Mössbauer Spectroscopy and SEM/TEM Study of the
Reductive Dissolution of Fe-Oxyhydroxides in Porous Media
EMSL Project ID
25675
Abstract
The microbially-mediated reductive dissolution of Fe(III)-oxyhydroxides plays an important role in regulating the redox state of subsurface soils and aquifers, a critical control on the mobility of redox-sensitive elements (U, Tc, Np) found as contaminants at many of the DOE sites (Lovley, 1997; Liger et al., 1999; Stumm et al., 1992). In order to further improve our understanding in this respect, we propose to study the kinetics of reductive dissolution of iron oxides and their transformation in well controlled experiments under active flow conditions using capillary tubes (100~250 m ID) with Fe oxyhydroxides-coated silica/borosilicate microspheres (20~180 m diameter). The proposed experiments will allow us to take full advantage of modern spectroscopic techniques to characterize the secondary mineral phases inside the anaerobic environment of the capillary tubes. By achieving a better understanding of these kinetic processes at the fine scale, it will be possible to apply these reaction rates to natural porous media so as to address the critical questions associated with contamination on the DOE sites.Experiments will consist of a combination of abiotic and biotic experiments. Abiotic experiments will rely primarily on the injection of ferrous sulfate or chlorite along the lines of the experiments conducted by Hansel et al. (2003). In these experiments, the aging (or "Ostwald ripening") of ferrihydrite to goethite and lepidocrocite was observed at relatively low Fe(II) concentrations (sub-millimolar), while secondary magnetite is observed at higher concentrations. Biotic experiments will be of two kinds: 1) "pseudo-biotic" experiments using AH2DS (the microbially-reduced form of the electron shuttle AQDS), and 2) experiments using actual Fe-reducing bacteria (most likely Geobacter). The characterization of the reaction products in the capillary tubes will be carried out as a post-mortem experiment. Particular emphasis will be on characterization and quantification of the secondary reaction products (goethite, lepidocrocite, magnetite, green rusts). Mössbauer spectroscopic analysis is well suited for this purpose, since it can provide information on the valence state, coordination number, crystal field strengths and magnetic ordering temperatures and is especially well suited for the analysis of Fe containing minerals with the advantage of greater sensitivity and applicability to poorly crystalline or amorphous materials over other traditional analytical techniques (Bancroft 1973; Dyar, et al., 2006). Fe-57 will be used to enhance the sensitivity to Mössbauer in several ways
• to prepare ferrihydrite for pseudo-biotic and biotic experiments in which various Fe-bearing secondary phases will form, and
• to prepare the injection solution for abiotic Fe(II) experiments, in combination with ferrihydrite prepared from isotopically "normal" Fe, so as to track the fate of sorbed Fe(II).
In addition, scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM) will be used as complementary analytical techniques for verification of the identity and morphology of secondary phases formed.
The project will involve the collaboration of EMSL scientists Dr. Ravi Kukkadapu for Mössbauer, Dr. Bruce Arey for SEM, and Dr. Chongmin Wang for TEM.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2007-06-01
End Date
2008-09-26
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members