Environmental Engineering Reference
In-Depth Information
To better understand U(VI) immobilization in subsurface fracture-flow systems,
Sani et al. [7] recently 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 due to
plugging of the fracture-flow reactors.
Biofilms Grown on Redox-Sensitive Surfaces
A surface that can exchange electrons with the cells in a biofilm is called a redox-
sensitive surface. A redox-sensitive surface can be a synthetic mineral such as
hematite or a mineral from contaminated sediment. The use of synthetic miner-
als allows us to have reproducible surfaces for well-controlled experiments with
high reproducibility. However, it is difficult to have homogeneous and well-defined
surfaces when we use natural contaminated minerals.
SRB Biofilms on Synthetic Minerals
In a previous study by Marsili et al. [87], the immobilization of U(VI) using biofilms
of
D. desulfuricans
G20 growing on synthetic carbonate-bearing minerals was stud-
ied to test whether U(VI) can be reduced in a subsoil formation by SRB biofilms
in the presence of carbonates in an efficient and sustainable way. To this end, three
FBCRs (Fig. 1.4), each filled with calcite, dolomite, or hematite, were operated
using lactate as the electron donor [87].
In the systems filled with carbonate-bearing minerals (calcite and dolomite),
the growth medium was not buffered, while 10 mM of carbonate was used to
buffer the system filled with hematite, a non-carbonate-bearing mineral. The total
amount of uranium accumulated in the biofilms increased linearly with time in
all three reactors (Fig. 1.7). In these experiments, 87.2% (calcite-filled column),
82.4% (dolomite-filled column), and 72.5% (hematite-filled column) of uranium
was removed from the bulk solution. The results demonstrate that the SRB biofilms
grown in all the reactors were able to immobilize uranium efficiently, despite the
presence of U-complexing carbonates when biofilms were grown on redox-sensitive
surfaces. Figure 1.8 shows HRTEM images of SRB biofilms grown on one redox-
insensitive surface (quartz) and several redox-sensitive surfaces (calcite, dolomite,
and hematite). The darker areas in the images show uraninite which was confirmed
using EDS [87]. In all cases, uranium was mostly reduced in the periplasmic space.
However, when the biofilm was grown on calcite, it was noticed that some of the U
was reduced in the extracellular material. This could be because the chemistries of
the extracellular matrices in biofilms grown on different minerals are different. The
immobilization of U in the extracellular matrix could be responsible for the higher
overall uranium removal efficiency in the column reactor filled with calcite.
Figure 1.9 shows a SEM image of biofilms growing on a dolomite surface.
Although the cells produced a significant amount of extracellular material, the TEM
image (Fig. 1.8c) shows that most of the uranium was immobilized within the
