Understanding the sorption behaviors of heavy metal ions in the interlayer and nanopore of montmorillonite: A molecular dynamics study.

The molecular-scale adsorption mechanism of heavy metal ions in the interlayer and nanopore regions of montmorillonite (MMT) were investigated by molecular dynamics simulations. Three typical heavy metals (zinc, cadmium, and lead) were selected as the model ions, and two types of MMT (Arizona and Wyoming) were considered. The results showed that Cd and Pb can form both inner- and outer-sphere complexes on Wyoming MMT, while Zn only formed outer-sphere complex due to the stronger hydration interaction of Zn than Cd and Pb. For Arizona MMT, all of the three ions only formed outer-sphere complexes on its interlayer and external basal surface in which the cations remained a fully hydrated state. The calculated diffusion coefficients of three cations in interlayer and nanopore indicated that their diffusion abilities were significantly impaired, implying that MMT adsorbents have a strong ability to fix and retard heavy metal ions. The derived results and mechanisms are instrumental to a profound understanding of the transport and retention of heavy metal elements in subsurface environments, and provide guidance for the management of heavy metal pollution.