Understanding Cd2+ Adsorption Mechanism on Montmorillonite Surfaces by Combining DFT and MD

The adsorption mechanism of Cd2+ on different cleavage planes of montmorillonite was investigated using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The most stable adsorption energies of Cd2+ on the (001) and (010) surfaces were -88.74 kJ/mol and -283.55 kJ/mol, respectively. On the (001) surface, Cd2+ was adsorbed on the centre of the silicon-oxygen ring by electrostatic interactions, whereas on the (010) surface, Cd2+ 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. Upon the partial substitution of Na+ by Cd2+, 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. Whereas, when Cd2+ completely replaced Na+, part of the Cd2+ moved from the inner-sphere surface complexes to the outer-sphere surface complexes owing to its competitive adsorption. In this case, its hydration number became 6.05, and the diffusion coefficient increased to 1.83 x 10(-10) m(2)/s. This study provides the theoretical background necessary for the development of montmorillonite-based adsorbents.

Automated summary

The adsorption mechanism of Cd2+ on different cleavage planes of montmorillonite has been investigated. The results show that the adsorption energies of Cd2+ on the (001) and (010) surfaces are -88.74 kJ/mol and -283.55 kJ/mol, respectively, and the adsorption mechanism is related to the electrostatic interactions and the interactions between orbitals. Upon the partial substitution of Na+, Cd2+ is adsorbed as inner-sphere surface complexes, while when Cd2+ completely replaces Na+, part of it moves to outer-sphere surface complexes.