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MONTE CARLO AND MOLECULAR DYNAMICS SIMULATION OF URANYL ADSORPTION ON MONTMORILLONITE CLAY

¹ Department of Chemistry, St. Lawrence University, Canton, NY 13617, USA
² Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA

^* E-mail address of corresponding author: jgreathouse{at}stlawu.edu

We performed Monte Carlo and molecular dynamics simulations to investigate the interlayer structure of a uranyl-substituted smectite clay. Our clay model is a dioctahedral montmorillonite with negative charge sites in the octahedral sheet only. We simulated a wide range of interlayer water content (0 mg H₂O/g clay – 260 mg H₂O/g clay), but we were particularly interested in the two-layer hydrate that has been the focus of recent X-ray absorption experiments. Our simulation results for the two-layer hydrate of uranyl-montmorillonite yield a water content of 160 mg H₂O/g clay and a layer spacing of 14.66 Å. Except at extremely low water content, uranyl cations are oriented nearly parallel to the surface normal in an outer-sphere complex. The first coordination shell consists of five water molecules with an average U–O distance of 2.45 Å, in good agreement with experimental data. At low water content, the cations can assume a perpendicular orientation to include surface oxygen atoms in the first coordination shell. Our molecular dynamics results show that UO₂(H₂O)²⁺ 5 complexes translate within the clay pore through a jump diffusion process, and that first-shell water molecules are exchangeable and interchangeable.

Key Words: Computer Simulation • Molecular Dynamics • Monte Carlo • Montmorillonite • Uranyl

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