard to human health, or to the environment when improperly treated, stored, transported, or disposed of, etc. [1,2]. The so-called Superfund, created by the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of the U S Congress, provides a mandate for the Environmental Protection Agency (EPA) to take actions in responses to hazardous releases of pollutants and to require responsible parties to contribute to the remediation. Massive government and private actions currently are underway to implement full site restorations, to develop new remediation technologies, and to avoid as much as possible problems arising from future disposal methods.

Electrokinetic soil processing is a n emerging remediation technique with the capability to decontaminate soils or slurries polluted with heavy metals, radionuclides, or certain organic compounds. The EPA general classification is that it is a physical remediation treatment for phase separation, although the process also may include pre-, concurrent-, or post-chemical treatments (3.41. This chapter contains a brief overview of the theoretical basis of electrokinetic soil processing, the results of some laboratory tests and engineering models, and a summary of experiences of some actual site applications. In principle, the contaminant may be a n inorganic/organic/ organometallic species and charged (ionic) /uncharged (polar/ nonpolar). The subject is a multidisciplinary one, encompassing basic electrochemistry, soil/colloid chemistry, and geotechnical/ environmental engineering.

Electrokinetic processing derives its name from one of the four major electrokinetic phenomena. These arise from the coupling between electrical and hydraulic flows and gradients in suspensions and porous (soil) media, which can be responsible for electroosmosis, streaming potential, electrophoresis, and migration or sedimentation

622

Water&

ELECTROOSMOSIS

C- Particles

ELECTROPHORESIS

T AH I

STREAMING POTENTIAL MIGRATION POTENTIAL

Figure 1. Four major electrokinetic phenomena in soils (adapted from Mitchell, 161)

potentials [5,6]. Electroosmosis and electrophoresis are the movement of pore water and charged particles, respectively, due to the application of an electrical field. Streaming potential and sedimentation potential are the generation of an electrical field due to the movement of an electrolyte under hydraulic potential and the motion of charged particles in a gravitational field, respectively (Figure 1).

Of the electrokinetic phenomena, electroosmosis has been of primary interest in geotechnical engineering because it is used in practice to dewater and to stabilize saturated fine-grained deposits. The proposed uses of electrokinetics in waste treatment and disposal may include: (1) dewatering of sludge slimes or dredged spoils, (2) electroosmotic flow barriers [7 ] , (3) leak detection systems for disposal facilities, (4) injection of grouts to create barriers, (5) to provide nutrients for biodegrading microcosm, (6) electrochemical in situ generation of reactants such as hydrogen peroxide for clean-up. and (7) decontamination of soils and ground waters 181. Electrokinetic soil processing involves not only electroosmosis and some electrophoresis but also the dispersal/entrapment of electrolysis products and their adsorption/desorption interactions, as outlined below.

623

THEORETICAL

Background

Electrokinetic processing of soils, by application of a direct current through a wet soil mass, results in the development of electrical, hydraulic, and chemical gradients. The formation of an acidic front at the anode from water electrolysis and the induced electroosrnotic flux of the pore fluid enable the removal of those contaminants that can be solubilized, desorbed from the soil, or simply carried by the pore fluid. The fundamental basis of each of these two main processes is described below.

When an electric potential is applied across a wet soil mass by immersion or placement of two or more electrodes, cations in the pore fluid are attracted to the cathode and anions to the anode (Figure 2). For a uniform, initial concentration of ions, the application of an electric field will only create instantaneously a uniform electric field. The current flow requires faradaic reduction and oxidation reactions at the cathode and anode, respectively, as well as the transport of ions in the solution phase (it is assumed normally that any movements of soil particles and/or colloids do not contribute significantly to the ionic current). Since there is generally a n excess of positively charged cations in the system, to neutralize the net negative charge on the soil particle surfaces, these double layer cations migrate toward the cathode carrying not only their waters of hydration but also producing, by viscous drag, an electroosmotic flux of the pore fluid. The net transport of water molecules by the waters of hydration of anions and cations

+ + + + + + ANODE CATHODE

NEGATIVE CLAY OR GLASS SURFACE

Figure 2. Schematic of electrolysis, adsorption/desorption, and electroosmotic flow.

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(5- 10 moles/Faraday maximum) is negligible compared to the induced electroosmotic flux (100-4,000 moles/Faraday). Counterions moving in the opposite direction tend to oppose this flow but the force exerted by the cations in the inter-particle regions is toward the cathode when the excess surface charge is negative. However, an excess of anions in the double layer for a positively charged clay surface would result in a reverse net flow from cathode to anode. The migrating ions reach a limiting velocity instantaneously and the electrical force is equal and opposite to the frictional drag (Stokes-Einstein Law). Notice that only a fraction of the total electrical current contributes to the electroosmotic force and, as the ionic strength of the pore fluid is increased, the fraction of the current carried by the double layer excess ions is diminished. With elapsed time, therefore, and if the electrode processes result in the formation of charged species, the movement of these electrode products and the original ions by transport and the advective (convective) flow of pore water combine to determine the product distribution and efficiency of remediation. Importantly, the downstream flux of hydrogen ions from the electrolysis of pore water at the anode desorbs, by displacement, adsorbates from sites on the clay surface 191. Basic metal hydroxides, carbonates, etc.. present a s insoluble salts or complexes, may be dissolved. Hence, the overall distribution of chemical species is complicated by ionic and convective t ransport , the electrolysis reactions, contaminant species adsorption/desorption energetics and kinetics, and localized acid/base reactions. The electrode reactions depend on the availability of reactants, the relative energetics and kinetics, and perhaps passivation or product accumulation effects.

The Electroosmotic Effect (Large Pore Theory)

The phenomenon of electroosmotic flow was first observed by the Russian scientist Reuss in 1808. I t has been applied practically in soil improvement and stabilization applications for over 50 years, e.g. [ 10- 121. One of the earliest explanations of electroosmosis is a model introduced by Helmholtz (1879) and later improved by Smoluchowski (1914). For modelling simplicity and experimental design, it is convenient to consider a single glass capillary. rather than a porous particle bed or membrane. The capillary (or clay particle)/liquid interface may be considered as a simple capacitor, with an excess of charges of one sign on the insulator substrate and an equivalent layer of oppositely charged ions in the liquid phase, as illustrated in Figure 2. We may ask what is the reason that insulator materials, such as glass or clay, have a surface charge? The majority of clay surfaces, a t neutral pH. exhibit a negative surface charge firstly because of isomorphous substitution in the underlying structure. Isomorphous substitution is the replacement of the primary atoms comprising the lattice structure by others of the same type but different valence and size resulting in deficiencies in charge. Clay particles, or glass, also will have surface ionizable functional groups at the broken edges of the lattice, e.g.,

625

Heterogeneous surface sites exist also and they may have cationic and anionic exchange capabilities. However, clay minerals have a net charge deficiency, the cation exchange values being larger and typically 5- 150 milliequivalents/ 100 gram [6]. Lower values are found for 1: 1 kaolinite minerals and higher values for 2: 1 montmorillonite minerals. The classical Helmholtz-Smoluchowski theory applies for the condition that the double layer thickness is small compared to the capillary diameter [5,6]. The thickness of the diffuse layer sometimes is approximated to the reciprocal Debye length, 1 / ~ . which is of the order 10 A and 100 A for 10-1 M and 10-3 M 1:l electrolyte in water a t 25°C. respectively [13]. A discussion of the fractional potential drop with diffuse layer thickness at metal electrodes has been presented in ref. [14]; 99.99% of the potential drop occurs in the distance 9 . 2 / ~ . The physical significance is that this estimate of diffuse layer thickness is no greater than 1 micron for a 10-5 M 1:l electrolyte and smaller a t higher interpore ionic concentrations. The double layer a t the surfaces of clays and soils will be complicated further by the surface heterogeneity, epitaxial crystal faces, adsorbed chemical species, etc. A comprehensive review of the equilibrium double layer and associated electrokinetic phenomena was written by Durkin and Derjaguin [ 151 and rigorous treatments of the classical theory for electrokinetic flow in narrow cylindrical capillaries have been presented by Rice and Whitehead [16]. Newman [17], and Koh and Anderson 1181. Original references are provided in [15-18], to which the reader is referred, and only a brief summary of the classical theories and earlier literature are included here.

In large pore theory, it is assumed that the double layers at clay surfaces do not overlap. Therefore, the theory is applicable for pores of the order of one micron or greater in soils. When the double layer has an excess of cations, for example, and an electrical field tangential to the surface perpendicular is applied, a net, directional force is exerted causing the cations to move. A convective flow of the liquid occurs and a steady state velocity is reached due to the momentum transfer from the cations to the liquid molecules. At steady state, the electrical force on a cation is equal and opposite to the frictional slip forces within the liquid. The electrical force is assumed to be transferred completely to mechanical energy and no allowance is made for heating or eddy current losses. Cations, in general, are more highly solvated than anions. The effective cross-sectional area of a solvated ion will be larger than that of an unsolvated ion, although no studies of electroosmotic differences due to the ion species nature are available. It is assumed, also, that the background ionic strength consists of equal concentrations of negative and positive ions whose motions result in

626

forces which are equal and thus cancel. The total electrical force in the axial direction z in a capillary, F,. is given by the product of the electrical field gradient, E,, and the local charge density due to excess ionic charge, p(r). The electrical field gradient is assumed to be uniform across the capillary in the radial direction, i.e., EZ z f(r).

The solution of the Poisson-Boltzmann equations for the local charge density p(r) of a double layer at the internal wall of a cylindrical capillary is given by [16],

in which K is the reciprocal Debye length, Io( 1 functions are modified Bessel functions of the first kind, a is the capillary radius with point distance from the axis, r, and yfo is the capillary wall potential. This choice of the potential plane requires careful consideration [5,15]. In double layer theory, it could represent 42, which is the plane potential a t the compact layer in the Gouy-Chapman-Stern model. However, it can be assumed that a quiescent layer of liquid exists at the surface and a hydrodynamic shear plane exists within the double layer. The boundary now is that where the liquid may move and the shear plane has a potential known as the zeta or electrokinetic potential, C,. In this model, the excess charge responsible for fluid motion resides between this boundary and the outer diffuse layer.

In the absence of any external pressure, for the capillary case, the equation of motion is found by equating the externally applied electrical force, F,, to the internally opposing viscous drag forces, when

in which v, represents the fluid velocity in the axial direction z, Fz is the electrical force, and q is the coefficient of viscosity. Traditionally, this equation has been solved with the boundary conditions which assume that the flow velocity will be maximum in the center of the capillary and zero at the wall,

627

There are important assumptions physically. The boundary condition at the surface of the capillary is referred to as the no slip condition. This condition generally has followed from experimental observations of Newtonian fluids, when the fluid which contacts a solid body is assumed to adhere to the boundary and have zero velocity, e.g. 1191. I t is an assumption which simplifies the problem and allows a mathematical treatment. The magnitude and profile of any surface discontinuity in flow is often not known. The solution for the velocity profile then reduces to,

This profile has a flat, maximum which extends across the central region of the capillary and rapidly approaches to zero at the wall (Figure 3). By way of contrast, in Figure 3, we show the profile obtained by numerical integration of equation (4) when full slippage is assumed. This constitutes the case when there is no friction at the interface between the capillary wall and the pore fluid. The physical significances of these integrals are as follows: (i) in the no slip model, the integral is based on the assumption that the electrical force is exerted between the surface potential plane of shear and the centre of the capillary, with a minimum velocity nearest to the wall. The mechanism by which the momentum is transferred from the double layer liquid, on which the electrical force is exerted, to the bulk of the liquid is not addressed. A Hagen-Poisseuille type of flow with a flattened profile results; (ii) for the full slip condition, only the liquid within the double layer ions is mobile. This produces a much smaller velocity profile that will be strongly dependent on the ionic strength for a constant surface charge. Obviously, the assumptions regarding slippage and momentum transfer are critical to the overall shape of the velocity profile. Precise measurements of surface charges and double layers in capillaries are difficult, however. More studies of the electroosmotic flow over wide concentration ranges and microscopic investigations of internal velocity profiles are needed. Cases in which the diffuse double layer region extends to the axis are presented in the literature 116,171, as well as that for a rectangular cross section 1201, elliptical pores [IS], nonuniformly charged walls I2 11, and parallel plates 1221. In view of the thinness of the double layer in moderately concentrated electrolytes and the fact that electroosmosis can indeed still occur, it is clear that the electrical force can be applied extremely close to the surface and the shear plane may be very close to the Stern layer. The volume transport for the capillary is obtained by integrating equation (6) for the cylindrical coordinates. If the diffuse layer thickness is a small fraction of the radius, the expression reduces to the classical Helmholtz-Smoluchowski result,

628

0.0020

0.0015

0.0010

0.0005

0

-0.0005

n 0 al u)

0 z v

- + - no Slip

-

- slip - $

I , I I 1 I I I _

Figure 3. Velocity profiles calculated numerically using eq. (6)-no slip condition and equation(4)-slip condition Q2 = 26 mV, 10-4 M KC1, EZ = 10 V/cm.

in which qe is the flow rate under electrical gradients, Ac is the capillary cross sectional area, and ke is the electroosmotic coefficient of permeability. In summary, this result is stated to be applicable to systems with large pores, dilute electrolytes, and it predicts that the rate of flow depends directly on the area, &. It is noted that ke is not a function of the pore size.

The above theory raises two major questions. The first, already discussed, concerns the role of slip/no slip at the wall surface and requires an experimental approach. The second concerns the nature of the field surrounding the ions in the double layer. This second question pertains to the effective force exerted on the ions (water) because the electrophoretic and relaxation force terms in the Debye- Huckel-Onsager conductance equation are neglected. These terms become increasingly significant at higher concentrations, e.g. 1231. As recognized by Koh and Anderson [18], the Debye-Huckel theory cannot be applied readily t o the double layer, since that region is not electroneutral. One should expect experimentally, then, that the normalized electrokinetic efficiency might diminish with increase in electrolyte and double layers concentration since the net electrical force is reduced by electrostatic drag. Since the ionic strength of the pore fluid will be increased to a concentrated level by the electrolysis reactions at practical current densities, the classical theory needs to be modified for concentrated electrolytes. A first approximation would be

629

to correct for the background ionic strength, however, a more complete theory would require quantification of any electrostatic drag created between the double layer excess charge and the wall potential as well.

Other Theories of Electroosmosis

An interesting subcase of the above is the small pore theory, sometimes known as Schmid theory 161. Unlike the preceding large pore theory, in which the assumption is made that the diffuse layer thickness is small compared to the capillary diameter, here the excess of ions is distributed throughout the entire void volume 1241, i.e. it is assumed that the double layers overlap. Hence, the electrical force can act more uniformly across the pore section, as is the case for an external hydraulic pressure. This condition is treated also by Rice and Whitehead [16], for example. The equation for the velocity, when Ka << 1. becomes

Velocity profiles across the capillary have a Poisseuille shaped flow and the expression predicts that the electroosmotic coefficient of permeability should vary with the square of the radius. In practice, it is found generally that this law is not as satisfactory as the Helmholtz- Smoluchowski approach for predicting electroosmotic behavior in soils. The failure of small pore theory may be because most clays have an aggregate structure with the flow determined by the larger pores [6].

Another theoretical approach is referred to as the Spiegler Friction theory [25,6]. Its assumption, that the medium for electroosmosis is a perfect permselective membrane, is obviously not valid for soils, where the pore fluid comprises dilute electrolyte. An expression is derived for the net electroosmotic flow, 9, in moles/Faraday,

in which the net flow refers to the difference between the actual solution flow and that due to transport of waters of hydration, C3 is the mole cm-3 concentration of free (unassociated) water, C1 is the mole cm-3 concentration of cations, and Xq is the friction coefficient between components i and j (W s2 cm-2 mole-1). Subscripts 1, 3 and 4 refer to the cations, the water molecules, and the solid ionic matrix (wall), respectively. This theory is of interest because it enables isolation of

630

parameters to quantify specific ion/water frictional drag. For a cation exchange resin at 25". Xi3 has a value 2.59 x 108 W s2 cm-2 mole-', which compares to X34 for Na+ ionjwater of 4.8 x lo6 W s2 cm-2 mole-l. Incorporation of this model with the classical one for electroosmosis could provide quantitative testing of the slip boundary condition.

In addition to the approach using phenomenological equations for modelling ion transport in soils, the theory of irreversible thermodynamics may be adapted to soils [26], as for the case of ion- exchange membranes. Spiegler [25] and Kedem and Katchalsky [27,28] are the prime examples of this approach to transport models. The detailed review by Verbrugge and Pintauro contains a number of other references to mathematical approaches for modelling the fundamental electrokinetic phenomena.

Electrical Models of Capillaries and Soils

Schufle et al. [29.301 and Rutgers and de Smet [31] have evaluated the total conductance of solutions contained in capillaries, to assess the excess surface conductance due to the presence of the electrical double layer. The model treats the system as two resistors in parallel; one comprising a bulk resistance afforded by an homogeneous cylinder of electrolyte and the other is attributable to the ion excess in the double layer at the capillary wall. A formula for the total resistance, R, is given by

where oo is the specific conductivity of the bulk solution, oS is a so- called specific surface conductivity (expressed as a 2-dimensional parameter, i.e. K a >> l), and 1 is the length of the capillary. Through an experimental measurement of the total resistance and accurate assessment of the dimensions and bulk conductivity, the surface conductivity may be derived. This equation is of interest since it may be applied to obtain the fraction of electrical energy that represents the maximum available for electroosmosis: in other words, the surface conductance is the fractional contribution of the total current applied to cause solution flow as the electrical energy is dissipated as mechanical and thermal losses.

Schufle et al. have proposed, from surface conductance results of dilute HC1 in small diameter glass capillaries, that long-range ordering of the structure of water may exist near interfaces (diams. ranged 2 mm - 5 microns). This controversial issue is inherently linked with models for electroosmosis, if dc methods are employed for the conductivity determination and a concurrent electroosmotic effect occurs (electrometer measurement methods have presumably been dc).

63 1

Higher activation energies for conductance in the more dilute solutions and with the smallest capillary diameters, as described by Schufle et al., are consistent overall with favorable conditions for electroosmosis. It would be of interest to compare the results of ac and dc methods for conductance of electrolytes in microcapillaries. Problems of defining the conductance of ions in double layer regions, as mentioned above, persist however. Also, the double layer itself could have some structuring effect on surface water molecules.

Modelling the electrical conductance of soils is complicated by a variety of factors. The simplest approach is to consider the porous medium to be comprised of a bundle of capillaries of constant radius, or with a range of radii. This does not provide a good model hydrodynamically, since it does not allow for the converging-diverging nature of the flow pathways 132). Other complications include the charge heterogeneity of the solid particles, colloid transport, the presence of organic chemical matter. etc. Conductance measurements have been used to predict the diffusion coefficients for salt movement through soils 1331. The conductance is considered to be due to a homogeneous solution and a surface process,

in which ECa is the specific conductance of the soil, ECw the pore fluid conductance, 8 is the volumetric water content, and ECs is the surface conductance. Tortuosity may be corrected for with a transmission coefficient. T. I t was thought that this model may not be valid for dryer soils. The surface conductance and transmission factor are assumed not to vary with change of the bulk solution strength and can be derived using simultaneous equations and measurements at two pore fluid concentrations. In this experimental study, good agreement was found for diffusion coefficients derived in this manner with those obtained by radiotracer methods. The surface conductance was typically 0.3 to 10% of the bulk conductances a t 1M salt concentrations and the transmission coefficient ranged 0.35-0.53. In ac conductance measurements, the conductances are a function of the frequency. Usually, the electrolysis reactions at the electrodes are neglected since their effective resistance is small in comparison to that of the soil matrix. High frequency ac conductance methods, which measure both conductance and capacitance, give information on porosity, particle shape and orientation, and dielectric constants, e.g. 134,351.

