Redox and Speciation Behavior of Plutonium in Mineralogically Complex Systems: The Role of Aluminum-Substitution in Iron (Oxyhydr)Oxides

Plutonium (Pu) is a radiotoxic element that is present in used nuclear fuel and reprocessing products, and at legacy contaminated sites. To address existing waste inventories and environmental contamination, it is essential to understand Pu fate and transport in subsurface environments, which is tied to its rich redox aquatic chemistry. Predictive geochemical models are necessary for the long-term performance assessments of nuclear waste repositories and radionuclide migration in contaminated sites. However, current models do not accurately predict actinide behavior under field conditions and legacy contaminated sites, and do not account for complexity in mineral assemblages, such as significant metal impurities or substitution. In this work, we studied mineralogically complex systems where Pu(V/VI) is the sorbate and Aluminum-substituted hematite (a-Fe2O3), or Aluminum-substituted goethite (a-FeOOH) are the sorbents. Using synchrotron-based high-energy resolution fluorescence detection X-ray absorption near-edge structure (HERFD-XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies targeting the M4 and L3 edges of Pu, we quantified of the extent of Pu surface-mediated reduction from Pu(V/VI) to Pu(IV), and the determined the Pu speciation in systems where controlled mineral complexity in iron oxides is induced. We found that Al-substitution in both hematite and goethite generates electronic changes and lattice distortions, and it slows down the sorption and ensuing reduction of Pu in short timescales. However, as the systems approached steady state, the extent of surface-mediated reduction to Pu(IV) was very similar to non-substituted iron (oxyhydr)oxides. We also found that Al-substitution in hematite and goethite leads to an increase in defects in the crystal morphology, which impacts the sequestration of Pu. In terms of Pu speciation, we found that at relatively high concentrations, precipitation of PuO2 nanoparticles occurs readily on the surface of corundum (a-Al2O3), while PuO2 nanoparticles and sorbed Pu species co-exist on the surface of hematite and Al-substituted hematite.