United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA542-F-96-014
September 1996
v>EPA A Citizen's Guide to
Phytoremediation
Technology Innovation Office
Technology Fact Sheet
What is phytoremediation?
Phytoremediation is the use of plants and trees to
clean up contaminated soil and water. Growing and,
in some cases, harvesting plants on a contaminated
site as a remediation method is an aesthetically
pleasing, solar-energy driven, passive technique that
can be used along with, or in some cases, in place of
mechanical cleanup methods.
Phytoremediation can be used to clean up metals,
pesticides, solvents, explosives, crude oil,
polyaromatic hydrocarbons, and landfill leachates.
How does phytoremediation work?
Phytoremediation (the term phyto- means plant) is a
general term for several ways in which plants are
used to clean up, or remediate, sites by removing
pollutants from soil and water. Plants can break
down, or degrade, organic pollutants or stabilize
metal contaminants by acting as filters or traps.
Some of the methods that are being tested are de-
scribed in this fact sheet.
Metals Remediation
At sites contaminated with metals, plants are used to
either stabilize or remove the metals from the soil
and ground water through two mechanisms:
phytoextraction and rhizofiltration.
Phytoextraction, also called phytoaccumulation, re-
fers to the uptake of metal contaminants by plant
roots into plant stems and leaves (Figure 1). Certain
plants absorb unusually large amounts of metals in
comparison to other plants. One or a combination of
these plants is selected and planted at a particular site
based on the type of metals present and other site
conditions. After the plants have been allowed to
grow for some time, they are harvested and either in-
cinerated or composted to recycle the metals. This
procedure can be repeated as many times as neces-
sary to bring contaminant levels in the soil down to
allowable limits. If plants are incinerated, their ash
must be disposed of in a hazardous waste landfill, but
the volume of ash will only be about 10% of the vol-
ume that would be created if the contaminated soil it-
self were dug up for treatment.
Metals such as nickel, zinc, and copper are the best
candidates for removal by phytoextraction because
they happen to be the favorites of the approximately
400 known plants that absorb unusually large
amounts of metals. Plants that absorb lead and chro-
mium are being studied and tested.
Rhizofiltration (rhizo- means root) has shown
promise for dealing with metals contamination in wa-
ter. Rhizofiltration is similar to phytoextraction, but
the plants to be used for cleanup are raised in green-
houses with their roots in water rather than in soil.
When the plants have developed a large root system,
contaminated water is collected from a waste site and
A Quick Look at Phytoremediation
Is an aesthetically-pleasing, passive, solar-energy driven cleanup technique.
Is most useful at sites with shallow, low levels of contamination.
Is useful for treating a wide variety of environmental contaminants.
Printed on Recycled Paper
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Figure 1. Uptake of Metals (Nickel) by Phytoextraction
Ni
Ni
Nickel is removed from soil by moving up into plant roots, stems, and leaves. The plant is then harvested
and disposed of and the site replanted until the nickel in the soil is lowered to acceptable levels.
brought to the plants where it is substituted for their
water source. The roots take up the water and the
contaminants along with it. As the roots become
saturated with contaminants, they are harvested and
disposed of. In addition to being useful for removing
metals from water, rhizofiltration may prove useful
for industrial discharge, agricultural runoff, acid
mine drainage, and radioactive contamination. For
example, sunflowers were used successfully to re-
move radioactive contaminants from pond water in a
test at Chernobyl, Ukraine.
Treating Organic Contaminants
Organic contaminants (those that contain carbon and
hydrogen atoms) are common environmental pollut-
ants. There are several ways plants can be used for
the phytoremediation of these contaminants: phyto-
degradation, enhanced rhizosphere biodegradation,
organic pumps, and phytovolatilization.
What Is An Innovative Treatment
Technology?
Treatment technologies are processes applied to
the treatment of hazardous waste or contaminated
materials to permanently alter their condition
through chemical, biological, or physical means.
Innovative treatment technologies are those that
have been tested, selected or used for treatment of
hazardous waste or contaminated materials but
lack well-documented cost and performance data
under a variety of operating conditions.
Phytodegradation is a process in which plants are
able to degrade (break down) organic pollutants. In
some cases, the pollutants degraded into simpler
molecules are used to help the plant grow faster (Fig-
ure 2). Plants contain enzymes, a broad category of
chemical substances that cause rapid chemical reac-
tions to occur. Some enzymes break down and con-
vert ammunition wastes, others degrade chlorinated
solvents such as trichloroethylene (TCE), and others
degrade herbicides.
Enhanced rhizosphere biodegradation takes place
in the soil surrounding the plant roots (the rhizo-
sphere) 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 di-
gest organic substances such as fuels or solvents that
are hazardous to humans and break them down into
harmless products in a process called biodegrada-
tion. 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. Biodegra-
dation is also aided by the way plants loosen the soil
and transport water to the area. The fact sheet en-
titled^ Citizen's Guide to Bioremediation describes
the biodegradation process in detail (see page 4).
