Iodine incorporation into calcium phosphate minerals at hydrothermal conditions

Abstract

Among the numerous radionuclides generated by nuclear fission, radioiodine (¹²⁹I) poses a long-term environmental and health threat due to its high mobility, solubility, radiotoxicity, and 15.7 Ma half-life. Even though relatively small amounts of radioiodine are generated per ton of spent fuel, its accumulation has exponentially increased since the inception of the nuclear age. Many approaches for the immobilization of radioiodine have been extensively investigated. These methods, however, are still limited by thermodynamic instability, high costs, slow kinetics, and volatile loss during vitrification. Given the relationship and similarities of iodine species with other anions present in apatite, such a mineral still represents a promising host for the safe disposal of radioiodine. However, the interaction between aqueous iodine species and the mineral apatite at hydrothermal conditions remains a knowledge gap. This work evaluated the immobilization of iodate (IO₃⁻) and iodide (I⁻) from solutions by calcium phosphate minerals (apatite and monetite) at 39 and 200 °C. The performed analysis of our experimental solids encompassed electron microprobe analysis (EMPA), scanning electron microscopy coupled with energy dispersion spectroscopy (SEM-EDS), X-ray diffraction (XRD), and synchrotron X-ray absorption spectroscopy (XAS). In addition, the experimental solutions were analyzed using UV–visible spectrophotometry (UV–Vis) before and after the experiments to track redox changes if any, of the iodine species initially used and to derive iodine concentrations. Iodine concentrations acquired from EMPA (for solids) and UV–Vis (for solutions) were used to calculate the apparent Nernst partition coefficient of iodine as $D^I=\frac{I_{\left(apatite\right)}}{I_{\left(fluid\right)}}$. The highest iodine concentration found in apatite was 6.0 wt% when an iodate-bearing NaCl solution was used as the crystal growth medium. In experiments where iodide-bearing solutions were used, iodine concentrations in apatite ranged from 0.04 to 0.45 wt%. Partitioning data show that IO₃⁻ is more compatible with apatite (D > 1) while I⁻ is less compatible (D < 1). The results indicate that iodine incorporation into apatite is favorable at oxidizing conditions.