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Nuclear Clean Up - Phytoremediation: Using plants to remove pollutants from the environment (including radionuclides)

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  • Nuclear Clean Up - Phytoremediation: Using plants to remove pollutants from the environment (including radionuclides)

    Phytoremediation: Using plants to remove pollutants from the environment (including radionuclides)

    Phytoremediation is defined as the use of green plants to remove pollutants from the environment or to render them harmless. Research into phytoremediation may offer the remediation solution for at least 30,000 contaminated sites in the U.S.

    Phytoremediation of metals: green and clean

    Soils and waters contaminated with toxic metals pose a major environmental and human health problem which is still in need of an effective and affordable technological solution. Microbial bioremediation has been somewhat successful for the degradation of certain organic contaminants, but ineffective at addressing the challenge of toxic metal contamination, particularly in soils. While organic molecules can be degraded, toxic metals are remediated by removal from soil. Therefore, the state of the-art technology for cleanup of toxic metal contaminated soils is the excavation and burial of the soil at a hazardous waste site at costs that can reach $1,000,000 per acre. In the U.S alone, the cost of cleaning up sites contaminated with toxic and radioactive metals is estimated to be $300 billion. The problem is even more acute abroad, particularly if large areas contaminated with radionuclides as a result of the Chernobyl nuclear disaster are taken into account.
    Phytoremediation of metals is a cost-effective "green" technology based on the use of specially selected metal-accumulating plants to remove toxic metals, including radionuclides, from soils and water . . .

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    "Green" Technology: Simple Concept and Cost-Effective

    Phytoremediation is the use of green plants to remove pollutants from the environment or render them harmless.

    "Current engineering-based technologies used to clean up soils?like the removal of contaminated topsoil for storage in landfills?are very costly," Kochian says, "and dramatically disturb the landscape."

    Kochian's cost-effective "green" technology uses plants to "vacuum" heavy metals from the soil through their roots. He says, "Certain plant species?known as metal hyperaccumulators?have the ability to extract elements from the soil and concentrate them in the easily harvested plant stems, shoots, and leaves. These plant tissues can be collected, reduced in volume, and stored for later use."

    While acting as vacuum cleaners, the unique plants must be able to tolerate and survive high levels of heavy metals in soils?like zinc, cadmium, and nickel.
    . . . .
    It Even Works With Uranium
    For soil contaminated with uranium, Kochian found that adding the organic acid citrate to soils greatly increases both the solubility of uranium and its bioavailability for plant uptake and translocation. Citrate does this by binding to insoluble uranium in the soil.
    "With the citrate treatment, shoots of test plants increased their uranium concentration to over 2,000 ppm?100 times higher than the control plants," he says. This demonstrates the possibility of using citrate?an inexpensive soil amendment?to help plants reduce uranium contamination.
    Recently, Kochian, with colleagues Lasat and Ebbs, identified specific agronomic practices and plant species to remediate soils contaminated with radioactive cesium or cesium-137.
    "Although the cause of cesium-137 contamination?aboveground nuclear testing?has been reduced, large land areas are still polluted with radiocesium," Kochian says. "Cesium is a long-lived radioisotope with a half-life of 32.2 years. It contaminates soils at several U.S. Department of Energy (DOE) sites in the United States. Projected costs of cleaning up these soils is very high?over $300 billion."
    Phytoremediation is an attractive alternative to current cleanup methods that are energy intensive and very expensive.
    In initial lab and greenhouse studies, Kochian's team showed that the primary limitation to removing cesium from soils with plants was its bioavailability. The form of the element made it unavailable to the plants for uptake.
    In a series of soil extraction studies, Kochian's team found the ammonium ion was most effective in dissolving cesium-137 in soils. This treatment increased the availability of cesium-137 for root uptake and significantly stimulated radioactive cesium accumulation in plant shoots.

    Later, Kochian did field studies with six different plant species in collaboration with Mark Fuhrmann, a DOE scientist at Brookhaven National Laboratory in Upton, New York. They found significant variation in the effectiveness of plant species for cleaning up contaminated sites.

    "One species, a pigweed called Amaranthus retroflexus, was up to 40 times more effective than others tested in removing radiocesium from soil. We were able to remove 3 percent of the total amount in just one 3-month growing season," says Kochian. "With two or three yearly crops, the plant could clean up the contaminated site in less than 15 years." . . .

    Rufus L. Chaney is at the USDA-ARS Environmental Chemistry Laboratory, Bldg. 007, 10300 Baltimore Ave., Beltsville, MD 20705-2350; phone (301) 504-8324, fax (301) 504-5048.

    "Phytoremediation: Using Plants To Clean Up Soils" was published in the June 2000 issue of Agricultural Research magazine.
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