Adsorption of Cs+ Ion into Di- and Tri-Octahedral Vermiculites as Demonstrated by Classical Molecular Dynamics Simulation

A Takeuchi, MATERIALS TRANSACTIONS, 62, 469-478 (2021).

DOI: 10.2320/matertrans.MT-M2020274

The intrinsic adsorption of a Cs+ ion into di- and tri-octahedral vermiculites without the presence of K+ ions was demonstrated by a classical molecular dynamics (MD) simulation. The calculation conditions included Coulomb and Born-Mayer-Huggins potentials, assisted by Lennard- Jones potentials under a constant pressure ensemble and valences from a force field for clays (CLAYFF) mainly as well as conventional valences. A monoclinic di-octahedral vermiculite crystal with a 6 x 3 x 1 supercell was created using crystallographic data from a monoclinic tri- octahedral vermiculite, followed by conversion to a rectangular supercell with periodic boundary conditions along the x-axis. The simulated rectangular supercell of the di- and tri-octahedral vermiculite maintained its crystalline structure for 1 ps at 298 K using a constant step of 0.1 fs. Vacancies with diameters of 0.15 nm, which is nearly equal to the ionic size of Cs+, or larger were found at the octahedral (O)-sheet only in the di-octahedral vermiculite simulated with valences from CLAYFF. The further MD simulations were performed by placing a Cs+ ion at a vacancy at the O-sheet of the simulated state of the di-octahedral vermiculite, revealing that a vacant site can be a candidate of adsorbing Cs+ ion. The low degree of crystallinity of the di-octahedral vermiculite because of the octahedral cationic vacancy and the tilting of hydroxyl (OH) group from perpendicular to (001) provided an additional site for absorbing Cs+ ion in the O-sheet. The simulation result of the di-octahedral vermiculite simulated with valences from CLAYFF suggested a novel mechanism for Cs+ ions to firmly adsorb into vermiculite without being desorbed again.

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