Conductance measurements are useful practically to monitor ionic distribution profiles in electrokinetic laboratory test cells and to characterize the electrical properties of soil samples.

632

RESULTS

Laboratory Studies

Many earlier geotechnical studies exist of the electroosmotic effect in soils and clays with an emphasis on dewatering; in this chapter we limit the discussion to the more recent activities aimed at decontamination of pollutants. Most laboratory tests have used a one- dimensional geometry and inert carbon electrodes.

MARIOTTE BOTTLE

ACQUISITION

w I TRANSDUCER I

IH

CARBON CARBON ENDCAP CATHODE ANODE

Figure 4. One-dimensional laboratory test apparatus,

Figure 4 presents a schematic diagram of a one-dimensional laboratory test for electrokinetic soil decontamination. Laboratory tests are highly recommended as a first step in any actual site remediation for feasibility testing, efficiency assessment, and optimization. The soil of interest, in a partially or fully saturated state, may be housed in a suitably inert, insulating cylindrical tube, of typically glass or plexiglass (note: some organic compounds can interact with plastics). In an open configuration, free ingress of deionized water or a processing fluid is permitted at the anode, with or without an hydraulic head. Using a Mariotte bottle to provide a small, constant 1-2 cm head, the typical hydraulic flow rate through a 10 cm diameter cylinder of 10 cm length packed with kaolinite clay might be of the order 0.5 to 2.0 ml/day. This quantity can increase as much as 100-200 fold to 50-100 mls/day by application of a small dc current ( 1 mfi to 5 mA). A closed configuration at the anode has been used for experiments investigating

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dewatering phenomena. To facilitate mathematical modelling, it is convenient to apply constant current and a small, constant hydraulic head across a saturated, homogeneous clay sample. The former is used to establish initial constant flux boundary conditions for electrogenerated products at the anode. The electrode reactions a t the cathode usually are more complex and may change appreciably over extended periods of electrolysis. Constant potential conditions might be applied at one electrode by use of a reference electrode and potentiostatic control. Alternatively, a two-electrode system can be used with a n approximately constant applied voltage. The choice, constant voltage or constant current, will affect the type of model equations used to predict complete clean-up or acid breakthrough (see below).

For sample preparation, a clay slurry usually is consolidated to a void ratio of about 2.0 to 3.0 in cylinders a t a maximum consolidation pressure of 200 kPa. The cylinders may serve also as the electroosmosis cells. Some variation in water content occurs across the specimens prepared in this manner and irregularities in consolidation/compaction cause fairly large variations in the typical electrokinetic parameters that are monitored. It is advisable always to test a minimum of two or more samples in view of this. Some physical characteristics of Georgia kaolinite are presented in Table 1.

The economics and efficiency of the process usually depend on the degree of solubility of the contaminant species and the electroosmotic flow rate, although some charged species may be separated by mainly migration in silty deposits. The electroosmotic flow rate, qe, is usually defined empirically from cell tests,

where ke = coefficient of electroosmotic permeability (cm2 5-1 V-1). Qe = electrical potential gradient (V cm- I), ki = electroosmotic water transport efficiency (cm3 A-l s-l), I = current (A), and (T = average conductivity (siemens cm-1). The parameter ke vanes from 1 x 10-4 to 10 x 10-5 cm2 s-1 V-1 for all soils and is higher at higher water contents. However, as noted above, it may vary as the ingress of charged electrode products alters the ionic strength of the pore fluid, or with clay surface composition. The flow parameter ki varies widely from 0 to 1.2 cm2 s-1 A-’ and is a function of the water content, the ionic strength, and the cation exchange capacity of the soil. Table 2 provides a general comparison of the coefficients of electroosmotic and hydraulic permeabilities for differing soil types. Hydraulic permeabilities can range six orders of magnitude from very porous sands to fine clays. Thus, electroosmosis is most advantageous for use in more nonporous clay media, where pump technologies are too slow to be practical.

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Table 1. Characteristics of Georgia kaolinite [91

Mineralogical Composition (Yo by weight)

Kaolinite Illite

Index Properties (ASTM D 4318)a

Liquid Limit (5) Plastic Limit (5)

Specific Gravity (ASTM D 854)b

YO Finer than -2 pm Size

Activity

Proctor Compaction Parameters

Maximum Dry Density, tons/m3 Optimum Water Content, Yo

Compression Index (C,)

Recompression Index (C,)

Permeability of Specimens Compacted at the Wet of Standard Proctor Optimum (x 10-8 cm/sec)c

98 2

64 3 4

2.65

90

0.32

1.37 31.0

0.25

0.035

6-8

aASTM Method for Liquid Limit, Plastic Limit, and Plasticity Index of

bASTM Method for Specific Gravity of Soils [D854-58 (197911

CFlexible wall permeability at full saturation

Soils (D 4318-83)

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Table 2. Comparison of the Coefficients of Electroosmotic and Hydraulic Permeabilities [6,9]

Material ke(10-5 cm2 s-1 V-1) kh(cm s-l)

Na-montmorillonite (1 70)' London clay (52) Kaolin (68) Clayey silt (32) Mica powder (50) Fine sand (26)

2.0 5.8 5.7 5.0 6.9 4.1

10-9

10-7

10-5 10-4

10-8

10-6

(*) indicates approximate water content in ?lo

For kaolinite clay at 50 k 10% water saturation, Acar et al. [8,36] and Putnam [91 have reported that the measured coefficient of water transport efficiency, ki, decreased from a maximum of about 1.8 L/A-hr a t 15 hours of processing to 0.2 L/A-hr at 150 hours, for an electrode current density 0.05 mA/cm2. In many tests, a short induction period with no flow occurred after switching on the current (this has even been observed in single capillary experiments 118, Fig. 41). The flow then quickly increased to a maximum value. Similar behaviour was found in some tests with low levels of contaminant present, e.g., for decontamination of 130 ppm Pbz+ ion from kaolinite a t a current density 0.04 mA/cm2, Hamed et al. [37,38] report a decrease in ki from a maximum of 2.7 L/A-hr. to approx. 0.4 L/A-hr. at 600 hours, Figure 5. These initial ki values are consistent with those found in earlier studies (61 but measured efficiencies will vary with the clay type and preparation, such as the degree of prewashing, the residual ionic salt content, water saturation, etc. The marked decrease in the water transport with time probably is caused mainly by the accumulation and reaction of ionic electrolysis products.

636

1 .o

a 0.8

2 0.6 c, 0.4 E .- 0.2

Y n

h

0

u)

a \

0 v

Figure 5.

pH Variations

" 0 200 400 600 800

TIME (hrs)

Representative flow behaviour at 0.037mA/cmz, porosity 0.7, with 120 ppm Pb(I1) loaded on kaolinite [37,38].

