Surface charge effects on Sr-hydroxyapatite degradation in water solution: Surface and penetration diffusion

Hydroxyapatite (HAP) can serve as a critical permeable reactive barrier for the in situ remediation of ⁹⁰Sr radionuclides from groundwater through promoting co-precipitation. The Sr²⁺ leaching associated with the structural degradation of Sr-HAP in the geological field environment is crucial for lowering groundwater radioactivity; thus, evaluating HAP chemical durability is critical. However, the degradation mechanism of Sr-doped HAP upon water solutions under the influence of surface charge effects remains unclear and requires further investigation. In this work, the surface degradation and penetration diffusion of Sr-doped HAP are systematically investigated based on density functional theory and dissolution experiments. Some specific issues such as the preparation process of single-phase ceramics and surface morphology are discussed in details, along with uncovering the relationship between Sr–O bond and Sr leaching activity. The results reveal that the degradation behavior of Sr-HAP ceramics follows a typical dissolution-precipitation process. The presence of unsaturated chemical bonds derived from incomplete [SrO₇] and [SrO₉] coordination polyhedra can enhance the ability of Sr-HAP to facilitate surface adsorption and contribute to the nonuniform distribution of electronic states at the nanoscale. Importantly, the hydrogen bonds from aqueous solution provide a strong upward pulling force for the surface adsorption sites to render the metastable surface structure rather than water molecule penetration diffusion, leading to the outer layer of the (0 0 1) surface spontaneously twists to form Schottky defects.