Contaminants can undergo several types of phenomena, which can be divided into two classes : those that would occur in the absence of the applied electric field, such as
• Adsorption-desorption of the contaminant from soil surfaces
• Precipitation-dissolution reactions
• Interactions between soluble chemical constituents in the pore water
The second class comprises those that occur because of the electric field, such as (1) plating of a metal on the electrode(s); and (2) electrolysis of water in the electrode compartments.
In the case of the second class, plating of a metal can be minimized by proper selection of the electrode. The nature of electrode reactions during electrokinetic treatment of soil depends on the material of the electrodes and the type and concentration of chemical species in the fluid around the electrodes . For electrokinetic remediation purposes, inert electrodes (i.e., electrode material such as carbon, platinum, or titanium, which do not take part in the reaction) are mostly used (see Section 18.12).
The soil particles and specialty clays have an active surface, which can interact with contaminants to their sorption or ion exchange. The large surface of some types of clays such as montmorillonite or illite increases this capacity of retention and the negative charges present usually in their structures attract cations, making their movement difficult. In this sense, the presence of other nontoxic ions (H+, Na+, etc.) competing for the same fixation sites, promoting their desorption, can help to increase the mobility of the contaminants.
On the other hand, when sorption/desorption is evaluated in soils, equilibrium isotherms are commonly used, assuming that the contaminants sorbed reach instantaneous equilibrium at all times. However, this assumption is only valid if movement of species is slow enough, but usually, in electrokinetic remediation, the migration velocity of species is high and therefore the kinetics of the sorption process is relevant to evaluate its effects on remediation.
A great variety of natural species is in a solid phase or in pore aqueous solution (e.g., CO32-, SO42-, S2-) in the natural soils and, if the contaminants react with them, precipitation can occur. Moreover, the acid or basic fronts or other ions generated or introduced in the electrode compartments can also produce precipitation or dissolution of the contaminants. Thus, the natural buffer capacity of the soils to remediate, as well as the ions proceeding of their own electrokinetic process, must be considered in order to know the effect on dissolution/precipitation and therefore on the contaminants' mobility and soil electric conductivity.
18.4.3. Movement of an Acid Front and an Alkaline Front in the Soil Compartment
The acid (H+ ions) generated at the anode advances through the soil towards the cathode by [11,20]:
• Ion migration due to electrical gradient
• Pore fluid advection due to electroosmotic flow
• Pore fluid flow due to any externally applied or internally generated hydraulic potential difference
• Diffusion due to the chemical gradients developed
The alkaline medium developed at the cathode first advances towards the anode by ionic migration and diffusion. However, the transport of OH- in the soil is overshadowed by any electroosmotic advection and neutralization by the H+ ions transported to this zone (generation of water takes place within this zone) [4,11].
According to Acar and Alshawabkeh , the extent of the "meeting zone" is controlled by the dissolution/precipitation chemistry of the species available in the soil; their interaction with the soil and with the hydrogen/hydroxide ions; and the electrochemistry of the species produced and/or injected in the electrode compartments. In fact, the acid-alkaline fronts that meet close to the cathode compartment within the soil, coupled with precipitation of hydroxide species in the high pH zone, result in the development of a low conductivity region close to the cathode.
As a consequence of different phenomena previously explained, acidification of the soil facilitates desorption of the contaminants. The driving mechanisms for the transport of the species in the soil are the same as those for the acid/base transport. As a result, cations tend to accumulate at the cathode and anions at the anode, and transfer of H+ and OH- ions across the medium is continuous. Extraction and removal are accomplished by electrodeposition, precipitation, or ion exchange at the electrodes or in an external extraction system placed in a unit cycling the processing fluid [11,21,22].
Extraction of contaminants by electrokinetic methods is based on the assumption that the contaminant is in the liquid phase in the soil pores  or in exchangeable positions in the solid phase, in contact with the previous one. Electroosmotic advection should be able to transport nonionic as well as ionic species through the soil towards the cathode. This is, perhaps, best achieved when the state of the material (dissolved, suspended, emulsified, etc.) is suitable for the flowing water to carry it through the tight pores of soil without causing a plug of concentrated material to accumulate at some point in the soil .
Polar organic molecules, ionic micelles, and colloidal electrolytes should migrate under the influence of an electric field, as well as being transported by the water. The size of these molecules or micelles and their tendency to agglomerate or be adsorbed onto soil surfaces are probably the main factors that control their removal from soil pores by electrokinetics . Removal of cationic species, on the other hand, should be enhanced by the electroosmotic flow of water as they migrate towards the cathode electrode compartment by the applied potential . In noncompact soils, concurrent colloidal transport may be significant in altering the microstructure of the porous medium and in facilitating the transport of adsorbed metal ions and organics .
The concentration of the metal on the soil strongly influences the retention energy and the operative removal mechanism. At low concentrations of metals, removal is most likely due to desorption. Numerous sorption sites on soils have a wide range of binding energies. At low metal concentrations, the high energy-binding sites are occupied first. At higher metal concentrations, the high energy-binding sites are completely occupied and the lower energy-binding sites begin to fill, resulting in a decrease in the average metal-soil binding energy. As a result, it is easier to remove a given percentage of the metal from the more highly contaminated soil. However, even for the highly contaminated soil, it becomes progressively harder to remove the remaining metals (i.e., metals associated with the high energy-binding sites). On the other hand, the formation of insoluble compounds on the soil surface or in the pore liquid can occur if the concentration of the metal is high; if the soil pH is alkaline; or if anions such as Cl- and SO42- are present in sufficient quantity .
The relative magnitude of contribution of either process (electroosmosis and electromigration) to decontaminate under a given set of initial and boundary conditions is determined by the soil and contaminant type, as well as their interaction, is yet unclear. At low concentrations of cations, electroosmotic water flow may contribute a significant percentage to, if not be totally responsible for, the overall decontamination process. At high concentrations of ionic species, electrolytic migration and intensity of electrochemical reactions may play more important roles than electroos-mosis in the decontamination process .
Detailed descriptions of the coupled transport processes of fluid, electricity, and contaminants during electrokinetics, and associated complicated features generated by various electrochemical reactions, have been given by several authors [3,7,9,12,25,26].
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