Q-294. Electrode-Based Approaches for Monitoring the in Situ Activity of Geobacter species during Bioremediation of Uranium-Contaminated Groundwater

K. H. Williams1,2, A. Franks3, K. P. Nevin3, M. J. Wilkins1, H. Elifantz3, P. J. Mouser3, L. N'Guessan3, P. E. Long4, D. R. Lovley3;
1Univ. of California, Berkeley, CA, 2Lawrence Berkeley Natl. Lab., Berkeley, CA, 3Univ. of Massachusetts, Amherst, MA, 4Pacific Northwest Natl. Lab., Richland, WA.

Experiments at the Rifle Integrated Field Challenge (IFC) site have demonstrated the ability to remove uranium from groundwater by stimulating the growth and activity of Geobacter species through acetate amendment. Prolonging the activity of these strains has prompted the development of in situ monitoring methods diagnostic of their activity. We have addressed this issue by developing an electrode-based sensor patterned after microbial fuel cells. We hypothesized that subsurface electrodes could be used to monitor rates of acetate oxidation, as evidenced by electron transfer to electrodes by anodophilic microbes, such as Geobacter species. During a recent field experiment, graphite electrodes serving as anodes were installed in boreholes downgradient from a region of acetate injection. Cathodes consisted of air-coupled graphite electrodes embedded at the ground surface and connected to the borehole electrodes via 560-Ohm resistors. Downgradient increases in current density (max. 50 mA m-2) tracked the delivery of acetate to the electrodes, whereas no change in current density was observed for an upgradient control. The removal of acetate accompanying a groundwater flush and the cessation of injection resulted in rapid decreases in current density. Little or no correlation existed between current density and the accumulation of reduced metabolic end products (e.g. Fe2+ and HS-) in the vicinity of the electrodes. Elevated current densities corresponded to the period of optimal uranium removal, with sustained values of 35-40 mA m-2 corresponding to the time when uranium concentrations were at or below 0.127 µM. Confocal laser microscopy of an electrode recovered during peak current density revealed a firmly attached biofilm ranging in thickness from 50-100 µm. Microbial community analysis of the same electrode detected sequences dominated by Geobacter strains (67%), along with a lower percentage mixture of gamma, beta, and alpha-proteobacteria and firmicutes. These results indicate the utility of electrode-based approaches for monitoring the in situ activity of Geobacter species during uranium bioremediation.