Phytoremediation
Description
Phytoremediation is a bioremediation
process that uses various types of plants to remove, transfer, stabilize,
and/or destroy contaminants
in the soil and groundwater.
There are several different types of phytoremediation
mechanisms. These are:
1. Rhizosphere biodegradation. In this process, the plant releases natural
substances through its roots, supplying nutrients to microorganisms in the
soil. The microorganisms enhance biological degradation.
2. Phyto-stabilization. In this process, chemical compounds produced by the
plant immobilize contaminants, rather than degrade them.
3. Phyto-accumulation (also called phyto-extraction). In this process, plant roots sorb 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 plantês 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.
4. Hydroponic Systems for Treating Water Streams
(Rhizofiltration). Rhizofiltration is
similar to phyto-accumulation, but the plants used for cleanup are
raised in greenhouses with their roots in water. This system can be used for ex-situ groundwater treatment. That is, groundwater is pumped
to the surface to irrigate these plants. Typically hydroponic systems utilize
an artificial soil medium, such as sand mixed with perlite or vermiculite. As
the roots become saturated with contaminants, they are harvested and disposed
of.
5. Phyto-volatilization. In this process, plants take up water containing organic
contaminants and release
the contaminants into the air through their leaves.
6. Phyto-degradation. In this process, plants actually metabolize and
destroy contaminants within plant tissues.
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.
The plants most used and studied are poplar
trees. The U.S. Air Force has used poplar trees to contain trichloroethylene (TCE)
in groundwater. In Iowa, EPA demonstrated
that poplar trees acted as natural pumps to keep toxic herbicides, pesticides,
and fertilizers out of the streams and groundwater. The US Army Corps of
Engineers has experimented with wetland plants to destroy explosive compounds
in the soil and groundwater. Submersed and floating-leafed species (coontail
and pondweed, and arrowhead, respectively) decreased trinitrotoluene (TNT)
to 5% of original concentration. Submersed plants were able to decrease Royal Demolition
Explosive (RDX) levels by 40%, and when microbial degradation was added,
RDX decreased by 80%. Sunflowers, using rhizofiltration, were used successfully
to remove radioactive
contaminants from pond water in a test at Chernobyl, Ukraine.
Limitations and Concerns
The toxicity and
bioavailability of biodegradation
products is not always known.
Degradation by-products may be mobilized in
groundwater or bio-accumulated in animals. Additional research is needed to
determine the fate of various compounds in the plant metabolic cycle to ensure
that plant droppings and products do not contribute toxic or harmful chemicals
into the food chain.
Scientists need to establish whether
contaminants that collect in the leaves and wood of trees are released when the
leaves fall in the autumn or when firewood or mulch from the trees is used.
Disposal of harvested plants can be a problem
if they contain high levels of heavy metals.
The depth of the contaminants limits
treatment. The treatment zone is determined by plant root depth. In most cases,
it is limited to shallow soils, streams, and groundwater. Pumping the water out
of the ground and using it to irrigate plantations of trees may treat
contaminated groundwater that is too deep to be reached by plant roots. Where
practical, deep tilling, to bring heavy metals that may have moved downward in
the soil closer to the roots, may be necessary.
Generally, the use of phytoremediation is
limited to sites with lower contaminant concentrations and contamination in
shallow soils, streams, and groundwater. However, researchers are finding that
the use of trees (rather than smaller plants) allows them to treat deeper
contamination because tree roots penetrate more deeply into the ground.
The success of phytoremediation may be
seasonal, depending on location. Other climatic factors will also influence its
effectiveness.
The success of remediation depends in
establishing a selected plant community. Introducing new plant species can have
widespread ecological ramifications. It should be studied beforehand and
monitored. Additionally, the establishment of the plants may require several
seasons of irrigation. It is important to consider extra mobilization of
contaminants in the soil and groundwater during this start-up period.
If contaminant concentrations are too high,
plants may die.
Some phytoremediation transfers contamination
across media, (e.g., from soil to air).
Phytoremediation is not effective for
strongly sorbed contaminants such as polychlorinated biphenyls (PCBs).
Phytoremediation requires a large surface
area of land for remediation.
Applicability
Phytoremediation is used for the remediation
of metals, radionuclides,
pesticides,
explosives, fuels, volatile organic
compounds (VOCs)
and semi-volatile
organic compounds (SVOCs). Research is underway to understand the role of
phytoremediation to remediate perchlorate, a contaminant that has been shown to
be persistent in surface and groundwater systems. It may be used to cleanup
contaminants found in soil and groundwater. For radioactive substances,
chelating agents are sometimes used to make the contaminants amenable to plant
uptake.
Technology Development Status
Phytoremediation is a broad technology type
that has been successfully demonstrated for some contaminants and is
experimental for others.
Web Links
http://www.itrcweb.org/Documents/PHYTO-3.pdf
http://www.frtr.gov/matrix2/section4/4-3.html
http://www.frtr.gov/matrix2/section4/4-33.html
http://www.rtdf.org/public/phyto/protocol/protocol99.htm
http://www.cluin.org/download/citizens/citphyto.pdf
http://el.erdc.usace.army.mil/phyto/
https://ert2.navfac.navy.mil/printfriendly.aspx?tool=Phytoremediation
http://t2.serdp-estcp.org/t2template.html#tool=Phytoremediation&page=Overview
(contains list of plants used for certain contaminants)
Other Resources and Demonstrations
See http://www.engg.ksu.edu/HSRC/phytorem/
for a list of other phytoremediation web resources.
At the Mixed Waste Management Facility at the
Savannah River Site (SRS), the U.S. Department of Energy has implemented an
extensive phytoremediation system for tritiated water. SRS has constructed a
retention pond to manage the groundwater as it discharges to the surface, and
it is using old growth forest to transpire the tritiated water. It has an
extensive pipeline system with sprinkler heads staged close to the ground.
http://www.rtdf.org/public/phyto/chlor_solv_management.pdf
for a list of 165 sites where phyto has been used. Also see http://www.rtdf.org/public/phyto/bib/print.cfm
See http://clu-in.org/products/intern/phytotce.htm
for a description of hydraulic capture of a TCE plume using poplar
trees.
See http://clu-in.org/download/remed/lasat.pdf
for a research paper on phytoremediation of metals.
See http://www.newswise.com/articles/researcher-use-trees-to-detect-contaminants-and-health-threats