The interaction of Pu(IV) with the hematite (001) terminations: a periodic boundary condition DFT study

The Enhanced Actinide Removal Plant (EARP), at Sellafield in the UK, is tasked with separating waste actinide species from an aqueous waste stream via base-induced hydrolysis of Fe(III). During this flocculation process ferrihydrite forms and the actinides interact strongly with the surface. It has been shown that Pu remains sorbed in the solid state over long periods of time, during which ferrihydrite undergoes transformation into hematite. It is of critical importance to the operations of future Geological Disposal Facility (GDF) technologies that the Pu@hematite system is studied to understand the binding strength and sorption mechanism. Here we present a comprehensive study of Pu(IV) binding to two well-established basal terminations of hematite using periodic DFT+U_eff. First, we outline our methodology and demonstrate correct prediction of the bulk hematite lattice parameters and electronic band gap, then we generate the (001)-Fe and (001)-O₃ terminations and demonstrate reasonable predictions of the surface energies, inter-layer spacings, and work functions. The (001)-Fe termination is then hydrated with a monolayer of water. We show that Pu(IV) forms multiple Pu-O bonds with both terminations at distances consistent with experimental EXAFS measurements. Density of states and charge density difference analysis reveals strong hybridisation between the Pu(5f) and O(2p) states supporting charge transfer as indicated by depleted charge density surrounding the Pu atom on the surface. Quantum Theory of Atoms in Molecules analysis shows that the Pu-O bonds are partially covalent, in agreement with our previous assessment of Pu bound to α-Fe₁₃, a prenucleation cluster to ferrihydrite (Fh), and Pu bound to ferrihydrite surfaces. The reaction energies for surface binding are significantly exothermic, even more so than was found in our previous analysis of the Pu@Fh surfaces, indicating that Pu(IV) should remain immobile and bound to hematite, particularly in oxygenated conditions as may be found in the GDF or subterranean environments.