What are
the consequences of metal-contaminated soils?
Soil contamination can have
dire consequences, such as loss of ecosystem and agricultural
productivity, diminished food chain quality, tainted water
resources, economic loss, and human and animal illness.
In extensive areas of eastern and central Europe, people
suffer from illnesses associated with elevated levels of
lead in the air, cobalt, arsenic, mercury, and cadmium in
the soil, and a food chain contaminated by metals related
to heavy industry. The Savannah River Site (SRS) is one
site in the U.S. that contains many polluted environments
that must be remediated to levels that pose negligible human
and ecological health risk.
In
situ immobilization techniques
In
recent years, attention has focused on the development of
in situ (in place) immobilization methods that are
generally less expensive and disruptive to the natural landscape,
hydrology, and ecosystems than are conventional excavation,
treatment, and disposal methods. In situ immobilization
of metals using inexpensive amendments such as minerals
(apatite, zeolite, or clay minerals) or waste by-products
(steel shot, beringite, iron-rich biosolids) is a promising
alternative to current remediation methods. This technique
relies on a fundamental understanding of the natural geochemical
processes governing the speciation, migration, and bioavailability
of metals in the environment. In polluted soils, metals
can be dissolved in solution, held on inorganic soil particles,
complexed with organic soil components, or precipitated
as pure or mixed solids. Soluble contaminants are subject
to migration with soil water, uptake by plants or aquatic
organisms, or loss due to volatilization into the atmosphere.
Metals in soil may be associated with various phases that
are reactive, semi-reactive or non-reactive. The risk to
the environment from contaminated soil cannot be assessed
by simply considering the total amount of potentially toxic
metals within the soil because these metals are not necessarily
completely mobile or biovailable.
The main goal of in situ remediation techniques is
to reduce the fraction of toxic elements that is potentially
mobile or bioavailable. Environmental mobility is the capacity
for toxic elements to move from contaminated materials to
any compartment of the soil or groundwater. Bioavailability
refers to the fraction of a contaminant that can be taken
into any biological entity, be it plant, earthworm, or human.
Depending on the chemical form in which a contaminant occurs,
it may range from being totally bioavailable to virtually
unavailable.
Objectives
of SREL research on in situ immobilizationof metals:
At SREL, the objectives of
research on in situ immobilization of metals include:
n
Evaluating the use of inexpensive, abundant materials
as stabilizing agents in metal-contaminated soils;
n
Determining the long-term efficacy of stabilizing agents;
n
Determining the influence of stabilizing agents on the
mobility, bioavailability, and toxicity of metals in soil;
n
Developing soil quality indices as tools in evaluating
the efficacy of remediation techniques and for monitoring
purposes.
Some
results of SREL research:
Soil amendments such as apatite,
zeolite, clay minerals, iron oxides, and alkaline biosolids
(waste by-products) were found to be suitable for remediating
metal-contaminated soil.
The amendments significantly
reduced the mobility of metals in soil, metal uptake by
plants, and metal phytotoxicity. However, the effectiveness
of these amendments varied. For example, iron oxide was
most effective for soils contaminated with arsenic, whereas
apatite was best at reducing the mobility of lead, cadmium,
and zinc. The alkaline biosolid played an important role
in stabilization of copper and nickel. Zeolite stabilizes
cadmium, zinc, lead, copper, and nickel in soil, especially
when metal levels re not high, but its efficacy might be
questionable.
Soil amendments used with
in situ remediation techniques decreased the mobility
of metals by increasing retention of metals in the non-mobile
solid phase. The influence of the stabilizing agents on
the mobility, bioavailability, and toxicity of metals can
be evaluated using newly developed availability indices
such as the modified distribution coefficient (Kmd), bioavailablity
factor (BF), recalcitrant factor (RF), and transfer factor
(TF), all of which give researchers information on the amount
of a metal contaminant that remains in the soil vs. the
amount that moves into solution or the food chain. For example,
Kmd is relatively low in heavily metal-contaminated soil,
meaning that metals are fairly mobile. SREL research has
shown that addition of certain amendments increased the
Kmd value, reducing the mobile fraction of metals in the
soil. In contrast, the bioavailability factor (BF) differs
for each element, with its value being dependent upon the
total concentration of the metal, the source of the metal,
and the properties of the soil. While a very mobile element
like cadmium can have a BF value of only 2% in uncontaminated
soils, the BF for cadmium can increase to as much as 50%
in polluted soils. The RF factor typically is lower in soils
with high concentrations of contaminants and low soil pH.
TF values, which indicate metal bioavailability, are usually
high in contaminated soils but decrease upon treatment.
