Email: fsrjr5@uaf.edu

My research involves the characterization of the passivating layer that forms on magnetite upon the reduction of heavy metal contaminants using the potentiostat based techniques of electrochemical impedance spectroscopy (EIS) and voltametry. EIS is used to characterize the interface that forms between an electrode, in this case magnetite, and an electrolytic solution in an electrochemical cell. In EIS, Bode and Nyquist plots are used to model the electrode-solution interface as an electrical circuit where components of the electrochemical cell are modeled as impedances.

Magnetite (Fe3+2Fe2+)O4, is a very common mineral in the environment, found in conditions including sands present in reducing groundwaters (1), deep sea sediments, and tops soils due to the activity of magnetotatic bacteria (MTB) (2). Not only is magnetite ubiquitous in many environmental systems, it is one iron oxide that largely dictates the fate and transport of trace metal and organic contaminants. Magnetite is electrochemically active, taking part in many relevant geochemical reactions because of the high conductivity of the crystal and the presence of Fe2+ contained within the lattice that can be readily oxidized. For instance, Cr(VI) is a mobile and toxic metal which can potentially contaminant groundwaters. Magnetite is electrochemically active in anoxic reducing conditions such as aquifers, and therefore is a prime reactive substrate for reducing Cr(VI) species to a less mobile and less toxic Cr(III) species (3).

It is proposed in literature that the surface of magnetite is oxidized to maghemite during redox reactions where trace levels of contaminants are reduced. The topotactic transformation of magnetite to maghemite at the mineral-aqueous interface is thought to create a passivating layer over the crystal. This newly formed layer insulates magnetite, enabling it to continue reducing contaminants (4). Numerous studies have focused on understanding the kinetics and proposing reaction mechanisms for the redox reactions of magnetite with trace contaminant species such as transition metals (1), Cr(VI) (3), TcO4- (5), and chlorinated ethylenes (6).

While many valuable studies on the reactivity of magnetite have been completed, surface structure, which is a major variable, has not been included into characterization of the reactivity and the proposition of mechanisms. Also, confirmation of the formed maghemite layer using experimental techniques sensitive to atomic-scale structure has not been completed. The types of heterogeneous redox reactions previously described take place at mineral-solution interfaces such that the composition of the exposed reactive structure will dictate the types of interactions and reactions that take place at the interface. The surface termination of magnetite is complex and is dependent on many solution conditions including pH and Eh. The reactivity of magnetite must be studied at the atomic-level to fully understand the mechanisms taking place at the mineral-solution interface.

My study is a complimentary to the structural characterization being completed by Dr. Sarah Petitto. Dr. Petitto works on characterizing the surface structure of the magnetite (100), (110), and (111) terminations using crystal truncation rod (CTR) diffraction. CTR is a powerful synchrotron X-ray based reflectivity technique used to model the atomic-scale surface termination of well ordered crystalline matter in 3-D space. I utilize EIS and other techniques to characterize the chemical aspects of magnetite reactivity in terms of the passivating layer, whereas Dr. Petitto utilizes CTR to characterize the structural aspects. These are two complimentary approaches with the combined goal of elucidating the role of magnetite in the reduction of trace levels of contaminant species in the environment.

References

1. White, A. F.; Peterson, M. L. Geochim. et Cosmochim. Acta. 1996, 60(20), 3799-3814.
2. Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure, Properties, Reactions,Occurrences and Uses, 2nd Ed; Wiley-Vch: Weinheim, 2003
3. Peterson, M. L.; White, A. F.; Brown, G. E.; Parks, G. A. Emviron. Sci. Technol. 1997, 31(5), 1573-1576.
4. Jolivet, J.-P.; Tronc, E. J. Col. and Inter. Sci. 1988, 12(2), 688-701.
5. Cui, D.; Eriksen, T. E. Environ. Sci. Technol. 1996. 30(7), 2263-2269.
6. Lee, W.; Batchelor B. Environ. Sci. Technol. 2002, 36, 5147-5154.