REDOX BEHAVIOR OF URANIUM on NANOPOROUS SURFACES OF ALUMINUM OXIDE

Microbial reduction of soluble U(VI) to sparingly soluble uraninite (UO₂) has received considerable attention as an efficient remediation technique for subsurface soil/sediment and groundwater in uranium contaminated 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) on the solid phase can be converted to an immobilized phase.

To investigate the effect of U(VI) sorption affinity on the redox reactivity of U(VI) sorbed on a mineral surface, U(VI) of 2.5~3 uM/g bound to the surfaces of nanoporous synthetic alumina (SSA of ~250 m²/g; average pore size of 2~3 nm) and non-nanoporous alpha alumina (SSA of ~10 m²/g; with some inter-crystal pore size of > 10 nm) are reduced by 1 mM hydroquinone (AH₂DS) over 1 day to 3 weeks in an anaerobic chamber. Reduction of ~100 mM dissolved U(VI) to UO₂ by 1 mM AH₂DS without a mineral is completed within 1 hour. XRD and TEM analyses confirmed the precipitation of nano-sized (~5 nm) UO₂. The reduction percentage of U(VI) sorbed on the surfaces of nanoporous and non-nanoporous alumina is estimated by comparing the U(VI) desorption percentage by anoxic bicarbonate solution of 10~1000 mM between 1 mM AH₂DS- reduced and unreduced samples, which will be confirmed by X-ray Absorption Spectroscopy. About 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. In contrast, U(VI) on nanoporous alumina was non-labile to reduction by 1 mM AH₂DS over 3 weeks. This is attributed to higher sorption affinity of U(VI) on nanoporous surfaces than non-nanoporous surfaces as illustrated by U(VI) desorption of ~80% from non-nanoporous alumina but ~3% from nanoporous alumina by 50 mM HCO₃. Given that the amount of U(VI) extracted by 50 mM bicarbonate is comparable to the amount of reduced U(VI) on alumina surfaces, the 50 mM bicarbonate extraction seems to be an accurate method to estimate the reducible U(VI) in natural soil and sediment. Bioremediation of U(VI) in DOE sites is likely effective only for labile solid phase U(VI) that is extractable by 50 mM bicarbonate.