Environmental Engineering Reference
In-Depth Information
H
2
or organics (acetate,
ethanol etc.)
U(VI)
N
2
MICROBIAL
CELL
U(IV)
NO
3
-
CO
2
+H
2
O
Figure 11.3.
Reduction of U(VI) and reoxidation of U(IV) by nitrate.
uranium removal is clearly very complex, and probably site/sediment specific.
Indeed, a recent study has even demonstrated that the stimulation of a natural
microbial community to immobilise U through bioreduction is possible with-
out the removal of nitrate (Madden et al.
2007
). Incubations with uranium-
contaminated sediments demonstrated nearly complete reduction of U(VI)
with very little loss of nitrate from pH 5.7 to 6.2 using methanol or glycerol
as a carbon source. The majority of the micro-organisms stimulated by these
enrichment conditions consisted of low G
C gram-positive bacteria most
closely related to Clostridium and Clostridium-like organisms (Madden et al.
2007
).
Finally, it is also obvious from several recent papers that nitrate can not only
inhibit U(VI) reduction (as noted in a majority of studies), but it can also have a
dramatic impact on pre-reduced sediments containing insoluble U(IV). When
added to such systems, nitrate (and oxygen also) can reoxidise the U(IV)
(
Fig. 11.3
), remobilising the uranium as U(VI) (Moon et al.
2007
). In the case of
technetium, reoxidation behaviour is more complex, with remobilisation
dependent on the nature of the oxidant (Burke et al.
2006
; Morris et al.
2008
).
The abundance of nitrate in the subsurface environments discussed provides
a selective pressure that favours denitrifiers among the soil micro-organisms
that can tolerate an acidic, nutrient-starved environment. In agreement with
the high nitrate concentrations observed in sediments from the Oak Ridge
Field Research Center (ORFRC), 16S rRNA gene sequences related to those of
nitrate-reducing bacteria, such as members of the Proteobacteria (including the
genera Sphingomonas, Acidovorax, Acinetobacter, Alcaligenes and Ralstonia), showed
a high relative abundance in the total and metabolically active fractions of
the microbial community (Akob et al.
2007
). In a parallel study, the microbial
community structure from ethanol-biostimulated sediments of a high-nitrate
(
รพ
130mM), low-pH, uranium-contaminated site at the ORFRC, taken at a time
point when denitrification was most likely, showed an abundance of betapro-
teobacterial clones in biostimulated sediments (Spain et al.
2007
). Multiple lines
of evidence suggest the dominance of Castellaniella species in these biostimu-
lated sediments and their role in nitrate removal in situ (Spain et al.
2007
).
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