Adsorption of Cs+ Ion into Di- and Tri-Octahedral Vermiculites as Demonstrated by Classical Molecular Dynamics Simulation
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 × 3 × 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.