Metal-Scavenging Plants to Cleanse the
ARS geneticist Yin Lin (left), agronomist Rufus Chaney
(center), and University of Maryland soil microbiologist Scott Angle examine
metal-accumulating Thlaspi plants in a growth chamber.
Rufus Chaney has his eye on plants with a lusty appetite for toxic heavy
Chaney foresees a day when these remarkable plants will be used to clean
contaminated soils at smelter and mining sites, landfills, nuclear waste dumps,
farmland, or any urban or rural site contaminated with lead, cadmium, zinc,
nickel, or radioactive isotopes such as uranium or cobalt.
The plants would take up the toxic metals or isotopes through their roots
and transport them to stems or leaves where they could be easily removed by
An Agricultural Research Service agronomist, Chaney says the cost of using
plants to clean polluted soil "could be less than one-tenth the price tag
for either digging up and trucking the soil to a hazardous waste landfill or
making it into concrete."
Chaney, a heavy-metals expert who works at the ARS Environmental Chemistry
Laboratory in Beltsville, Maryland, says the cost could be further offset by
recovering heavy metals from the plants and selling them. The metal-scavenging
plants, called hyperaccumulators, would be grown and harvested like hay, Chaney
says. "Burning the hay allows recovery and recycling of the metals. The
ash is similar to commercial ore."
Chaney calls the process green remediation. He says that without
intervention, heavy metals stay in soil for centuries.
Scott Angle, who collaborates with Chaney at the University of Maryland,
says that while the plants researchers are seeking are not currently available,
prototype plants are being tested, and the search for hyperaccumulator genes
Angle is part of Chaney's hyperaccumulator team that also includes graduate
student Sally L. Brown, chemist Faye A. Homer, geneticist Yin Li, and
technician Carrie E. Green.
Angle is hopeful that plants effective at storing zinc, cadmium, and nickel
will become a reality in 5 to 10 years, although he is not as hopeful for
plants that take up lead. He says the DuPont Environmental Biotechnology
Laboratory is studying removal of lead with selected strains of ragweed.
When it comes to lead removal, ragweed and Thlaspi rotundifolium are
better than other plants. But right now, they're too slow, as are the zinc and
cadmium hyperaccumulator plants such as Alpine pennycress, Thlaspi
caerulescens. Pennycress is a wild herb found on zinc- and nickel-rich
soils in many countries.
Historically, hyperaccumulator weeds have been used in Europe by prospectors
as signs of the presence of metal ore. The weeds occur in the Alpine areas of
Central Europe, as well as in our Rocky Mountains.
"They have spread more widely in Europe and the United States in the
past 10,000 years, having survived the glaciers in the Alpine areas. They
colonize areas where other plants have died off because of high zinc
levels," Chaney says.
"But these plants are usually low-growing, scrawny weeds and take in
the metals too slowly to be practical," Angle says.
As a microbiologist, Angle is focusing on improving the plants' rate of
metal uptake by removing otherwise beneficial mycorrhizal fungi from their
roots. He believes the fungi hinder the plants' ability to take in more metal.
Of the species tested at Pig's Eye Landfill in St. Paul, Minnesota,
pennycress was the best at taking in cadmium, zinc, and lead. Pennycress has
proven especially good at removing zinc and cadmium, accumulating up to 30,000
parts per million (ppm) of zinc in its leaves without yield reduction. Most
plants experience zinc toxicity by the time they reach 500 ppm zinc.
Pennycress can take in zinc at the rate of 125 kilograms per hectare (kg/ha)
per year (108 pounds/acre), if fertilized and managed carefully. It takes in
cadmium at the rate of 2 kg/ ha per year (1.7 pounds/acre).
"If you have a site with 2,000 kg/ ha zinc, it would typically have 20
to 30 kg/ha cadmium as well. That means the pennycress would take about 16
years to remove both," Chaney says.
A hyperaccumulator bioengineered to be high yielding could take in 500 kg/ha
zinc each year and 6 to 8 kg of cadmium. That would reduce the time to about 4
years, making it more practical, Chaney says.
Currently, trials are being held at a town park in Palmerton, Pennsylvania,
to test ways to remove zinc and cadmium from soils.
Beverly Kershner, a soil scientist who coordinates and manages the Palmerton
site for the Zinc Corporation of America, says the site is thought to have been
contaminated by a zinc smelter that operated in Palmerton from 1890 through
1980before modern environmental regulations existed.
