Adsorption of Cs⁺ on Pyrophyllite (001), (100) and (010) Surfaces: A First Principles Study

Due to the long half-life, high fission yield and elevated radiotoxicity, ¹³⁷Cs is considered as one of the most critical radionuclides in deep geological waste disposal and remediation of radioactive contamination. In this paper, the adsorption behavior of Cs⁺ on pyrophyllite surfaces has been investigated using the first-principles method. First, Cs⁺ adsorption at distinct sites on the pyrophyllite (001) basal surface was compared to elucidate the interaction mechanism. Next, Bader charge analysis was employed to identify the origin of different adsorption behaviors on pyrophyllite and kaolinite siloxane surfaces. Finally, the adsorption of Cs⁺ on the (100) and (010) edge surfaces of pyrophyllite was examined. First-principles calculations indicate that Cs⁺ can be adsorbed on the (001) basal surface in the form of mono-, bi-, and tri-dentate complexes, with adsorption energies from −0.540 eV to −0.348 eV. The hollow site at the center of six-membered ring is the most favorable for immobilizing Cs⁺. The charge density difference and Bader charge analysis reveal a small charge polarization of Cs⁺ and surface oxygen atoms. On the (100) edge surface, the strongest binding (−1.002 eV) occurs adjacent to ≡Al(OH₂) groups, whereas attachment near ≡Si(OH) groups is relatively weaker (−0.527 eV). For the (010) edge surface, multidentate complexes are formed at the H₁ and H₂ cavities with energies of −1.049 eV and −0.679 eV, respectively. Compared with (001) basal surface, the adsorption configurations of Cs⁺ on (010) and (100) edge surfaces are more stable. The adsorption stability of Cs⁺ on edge surfaces is closely related to the distribution of oxygen and hydrogen atoms surrounding the adsorption sites. Specifically, the more oxygen atoms and the fewer hydrogen atoms near the adsorption sites, the more stable the adsorption configuration. These findings provide a quantitative basis for exploiting pyrophyllite edge sites in Cs⁺ immobilization strategy.