Uranium Complexes Formed at Hematite Surfaces Colonized
by Sulfate-Reducing Bacteria
ANDREW L. NEAL, JAMES E, AM0NETTE, BRENT M, PEYTON, AND GILL G, GEESEY
Department of Microbiology, Montana State University, Bozeman Montana
59717-3520,
Center for Biofilm Engineering, Montana State University, Bozeman, Montana
59717-3980,
Environmental Molecular Sciences Laboratory, Pacific Northwest National
Laboratory, Richland, Washington 99352, and Center for Multiphase Environmental
Research, Washington State University, Pullman, Washington 99164
Modeling uranium (U) transport in subsurface environments requires a thorough
knowledge of mechanisms likely to restrict its mobility, such as surface
complexation, precipitation, and colloid formation. ln closed systems,
sulfate-reducing bacteria (SRB) such as Desulfovibrio spp, demonstrably
affect U immobilization by enzymatic reduction of U(VI) species (primarily
the uranyl ion, UO22+, and its a complexes) to U(IV).
However, our understanding of such interactions under chronic U(VI) exposure
in dynamic systems is limited. As a first step to understanding such interactions,
we performed bioreactor experiments under continuous flow to study the
effect of a biofilm of the sulfate-reducing bacterium Desulfovlorio
desulfuricans attached to specular hematite (a.-Fe203)
surfaces on surface-associated U(VI) complexation, transformation, and
mobility. Employing real-time microscopic observation and X-ray photoelectron
spectroscopy (XPS), we show that the characteristics of the (VI) complex(es)
formed at the hematite surface are influenced by the composition of the
bulk aqueous phase flowing across the surface and by the presence of surface-associated
SRB, The XPS data further suggest higher levels of U associated with hematite
surfaces colonized by SRB than with bacteria-free surfaces. Microscopic
observations indicate that at least a portion of the U(VI) that accumulates
in the presence of the SRB is exterior to the cells possibly associated
with the extracellular biofilm matrix. The U4f7/2 core-region
spectrum and U5f2 valence-band spectrum provide preliminary
evidence that the SRB-colonized hematite surface accumulates both U(VI)
and U(IV) phases, whereas only the U(VI) phase(s) accumulates on uncolonized
hematite surfaces. THe results suggest that mineral surfaces exposed to
a continuously replenished supply of U(VI)-containing aqueous phase will
accumulate U phases that may be more representative of those that exist
in U-contaminated aquifers than those which accumulate in cloased experimental
systems. These phases should be considered in models attempting to predict
U transport through subsurface environemnts.
SREL Reprint #2763
Neal, A. L., J. E. Amonette, B. M. Peyton and G. G. Geesey. 2004. Uranium complexes formed at hematite surfaces colonized by sulfate-reducing bacteria. Environmental Science & Technology 38:3019-3027.
