Exploring the Degradation Behavior of Ce-Monazite in Water Solution through Adsorption and Penetration Kinetics

Abstract

Monazite (LnPO4, Ln = La–Gd) is an important light lanthanide phosphate for the application of ionic conductors, coatings, and diffusion barriers and the safe disposal of high-level radioactive wastes, and thus evaluating its chemical durability is critical. However, the degradation mechanism of monazite upon a water solution has not yet been fully established. In this work, the effects of water on the detailed structure and properties of CePO4 monazite are systematically investigated based on density functional theory. Some specific issues such as surface hydrolysis reaction and the nucleation of rhabdophane precipitation are discussed in detail and compared with the available dissolution experimental results. The results reveal that the interaction between water molecules and the (010) surface is dominated by the physisorption of the Ce atoms rather than [PO4] tetrahedron. The bilayer water with 16 H2O molecules can describe the fundamental forces at the water–monazite interface associated with water–solid interaction and water–water interaction. The penetrating water molecules play a key role in corroding down the whole (010) surface to form the Schottky defect pairs of Ce and [PO4]. Importantly, these dissolved ions are easily adsorbed on the defective area of the (010) surface, which contributes to the nucleation of CePO4·0.667H2O rhabdophane precipitation. Moreover, some potential diffusion pathways for the H2O molecule to migrate on the surface and penetrate into the inner (010) matrix are discussed.