BIOAUGMENTATION WITH ACTlNOMYCETES CB1190 TO ENHANCE PHYTOREMEDIATION OF 1.4-DIOXANE
|S.L. Kelley, P.J.J. Alvarez, E.W. Aitchison, and J.L. Schnoor, Department of Civil and Environmental Engineering, 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 has a boiling point of 101°C at 1 atm and a melting point of 11.8°C, so it exists as a liquid at room temperature and is fully miscible in water. It has a low Henry's Law Constant of
4.88 x 10-6 atm-m3/mol, but a moderate vapor pressure of 38.0 mm Hg at 25°C. Thus it has a tendency to volatilize (slowly) from dry soils. Dioxane is one of the most recalcitrant xenobiotics in subsurface environments. Its half-life in soils and ground water is on the order of years. The log Kow of 1,4-dioxane is -0.27, which indicates a very low partitioning into the organic carbon fraction of soil. Therefore, it is expected to be highly mobile in soil.
Phytoremediation, the use of plants to remove environmental pollutants from contaminated sites, shows great promise as an approach to hazardous waste management. Plants can enhance the removal of xenobiotics by at least two mechanisms: (1) direct uptake and, in some cases, in-plant transformation to less toxic metabolites; and (2) stimulation of microbial activity and biochemical transformations in the root zone through the release of exudates and enzymes. This latter mechanism, however, is not very effective for removing recalcitrant xenobiotics, such as 1,4dioxane. To this end, bioaugmentation of the rhizosphere with specialized microorganisms might enhance the biofiltration capabilities of the root zone. In this research, soil microcosms were used to study the potential for bioaugmentation with Actinomycetes CB 1190 to enhance bioremediation of 1,4-dioxane in the poplar (Populus sp.) rhizosphere. Emphasis was placed on evaluating the effect of root exudates on biodegradation of 1,4-dioxane.
Microcosms were prepared in 250 mL serum bottles capped with Mininert valves. The microcosms contained 20 g of Nodeway soil from a site near Amana, Iowa (2% organic matter), and 200 mL of basal salts medium (BSM) spiked with 1-4,dioxane (100 mg/L). The 1,4-dioxane degrading Actinomycetes CB 1190 that was used for bioaugmentation studies was provided by Rebecca Parales, Department of Microbiology, The University of Iowa. The Actinomycetes CB 1190 was grown in basal salts medium broth on 1000 mg/L tetrahydrofuran (THF), a compound with a similar structure that is preferred by the CB 1190. Dioxane degradation was monitored by gas chromatography.
CB 1190 degraded 100 mg/L 1,4-dioxane in incubations without soil within one month. These incubations were used as positive controls to ensure the viability of CB 1190 and to obtain baseline data for biodegradation kinetics in the absence of soil and root exudates. Dioxane was not removed in sterile controls or in viable microcosms not amended with CB 1190, indicating that the indigenous soil population did not have the ability to degrade 1,4-dioxane. All microcosms amended with CB 1190 degraded 1,4-dioxane, suggesting the feasibility of bioaugmentation to degrade this recalcitrant compound. The presence of soil reduced the degradation rate relative to the positive controls in mineral medium. This is a common observation with the biodegradation of organics often attributed to reduced bioavailability. Interestingly, root extract (poplar roots ground with a mortar and pestle and filtered through 0.22 (m membrane filter) stimulated degradation of 1,4-dioxane in soils, and 100 mg/L of the 1,4-dioxane was removed within 45 days. This suggests that the rhizosphere might be a favorable environment for 1,4-dioxane biodegradation.
Rapid uptake of 1,4-dioxane by hybrid poplar trees coupled with Actinomycetes CB 1190 degradation makes phytoremediation an attractive alternative at dioxane-contaminated sites. Bioaugmentation of the poplar rhizosphere with CB 1190 may enhance soil transformation of 1,4-dioxane in such applications.
Key words: biodegradation, bioaugmentation, 1,4-dioxane, phytoremediation, root exudates
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