Biogeochemical Mechanisms Controlling Reduced Radionuclide Particle Properties and Stability
Sponsor: DOE Office of Biological and Environmental Research Natural and Accelerated Bioremediation Research Program (NABIR)
Contacts: Jim Fredrickson
Uranium and technetium, common subsurface contaminants on DOE sites, are problematic because they exist as highly soluble and mobile ions, U as U(VI) carbonate complexes and Tc as Tc(VII)O4-, in oxidized groundwaters. Laboratory research has demonstrated that dissimilatory metal-reducing bacteria (DMRB) can effectively reduce U(VI) and Tc(VII) to insoluble UO2 and TcO2 phases. Bioreductive immobilization of these contaminants offers considerable promise for in situ remediation of contaminated DOE sites. A major focus of the NABIR program is on "understanding the role of microorganisms in long-term immobilization of contaminants in place, and the potential for their remobilization".
Our previous NABIR research findings demonstrated that UO2 nanoparticles are present in the cell periplasm of metal-reducing Shewanella sp. and that localization of UO2 to this compartment protects U(IV) against oxidation by Mn(III,IV) oxides. More recently, we have established that UO2 and TcO2 particles of relatively uniform nanometer-size are present on the outside of the cells either as a result of reduction by externalized bioreductants or transported from the periplasm, or both. Preliminary investigations with a genetic mutant of MR-1 suggest that a protein secretion pathway that is common to many Gram-negative bacteria plays a critical role in the formation of the extracellular reduced nanoparticles. This is a rather remarkable finding suggesting either the transfer of electrons directly to U(VI) outside of the cell and/or transport of a solid phase across the outer membrane of bacteria. This finding has important implications for the long-term immobilization of contaminants and potential for their remobilization. External nanoparticles may be subject to a variety of processes including oxidation and transport. This research will focus on defining the mechanisms by which Gram-negative metal-reducing bacteria form external U and Tc nanoparticles and on their subsequent biogeochemical behavior.