Electro-Chemical Technologies for In-Situ Restoration of Contaminated Subsurface Soils
Associate Professor, Department of Civil & Environmental Engineering,
Lehigh University, Bethlehem, PA 18015, USA
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INVITED PAPER
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Electro-Chemical Technologies for In-Situ Restoration of Contaminated Subsurface Soils
Associate Professor, Department of Civil & Environmental Engineering, | |
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INVITED PAPER |
ABSTRACT
Electrochemical treatment of contaminated subsurface is probably one of the most promising insitu soil decontamination technologies capable of removing organic and inorganic contaminants and radionuclides. By using a low level direct current (DC) electrical potential difference across electrode pairs placed in ground, contaminants in the soil are moved under the action of the electrical field. Extraction of contaminants by electrokinetic method is based on the assumption that the contaminant is in liquid phase in the soil pores. This involves contaminant adsorption, transport, capture, and removal from soils. The process is most effective when the transported substances are either ionic, surface charged or in the form of small micelles with little drag resistance.
There are metal and alkaline earth species, which tend to remain ionic under a large range of pH and redox potential values. Those substances are expected to electromigrate and separate even under high pH and variable redox conditions unless they become adsorbed onto soil surfaces and clay interstices. For those species that tend to precipitate at high pH, shorter istances of electromigration may be provided whereby the contaminant may be captured in pre-constructed reactive zones before they precipitate in the soil. Electrochemical containment in this manner is considered an alternative to the treatment processes, which may otherwise require additional ex-situ remediation techniques to extract heavy metals from, contaminated soils when solubilizing agents are used. Electrochemical containment of Pb using reactive filter beds in front of the electrodes is illustrated in this paper.
Furthermore a modified mathematical model, closely following an existing model is developed and solved numerically for PbNO3 for multi-species transport under transient electric field. Coupled potential differences (chemical and electrical potentials) are incorporated in the model. A set of differential equations addressing mass and charge balances is formulated. Chemical reactions, including electrolysis, water auto-ionization, sorption/description and precipitation reactions, are described using algebraic equations. The varying electric conductivity, electric field, current and voltage are also simulated in the model. The modified model handled the changing electrical field due to re-distribution of charged concentration in the soil under constant potential between the electrodes. The model also incorporated effective ionic mobility to describe the variations in conductivity of soil with finite concentration of ions rather than infinitely dilute systems.
KEYWORDS: contaminants, contaminated soils, electrochemical treatment, insitu treatment, remediation
