HR: 0830h
AN: NS51B-05    [Abstracts]
TI: Proton Nuclear Magnetic Resonance: A Novel Approach for Monitoring In Situ Iron Mineralization Processes.
AU: * Keating, K
EM: kkeat@pangea.stanford.edu
AF: Geophysics Department, Stanford University, Mitchell Earth Sciences Building, Room 360 397 Panama Mall, Stanford, CA 94305-2251 United States
AU: Knight, R
EM: knight@stanford.edu
AF: Geophysics Department, Stanford University, Mitchell Earth Sciences Building, Room 360 397 Panama Mall, Stanford, CA 94305-2251 United States
AU: Borch, T
EM: borch@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall Braun Hall, Building 320 , Stanford, CA 94305-2115 United States
AB: Transformation or dissolution of iron minerals resulting from (a)biotic reduction can have a pronounced impact on the fate and transport of nutrients and contaminants in terrestrial environments. Accordingly, non-invasive in situ measurements of such geochemical reactions are of significant interest. Proton nuclear magnetic resonance (NMR) relaxation time measurements can be used to probe the molecular-scale physical and chemical environment of water in the pore-space of geological materials. In this study, we present a novel method based on proton nuclear magnetic resonance (NMR) relaxation time measurements for in situ monitoring of iron mineralization processes. Laboratory NMR measurements were used to monitor changes in the chemical (oxidation state) and mineralogical form of iron oxides during abiotic reduction reactions by ferrous iron. Specifically, columns packed with ferrihydrite-coated quartz sand (1 wt % Fe) were reacted with anaerobic media containing ferrous iron under advective flow conditions at circum neutral pH. The abiotic reduction of ferrihydrite with aqueous ferrous ion has previously been shown to result in the intermediary precipitation of goethite and lepidocrosite followed by dissolution and re-precipitation of magnetite (Hansel et al., 2003). The NMR relaxation measurements, obtained over a 24 hour period, were shown to be very sensitive to changes in the mineralogical form of iron minerals. A 60 % decrease in the relaxation time was observed as ferrihydrite converted to magnetite which is in agreement with NMR control studies of ferrihydrite - magnetite mixtures. Intermediary mineral precipitation was indicated by an increase in the relaxation time following the introduction of ferrous iron to the system. These results demonstrate the potential of NMR field instruments as an in situ method for monitoring geochemical reactions.
DE: 0400 Biogeosciences
DE: 0614 Biological effects
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly