Environmental Engineering Reference
In-Depth Information
C
A
B
Fig. 1.2
A mineral with attached cells on the surface: (
a
) a few cells are attached to the surface;
(
b
) the cells increase in number and start to cover the surface; (
c
) the surface is covered by layers
of cells
Lewandowski and Beyenal [83] and by others [86]. The biofilm on the surface
does not have to be very thick: the thickness can vary from a few micrometers to
several hundred micrometers. We expect that when cells start growing on a surface,
the metabolic activity of the cells will affect the abiotic and redox reactions.
For example, the surface-associated cells can consume the available oxygen,
which lowers the redox potential, influences the abiotic reactions and generates
concentration gradients. These gradients will control the activity of the cells (biotic
reactions) and the abiotic and redox reactions and will influence the bulk chemistry.
The biofilms can also exchange electrons with the surface they are growing on,
and these electron exchanges can influence uranium mobility. Therefore uranium
immobilization must be investigated for redox-sensitive and redox-insensitive
surfaces. Redox-insensitive surfaces do not exchange electrons with biofilms.
In the past several years, using sulfate-reducing biofilms as a model, researchers
including our group have systematically studied the feasibility of U(VI) immobi-
lization in biofilms growing on redox-sensitive and -insensitive surfaces with and
without carbonate in the medium [18, 87, 73]. Recently, we have been studying ura-
nium immobilization using dissimilatory iron-reducing bacteria (DIRB)
Shewanella
oneidensis
MR-1 and other naturally growing species in uranium-contaminated
subsurface sediments.
1.3.2 Uranium Immobilization Mechanisms
Using Sulfate-Reducing Biofilms
Sulfate-reducing bacteria (SRB) are anaerobic microorganisms that utilize a vari-
ety of organic substrates as electron donors and sulfate as the terminal electron
acceptor, resulting in the production of sulfide [88]. Due to the differences in solu-
bility of metal sulfates and metal sulfides, SRB have been used to immobilize metals
in groundwater and wastewater [88, 89]. Specifically, heavy metal immobilization
using SRB biofilms has been shown to be an effective method in bioremediation.
It has been reported that SRB also have the capabilities to reduce and immobilize
metals and radionuclides, such as Cr(VI), U(VI) and Tc(VII) [35, 90].
Among the mechanisms we have discussed (Fig. 1.1), at least three are involved
in U(VI) immobilization by SRB [18]: (i) biosorption of U(VI) to cell surfaces and
