Geoscience Reference
In-Depth Information
However, an important aspect is the added volume necessary to include the support material
in the medium. Resins contain only about one quarter of the total amount of iron per gram in
comparison with commercial iron, and the presence of the support increases the global mass by
a factor of five. Even so, studies have shown that equivalent weights of nanoparticles supported
on resins may reduce 20 times more Cr(VI), based on the iron present, than particles of granular
iron, and this disparity increases when the concentration of the pollutant reduces.
However, it is difficult to determine the rate of corrosion of the iron particles, once various
experiments have shown that both the support material and the method of preparation cause a
significant effect on the electrochemical reaction of the composite materials (Henderson and
Demond, 2007). Finally, the hydraulic conductivity is an important aspect for permeable barriers.
Groundwater flow is highly sensitive to changes in permeability, and an installed barrier becomes
useless if the hydraulic conductivity of the barrier is sufficiently different from the surrounding
environment as to redirect the flow of pollutants to the outside of the barrier. It is expected that
the behavior of supported nanoparticles be similar to the particles of granular iron, which has not
been considered an important parameter in existing barriers.
1.8
ELECTROKINETICS
EK is based on the application of low-density electric currents between electrodes placed in
the soil originating the migration of ions towards the corresponding electrode. In addition, sev-
eral combinations of these technologies have been applied. The term electrokinetic remediation
comprises, in fact, a number of different technologies. We consider essentially two major variants.
The first is based on the removal of ions (and/or polar organic compounds) using electromi-
gration; in this method, the ions are mobilized under the direct action of an electric field. This
approach was developed between 1958 and 1981 and has been marketed since 1988. The objec-
tive of the technology is to promote electromigration with external recirculation of the electrolyte
pumped from compartments involving the electrodes created by physical walls permeable to ions
but able to contain the anolyte and the catholyte. The second variant, initiated by Casagrande in
1947 and later developed by Honig in 1987 (Lageman
et al
., 2005), favors the electroosmosis
and is based on the movement of water through the electric double layer formed in the porous
medium. Pollutants are transported through the water layer moving towards the cathode or in the
direction of an adsorbent medium placed in its path, without recirculation of electrolytes. This
approach was initiated at the Massachusetts Institute of Technology (MIT) in 1989. Of these two
approaches, electromigration is gradually displacing the electroosmotic approach.
When electrodes submitted to a potential are submerged into an aqueous solution in a humid
soil, the first phenomenon that occurs is the electrolysis of water. The solution becomes acidic
at the anode due to hydrogen ion production and release of oxygen, and alkaline at the cathode
may fall to values below 2, while pH of the cathode can exceed 12, depending on the intensity of
the applied current. At the cathode:
H
2
O
+
e
−
→
½H
2
+
OH
−
(1.16)
At the anode:
H
2
O
→
2H
+
+
½O
2
+
2e
−
(1.17)
Most soils are conductive due to the presence of ions dissolved in the soil water, such as
calcium, sodium, potassium, carbonates, fatty acids, nitrates, phosphates, sulfates and chlorides.
A moisture content of 5% is sufficient to allow movement of these ions. Then, under the action
of electric current, an acid front moves from the anode to the cathode by migration and advection
causing desorption of contaminants from the soil. Since the H
+
ion has a much higher mobility
than other ions, it will carry a disproportionately higher fraction of total current. The process
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