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Green Cleaners

31 January 2001 7:00 pm
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Green power. Brake ferns--growing here on a wall of the University of Florida campus--may be a commercially viable way to clean up contaminated sites.

Ferns may be the key to cheap cleanup of soils contaminated with arsenic. In the 1 February issue of Nature, scientists report that brake fern (Pteris vittata) squirrels away so much of the toxic element that arsenic concentrations in the fern's fronds can be more than 100 times greater than in surrounding soil.

Arsenic occurs naturally in many environments, but soils contaminated from old copper smelters or wood treatment factories can have levels that are thousands of times higher. Long-term exposure to arsenic is thought to cause bladder, liver, and skin cancer; high doses interfere with cells' energy metabolism and can be deadly. Currently, removing the soil is the only method for cleaning up contaminated sites, but it can cost up to $2.5 million to dig up and safely truck away the top 30 centimeters of soil from a site the size of a hectare, says Rufus Chaney, a research agronomist with the U.S. Department of Agriculture (USDA) in Beltsville, Maryland. "We haven't cleaned up large sites for arsenic contamination because it's prohibitively expensive," he says.

In the search for a cheaper, easier solution, Lena Ma, a soil scientist at the University of Florida in Gainesville, turned to plants. She and her colleagues tested 14 species growing at a site where wood had been pressure-treated with a preservative that contained copper, chromium, and arsenic. Soil levels of arsenic averaged about 150 parts per million (ppm), whereas a nearby uncontaminated site had levels under 8 ppm.

Most plant species the team tested had less arsenic than the contaminated soil they were growing in, but the first fern the team examined contained 4300 ppm. "We were shocked," says Ma. The shock quickly gave way to delight. Her graduate student and co-author Kenneth Komar "was dancing around--it's like hitting the lottery," she adds. Subsequent experiments in the greenhouse showed that in just 2 weeks, fronds of brake fern, an Asian plant introduced into Florida more than 100 years ago, can accumulate 100 times more arsenic than is in the soil. The skyrocketing levels don't seem to affect the plant.

Decontaminating soils using plants, a method known as phytoremediation, requires planting, growing, and harvesting a species that "hyperaccumulates" the contaminant. Finding a plant that can do so for arsenic "is an important discovery," USDA's Chaney says, and one that will be commercially valuable. "This plant gives a whole new opportunity" to clean up arsenic waste, says Chaney.

Related sites

Lena Ma's home page

Phytoremediation research at the U.S. Army Corps of Engineers

Phytoremediation: Using plants to clean up soils

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