Environmental Engineering Reference
In-Depth Information
U(VI)
U(IV)
Direct reduction
Indirect reduction
Fe(II), H
2
S
U(VI)
e
-
e
-
U(IV)
Fe(III), SO
4
2-
Bacterial Cell
U(VI)
Biosorption
U(VI)
U(VI)
Bioaccumulation
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
Active site
Ligand
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
U(VI)
Polyphosphate body
U(VI)
Bioprecipitation
Needle-like fibril
Fig. 1.1
Reducing and non-reducing mechanisms during the bioremediation of uranium using
bacterial cells: direct and indirect reduction, biosorption, bioprecipitation, and bioaccumulation
[32, 35, 3, 44, 27]
by disproportionation of the unstable U(V) complexes to U(IV) and U(VI) was
the likely mechanism of uranium reduction. Due to the insoluble nature of U(IV)
dioxide (UO
2
), examination of the U(IV) deposition site provides an indication
of the location of the enzymes responsible for U(VI) reduction. Although urani-
nite deposits within the cytoplasm in
Pseudomonas
sp.,
D. desulfuricans
, and
D. aspoensis
have been reported [39, 3, 40], most research has found that insoluble
U(IV) accumulates in the periplasm and on the outside of both Gram-negative
and Gram-positive bacterial cells [41, 42, 37, 43], suggesting that U(VI) does not
generally have access to intracellular enzymes. The enzymes responsible for U(VI)
reduction would be electron-carrier proteins or enzymes exposed to the outside of
the cytoplasmic membrane, within the periplasm, and/or in the outer membrane
[27]. Recently,
c
-type cytochromes have been shown, in vitro and in vivo, to play
an important role in the U(VI) reduction process, as summarized in Table 1.2.
Biosorption
Biosorption is defined as the metabolism-independent immobilization of heavy met-
als and radionuclides by physiochemical mechanisms (Fig. 1.1) [3, 44]. Although
the biosorption of metal species is a metabolism-independent process and thus can
be carried out by both living and dead microbial biomass, metabolic activity may
