Superfund Research Program
The Use of Poplar Trees to Remediate Chlorinated Organic Contaminants
In recent years a great deal of interest has developed in the use of trees to clean up the chemicals found at hazardous waste sites. As a result of natural processes fueled by solar energy, trees offer great potential for environmentally sound and cost-effective clean-up of contaminated soil and groundwater. What makes trees so useful are their root systems - some tree species penetrate large volumes of soil in search of water. In addition to drawing water and nutrients out of the ground, some trees also take up organic contaminants and metabolize them to harmless products, providing the foundation for a biologically based method of pumping organic pollutants out of soil and water, and degrading them by tree enzymes.
This promising technology - known as phytoremediation - is still relatively young and in need of more research, especially in its use for treating organic pollutants. Scientists at the University of Washington are exploring the use of hybrid poplar trees to clean-up soil and water contaminated with chlorinated solvents. Recently the researchers used a hybrid poplar tree that grows at a remarkable rate - up to 10-15 feet per year - and has the ability to remove and degrade trichloroethylene (TCE) and certain other chlorinated organic solvents from soil and water. To pave the way for implementing this new technology in the field, the researchers carried out laboratory studies that assessed the capability of the hybrid poplar to metabolize TCE to innocuous products. Greenhouse studies were also conducted to determine if the tree could withstand the levels of TCE that are typically found on hazardous waste sites.
In initial laboratory experiments the scientists demonstrated that axenic poplar tissue cultures free of competing bacteria, microflora, and microfauna are capable of degrading TCE to carbon dioxide and salts. These findings are an indication of how a poplar tree would treat the TCE it took up through its root system. Further studies with cuttings from the hybrid poplar tree, Populus deltoides X P. trichocarpa, showed that TCE was not toxic to the trees, even at concentrations much higher than those usually found at hazardous waste sites. Building upon these successes in the laboratory, the scientists developed experimental, outdoor test plots with young hybrid poplars to test this tree's ability to work in the field.
A three-year field trial was conducted in collaboration with Occidental Chemical Corporation in Fife, Washington to test the hybrid poplar's efficiency in taking up TCE from groundwater, as well as the tree's ability to process the toxic compound without releasing harmful by-products into the environment. The field trial set-up utilized a series of polyethylene-lined plots of ground (cells) that were exposed on an ongoing basis to TCE-contaminated water. Four cells were planted with rooted cuttings of hybrid poplars; two of these cells were dosed daily with water containing about 15 ppm TCE, and the other two cells received only water. A fifth cell, which did not have trees and served as a non-vegetated control, also received water containing the same amount of TCE that was added to the test cells.
The water, soil, trees and transpired gases were monitored to track the fate of TCE. In the cells containing trees, only about 2 to 4 % of the TCE remained in the effluent as compared to 68% in the non-vegetated control cell. The field trial demonstrated that over 95 % of the added TCE was removed from the vegetated cells simply by planting the trees and letting them grow. Moreover, the trees did not release TCE into the air, as no measurable TCE was detected in the air immediately surrounding the leaves or in the general atmosphere above the tree canopy. Further studies are ongoing to confirm that the trees are completely degrading TCE to chloride salts.
Similar results were obtained with the carcinogenic solvent, carbon tetrachloride. About 95% of the material was removed from the irrigation water of the test cells and no carbon tetrachloride was detected in the trees' transpirate. These results suggest that this approach to bioremediation may be broadly applicable to a variety of common soil and groundwater contaminants.
These studies show that the use of hybrid poplars can be considered as a low cost alternative or supplement to current methods of remediation for chlorinated organic solvents, which are among the most common soil and groundwater contaminants at hazardous waste sites across the country. The use of these poplar trees as a biological method for treating contaminated soil and water offers a number of advantages over conventional technologies: 1) the trees are highly effective at removing and degrading pollutants from soil and water fueled only by "solar-power"; 2) the technology is inexpensive to implement and maintain and may even produce aesthetic benefits; 3) this approach serves as a "green solution" to pollution abatement and has high public approval.
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To learn more about this research, please refer to the following sources:
- Gordon MP, Choe N, Duffy J, Ekuan G, Heilman PE, Muiznieks IA, Newman LA, Ruszaj M, Shurtleff BB, Strand SE, Wilmoth J. 1997. Phytoremediation of Trichloroethylene with Hybrid Poplars. In: Phytoremediation of Soil and Water Contamination: ACE Symposium Series 664. American Chemical Society, pp.177-185.
- Newman LA, Strand SE, Choe N, Duffy J, Ekuan G, Ruszaj M, Shurtleff BB, Wilmoth J, Heilman PE, Gordon MP. 1997. Uptake and biotransformation of trichloroethylene by hybrid poplars. Environ Sci Technol 31(4):1062-1067.
- Schmiedeskamp M. 1997. Pollution-purging poplars: trees that break down organic contaminants. Scientific American December:46.
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