Effect of Mn(II) on the Structure and Reactivity of Biogenic Uraninite

Abstract

The efficacy of a site remediation strategy involving the stimulation of microbial U(VI) reduction hinges in part upon the long-term stability of the product, biogenic uraninite, toward environmental oxidants. Geological sedimentary uraninites (nominal formula UO 2 ) reportedly contain abundant cation impurities that enhance their resistance to oxidation. By analogy, incorporation of common groundwater solutes into biogenic uraninite could also impart stability-enhancing properties. Mn(II) is a common groundwater cation, which has a favorable ionic radius for substitution reactions. The structure and reactivity of Mn(II)-reacted biogenic uraninite are investigated in this study. Up to 4.4 weight percent Mn(II) was found to be structurally bound in biogenic uraninite. This Mn(II) incorporation was associated with decreasing uraninite particle size and structural order. Importantly, the equilibrium solubility of Mn-reacted uraninite was halved relative to unreacted uraninite, demonstrating changes in thermodynamic properties, while the dissolution rate was up to 38-fold lower than that of unreacted biogenic uraninite. We conclude that structural incorporation of Mn(II) into uraninite has an important stabilizing effect, leading to the prediction that other groundwater solutes may similarly stabilize biogenic uraninite.

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

“…Similarly, biogenic UO 2 was produced via the enzymatic reduction of U(VI) by S. putrefaciens and G. sulfurreducens in the absence of Fe(III) and shows the same salient features as that of nanoparticulate UO 2 produced by biogenic magnetite in this study and reported elsewhere Bargar et al, 2008;Sharp et al, 2009;Veeramani et al, 2009).…”

“…the number of U atoms bound at the surface of the particle becomes comparable to the number of U atoms within the interior of the particle, resulting in a decrease in the average U-U CN (4.1-6.1), which is consistent with EXAFS in the present study. This under-coordination is consistent with the nanoparticulate size and consequential high proportion of surface-exposed atoms (O'Loughlin et al, 2003;Schofield et al, 2008;Veeramani et al;Sharp et al, 2009). The same EXAFS results were observed for both unpasteurized and pasteurized magnetite samples, confirming that an abiotic process leads to uraninite formation.…”

“…The phase composition was determined by analyzing selected area electron diffraction (SAED) patterns and fast Fourier transforms of HRTEM images on the micrometer and nanometer scale, respectively. The interpretation of HRTEM images, SAED patterns and diffractograms were performed according to the method described by Veeramani et al (2009) using the known electron-optical parameters and crystallographic data for several phases containing iron or uranium (ICSD 2003). Briefly, the interpretation of electron diffraction patterns was based on the spacing of the diffraction rings.…”

Speciation-Dependent Kinetics of Uranium(VI) Bioreduction

“…The uranium product formed by Shewanella oneidensis MR-1 through bioreduction has previously been identified by means of synchrotron based X-ray powder diffraction (SR-PD), extended X-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (HR-TEM) Schofield et al 2008;Ulrich et al 2009;Ulrich et al 2008;Veeramani et al 2009). The combined results indicated extracellular nanoparticles of 2-5 nm diameter with a mineral structure and composition homologous to stoichiometric UO 2+x (with x < 0.05) and were consistent with bacteriogenic UO 2 described in other studies, e.g., by Burgos et al (2008) …”

Structural Similarities between Biogenic Uraninites Produced by Phylogenetically and Metabolically Diverse Bacteria

“…As such, it provides justification for geochemical models for bacterial products based on a single set of rate and thermodynamic constants (26,30).…”