Relationship between Mineralogically Complex Iron (Oxyhydr)oxides and Plutonium Sorption and Reduction: A High-Energy Resolution X-ray Absorption Spectroscopy Perspective

To facilitate the continued use of commercial nuclear power and address environmental contamination, it is essential to understand the fate and transport of plutonium (Pu) in (sub)surface environments. Current geochemical models do not account for complexity in mineral assemblages, such as metal substitution or the role of nanoscale crystallite sizes. In this work, we studied mineralogically complex systems where Pu(V) was the sorbate and Al-substituted or nanoscale iron (oxyhydr)oxides were the sorbents. Using M₄-edge and L₃-edge high-energy resolution fluorescence detection X-ray absorption near-edge structure (HERFD-XANES) spectroscopy, we probed the electronic configuration of Pu, quantified the extent of Pu surface-mediated reduction, and explored Pu speciation. Our results indicate that nanoscale iron oxides exert a greater degree of control over the redox behavior of Pu than Al-substituted iron (oxyhydr)oxides under circumneutral pH and oxic conditions. This is due to the dependence of Pu surface-mediated reduction on an initial sorption step, which is greater with the increased specific surface area and reactivity of nanoscale crystallites.