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 90 Sr radionuclides from groundwater through promoting co‐precipitation. The Sr 2+ 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 [SrO 7 ] and [SrO 9 ] 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.