ELECTROKINETIC PROCESSING OF SOIL
Electric fields as well as electron transfer processes have been used for the
treatment of soil and groundwater containing organic or inorganic pollutants.
The underlying electrokinetic mechanisms have been discussed in Chapter 2
(Section 2.10) and include electro-osmosis (the movement of a liquid in a pore
due to an electric field), electrophoresis (the movement of a charged particle in
an electric field), streaming potential (the production of an electric field due to
the movement of an electrolyte under a hydraulic potential), and sedimentation
potential (the production of an electric field due to the movement of charged
particles caused by gravity). 341 342 When suitable anodes and cathodes (e.g., made
of graphite or Ebonex) are strategically buried in the ground or placed in contact
with a slurry and an electric field from a DC source is applied (typically in
the range 40-200 V), one or more of these phenomena occur and are used for
the removal of the pollutant(s). The general technique has been called
electroreclamation, electro-osmotic purging, electroremediation, electrorestoration, or
electrokinetic processing in various contexts.
This applied electric field produces inside a charged soil pore filled with a
liquid, a drag interaction between the charged outer layer of the liquid, and its
bulk. The liquid then moves along the potential gradient to wells or reservoirs,
where it can be collected and removed. This phenomenon is called electroosmotic
transport. As can be expected, the velocity of the liquid due to this
transport is proportional to the applied electric field and to the zeta potential
of the pore surface that arises from physical and chemical interactions as well
as lattice imperfections; this potential has been found to be negative in wet silts
and clays. 341 343
Simultaneously, water or a purging solution is fed into the soil to aid in the
removal of the undesired species by flushing them out by virtue of the electroosmotic
effect. This occurs in soils with low hydraulic conductivities like kaolin,
sand-clay mixtures, and the like, and to prevent discontinuities in the soil
that would create sites with very high electrical resistivities, thus rendering
this technique ineffective. This purging stream can also be used to increase the
acidity or basicity of the soil, to increase or decrease the solubility of a given
species, to form complexes, and so forth. TM
In addition, chemical reactions occurring at the electrodes primarily produce
H2(g ) and OH- (aq) at the cathode and O2(g ) and H*(aq) at the anode as it
normally occurs during water electrolysis. These charged species (H § and OH-),
along with other ions encountered in the medium, are attracted to the electrodes
of opposite polarity and migrate, creating an acid front and a basic front.
The movement of these fronts is aided by concentration gradients that promote
diffusion and is known to be dominated by the transport of the proton,
which neutralizes the base front and impedes its advance toward the anode. 34
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