Electrochemical processes, soil treatment

The electrochemical soil decontamination process is designed to treat organic compounds and heavy metals. It utilizes induced electrical currents to establish chemical, hydraulic, and electrical gradients designed to extract contaminants for soils. Treatment may be accomplished in situ or on site in lined cells. 

Clarke RL, Lageman R, Smedley SI. (1997). Some practical applications of integrated electrochemical techniques used on remediation, recycling and resource recovery. 1. Electrokinetic treatment of soils and sediments. Fourth European Electrochemical Processing Conference Electrochemical Processing—The Versatile Solution, April 14-18, Barcelona, Spain. 

Huang CP, Cha D, Chang J, Qiang Z, Sung Me, Chiang YC. (1999). Electrochemical Processes for In-Situ Treatment of Contaminated Soils. Final Progress Report, September 1998 to May 1999, Project ID 54661, Department of Energy. 

Mikkola et al (2008) employed an approach of generating the necessary oxidizing agents (i.e., HO- and S20i ) electrochemically within a treatment system.That study aimed at evaluating the feasibility of an innovative EK process for an in situ application against fuel-contaminated soil. However, no experimental data regarding the remediation of contaminants were reported in that preliminary work. 

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

This chapter has shown the complexity of the chemical and biological processes around wetland plant roots and the effects of the extreme electrochemical gradient between the root surface and surrounding soil. Models of nutrient uptake by plants in aerobic soil, which treat the root as a simple sink to which nutrients are delivered by mass flow and diffusion but the root not otherwise influencing the surrounding soil, work reasonably well for the more soluble nutrient ions. However, the complexity of the wetland root environment is such that such models are inadequate and more elaborate treatments are necessary. Many of the mechanisms involved are still poorly defined and speculative, but their potential significance is clear. 

Principles and applications of electrochemical remediation of industrial discharges are presented by Pallav Tatapudi and James M. Fenton. Essentials of direct and indirect oxidation and reduction, membrane processes, electrodialysis, and treatment of gas streams, and of soils, are complemented by discussions of electrode materials, catalysts, and elements of reactor design. 

The electrochemical removal of contaminants may not be always possible or practical. For instance, a site may be too polluted to be treated to the acceptable level by any of the technologies, but it is critical to reduce the risk posed by the site contamination. The common approach used for risk reduction is stabilization and solidification (or immobilization) technology. In this approach, contaminants are transformed into a form that does not allow them to be released into the environment. Electrochemical approach may be used to stabilize the contaminated soils at a low cost and it will serve as an interim or pretreatment process to permanent treatment technologies. 

A well-documented field application of electrochemical remediation is reported to address the problem of chlorinated solvent (TCE) in clay soil at the DOE site in Paducah, Kentucky. This process is known as Lasagna (Terran Corporation, Beavercreek, OH) and it combines the electro-osmotic transport of TCE in pore water and degrades it in vertical curtains installed along the flow path within the soil that are filled with iron filings and kaolin clay. Pore water accumulated at the cathode is recycled by gravity back to the anode as makeup water and neutralize the acid formed at the anode. Overall, the treatment is found to be effective. The implementation and performance results are presented in detail in Chapter 30. The same Lasagna process is implemented very recently at another site contaminated with TCE in Fonde du Lac, Wisconsin, and performance is being monitored (Chapter 30). 

---