Quantum-Mechanical Methods for Quantifying Incorporation of Contaminants in Proximal Minerals

Abstract

Abstract:Incorporation reactions play an important role in dictating immobilization and release pathways for chemical species in low-temperature geologic environments. Quantum-mechanical investigations of incorporation seek to characterize the stability and geometry of incorporated structures, as well as the thermodynamics and kinetics of the reactions themselves. For a thermodynamic treatment of incorporation reactions, a source of the incorporated ion and a sink for the released ion is necessary. These sources/sinks in a real geochemical system can be solids, but more commonly, they are charged aqueous species. In this contribution, we review the current methods for ab initio calculations of incorporation reactions, many of which do not consider incorporation from aqueous species. We detail a recently-developed approach for the calculation of incorporation reactions and expand on the part that is modeling the interaction of periodic solids with aqueous source and sink phases and present new research using this approach. To model these interactions, a systematic series of calculations must be done to transform periodic solid source and sink phases to aqueous-phase clusters. Examples of this process are provided for three case studies: (1) neptunyl incorporation into studtite and boltwoodite: for the layered boltwoodite, the incorporation energies are smaller (more favorable) for reactions using environmentally relevant source and sink phases (i.e., ΔE rxn (oxides) > ΔE rxn (silicates) > ΔE rxn (aqueous)). Estimates of the solid-solution behavior of Np to predict the limit of Np-incorporation into boltwoodite (172 and 768 ppm at 300 °C, respectively); (2) uranyl and neptunyl incorporation into carbonates and sulfates: for both carbonates and sulfates, it was found that actinyl incorporation into a defect site is more favorable than incorporation into defect-free periodic structures. In addition, actinyl incorporation into carbonates with aragonite structure is more favorable than into carbonates with calcite structure; and (3)

Uranium fate during crystallization of magnetite from ferrihydrite in conditions relevant to the disposal of radioactive waste

“…The equivalent model from atomistic simulations (Kerisit et al, 2011) does not distinguish between axial and equatorial O, however, the U-O distance reported (2.13 Å) corresponds well with the average distance reported here (2.11-2.22 Å) and from quantum-mechanical modelling (2.17 Å) (Shuller-Nickles et al, 2014). The Fe coordination observed here agrees well with quantummechanical modelling (Shuller-Nickles et al, Parameters were tied in a given fit.…”

“…The atomistic approach modelled the structure using incorporated U(IV), U(V) and U(VI) atoms in both octahedral and tetrahedral sites (Kerisit et al, 2011). In contrast, the quantum mechanical modelling used specific incorporation modes including: U(VI) incorporation into an octahedral site coupled with either octahedral or tetrahedral vacancies; and U(IV) incorporation into an octahedral site with either creation of an octahedral vacancy or Fe(III) reduction to Fe(II) to maintain charge balance (Shuller-Nickles et al, 2014). In the latter study, incorporation of U(VI) in an octahedral site in place of an Fe(III), with creation of an octahedral Fe(III) site vacancy was considered the most energetically favourable.…”

The Application of Periodic Density Functional Theory to the Study of Uranyl-Containing Materials: Thermodynamic Properties and Stability

“…The hydration and corrosion of the SNF under oxidizing conditions will result in the dissolution of the uranium dioxide composing the SNF matrix and the formation of uranyl secondary mineral phases [42][43][44][45][46][47][48][49][50][51][52][53][54]. Therefore, the formation and stability of uranyl minerals will determine the release of U(VI) and other actinide elements from the HLWR container and subsequently from the repository to the biosphere [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70].…”

Theoretical Studies of the Structural, Mechanical and Raman Spectroscopic Properties of Uranyl-Containing Minerals

“…The theoretical solid-state methods may be used, in conjunction with experimental techniques, as an interpretative tool of the experimental structural and vibrational data or as a predictive tool to determine the structural, vibrational, mechanical and thermodynamic properties of these substances. The understanding of the structures of these compounds is very important itself to characterize them and to evaluate the possible incorporation of transuranic elements and fission products into the structures of uranyl minerals [8,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41].…”

“…The release and the concomitant environmental impact of the fission products and transuranic elements present in the SNF to the biosphere [8,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] can be diminished by retention processes of these contaminants in the crystal structures of these secondary phases. The precise knowledge of their unit cells [27,28] is essential because it may be a used to understand and evaluate the incorporation of these elements into their crystal structures [8,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. Besides, the long-term performance assessment of DGRs requires the development of identification procedures and the characterization of the physical properties of these alteration products, which is a great challenge from the experimental point of view not only because these materials are very complicated involving the most elements of the periodic table but also due to its radiotoxicity [1].…”