Environmental Engineering Reference
In-Depth Information
H
2
or organics (acetate,
ethanol etc.)
Fe(III)
Tc(IV)
MICROBIAL
CELL
Tc(VII)
CO
2
+H
2
O
Fe(II)
Figure 11.2.
Indirect reduction of Tc(VII) mediated by Fe(II).
Similar trends were also noted in sediments collected from the Drigg low-
level radioactive waste storage site (Wilkins et al.
2007
), confirming a generic
indirect mechanism for Tc(VII) reduction that is not strongly site specific.
Interestingly, in both studies, the reduced insoluble Tc was surprisingly resist-
ant to remobilisation by strong oxidising agents such as nitrate, consistent
with data from other parallel studies (Burke et al.
2006
).
Finally, the interplay between nitrate and the reduction of Tc(VII) was
explored in more detail in microcosms prepared from sediments from the US
Department of Energy Field Research Center (FRC) in Oak Ridge, Tennessee, USA
(McBeth et al.
2007
). Here the impact of 0, 10 and 100mM added nitrate on the
progression of a range of terminal electron accepting processes and
99
Tc immo-
bilisation was assessed. In the nitrate unamended and 10mM nitrate amended
systems, bioreduction proceeded and extractable Fe(II) ingrowth and concomi-
tant Tc(VII) removal was observed. Interestingly, the relatively low (10mM)
addition of nitrate seemed to augment the development of bioreducing condi-
tions. In contrast, in the 100mM nitrate amended system, Fe(II) ingrowth was
limited and no Tc(VII) removal occurred, suggesting strong inhibition of micro-
bial metal reduction at high, but nuclear site relevant concentrations of NO
3
.
Bacterial community changes induced by uranyl or
sodium nitrate treatments and the fate of the added U(VI)
The fate and transport of uranium are governed by the contrasting chemistry
of U(IV) and U(VI). U(VI) generally forms soluble, and thus mobile, complexes
with carbonate and hydroxide, while U(IV) precipitates as the highly insoluble
mineral uraninite. Many studies have focused on in situ bioremediative stimu-
lation of native U(VI)-reducing bacteria by the addition of different organic
electron donors for aqueous U(VI) reduction such as acetate, lactate, glucose
and ethanol to uranium contaminated waters and sediments (Holmes et al.
2002
; Anderson et al.
2003
; Suzuki et al.
2003
; North et al.
2004
; Brodie et al.
2006
; Nyman et al.
2006
).