The occurrence of pH gradients in electrochemical processing of soils has been noted by a number of earlier investigations concerning dewatering [11,12,39-451 and Acar et al. 18,361 and Putnam [9] have made detailed studies of the development of pH gradients in electrokinetic cells with open anode geometry. Attempts to model the early stages of acid-base distributions were made by Acar and coworkers [461. The primary electrolysis reaction a t an inert anode is the electrolysis of water, provided that very low concentrations of other oxidizable species are present,

and, a t the cathode,

4 H2O + 4e- + 2 H2 + 40H- 114)

The production of H+ ion at the anode decreases the pH at this electrode. at a rate depending on the current density employed and the through-volume flow of pure liquid. The resulting voltage drop at the electrodes should always exceed that necessary to electrolyze molecular water (2 2V),

637

4H20+4e- -) 2H2+40H- E O = - 0.8287 V

2H20 + 0 2 + 4 H + + 4 e - E O = - 1.229V ~~~

Net 6H2O -+ 2H2 + 0 2 + 4H+ + 40H- E O = - 2.057 V

and, in the event of an open boundary condition a t the anode (free ingress of water), there is an ample supply of water to sustain the primary electrolysis reactions; a t the anode equation (13) and at the cathode equation (14). The presence of other electroactive species in the system will alter the faradaic efficiency of these primary reactions. For example, organic compounds might be oxidized at the anode. Metal ions, hydrogen ion and dissolved oxygen might be reduced at the cathode. The contaminant of interest may or may not be electrolyzable. Overall, the quantity of water electrolyzed is very small relative to the net flux generated by the electroosmotic effect. As the faradaic reaction proceeds, the electrogenerated hydrogen and hydroxide ions contribute to the ionic species already present in the pore fluid as ionic current carriers and, hence, a nonlinear ionic strength conductor is produced. Typically, the residual ionic strength of pore fluid in a kaolinite clay is millimolar or less of chiefly sodium sulfate [9]. Sites requiring remediation may vary widely in their pore fluid, ionic strength and chemical constituents present, both natural and extraneous.

The hydrogen ion generated a t the anode has an essential role in electrokinetic processing, since a low pH can achieve displacement of adsorbed contaminant species at charged sites on the clay surface (this is analogous to flushing a n ion exchange resin with acid to remove adsorbates). At low levels of contamination, many toxic species are likely to be distributed mainly in an adsorbed form rather than in the pore fluid. For example, the adsorption isotherms for Pb2+ ion and phenol on Georgia kaolinite are shown in Figure 6 (37,471. This mineral can adsorb about 1.100 pg of Pb2+ ion per gram of dry clay, which limiting quantity is usually defined as the cation-exchange capacity (for Pb2+, 1.06 milliequivalents/lOO g dry clay). The phenol adsorptive capacity of kaolinite similarly is about 1 mg phenol per gram dry clay. Flushing the soil mass with an acidic solution causes the displacement of adsorbed species, e.g..

2H+ + Pbz+(clay)2- + 2H+(clayI2- + Pb2+ (15)

In the case of phenol, hydrogen ion will protonate the molecule, which can enhance its transport to the cathode. Other positively charged ions introduced at the anode, such as NH4+, Na+. Zn2+, etc., could also

638

- 1 0 4 ~ P 0,

c 0

v)

2 1 0 - - - d

- - 1 - 1 1 1 1 1 1 1 1 1 1 1 l 1 1 1 1 1 1 111111111 I11111,1l lW a

achieve displacement of adsorbed cationic contaminants but might themselves be unsatisfactory additives to the soil.

h 3 1 0 ~ 7 0)

v

4

10-1 1 10 lo2 lo3 Equilibrium Concentration (ppm)

a

Figure 0. Adsorption isotherms for Pb(II) ion and phenol on kaolinite [37,47].

The rate at which H+ ion is injected into the system may be controlled by varying the current density. For example, assuming the reaction in eqn. (13) is 100% faradaic efficient, the rate of H+ ion production is equal to twice the molar rate of water molecule electrolysis, kHzO. Thus, the steady state concentration of H+ ions will be given by,

2 kHZO (moles/sec) Qt (L/sec) (16)

Molar concentration =

where Qt is the net flux of water passing the anode. electrolysis rate is calculated with equation (1 7).

The water

= 1 = 5.18 x I moles/sec kHzo 2F

639

7

6 -

5 -

if I is the total current in amperes. For example, the solution leaving the anode will be pH 1.83 for 1 square meter of electrode, 0.25 amps current, and 15 L/day water ingress (note that, as discussed above, the electroosmotic flow rate may decrease with time and this complicates species distribution in time).

The minimum pH required to desorb and solubilize contaminants may be determined in laboratory experiments with samples from a site. In practical operations, it may be desirable to control the H+ ion production rate, depending on the buffer capacity of the contaminated soil because, a t low pH values, the clay itself will undergo acidic dissolution, e.g. [48,491; so excessive current densities can waste energy and cause chemical changes to the soil. Figure 7 illustrates pH profiles determined in a 130 ppm Pb2+ ion decontamination from kaolinite. a t short and long process durations. Note that a t short process times, the H+ ion disperses rapidly throughout the specimen due to advection (the electroosmotic, convective flux), migration (charged species transport in an electrical field), and diffusion. The pH determined by in situ insertion of a glass electrode into the soil results in a quasi-thermodynamic determination of the H+ ion activity, since

Initial Pore Fluid pH 0-

, -o - - _-- --- - - - - -- 0 0 0 0 : 0

a ’&-n-‘ I ./- I I .

Figure 7. pH profiles of test cells after 388 A-hr/rns (circles) at 0.012 mA/cm2 and 1982 A-hr/rns (triangles) at 0.037 mA/cm2; open symbols- pore fluid: full-in situ.

640

solution phase H+ ions, double layer and adsorbed H+ ions may all contribute to the measured activity. The particulate matter in contact with the glass membrane has an additional unknown influence on the membrane potential. The close concurrence of in situ and pore fluid pH levels close to the anode indicates that the clay ion exchange capacity has been exceeded, whereas closer to the cathode, the large divergencies in these values reveal that the clay surface has not been fully saturated with H+ ions.

To conclude this section, some comments are included concerning the nature and role of the cathode processes. Some metal ion contaminants, such as Pb2+ ion, when desorbed by downstream H+ ion will meet the zone of upstream OH- ions. This can cause local precipitation of gelatinous metallic hydroxides, possibly decreasing the cross-sectional area available for flow, as well as decreasing the ionic species available for transport, e.g.,

Pb2+ + 20H- + Pb(OH)2 (18)

soluble, conducting insoluble, nonconducting

Since lead is amphoteric, in stronger base the Pb2+ ions will exist as soluble plumbite ion, from which it can be electrodeposited [50]. Not all metal contaminants can be electrodeposited from aqueous solution but most precipitate as hydroxides (exceptions are alkali metals, some alkaline earths, NH4+). Hydrogen ion, meeting the upstream base, reacts exothermically producing nonconducting, molecular water.

