XAFS and X-Ray and Electron Microscopy Investigations of Radionuclide Transformations at the Mineral-Microbe Interface 1 Ken Kemner, 1 Ed O’Loughlin, 1 Shelly Kelly, 1 Bruce Ravel, 1 Maxim Boyanov, 1 Deirdre Sholto-Douglas, 2 Barry Lai, 3 Russ Cook, 4 Everett Carpenter, 5 Vince Harris, 6 Ken Nealson (1) Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439, (2) Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, (3) Electron Microscopy Center, Argonne National Laboratory, Argonne, Illinois 60439, (4) Virginia Commonwealth University, Richmond, Virginia 23284, (5) Northeastern University, Boston, Massachusetts 02115, (6) University of Southern California, Los Angeles, California 90007 Abstract. The microenvironment at and adjacent to surfaces of actively metabolizing cells, whether in a planktonic state or adhered to mineral surfaces, can be significantly different from the bulk environment. Microbial polymers (polysaccharides, DNA, RNA, and proteins), whether attached to or released from the cell, can contribute to the development of steep chemical gradients over very short distances. It is currently difficult to predict the behavior of contaminant radionuclides and metals in such microenvironments, because the chemistry there has been difficult or impossible to define. The behavior of contaminants in such microenvironments can ultimately affect their macroscopic fates. We have successfully performed a series of U L III edge x-ray absorption fine structure (XAFS) spectroscopy, hard x-ray fluorescence (XRF) microprobe (150 nm resolution), and electron microscopy (EM) measurements on lepidocrocite thin films (~1 micron thickness) deposited on kapton films that have been inoculated with the dissimilatory metal reducing bacterium Shewanella oneidensis MR-1 and exposed to 0.05 mM uranyl acetate under anoxic conditions. Similarly, we have performed a series of U L III edge EXAFS measurements on lepidocrocite powders exposed to 0.05 mM uranyl acetate and exopolymeric components harvested from S. oneidensis MR-1 grown under aerobic conditions. These results demonstrate the utility of combining bulk XAFS with x-ray and electron microscopies. Keywords: XAFS, uranium, x-ray microscopy, biogeochemistry PACS: 91.62.+g INTRODUCTION Uranium exists in most oxidizing environments as U(VI) in the uranyl ion (UO 2 2+ ), which is generally water soluble. An exception, uranyl phosphate, is typically highly insoluble. Uranium is also often found as U(IV) in highly insoluble uraninite (UO 2 ) in reducing environments such as groundwater. Microorganisms can affect the mobility of uranium and other metals via electron transfer reactions resulting from microbial respiration. They can also affect the mobility of uranium via production of a variety of exopolymeric substances (EPS). Although EPSs may remain attached to the cell membrane (capsular material), many are freely released from the bacteria. Because they typically have a net negative charge, exuded EPS can travel relatively unimpeded through porous media in which many mineral surfaces are negatively charged. However, some functional groups associated with EPS can also dissolve mineral surfaces, complex cations, and reincorporate the ions into solution. These processes affect the chemistry of elements that are constituents of or are sorbed to the solid phase. In addition to affecting the bioavailability of contaminant elements, the presence or absence of EPS can affect the bioavailability of macro- and micronutrients, an influence ultimately manifested in changes in the metabolic state of a microbial species. The interactions of metal contaminants with geosurfaces and with EPS have been studied, but very little work has been done to directly probe the interaction of a contaminant metal with the microbe- geosurface interface. A better understanding of this