Electrokinetics
Description
Electrokinetic
remediation is a process in which a low-voltage direct-current electric
field
is applied across a section of contaminated soil to move contaminants.
The principle of electrokinetics remediation is similar to a battery.
After
electrodes (a cathode and anode) are introduced and charged, particles
(e.g., ions)
are mobilized by the
electric current. Ions and water move toward the electrodes.
Each
electrode assembly contains water, a pump, and an electrode. The outer
casing
of the electrode is made of porous
ceramic that allows electrical current and water to pass. The porous
casing
contains water, which is held under pressure so it does not flow out
and
saturate adjacent soils. The ions flow through the casing, where they
are then
removed for treatment.
Another
similar process for transporting contaminants through the soil using
electrical
current is called electroosmosis. In contrast to electrokinetics,
electroosmosis is the movement of a liquid containing ions. As the
electric
current is applied to the soil, water in the soil pores flows between
the
electrodes. Sometimes these two processes are used together to enhance
removal.
This
basic electrokinetic technology is sometimes combined with in-situ
treatment technology, such as the Lasagna Technologyª. In this
process, the
contaminants are moved by electroosmosis through treatment zones. As
the
electric charge is varied, the contaminants reverse direction. Chemical
oxidation could occur, with chemical additives delivered by fracturing
in
conjunction with oxidant solution injection.
Limitations
and Concerns
The
effectiveness is sharply reduced for wastes with a moisture content of
less
than 10 percent.
Recent
tests at the Livermore National Laboratory have indicated problems when
electrokinetics is applied to dense non-aqueous phase
liquid (DNAPL),
because the system gets clogged.
In
unsaturated
soils, the addition of water could potentially wash contaminants out of
the
area of influence.
The
presence of buried metallic or insulating material can induce
variability in
the electrical conductivity of the soil.
Metallic
electrodes may dissolve as a result of electrolysis and introduce
corrosive
products into the soil. Electrodes made of inert materials such as
carbon,
graphite, or platinum should be used.
Electrokinetics
is most effective in clays because clay particles have a negative
surface
charge.
The
solubility and desorption
potential of the contaminants may limit the success of the technology.
Applicability
Electrokinetics
is primarily used to remove metals and radionuclides
in low permeability
soils. It may also be used for organic
compounds, including VOCs and pesticides, although as noted above,
there have
been some problems with this application.
Technology
Development Status
This
technology is being field demonstrated at several locations. Bioremediation
in LasagnaTM treatment
zones has
been demonstrated in the laboratory, but it requires further
development.
Electrokinetic remediation has been applied successfully to soils with
high
soil moisture. It has been used commercially in Europe, but there are
no
commercial applications in the U.S. Sandia National Laboratories (SNL)
is
trying to extend electrokinetic remediation technology to unsaturated
soils.
The SNL electrode design allows water to enter the soil at the anode,
replenishing the pore water adjacent to the electrode casing, but never
saturating the soil. The SNL design for low-moisture soils is in the
pilot
stage.
Web
Links
http://www.frtr.gov/matrix2/section4/4-4.html
http://www.sandia.gov/Subsurface/factshts/ert/ek.pdf
Other
Resources and Demonstrations
See
http://www.epa.gov/swertio1/download/remed/electro.pdf
for a description of current research projects.
Also
see the Remediation Technologies Development Forum (RTDF) at http://www.rtdf.org/public/lasagna/lastechp.htm
for a current list of relevant publications. Also see ÒEmerging
Technologies
for the Remediation of Metals in Soil: Electrokinetics,Ó
Interstate Technology
and Regulatory Council (ITRC), December, 1997 at http://www.itrcweb.org/Documents/MIS-4.pdf.
See
http://costperformance.org/profile.cfm?ID=246&CaseID=246
for an SNL demonstration in an unlined chromic acid pit.