Reduction of Contaminants in Soil and Water by Direct Electric Current (original) (raw)

Applying a low direct current electrical field across a porous medium, such as wet soil induces migration of ionic and surface charged constituents and flow of the pore water to a directed location within the field. Commonly referred to as “electrokinetics”, this process is a proven, sustainable technology that can transport water and substances residing in or near the aqueous phase in clayey soil at a significantly higher rate than hydraulic methods. Harmful heavy metal contaminants and radioactive materials, as well as some immiscible fluids can thus be removed in-situ. The process is especially useful in applications where pump-and-treat methods are impractical due to the low permeability of the medium, or in cases where the contaminants persist owing to their affinity to the solid phases in clay soils. Moreover, the application of direct electric current in soils have been used successfully to transport reactive agents, such as nano-particle slurries to enhance in-situ reactions that convert contaminants, or chelating or surfactant agents to solubilize the contaminants so they can be transported with water advection. More recently, evidence was provided that shows how direct electric current can contribute to success of the desired transformation reactions by not only providing the “driving force” necessary to deliver active reagents, but also by lowering the energy for the redox reactions to occur. This enhancement of transformation reactions was attributed to the double-layer polarization of the clay surfaces leading to Faradaic processes under the applied electric field. In most field situations, the contaminants are found adsorbed onto soil surfaces, ironoxide coatings, soil colloids and natural organic matter. Most contaminants are retained in clay interstices as hydroxycarbonate complexes, or present in the form of immobile precipitates and products in soil pore throats and pore-pockets that “lace” the vadose zone. This exacerbates clean-up efforts as the available technologies, such as in-situ bioremediation, chemical treatment or the traditional pump-and-treat method may not be able to treat the entire site effectively in low permeability soils. Electrokinetics treatment, when designed to properly address the site specific features, can potentially reduce the subsurface pollution by transporting and/or transforming contaminants, and enhance resource recovery by extracting trapped materials (i.e., heavy metals, oils and petroleum) which may not be extractable by other means. This chapter first provides an overview of the use of direct electric current for environmental mitigation in the subsurface, and discusses with examples the application of direct electric current to: (i) transport of environmental contaminants for the purpose of extracting them out of subsurface soil and water; (ii) transformation of environmental contaminants for the purpose of their destruction in-situ or conversion to environmentally less toxic/benign compounds; and (iii) recovery of resource by extraction and accumulation of usable materials from subsurface soil and water.