Environmental Engineering Reference
In-Depth Information
A column filled with sediments from the contaminated sites was used in a FBCR
(Fig. 1.4b). The FBCR was stimulated with lactate, succinate, malate, and fumarate
in artificial groundwater. Samples were taken periodically from the column outlet
and analyzed. The total uranium accumulated in the biofilm reactor over time is
shown in Fig. 1.12. We found that 63.5% of the uranium was removed in around 40
days, which is somewhat lower than what we expected based on previous studies
using pure culture and synthetic minerals in the laboratory. During the U(VI) biore-
mediation in this biofilm reactor, we found that Fe(III) and Mn(IV) were released
from the sediments. Both Fe(III) and Mn(IV) have been shown to have the capabil-
ity of competing with U(VI) as electron acceptors, which may have contributed to
the discrepancies between these results and those of previous studies using minerals
containing no Fe or Mn oxides. In addition to the mechanisms of U immobilization
and remobilization illustrated in Fig. 1.11, the release of Mn(IV) and Mn(II) may
have similar effects to those of Fe(II) and Fe(III) during U immobilization using
indigenous biofilms grown on natural sediments. Extensive research work will be
needed to elucidate the complex interactions between biofilms and redox-sensitive
minerals.
1.4 Conclusion
Uranium is one of the most common radionuclides in soils, sediments, and ground-
water at the DOE contaminated sites and must be remediated. Because indigenous
microorganisms are readily available at the contaminated sites, bioremediation
through natural attenuation by microbial processes has become a preferred strat-
egy for in situ uranium remediation. The basic concept of uranium bioremediation
is to immobilize U(VI) by harnessing indigenous microorganisms in groundwater
and aquifer sediments to reduce U(VI) and form sparingly soluble U(IV) minerals,
which has been shown to be feasible by multidisciplinary researchers.
Most uranium immobilization studies have been conducted in the presence of
suspended microorganisms or enriched sediments, eventually spiked with micro- or
nano- particles of other minerals. However, in natural soils and water-sediment inter-
faces, microorganisms are commonly found in the form of surface-associated cells,
or biofilms. In the past several years, using SRB biofilms as a model, the feasibility
of U(VI) immobilization in biofilms growing on redox-insensitive surfaces (quartz
or glass slides) and redox-sensitive surfaces (carbonate-bearing minerals) has been
studied. In the absence of bicarbonate, SRB biofilms have been shown to immobilize
U(VI) for significant amounts of time as the result of both enzymatic and chemical
reduction of U(VI) to insoluble uraninite. In the presence of carbonate buffer, the
uranium removal extent and rate were satisfactory in the SRB biofilm reactors after 5
months of operation. In addition, SRB biofilms grown on carbonate-bearing miner-
als have been shown to be able to immobilize uranium and the precipitated uranium
has been shown to be stable as long as sulfate-reducing conditions are satisfied in
the biofilm reactor over a long period of time (4-5 months).
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