Investigating mineral-metal-microbe interactions with hard x-ray radiation

Kenneth M. Kemner
Molecular Environmental Science Group
Argonne National Laboratory, Argonne, IL 60439

The microenvironment within, at, and adjacent to actively metabolizing cell surfaces can be significantly different from the bulk environment. The behavior of nutrient elements and contaminants in such microenvironments can affect fundamental biochemical and biogeochemical processes that influence the macroscopic fates of contaminants. Because biologically driven redox changes to metals and radionuclides result in changes in the oxidation state, chemical environment, or structural characteristics of the element of interest, x-ray absorption spectroscopy (XAS) techniques can provide significant insight into these transformations. Additionally, to adequately determine the oxidation state and chemical speciation of elements within microbes and at microbe-mineral interfaces, the dimensions of the x-ray probe must enable the majority of the x-rays to be positioned at these locations. Recent advances in synchrotron-based x-ray imaging and microspectroscopy have resulted in the development of a class of techniques that enable this. For example, elemental distributions in a microbial system can be mapped by using an x-ray fluorescence (XRF) microprobe, where the electron beam of an energy dispersive x-ray microanalysis experiment is replaced by an apertured or focused x-ray beam from a synchrotron source. With the aid of synchrotron x-ray sources, x-ray microprobe techniques can be combined with x-ray spectroscopic techniques by using a monochromatic x-ray beam to record x-ray absorption spectra at the absorption edges of the element of interest at specific locations in the sample. Because such spectra are sensitive to the chemical and structural environment of the element, such “spectroscopic imaging” experiments can substantially improve understanding of the system or process of interest. We have developed and used x-ray absorption spectroscopy and micro(spectro)scopy (~100 nm resolution) to investigate Fe and U biogeochemical transformations by a number of microbial species and communities. Results include 1) elemental analysis of single planktonic and surface-adhered cells (Pseudomonas fluorescens) and their tolerance to heavy metal exposure, 2) identification of the distribution and average valence state of U relative to cells (Shewanella oneidensis MR-1) adhered to iron hydroxide films, 3) identification of the co localization of U and Fe in extracellular structures produced by S. oneidensis MR-1, and 4) characterization of uranium transformations in biostimulated contaminated subsurface sediments. An explanation of synchrotron-based x-ray absorption spectroscopy, x-ray fluorescence microscopy, and their use in these studies will be presented.

This work was supported by the US Department of Energy (US DOE), Office of Science (OS), Biological and Environmental Research, Environmental Remediation Science Program. The Advanced Photon Source is funded by the US DOE OS Basic Energy Science Division. MRCAT/EnviroCAT is also supported by its member institutions.