WEST LAFAYETTE, Ind. -- Plants have been quietly cleaning up behind us, recycling waste carbon dioxide into life-giving oxygen for as long as humans have been breathing.
Now Purdue University researchers are prodding them to take the cleanup a step further, to become phytoremediators -- plants that collect heavy metals and radioactive waste from polluted water and contaminated soils.
"Phytoremediation is only five or 10 years old," says Purdue horticulture Professor Peter Goldsbrough, "but there's a lot of hope that eventually plants can be used to restore sites and remove pollutants at a fairly low cost."
Initial commercial efforts at phytoremediation have been promising, but officials from the U.S. Department of Energy (DOE), which owns most toxic sites in the United States, are unsure about how well current processes will work on large projects -- and how much a large-scale operation would cost.
In one multinational effort, a New Jersey company is using plants to clean up nuclear contamination from the April 1986 nuclear accident at Chernobyl, Ukraine.
Using traditional technology to clean contaminated soil (such as removing, chemically treating or heating it) would cost several hundred dollars per ton of soil, according to DOE officials. That puts cleanup costs for contaminated soil in the United States at hundreds of billions of dollars. If phytoremediation reduced those cleanup costs by even 10 percent, an estimate Goldsbrough considers plausible, it would save taxpayers billions.
Goldsbrough, working with researchers from Australia, has been studying how plants take up and store cadmium, a toxic heavy metal. It's both a natural and industrial contaminant. Cadmium recently was found at high levels in livers of British sheep grazing on a pasture fertilized for many years with sewage sludge. According to findings reported at the annual meeting of the British Society of Animal Science, cadmium levels in the sheep livers were high enough to cause concern but probably not high enough to cause serious health problems for people who ate the contaminated organ meat.
According to the World Health Organization, cadmium can damage human kidneys and other organs. High doses of cadmium are debilitating.
Cadmium also can be toxic to plant tissue, but plants generally find ways to detoxify the heavy metal before it causes damage.
In research published in the journal Plant Physiology, Goldsbrough and his Australian collaborators described a mutant strain of Arabidopsis thaliana (a plant in the mustard family, often used in laboratory testing) that can't defend itself against cadmium. Then they figured out why it was so susceptible to cadmium poisoning.
"Normal Arabidopsis plants protect themselves by linking the toxic metal to a peptide once it's inside the plant," Goldsbrough says. The bound, nontoxic form of cadmium then gets dumped into a cell vacuole, a sort of bubble inside a cell that cordons off a section and holds the toxins at bay.
The mutant plants lacked the peptides that tie up and detoxify the heavy metal.
In subsequent related research, Goldsbrough and Purdue co-workers identified genes in Arabidopsis for other peptides that can bind and detoxify toxic metals.
The DNA that Goldsbrough has identified could be manipulated to produce plants able to mine large quantities toxic metals from soil. And what he's learned about Arabidopsis could be applied to other plants. Once breeders develop metal-mining plants, EPA Superfund-site farmers might clean up contaminated sites after a few cropping seasons.
The same DNA responsible for a plant's ability to pull toxic metals from soil could be manipulated to coerce carrot, corn, apple or rutabaga roots to reject heavy metals, Goldsbrough says. If grown in areas where soil is contaminated, such bioengineered fruits and vegetables would hold fewer toxins, and people eating them would have healthier diets.
While Goldsbrough prods vegetables to change the way they take up toxic metals, another Purdue researcher pushes tobacco plants to clean up radioactive waste.
Purdue botany Professor Mary Alice Webb uses tobacco seedlings to take up strontium, a contaminant made famous by the Chernobyl accident. Strontium 90 is one of the radioactive wastes that billowed up in the smoke of the Chernobyl fire, then fell and contaminated land around the blown reactor. In humans, radioactive strontium can cause bone tumors and, possibly, leukemia.
Webb hadn't set out to clean up radioactive waste. Her work started with calcium, but led to strontium. She was, and still is, fascinated by the way certain plants take up and store calcium. Plants need a large amount of calcium for normal growth and development, but too much can kill them.
"Most plants have cells that serve as calcium dumps," Webb says. "Plants have a tremendous ability to store calcium oxalate." Some plants contain almost half as much calcium by weight as does an average human body, and all without a calcium-rich skeleton.
Webb found a way to isolate bunches of calcium oxalate crystals from grape leaves and to study how they formed. Her work was reported in The Plant Journal.
As she worked with calcium, Webb became aware of research that showed how strontium can mimic calcium in biological systems.
"I theorized that since calcium and strontium are so similar, plant cells might take up strontium and incorporate it into calcium oxalate crystals, and that should increase the plants' capacity for strontium," Webb says.
She and undergraduate Christina Rinderle found that tobacco picks up strontium just as it does calcium. They already have grown tobacco seedlings that take up more than 1 percent of their dry weight in strontium. If they can find ways to increase the percentage, they could create super-plants that would more quickly pull large quantities of strontium out of contaminated soil and water. Their work would supplement ongoing commercial efforts to use plants for toxic cleanup.
Phytotech, a phytoremediation company based in New Jersey, has used floating rafts of sunflowers to clean up radioactive Chernobyl water. Dangling sunflower roots pull both cesium 137 and strontium 90 out of the water. After they've done their work, the sunflowers are disposed of as radioactive waste. The estimated cost for the project is $2 to $6 per thousand gallons of water. Phytotech researchers also used Indian mustard to clean cesium and strontium out of soil contaminated by Chernobyl fallout.
Sources: Peter Goldsbrough, (765) 494-1334; e-mail, firstname.lastname@example.org
Mary Alice Webb is on sabbatical. You can reach her via e-mail at email@example.com
Writer: Rebecca J. Goetz, (765) 494-0461; e-mail, firstname.lastname@example.org
Purdue News Service: (765) 494-2096; e-mail, email@example.com