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Biological uranium (VI) reduction in suspended culture and fixed-media bioreactor systems using indigenous isolates of uranium (VI) reducing bacteria
Tailing dumps and process waste stockpiles at uranium mining sites and nuclear power
processing facilities contain significant levels of uranium. Uranium in the tailing dumps can
exist either as U(VI) or U(IV) depending on the pH and redox conditions within the dump.
However, it is desirable to keep uranium in the dump sites in its tetravalent form, U(IV),
since the hexavalent form, U(VI), is highly mobile and very toxic to aquatic life forms and
humans. Natural attenuation processes such as bacterial reductive/precipitation and
immobilization of soluble uranium emerge as viable method for remediating U(VI)
contaminated sites. For example, dissimilatory metal-reducing bacteria (DMRB) have been
investigated for their capability to remove uranium from aqueous solutions. These bacteria
were able to use U(VI) as an electron acceptor thereby reducing U(VI) to U(IV) which is
easier to remove from solution by precipitation.
In this study, the efficiency of indigenous culture of bacteria from the local contaminated site
in reducing U(VI) was evaluated using both batch and continuous flow bioreactor systems.
Because the stability of uranium in the tailing dumps and stockpiles of uranium concentrate at
uranium mining fields is affected by the pH, redox potential, the presence of complexing
anions in the waste rocks, toxic metals, organics, inhibitors, and chelators, the effect of these
factors in U(VI) bioremediation process was also evaluated in this study. Batch kinetics studies showed near complete U(VI) removal of up to 400 mg/L. Experiments on suspended
culture bioreactor system conducted in 10 L Erlenmeyer’s flask under shock loading
conditions also showed U(VI) removal of up 400 mg/L. Higher U(VI) removal rates achieved
in a suspended culture system operated without re-inoculation were associated with
continuous addition of nutrients and glucose in a bioreactor over time. This demonstrate the
effectiveness of carbon source and nutrients in enhancing U(VI) reduction process in
bioreactor systems.
Further experiments were conducted in a fixed-film, continuous flow bioreactor system to
evaluate the capacity of the indigenous mixed culture in reducing U(VI) under oxygen
stressed and nutrient deficient conditions. The experiments in the fixed-film bioreactor
system were conducted using columns with four equally spaced intermediate sampling ports
along the length to facilitate finite difference modelling of the U(VI) concentration profile
within the column. Near complete U(VI) removal of up to 85 mg/L was achieved in the
fixed-film bioreactor operated without organic carbon source. At higher U(VI) feed
concentration of 100 mg/L the bioreactor system was able to achieve the removal efficiency
of 60%. A sterile control column on the other hand showed insignificant U(VI) removal over
time, indicating U(VI) removal by biochemical processes. The shift in microbial culture was
monitored in the fixed-film bioreactor after 99 days of exposure to U(VI) using the 16S
rRNA genotype fingerprinting method.
The fate of U(VI) within a complex biofilm structure was predicted and evaluated using
mathematical modelling. The mathematical model developed in this study for describing the
biofilm system incorporated both the mass transport kinetics, microbial growth kinetics, and
reduction kinetics, thus the diffusion-reduction equation. The model successfully captured the
trends of U(VI) removal within the biofilm for different loading conditions. The validity of
the model in predicting U(VI) reduction within the bench-scale biofilm reactor at various
U(VI) concentrations demonstrated the feasibility of the model in predicting field scale
system and improving design and operation of site for clean-up.