1. It produces an acid in the anode compartment that is transported across the soil
and desorbs contaminants from the surface of soil particles.
2. It initiates electromigration of species available in the pore fluid and those
introduced at the electrodes.
3. It establishes an electric potential difference which may lead to electroosmosisgenerated flushing of different species.
In order to remove the contaminants from the soil by electrokinetics, the contaminants should exist in pore water in dissolved ionic form so that they are transported
to either the cathode or anode. In a study by Reddy and Chinthamreddy (2003), they
found that in order to improve the performance of electrokinetics, the following can
be done: changing the operating conditions such as switching the electrodes,
prolonging the processing time, and increasing electric gradient or by controlling
the reservoir fluid pH.
Trivalent chromium behaves differently under electrokinetics in different soil
types. In a study conducted by Reddy and Chinthamreddy (1999), they found that
during the electrokinetics of both Cr(III) and Cr(VI), the pH near the anode
decreased to a value between 2 and 3, and at the cathode the pH increased between
11 and 12; due to the high pH at the cathode, a precipitate of chromium hydroxide
was formed that clogged the pore space of the cathodic base front. The limitation of
this process is the near-anode focusing effect which results in the formation of a
precipitate layer block around the anode resulting in the reduction of efficiency with
time (Shen et al. 2007; Li et al. 2011).
The two most common occurring valence states of chromium trivalent exist in the
form of cationic hydroxides such as Cr(OH) 3 which will migrate toward the cathode
during electrokinetic remediation (Fig. 2.5). However, chromium(VI) exists as
CrO 4
2À at high pH and as HCrO 4
À at low pH and other forms of oxyanions such
as CrO 4
2À which migrate toward the anode; however it is adsorbed by the soil in the
low pH regions, and it stops the complete removal of Cr (VI) from the soil.
Electrokinetics is highly dependent on the acidic condition which favors the
re-solubilization of heavy metal precipitated contaminants into the solution phase
which makes it easier to transport; this can be done by acidification.
The problem encountered during electrokinetic mobilization and recovery of
metallic species in soil is mainly due to accumulation of precipitates and increase
Cr
3+
CrO 4
–
Pb
++
H
+
OH
–
Cathode
Anode
Fig. 2.5 Movement of ionic
species under an
electrokinetic gradient
2 Advances in Bioremediation of Toxic Heavy Metals and Radionuclides in. . .
39
and desorbs contaminants from the surface of soil particles.
2. It initiates electromigration of species available in the pore fluid and those
introduced at the electrodes.
3. It establishes an electric potential difference which may lead to electroosmosisgenerated flushing of different species.
In order to remove the contaminants from the soil by electrokinetics, the contaminants should exist in pore water in dissolved ionic form so that they are transported
to either the cathode or anode. In a study by Reddy and Chinthamreddy (2003), they
found that in order to improve the performance of electrokinetics, the following can
be done: changing the operating conditions such as switching the electrodes,
prolonging the processing time, and increasing electric gradient or by controlling
the reservoir fluid pH.
Trivalent chromium behaves differently under electrokinetics in different soil
types. In a study conducted by Reddy and Chinthamreddy (1999), they found that
during the electrokinetics of both Cr(III) and Cr(VI), the pH near the anode
decreased to a value between 2 and 3, and at the cathode the pH increased between
11 and 12; due to the high pH at the cathode, a precipitate of chromium hydroxide
was formed that clogged the pore space of the cathodic base front. The limitation of
this process is the near-anode focusing effect which results in the formation of a
precipitate layer block around the anode resulting in the reduction of efficiency with
time (Shen et al. 2007; Li et al. 2011).
The two most common occurring valence states of chromium trivalent exist in the
form of cationic hydroxides such as Cr(OH) 3 which will migrate toward the cathode
during electrokinetic remediation (Fig. 2.5). However, chromium(VI) exists as
CrO 4
2À at high pH and as HCrO 4
À at low pH and other forms of oxyanions such
as CrO 4
2À which migrate toward the anode; however it is adsorbed by the soil in the
low pH regions, and it stops the complete removal of Cr (VI) from the soil.
Electrokinetics is highly dependent on the acidic condition which favors the
re-solubilization of heavy metal precipitated contaminants into the solution phase
which makes it easier to transport; this can be done by acidification.
The problem encountered during electrokinetic mobilization and recovery of
metallic species in soil is mainly due to accumulation of precipitates and increase
Cr
3+
CrO 4
–
Pb
++
H
+
OH
–
Cathode
Anode
Fig. 2.5 Movement of ionic
species under an
electrokinetic gradient
2 Advances in Bioremediation of Toxic Heavy Metals and Radionuclides in. . .
39