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Iron vacancies accommodate uranyl incorporation into hematite

DOI: 10.1021/acs.est.8b00297 DOI Help

Authors: Martin E. Mcbriarty (Pacific Northwest National Laboratory) , Sebastien Kerisit (Pacific Northwest National Laboratory) , Eric J. Bylaska (Pacific Northwest National Laboratory) , Samuel Shaw (The University of Manchester) , Katherine Morris (The University of Manchester) , Eugene S. Ilton (Pacific Northwest National Laboratory)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Environmental Science & Technology

State: Published (Approved)
Published: May 2018
Diamond Proposal Number(s): 7367 , 7593 , 8070

Open Access Open Access

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 (α-Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite 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 synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite crystal structure 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 low temperature hydrothermal conditions.

Subject Areas: Environment, Earth Science, Engineering


Instruments: B18-Core EXAFS

Other Facilities: SSRL; APS