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extracellular biopolymers; (ii) chemical reduction of U(VI) by microbially gener-
ated H 2 S (indirect reduction); and (iii) enzymatic reduction in which U(VI) acts as
the terminal electron acceptor (direct reduction). Mohagheghi et al. [91] hypoth-
esized that the combination of biosorption and chemical reduction processes was
responsible for U(VI) reduction. Lovley and coworkers [77], though, showed that
the enzymatic reduction of U(VI) by sulfate-reducing bacteria was much faster than
chemical reduction by sulfide, indicating that enzymatic reduction may be the dom-
inant form of U(IV) immobilization in sulfidogenic environments. However, these
conclusions were reached with respect to bicarbonate-buffered systems, in which
it has been demonstrated that U(VI) is almost entirely complexed in the forms
of UO 2 (CO 3 ) 2 2 and UO 2 (CO 3 ) 4 3 [69], increasing the difficulty of U(VI) reduc-
tion by microbially generated H 2 S and resulting in bioreduction as the prevailing
U(VI) reduction mechanism [37, 34]. Interestingly, Spear et al. [92] showed that in a
bicarbonate-buffered system growing cells of D. desulfuricans reduced U(VI) faster
than U(VI) reduction under nongrowth conditions, suggesting that microbially gen-
erated sulfide may have been responsible for the increased rate of U(VI) reduction.
1.3.3 Uranium Immobilization Mechanisms Using DIRB Biofilms
In most DIRB biofilm studies, S. oneidensis MR-1 [93] has been used as the
model biofilm-forming facultative anaerobic microorganism. There have been some
studies of MR-1 biofilms, primarily focused on the structure and metabolism within
the biofilms [94-96]. However, hitherto very few studies on U(VI) immobilization
using Shewanella biofilms have been available. Recently, Sani et al. [7] reported
their results on U(VI) removal by biofilms of S. oneidensis MR-1 in fracture-flow
reactors. S. oneidensis biofilms were shown to have limited U(VI) immobilization
capacity in both flow and batch modes. In a recent paper, McLean et al. [95]
studied the kinetics and stratification of anaerobic metabolism within live biofilms
of S. oneidensis MR-1 through a combination of noninvasive NMR microscopic
imaging/spectroscopy and confocal imaging tools. It was suggested that, even
under bulk aerobic conditions, MR-1 biofilms have the ability to perform anaerobic
reduction as oxygen becomes scarce with depth in the biofilm: thus, U(VI) may
be immobilized within the biofilm matrix and remain immobilized as long as the
active biofilms are maintained and oxygen is depleted within the matrix.
In uranium bioremediation using DIRB biofilms, biosorption and bioreduction
are the two main processes contributing to U(VI) immobilization. Both of these
processes have been discussed earlier (Fig. 1.1). Here we want to emphasize the
bioreduction process of MR-1. At least two distinct pathways have been proposed
for the transfer of electrons to the mineral substrate by MR-1: (i) the direct transfer
of electrons from the cell surface at the mineral-microbe interface through a network
of c -type cytochromes [97, 98] localized in the periplasm [99], outer membrane
[100, 71], and nanowires (pilus-like assemblages) [101] and (ii) the indirect transfer
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