The interaction of Pu(IV) with low index ferrihydrite surfaces: a periodic boundary condition DFT study

Nanoparticulate ferrihydrite (Fh) has a strong affinity towards environmental contaminants, particularly radionuclides. Recently, Pu(IV) was found experimentally to form a tetradentate inner-sphere surface complex with Fh, motivating the present study of the interaction of Pu(IV) with Fh(100), Fh(110) and Fh(120) surfaces using DFT+U_eff. Prior to introduction of Pu(IV), we first discuss the effects of spin arrangement and the choice of U_eff on bulk Fh. The relaxed lattice parameters agree well with previous experiments and simulations, and band gaps (direct/indirect) are determined. The work function, bare and hydrated surface energies of the three terminations are in agreement with previous studies, though we highlight the need for further experimental work in this area. Multidentate binding to the Fh surfaces is highly favorable, with Pu(IV) surface complexation energies significantly exothermic (-3.01 to -6.24 eV). Average Pu-O and Pu-Fe distances are within 0.31 Å of EXAFS measurements, for the lowest energy complexes. Pu(IV) binding is tetradentate on Fh(110) and Fh(120) and tridentate on Fh(100). Surface complex stability depends on the charge of the Pu, indicating primarily ionic Pu-O bonds, though Pu(f) and O(p) states hybridise in the bonding region of the valence band. The Pu-O interactions are determined as partially covalent using the quantum theory of atoms in molecules, consistent with our previous findings for Pu(IV) bound to the Fe₁₃ Keggin cluster. Our work supports recent experimental evidence that Pu(IV) uptake begins via the Keggin and remains bound through transformation to Fh.