This reaction, similarly, reduces the ionic strength available for conduction and thereby assists maintaining the electroosmotic effect. Eventually, however, with sustained electrolysis and the downstream loss of OH- ions, the upstream base will be completely neutralized. The cell then will mainly transport H+ ion from the anode to the cathode where it is reduced preferentially to those metal ions with more negative reduction potentials,

As for the case at the anode, other electrochemical reactions can be chosen a t the cathode by flushing, or choice of electrode material.

64 1

Acid-base effects on the soil surface conductivity and efficiency of electroosmosis need to be understood and quantified better.

It is possible to control the anode current density such that the influx of H+ ions just saturates the available surface sites and not add to the pore fluid concentration. In this way, the excess H+ ion in the pore fluid available for migration is minimal and the electroosmotic advective flux should be a maximum possible. Using a n initial cell flow rate of 80 mL/day for 80 cm2 area electrode, we estimate that a current density of 0.007 mA/cm2 should be optimum if the ion-exchange capacity is 1 milliequivalent/lOO g clay and the porosity is 0.7. With changing flow, the current density might be adjusted to maintain the H+ ion influx constant. Higher current densities may be needed, however, if migration is needed to remove contaminant excesses in the pore fluid when the surface sites are fully saturated.

At the completion of remediation. the processed volume of soil will have been acidified and natural re-equilibration may be possible. Alternatively, the area can be treated with acceptable alkalis such as lime or ammonia. The system total acidity is determined best with an acid-base titration. If alternative reactions are chosen at the electrodes to the production of acid and base, the effects of these on the whole system need to be evaluated in terms of the toxicity of electrode reactants and products, their influence on the soil/contaminant ion- exchange efficiency, the effectiveness of electroosmostic flushing, solubilities, etc. Leach tests can be used to assess if the residual contaminant achieves a level acceptable for ground water quality.

Conductivity

In electrokinetic tests, the apparent conductivity, Ka, can be estimated from the electrical potential drop across the electrodes and the current,

Ka(siemens/cm) = It(amp) Lkm] / Vt(vo1t) AJcm2) (21)

where It is the current, Vt is the applied cell voltage, L is the specimen length, and & is the cross-sectional area. It is assumed that overvoltages due to electrolysis reactions are small relative to the potential drop across the soil and can be neglected. In laboratory tests with kaolinite, for example, at 0.05 mA/cm2, the value of & decreased from about 110 to 50 pS/cm [91. For experiments over longer periods, with kaolinite loaded with 130 ppm Pb2+ ion [37,38] or 100 ppm Cd2+ ion, the & values ranged 80-15 pS/cm and 100-13 pS/cm, respectively [51]. Higher initial conductances result from higher loadings of ionic contaminant. Note that the trend in conductance correlates well with the decrease in water transport efficiency and pH. Pore fluid conductances are higher than in situ soil measurements (uncorrected for porosity) and vary in time across the cell from anode to cathode.

642

Highest values are found close to the anode, where H+ ion has transported into the soil from the anodic water electrolysis reaction. In tests conducted for an intermediate time, a minimum conductance is found close to the cathode, presumably because of the acid-base neutralization reaction, eqn. (19). and possibly clogging, eqn. (18). With sustained processing, however, the conductance is high in the anode section (high H+ ion and anion concentration) and low close to the cathode (anion depleted region). Figure 8.

w g- 10,000

1,000 v "r

,oo i-" ua_s/' 0

" 0 0.2 0.4 0.6 0.8 1.0

NORMALIZED DISTANCE FROM ANODE

Conductivity profiles found in situ with 388 A-hr/ms (full circles) at 0.012 mA/cm2 and 1982 A-hr/ms (open circles) at 0.037 mA/cm2 processing, 144 and 117 ppm Pb(II), respectively [37,381.

Electrochemical Modelling of the Electrokinetic Process

The purpose of modelling is to develop predictive equations to assist in establishing the optimal processing conditions and the fullest extent of remediation. Verbrugge and Pintauro have provided a review of transport models for ion-exchange membranes and much of this theory might be adapted to electrochemical soil processing [26]. Two fundamental approaches have been identified in this review: the first is based on the Nemst-Planck flux equations and hopes to provide a detailed molecular-level account of the processes: the second approach uses irreversible thermodynamics, without specifying molecular-level interactions. This latter approach uses the Stefan-Maxwell transport equations and concentrated solution theory. In general, it is desirable to obtain a kinetic model to predict the overall system behaviour

643

because this will be the most useful practically for planning and engineering electrokinetic site remediations.

Acar and coworkers I461 and Shapiro et al. [521 have presented general models based on the first of these two approaches. These models predict that the contaminant and the electrolysis products a t inert electrodes will be transported and dispersed by advection, migration, and diffusion. Modelling in this manner provides only a first-order, mathematical framework to examine the flow patterns and chemistry generated in the process: adsorption/desorption kinetics, ac id /base chemical react ions, complex equilibria, and precipitation/solubility factors may heavily influence the model accuracy and outcome of any site remediation. Two approaches for mathematic modelling are the use of analytical solutions or numerical, finite element methods (FEM). Both models require adequate definitions for the boundary conditions (nature of electrolyses, flow behaviour).

Total mass transfer, qt, due to electrical and material gradients in a soil, according to the Nernst-Planck equation, is

where n = charge on the ion, F = Faraday's constant, R = Universal gas constant, T = temperature, Cp = electrical potential, DJ = diffusion coefficient of species J. z = flow direction, v, = average seepage velocity, p = porosity, and A = average cross-sectional area. The three terms in equation (22) represent chemical diffusion, advective (electro-osmotic + hydraulic) flow, and ion migration, respectively. Acar et al. [46] solved this equation analytically for hydrogen ion transport downstream from the anode. I t was assumed the advective flow, v,. was constant. With this condition and a constant boundary flux, the local H+ ion concentration becomes,

(z + kt) + exp + erfc (z-kt) (I3 1 2(D*t)'12} ( 2 3 )

c(z,t) = co erfc { 2(D*t) ' I2

in which k, taken to be constant, is a linear combination of the migration, electroosmotic and hydraulic velocities. Adsorption/ desorption kinetic effects can be approximated with a retardation coefficient, R, but the role of the nature of the surface species on the electro-osmotic effect is not known. Since it is known now, from numerous bench studies, that the total fluid flow rate decreases with time of processing, numerical methods are preferred for modelling. Approximate models could be employed with bulk constants derived from laboratory tests and the effects of diffusion (slow kinetically) might

644

be omitted. The decrease in the total flow with time presumably is linked with the enhancement of pore fluid ionic strength due to the formation of H+ and OH- ions at the boundaries. Acar et al. (461 provided also an analytical solution for the upstream transport of base from the cathode and discuss some physical consequences of this.

