Environmental Engineering Reference
In-Depth Information
7. After a few days of operation, biofilm should be visible on the packing material;
its presence can be confirmed by sulfide production when SRB are used. The
time needed for a visible biofilm growth depends on the amount of the substrate
and the microorganisms added to the reactors.
1.3.5 Uranium Immobilization Using Biofilms Grown
on Various Surfaces
Biofilms grown on both redox-insensitive and redox-sensitive surfaces have been
used in uranium immobilization studies. As redox-insensitive surfaces, quartz and
glass have been used to evaluate U(VI) immobilization by SRB biofilms [18, 73] and
S. oneidensis
MR-1 biofilms in flat plate reactors, and to evaluate U(VI) removal by
biofilms of
S. oneidensis
MR-1 in fracture-flow reactors [7]; as redox-sensitive sur-
faces, synthetic minerals such as hematite, calcite, and dolomite have been used
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 [87]. In addi-
tion to well-controlled laboratory conditions, natural sediments with biofilms of
stimulated indigenous bacteria have also been evaluated for U(VI) immobilization
[108, 66, 109].
1.3.5.1 Biofilms Grown on Redox-Insensitive Surfaces
A redox-insensitive surface is a surface that does not exchange electrons with
biofilms. We generally use quartz as a redox-insensitive surface. When a redox-
insensitive surface is used, electrons are only exchanged among electron donors,
electron acceptors and redox mediators.
SRB Biofilms on Quartz Surface
Beyenal et al. [18] measured U(VI) immobilization and microbial activity using
SRB biofilms composed of
D. desulfuricans
G20 grown on quartz surfaces in flat
plate reactors (Fig. 1.5). Lactate and sulfate were used as the electron donor and the
electron acceptor, respectively. A medium that minimizes metal complexation was
used. Microbial activity was determined by measuring the input and outlet lactate
concentrations or by measuring the H
2
S production in the biofilm. Since it was
also expected that H
2
S could abiotically reduce U(VI), H
2
S profiles measured using
microelectrodes in the biofilms were selected as an indicator of microbial activity
and H
2
S production.
The H
2
S concentrations and flux were found to be higher when the reactor
was not fed with U(VI) (Fig. 1.5a). After inactivating bacterial metabolism with
NaN
3
(after which no H
2
S was produced), the U immobilization was quantified.
The total amount of U immobilized in biofilms exposed to U(VI) was higher than
