Solid-Phase
Separation
Description
Solid-phase
separation techniques concentrate contaminants, such as
metals and
radionuclides, through physical and chemical processes. There are
several types
of techniques for separating solids from soil. These processes are
described
below.
0.
Gravity
Separation. Gravity
separation capitalizes on the density
difference between metals and soil. When the metals-contaminated soil
is
suspended in water, denser materials such as metals sink and are
removed.
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Sieving/Physical
Separation. Sieving and
physical
separation are elementary separation processes. They use sieves and
screens to
separate different size particles. Most organic and inorganic contaminants tend
to bind to the
fine fraction of a soil (i.e., clay and silt). Thus, separating the
fine clay
and silt particles from the coarser sand and gravel soil particles
concentrates
the contaminants into a smaller volume of soil that can then be further
treated
or disposed.
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Dry Soil
Separation. Dry soil
separation separates radioactive particles
from clean soil particles. In this process, radionuclide-contaminated
soil is
excavated and screened to remove large rocks. Large rocks are crushed
and
placed with soil on a conveyor belt, which carries the soil under
radiation
detectors that measure and record the level of radiation in the
material.
Radioactive particles are tracked and mechanically diverted through
automated
gates, which separate the soil into contaminated and clean segments.
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Magnetic
Separation. In magnetic
separation, a magnetized medium such as magnetized
steel wool is used
to separate slightly magnetic radioactive particles from
soil. All uranium
and plutonium compounds are slightly magnetic while most soil is
nonmagnetic.
Soil is mixed with water to form a slurry, and the slurry is
passed through the
magnetic medium.
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Chemical
Leaching. Chemical leaching processes use weak
acids such as
vinegar to dissolve and wash the metals from the soil. The metals
recovered by
the process can possibly be recycled.
Limitations
and Concerns
Fugitive
dust from dry soil screening and separation is a major concern.
Gravity
separation processes rely on a difference in particle density. The
specific
gravity of particles will affect the settling rate and process
efficiency.
Additionally, settling velocity is dependent on the viscosity of the suspending
fluid, which must
be known to estimate process efficiency and to size equipment.
Special
measures may be required to mitigate odor problems resulting from
organic sludge that undergoes
septic conditions.
If
it is intended for soil to be returned to its original location, a
rigorous
testing system should be established to ensure that the soil does not
contain
low-levels of contamination.
Magnetic
separation may leave a small suspension of radioactive materials in a slurry, which may be more
difficult to
remediate than the original soil contamination.
Applicability
Separation
techniques are used to treat soil. The target contaminant groups for
separation
processes are semi-volatile organic compounds (SVOCs),
fuels, and inorganics (including radionuclides). Dry soil
separation
effectively treats soils contaminated with gamma-emitting
radionuclides,
including Th, U, Cs-137, Co-60, Pu-239, Am-241 and Ra-222. Separation
is also
useful when heavy metal contaminants
occur as particulates
(e.g.,
in small-arms ranges).
Technology
Development Status
Physical
separation techniques are generally proven. Dry soil separation can
substantially reduce the volume of radioactive waste (by more than
90%), and it
has been used on a commercial scale both at Johnston Atoll in the South
Pacific
and at the Energy DepartmentÕs Savannah River site in South
Carolina. Magnetic
separation is a newer, experimental separation process.
Web
Links
http://www.frtr.gov/matrix2/section4/4-18.html
Other
Resources and Demonstrations
See
Precipitation and Soil Washing.
See
http://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminants-on-Ranges/ER-199513/ER-199513 and https://portal.navfac.navy.mil/portal/page/portal/navfac/navfac_ww_pp/navfac_nfesc_pp/environmental/erb/resourceerb/rpt-sar-imple.pdf
for the use of different separation technologies at small arms ranges.