Environmental Engineering Reference
In-Depth Information
The basic concept of uranium bioremediation through U(VI) immobilization is
to harness indigenous microorganisms to reduce U(VI) and form sparingly soluble
U(IV) minerals. This U(VI) bioremediation model has been shown to be feasi-
ble by multidisciplinary researchers. The reduction of U(VI) by microorganisms
in groundwater and aquifer sediments has been demonstrated in the laboratory;
however, electron donors are usually limited in natural aquifer systems [57-61].
Electron donors such as acetate, lactate, or ethanol are typically provided to stim-
ulate the U(VI) bioreduction during
in situ
U(VI) bioremediation in field studies.
Contaminated sites managed by the U.S. Department of Energy Uranium Mill
Tailing Remedial Action (UMTRA) program at Oak Ridge and Rifle have been
extensively studied. Representative field studies on the
in situ
bioremediation of
U(VI) are summarized in Table 1.4.
1.2.2.3 Redox, Abiotic and Biotic Reactions
In order to enhance bioremediation efficiency in the field, a thorough knowledge
of the local geochemistry and microbial metabolic activities is required because
the mobility and fate of uranium in the environment are mainly controlled by
redox reactions and biotic and abiotic processes. Redox reactions and abiotic pro-
cesses are governed by geochemical parameters such as reduction potential E
h
,pH,
temperature and ligand concentrations [32, 27].
Redox Reactions of U(VI) and U(IV)
The redox reactions of U(VI) and U(IV) typically result in substantial changes
in uranium solubility and bioavailability through: (i) immobilization of uranium
when U(VI) is reduced to U(IV), forming insoluble U(IV)-bearing minerals such
as uraninite UO
2+x
and decreasing uranium bioavailability [62], and (ii) mobiliza-
tion of uranium when U(IV) is oxidized to U(VI), increasing uranium solubility and
bioavailability. In natural environments, the E
h
of the U(IV)/U(VI) couple should
fall in the range of -0.042 to 0.086 against the standard hydrogen electrode poten-
tial (V
SHE
), depending on the Ca
2+
and CO
2
3
concentrations [63]. For the reduction
of U(VI), the development of a low redox potential is essential but not sufficient
because other geochemical factors also play important roles in uranium solubil-
ity [64]. For example, under slightly acidic conditions, uranium immobilization is
expected to be more sustainable. However, the pH should not be too low, because
U(IV) may become soluble in an environment with a pH lower than 4 [32].
Abiotic Processes that Control Uranium Mobility
Abiotic U(IV) oxidation occurs when O
2
,NO
3
, Fe(III), and Mn(IV) are reduced
[65-67]. H
2
S, H
2
, and Fe(II) reduce U(VI) at a slower rate compared to oxida-
tion by O
2
, Fe(III) and Mn(IV), and the reduction can be inhibited by uranium
complexes. In fact, complexation with other chemical species such as carbonate is
another important factor that determines uraniummobility and fate in environmental
