Soil
Washing
Description
Soil
washing uses water to remove contaminants from soils.
The process works by
either dissolving or suspending contaminants in the wash solution. It
is often
used in conjunction with other physical separation techniques. (See the
description of Separation).
Soil
washing separates soil by particle size. Most organic and inorganic contaminants tend
to bind and sorb to clay, silt, and
organic soil
particles. Most silt and clay are stuck to larger particles (i.e., sand
and
gravel). Washing separates the small particles from the large particles
by
breaking adhesive bonds. The separated material is smaller in volume
and is more
easily disposed.
Limitations
and Concerns
Soil
washing is a technique of concentrating contaminants through
separation. It
does not destroy or immobilize the contaminants. Consequently, the
resulting
concentrated soil must be disposed of carefully.
The
ÒcleanÓ portion of separated soil must be analyzed for
residual contamination
before it is disposed of as clean material. Sites using soil
washing
often have an on-site capability to
test samples of treated soil before it is released as clean. For example,
see the
descriptions of X-Ray
Fluorescence and Laser-Induced
Fluorescence
characterization technologies.
Soil
contaminated with both metals and organic compounds make formulating a
single
suitable washing solution difficult. In this case, sequential washing,
using
different wash formulations may be required.
Some
washing formulation may have a tendency to change the mobility of
certain
metals, and thus fully characterizing and understanding the site is of
utmost
importance.
High
organic content of the soil may require pretreatment.
Wash
water requires treatment before it can be discharged, as it is usually
not
completely free of smaller particles or organic particles.
Soil
washing processes for soil contaminated with volatile organic compounds (VOCs) may require emission
controls.
Measuring
contaminant concentration in air particulates during excavation and
treatment
is an important step to ensure that a wider population is not being
unduly
exposed.
Applicability
Soil
washing systems are used on soils contaminated with semi-volatile
organic
compounds (SVOCs), fuels, and heavy
metals,
including radionuclides. The technology can be
used on
selected VOCs and pesticides.
Technology
Development Status
Soil
washing is used extensively in Europe. Commercialization in the United
States
is not as extensive.
Web
Links
http://www.frtr.gov/matrix2/section4/4-19.html
http://www.clu-in.org/download/citizens/soilwashing.pdf
http://www.clu-in.org/techfocus/default.focus/sec/Soil_Washing/cat/Overview
Other
resources
See the technology description of Separation.