Environmental Engineering Reference
In-Depth Information
1.2.2 Bioremediation of Uranium
Bioremediation is defined as the use of microbes to detoxify contaminants present
in the environment [28]. There are a number of
ex situ
and
in situ
bioremediation
methods currently available [29]. Bioremediation usually works by either trans-
forming or degrading contaminants into nonhazardous or less hazardous chemicals,
so-called biotransformation and biodegradation, respectively [30]. Unlike organic
compounds, metals and radionuclides such as uranium cannot be biodegraded [27].
Uranium bioremediation immobilizes uranium by converting soluble U(VI) species
into insoluble U(IV) species. In general, laboratory tests and
ex situ
bioremedia-
tion applications have shown that microorganisms can change the oxidation states
of various heavy metals and radionuclides, either increasing contaminant mobility
to provide a route for removal from solid matrices such as soils, sediments, dumps
and industrial waste, or immobilizing the metals and radionuclides, which enables
transformation into insoluble, chemically inert forms which are thus removed from
the aqueous phase [31, 27].
1.2.2.1 Uranium Bioimmobilization Mechanisms
Within the subsurface, microbial activity can be harnessed for uranium immobi-
lization through various processes [32, 3, 12, 27]: (i) direct or indirect reduction
of U(VI) to U(IV); (ii) biosorption of uranium onto the cells; (iii) precipitation of
uranium by organic complexing ligands produced by the cells; (iv) bioaccumulation
of uranium in the cytoplasm through chelating to polyphosphate bodies or form-
ing needle-like fibrils. A schematic illustration of the various mechanisms of U(VI)
bioremediation using bacterial cells is shown in Fig. 1.1.
Uranium Reduction
The microbial reduction of soluble U(VI) to insoluble U(IV), preventing its
migration with groundwater, has been proposed for the immobilization of uranium
[33, 34]. The reduction can be carried out directly by microbes or indirectly through
electron transfer by metal-reducing bacteria [35]. Much research is going into efforts
to develop
in situ
subsurface bioremediation technology for the U.S. Department
of Energy Contaminated Sites. The first report on the microbial reduction of U(VI)
appeared around 50 years ago: the reduction was demonstrated in crude extracts
from
Micrococcus lactilyticus
(reclassified as
Veillonellar alcalescens
) [36].
Lovley and coworkers did pivotal work in establishing the reduction of U(VI) by
dissimilatory metal-reducing bacteria (DMRB) such as
Desulfovibrio desulfuricans
and
Shewanella oneidensis
[33, 37, 34]. Currently, more than 25 phylogenetically
diverse species of prokaryotes are known to be capable of U(VI) reduction [12].
Table 1.1 shows selected bacteria that are reported to be involved in U(VI) reduction.
The mechanism of electron transport during U(VI) reduction by DMRB has
not been conclusively elucidated [27]. Recently, Renshaw et al. [38] demonstrated
that, in
G. sulfurreducens
, single-electron reduction of U(VI) to U(V) followed
