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Uranyl Surface Complexes in a Mixed-Charge Montmorillonite: Monte Carlo Computer Simulation and Polarized XAFS Results

Published online by Cambridge University Press:  01 January 2024

Jeffery A. Greathouse*
Affiliation:
Department of Chemistry, St. Lawrence University, Canton, NY 13617, USA
Hannah R. Stellalevinsohn
Affiliation:
Department of Chemistry, St. Lawrence University, Canton, NY 13617, USA
Melissa A. Denecke
Affiliation:
Forschungszentrum Karlsruhe GmbH, Institut für Nukleare Entsorgung, Postfach 3640, 76021 Karlsruhe, Germany
Andreas Bauer
Affiliation:
Forschungszentrum Karlsruhe GmbH, Institut für Nukleare Entsorgung, Postfach 3640, 76021 Karlsruhe, Germany
Roberto T. Pabalan
Affiliation:
Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
*
*E-mail address of corresponding author: [email protected]
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Abstract

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We report a combined experimental and theoretical study of uranyl complexes that form on the interlayer siloxane surfaces of montmorillonite. We also consider the effect of isomorphic substitution on surface complexation since our montmorillonite sample contains charge sites in both the octahedral and tetrahedral sheets. Results are given for the two-layer hydrate with a layer spacing of 14.58 Å. Polarized-dependent X-ray absorption fine structure spectra are nearly invariant with the incident angle, indicating that the uranyl ions are oriented neither perpendicular nor parallel to the basal plane of montmorillonite. The equilibrated geometry from Monte Carlo simulations suggests that uranyl ions form outer-sphere surface complexes with the [O=U=O]2+ axis tilted at an angle of ~45° to the surface normal.

Type
Research Article
Copyright
Copyright © The Clay Minerals Society 2005

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