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Pore-Scale Characterization of Biogeochemical Controls on Iron and Uranium Speciation under Flow Conditions
journal contribution
posted on 2012-08-07, 00:00 authored by Carolyn I. Pearce, Michael J. Wilkins, Changyong Zhang, Steve
M. Heald, Jim K. Fredrickson, John M. ZacharaEtched silicon microfluidic pore network models (micromodels)
with
controlled chemical and redox gradients, mineralogy, and microbiology
under continuous flow conditions are used for the incremental development
of complex microenvironments that simulate subsurface conditions.
We demonstrate the colonization of micromodel pore spaces by an anaerobic
Fe(III)-reducing bacterial species (Geobacter sulfurreducens) and the enzymatic reduction of a bioavailable Fe(III) phase within
this environment. Using both X-ray microprobe and X-ray absorption
spectroscopy, we investigate the combined effects of the precipitated
Fe(III) phases and the microbial population on uranium biogeochemistry
under flow conditions. Precipitated Fe(III) phases within the micromodel
were most effectively reduced in the presence of an electron shuttle
(AQDS), and Fe(II) ions adsorbed onto the precipitated mineral surface
without inducing any structural change. In the absence of Fe(III),
U(VI) was effectively reduced by the microbial population to insoluble
U(IV), which was precipitated in discrete regions associated with
biomass. In the presence of Fe(III) phases, however, both U(IV) and
U(VI) could be detected associated with biomass, suggesting reoxidation
of U(IV) by localized Fe(III) phases. These results demonstrate the
importance of the spatial localization of biomass and redox active
metals, and illustrate the key effects of pore-scale processes on
contaminant fate and reactive transport.