30    ELECTROKINETIC SOIL PROCESSING: AN EMERGING
TECHNOLOGY
Robert E. Marks, Director of Engineering
Yalcin B. Acar, President
Robert J. Gale, Vice President
Electrokinetics Inc.
Louisiana Business & Technology Center
Louisiana State University
Baton Rouge, Louisiana 70803-6100
INTRODUCTION
Electrokinetic soil processing (ESP) is an in situ separation and removal technique used for extraction of heavy metals and some organic compounds from soils. The feasibility of the process has been
demonstrated at bench scale and in limited field testing to be applicable to removal of these contaminants from low permeability clays.
The technology uses electricity to affect chemical concentrations and water flow through the pores
of the clay. The application of low level direct currents (in the order of 50-2,000 yA/cm2), results in
electro-osmosis (EO) which generates a flow of fluid between the clay particles. If the medium is a
mixture of sedimentary particles then the sediments move under the influence of a direct current and
this effect is called electrophoresis. The application of a hydraulic head results in a streaming potential (movement of water under hydraulic potential) and the generation of a low level current. The flow
generated by equivalent hydraulic and electrical potential differences in saturated clays may be
approximately 1/1000 of the flow generated by electro-osmosis. Finally migration potential is settling
of particles due to gravity and this movement also generates a low level direct current. These coupling
effects are more important in fine-grained soils with lower coefficients of permeability.
Electrokinetic Phenomena in Soils
Figure 1 shows the electrokinetic representations of each of the four processes. The first process
which is the most powerful driving force in ESP is shown in Figure 1.1. The applied voltage and direct
current results in the production of an acid front which causes pore fluid movement across the cell.
The positively charged ions (cations) move towards the negatively charged cathode and vice versa.
Figure 1.2 shows the electrophoresis process where application of a direct current to the negatively
charged clay suspension causes migration of the particle to the positively charged anode. An electrical
potential is produced when hydraulic gradient induces water flow through saturated clay and is
termed streaming potential and is shown in Figure 1.3. Figure 1.4 shows downwards particle movement in a clay suspension and this produces an electrical gradient which is called migration potential.
The most important processes for commercial removal of contaminants in saturated clays and sediments are electro-osmosis and electrophoresis.
Electrokinetic Process for Contaminant Removal from Clay-based Sites
A schematic diagram for the removal of contaminants from clayey soils is shown on Figure 2. This
diagram is a typical representation for removal of contaminants from a soil site contaminated with
toxic metals. When the electrokinetic soil process (ESP) is initiated, electrolysis of water occurs at the
anode and cathode. Hydrogen ion (H + ) is produced at the cathode and hydroxyl ion (OH) is
produced at the cathode. The electrolysis reactions result in acidic and basic domains at the anode and
cathode respectively. The acid front increases the solubility of the metals and metal oxides present in
the contaminated soils and increases recovery at the cathode. The electrical power supply, conditioning fluid containers and any purification facilities are located adjacent to the site. The site may be
47th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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