4.1 Introduction
Electrokinetic remediation, also known as electromigration, electrokinetic soil
processing, electroreclamation, or electrochemical decontamination, uses a direct
electric current (DC) to remediate organic compounds, heavy metals, radionuclides,
or mixed organic and inorganic waste from soils, groundwater, etc. (Lima et al.
2017) When the DC is applied, several phenomena occur, such as electroosmosis,
electrophoresis, and electromigration, which affect the migration of substances in
both ionic and colloidal forms (Ho et al. 1999a; Gomes et al. 2015). Moreover, due
to the existing protonic and hydroxyl groups on the surface of the minerals forming
the sedimentary rocks, the sediment surface has an electric charge, which depends on
the pH and ionic strength of the surrounding solution (electrolyte) (Saleh et al. 2006).
The application of an external electric field leads to the release of ions from the
electric double layer and their migration towards the opposite charged electrode
(electromigration). With increasing pH (around the cathode), the surface charge of
soil particles changes from positive, through neutral, to negative. An acidic environment is produced in the area of the anode, which triggers desorption of the
contaminants from the soil surface and pores. In an environment with low hydraulic
conductivity or high heterogeneity, the released contaminants are transported in the
electric field by electrokinetics (Ho et al. 1999b).
Electrogeochemical processes can be applied to chemicals that can undergo
oxidation-reduction changes or changes that are initialized by these reactions.
Typical example of it is reduction or oxidation of metals and by changing their
valence they precipitate from groundwater or substances that can be decomposed or
chemically modified by these processes (a typical example is reductive
dehalogenation of chlorinated ethenes-ClE). While in the case of metal
coprecipitation, the rock environment may become gradually clogged with newly
formed minerals; in the case of hydrogenation of ClE, this effect will be minimal as
the chemically treated contaminant does not form a mineral phase. However, this
may occur by the interaction of groundwater with the rock matrix.
Remediation technologies are very often based on a combination of various
processes and the use of different reactive materials. Zero-valent iron is a promising
reactant for the removal of various contaminants from groundwater because of a high
reductive capacity and eco-friendliness, i.e., production of nontoxic iron oxides after
the removal of the pollution (Černík et al. 2019; Johnson et al. 2009; Kanel and Choi
2007; Macé et al. 2006). Under the appropriate conditions, the use of nZVI as a
remediation agent can be highly effective, but the effect lasts for a relatively short
time and acts over a relatively small radius from the application point (Bennett et al.
2010). There is abundant evidence that the mobility of nZVI in the porous media is
limited under almost all conditions (Bennett et al. 2010). Nanoscale ZVI also has a
tendency to aggregate quickly and settle into pores mainly because of magnetic
attractive forces (Lowry and Johnson 2004). Therefore, nanoscale iron particles are
often modified by several methods, e.g., they are coated with various polymers (Park
et al. 2009; Schrick et al. 2004; Wang et al. 2010; Zhang et al. 2009), a protective
66
M. Černík et al.
Electrokinetic remediation, also known as electromigration, electrokinetic soil
processing, electroreclamation, or electrochemical decontamination, uses a direct
electric current (DC) to remediate organic compounds, heavy metals, radionuclides,
or mixed organic and inorganic waste from soils, groundwater, etc. (Lima et al.
2017) When the DC is applied, several phenomena occur, such as electroosmosis,
electrophoresis, and electromigration, which affect the migration of substances in
both ionic and colloidal forms (Ho et al. 1999a; Gomes et al. 2015). Moreover, due
to the existing protonic and hydroxyl groups on the surface of the minerals forming
the sedimentary rocks, the sediment surface has an electric charge, which depends on
the pH and ionic strength of the surrounding solution (electrolyte) (Saleh et al. 2006).
The application of an external electric field leads to the release of ions from the
electric double layer and their migration towards the opposite charged electrode
(electromigration). With increasing pH (around the cathode), the surface charge of
soil particles changes from positive, through neutral, to negative. An acidic environment is produced in the area of the anode, which triggers desorption of the
contaminants from the soil surface and pores. In an environment with low hydraulic
conductivity or high heterogeneity, the released contaminants are transported in the
electric field by electrokinetics (Ho et al. 1999b).
Electrogeochemical processes can be applied to chemicals that can undergo
oxidation-reduction changes or changes that are initialized by these reactions.
Typical example of it is reduction or oxidation of metals and by changing their
valence they precipitate from groundwater or substances that can be decomposed or
chemically modified by these processes (a typical example is reductive
dehalogenation of chlorinated ethenes-ClE). While in the case of metal
coprecipitation, the rock environment may become gradually clogged with newly
formed minerals; in the case of hydrogenation of ClE, this effect will be minimal as
the chemically treated contaminant does not form a mineral phase. However, this
may occur by the interaction of groundwater with the rock matrix.
Remediation technologies are very often based on a combination of various
processes and the use of different reactive materials. Zero-valent iron is a promising
reactant for the removal of various contaminants from groundwater because of a high
reductive capacity and eco-friendliness, i.e., production of nontoxic iron oxides after
the removal of the pollution (Černík et al. 2019; Johnson et al. 2009; Kanel and Choi
2007; Macé et al. 2006). Under the appropriate conditions, the use of nZVI as a
remediation agent can be highly effective, but the effect lasts for a relatively short
time and acts over a relatively small radius from the application point (Bennett et al.
2010). There is abundant evidence that the mobility of nZVI in the porous media is
limited under almost all conditions (Bennett et al. 2010). Nanoscale ZVI also has a
tendency to aggregate quickly and settle into pores mainly because of magnetic
attractive forces (Lowry and Johnson 2004). Therefore, nanoscale iron particles are
often modified by several methods, e.g., they are coated with various polymers (Park
et al. 2009; Schrick et al. 2004; Wang et al. 2010; Zhang et al. 2009), a protective
66
M. Černík et al.