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
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:
Evaluating the use of inexpensive, abundant materials as stabilizing
agents in metal-contaminated soils;
Determining the long-term efficacy of stabilizing agents;
Determining the influence of stabilizing agents on the mobility,
bioavailability, and toxicity of metals in soil;
Developing soil quality indices as tools in evaluating the efficacy
of remediation techniques and for monitoring purposes.
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.