Chapter 2

Reduction of Contaminants in Soil and Water By Direct Electric Current

Sibel Pamukcu, Ehsan Ghazanfari and Kenneth Wittle

2.1 Introduction

Applying a low direct current electrical field across a porous medium, such as wet soil induces migration of ionic and surface charged constituents and flow of the pore water to a directed location within the field. Commonly referred to as "electrokinetics", this process is a proven, sustainable technology that can transport water and substances residing in or near the aqueous phase in clayey soil at a significantly higher rate than hydraulic methods. Harmful heavy metal contaminants and radioactive materials, as well as some immiscible fluids can thus be removed in-situ. The process is especially useful in applications where pump-and-treat methods are impractical due to the low permeability of the medium, or in cases where the contaminants persist owing to their affinity to the solid phases in clay soils. Moreover, the application of direct electric current in soils have been used successfully to transport reactive agents, such as nano-particle slurries to enhance in-situ reactions that convert contaminants, or chelating or surfactant agents to solubilize the contaminants so they can be transported with water advection. More recently, evidence was provided that shows how direct electric current can contribute to success of the desired transformation reactions by not only providing the "driving force" necessary to deliver active reagents, but also by lowering the energy for the redox reactions to occur. This enhancement of transformation reactions was attributed to the double-layer polarization of the clay surfaces leading to Faradaic processes under the applied electric field.

In most field situations, the contaminants are found adsorbed onto soil surfaces, iron-oxide coatings, soil colloids and natural organic matter. Most contaminants are retained in clay interstices as hydroxycarbonate complexes, or present in the form of immobile precipitates and products in soil pore throats and pore-pockets that "lace" the vadose zone. This exacerbates clean-up efforts as the available technologies, such as in-situ bioremediation, chemical treatment or the traditional pump-and-treat method may not be able to treat the entire site effectively in low permeability soils. Electrokinetics treatment, when designed to properly address the site specific features, can potentially reduce the subsurface pollution by transporting and/or transforming contaminants, and enhance resource recovery by extracting trapped materials (i.e., heavy metals, oils and petroleum) which may not be extractable by other means. This chapter first provides an overview of the use of direct electric current for environmental mitigation in the subsurface, and discusses with examples the application of direct electric current to:

2.2 Overview of Direct Electric Current in Subsurface Environmental Mitigation

Electrically induced migration of ions and water is a proven method of externally forced mass transport in clay soils for contaminant remediation purposes (Pamukcu and Wittle, 1992; Probstein and Hicks, 1993, Lageman, 1993; Acar and Alshawabkeh, 1993, 1996a; Acar et al., 1994; Hicks and Tondorf 1994; Eykholt and Daniel, 1994; Shapiro et al., 1995; Yeung et al., 1996; Alshawabkeh and Acar, 1996b; Electorowicz and Boeva, 1996; Reddy and Parupadi, 1997; Dzenitis, 1997; Chilingar et al., 1997). While electoosmosis is analogous to soil washing, ion-migration is probably the primary mechanism of mass transport when the contaminants are ionic or surface charged. Relative contribution of electroosmosis and ion migration to the total mass transport varies according to soil type, water content, ion species, and their concentration. In silts and low-activity clays, electroosmotic flow reaches maximum in comparison to hydraulic flow. But the mass transported by ionic migration is always much higher than the mass transported by electroosmotic advection (Acar and Alshawabkeh, 1993). The effect of electroosmosis decreases significantly when pH and zeta potential drops in the later stages of a sustained electrokinetic process under a constant electric potential (Hamed et al., 1991 and Pamukcu et al., 1997). When micelles (i.e., charged aggregate of molecules or particles) are formed with other species in the processing fluid, or when slurry masses are treated, electrophoresis may become significant (Pamukcu et al., 1995).

Chilingar and co-workers (1997) reviewed and evaluated the electro-bioremediation technologies for remediation of soils contaminated with hydrocarbons and metals. They found many successful applications of the combined technologies (Loo et al., 1994) including: (i) primarily passive biotreatment for degradation of gasoline and diesel in the soil and groundwater, (ii) combination of biodegradation and electrokinetic transport with a hot air venting system and ultraviolet light bio-control system for degradation of gasoline in the clayey soil, (iii) closed recovery system consisting of special enhanced bioremediation for treatment of soil and ground water for a site contaminated with gasoline.

2.2.1 Theoretical Considerations: Transport of Charged Species - Electromigration

As in many electrochemical systems, flow of electric current through a network of a multi-phase material, such as wet soil occurs in different phases simultaneously: in the bulk liquid (electrolyte in the pores), on the surface of the solid (clay particles), and in the interface layer(s) between the solid and the liquid. Flow of the current can be achieved by ionic conduction through the liquid phase and electronic conduction through the solid phase and the interface layer(s). The electronic conduction orthogonal to and along the interface layer(s) takes place via charge transfer. In the classical treatise of "electrokinetic phenomenon" in colloidal systems (Hunter, 2001; Lyklema, 1995), it is this interface, known as the electric double layer (EDL) or diffuse double layer (DDL) which plays a critical role in the coupling between the ion motion and the fluid flow. The double layer intrinsically connects the solid and the liquid phase, and mediates the relative motion between the liquid and solid phase through (i) accumulation of charge density; (ii) transport of charge and ions along surface; and (iii) passage of charge to the surrounding electrolyte (Bard and Faulkner, 1980).

The bulk transport of ions in electrochemical systems without the contribution of advection by water is described by Poisson-Nernst-Planck (PNP) equations (Rubenstein, 1990). The well-known Nernst-Planck equation describes the processes of diffusion, the process that drives the ions from regions of higher concentration to regions of lower concentration; and electromigration (also referred to as ion-migration), the process that launches the ions in the direction of the electric field (Bard and Faulkner, 1980). Since the ions themselves contribute to the local electric potential, Poisson's equation that relates the electrostatic potential to local ion concentrations is solved simultaneously to describe this effect. The electro-neutrality assumption simplifies the mathematical treatise of bulk transport in most electrochemical systems. Nevertheless, this "no charge density accumulation" assumption does not hold true at the inter-phase regions of electric double layer between the solid and liquid, hence the cause of most electrokinetic phenomenon in clay-electrolyte systems.

The analysis of mass transport by diffusion under chemical (?C/?x) gradient and by migration under electrical (?F/?x) gradient in dilute solutions - for which the interactions between individual species can be neglected - is described by the Nernst-Planck [Eq. 2.1] and the Poisson's [Eq. 2.2], together referred to as the PNP equations:

(2.1)

(2.2)

where, C, D*, u and z are the concentration, diffusion coefficient, mobility and charge number of a single species i, respectively, and F is the Faraday's constant (i.e., a mole of charge); F is the electrostatic potential; es is the permittivity of the solvent, and ? is the charge density.

For many electrochemical systems the local electroneutrality condition is used, which sets the left-hand side of equation 2.2 to zero for zero charge density. The mathematical implication is then that the electrical potential satisfies the Laplace equation [?2F=0] hence uniform concentration distribution and uniform conductivity within the electrochemical cell. Yet in real electrochemical systems with concentration gradients, the current density of the system can be affected to cause current flow in the opposite direction of the electric field (Newman, 1991). Using the PNP equations and the electroneutrality condition, it can be shown mathematically that the concentration gradients give rise to a spatial variation of conductivity, where a diffusion potential arises to ensure ion movement at the same speed to overcome the...