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Materials Science Division |
HighlightsMicrobial Transformations of Fe(III) Oxides and Oxyhydroxides: Characterization and Reactivity of Biogenic Fe(II) MineralsMicrobial Transformations of Fe(III) Oxides and Oxyhydroxides: Characterization and Reactivity of Biogenic Fe(II) Minerals Scientific Achievement The
bioreduction
of Fe(III) oxides and hydroxides may result in the production of a
suite of
Fe(II)-bearing secondary mineralization products (Fe(II)SMPs),
including
magnetite, siderite, vivianite, green rusts, and ferrous hydroxy
carbonate. In an
effort to identify the factors
controlling the formation of specific Fe(II)SMPs, we examined the
Fe(II)SMPs
formed from the bioreduction of a series of natural and synthetic
Fe(III)
oxides (including akaganéite, feroxyhyte, ferric green rust,
ferrihydrite,
goethite, hematite, lepidocrocite, and maghemite) by Shewanella
putrefaciens
CN32, a dissimilatory Fe(III)-reducing bacterium. Our results showed that
the types of
Fe(II)SMPs that formed were dependent on both the mineralogy of the
parent
Fe(III) oxide as well as solution composition. For example, the
presence of
sufficient concentrations of phosphate resulted in the formation of
micron-sized green rust or vivianite instead of nanoparticulate
magnetite. The
use of SEM was
crucial in identifying the presence of specific Fe(II)SMPs and
complemented
parallel analyses using XRD and 57Fe Mössbauer
spectroscopy. In
particular, SEM imaging was essential for
identifying the presence of ferrous hydroxy carbonate (FHC) in many of
our
samples. The
presence of FHC was not
indicated by XRD analysis, but due to its distinctive morphology, was
easily
identified by SEM imaging. The
quality
of the Mössbauer analyses of these samples was therefore improved
knowing to
include FHC as one of the components in modeling the spectra. Significance Fe(II)SMPs provide a reservoir of reducing capacity in the subsurface that may contribute to the reduction of contaminants such as U(VI). However, there are substantial differences in the ability of different Fe(II)SMPs to reduce soluble U(VI) to insoluble U(IV). Thus, to understand the potential for the reduction of U(VI) by Fe(II)SMPs in subsurface environments we need to identify the key microbial and geochemical parameters that control the types of Fe(II)SMPs that form. The results described above reflect progress in year one of a five year project examining the “Coupled Microbial, Geochemical, and Mineralogical Controls on Biogenic Fe(II) Speciation and Reactivity.” Studies over the remaining four years will examine the role of microbial physiology and geochemical conditions on the formation of Fe(II)SMPs and their interactions with U(VI). Performers 
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