Fundamental formulation of electrokinetic extraction of contaminants from soil

Abstract

Electro-osmosis and ionic migration are the basic cleanup mechanisms in the electrokinetic extraction of contaminants from fine-grained soils. These are coupled flows as the flows of fluid and contaminants are driven by an externally applied electrical gradient. Moreover, other electrochemical reactions will occur simultaneously during the process. The most pronounced effect is the generation of pH gradient in the soil. The change of pH in the pore fluid can have a significant impact on the degree of sorption and desorption of chemicals on soil particle surfaces, complexes formation and precipitation of chemical species, and dissociation of organic acids; thus affecting the feasibility and efficiency of the cleanup technique tremendously. An attempt is made to formulate the coupled flows of ionic contaminants and the resulting change of pH in the pore fluid during the electrokinetic extraction process. The coupled flows of contaminants are formulated by the formalism of nonequilibrium thermodynamics. The pH is determined as a function of time and space by maintaining electrical neutrality throughout the system all the times. A numerical model NEUTRAL is developed to simulate the processes. The good agreement between computed and experimental results published in the literature indicates that the approach is a valid step toward a better understanding of the physics and chemistry involved during electrokinetic treatment of contaminated soils. Key words : electrokinetics, in situ remediation, contaminated soil, coupled flows, electrochemistry, nonequilibrium thermodynamics.