In the Palmerton area, zinc contamination causes serious problems with
growing lawns and crops.
Chaney says that the high zinc content of the soil20,000
kg/hainterferes with absorption of the accompanying cadmiumabout
200-300 kg/haby plants. This keeps the amount of cadmium in garden
vegetables very low.
Sally Brown, a University of Maryland agronomy student,
prepares acid-digested samples of hyperaccumulator plants for metals
The low levels in edible plants was reflected in blood and urine samples
taken from Palmerton residents: Scientists found no signs of cadmium related
problems in long-term Palmerton residents, including home gardeners.
Chaney says that there has been concern about toxicity of cadmium because of
reports of kidney tubular dysfunction and rickets (osteomalacia) among Japanese
farm families eating rice grown on soils contaminated with cadmium from mining
There have been no such reports about zinc. And at Palmerton, blood and
urine samples showed no signs of zinc toxicity.
"Blood and urine sampling done as part of the Palmerton project
suggests that the Japanese situation was a highly unusual case. The rice-based
diets and malnourished subsistence of farm families reflects a unique situation
of high cadmium intake by people with little or no intake of other minerals
that would interfere with cadmium absorption to the blood," Chaney says.
"The Palmerton soil had 50 times as much cadmium as those that produced
the rice that affected the Japanese. But most industries that release cadmium
also release zinc," he adds, "so the same protective effect would
The most difficult and extensive soil contamination problem for Chaney and
other researchers working with hyperaccumulators is lead contamination, says
Angle. Both he and Chaney, who is working on other techniques for soil lead
remediation, believe that lead is not a metal to begin hyperaccumulator work
with. It tends to accumulate in the fibrous roots of plants, making it very
difficult to remove by harvesting.
Chaney is engaged in discussions with the U.S. Department of Energy, the
U.S. Department of Defense, and the U.S. Environmental Protection Agency to
develop hyperaccumulators to clean hazardous and nuclear waste sites, using
plant species that accumulate uranium, cesium, strontium, and other radioactive
"Plants that accumulate high levels of uranium and cobalt have been
found, but no work has been done to maximize this removal," Chaney says.
Samples extracted from hyperaccumulator plants are analyzed for
concentrations of zinc and cadmium by University of Maryland graduate student
Screening for Genes
Li is working with Chaney and Angle to do breeding studies as part of a
search for the gene or genes that allow pennycress to hyperaccumulate metals.
He says that one way to create the plants he is seeking is to find the
responsible gene or genes and insert them into high-yielding plants. The other
option is to use traditional breeding to produce Alpine pennycress plants that
grow faster and taller.
The varieties Chaney works with currently grow only to 8 to 12 inches.
Alan J.M. Baker of the University of Sheffield in the United Kingdom, who is
cooperating with Chaney and Angle in the hyperaccumulator project, has been
collecting Thlaspi strains from across Europe. He recently gave Chaney
the last of the seeds needed to make genetic crosses of various metal-storing
strains of Alpine pennycress.
"You have to cross them to determine the number of genes involved and
their characteristics," Chaney says, "before you can search for the
genes and try to move them into other plant species."
Scott Angle, soil microbiologist at the University of Maryland,
examines the dense roots of a metal-scavenging Thlaspi plant.
Related cooperative research is under way between the ARS Appalachian Soil
and Water Conservation Research Laboratory in Beckley, West Virginia, and the
Virginia Polytechnic Institute and State University (VPI & SU) in
David C. Martens and Sam Ha of VPI & SU are collaborating with Virupax
C. Baligar and Ralph B. Clark of ARS, Xiaoe Yang, from Zhejiang Agricultural
University in Hangzhou, China, works at Beckley with Baligar and Clark. She
uses a tiny plant called Arabidopsis thaliana to search for
hyperaccumulating and excluding plants.
Metal-excluding plants would ensure that at least the edible parts of plants
would be free of toxic metals.
Arabidopsis is used as a model plant by molecular biologists because
it has a small number of genes and a short life cycle.
From 200,000 seeds that were chemically treated to induce mutations, Yang
was able to isolate two plants that were sensitive to nickel toxicity and two
that were moderately tolerant to nickel toxicity. The moderately tolerant
mutants had smaller and thicker leaves than the wild type.
Currently, Yang's studies are under way to understand the mechanisms
responsible for storing or excluding nickel in Arabidopsis. Ha, a
molecular geneticist, is using the mutants to find the genes involved.
