40    OPTIMIZATION OF TREATMENT
PARAMETERS DURING BIOREDUCTIVE
PRECIPITATION OF URANIUM USING A BENCH
SCALE FERMENTER
Zabin F. Rahman, Kevin G. Robinson, and Gregory D. Reed
Department of Civil and Environmental Engineering
219A Perkins Hall
The University of Tennessee
Knoxville, Tennessee 37996
ABSTRACT
Evaluation of a bench-top anaerobic bioreactor for microbially-mediated reductive precipitation of hexavalent uranium was addressed in this work. Selected treatment parameters were studied to determine the impact of each on treatment performance. Under the conditions tested, soluble uranium was effectively removed from solution.
INTRODUCTION
Traditional treatment practices for uranium-bearing waste streams at many Department of Energy (DOE) facilities generate significant quantities of secondary sludges which are classified as
mixed wastes. Currently no economical technology exists for the permanent disposal of these
mixed-waste sludges. Selective removal of uranium, the principal radioactive component in
many mixed-waste sludges, would allow the remaining waste stream to be treated as a Resource
Conservation and Recovery Act (RCRA) waste by conventional treatment. In addition, volume
reduction would make the storage or disposal of radioactive uranium sludge more economical.
Conceivably, uranium could be recycled with minimal additional processing.
The recent discovery that certain microorganisms can enzymatically catalyze reduction of
aqueous uranium provided a potential alternative treatment option for uranium removal. In a nutrient limiting environment, anaerobic sulfate reducing bacteria use uranium as an electron sink
for respiratory metabolism. The reduced form of uranium is sparingly soluble in water and tends
to precipitate quickly under favorable conditions. Since microbial reactions are induced by target
specific enzymes, this bioreduction method can selectively remove uranium from aqueous waste
streams. Batch experiments conducted to evaluate the scientific basis of this technology revealed
that a sulfate reducing bacteria, Desulfovibrio desulfuricans, successfully reduced hexavalent
uranium to the tetravalent state in 25-mL pressurized test tubes under anaerobic conditions. To
effectively use this bioreduction process as a uranium removal technique, additional information
concerning the technological aspects of treatment is required.
The focus of this research was to scale-up the uranium bioreductive precipitation process using
a bench top anaerobic bioreactor and to evaluate biological performance under nongrowth conditions. The influence of treatment parameters such as the extent of anaerobic gas input, reactor
temperature, the rate of mixing during uranium reduction, and uranium loading were evaluated
using a synthetic test solution containing 0.03 M bicarbonate mixed with depleted uranium
(aqueous) and a bacterial inoculum.
52nd Purdue Industrial Waste Conference Proceedings, 1997, Ann Arbor Press. Chelsea, Michigan 48118. Printed in
U.S.A.
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