Sulfurospirillum barnesii is capable of anaerobic growth
using ferric iron or arsenate as electron acceptors. Cell
suspensions of S. barnesii were able to reduce arsenate
to arsenite when the former oxyanion was dissolved in solution,
or when it was adsorbed onto the surface of ferrihydrite,
a common soil mineral, by a variety of mechanisms
(e.g., coprecipitation, presorption). Reduction of Fe(III) in
ferrihydrite to soluble Fe(II) also occurred, but dissolution of
ferrihydrite was not required in order for adsorbed
arsenate reduction to be achieved. This was illustrated by
bacterial reduction of arsenate coprecipitated with
aluminum hydroxide, a mineral that does not undergo
reductive dissolution. The rate of arsenate reduction was
influenced by the method in which arsenate became
associated with the mineral phases and may have been
strongly coupled with arsenate desorption rates. The extent
of release of arsenite into solution was governed by
adsorption of arsenite onto the ferrihydrite or alumina
phases. The results of these experiments have interpretive
significance to the mobilization of arsenic in large
alluvial aquifers, such as those of the Ganges in India
and Bangladesh, and in the hyporheic zones of contaminated
streams.
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