Fundamentals of electrokinetics - In-Situ Remediation of Arsenic-Contaminated Sites - page 96

Even though the concentrations of contaminants are identical, the overall performance of the
electrokinetic process appears to be totally different depending on their fractionation in soils.
If contaminants exist as loosely bound fractions such as (1)-(4), for example, they tend to be
relatively easily removed by the process. On the contrary, contaminants associated with organics
or in crystal lattices such as (5)-(7) cannot be effectively removed or separated from soils (Kim
et al ., 2009a).

5.3.1.3 Voltage and current level
The electric field strength affects electromigration and electroosmosis, as shown in Equations
(5.3) and (5.9) . The common range of the electric field strength frequently used is 1-100 V m − 1
(Alshawabkeh et al ., 1999; Page and Page, 2002; Virkutyte et al ., 2002). The level of electric
current used in most implementation is in the order of a few tens of milliamperes per square
centimeter. The electric current intensities control the rate of electrolysis of water through the
reaction of Equations (5.14) and (5.15) (Hamed et al ., 1991; Kim and Kim, 2002). The rate of
H + generation ( R H ) can be related to the rate of water electrolysis ( R w ) in the anode based on
Equation (5.11) :

R H [mole s − 1 ] I / F (5.14)

where I (A) is the current intensity and F is the Faraday constant [96,485 (A s) mole − 1 ]. Similarly,
the rate of OH − generation ( R OH ) can be calculated by the rate of water electrolysis ( R w )inthe
cathode based on Equation (5.15) :

= 2 × R w = 2 × ( I / 2F) =

R OH [mole s − 1 ] = R w = I / F (5.15)

Although a high current intensity can generate more acid and increase the rate of transport to
facilitate the contaminant removal process, it increases power consumption tremendously as power
consumption is proportional to the square of electric current. An electric current density in the
range of 1-10 A m − 2 has been demonstrated to be the most efficient for the process (Alshawabkeh
et al ., 1999; Page and Page, 2002; Virkutyte et al ., 2002). An optimum electric field strength and
electric current density should be evaluated based on soil properties, electrode distance, and time
requirements of the process. Details will be discussed in later sections.

5.3.2 Practical consideration for optimization of operation and design
of electrokinetic remediation
5.3.2.1 Electrode
When one designs the electrodes for the electrokinetic process, the distance between anode and
cathode is a crucial parameter because it controls the electric field strength applied and the duration
of the process. The longer the distance considered, the fewer the number of electrodes required.
Accordingly, the cost for production and installation of electrodes can be saved. On the other
hand, the increase in time required by the process cannot be avoided, and the cost of operation
must be increased. Therefore, duration of the process and cost should be considered when the
distance between electrodes is determined. The time required by the process is affected by the
velocity of contaminant transport as well as the distance between electrodes, and one must know
the relation between time requirements and electrode distance when designing the appropriate
distance of electrodes. The velocity, V [m s − 1 ], of contaminant transport under the applied electric
field, E [V m − 1 ], can be calculated by electromigrative velocity, V em [m s − 1 ], in Equation (5.3)
and electroosmotic velocity, V eo [m s − 1 ], in Equation (5.4) :

V = V em + V eo (5.16)

Substituting Equations (5.2 -5.4) , and (5.10) into Equation (5.16) :

V = ( nτu i + k eo ) E (5.17)