The Several Different Types of Phytoremediation
- Phyto-accumulation
(also called phyto-extraction). In this process, plant roots sorbs the
contaminants along with other nutrients and water. The contaminant mass is
not destroyed but ends up in the plant shoots and leaves. This method is
used primarily for wastes containing metals. At one demonstration site,
water-soluble metals are taken up by plant species selected for their
ability to take up large quantities of lead (Pb). The metals are stored in
the plant’s aerial shoots, which are harvested and either smelted for
potential metal recycling/recovery or are disposed of as a hazardous waste.
As a general rule, readily bioavailable metals for plant uptake include
cadmium, nickel, zinc, arsenic, selenium, and copper. Moderately
bioavailable metals are cobalt, manganese, and iron. Lead, chromium, and
uranium are not very bioavailable. Lead can be made much more bioavailable
by the addition of chelating agents to soils. Similarly, the availability of
uranium and radio-cesium 137 can be enhanced using citric acid and ammonium
nitrate, respectively.
- Phytostabilization
is the use of certain plant species to immobilize contaminants in the
soil and ground water through absorption and accumulation by roots,
adsorption onto roots, or precipitation within the root zone of plants (rhizosphere).
This process reduces the mobility of the contaminant and prevents migration
to the ground water or air, and it reduces bioavailability for entry into
the food chain. This technique can be used to reestablish a vegetative cover
at sites where natural vegetation is lacking due to high metals
concentrations in surface soils or physical disturbances to surficial
materials. Metal-tolerant species can be used to restore vegetation to the
sites, thereby decreasing the potential migration of contamination through
wind erosion and transport of exposed surface soils and leaching of soil
contamination to ground water.
- Rhizofiltration
(rhizo- means root) is the adsorption or precipitation onto plant roots or
absorption into the roots of contaminants that are in solution surrounding
the root zone. Rhizofiltration is similar to phytoextraction, but the plants
are used primarily to address contaminated ground water rather than soil.
The plants to be used for cleanup are raised in greenhouses with their roots
in water rather than in soil. To acclimate the plants once a large root
system has been developed, contaminated water is collected from a waste site
and brought to the plants where it is substituted for their water source.
The plants are then planted in the contaminated area where the roots take up
the water and the contaminants along with it. As the roots become saturated
with contaminants, they are harvested. For example, sunflowers were used
successfully to remove radioactive contaminants from pond water in a test at
Chernobyl, Ukraine.
-
Phytovolatilization is the uptake and transpiration of a
contaminant by a plant, with release of the contaminant or a modified form
of the contaminant to the atmosphere from the plant. Phytovolatilization
occurs as growing trees and other plants take up water and the organic
contaminants. Some of these contaminants can pass through the plants to the
leaves and evaporate, or volatilize,
into the atmosphere. Poplar trees at one particular study site have been
shown to volatilize 90% of the TCE they take up.
-
Phytodegradation, also
called phytotransformation, is the
breakdown of contaminants taken up by plants through metabolic processes
within the plant, or the breakdown of contaminants external to the plant
through the effect of compounds (such as enzymes) produced by the plants.
Pollutants (complex organic molecules) are degraded into simpler molecules
and are incorporated into the plant tissues to help the plant grow faster
(Figure 2). Plants contain enzymes, complex chemical substances (proteins) that cause
rapid chemical reactions to occur. Some enzymes breakdown and convert
ammunition wastes, others degrade chlorinated solvents such as
trichloroethylene (TCE), and others degrade herbicides.
6.Rhizodegradation, also called enhanced rhizosphere biodegradation, phytostimulation, or planted-assisted
bioremediation/degradation, is the breakdown of contaminants in the soil
through microbial activity that is enhanced by the presence of the root zone
(the rhizosphere) and is a much slower
process than phytodegradation. Microorganisms (yeast, fungi, or bacteria)
consume and digest organic substances for nutrition and energy. Certain
microorganisms can digest organic substances such as fuels or solvents that are
hazardous to humans and break them down into harmless products in a process
called biodegradation. Natural substances released by the plant roots—sugars,
alcohols, and acids—contain organic carbon that provides food for soil
microorganisms and the additional nutrients enhance their activity.
Biodegradation is also aided by the way plants loosen the soil and transport
water to the area.
7. Hydraulic
Control. In this process, trees indirectly remediate by
controlling groundwater movement. Trees act as natural pumps when their roots
reach down towards the water table and establish a dense root mass that takes up
large quantities of water. A poplar tree, for example, pulls out of the ground
30 gallons of water per day, and a cottonwood can absorb up to 350 gallons per
day.
(Taken from www.geocities.org/techtree/ttdescript/phytrem.htm;
and A Citizen’s Guide to Bioremediation, April 1996,EPA 542-F-96-007)
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