Electrochemical processes, soil

In the 1960s and 1970s the use of soil geochemistry in mineral exploration led to the discovery of the now commonly observed twin-peak or rabbit-ear geochemical anomaly (Govett 1976). From the time rabbit-ear anomalies were first described they were attributed to electrochemical processes but the theory proposed at that time could only... 

The Mechanism. A closer look reveals that the processes in such an operation involve electro-osmosis (Section 6.11.3). The contaminants in the soil are adsorbed on the surface of the soil particles, and there is an equilibrium between what is adsorbed and what is dissolved in the aqueous solution in the pores of the soil. One can present the electrochemical processes in a schematic form as shown in Fig. 15.30. 

Electroremediation — Electrochemical process for in-situ decontamination and restoration of polluted soils, sludge, or other solid wastes. It is also currently known as electroreclamation, electrorestoration, or electrokinetic remediation. The technology involves the application of a low-intensity direct current across inert electrode pairs... 

During the weathering process, elements can disperse from source mineralisation by a variety of chemical processes. For reasons discussed below, electrochemical processes are increasingly thought to be the primary transport mechanism in environments of thick, young, exotic (i.e., transported) overburden. They are also likely to operate in other environments but their dominance as a transport mechanism is less certain. This chapter presents the principles behind electrochemical masj transport and discusses the role of natural geoelectrochemical processes in the formation of selective leach and conventional geochemical soil anomalies. 

Perhaps the most detailed accounts of field studies presently available are those conducted by Lagemem and coworkers [83,84]. Table 3 contains a summary of the sites and the metal contaminants remediated. The anode and cathode housings are interconnected in this process and form two separate circulation systems filled with different chemical solutions, details of which are not provided in the publications. However, the extent of remediation of many of the metal contaminants tested Is quite high. These studies demonstrate that electrochemical processing of soils is a viable and practical technology. 

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. 

Mohamed AMO. (1996). Remediation of heavy metal contaminated soils via integrated electrochemical processes. Waste Management 16(8) 741-747. 

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. 

Acar YB, Gale RJ, Putnam G, Hamed J. (1989). Electrochemical processing of soils Its potential use in environmental geotechnology and significance of pH gradients. 2nd... 

Factors Affecting Corrosion in Soils Since corrosion is an electrochemical process, it requires a potential difference between two points electrically connected and immersed in an electrolyte. Electrons flow from the anodic area through the metallic path to the cathodic area to complete the circuit. The anodic area is the most negative in potential sind is the area that corrodes through the loss of metal ions to the electrolyte. 

According to Lohner [4], the potential benefits of electrokinetic and electrochemical processes coupled with bioremediation include enhancement of pollutant bioavaUabUity by means of electrokinetic mobilization, increase of restricted soil bacteria mobility by electrokinetic transport processes, electrokinetic-induced mass transfer and transport of ionic electron acceptors and nutrients, and electrochemical production of limited electron donors (H2) and acceptors (O2). 

Ruiz D, Anaya JM, Ramirez V, Alba GI, Garcia MG, Carrillo-Chavez A, Teutli MM, Bustos E (2011) Soil Arsenic removal by a permeable reactive barrier of iron coupled to an electrochemical process, hit J Electrochem Sci 6 548-560... 

Another approach for reducing corrosion is to employ mechanisms that can modify the electrochemical processes that consume materials. Cathodic protection, either through the use of sacrificial anodes or an impressed current system, can convert a material that normally will corrode quite readily into a material that resists corrosion. This approach, which is the topic of Chap. 13, works very well for protecting fixed assets in contact with potentially corrosive environments such as soils, seawater, or any other electrolytically conducting medium. 

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. 

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. 

---