layer made of biodegradable oil (Quinn et al. 2005), natural gums (Padil Vinod et al.
2017), etc. Another method for improving the effectiveness of nZVI is to use it in
combination with another remedial method, e.g., biodegradation (Klimkova et al.
2011; Němeček et al. 2016).
A new way of increasing the effectiveness of nZVI is to support the remediation
with a DC electric field (Černík et al. 2019) mentioned above. The use of a direct
current can help to overcome the poor mobility of nZVI (Gomes et al. 2015). With an
increasing pH (around the cathode), the surface charge of the nZVI particles changes
from positive to negative. The larger the magnitudes of the surface charge, the larger
the repulsive forces and smaller the tendency for aggregation. Moreover, the negative surface charge forestalls nanoparticle adsorption on the negatively charged
minerals (Morais et al. 1976; Sposito 1998). Similarly, the field induces migration
of contaminants from their source to the treatment zone.
The influence of ZVI and DC can be interpreted by a conceptual geochemical
model. Unaffected groundwater, generally, has moderate E h and slightly positive
and neutral pH. Sometimes, the water shows a more reductive E h , indicating the
ongoing biological reductive dechlorination of contaminants or other biologically
enhanced reductive processes. The application of nZVI leads to a sharp decrease in
the E h , sometimes to the limit of water stability, and an increase in the pH depending
on the buffer capacity (mainly carbonate content) of the aquifer. After depletion of
the reduction capacity of nZVI (by its oxidation), the conditions return to a moderate
oxidation state (the kinetics of the process depend on the balance of the substances
with an oxidation potential brought into the reaction zone, and not on the level of
contamination). As it was mentioned above, the application of the DC field changes
the conditions in the reaction zone, depending on the distance of the electrodes.
Anoxic conditions are not usually reached on the cathode because of the increase in
the pH, even when there is a significant decrease in the E h . Conversely, in the
vicinity of the anode, despite the increase in the E h , anoxic conditions are often
reached as a result of a significant decrease in the pH (Fig. 4.1).
4.2 Electrokinetic Remediation
4.2.1 Basic Principles of the Action of an Electric Field
on Water Parameters
The content of dissolved substances in groundwater determines its electrical conductivity, which determines the current density, i.e., the number of electrons that can
be transmitted through the rock environment at a certain voltage. However, if the
practical decomposition voltage on an electrode is exceeded, electrolysis of water
occurs according to the following equations (Fig. 4.2):
4 Combination of Electrokinetics and nZVI Remediation
67
2017), etc. Another method for improving the effectiveness of nZVI is to use it in
combination with another remedial method, e.g., biodegradation (Klimkova et al.
2011; Němeček et al. 2016).
A new way of increasing the effectiveness of nZVI is to support the remediation
with a DC electric field (Černík et al. 2019) mentioned above. The use of a direct
current can help to overcome the poor mobility of nZVI (Gomes et al. 2015). With an
increasing pH (around the cathode), the surface charge of the nZVI particles changes
from positive to negative. The larger the magnitudes of the surface charge, the larger
the repulsive forces and smaller the tendency for aggregation. Moreover, the negative surface charge forestalls nanoparticle adsorption on the negatively charged
minerals (Morais et al. 1976; Sposito 1998). Similarly, the field induces migration
of contaminants from their source to the treatment zone.
The influence of ZVI and DC can be interpreted by a conceptual geochemical
model. Unaffected groundwater, generally, has moderate E h and slightly positive
and neutral pH. Sometimes, the water shows a more reductive E h , indicating the
ongoing biological reductive dechlorination of contaminants or other biologically
enhanced reductive processes. The application of nZVI leads to a sharp decrease in
the E h , sometimes to the limit of water stability, and an increase in the pH depending
on the buffer capacity (mainly carbonate content) of the aquifer. After depletion of
the reduction capacity of nZVI (by its oxidation), the conditions return to a moderate
oxidation state (the kinetics of the process depend on the balance of the substances
with an oxidation potential brought into the reaction zone, and not on the level of
contamination). As it was mentioned above, the application of the DC field changes
the conditions in the reaction zone, depending on the distance of the electrodes.
Anoxic conditions are not usually reached on the cathode because of the increase in
the pH, even when there is a significant decrease in the E h . Conversely, in the
vicinity of the anode, despite the increase in the E h , anoxic conditions are often
reached as a result of a significant decrease in the pH (Fig. 4.1).
4.2 Electrokinetic Remediation
4.2.1 Basic Principles of the Action of an Electric Field
on Water Parameters
The content of dissolved substances in groundwater determines its electrical conductivity, which determines the current density, i.e., the number of electrons that can
be transmitted through the rock environment at a certain voltage. However, if the
practical decomposition voltage on an electrode is exceeded, electrolysis of water
occurs according to the following equations (Fig. 4.2):
4 Combination of Electrokinetics and nZVI Remediation
67