Geological Society of America

T18 Structure, Properties, and Geochemistry of Nanoparticles, Nanoclusters, and Nanocomposites in Biogeochemical Systems I: In Honor of Benjamin Gilbert, Recipient of the 2010 MSA Award

Sunday, 31 October 2010
8:00 AM - 12:00 PM
Mile High Ballroom 2AB/3AB

REDUCTION OF U(VI) BY MICROORGANISMS: FORMATION OF NANOPARTICULATE URANINITE AND OTHER PRODUCTS

Paper number: T18-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Rizlan Bernier-Latmani Presenting Author
Ecole Polytechnique Federale de Lausanne, Lausanne, N/A
Harish Veeramani
Ecole Polytechnique Federale de Lausanne, Lausanne, N/A
Eleanor Schofield
Stanford Synchrotron Radiation Lightsource, Menlo Park, CA
Jonathan O. Sharp
Colorado School of Mines, Golden, CO
Daniel S. Alessi
Ecole Polytechnique Federale de Lausanne, Lausanne, N/A
Benjamin Uster
Ecole Polytechnique Federale de Lausanne, Lausanne, N/A
Elena I. Suvorova
Ecole Polytechnique Federale de Lausanne, Lausanne, N/A
John R. Bargar
Stanford Synchrotron Radiation Lightsource, Menlo Park, CA

Abstract

Microbially-mediated U(VI) reduction leads to the formation of various U(IV) species, including a U(IV) oxide, uraninite (UO₂). Biogenic uraninite produced by Shewanella oneidensis MR-1 was nanoparticulate (~ 3 nm in diameter) and displayed remarkable order and structural homology to stoichiometric UO₂. The nanoparticles are produced both in the bacterial periplasmic space as well as extracellularly. A systematic study of the effect of phylogenetic diversity on U(VI) reduction revealed that , under the same chemical conditions, there was little difference in the U(IV) product regardless of the physiological and metabolic characteristics of the bacteria.

However, the product of this reduction depended strongly on the geochemical conditions in which the biological process took place. At low cation and phosphate concentrations, nanoparticulate UO₂ was the dominant product. In contrast, the presence of increasing concentrations of divalent cations promoted the formation of non-uraninite products. These included U(IV) associated with phosphate in a surface complexation arrangement as well as nanoparticulate crystalline and possibly amorphous U(IV)-phosphate phases. We refer to the non-uraninite products as 'monomeric U(IV)' due to the absence of a characteristic U-U shell in the EXAFS spectra of these compounds.

We developed a method to easily quantify the relative contribution of monomeric U(IV) and UO₂ in samples containing U(IV). We found that, under all the conditions tested, a measurable contribution of monomeric U(IV) was recorded. These findings suggest a more complex speciation of biogenic U(IV) than previously envisaged and provide fertile ground for the investigation of U(IV) geochemistry.