![]() ![]() The DFT calculations were performed with the Dmol3 module of the Materials Studio 2017 software. The model was constructed to obtain a (001) surface and a (010) surface with 81 and 93 atoms, respectively. In the (010) surface ( Figure 1B), the exposed Si and Al form hydroxyl groups, such as ≡Si-OH and ≡Al-OH. Consequently, competitive adsorption exists between the cations and the surface. Substituting Al 3+ by Mg 2+ in the AlO 6 octahedral layer renders the surface negatively charged, which can become neutral upon absorbing cations from the environment. The montmorillonite (001) surface is composed of two sheets of tetrahedral (T) SiO 4 enclosing a sheet of octahedral (O) AlO 6 resulting in a TOT structure ( Figure 1A). To study the adsorption mechanism of Cd 2+ on different crystal planes of montmorillonite, the (001) and (010) surfaces were constructed, and a vacuum layer of 15 Å was added above the surface to avoid periodic influence ( Figure 1). To meet the structural requirements of montmorillonite, 2 × 1 × 1 supercrystal cells were constructed, lattice substitution was set, and the chemical formula of the final cell was Na 0. The Wardle model was adopted for the montmorillonite crystal cells. Conversely, MD simulations can be applied to systems with a large number of molecules but cannot explore the mechanism of the chemical bonding. However, DFT has limited accuracy in the presence of a large number of molecules due to the numerous calculations involved. ![]() relied on MD to study the adsorption of NH 4 + between montmorillonite layers and found that the interaction of NH 4 + and water molecules between these layers were affected by surface lattice substitution, charge, and the content of water stratification. ![]() concluded via DFT that the selectivity of flotation agents to minerals and ions is related to the symmetry of the molecular orbitals. investigated the adsorption of Ca(OH) + on montmorillonite (001) and (010) surfaces by DFT, and the results revealed that the cations were adsorbed on the (001) surface electrostatically and on the (010) surface via hydrogen bonds. Recently, density functional theory (DFT) calculations and molecular dynamics (MD) simulations have been employed to understand the ion adsorption mechanism. In most of the studies, montmorillonite adsorption of heavy metal ions has been investigated on the macroscopic level only a limited number of studies have discussed the adsorption mechanisms at the microscopic scale. reported the synthesis of a waste-paper montmorillonite composite aerogel to adsorb Cd 2+ by ambient pressure drying technology followed by modification with NaOH or H 2O 2, and the NaOH-modified aerogel revealed a maximum adsorption capacity of 232.5 mg/g. The adsorption process was endothermic, and Pb 2+ was removed by 99.45%. investigated the removal of Pb 2+ from aqueous solutions by nano illite/smectite clay and revealed that the adsorption patterns followed the Langmuir adsorption isotherm models. reported the synthesis of chitosan-2D montmorillonite with good adsorption properties for Pb 2+ by the stripping and modification of montmorillonite. Currently, adsorbent materials are mainly obtained by changing their particle size or composition. Montmorillonite and other clay minerals have been used as adsorption materials due to their low cost, easy access, large specific surface area, and strong surface charge. This study provides the theoretical background necessary for the development of montmorillonite-based adsorbents. ![]() In this case, its hydration number became 6.05, and the diffusion coefficient increased to 1.83 × 10 −10 m 2/s. Whereas, when Cd 2+ completely replaced Na +, part of the Cd 2+ moved from the inner-sphere surface complexes to the outer-sphere surface complexes owing to its competitive adsorption. Upon the partial substitution of Na + by Cd 2+, Cd 2+ was adsorbed on the (001) surface as inner-sphere surface complexes, with a hydration number of 5.01 and a diffusion coefficient of 0 m 2/s. On the (001) surface, Cd 2+ was adsorbed on the centre of the silicon–oxygen ring by electrostatic interactions, whereas on the (010) surface, Cd 2+ was adsorbed between two ≡Al–OH groups and formed two covalent bonds with O, which was mainly due to the interaction between the Cd s and O p orbitals. The most stable adsorption energies of Cd 2+ on the (001) and (010) surfaces were −88.74 kJ/mol and −283.55 kJ/mol, respectively. The adsorption mechanism of Cd 2+ on different cleavage planes of montmorillonite was investigated using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. ![]()
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