Long-Term Dynamics of Uranium Reduction/Reoxidation under Low Sulfate Conditions
The biological reduction and precipitation of uranium has shown potential to prevent uranium migration from contaminated areas. Although previous research has shown that uranium bioremediation is maximized during iron reducing conditions, little research has been performed to understand how long iron/uranium reducing conditions can be maintained. Similarly, questions remain about the stability of the bioreduced uranium and that of the uranium-reducing microbial population after iron/uranium biostimulation conditions are terminated and an oxidant (i.e. oxygen) is introduced into the previously reduced zone. To gain further insights into these processes, columns, packed with sediment containing iron as Fe-oxides (mainly Al-goethite) and silicate Fe (Fe-containing clays), were operated in the laboratory under field-relevant flow conditions to measure the long-term (> 200 d) removal efficiency of uranium from a simulated groundwater during biostimulation with acetate under low sulfate conditions. The biostimulation experiments were then followed by reoxidation of the reduced sediments with oxygen. During biostimulation, Fe(III) reduction occurred simultaneously with U(VI) reduction. Both Fe-oxides and silicate Fe(III) were partly reduced, and silicate Fe(III) reduction was detected only during the first half of the biostimulation phase while Fe-oxide reduction occurred throughout the whole biostimulation period. Mössbauer measurements indicated that the biogenic Fe(II) precipitate resulting from Fe-oxide reduction was neither siderite nor FeS0.09 (mackinawite). U(VI) reduction efficiency increased throughout the bioreduction period, while the Fe(III) reduction gradually decreased with time. Effluent Fe(II) concentrations decreased linearly by 30% over the final 100 days of biostimulation, indicating that bioreducible Fe(III) in the sediment was not exhausted at the termination of the experiment. Even though Fe(III) reduction did not change substantially with time, microorganisms not typically associated with Fe(III) and U(VI) reduction (including methanogens) became a significant fraction of the total microbial population during long-term biostimulation, meaning that most acetate was utilized for other biological processes than Fe(III) and U(VI) reduction. Selected columns were reoxidized after 209 days by discontinuing acetate addition and purging the influent media with a gas containing 20% oxygen. Uranium reoxidation occurred rapidly with 61% of the precipitated uranium resolubilized and transported out of the column after 21 days and virtually all of the uranium being removed by day 122. During the first 21 days of reoxidation, the Fe(III) and U(VI) reducing microbial population remained at pre-oxidation levels (even though the methanogen population decreased by 99%) indicating that short-term disruptions in biostimulation (equipment failure, etc.) would not negatively affect the uranium reducing microbial population.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 949107
- Report Number(s):
- PNNL-SA-57288; GCACAK; 25608; KP1504010; TRN: US0901726
- Journal Information:
- Geochimica et Cosmochimica Acta, 72(15):3603-3615, Vol. 72, Issue 15; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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