Mechanism Study of Cu(II)/Cd(II)/Ni(II) Ions Adsorption by Montmorillonite-Based Geopolymers: Molecular Dynamics Simulation and Structural Characterization

Abstract

In this study, a high temperature-alkali modified montmorillonite-based geopolymer ( SMT-OH) was successfully prepared. Adsorption experiments show that the adsorption reached the maximum (Cu (122.5 mg/g), Cd (224.0 mg/g), Ni (141.0 mg/g) when the pH was 7.0, the concentration was 200 mg/l and the dosage was 0.03 g (0.15 g, 0.15 g). Three heavy metals adsorption behavior of SMT-OH followed the Redlich-Peterson model and the pseudo-second-order kinetics. The internal diffusion model fitting results demonstrated that three heavy metals removal by SMT-OH was dominated by the phys-chemical adsorption. Characterization analyses revealed that the amorphous Si–O, -OH, and Na⁺ on the surface of SMT-OH enhanced the adsorption of heavy metal ions. In addition, characterization indicates that Cu(II) adsorption mainly occurs through ion exchange and surface coordination, Cd(II) primarily through surface co-precipitation, while Ni(II) involves both surface coordination and co-precipitation. Density functional theory (DFT) calculations indicate that the O-top site is the most stable for Cu(II) and Ni(II), while the bridge site is preferred for Cd(II). Molecular dynamics (MD) simulation results show that these ions are adsorbed on the surface of SMT-OH at a distance of 2 ~ 3 Å and reduce the diffusion coefficient of heavy metal ions. The simulation results provided strong supporting evidence for experimental conclusions.

Graphical Abstract