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Microbial Reduction of Arsenic-Doped Schwertmannite by <i>Geobacter sulfurreducens</i>
The fate of AsĀ(V) during microbial reduction by <i>Geobacter
sulfurreducens</i> of FeĀ(III) in synthetic arsenic-bearing schwertmannites
has been investigated. During incubation at pH7, the rate of biological
FeĀ(III) reduction increased with increasing initial arsenic concentration.
From schwertmannites with a relatively low arsenic content (<0.3
wt %), only magnetite was formed as a result of dissimilatory iron
reduction. However, bioreduction of schwertmannites with higher initial
arsenic concentrations (>0.79 wt %) resulted in the formation of
goethite.
At no stage during the bioreduction process did the concentration
of arsenic in solution exceed 120 Ī¼gL<sup>1</sup>, even for
a schwertmannite with an initial arsenic content of 4.13 wt %. This
suggests that the majority of the arsenic is retained in the biominerals
or by sorption at the surfaces of newly formed nanoparticles.Subtle differences in the As <i>K</i>-edge XANES spectra
obtained from biotransformation products are clearly related to the
initial arsenic content of the schwertmannite starting materials.
For products obtained from schwertmannites with higher initial As
concentrations, one dominant population of AsĀ(V) species bonded to
only two Fe atoms was evident. By contrast, schwertmannites with relatively
low arsenic concentrations gave biotransformation products in which
two distinctly different populations of AsĀ(V) persisted. The first
is the dominant population described above, the second is a minority
population characterized by AsĀ(V) bonded to four Fe atoms. Both XAS
and XMCD evidence suggest that the latter form of arsenic is that
taken into the tetrahedral sites of the magnetite.We conclude
that the majority population of AsĀ(V) is sorbed to
the surface of the biotransformation products, whereas the minority
population comprises AsĀ(V) incorporated into the tetrahedral sites
of the biomagnetite. This suggests that microbial reduction of highly
bioavailable AsĀ(V)-bearing FeĀ(III) mineral does not necessarily result
in the mobilization of the arsenic