Results of FEM simulations for acid-base distributions are provided in refs. [36.46.53,54]. Shapiro et al. [52] similarly have calculated pH profiles for the downstream H+ ion flux but included multicomponent ion fluxes, when two kinematic waves result. Neither model includes any complications arising from nonlinear, radial concentration fluxes or from the acid-base neutralization reaction eqn. (19). The latter will create a heated, dilute zone of neutral pH close to the cathode.

Some mathematical modelling papers have been published also concerning the use of electroosmotic processes for ground water pollution control, e.g. 155-581.

Selected Examples of Bench Scale Tests

Runnels and Larson have studied the use of electromigration to remove Cu2+ from pure quartz. silty sand [59]. Applied voltages were maintained constant at fairly low levels, 1.5-2.5 V, and currents ranged 10-50 @ cm-2. At 0.01 M CuSO4 concentration, pHs in the range 2- 4.50, about 7-53% of the copper was removed by processing to 30 days. These authors did not attempt optimization of the process but concluded that electromigration held promise a s a remediation technique to prevent further contamination of ground waters. Daniel and Eykholt 153,601 similarly attempted the recovery of Cu2+ ions. Tests employed voltages of 0-5 V, 0-320 ppm Cu2+ ion on kaolinite clay, and the test cell used anode and cathode reservoirs. Variations in soil pH similar to those reported by Acar et al. were found I8,36,46] but a high accumulation of basic Cu2+ salt resulted in the soil adjacent to the cathode reservoir. I t is concluded that future efforts were needed to reduce the high pH values at the cathode, Power consumption was typically 7 kWhr m-3.

Pamukcu et al. (611 have attempted to remove zinc ions by electroosmosis from kaolinite clay. The experimental cell was designed to have relatively large volume cathode and anode electrolyte chambers. The total zinc concentrations were found to increase from 175 mg/L to 745 mg/L in the anode chamber and to 440 mg/L in the cathode chamber. Zinc ion. similarly to lead, is amphoteric in aqueous media and can form negatively charged zincates. which accounts for the enhancement of zinc at the anode. Quantitative interpretation of these experiments was complicated due to periods with no current application and the addition of NH4OH and NaCl solutions to the anode chamber.

Hamed et al. t37.38.511 have investigated the electrokinetic remediation of kaolinite containing Pb2+, Cd2+, and Cr3+ ions. High removal efficiencies. to 95% were achieved for Pb2+ ions present to

645

1000 ppm levels, Figure 9. In all tests, however, the section closest to the cathode had a large amount of precipitated hydroxide salt. Also, the removal efficiencies were > 92% for Cd2+ ions at 120 ppm. However, Cr3+ ions loaded at 120 ppm of dry soil indicated only 60-70% removal efficiencies, which may reflect both difficulties in desorbing this trivalent species and increased complex ion equilibria. Many metal ions react with water producing hydroxide complexes, which will adsorb or precipitate,

C$+ + H2O --L Cr(OHI2+ + H+

Cr(OHI2+ + H20 .-L Cr(OH)2+ + H+ etc.

Cr(III), in addition, is amphoteric, forming chromites with base,

Cr(OH)3 + OH- + Cr(OH)4- (26)

--z

zz u - 0

0

2% 1 .o

0 0.2 0.4 0.6 0.8 1 .o NORMALIZED DISTANCE FROM ANODE

F'igure 9. Profile of Pb(II] after processing 388 and 1982 A-hr/ms at 0.012 and 0.037 mA/cm2, respectively. Full circles indicate the shorter processing condition and note accumulation due to displacement 137,381.

646

Further complications ensue if Cr(V1) species are present. The standard potentials for Cr(VI)/Cr(III) couples are strongly dependent on the pH.

H Cr 0 4 - + 7H+ + 3e- e- Cr3+ + 4H20 E" 1.38 V

Cr 0 4 2 - + 4H20 + 3e- == Cr(OH)3 + 50H- Eo -0.11 V

Tables of acid/base interactions and thermodynamic references are available for predictions of homogeneous behaviour, e.g. [62.63]. Another complication is the result of soil chemistry. This can be especially important at trace levels when speciation is difficult because analytical methodologies may not have the required molecular sensitivities.

A key aspect of the process recommended by Acar et al. I641 is to continue electrolysis for a sufficient time to completely acidify the soil mass. The effects of hydroxide ion formation at the cathode can be alleviated further by the addition of a weak acid. Anions of this weak acid, for example acetic acid, will migrate upstream but upon meeting the downstream H+ ions, associate to form nonconducting molecules,

H+ + CH3 COO- CH3 COOH (27)

Thus, the ionic strength increase is offset by the formation of associated acid. Acetic acid additionally is a good choice of cathode depolarizer since most metal acetate salts are soluble.

In addition to metals, a number of laboratory tests have explored the feasibility of decontaminating organic pollutants from soils. Shapiro et al. [52] have demonstrated that phenol at 450 ppm and 0.5 M acetic acid can be removed in excess of 90% by the passage of 1.2 pore volumes of effluent. Acar et al. 1471 similarly have studied phenol remediation at a loading 500 pg/g dry kaolinite using electrokinetic soil processing and reported > 95% recovery in the effluent by 2.0 pore volumes. In this latter study, the energy expenditures were calculated to be in the range 18-39 kwh per cubic meter of soil processed. Bruell et al. [661 were able to successfully remove BTEX [benzene, toluene, ethylene, and xylene) compounds in gasoline as well as trichloroethylene on kaolinite at concentrations below the aqueous solubility limit. Studies currently are underway at LSU to remediate soils contaminated with higher levels of nonpolar organics using micelle additives (671.

U.S. Patents using electrical currents/voltages for soil remediation have been issued to Probstein et al. [651 and Acar and Gale 1641.

The problems of radionuclide contamination of soils are particularly serious in the U.S. [68-701. There are 33 radioactively contaminated

647

sites listed or proposed'for listing on the National Priorities List. These were caused by uranium mining, the commercial radium industry, or the Federal nuclear weapons research and production programs. The quantity of contaminated soils is very large and since the radioactivity, unlike organic materials, cannot be destroyed and is decreased only by natural decay, the development of remediation technologies has focussed on separation/concentration techniques. Consequently, the EPA has initiated the Volume Reduction and Chemical Extraction (VORCE) project to select technologies that can reduce the volume of radioactively contaminated soil [7 1 I . Electrokinetic soil treatment has been included as a potential VORCE technology 1721. and as a low cost, in situ method may be favored over soil/acid washing approaches. Bench scale decontamination of uranyl ion from kaolinite has been demonstrated and a yellow uranium hydroxide precipitate collected at the cathode (731. Thorium salts showed limited removal a t the same operating conditions, perhaps because of its larger ionic charge and tendency to hydrolyze. Radium was extremely insoluble and non- extractable from kaolinite. Clearly, the specific chemical properties of each species has to be considered and perhaps pre- or post-chemical treatments used in conjunction with the electrokinetic process.

