How Does Uranium Adsorb on (010) Pyrophyllite Under Alkaline Conditions? An In Silico Study

Deep geological waste repositories must ensure that radionuclides from high-level waste are contained safely, despite the evolution of extreme geochemical conditions of (hyper)alkaline pH (>10) and salinity over geological time scales. Over time, the chemistries of clay minerals (used as geotechnical barriers) and uranium are altered, potentially leaching harmful radionuclides to the environment. However, Ca²⁺ was reported to aid in U(VI) retention at pH > 10. Herein, two-dimensional periodic density functional theory calculations in combination with COSMO implicit solvation are carried out to elucidate the retention mechanisms of [UO₂(OH)₃(H₂O)]⁻ and [UO₂(OH)₄]^2– on (010) pyrophyllite under (hyper)alkaline conditions. By considering the protonation equilibria of surface sites and environmental speciation of uranium, the structure and properties of (010) pyrophyllite and the uranyl retention mechanisms have been investigated. Representative of increasing interface pH, three edges of (010) are proposed: 010_H, 010_1Ca, and 010_2Ca. Ca²⁺–bound surfaces (010_1Ca, 010_2Ca) are consistent with protonation equilibria of SiOH and Al(OH₂)₂ sites above pH 8, whereas 010_H fails to describe alkaline conditions. The Ca²⁺ ions bridge the adsorbed U(VI) species on 010_1Ca or 010_2Ca, and their geometries agree with EXAFS structures from the literature, exhibiting a similar ν_stretch for the uranyl bonds. A correlation of energetics and U(VI) adsorption to surface speciation and batch-sorption experiments from the literature is presented to quantitatively distinguish the pH ranges of the proposed edge models. This study highlights the importance of surface and solute chemistries at the interface in building computational models.