APATITE CRYSTALLIZATION AND IMMOBILIZATION OF IODINE FROM HYDROTHERMAL SOLUTIONS

Nuclear energy production faces a critical challenge in the safe storage of spent fuel, particularly in dealing with radionuclides such as iodine-129 and uranium. Apatite is considered to be a promising backfill material due to its ability to incorporate various anions and cations and its relatively low solubility. We investigated multiple methods regarding the precipitation and synthesis of phosphate phases using the mineral brushite as the initial material and multiple electrolyte solutions for a suitable conversion pathway for apatite crystallization at hydrothermal conditions that could potentially favor the incorporation of an element in question. This crystallization method was use by authors in a recent publication (Jimenez-Arroyo et al., 2023) and high apatite-uranium partition coefficients at a wide range of temperatures were shown. Therefore, we are hereby further testing this methodology for the immobilization of iodate (IO₃^-) and iodide (I^-) from hydrothermal solutions at 39 and 200 °C. The performed analysis to our experimental solids encompassed electron microprobe (EMPA), scanning electron microscopy with energy dispersion spectroscopy (SEM-EDS), X-ray diffraction (XRD), and atom probe tomography (APT). Additionally, the experimental fluids 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 as well to derive iodine concentrations. Iodine concentrations acquired from EMPA (in solids), and UV-Vis (fluids) were then used to calculate apparent Nernst partition coefficient of iodine as D^I = I _crystal / I _fluid. The highest iodine concentration found in apatite was 5.6 wt.% when an iodate-bearing solution was used. In experiments where iodide-bearing solutions were used, iodine concentrations in apatite ranged from 0.04 to 0.45 wt.%. Partitioning data indicates that IO₃^- is more compatible with apatite (D>1) while I^- is less compatible (D<1). Suggesting then, that iodine incorporation into apatite is favorable at oxidizing conditions.

Jiménez-Arroyo, Á., Gabitov, R., Migdisov, A., Lui, J., Strzelecki, A., Zhao, X., Guo, X., Paul, V., Mlsna, T., Perez-Huerta, A., Caporuscio, F., 2023. Uranium uptake by phosphate minerals at hydrothermal conditions. Chemical Geology, p.121581.