Environmental Engineering Reference
In-Depth Information
of electrons from the cells to the mineral surface without direct contact through low-
molecular-weight soluble redox mediators or electron shuttles [102, 103]. Studies
have shown that redox shuttles such as quinone-containing humic acids can pro-
mote the reduction of Fe(III) oxides [104]. In U(VI) immobilization, humics have
been demonstrated to play two functional roles: enhancing U(VI) bioreduction and
increasing U(IV) solubilization and reoxidation by forming U(IV)-humics com-
plexes [105]. In addition to these exogenous electron shuttles, endogenous electron
shuttles, such as flavins secreted by
S. oneidensis
MR-1 in both planktonic cul-
tures and biofilms, have been identified recently, and their potential to mediate
extracellular electron transfer for mineral reduction has been confirmed [106, 107].
1.3.4 Biofilm Reactors for Studying Uranium Immobilization
Our research group generally uses two types of reactors in uranium immobilization
studies: (i) flat plate reactors and (ii) fixed bed column reactors (FBCRs). The selec-
tion of the reactor depends on the research questions that need to be addressed. For
example, if we are interested in the depth profiles of sulfite in SRB biofilms, a flat
plate reactor will be used because this reactor allows us to monitor the location of
the microelectrode and image the biofilm structure simultaneously. However, if we
are only interested in quantifying uranium immobilization in biofilms, FBCRs are
preferred because we can select a wide range of minerals to study how mineral type
affects uranium immobilization.
1.3.4.1 Flat Plate Reactor
A flat plate reactor is typically a rectangular channel used to grow biofilms for
quantifying relations between biofilm structure and activity [83]. This type of reac-
tor allows us to (i) grow biofilms under well-defined hydrodynamic conditions, (ii)
use a variety of microscopic techniques to quantify biofilm structure, and (iii) use
microsensors to quantify chemical gradients in the biofilms.
A flat plate reactor typical of those we use in our biofilm studies is shown in
Fig. 1.3. The polycarbonate channel is 2.5 cm wide, 4.0 cm deep and 34.5 cm long.
The working volume of the reactor is around 150 mL including the tubing volume.
All fittings except the output line have a 3/8” opening width with a 1/8” plastic
pipe thread centered and placed near the edge. The output line is placed above the
recycle line, and the latter is used to control the total flow rate by recycling part of
the effluent to the reactor. The residence time in the reactor is determined by the
flow rate in the nutrient line only because the flow in the recycle line does not leave
the system and hence has no effect on the residence time.
The following protocol is used to operate the flat plate reactor to grow biofilms:
1. Autoclave the tubings, reactor, flow breakers and connectors.
2. Sterilize the entire system by fully filling the reactor with bleach (20% v/v) and
recycling for at least 2 h.
