HR: 17:15h
AN: B24D-06    [Abstracts]
TI: Assessing the Geochemical Reactivity of Fe-DOM Complexes in Lacustrine Sediments Using Nitroaromatic Probe Compounds
AU: * Hakala, J
EM: hakala.2@geology.ohio-state.edu
AF: Division of Global and Environmental Change, School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, United States
AU: Fimmen, R L
EM: fimmen.2@osu.edu
AF: Division of Global and Environmental Change, School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, United States
AU: Chin, Y
EM: yo@geology.ohio-state.edu
AF: Division of Global and Environmental Change, School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, United States
AU: Agrawal, S G
EM: agrawal.45@osu.edu
AF: Division of Global and Environmental Change, School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, United States
AU: Ward, C P
EM: ward.518@osu.edu
AF: Division of Global and Environmental Change, School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, United States
AB: The complexation environment of Fe in anoxic sediment porefluids is important for understanding its role in biogeochemical reactions in these systems. These include the microbial dissolution of iron-bearing sediments and the degradation of hydrophobic pesticides. We studied the effect of naturally occurring Fe-DOM complexes on the reduction of two nitroaromatic compounds (NACs) as a surrogate for probing electron-transfer kinetics, and monitored the geochemical parameters affecting reactivity. We found that Fe(II) was necessary for rapid NAC reduction (< 24 hr), and observed faster reduction with increased pH. NAC reduction in `pH-adjusted' porefluids (acidified to pH 2.5 after porefluid extraction and raised to the native pH (between 6.5 to 7.8) prior to reaction) was similar to that observed in model systems containing Fe(II) and aquatic fulvic acids. Conversely, NAC reduction in fresh, unaltered porefluids was slower than that observed in `pH-adjusted' porefluids, indicating that the natural metal redox speciation differs between fresh and `pH-adjusted' samples. Electrochemical scans demonstrate a vertical gradient in concentrations of Fe(III), Fe(II), and Mn(II) from 0 to 30 cm depth. Furthermore, the electrochemical data indicate that ferric iron in these systems is predominantly complexed with organic ligands, with a half-cell potential of -0.45 V. The magnitude of this ferric-organic peak decreases when porefluids are pH- adjusted for short durations (acidified to pH 2.5 and re-raised to circumneutral levels) and disappears completely when the acidified porefluid is allowed to equilibrate overnight. These data show that pH-adjustment of porefluids presumably alters both their complexation chemistry and reactivity towards NACs, and shows how small changes in Fe complexation chemistry potentially affects electron transfer reactions in anoxic environments.
DE: 0400 BIOGEOSCIENCES
DE: 0404 Anoxic and hypoxic environments (4802, 4834)
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 0461 Metals
DE: 0497 Wetlands (1890)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting