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Plants Perform "Green Clean" of Toxic Sites

Brian Handwerk
for National Geographic News
September 24, 2004
 
Many plants employ natural processes to clean contaminated soil and groundwater. One such process is phytoremediation, by which plants remove heavy metals, such as mercury and lead, from soil. Researchers are now helping plants do a quicker, better job of rehabilitating polluted sites through phytoremediation.

Guy R. Lanza is a microbiology professor and director of the environmental sciences program at the University of Massachusetts in Amherst.

Among other projects, Lanza studies how phytoremediation technology can be applied to New England's Blackstone River corridor, an early birthplace of the U.S. industrial revolution.



Hundreds of years of pollution have left riverside sites in Massachusetts and Rhode Island contaminated with what Lanza dubs "a complicated chemical soup." Conventional cleanup methods of excavating and removing toxic silt and soil are not practical—the contamination zone is simply too enormous.

Taking a cue from nature, Lanza and his colleagues visited a contaminated pond and studied the wetland vegetation surviving that environment.

"We're trying to isolate, and hopefully manipulate, the plant systems to do more of what we want them to do," Lanza said. "In this case to remove these toxic metals." The researcher said his investigation shows that removing heavy metals like lead, cadmium, zinc, and arsenic with plants is feasible.

Industrial Legacy

At Ford Motor Company's Rouge Center in Dearborn, Michigan, decades of steelmaking left soil contaminated with highly carcinogenic compounds known as polycyclic aromatic hydrocarbons, or PAHs.

With funding from the automaker, Michigan State University researcher Clayton Rugh and University of Michigan-Dearborn researcher John Thomas developed a phytoremediation cleanup system. Their approach used native plants, which also boosted a sitewide initiative to restore the facility's native wildlife habitat.

The project is now in its fourth summer.

Rugh said ten plant species were the standout performers, among them New England aster, joe-pye weed, and leadplant.

The site's plantings are attracting insects, birds, and other wildlife while aggressively accelerating the natural degradation process of toxins.

"If left undisturbed, it would take decades or centuries for these contaminants to naturally decompose," Rugh said. "What our research is indicating is that we can achieve at least 50 percent degradation in three to five years—and that's at the very least. Some [species] seem to be approaching 70 percent in just three growing seasons."

Other companies, including British Petroleum, Chevron, and DaimlerChrysler, have similar projects.

Explosive Cleanup

Sponsors of phytoremediation research include the U.S. Army and the U.S. Department of Defense. Programs there have tapped plants to help with the thorny problem of ordnance disposal.

Firing and training ranges are contaminated with the toxic remains of partially exploded rounds. The rounds include a chemical commonly known as RDX.

Mike Reynolds serves with the Army's Engineer Research and Development Center's Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire. He said his group has recently begun research linked to solving the problem.

"On live-fire ranges or in their bordering areas, you have unexploded ordnance, which can be dangerous," Reynolds said. "There are just not a lot of [cleanup] alternatives."

Lee Newman, a researcher at the University of South Carolina's Arnold School of Public Health, is collaborating on the project. She notes that while explosive residues like TNT have been successfully remediated in the past, RDX is proving a bit tougher.

"One of the big concerns with some of the more recalcitrant compounds [would be] if the plants start accumulating high levels of explosives but don't break them down," Newman said.

The plants would "never become explosive," Newman said. "But they could become toxic and die. And [RDX] may go right back into the ground again."

"In a sense, what we propose to do with RDX in surface soils is tie RDX molecules permanently into soil organic matter, and they won't really exist as RDX anymore," Reynolds explained.

While many phytoremediation applications show promise, the technique faces an uncertain future as an alternative and inexpensive entrant in the very profitable field of contaminant remediation.

"People love the idea of using something natural to clean the environment rather than excavating and hauling things away," Newman said. "But as a field we need to convince regulators that what we're doing is based on good science and that we can do something effective," she explained.

"Some very progressive firms are now offering phytoremediation as an option, but it's still not one of the main options," she added. "If [companies] use a new technology and it doesn't work, they are going to be in a lot of trouble and also a money-losing situation."

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