Influence of the goethite (α-FeOOH) surface on the stability of distorted PuO2 and PuO2-x phases

Experiments by [Powell, B. A., Dai, Z. R., Zavarin, M., Zhao, P. H., Kersting, A. B.: Stabilization of plutonium nano-colloids by epitaxial distortion on mineral surfaces. Environ. Sci. Technol. 45, 2698 (2011).] deduced the heteroepitaxial growth of a bcc Pu4O7 phase when sorbed onto goethite from d-spacing measurements obtained from selected-area electron diffraction (SAED) patterns. The structural and/or chemical modification of Pu(IV) oxide (PO) nanocolloids upon sorption to goethite, in turn, affects colloidal-transport of Pu in the subsurface. In this study, molecular simulations were applied to investigate mechanisms affecting the formation of non-fcc PO phases and to understand the influence of goethite in stabilizing the non-fcc PO phase. Analyses of the structure, chemistry, and formation energetics for several bulk PuO2 and PuO2-x phases, using ab initio methods, show that the formation of a non-fcc PO can occur from the lattice distortion (LD) of fcc PuO2 upon sorption and formation of a PO-goethite interface. To strain and non-uniformly distort the PuO2 lattice to match that of the goethite substrate at ambient conditions would require 88 kJ/mol Pu4O8. The formation of a hypostoichiometric PuO2-x phase, such as the experimentally-deduced bcc, Ia (3) over bar Pu4O7 phase, requires more O-poor conditions and/or high energetic inputs (>+365 kJ/mol Pu4O7 at O-rich conditions). Empirical methods were also applied to study the effect of lattice distortion on sorption energetics and adsorbate particle growth using simple heterointerfaces between cubic salts, where KCl clusters (notated as KClLD) of varying size and lattice mismatch (LM) were sorbed to a NaCl cluster. When the lattice of a KClLD cluster has <15% LM with that of a NaCl substrate, the sorption of KCl LD onto NaCl is exothermic (<-80 kJ/mol) and the KCl LD cluster can reach sizes of similar to 2-5 nm on the NaCl substrate. These models suggest that the lattice of a fcc PuO2 particle can distort upon formation of a heterointerface with goethite to lower LM, in turn better enabling the growth of the PO adsorbates and yielding more exothermic adsorption energies. A more detailed understanding of the interfacial environment between PO and goethite is obtained through structural, chemical, and energetic analyses on modeled PuO2 (110)-and PuO2-x (110)-goethite (001) heterointerfaces. Structural analyses of the heterointerfaces continue to support that the lattice of PO is strained to better match that of goethite and thus lead to the formation of a non-fcc PO phase. When the lattice of the PO (110) surface is distorted to match that of the goethite (001) surface, the alignment and d-spacings from simulated electron diffraction patterns for the PO-goethite heterointerfaces reproduce experimental observations. Non-fcc PO thin-films are also found to be stabilized through the formation of an interface with goethite, as the work of adhesion for the PuO2- and PuO2-x-goethite interfaces are 1.4 J/m(2) and 2.0 J/m(2), respectively. Analyses of electron and charge density of the heterointerfaces also show that covalent-to polar-covalent bonding at the interface promotes the stabilization of a PO-goethite interface. The results from these models contribute to experimental observations, providing further understanding of how the goethite substrate influences the formation and stabilization of a non-fcc PO phase. Furthermore, the information from this study aids in better understanding processes at mineral-water interfaces that influence actinide transport.