Reduction of Uranyl in the Interlayer Region of Low Iron Micas under Anoxic and Aerobic Conditions

The role of the interlayer region of three different Fe^II-poor micas (2.8, ∼0.02, and 0.01 Fe^II atom %) in the reduction of U^VI was investigated at pH ∼4.3, low ionic strength, and 23 °C under both anoxic and aerated conditions in batch reactors for 1 to 10 days. U diffusion profiles were mapped with X-ray microscopy, and the oxidation state of U was tracked with XANES as a function of distance from the crystal edge. Peak U concentrations only reached about 0.006−0.0002 atom percent, where the full-width-at-half-maximums of the diffusion fronts were roughly 25−50 μm. Results for anoxic conditions indicated appreciable reduction of uranyl in all three micas with no measurable dependency on distance from the edge. Under aerobic (open to the atmosphere) conditions, U was still partially reduced, although to a lesser degree than for anoxic conditions, and U^VI/U_tot increased with time. Interestingly, reoxidation reached a plateau after about 3−10 days, leaving an appreciable proportion of U reduced. It is likely that measurable reduction occurred, despite low Fe^II, because U concentrations were even lower, where Fe^II/U ratios ranged from about 10⁴−10². A coupled diffusion-reduction model was developed that successfully simulated the proximal portion of the diffusion front. The results show that reduction of U^VI in the interlayer of low Fe^II micas is, in principle, viable and that under aerobic conditions reduction is initially faster than reoxidation. This extends the range of possible environments where heterogeneous reduction of U^VI by micas might occur.