Nano-scale Structure and Stability of Biogenic Uranium(IV) Oxide

Ulrich, K.U., Schofield, E.J., Bargar, J.R., Sharp, J.O., Veeramani, H., Bernier-Latmani, R., Singh, A. and Giammar, D.E. (2008) Nano-scale Structure and Stability of Biogenic Uranium(IV) Oxide. In: Goldschmidt 2008, Vancouver, Canada, 13-18 July 2008, A966.

Abstract

The chemical stability of biogenic U(IV) oxide, a product of environmental bioremediation, is a seminal issue for successful U immobilization. Fundamental differences in particle size and crystal structure with synthetic UO2+x (x~0), such as disorder and hyperstoichiometry (0<x<0.25), are expected to substantially impact the stability of biogenic UO2 in groundwater. Biogenic UO2 nanoparticles (~3.5 nm diameter) formed by Shewanella oneidensis MR-1 both at pH 6.3 and 8.0, were compared to their abiotic analog with respect to local and long-range atomic and nano-scale structures as well as dissolution kinetics under environmentally relevant conditions. Synchrotron-based powder diffraction, X-ray absorption spectroscopy, and transmission electron microscopy provided structural insight. Dissolution rates were quantified by continuous flow-through experiments under anoxic or oxidizing conditions. The biogenic and synthetic UO2 exhibited similar equilibrium solubility and the lowest dissolution rates under reducing, near neutral pH conditions. The rates increased loglinearly with decreasing pH. Similar surface area-normalized rates for the two solids suggest similar reactive surface site densities. This finding is consistent with the discovered structural homology of the two solids with the interior of the particles being consistent with an uncompressed fcc lattice, whereas a ~1 nm thick outer zone exhibits local static disorder similar to that in UO2+x. The presence of carbonate increased the dissolution rate of biogenic UO2 almost to the value obtained under oxidizing conditions, and as much greater than that of synthetic UO2. Thus, biogenic UO2 particles are more susceptible to surface oxidation by water and reactive oxidants originating from α- radiolysis of adjacent water. Even in anoxic aquifers, UO2 dissolution may be controlled by thermodynamics of surface U(VI) rather than U(IV) phases.

• Item Type: Conference Proceedings
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Veeramani, Dr Harish
• Authors: Ulrich, K.U., Schofield, E.J., Bargar, J.R., Sharp, J.O., Veeramani, H., Bernier-Latmani, R., Singh, A., and Giammar, D.E.
• College/School: College of Science and Engineering > School of Engineering > Infrastructure and Environment

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