Iodate Incorporation into Apatite: An Experimental Study

Radioiodine (¹²⁹I) poses a long-term environmental and health risk due to its high mobility, solubility, radiotoxicity, and long half-life. Owing to chemical similarities between iodine species and other anions accommodated in apatite, this mineral represents a promising host for radioiodine immobilization. Nevertheless, the mechanisms of iodine incorporation into the apatite structure remain insufficiently understood. In this study, we evaluated the immobilization of iodate by Cl-OH and hydroxyapatite under hydrothermal conditions. Experimental solids were characterized using electron microprobe, scanning electron microscopy, X-ray diffraction, synchrotron X-ray absorption spectroscopy, and atom probe tomography. Experimental solutions were analyzed by ultraviolet–visible spectrophotometry and ion chromatography. The highest iodate concentration measured in apatite was 6.0 wt.% when a 0.1 M NaIO₃ solution was used as the crystal growth medium. However, the ratio of iodate incorporated into the solid relative to its concentration in solution increased with decreasing aqueous iodate concentration, indicating enhanced compatibility of iodate within apatite at ppm-level aqueous concentrations. These results suggest that iodate may substitute for both phosphate and hydroxyl (or chloride) in the apatite structure.