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EMERGING TECHNOLOGY SUMMARY: THEORETICAL AND EXPERIMENTAL MODELING OF MULTI-SPECIES TRANSPORT IN SOILS UNDER ELECTRIC FIELDS
Citation:
Acar, Y. B., A. N. Alshawabkeh, AND R A. Parker*. EMERGING TECHNOLOGY SUMMARY: THEORETICAL AND EXPERIMENTAL MODELING OF MULTI-SPECIES TRANSPORT IN SOILS UNDER ELECTRIC FIELDS. EPA/600/SR-97/054, 1997.
Impact/Purpose:
present information
Description:
This project investigated an innovative approach for transport of inorganic species under the influence of electric fields. This process, commonly known as electrokinetics uses low-level direct current (dc) electrical potential difference across a soil mass applied through inert electrodes placed in an open flow arrangement. The application of low-level dc current across electrodes placed in the soil mass causes physiochemical and hydrological changes in the soil-water-electrolyte medium leading to contaminant transport and removal. The feasibility and efficiency of transporting lead under electric fields was investigated in this study at pilot scale in three one-ton Georgia kaolinite specimens spiked with lead nitrate solution. Electrode spacing was set at 72.4 centimeters (cm). Tests were conducted on specimens with a lead nitrate concentration of 856 milligrams per kilogram (mg/kg) to 5,322 mg/kg. Pore pressures and temperatures developed across the soil mass, electric potential distributions, pH distributions, and lead transport were investigated. The results demonstrate that heavy metals and species that are solubilized in the anodic acid front can be efficiently transported by electromigration under an electric field applied across electrodes placed in soils. After 2,950 hours of processing and an energy expenditure of 700 kWh/m3, 55% of the lead removed across the soil was found precipitated within the last two cm close to the cathode, 15% was left in the soil before the 2 cm zone, 20% was found precipitated on the fabric separating the soil from the cathode compartment, and 10% was unaccounted for. Overall, The project adequately demonstrated the potential applicability of the process, and it appears that the process is appropriate for testing on a larger scale. A theoretical model was also developed for multi-component species transport under coupled hydraulic, electric, and chemical potential differences. A mass balance of species and pore fluid coupled with a charge balance across the medium resulted in a set of differential equations. Sorption, aqueous phase and precipitation reactions were modeled by a set of algebraic equations. Instantaneous chemical equilibrium conditions were assumed. Transport of H+, OH", and Pb2+ ions, the associated chemical reactions, electric potential, and pore pressure distribution across the electrodes in electrokinetic remediation were modeled. Model predictions of acid transport, lead transport, and pore pressure distribution displayed agreement with the pilotscale results validating the formalisms offered for multi-component transport of reactive species under an electric field. The model also bridges the gap between the electrochemistry and mechanics in electroosmotic consolidation of soils.