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In-Depth Information
was then monitored as anoxia developed in the microcosms (Burke et al.
2005
).
As expected, the microcosms progressed through a cascade of terminal electron
accepting processes during which time over 99% of the Tc was removed from
the pore waters. In sterile controls, Tc remained in solution (presumably as
Tc(VII)), indicating that removal to sediments was biologically mediated
(Burke et al.
2005
). A detailed analysis of geochemical indicators demonstrated
that Tc removal occurred as Fe(III)-reducing conditions developed after the
consumption of most of the nitrate and accumulation of Mn(II) in pore waters.
Pure culture microcosms were established by inoculating sterilised mixtures
of sediment slurry and river water with cultures of either a nitrate-reducing
bacterium (Pseudomonas stutzeri), an Fe(III)-reducing bacterium (Shewanella sp.) or
a sulphate-reducing bacterium (Desulfovibrio desulfuricans sp. Essex), all with the
addition of an appropriate electron donor. The generation of Fe(II) and the
concomitant removal of Tc occurred only in the presence of Shewanella and
Desulfovibrio spp., which suggested that Tc removal is linked to Fe(II) ingrowth
in these sediments (Burke et al.
2005
). The 16S rRNA gene analysis confirmed
the presence of organisms related to known nitrate- (Rhodobacter capsulatus),
sulphur/metal- (Pelobacter sp.) and sulphate-reducing bacteria (Desulfovibrio senezii)
in the sediments. Additionally, Geobacteraceae-specific primers were used to
detect Fe(III)-reducing bacteria from this phylogenetic group. Thus, there was
a complex range of Fe(III)- and sulphate-reducing bacteria present in the sedi-
ments that could have been responsible for production of Fe(II), or potentially
sulphide, and the subsequent indirect reduction of Tc(VII) mediated by these
reduction products. X-ray absorption spectroscopic analysis from progressive
anoxia samples spiked with 1000
m
MTcO
4
confirmed that TcO
4
removal was
due to reduction to hydrous Tc(IV)O
2
in Fe(III)- and sulphate-reducing estuarine
sediments (Burke et al.
2005
).
In addition to working with estuarine sediments that initially contained
no background radioactivity, we have also worked extensively with sediments
from (or representative of ) several 'nuclear' sites. For example, microcosm
experiments containing soil samples representative of the UKAEA site at
Dounreay have been performed with unamended sediments, carbonate buf-
fered sediments and microcosms amended with EDTA, a complexing ligand
used in nuclear fuel cycle operations (Begg et al.
2007
). During the development
of anoxia mediated by indigenous microbial populations, Tc(VII)O
4
was again
removed from solution, during periods of microbial Fe(III) reduction when
Fe(II) was growing into the microcosms. A pivotal role for biogenic Fe(II) in
Tc(VII) reduction and precipitation (
Fig. 11.2
) was confirmed in microcosms
which had been prereduced to the point that Fe(III) reduction dominated, and
then sterilised by autoclaving. In these sterile Fe(III)-reducing sediments, the
Tc-spike was removed from solution to below the liquid scintillation counting
detection limit (
>
98%) over 21 days.
