Geoscience Reference
In-Depth Information
If L is the distance of electrodes, then the time requirement ( t ) of the process is expressed as:
t
=
L/V
=
L/ (( nτu i
+
k eo ) E )
(5.18)
From a practical perspective, Equation (5.18) is not proper because the interaction between
soil particles and contaminants is not considered. The transport of contaminants in soil media is
delayed due to interactions such as sorption and precipitation. Therefore, the retardation (delaying)
factor ( R d ) should be taken into account:
t
=
( R d L ) / (( nτu i
+
k eo ) E )
(5.19)
The retardation factor is influenced by characteristics of the soil and contaminant and should
be evaluated prior to implementation of the process. As shown in Equation (5.19) , there are three
parameters ( t , L , and E ) to be determined when the electrokinetic process is designed, and one
can be optimized by changes in the other two.
In addition to the distance between electrodes, electrode material is important. Chemically
inert and electrically conducting materials such as graphite, coated titanium stainless steel, or
platinum have been usually used as anodes to prevent dissolution of the electrode and generation
of undesirable corrosion products in an acidic environment. Any conductive materials that do
not corrode in a basic environment can be used as cathodes. When one selects the material for
electrodes, the following aspects must be considered (Alshawabkeh et al ., 1999):
Chemically inert and electrically conducting material.
Availability of the material.
Easy fabrication to the form required for the process.
Easy installation in field.
Costs for material, production or fabrication, and installation.
Another consideration for electrode design is the spacing between electrodes with the same
polarity. The spacing of electrodes is subjected to the configuration of electrodes. Overall, there
are two kinds of electrode arrangements applied: one-dimensional (1-D) and two-dimensional
(2-D) arrays. In order to design an effective and efficient configuration of electrodes, several
aspects must be considered:
Electrically effective and ineffective spots or areas.
Number and cost of electrodes required per unit area.
Processing time required.
Figure 5.3 shows representative 1-D and 2-D electrode configurations. The l-D electrode con-
figuration is the easiest method to install, and the electric field is produced linearly. Depending
on the spacing of electrodes, spots or areas of inactive (ineffective) electric field can be varied
in this configuration. However, there are drawbacks to this configuration. Because the numbers
of anode and cathode are identical, it is difficult to collect wastewater that contains contaminants
from the effluent systems of electrode compartments. Additionally, more electrodes are required,
compared with other configurations. To overcome the drawbacks of the 1-D configuration, the
2-D arrangement can be considered. Triangular, square, or hexagonal electrode configurations
can be used for 2-D implementation. Depending on the type of contaminants targeted, the place of
anode or cathode is determined. When cationic contaminants, such as heavy metals, are removed
by the electrokinetic process, the cathode is placed at the center, and the anodes are placed on
the perimeter to maximize the spread of the acidic environment generated by the anodes and to
minimize the extent of the basic environment generated by the cathode. The 2-D configurations
of electrodes generate nonlinear electric fields, that is, the electric potential gradient and electric
current density gradually increases towards the center of the configuration, and the transport of
contaminants is enhanced as the processing time increases. Compared with the 1-D array, the col-
lection of wastewater is easy due to one effluent system being placed at the center. The numbers
 
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