Changes in the pH occurred only after exceeding the decomposition voltage,
where the electrolytic decomposition of groundwater occurred, and the water with
the changed pH subsequently spread through the aquifer because of dispersion,
electromigration, and groundwater flow.
The long-term conditions of E h and pH throughout the polygon stabilized
approximately on the boundary of the oxidation and reduction conditions, i.e.,
outside the stability field of Fe(OH) 3 (Fig. 4.6b). The stability field of Fe(OH) 2 or
Fe(OH) 4
À at the cathode is achieved at a pH greater than 11. The groundwater
around anode does not reach a very low pH and the conditions correspond to the
stability field of dissolved Fe
2+ ions, i.e., the reduction conditions are created. This
state can be interpreted not only by the anode dissolution and subsequent release of
Fe
2+ into the groundwater, but also by the gradual dissolving of iron from the rock
matrix. In the treated area, the content of the dissolved iron, which can enter into
reactions with the contaminant, gradually increased. Moreover, changes in the E h are
independent of the groundwater flow.
The spread of the pH changes in the aquifer is then slower because of these
processes (Fig. 4.8). After disconnecting the DC current (at day 88), the system had
relatively large inertia due to rapid consumption of the electrodes.
Figure 4.9 shows areal representation of E h , pH, and conductivity at the pilot test
polygon in three time periods (8, 87, and 172 days). In all three cases, the situation is
stable in time with well distinguished areas around anode and cathodes.
Due to the application of DC field, overall concentration of ClE decreased
significantly (Fig. 4.10, Table 4.1). Around anode (IS10), the ClE total concentration
decreased to 50% of the original value, comparable with the decrease in all individual ClEs. However, the drop in ClE is not permanent and very strong rebound effect
was observed (t ¼ 172 d). This rebound is probably due to the release of ClE from
contaminated sediments rather than by the inflow of the contaminated water from the
surroundings. The sum of ClE in the well IS10 returned to 83% of the original value
with a slightly increased ration of DCE (from 63 to 69%).
4.3 Synergic Action of nZVI and a DC Field
4.3.1 Laboratory Reactor Test
In order to study the oxidation-reduction processes of the contaminant removal,
laboratory reactor tests are the most reliable experiments. There are different setups
of the reactors (Fig. 4.11). Glass reactors are equipped with well-sealed inputs or
outputs (ports). Mechanical mixers, sampling points, or electrodes are permanently
installed inside the reactor, so that the sampling process itself is realized without
access to air in the reactor. The experiment can be run in a simple reactor, where the
samples of contaminated water can be taken in regular intervals or the reactor is
directly connected to gas chromatography. In case of electrokinetic experiments, the
reactor can be equipped with the DC electrodes. The most exact measurement, where
4 Combination of Electrokinetics and nZVI Remediation
75
where the electrolytic decomposition of groundwater occurred, and the water with
the changed pH subsequently spread through the aquifer because of dispersion,
electromigration, and groundwater flow.
The long-term conditions of E h and pH throughout the polygon stabilized
approximately on the boundary of the oxidation and reduction conditions, i.e.,
outside the stability field of Fe(OH) 3 (Fig. 4.6b). The stability field of Fe(OH) 2 or
Fe(OH) 4
À at the cathode is achieved at a pH greater than 11. The groundwater
around anode does not reach a very low pH and the conditions correspond to the
stability field of dissolved Fe
2+ ions, i.e., the reduction conditions are created. This
state can be interpreted not only by the anode dissolution and subsequent release of
Fe
2+ into the groundwater, but also by the gradual dissolving of iron from the rock
matrix. In the treated area, the content of the dissolved iron, which can enter into
reactions with the contaminant, gradually increased. Moreover, changes in the E h are
independent of the groundwater flow.
The spread of the pH changes in the aquifer is then slower because of these
processes (Fig. 4.8). After disconnecting the DC current (at day 88), the system had
relatively large inertia due to rapid consumption of the electrodes.
Figure 4.9 shows areal representation of E h , pH, and conductivity at the pilot test
polygon in three time periods (8, 87, and 172 days). In all three cases, the situation is
stable in time with well distinguished areas around anode and cathodes.
Due to the application of DC field, overall concentration of ClE decreased
significantly (Fig. 4.10, Table 4.1). Around anode (IS10), the ClE total concentration
decreased to 50% of the original value, comparable with the decrease in all individual ClEs. However, the drop in ClE is not permanent and very strong rebound effect
was observed (t ¼ 172 d). This rebound is probably due to the release of ClE from
contaminated sediments rather than by the inflow of the contaminated water from the
surroundings. The sum of ClE in the well IS10 returned to 83% of the original value
with a slightly increased ration of DCE (from 63 to 69%).
4.3 Synergic Action of nZVI and a DC Field
4.3.1 Laboratory Reactor Test
In order to study the oxidation-reduction processes of the contaminant removal,
laboratory reactor tests are the most reliable experiments. There are different setups
of the reactors (Fig. 4.11). Glass reactors are equipped with well-sealed inputs or
outputs (ports). Mechanical mixers, sampling points, or electrodes are permanently
installed inside the reactor, so that the sampling process itself is realized without
access to air in the reactor. The experiment can be run in a simple reactor, where the
samples of contaminated water can be taken in regular intervals or the reactor is
directly connected to gas chromatography. In case of electrokinetic experiments, the
reactor can be equipped with the DC electrodes. The most exact measurement, where
4 Combination of Electrokinetics and nZVI Remediation
75