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A Conceptual Model for the Influence of Biofuels on the Anaerobic Biodegradation of Benzene in Aquifer Sediment
Citation:
WILSON, JOHN T. AND C. J. ADAIR. A Conceptual Model for the Influence of Biofuels on the Anaerobic Biodegradation of Benzene in Aquifer Sediment. To be Presented at International Symposium on Bioremediation and Sustainable Environmental Technologies, Reno, NV, June 27 - 30, 2011.
Impact/Purpose:
Presentation for the International Symposium on Bioremediation and Sustainable Environmental Technologies - (Reno, NV; June 27 -30, 2011)
Description:
Background/Objectives: In some spills of petroleum hydrocarbons, benzene is degraded through a fermentation reaction to form carbon dioxide and methane. In ground water where the soluble electron acceptors such as oxygen, sulfate, or nitrate are depleted, and where iron(II) and manganese(IV) minerals are depleted, the fermentation reaction to methane is the important mechanism for natural attenuation of benzene and other BTEX compounds. During the fermentation of BTEX compounds, acetate and molecular hydrogen accumulate in the ground water. The fermentation reaction is less thermodynamically favorable at low concentrations of benzene, low pH, and high concentrations of acetate and hydrogen. To serve as a natural attenuation mechanism, the reaction must consume benzene at concentrations near the MCL for benzene. If the concentration of benzene is 5 μg/L, the concentration of acetate is 200 mg/l and the pH is 6.0, the reaction is not feasible at hydrogen concentrations greater than 1000 ppm by volume in a gas phase in equilibration with the ground water. Biofuels such as ethanol, n-propanol, n-butanol, and iso-butanol also undergo a fermentation reaction to produce acetate and hydrogen. During their anaerobic degradation, they may produce concentrations of acetate and hydrogen that make the fermentation of benzene unfeasible. If this is the case, the natural biodegradation of benzene will be delayed until the biofuels are consumed. Approach/Activities: Aquifer sediment from a fuel spill in Elizabeth City, North Carolina was amended with ethanol, n-propanol, nbutanol, and iso-butanol as well as BTEX compounds at low concentrations near the MCL for benzene. Concentrations of hydrogen and methane were monitored in the headspace of the microcosms, and concentrations of biofuels and BTEX compounds were monitored in the pore water. Results/Lessons Learned: As long as the biofuels were present in the porewater, the pH of the pore water varied form 5.6 to 6.3, and the concentration of hydrogen varied from 100 to 10,000 ppm. After the microcosms were fully acclimated to anaerobic biodegradation of the hydrocarbons, the concentrations of acetate accumulated to near 100 mg/L and the concentrations of hydrogen were generally near 1000 ppm in the headspace. In control microcosms that were amended with BTEX compounds but no biofuels, the BTEX compounds were degraded after four months. In microcosms with biofuels, there was little degradation of benzene or BTEX compounds for at least 11 months. It may be possible to predict whether benzene will degrade in ground water from concentrations of acetate and hydrogen. The Gibbs free energy for fermentation of the biofuels predicts that as long as the biofuel is available at concentrations of a few mg/L, the concentrations of hydrogen will make the fermentation of benzene thermodynamically unfeasible. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy. NOTE: Download of presentation slides is not available.