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

Abstract

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+, Cd2+ was adsorbed on the (001) surface as inner-sphere surface complexes, with a hydration number of 5.01 and a diffusion coefficient of 0 m2/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 × 10−10 m2/s. This study provides the theoretical background necessary for the development of montmorillonite-based adsorbents.