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:
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/
http://www.ert2.org/phytoremediation.aspx#tool=phytoremediation&page=Overview
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.erdc.usace.army.mil/pls/erdcpub/!www_fact_sheet.capability_page?ps_capability_numb=50911&tmp_Main_Topic=&page=SUCCESS#SUCCESS for DoD projects.
See http://www.newswise.com/articles/researcher-use-trees-to-detect-contaminants-and-health-threats
.