Environmental Engineering Reference
In-Depth Information
conditions, abiotic uranium reduction is inhibited by the formation of the U(VI)-
CO 3 complexes that exist in groundwater. There are several bacteria capable of
reducing U(VI)-CO 3 complexes enzymatically to uraninite. The abiotic reoxida-
tion of uranium by Fe(III) and Mn(IV) oxides has been well demonstrated in the
laboratory and in the field [12].
Biotic, Abiotic and Redox Conditions in Biofilms
In U(VI) bioremediation research, most of the studies have been conducted in the
presence of suspended microorganisms or enriched sediments [75-78], eventually
spiked with micro- or nano-particles of other minerals [69, 67]. However, in natural
soils and water-sediment interfaces, microorganisms are commonly found in the
form of surface-associated cells, or biofilms [79-81].
The metabolic activity of cells in biofilms, which is expected to be different from
that of suspended cells, can affect the abiotic and redox reactions controlling the
mobility of U(VI) in the environment. For example, when oxygen is introduced into
a suspended cells system, one can expect to observe the reoxidation and remobiliza-
tion of immobilized uranium in the system; however, when a biofilm is exposed to
oxygen, some of the uranium near the interface of the biofilm and the bulk solution
may be mobilized due to reoxidation while uranium in the deeper portion of the
biofilm remains immobilized because oxygen does not reach there and the reducing
conditions are still present inside the biofilm [82, 83]. In addition, the biotic reduc-
tion of nitrate to nitrogen occurring after the depletion of oxygen produces nitrite,
nitrous oxide and nitric oxide and these intermediately oxidize U(IV) [84, 85, 66].
These situations can be important, especially if the contaminated site is mostly
aerobic. The development of biofilm on surfaces may create conditions for nitrate
reduction. All these combined biotic, abiotic and redox conditions pose challenges
in predicting the mobility of uranium in biofilms. U mobility has been frequently
described for bulk solutions and for planktonic cultures. However, uranium immo-
bilization in subsurface-attached biofilms with heterogeneous local conditions can
be significantly different from that in bulk conditions.
1.3 Biofilms Immobilizing Uranium
1.3.1 Definition of Biofilms
Figure 1.2 illustrates our definition of biofilms related to bioremediation. In
Fig. 1.2a, single cells are shown attached to a surface. The presence of even a few
cells will affect abiotic and redox reactions on the surface. When the cell number
increases (Fig. 1.2b) the surface-associated bacteria will cover a significant amount
of surface (Fig. 1.2c) and begin to control the chemistry on the mineral surface.
Throughout this chapter we will use the term “biofilm” to refer to surface-associated
cells as shown in Fig. 1.2. This definition is not new and has been described by
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