Predicting equilibrium uranium isotope fractionation in crystals and solution

Abstract:

Despite the rapidly growing interest in using ²³⁸U/²³⁵U measurements as a proxy for changes in oxygen abundance in surface and near-surface environments, the present theoretical understanding of uranium isotope fractionation is limited to a few simple gas-phase molecules and analogues of dissolved species (e.g., 1,2,3). Understanding uranium isotope fractionation behavior in more complicated species, such as crystals and adsorption complexes, will help in the design and interpretation of experiments and field studies, and may suggest other uses for ³⁸U/²³⁵U measurements. In this study, a recently developed first-principles method for estimating the nuclear volume component of field shift fractionation in crystals and complex molecular species (4) is combined with mass-dependent fractionation theory to predict equilibrium ³⁸U/²³⁵U fractionations in aqueous and crystalline uranium compounds, including uraninite (UO₂).

The nuclear field shift effect, caused by the interaction of electrons with the finite volume of the positive charge distribution in uranium nuclei, is estimated using Density Functional Theory and the Projector Augmented Wave method (DFT-PAW). Tests against relativistic electronic structure calculations and Mössbauer isomer shift data indicate that the DFT-PAW method is reasonably accurate, while being much better suited to models of complex and crystalline species. Initial results confirm previous predictions that the nuclear volume effect overwhelms mass depdendent fractionation in U(VI)-U(IV) exchange reactions, leading to higher ²³⁸U/²³⁵U in U(IV) species (i.e., for UO_{2 xtal} vs. UO₂²⁺_aq, ln α^NV ≈ +1.8‰ , ln α^MD ≈ –0.8‰, ln α^Total ≈ +1.0‰ at 25ºC). UO₂ and U(H₂O)₉⁴⁺, are within ~0.4‰ of each other, while U(VI) species appear to be more variable. This suggests that speciation is likely to significantly affect natural uranium isotope fractionations, in addition to oxidation state. Tentatively, it appears that uranyl-type (UO₂²⁺-bearing) structures will tend to have higher ²³⁸U/²³⁵U than uranate-type structures that lack strong U=O bonds.

References: 1. Bigeleisen (1996) JACS 118:3676; 2. Schauble (2006) Eos 87:V21B-0570; 3. Abe et al. (2008) J Chem Phys 128:144309, 129:164309, & Abe et al. (2010) J Chem Phys 133:044309; 4. Schauble (2013) PNAS 110:17714.