electrical force on the liquid and the surface of the soil particles (Page and Page
2002). Electroosmosis has been found to be effective in removing cation at low
concentrations. The rate of electroosmotic flow in the soil is described by Darcy’s
law for hydraulic flow (Page and Page 2002):
q A ¼ Àk e EA
ð2:5Þ
where k e is the coefficient of electroosmotic permeability (or conductivity), E is the
electric field strength or negative potential gradient, and A is the total cross-sectional
area normal to the flow direction.
Electrophoresis refers to the transport of charged particles under the influence of
an electric current, these charged particles colloid in soil-liquid mixture, and it is an
important mechanism in remediation of sludge. The drawback of this mechanism is
that in compact systems like clay soil, movement of contaminants is restrained.
However electrophoresis is important in remediating colloids that have contaminants
adsorbed to them (Pamukcu and Wittle 1992; Reddy and Parupudi 1997).
Other mechanisms that are involved in electrokinetics are diffusion which plays a
significant role in cationic and anionic contaminant transport, advection which
moves soil moisture or groundwater due to hydraulic forces, and finally convection
which is responsible for the movement of soil moisture or groundwater due to
buoyancy forces. Trivalent chromium migrates toward the cathode due to
electromigration, and CrO 4
2À and Cl
- migrate toward the anode due to
electromigration, and the negative-charged colloids move due to electrophoresis.
The electrolysis reaction at the electrodes generates hydrogen ions (H
+
) and
oxygen gas at the anode and hydroxyl ions (OH
À
) and hydrogen gas at the cathode.
The oxygen gas produced at the anode and the hydrogen gas produced at the cathode
escape out of the soil. The hydrogen ions in the anode attempt to migrate through the
soil toward the cathode, whereas the hydroxyl ions in the cathode attempt to migrate
through the soil toward the anode. The degree at which the H
+ and OH
- ions migrate
depends on the buffering capacity of the soil. An acid front is produced at the anode,
and at the cathode a base front is produced, and these two fronts move toward
opposite directions. Page and Page (2002) have found that the acid front moves
faster than the base front due to the fact that the mobility of H
+ exceeds that of OH
À ,
and electroosmotic flow is generally toward the cathode.
During electrokinetics the pH of the soil becomes acidic with the reading at the
anode dropping to around 2 and the pH at the cathode increases to above 10. The rate
of acid and base production depends on the current density (Castillo et al. 2012).
Precipitated hydroxides occur at the point where the pH change occurs as the
solubility of metal ions is at a minimum. In order to enhance the electrodes, reduce
the pH at the cathode, and increase the pH at the anode, an alkaline solution needs to
be added at the anodic compartment, and an acid solution needs to be added to the
cathode compartment.
According to Acar et al. (1995), the application of electric current has the
following effects:
38
E. M. Nkhalambayausi-Chirwa et al.
2002). Electroosmosis has been found to be effective in removing cation at low
concentrations. The rate of electroosmotic flow in the soil is described by Darcy’s
law for hydraulic flow (Page and Page 2002):
q A ¼ Àk e EA
ð2:5Þ
where k e is the coefficient of electroosmotic permeability (or conductivity), E is the
electric field strength or negative potential gradient, and A is the total cross-sectional
area normal to the flow direction.
Electrophoresis refers to the transport of charged particles under the influence of
an electric current, these charged particles colloid in soil-liquid mixture, and it is an
important mechanism in remediation of sludge. The drawback of this mechanism is
that in compact systems like clay soil, movement of contaminants is restrained.
However electrophoresis is important in remediating colloids that have contaminants
adsorbed to them (Pamukcu and Wittle 1992; Reddy and Parupudi 1997).
Other mechanisms that are involved in electrokinetics are diffusion which plays a
significant role in cationic and anionic contaminant transport, advection which
moves soil moisture or groundwater due to hydraulic forces, and finally convection
which is responsible for the movement of soil moisture or groundwater due to
buoyancy forces. Trivalent chromium migrates toward the cathode due to
electromigration, and CrO 4
2À and Cl
- migrate toward the anode due to
electromigration, and the negative-charged colloids move due to electrophoresis.
The electrolysis reaction at the electrodes generates hydrogen ions (H
+
) and
oxygen gas at the anode and hydroxyl ions (OH
À
) and hydrogen gas at the cathode.
The oxygen gas produced at the anode and the hydrogen gas produced at the cathode
escape out of the soil. The hydrogen ions in the anode attempt to migrate through the
soil toward the cathode, whereas the hydroxyl ions in the cathode attempt to migrate
through the soil toward the anode. The degree at which the H
+ and OH
- ions migrate
depends on the buffering capacity of the soil. An acid front is produced at the anode,
and at the cathode a base front is produced, and these two fronts move toward
opposite directions. Page and Page (2002) have found that the acid front moves
faster than the base front due to the fact that the mobility of H
+ exceeds that of OH
À ,
and electroosmotic flow is generally toward the cathode.
During electrokinetics the pH of the soil becomes acidic with the reading at the
anode dropping to around 2 and the pH at the cathode increases to above 10. The rate
of acid and base production depends on the current density (Castillo et al. 2012).
Precipitated hydroxides occur at the point where the pH change occurs as the
solubility of metal ions is at a minimum. In order to enhance the electrodes, reduce
the pH at the cathode, and increase the pH at the anode, an alkaline solution needs to
be added at the anodic compartment, and an acid solution needs to be added to the
cathode compartment.
According to Acar et al. (1995), the application of electric current has the
following effects:
38
E. M. Nkhalambayausi-Chirwa et al.