PNNL

Abstract

Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (-Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped material depend on the local coordination environment of incorporated uranium. While measurements of uranium coordination have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate EXAFS spectra which were fitted to experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite lattice accommodates a trans-dioxo uranyl-like configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies situated at opposing corner-sharing lattice sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics over thermodynamics in determining the state of an impurity incorporated in a host phase under hydrothermal conditions.