Botanical cleanup crews: using plants to tackle polluted water and soil - phytoremediation

Science News, July 20, 1996 by Tina Adler

Rafts with sunflowers growing on them float on a small pond at the Chernobyl nuclear accident site in the Ukraine. No, it's not some touching monument to the 1986 disaster. The plants are helping to clean the pond; their roots dangle in the water to suck up the radionuclides cesium 137 and strontium 90.

This sunflower project is one of many international efforts at phytoremediation-the use of plants to absorb pollutants from air, water, and soil. In the United States, both government agencies and private companies, including Exxon Corp. and DuPont are testing a variety of plants to see if they can do some of the dirty work of cleaning up such pollutants as radioactive material, lead, selenium, and oil. Many plants, it turns out, have a taste for these stubborn contaminants.

"To survive, plants have evolved sophisticated metabolic and sequestration mechanisms to detoxify a wide variety of chemical substrates," explains Scott Cunningham of DuPont Central Research and Development in Newark, Del. The plants are also loaded with microbes and fungi that help break down the chemicals. Cunningham spoke in May at a conference on phytoremediation held in Arlington, Va.

To many academic and industry researchers, including environmentalists, phytoremediation looks promising, although even bright-eyed sunflowers have yet to convince these observers that they are ready for the big time.

The Chernobyl sunflower project began in 1994. That summer, researchers from Phytotech, a phytoremediation company in Monmouth Junction, N.J., and their government and university colleagues installed the rafts. Together, they held 24 sunflowers and dotted a 75-square-meter pond located 1 kilometer from the Chernobyl reactor, says Burt Ensley, Phytotech's president.

The plants preferentially absorb cesium and strontium from a mixture of metals, he notes. The plants don't metabolize the radionuclides, but the cesium stays in the roots and most of the strontium moves to the shoots. The company disposes of the plants as radioactive waste after about 3 weeks on the pond.

The investigators started with too few flowers to clean the pond completely, Ensley acknowledges. This summer, they installed 50 to 60 sunflowers, which should clean the pond in a couple of weeks, he asserts. Ensley estimates that removing radioactive metals with sunflowers costs $2 to $6 per thousand gallons of water, much less than existing technologies. However, to avoid recontaminating the pond, the ground nearby must be decontaminated at the same time. For 2 years, Phytotech scientists have been removing cesium and strontium from soil on one-quarter acre of the Chernobyl site by growing Indian mustard (Brassica juncea). In the United States, almost all radioactive sites belong to the Department of Energy. Prior to the Chernobyl sunflower project, Phytotech researchers experimented with pumping contaminated groundwater into containers of sunflowers at a DOE uranium-processing plant in Ashtabula, Ohio. Within 24 hours, the plants reduced the concentration of uranium in the water from 350 parts per billion (ppb) to less than 5 ppb, which meets the legal limits for groundwater, Ensley says.

This summer, Phytotech and DOE researchers began a project using sunflowers to remove uranium from contaminated springs at the Oak Ridge (Tenn.) National Laboratory.

"I've heard of uranium contamination at DOE sites of 100 parts per million [ppm], and we couldn't clean that up. We could go up to 2,000 ppb," Ensley says. Are DOE managers of tainted sites clamoring for Phytotech's help? Not quite, says Ensley. They have a bias against new technology and worry about its costs, he contends.

"What [Ensley] has is wonderful," but it's still just "gee-whiz science," asserts Rashalee Levine of DOE's Office of Technology Development in Germantown, Md. In general, phytoremediation performs very well on a small scale, she says, but she is waiting to see how it handles big jobs. Plants take a lot of space to cultivate and tend to work slowly, she notes. Also, "it remains to be seen how much it will cost." Her office received a cut this fiscal year in its funding for phytoremediation projects. The agency is supporting research on the use of plants on six small sites contaminated with cadmium, zinc, cesium, strontium, uranium, or some combination of these.

Compared to the radionuclides, lead presents a particularly sticky problem for the environment and for phytoremediation researchers. It forms strong bonds with minerals and organic matter in the soil. Plants absorb only a little lead, and it doesn't move beyond the roots. Scientists have recently circumvented this difficulty by watering the plants with a solution containing lead-chelating agents. These organic molecules wrap themselves around lead atoms and allow the lead to dissolve in water so plants can absorb it better. Phytotech has applied for a patent on the use of chelators for lead removal. The company is using plants and chelator solutions to clean up a handful of sites contaminated with lead. Most recently, it began growing Indian mustard on a quarter-acre patch of a former battery recycling plant in Trenton, N.J.