Geoscience Reference
In-Depth Information
5.3.2.2 Electrolyte chemistry and enhancement scheme
As discussed in Section 5.3.1.2, contaminants exist in different chemical forms in soil media
depending on environmental conditions. Among those different forms, only dissolved fractions
of contaminants are mobile and can be removed by the electrokinetic process. Therefore, other
forms of contaminants should be transformed to their soluble and dissolved forms. However, the
transformation processes in which the chemical forms of contaminants are changed are considered
to be contaminant specific, reversible, and dependent on environmental conditions. The acidic
condition is favorable in desorption and mobilization of cationic contaminants, such as heavy
metals, while the basic environment aids in solubilization of anionic contaminants such as arsenite,
arsenate, and fluoride. Therefore, the pH and composition of the electrolyte solution must be
adjusted because the transformation processes of contaminants in the soil can be improved or
hindered by electrolyte chemistry.
A variety of enhancement schemes have been proposed and tested to facilitate electrokinetic
extraction of contaminants, as briefly addressed in Section 5.2.3.3. Normally, enhancement
schemes can be embodied by injecting reagents, called as enhancing or enhancement agents,
into the soil or electrolyte solutions. The enhancement agents are needed to control the soil
chemistry and to promote solubilization (mobilization) of contaminants, and finally improve
the overall efficiency of the process. Alshawabkeh et al . (1999) summarized the important
characteristics of enhancement agents:
They should not form insoluble salts with the contaminant within the range of pH values
expected to develop during the process.
They should form soluble complexes with the contaminant that can electromigrate efficiently
under the electric field applied.
They should be chemically stable over a wide range of pH values.
They should have a higher affinity for the contaminant than the soil particle surface.
They and the resulting complexes should not have a strong affinity for the soil particle surface.
They should not generate toxic residue in the treated soil.
They should not generate an excessive quantity of wastewater or the end products of the
treatment process should be amenable to concentration and precipitation after use.
They should be cost-effective including reagent cost and treatment costs for the waste collected
and/or wastewater generated.
They should not induce excessive solubilization of soil minerals or increase the concentrations
of any regulated species in the soil pore fluid.
If possible, they should complex with the target species selectively.
The enhancement schemes that have been most frequently used include enhancement agents
for catholyte neutralization, membrane enhancement, and chelating or complexing agents. The
common chelating or complexing agents tested are citric acid, EDTA, and surfactants. However,
special caution should be exercised when enhancement schemes are considered because some
agents significantly affect the zeta potential of the soil surface, and in turn, the direction of
electroosmosis is changed (Eykholt and Daniel, 1994; Kim, 2001; Kim et al ., 2002a). The overall
efficiency of electrokinetics can be influenced by unexpected variation of zeta potential and
electroosmosis.
5.3.2.3 Type of electricity
The selection of a suitable type of electricity is important to improve the performance of the
electrokinetic process. Figure 5.4 shows typical types of electricity that can be applied for elec-
trokinetic remediation. The full-wave DC has usually been applied in electrokinetic removal of
inorganic contaminants, such as heavy metals, arsenic species, and radionuclides. For the sake of
extracting organic contaminants, however, AC can be used for the electrokinetic process because
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