Electro-kinetic extraction of contaminants from polluted soil

Abstract

Electro-osmosis is an effective driving force which moves fluid through a fine-grained soil of low hydraulic conductivity. When electrodes are installed in a wet soil, fluid will be moved from the anode to the cathode by the applied electric field. Ionic migration will cause ions in the pore fluid of the soil to be migrated by the electric field in directions controlled by the charge of the ions, even if the fluid is stationary. Combining electro-osmosis with ionic migration results in the possibility of electro-kinetic removal of inorganic and organic contaminants from polluted soils. However, other electrochemical reactions, such as electrolysis, ion diffusion, development of pH and osmotic gradients, mineral decomposition, ion exchange, precipitation of salts and secondary minerals, hydrolysis, oxidation, reduction, physical and chemical adsorption, and fabric change, etc., will occur simultaneously. Hence the physics and chemistry involved in the removal mechanisms are very complex and have yet to be understood before the technique can be fully utilized. This paper will present the theoretical and laboratory experimental research being performed at Texas A&M University to better understand these complex transport phenomena. The removal process will involve the coupled flows of fluid, electricity, and contaminants under simultaneously imposed hydraulic, electrical, and chemical gradients. The formalism of non-equilibrium thermodynamics is used for the formulation of the flow processes. The change in pH and the resulting change in chemical state of the contaminants are also included in the theoretical formulation. A new experimental apparatus has been designed, fabricated, and assembled. Experiments are performed on kaolinite consolidated from slurry to evaluate the validity of the theoretical formulation. Moreover, the effects of the following parameters on the viability, feasibility, practicality, and economics of this potential environmental cleanup technology are evaluated: soil type and pore size, type and concentration of contaminants, type of purging solution, cost effectiveness, and other electrochemical effects induced by the applied electric field.