Environmental Engineering Reference
In-Depth Information
Although not the focus of such intensive recent research, the fate of uranium
in complex natural systems without the addition of organic substances is
also of great environmental importance, in order to predict the potential
risks of uranium migration within piles, tailings and depository sites and
to prevent their spread via groundwater flow. Thus, different microcosm
experiments were performed in order to investigate the influence of uranyl
or sodium nitrate on the natural bacterial community of a uranium mining
waste site in Germany under acidic and oligotrophic conditions (Geissler &
Selenska-Pobell
2005
; Geissler
2007
; Selenska-Pobell et al.
2008
).
One of the most important observations obtained from the analyses of the
uranyl and sodium nitrate treated subsamples was the extremely high diver-
sity of the indigenous bacterial community and the strong changes in commu-
nity structure noted by increasing the uranyl or sodium nitrate concentrations,
as well as a strict dependence on aeration conditions (Geissler & Selenska-Pobell
2005
; Geissler
2007
; Selenska-Pobell et al.
2008
). After longer incubations, even
with higher U(VI) concentrations, uranium sensitive populations were esta-
blished under aerobic as well as under anaerobic conditions. This indicated
that U(VI) was no longer bioavailable in these long-term experiments (Geissler &
Selenska-Pobell
2005
; Geissler
2007
; Selenska-Pobell et al.
2008
). Surprisingly,
no U(VI) reduction was observed under anaerobic oligotrophic conditions even
after longer incubations, when all the available nitrate was depleted (Geissler
2007
; Selenska-Pobell et al.
2008
). Time-Resolved Laser-induced Fluorescence
Spectroscopic (TRLFS) analysis demonstrated that, in the uranyl nitrate treated
sample incubated for 14 weeks under reducing conditions, most of the added
U(VI) was bound by phosphate phases of biotic origin. U(VI) added to this
sample was bound in mixed organic and inorganic phosphate compounds,
suggesting that, at this site, U(VI)-phosphate chemistry may have a major role
in controlling uranium fate.
Recent publications have also shown that representatives of Rahnella spp.
recovered from uranium contaminated samples are able to immobilise U(VI)
via the secretion of orthophosphate (Martinez et al.
2007
) leading to the pre-
cipitation of meta-autunite-like mineral phases under laboratory conditions
(Beazley et al.
2007
). Moreover, it was demonstrated that various bacteria,
phylogenetically unrelated to Rahnella spp., are able to protect themselves
against potentially toxic U(VI) at acidic pH by secretion of inorganic phosphate
groups which are involved in precipitation of uranyl hydrophosphate-like phases
(Macaskie et al.
1992
) or of meta-autunite-like compounds (Merroun et al.
2006
;
Jroundi et al.
2007
;Martinezet al.
2007
) when studied in laboratory conditions.
The results of the TRLFS spectra of the uranyl nitrate treated soil samples
differed significantly from those of the meta-autunite-like phases produced
by different bacterial strains in the presence of U(VI) observed in fully defined
and relatively simple microbiological media under laboratory conditions
