Aiming to identify the complexing mechanisms of heavy metal cations on edge surfaces of 2:1-type clay minerals, systemic first-principles molecular dynamics (FPMD) simulations were conducted and the microscopic structures and complex free energies were obtained. Taking Cd(II) as a model cation, the structures on both (010) and (110) edges of the complexes were derived for the three possible binding sites (≡SiO, ≡Al(OH)2/≡AlOH≡AlSiO, and vacant sites). The stable complexes adsorbed on the three binding sites on both terminations had similar structures. The free energies of the complexes on (010) edges were calculated by using the constrained FPMD method. The free energies of complexes on the ≡SiO and ≡Al(OH)2 sites were similar and they were both significantly lower than the free energy of the complex on the octahedral vacant site. In association with the concept of high energy site (HES) and low energy site (LES) in the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange (2SPNE SC/CE) sorption model, the vacant site was assigned to HES and the other two sites to LES, respectively.

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