Trees can act as organic pumps when their roots
reach down toward the water table and establish a
dense root mass that takes up large quantities of wa-
ter. Poplar trees, for example, pull out of the ground
30 gallons of water per day, and cottonwoods can ab-
sorb up to 350 gallons per day. The pulling action
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caused by the roots decreases the tendency of surface
pollutants to move downward towards ground water
and into drinking water. Poplars planted along stream
beds in agricultural areas reduce the amount of ex-
cess fertilizer and herbicides that get into the streams
and ground water. In another similar application,
trees planted on top of landfills as organic substitutes
for the traditional clay or plastic caps, suck up rain-
water that could otherwise seep through the landfill
and come out the bottom as contaminated "leachate."
Phytovolatilization occurs as growing trees and
other plants take up water and the organic contami-
nants in it. Some of these contaminants can pass
through the plants to the leaves and evaporate, or
volatilize, into the atmosphere. Poplar trees, for ex-
ample, volatilize 90% of the TCE they suck up.
Does phytoremediation work at every
site?
Phytoremediation can be used to clean up metals,
pesticides, solvents, explosives, crude oil,
polyaromatic hydrocarbons, and landfill leachates.
Phytoremediation is used in combination with other
cleanup approaches as a "finishing" step. Although
phytoremediation is significantly slower than me-
chanical methods, and is limited to the depth that the
roots can reach, it can clean out the last remains of
contaminants trapped in the soil that mechanical
treatment techniques sometimes leave behind.
Generally, the use of phytoremediation is limited to
sites with lower contaminant concentrations and
contamination in shallow soils, streams, and ground
water. However, researchers are finding that the use
of trees (rather than smaller plants) allows them to
treat deeper contamination because tree roots pen-
etrate more deeply into the ground. Contaminated
ground water very deep underground may be treated
by pumping the water out of the ground and using it
to irrigate plantations of trees.
Further research is needed to study the effects on the
food chain that could occur if insects and small ro-
dents eat the plants that are collecting metals and are
then eaten by larger mammals. Also, scientists still
need to establish whether contaminants can collect in
the leaves and wood of trees used for phytoremedia-
tion and be released when the leaves fall in the au-
tumn or when firewood or mulch from the trees is
used.
Where has it been used?
Phytoremediation has been successfully tested in
many locations. In Iowa, poplar trees planted along a
stream bank between a corn field and the stream
acted as natural pumps to keep toxic herbicides, pes-
ticides, and fertilizers out of the streams and ground
water. When the trees were three years old, research-
ers tested the levels of the nitrate contamination in
the ground water at the edge of the cornfield and
found it to be 150 milligrams per liter (mg/L). The
ground water among the poplar trees along the
stream bank, however, had nitrate concentration of
only 3 mg/L—well under the EPA nitrate limit of 45
mg/L in drinking water. The table on page 4 lists
some phytoremediation projects.
Figure 2. Destruction of Organic Contaminants by Phytodegradation
Enzymes in plant roots break down (degrade) organic contaminants. The fragments are incorporated into
new plant material.
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Table 1. Examples of Sites Testing Phytoremediation*
Location
Ogden, UT
Portsmouth, VA
Milan, TN
Aberdeen, MD
Application
Phytoextraction
Rhizofiltration
Phytodegradation
Phytodegradation
Organic Pumps
Phytovolatilization
Rhizofiltration
Contaminants
Petroleum hydro-
carbons
Petroleum
Explosives wastes
Trichloroethylene
Trichloroethane
Medium
Soil
Ground water
Soil
Sediment
Ground Water
Plant
Alfalfa, Poplar
Juniper, Fescue
Grasses
Clover
Duckweed
Parrot feather
Poplar trees
"Not all waste types and site conditions are comparable. Each site must be individually investigated and tested.
Engineering and scientific judgment must be used to determine if a technology is appropriate for a site.
For More Information
The publications listed below can be ordered free of charge by faxing your request to NCEPI at 513-489-
8695. If NCEPI is out of stock of a document, you may be directed to other sources. If you choose, you may
write to NCEPI at:
National Center for Environmental Publications and Information (NCEPI)
P.O. Box 42419
Cincinnati, OH 45242
• "Tree Buffers Protect Shallow Ground Water at Contaminated Sites," Ground Water Currents
(newsletter), December 1993, EPA542-N-93-011.
Recent Developments for In Situ Treatment of Metal Contaminated Soils, (Available Fall 1996), EPA
542-R-96-008.
• A Citizen's Guide to Bioremediation, April 1996, EPA 542-F-96-007.
• Soil Stabilization Action Team, April 1996, EPA 542-F-96-01 Od.
• "Mother Nature's Pump and Treat," by Kalle Matso in Civil Engineering, October 1995, pages 46-49.
• "The Green Clean," by Kathryn Brown Sargeant in BioScience, October 1995, pages 579-582.
NOTICE: This fact sheet is intended solely as general guidance and information. It is not intended, nor can it be relied upon, to create any rights enforceable
by any party in litigation with the United States. The Agency also reserves the right to change this guidance at any time without public notice.
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