Local structural information of Cs in smectite hydrates by means of an EXAFS study and molecular dynamics simulations

To elucidate the behavior of Cs atoms and the adsorption sites for Cs atoms in clays, molecular structural information around Cs atoms adsorbed in bentonite (montmorillonite-beidellite) at pH 3.9, 7.1, and 9.8 has been investigated in extended X-ray absorption fine structure (EXAFS) spectroscopy using a crystalline Cs₂SO₄ as a model compound. Molecular dynamics (MD) simulations were also performed on beidellite. The basic cube cell used for MD simulations contained 3972 atoms, including two Cl atoms in the interlayer water. In EXAFS analyses, the first coordination shell formed by oxygen of water molecules was observed at a distance of 3.15-3.16 Å, with the coordination number of 7.0-7.4. The second coordination shell consisted of 6O atoms forming clay minerals at a distance of 3.59-3.62 Å. In MD simulations, many Cs atoms were found to roam much more freely over the mineral surface, and even in the interlayer water. The results suggested that the inner-sphere surface complexes coexisted with the outer-sphere surface complexes, and that the adsorbed sites were positioned nearby the edge of the basal oxygen hexagonal cavity on tetrahedral sheets in Kunipia F bentonite.