Environmental Engineering Reference
In-Depth Information
settings [12, 27]. Mobile aqueous complexed species and insoluble uranium-bearing
minerals are often formed by complexation [68]. As an example of the importance of
complexation, it has been clearly demonstrated in the laboratory [18, 69, 34] and in
aH
2
S-rich stratified water column in the Black Sea [70, 12] that U(VI)-CO
3
com-
plexes UO
2
(CO
3
)
2
2
and UO
2
(CO
3
)
4
3
, the dominant aqueous uranium species in
most surface and subsurface settings, are not reduced homogeneously by chemical
reductants such as sulfide and molecular hydrogen.
Biotic Processes that Control Uranium Mobility
U(VI)-reducing microorganisms typically reduce U(VI) or U(VI)-CO
3
complexes
to form U(IV) oxides such as uraninite in the periplasmic space or nanoparticles
in the extracellular material [71-73]. In natural systems, the mobility of uranium
is determined by the interplay between biotic and abiotic processes [74, 27]. The
electrons from the microbial oxidation of lactate can reduce U(VI):
2UO
2
+
+
lactate
−
+
acetate
−
+
HCO
3
5H
+
2H
2
O
→
2UO
2
+
+
(1.1)
In principle, the oxidation of an electron donor coupled to the reduction of an
electron acceptor with a higher redox potential is more favorable, because electrons
always flow from the low redox potential to the high redox potential and this is
favorable for microbial energy conservation. Generally oxygen is the ultimate elec-
tron acceptor because of its high redox potential. However, if oxygen is not present,
in the presence of nitrate, nitrate is expected to be the next electron acceptor for
microbial respiration:
NO
3
2H
+
+
2e
−
→
NO
2
+
+
H
2
O
(1.2)
Under sulfate-reducing conditions, the microbes involved in the process of metal
removal gain energy by coupling the oxidation of organic compounds with the
reduction of sulfate ions, generating hydrogen sulfide as a by-product [27]:
SO
2
4
2 lactate
−
+
H
+
→
2 acetate
−
+
HS
−
+
+
+
2CO
2
2H
2
O
(1.3)
HS
−
+
H
+
⇔
H
2
S
(1.4)
HS
−
⇔
S
2
−
+
H
+
(1.5)
Microbially generated H
2
S dissolves in water and, being a diprotic acid, dis-
sociates to bisulfide (HS
-
) and sulfides (S
2-
) that can reduce uranium [18]. In the
presence of iron, the formation of iron sulfide minerals may decrease the amount of
S
2-
available for uranium reduction by sulfate-reducing bacteria.
Recent studies have demonstrated that redox transformations of uranium are
governed by kinetic factors that are strongly controlled by microbial activity
[12, 27]. Although abiotic uranium oxidation proceeds efficiently under aerobic
