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Biogeochemical Processes Controlling Microbial Reductive Precipitation of Radionuclides

Jim Fredrickson, Principal Investigator

Anaerobic Biotransformation and Mobility of Pu and of Pu-EDTA, poster.
Biogeochemical Processes Controlling Microbial Reductive Precipitation of Radionuclides. Download poster [PDF, 1.15MB].

Uranium (U) and technetium (Tc) are common subsurface contaminants on U.S. Department of Energy (DOE) sites. These elements are problematic because they exist as highly soluble and mobile ions - U as U(VI)-carbonate complexes and Tc as Tc(VII)04- in oxidized groundwaters. Dissimilatory metal-reducing bacteria can reduce U(VI) and Tc(VII) to insoluble UO2 and TcO2 phases, respectively. Bioreductive immobilization of these contaminants offers considerable promise for in situ remediation of contaminated DOE sites. A major focus of the Environmental Remediation Sciences Program (ERSP) program is on "understanding the role of microorganisms in long-term immobilization of contaminants in place, and the potential for their remobilization." Considerable emphasis has been placed on defining the specific biological and biogeochemical mechanisms of contaminant reduction within the NABIR program, yet relatively little is known regarding the biological factors controlling the form and behavior of the bioreduced contaminants.

Our previous NABIR research findings demonstrated that UO2 nanoparticles are present in the cell periplasm of metal-reducing Shewanella species 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 transported from the periplasm via a protein secretion pathway that is common to many Gram-negative bacteria. This is a rather remarkable finding and the first report we are aware of describing the transport of a solid phase across the outer membrane of bacteria. This process has important implications for the long-term immobilization of contaminants and potential for their remobilization. Once nanoparticles are secreted from cells, they may be subject to a variety of processes, including oxidation and transport. The Biogeochemical Processes Controlling Microbial Reductive Precipitation of Radionuclides project team at Pacific Northwest National Laboratory (PNNL) is focused on defining the mechanisms by which Gram-negative, metal-reducing bacteria export contaminant nanoparticles from the periplasm and on the subsequent biogeochemical behavior of the externalized nanoparticles.

Information regarding the Biogeochemical Processes Controlling Microbial Reductive Precipitation of Radionuclides project was obtained from its ERSP project page.

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