United States
Environmental Protection
Agency
Office of Solid Waste
and Emergency
Response (5102G)
EPA 542-F-98-002
October 1996
www.ciu-m.com
v>EPA Recent Developments for In Situ Treatment
of Metal Contaminated Soils
Fact Sheet and Order Information
This report (EPA 542-R-97-004) provides status information on four in situ technologies for treating
soil contaminated with metals:
Electrokinetic remediation Phytoremediation Soil flushing Solidification/stabilization
The report discusses different techniques currently in practice or under development; identifies
vendors and summarizes performance data; and discusses technology attributes that should be
considered during early screening of potential remedies.
SUMMARY
Metals are prevalent at most Superfund sites. Almost
three-quarters of these sites with signed Records of
Decision (ROD) are contaminated with metals as the sole
contaminant group (approximately 16%) or in combination
with other contaminant groups such as volatile or semi-
volatile organic compounds. In situ remedies should be
capable of providing cost savings, when compared with
traditional disposal methods, and are being selected more
frequently at Superfund sites. However, relatively few
alternatives exist for the in situ treatment of metals.
This report provides hazardous waste cleanup
professionals with a status update on available and
promising technologies for in situ remediation of soil
contaminated with heavy metals. The report is intended
to assist in screening new technologies early in the remedy
evaluation and selection process.
Electrokinetics
Electrokinetic remediation relies on the application of low
density direct current between electrodes placed in the
soil. Contaminants are mobilized in the form of charged
species, particles, and ions. Several organizations are
source of
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,-- 'plume' of
contaminants.
direction of
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Contaminated Plume Stopped by Electrokinetic Fence
developing technologies for the enhanced removal of
metals by transporting contaminants to the electrodes
where they are removed and subsequently treated above
ground.
Experience with this technology is limited to bench and
pilot scales, with the notable exception of a metal removal
process that has been commercially operated by a single
vendor in Europe and recently licensed in the United
States. Limited performance data from this vendor
illustrate the potential for achieving removals greater than
90 percent for some contaminants.
A broad range of metals is potentially treated by this
technology. The commercial applications in Europe
treated copper, lead, zinc, arsenic, cadmium, chromium,
and nickel. There is also potential applicability for
radionuclides and some types of organic compounds.
Electrokinetics also can be used to slow or prevent
migration of contaminants by configuring cathodes and
anodes in a manner that causes contaminants to flow
toward the center of a contaminated area of soil. The
practice has been named "electrokinetic fencing."
Phytoremediation
This technology is in the early stage of commercialization,
and in the future may provide a low cost option under
specific circumstances. At the current stage of
development for treatment of metals, this process is best
suited for sites with widely dispersed contamination at low
concentrations where only treatment of soils at the surface
(that is, within depth of the root zone) is required.
Two basic approaches for metals remediation include
phytoextraction and phytostabilization. Phytoextraction
relies on the uptake of contaminants from the soil and
their translocation into aboveground plant tissue, which is
harvested and treated. Although hyperaccumulating trees,
shrubs, herbs, grasses, and crops have potential, crops seem
to be most promising because of their greater biomass
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production. Nickel and zinc appear to be the most easily
absorbed, although preliminary tests with copper and
cadmium are encouraging. Significant uptake of lead,
a commonly occurring contaminant, has not been
demonstrated in any of the plants tested thus far.
However, one researcher is experimenting with soil
amendments that make lead more available for uptake
by plants,
Soil Flushing
Soil flushing involves extraction of contaminants from
soil using water or other suitable aqueous solutions.
Although additives such as acids and chelating agents
have had limited commercial use for full-scale ex situ
soil washing projects, they have not been demonstrated
as feasible for in situ applications.
Soil flushing has been selected at seven Superfund sites
with metals present; however, at six of those sites,
organic contaminants are the primary targets. Soil
flushing is most effective in removing hexavalent chrome,
Spray Application
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Pump
Graundwaler
Treitment
Pump
~
Groundwater
Treatment
Contaminated Area
Lcachate
Collection
1
Low Permeability
Zone
due to its solubility in water. Two soil flushing remedies
are currently ongoing at Superfund sites, with some
preliminary data available from a hexavalent chrome
application.
Solidification/stabilization
This process (also referred to as immobilization) changes
physical characteristics of the waste in order to immobilize
contaminants. Metals are commonly remediated by ex situ
solidification with pozzolans and possibly other additives.
This technology also can be adapted to an in situ technique
through the use of various proprietary- augers that provide
mechanisms for reagent delivery and mixing. In situ
treatment likely will have a cost advantage over
ex situ applications for larger volumes and for depths
greater than 10 feet. However, in situ immobilization is
rarely selected for Superfund use, largely because of
concerns with thoroughness of mixing and long-term
Typical Soil Flushing System (Surface Sprinklers)
A second in situ solidification technique involves
vitrification where an electrical current is passed between
electrodes to melt soil and incorporate metals into a
vitrified product. This technology is commercially available
and has been successfully used at two Superfund sites.
Both processes are broadly applicable to a range of
metals. Vitrification uses a hood to capture mercury and
other volatile metals, such as lead and arsenic, which may
be partially vaporized during operations. Vitrification is
best suited for wastes that are difficult to treat, such as
mixtures of organics and metals.
ORDER INFORMATION
This report (EPA 542-R-97-004) is available free from the
EPA National Center for Environmental Publication and
Information (NCEPI) at 800-490-9198 or 513-489-8190, or
fax 513-489-8695. It is also viewable and downloadable
from the Internet at http://www.clu-in.com
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