Short reviews of the available bench-scale and pilot-scale studies of electrokinetic soil remediation have been presented by Acar et al. [74].

Field Scale and Pilot Tests

One of the earliest studies of alkali metal ion removal from soils was carried out by Puri and h a n d 175). h r i proposed that monovalent ions will move faster than divalent due to the former's higher dissociation from sites on the clay surface. The volume treated was relatively small, 4.5 m2 and 0.3 m depth. Jacobs and Mortland demonstrated in a later bench study that Na', K+, Mg2+, and Ca2+ ions are leached from Wyoming bentonite by electroosmosis [76], the monovalent ions eluting a t a faster rate than the divalent, in agreement with Puri's suggestion. Krizek et al. [42] showed similarly that the soluble ions content increased appreciably in eluent from electroosmotic consolidation of polluted dredgings, but reported that heavy metals were not extracted. This result may have been due to the formation of insoluble hydroxides. On the other hand, Segall et al. I771 discuss the process leachate obtained from electroosmotic dredgings in a dewatering project and note an increase in heavy metals, organic materials, and alkalinity. All of the processing conditions were not available from this study but the distance between electrodes was 3-5 metres. Hamnet (781 also has studied the reclamation of agricultural soils by removal of unwanted salts by electroosmosis and a U.S. Patent exists for a process termed electrodrainage to reduce alkali and saline salts in soils [791. An interesting use of electrochemical processing is for exploring for minerals in deep soil deposits [80]. Shmakin notes that this method has been used in the USSR since the early 1970s for Cu, Ni, Co and Au prospecting. A porous, ceramic pot with nitric acid is placed as a

648

cathode vessel and the migrating ions are extracted and analyzed. The quantities of extracted metals and their rates of accumulation can be correlated to the ore compositions and the distance from the cathode to the deposits.

A field study was attempted for control of radionuclide migration, specifically 9oSr species [Sl]. The area treated was 11 x 5 metres with 25 anodes and a central cathode. Although a slight accumulation of 9oSr was found at the cathode, the study was largely inconclusive since speciation and a comprehensive analytical survey of contaminant transport were not attempted. Another inconclusive study is that of Banerjee et al. [821 for decontamination of a chromium site. Combined hydraulic and electrical potentials were applied and steel electrodes were used, which could react chemically with the effluent. The complexities of chromium speciation have been alluded to above. However, it is stressed that for field and pilot study results to be meaningful, extensive monitoring of the contaminant and its speciation is necessary before, during, and subsequent to the electrochemical process.

Perhaps the most detailed accounts of field studies presently available are those conducted by Lageman and coworkers [83,84]. Table 3 contains a summary of the sites and the metal contaminants remediated. The anode and cathode housings are interconnected in this process and form two separate circulation systems filled with different chemical solutions, details of which are not provided in the publications. However, the extent of remediation of many of the metal contaminants tested is quite high. These studies demonstrate that electrochemical processing of soils is a viable and practical technology.

There is a need to engineer field studies better in terms of the placement of electrodes, the maintenance of electrode conditions, and the separation/concentration of contaminants. Feasibility studies and practical data used in electrokinetic dewatering technologies may be of assistance, e.g. [85]. Undoubtedly, however, chemical changes in the soil mass must be carefully monitored and understood for efficacious remediation.

Interest in soil decontamination by electrokinetic processing has been increasing steadily, as shown by the volume of scientific studies. There have been two workshop/conferences dedicated to this topic in the U.S. in recent years [86,87] and a number of companies now have offered specialized services in this area. The technology is an emerging one and it is not yet fully mature. There exists a need to conduct further basic and applied studies before it achieves its full potential. In environmental remediation, the site chemistry before and after any application of technical processes need to be fully evaluated. It is recommended that bench scale, laboratory tests be undertaken prior to any site work to help optimize the process parameters. Although the

Table 3.

Field and Laboratory Studies of Electrokinetic Soil and Sludge Processing I83.841

Site

~ ~~ ~ ~~

Contaminant (ppm) After processing (ppm) Removal YO

Weser River Mud Germany

Cd Q1 Pb Ni Zn Cr

As Hg

10 143 173 56 90 1 72 0.5 13

5 41 80 5 54 26 0.2 4.4

50 71 54 91 94 64 60 66

Paint factory] (Sediment)

Pb > 3204 a 510

587 158

> 82 69

Galvanising plant2 (Sandy clay1

Zn 2080 1624 22

Timber impregnation plant3 As 143 (heavy clay)

c 61 57

~ ~ ~~

1 Cathode-anode 2m; 26 sampling locations: mean of 9 values 2 Cathode-anode 1.5 m; soil resistivity 5R cm falling to 2.5R cm: c.d. 0.8 mA/cm2 average: mean of 12 values

o\ P W 3 Clay resistivity 10R cm falling to 5R cm: mean of 10 values: c.d. 0.4 mA/cm2 average, 96 days.

650

fundamental principles of the process are now better understood and they can be expressed in theoretical formalism, it is essential to conduct studies in assessment of the theoretical models. We note that much of the basic data on sorption, diffusion, dispersion, and migration in capillaries and soils obtained in the last three decades are still insufficient to accurately predict the conduction phenomena in soils, in part due to the added complexity of the electrode reactions: however, in order to provide engineering design/analysis guidelines, implementation procedures and to assess the technical feasibility of alternatives, it is essential to provide a well-established theoretical basis. Nevertheless, the method is highly effective in many laboratory tests and has the potential to provide a low cost, in situ acid wash, which may be effective for remediating many sites contaminated with inorganic / organic pollutants.

ACKNOWLEDGEMENTS

Electrokinetic studies at LSU have been supported by the Board of Regents of the State of Louisiana (LEQSF RD (86-89)BlO). the National Science Foundation (MSS-9014711). the Hazardous Waste Center of LSU, the Environmental Protection Agency, and Electrokinetics. Inc. Funds provided by these agencies and organizations are gratefully acknowledged. The authors acknowledge the following graduate students at LSU: A. Alshawabkeh, J. Hamed, S . Puppala, G. Putnam. N. Tran, and A. Ugaz as well as assistance in preparing this chapter from the office and drafting staff of the LSU Chemistry Department, Henry Hurtado and Barbara Marquette.

65 1

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