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

Science News, July 20, 1996 by Tina Adler

The soil has lead concentrations between 500 and 1,000 ppm, about twice New Jersey's legal limit for lead in residential land.

The company disposes of lead-rich plants at hazardous-waste treatment facilities. Eventually, Ensley would like to give the plants to lead smelters, who could extract and reuse the metal. However, smelters won't take plants that have high concentrations of multiple chemicals, he notes. Researchers at DuPont spend about $1 million a year on phytoremediation research. Using chelator solutions, they report that they can make almost any plant in a greenhouse setting absorb a significant amount of lead, even crops such as corn and peas, which don't normally extract metals from soil. They find that the best plants for lead removal grow big and fast, absorb a lot of water, and tolerate slightly poor soil.

Cunningham reports that ethylenediaminetetraacetic (EDTA) salt is one of the most promising chelators. Manufacturers use EDTA salt and closely related compounds as food preservatives.

DuPont's team has yet to test chelators in the field, but it may begin doing so next summer, Cunningham says. It first needs to make sure that the chelator solutions don't free more lead from the soil than the plants can absorb, because the excess could seep into the groundwater. The DuPont scientists haven't yet decided on the best chelators and plants to employ.

One of the most common groundwater pollutants is trichloroethylene (TCE), once used in abundance for dry cleaning and as a degreasing agent but now banned.

Washington alone has about 10,000 sites contaminated with TCE, according to the state's Department of Ecology.

Fortunately, poplar trees appear to have a fondness for TCE. Moreover, their roots can reach depths of 40 to 50 feet, making them particularly well suited for cleaning groundwater.

With funding from the manufacturing company Occidental Chemical, Milton T. Gordon of the University of Washington in Seattle and his colleagues last year began growing 18 poplars in large, sealed containers into which they pump water containing TCE concentrations of 50 to 70 ppm. The trees are removing 95 percent of the chemical. Most contaminated groundwater has TCE concentrations below 25 ppm.

The Washington team plans to investigate how much of the chemical entering the trees stays in the roots, stems, and leaves. The TCE that the trees transpire into the air degrades quickly, as sunlight breaks down the chemical.

When the trees have exhausted their capacity to take up TCE, they may be burned or pulped, which would probably render the chemical harmless, Gordon says.

Soil in parts of the western United States has a natural abundance of selenium, an important nutrient for animals and humans that in high doses is toxic. Drainage water from agricultural fields often becomes rich in selenium.

Growers then send the water to their evaporation ponds, where the selenium reaches even higher concentrations, which are deadly to wildlife.

Plants, however, "are very effective at removing selenium from contaminated soils," asserts Norman Terry of the University of California, Berkeley. They not only absorb the chemical, they also turn some of it into the less toxic dimethyl selenide gas. Selenate, the common form of selenium in soil, is about 600 times more toxic than dimethyl selenide gas.