HR: 18:05h
AN: NS44A-06    [Abstracts]
TI: Electrical Signatures Associated with Abiotic and In Vitro Dissimilatory Iron Reduction
AU: * Regberg, A B
EM: aregberg@geosc.psu.edu
AF: The Pennsylvania State University, 302 Hosler Bldg, University Park, PA 16802, United States
AU: Brantley, S L
EM: brantley@eesi.psu.edu
AF: The Pennsylvania State University, 2217 EARTH & ENGR SCIENCES, University Park, PA 16802, United States
AU: Singha, K
EM: ksingha@geosc.psu.edu
AF: The Pennsylvania State University, 0311 DEIKE BLDG, University Park, PA 16802, United States
AU: Tien, M
EM: mxt3@psu.edu
AF: The Pennsylvania State University, 303 ALTHOUSE LAB, University Park, PA 16802, United States
AB: Several researchers have described anomalous electrical signatures associated with bacterial activity in anoxic zones in aquifers containing organic contaminants. It is thought that these signals can be attributed to (bio)geochemical changes caused by the oxidation of organic contaminants and the reduction of associated species like iron oxides. We report laboratory observations of changes in electrical conductivity (EC) that can be attributed to specific (bio)geochemical reactions involving reductive dissolution of iron oxides enzymatically and nonenzymatically. Abiotic reduction of ferrihydrite by ascorbic acid in batch experiments causes a cumulative 20- 40% increase in measured conductivity, (EC increases by ~300 μ S/cm). This change can be attributed to a decrease in conductivity (Δ EC) from increasing proton activity (Δ pH = 3.25 --> 5.07, Δ EC = -200 μ S/cm) and an increase in dissolved Fe(II) (Δ [Fe] = 2.2 - 3.3 mM, Δ EC = 400 -700 μ S/cm). Conductivity is presumably unaffected by Fe(II) sorbed to the ferrihydrite. Rates calculated from this method are comparable to literature rates for similar experiments. In a similar in vitro system, total membrane fractions from Shewanella oneidensis MR-1 were used to reduce ferrihydrite in the presence of formate and HEPES buffer. A 10 - 15% increase in conductivity was observed in the batch experiment (Δ EC = ~280 μ S/cm). This Δ EC is attributed to an increase in the concentration of de-protonated HEPES as well as carbonate ion as formate is oxidized. Fe(II) released in this system is quickly sorbed onto the ferrihydrite surface and is not thought to change conductivity. Despite the sorption of iron in these in vitro experiments, conductivity changes measurably and documents the rate of the reaction. Accessory changes like buffer de- protonation play an important role in interpreting the electrical signals caused by dissimilatory iron reduction. In order to accurately interpret field data it is necessary to anticipate these changes and attempt to monitor them chemically. Through this work we hope to link chemical changes caused by bacterial activity in the lab to electrical anomalies measured in the field. By quantifying the changes in conductivity, we will investigate rates and distributions of bacterial activity at the field scale.
DE: 0416 Biogeophysics
DE: 1009 Geochemical modeling (3610, 8410)
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly