Theoretical insights into the adsorption mechanism of Cd(II) on the basal surfaces of kaolinite.

Retardation of Cd(II) migration is an ongoing concern for environmental remediation, but a prevalent obstacle of the procedure originates from the lack of an atomic-scale description of the inherent mechanism for Cd(II) adsorption at mineral-water interfaces. Herein, we performed first-principles calculations and ab initio molecular dynamics (AIMD) simulations to explore the adsorption mechanism of Cd(II) on the basal surfaces of kaolinite. Representative monodentate and bidentate Cd(II) complexes were constructed on the Kln-Al(001) and Kln-Si(001̅) surfaces. The results showed that bidentate coordination of Cd(II) on the Kln-Al(001) surface was superior to all other studied models due to the favorable formation energy and better agreement with EXAFS data. The calculated electron density difference revealed the charge transfer from surface oxygen (Os) to Cd(II) upon adsorption. In particular, partial density of states (PDOS) analysis indicated that the Cd-Os bond exhibited covalent characteristics, attributed to the overlaps of Cd-5p and Os-2p orbitals in the valence band. Furthermore, radial distribution functions supported by AIMD simulations were employed to confirm the structural features of Cd(II) coordination shell at kaolinite-water interfaces. This theoretical study provides insightful guidance for future Cd(II) research to improve current assessments of contaminant remediation.