Evolution of Fe(II) Mineralization Under Hydrodynamic Conditions Induced by Dissimilatory Iron-Reducing Bacteria
EMSL Project ID
2359
Abstract
The reducing capacity of soils and sediments for contaminants will be dictated, in part, by the Fe(II) bearing solids produced by dissimilatory iron reducing bacteria (DIRB). A primary factor controlling the nature of the resultant Fe(II) phase is the Fe(II) supply rate and concentration, and its subsequent surface reactions. As such, the nature and reactivity of these products should differ when a flow (solute transport) component is introduced. Accordingly, we investigated Fe solid phase transformations upon microbial iron reduction under hydrodynamic (flow) conditions. Experiments were conducted using packed mineral beds of a common, iron-reducing bacterium (Shewanella putrefaciens, strain CN32) and ferrihydrite-coated silica within an artificial groundwater matrix (PCO2=0.02 atm, pH=7) supplemented with ~3 mM lactate. Solids were characterized as a function of time and flow path (2-cm intervals) using x-ray absorption fine structure (XAFS) spectroscopy. Mineral morphology and spatial relationships between microbes and minerals were resolved using epifluorescence, high resolution transmission electron (HRTEM) and field emission scanning electron (FESEM) microscopies. According to XAFS results, microbial Fe(III) reduction within an artificial groundwater matrix under hydrodynamic flow conditions results in the conversion of ferrihydrite to goethite and magnetite. Sequestration of Fe(III) within goethite and magnetite results in decreased Fe(III) bioavailability and subsequent decreased rates of microbial reduction. On the basis of HRTEM images, magnetite precipitation occurs via topotactic conversion of ferrihydrite and is correspondingly associated with the ferrihydrite surface. Conversely, goethite is formed via dissolution/reprecipitation and is concentrated on the cell envelope. Mössbauer spectroscopy has proven to be an excellent means for determining the oxidation state and coordination of iron in heterogeneous material. We propose to use this technique to support our XAS analyses of Fe, providing confirmation of the presence of goethite, ferrihydrite, and magnetite throughout the columns and over time. Due to the complexity of XAS Fe spectra, confirmation of XAS results using Mössbauer spectroscopy is essential. We propose to collaborate with Dr. Ravi K. Kukkadapu in performing Mössbauer analyses at the Environmental Molecular Science Laboratory (EMSL), Pacific Northwest National Laboratory.
Project Details
Project type
Exploratory Research
Start Date
2001-06-01
End Date
2002-09-05
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
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