Environmental Engineering Reference
In-Depth Information
250
1200
Without U
With U
1000
U(VI) delivered to the
flat plate reactor
200
800
150
600
100
400
50
200
0
0
0
5
10
15
20
25
30
35
0
100
200
300
400
500
600
Distance from bottom (µm)
Time (week)
B
A
Fig. 1.5 ( a )H 2 S concentration profiles in three-week-old biofilms grown without U(VI) and
with U(VI). ( b) Uranium accumulated in SRB biofilms. Reprinted “in part” with permission from
Beyenal et al. [18]. Copyright 2007 American Chemical Society
that measured in biofilms exposed to both U(VI) and NaN 3 (8.5
±
1.1 vs. 6.2
±
mol/cm 2 ), suggesting that some of the microbially generated sulfide may have
reacted with U(VI).
To separately determine the effect of sulfide on U(VI) reduction, Beyenal et al.
performed abiotic experiments in batch reactors containing U(VI) and sulfide in
PIPES buffer (30 mM, pH 7) [18]. Under anoxic conditions, both the sulfide and
U(VI) concentrations decreased with time, which was not observed in the U(VI)-
and sulfide-free control experiments [18]. This demonstrates that U(VI) is abioti-
cally reduced by sulfide in the absence of carbonate buffer. It was also found that
when the medium was buffered with 30 mM carbonate, sulfide did not reduce U(VI)
[18]. Recently, other research groups have shown that microbially generated sulfide
can reduce uranium in a medium with 15 mM carbonate buffer, and that the lower
the carbonate buffer strength, the higher the uranium reduction rate [110].
The mass of the U immobilized in the sulfate-reducing biofilmwas also estimated
to quantify the immobilization dynamics. The total amount of U immobilized in
the biofilm was found to increase linearly with time during the operation of the
reactor (Fig. 1.5b). U removal of at least 88% was obtained consistently using a
sulfate-reducing biofilm in the reactor.
It has been shown that SRB biofilms have the ability to immobilize U(VI) for
significant amounts of time as the result of both enzymatic and chemical reduction
of U(VI) to uraninite. However, natural groundwater is usually buffered with car-
bonates, where the chemical reduction of U(VI) was considered difficult because of
the formation of complexes with carbonates. In addition to SRB it has been reported
that DIRB can reduce U(VI) to U(IV) enzymatically in the presence of a carbonate
buffer [111, 41].
Marsili et al. [73] tested the ability of SRB biofilms to remove U(VI) from
contaminated carbonate-buffered groundwater, in flat plate reactors (Fig. 1.3) with
biofilms of D. desulfuricans G20 grown on glass slides operated for 5 months at
0.6
μ
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