Redox Behavior of Uranium at the Nanoporous Aluminum Oxide-Water Interface: Implications for Uranium Remediation
- Department of Geoscience, University of Wisconsin – Madison, Madison, Wisconsin 53706, United States
- Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
Microbial reduction of soluble U(VI) to sparingly soluble uraninite (UO2) has received considerable attention as an efficient remediation technique for the subsurface uranium contamination in DOE facilities. An important unanswered question is whether U(VI) sorbed on a reactive mineral surface is subject to efficient reduction reaction or not, as is the case for aqueous U(VI). Answering this question is important because the efficiency of in-situ bioremediation would depend on how effectively and rapidly U(VI) bound to sediment or soil can be converted to an immobilized phase. To investigate the potential linkage between U(VI) sorption affinity and the redox reactivity of U(VI) sorbed on a mineral surface, U(VI) of 2~5 uM/g bound to the surfaces of nanoporous synthetic alumina (SSA of 230 m2/g; nanoporosity of 145 uL/g < 5 nm), mesoporous activated alumina (SSA of 147 m2/g; nanoporosity of 90 uL/g < 5 nm), and non-nanoporous alpha alumina (SSA of 8 m2/g; nanoporosity of 5 L/g < 5 nm) were reduced by 1 mM hydroquinone (AH2DS) over 1 day to 3 weeks in an anaerobic chamber. Reduction of ~100 uM dissolved U(VI) in 2 mM HCO3 to UO2+x by 1 mM AH2DS is completed within 1 hour. XRD and TEM analyses confirmed the precipitation of nano-sized (<~5 nm) UO2+x. The extent and kinetics of reduction of U(VI) sorbed on the surfaces of aluminum oxides is semi-quantitatively estimated by comparing the U(VI) desorption percentages by anoxic sodium bicarbonate solution of 10~1000 mM between 1 mM AH2DS- reduced and unreduced samples, given that only U(VI) not U(IV) on the aluminum oxides is supposed to react with sodium bicarbonate. The anoxic bicarbonate extraction results with a remarkable difference in U(VI) desorption % between reduced and unreduced samples indicate that roughly a half of U(VI) sorbed on non-nanoporous alumina is reduced by 1 mM AH2DS in 1 day, then the reduction of sorbed U(VI) is nearly completed in 2 days, while there seemed to be only a partial and incomplete U(VI) reduction on the surface of nanoporous or mesoporous alumina over almost 3 weeks, as indicated by a minor difference in U(VI) desorption %. X-ray absorption spectroscopy analysis of duplicate samples shows a consistent result that U(VI) sorbed on non-nanoporous alumina was completely reduced by 1 mM AH2DS, within 3 days, while only 50% and 30% of U(VI) sorbed on mesoporous and nanoporous alumina were reduced within 3 days, respectively, and there was no further U(VI) reduction reaction for 2 weeks, suggesting rapid U(VI) redox equilibration on the surface of aluminum oxides. The partial and incomplete reduction of U(VI) on nanoporous or mesoporous alumina, which has been frequently observed from bioreduction of U(VI) in natural soil and sediment in laboratory or field-scale experiments, is attributed to higher sorption affinity of U(VI) on those nanoporous or mesoporous surfaces as illustrated by U(VI) desorption of 8 and 25 % from nanoporous and mesoporous alumina by 50 mM HCO3, compared to 90% desorption from non-nanoporous alumina for ~3 weeks.
- Research Organization:
- Univ. of Wisconsin, Madison, WI (United States); Argonne National Laboratory (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Biological and Environmental Research (BER)
- DOE Contract Number:
- SC0002150
- OSTI ID:
- 1096426
- Report Number(s):
- DOE-UW-02150-1; 26XI
- Journal Information:
- Environmental Science and Technology, Vol. 46, Issue 13; ISSN 0013-936X
- Publisher:
- American Chemical Society (ACS)
- Country of Publication:
- United States
- Language:
- English
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