Adsorption and removal of metal ions by smectites nanoparticles: Mechanistic aspects, and impacts of charge location and edge structure

Clay minerals are efficient adsorbents for metal ions and have been widely used to control heavy metals, while a number of critical issues remain elusive. In this study, the fully flexible models for smectites (montmorillonite and beidellite) nanoparticles are developed and then subject to molecular dynamics simulations. Edge rather than basal and interlayer surfaces show much higher adsorption efficacy, and therein univalent metal ions always cause excessive adsorption. For montmorillonite, inner-sphere Pb2+ ions emerge and predominate only at edge surfaces, and have higher stability than inner-sphere Na+ ions, manifesting the central role of edge surfaces to remove heavy metals. The peculiar distribution of edge-O atoms causes similar coordination environments for inner- and outer-sphere metal ions, and inner-sphere metal ions are preferred significantly due to bonding with edge-hydroxyls. Metal ions with smaller radii are more favorable to adsorb at edge surfaces, and those with considerable hydration effects (e.g., heavy metals) can be preferred. Edge surfaces of montmorillonite rather than beidellite are more efficient for adsorption, mainly as a result of distinct adsorption behaviors at basal surfaces that show reversed trends. (010) rather than (110) edges are superior for adsorption, which is caused mainly by structural differences. (010) edges are more exposed to adsorbates, and cleavage of smectites nanoparticles along (010) edges enhances removal of heavy metals. Diffusion, mobility and stability of adsorbed metal ions are also discussed, which further understanding of adsorption at edge surfaces. The findings are consistent with experimental observations available and provide new insights to the complicated processes at clay minerals/water interfaces including control of heavy metals. [Display omitted] •Fully flexible models for smectites nanoparticles with edges have been developed.•Edges prefer inner-sphere adsorption and play a central role to control heavy metals.•Ionic radius (decisive) and hydration effect affect adsorption of metal ions at edges.•Edges of montmorillonite rather than beidellite are more efficient for adsorption.•(010) rather than (110) edges are obviously superior for adsorption.