PHYTOREMEDIATION OF 1,4-DIOXANE BY HYBRID POPLARS
|E.W. Aitchison, J.L. Schnoor, S.L. Kelley, and P.J.J. Alvarez, Department of Civil and Environmental Engineering, 125 Engineering Research Facility, 330 South Madison Street, The University of Iowa, Iowa City, IA 52242||
1,4-dioxane, a suspected carcinogen, is widely used as a solvent in paints, varnishes, lacquers, cosmetics, and deodorants. It exists as a liquid at room temperature, is fully miscible in water, and is expected to be highly mobile in soil. Its half-life in soils and ground water is on the order of years, while its half-life in the atmosphere in the presence of NO and hydroxyl radicals is only 6.7-9.6 hours. Therefore, 1,4-dioxane volatilization into the atmosphere by plant transpiration could be a desirable result.
In this research we assessed the capacity of hybrid poplar trees (Populus deltoides nigra, DN34, Imperial Carolina) for uptake and translocation of 1,4-dioxane using l4C-labeled dioxane in hydroponic experiments. Plants can enhance the removal of xenobiotics by at least two mechanisms: (1) direct uptake and, in some cases, in-plant transformations to less toxic metabolites; and (2) stimulation of microbial activity and biochemical transformations in the root zone through the release of exudates and enzymes. Advantages of hybrid poplar trees as phytoremediation tools include:
Cuttings, identical male clones from adult Imperial Carolina hybrid poplar trees, were allowed to root hydroponically for approximately two weeks in an inorganic nutrient solution. Seven rooted cuttings were individually planted into 280 mL modified screw top culture flasks. Two of the planted cuttings were excised at the stem, thus preventing transpiration during the course of the experiment. These excised cuttings were used to determine 1,4-dioxane sorption to roots and degradation by root-associated microorganisms. The seven planted reactors, plus one non-planted control reactor, were then spiked with 1,4-dioxane and its ring-labeled equivalent to obtain an aqueous concentration of 20 ppm. After 8 days in a growth chamber, reactors were sacrificed for analysis of plant residues. To quantify the 14C activity in the plant, the biomass was separated into roots, leaves, and stems and weighed promptly.
1,4-dioxane was readily taken-up by the hybrid poplar tree cuttings from hydroponic solution. After 8 days, the following results were obtained:
The results indicate that degradation of 1,4-dioxane by indigenous root-zone micro-organisms is minimal in comparison to plant uptake. The majority of 1,4-dioxane taken up into the plant was volatilized (average = 77%), with the remaining mass concentrated primarily in the stem.
Rapid uptake of 1,4-dioxane by hybrid poplar trees makes phytoremediation appear as an attractive alternative at dioxane-contaminated sites. Further research will examine poplar removal of 1.4-dioxane from contaminated soil.
Key words: phytoremediation, 1,4-dioxane
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