Yang also tested wheat, corn, cabbage, white clover, and ryegrass for their
abilities to store or exclude cadmium, zinc, and nickel. She found that
ryegrass accumulated the least amounts of cadmium in its shoots but is an
accumulator of nickel. White clover is a nickel excluder.
"Chaney's work has important implications for our efforts to restore
the environment," says Don Bills, director of the ARS Natural Resources
Institute at Beltsville. "We need new, innovative methods to solve some of
our worst pollution problems. The concept of manipulating plant genes that
regulate toxic metal uptake is cutting-edge research. This is part of ARS'
larger effort to maintain a productive agricultural environment that will
sustain American agriculture indefinitely."
By Don Comis, ARS.
L. Chaney is with the USDA-ARS Environmental Management and Byproduct
Utilization, 10300 Baltimore Avenue, Bldg 007 BARC-West, Beltsville, MD,
20705-2350; (301) 504-8324 ext. 447, fax (301) 504-5048.
Baligar is at the USDA-ARS
Perennial Crops Laboratory, 10300 Baltimore Avenue, Bldg 001 BARC-West,
Beltsville, MD, 20705-2350; phone (301) 504-6492 , fax (301) 504-5823.
"Metal-Scavenging Plants to Cleanse the Soil"
was published in the November 1995 issue of
Agricultural Research magazine.
Growing Lawns on High-Zinc Soils
Just outside the Borough of Palmerton, Pennsylvania, nestled against Blue
Mountain, ARS agronomist Rufus Chaney has test plots that compare the growth of
various turf grasses with different soil additives on high-zinc soils. Chaney
used his considerable experience with heavy metals and sewage sludge research
to tailor the project to local conditions.
He has found that mixing high-iron and high-lime sewage sludge compost into
high-zinc soil reduces zinc toxicity to grasses because the soil zinc binds to
the compost iron. This decreases zinc uptake by plants.
"Also, the grasses more easily obtain adequate iron, which further
reduces their zinc uptake," Chaney says. "Tall fescue varieties did
very nicely on soil treated with the sludge compost."
Revival FieldLinking Art and Science
The research and testing of a new "green remediation" technology
has advanced from laboratory to field, thanks to an important contribution from
an unlikely sourcean artist. Green remediation refers to the use of
plants to remove heavy metals from contaminated soil.
When New York sculptor Mel Chin read about hyperaccumulator plants, he was
struck by the poetic nature of this process. So he conceived a sculpture in
which plants and biotechnology would replace chisels and marble. Chin says the
aesthetic of "Revival Field," the name for the field trials project,
"relates to my interest in alchemy and my understanding of transformative
processes and the mutable nature of materials. The contaminated soil is
transformed back into rich earth, capable of sustaining a diverse
Chin says his sense of "responsibility to the scientific advancements
that could make this change possible" led him to Rufus Chaney, who was
doing lab research on hyperaccumulators.
The initial phase of the Revival Field project enabled Chaney's laboratory
research to be tested for the first time on actual contaminated sites.
After lengthy and difficult negotiations, Chin was able to secure a location
for the first trial at Pig's Eye Landfill in St. Paul, Minnesota. The site is
contaminated with cadmium, zinc, and lead. With funding from the National
Endowment for the Arts and the Walker Art Center in Minneapolis, this
innovative project began.
Following Chaney's scientific plan, Chin designed a circular field with
replicated plantings to analyze the use of six hyperaccumulator and
metal-tolerant plants and a variety of soil treatments. Two main walkways
divided the field like the crosshair of a rifle scope, symbolizing a targeting
of the earth for cleanup.
The Minnesota field trial was active from 1990 to 1993. It showed that
Alpine pennycress was best at taking in heavy metals, although neither it nor
any of the other plants took in metals fast enough to achieve significant
cleansing in 3 years.
A second similar test is currently under way near a Superfund site in
Palmerton, Pennsylvania. Superfund refers to areas placed on a national
priorities list for U.S. Environmental Protection Agency cleanup.
Revival Field is by design a public project, and both collaborators have
sought to make information available to a wide audience through lectures,
articles, and art exhibitions.
The project has already generated considerable interest among scientists,
artists, and environmentally concerned citizens.
Chin says, "The next phase of Revival Field will depend on the needs
and directives of Rufus Chaney and a growing body of like-minded specialists
from around the world. Art and science will continue their integrated
cooperation in an active response to a problem that threatens the health of
communities everywhere." By Don Comis, ARS.