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
helps to remove metals, radionuclides and organics in low permeability soils.
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 Cooperation Work Group (ITRC), December, 1997. See http://www.itrcweb.org/Documents/MIS-4.pdf.
See
http://www.osti.gov/bridge/product.biblio.jsp?query_id=2&page=0&osti_id=576168
and http://costperformance.org/profile.cfm?ID=246&CaseID=246
for an SNL demonstration in unlined chromic acid pit.
See http://207.86.51.66/download/rtdf/lasagna/las-fin.pdf for a final report on the use of LasagnaTM at Paducah Gaseous Diffusion Plant in Paducah, Kentucky.