Phytoremediation is best applied at sites with shallow
contamination by organic, nutrient, or metal pollutants.
Phytoremediation is well-suited for use at very large
field sites where other methods of remediation are not
cost-effective or practicable; at sites with low
concentrations of contaminants where only "polishing
treatment" is required over long periods of time; and in
conjunction with other technologies where vegetation is
used as a final cap and closure of the site. There are
limitations to the technology that need to be considered
carefully before it is selected for site remediation. These
include: limited regulatory acceptance, long duration of
time sometimes required for clean-up to below action
levels, potential contamination of the vegetation and
food chain, and difficulty establishing and maintaining
vegetation at some toxic waste sites. This detailed
report discusses the current status of phytoremediation
to treat soils and ground water. Several field
demonstration summaries are presented, with such
information as: participants, compounds treated, site
characteristics, results, and contacts.
An emerging technology for cleaning contaminated
soils and shallow ground water is phytoremediation, an
environmentally friendly, low-cost, and low-tech
process. Phytoremediation encompasses all
plant-influenced biological, chemical, and physical
processes that aid in the uptake, degradation, and
metabolism of contaminants by either plants or
free-living organisms in the plant's rhizosphere. Aphytoremediation system can be viewed as a biological,
solar-driven, pump-and-treat system with an extensive,
self-extending uptake network (the root system) that
enhances the soil and below-ground ecosystem for
subsequent productive use.
Mechanisms of Phytoremediation
Plants and bacteria can form
specific associations in which the plant provides the
bacteria with a specific carbon source that induces the
bacteria to reduce the toxicity of the contaminated soil.
Alternatively, plants and bacteria can form nonspecific
associations in which normal plant processes stimulate
the microbial community, which in the course of
normal metabolic activity degrades contaminants in
soil. Plants can provide carbon substrates and nutrients,
as well as increase contaminant solubility. These
biochemical mechanisms increase the degradative
activity of bacteria associated with plant roots. In
return, bacteria can augment the degradative capacity
of plants or reduce the toxicity of the contaminated soil.
During phytoremediation, PAHs that are resistant to
degradation may adsorb to the surfaces of plant roots,
making the roots an important sink for specific PAHs.
Tall fescue and alfalfa were grown in a greenhouse
under controlled conditions, and roots were harvested
at three growth stages: vegetative, flowering, and
mature. Naphthalene adsorption to the various plant
roots was then evaluated. Results show that the mass of
naphthalene volatilized was the largest component of
the mass balance (32-45%). The mass in solution was
usually greater than that adsorbed to the roots. The
affinity of naphthalene for alfalfa roots was greater thanthat for tall fescue roots, but fescue roots were present
in much greater quantities in the soil compared with
alfalfa. Naphthalene adsorption on the roots of both
plant species increased with plant age.
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