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

抄録

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.

Fig. 5 (a), (b) Polyhedral views of the di-octahedral vermiculite simulated with CLAYFF valences without drawing the O ion and containing Cs⁺ ion in the vacancy at its states of t = 0 and 5 ps. (c), (d) The trajectories of Cs⁺, drawn with red curves on the Cs⁺ as a ball view, together with those of Si and Al ions drawn with red and green curves, respectively, of the di-octahedral vermiculite at t = 0 and 5 ps.