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(376) Simulating the biodegradation of toluene in sediments for aerobic and denitrifying conditions.
Jaffe, Peter*,1, Kim, Hyun-su1, 1 Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, USA
ABSTRACT- Heterotropic bacteria can degrade organic substrates utilizing different terminal electron acceptors. The sequence of electron acceptor utilization depends on the energy yield of the individual reaction pathway, and which decreases as the redox potential decreases. Due to these differences in energy yield, and an inhibiting activity of oxygen on some enzymatic processes, the simultaneous utilization of oxygen and nitrate as terminal electron acceptor may not occur for many degradation processes, unless the dissolved oxygen is below some threshold concentration. Sand column experiments, utilizing toluene as a carbon source, showed the simultaneous utilization of oxygen and nitrate as electron acceptors in regions where the oxygen concentration was significantly higher (1.1 mg/L) than the above mentioned threshold concentration. Results from aerobic and anaerobic plate counting analyses showed simultaneous growth of aerobes and denitrifiers in the zone of the column where simultaneous utilization of oxygen and nitrate was observed. From these observations, it was postulated that the porous media contained oxygen-free micro-locations where the denitrifiers were able to degrade the toluene. To simulate the observed dynamics, a dual biofilm model was implemented. This model formulation assumes that the biofilm is composed of two distinct layers, where the outer layer is colonized by aerobic bacteria and the inner layer by denitrifying bacteria. The thickness of the aerobic layer is such that oxygen is depleted at the boundary of these two layers, resulting in an oxygen-free microlocation that allows denitrification to proceed, even though oxygen is still present in the bulk-fluid phase. Model analyses indicated that changes in physical, chemical, and hydrologic parameters could change the length and location of the zone where oxygen and nitrate are utilized simultaneously.
Key words: toluene, aerobic, nitrate reducing, modeling
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