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 FeII-poor micas (2.8, ∼0.02, and 0.01 FeII atom %) in the reduction of UVI 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 UVI/Utot 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 FeII, because U concentrations were even lower, where FeII/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 UVI in the interlayer of low FeII 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 UVI by micas might occur.