EVALUATING CALCIUM PHOSPHATE MINERALS AS A SUPPLEMENT TO ENGINEERED BACKFILL MATERIALS TO ENTRAP RADIOACTIVE IODINE

Iodine-129 is a radionuclide produced from nuclear waste that is both radiotoxic and has a long half-life. Iodine is soluble in aqueous solutions and poorly absorbed on clay minerals (a widely accepted component for engineering barriers). Therefore, it is imperative to study mechanisms to immobilize I-129 to prevent health and environmental disasters in the future. Calcium phosphate minerals have shown favorable results to immobilizing various cationic and anionic radionuclides and have a potential in capturing iodine by substitution to phosphate or halogen crystallographic site (in case of apatite) (Rigali et. al., 2016). In our experimental study, since the thermal peak of the waste canisters is expected to be ~300°C (Greenburg and Wen 2013), brushite was transformed into apatite in an aqueous environment at temperatures ranging from 25˚ C to 250˚ C. Apatite was precipitated in multiple concentrations of iodate (IO3-) ions. After conducting the experiment, samples were separated into solid crystals and fluid. Solids were analyzed with scanning electron microscope (SEM) encompassed electron microprobe (EMPA), energy dispersion spectroscopy (EDS), and X-ray diffraction (XRD). The fluids were tested with UV-Visible Spectrophotometry (UV-Vis) to determine IO3- concentrations. The intensity peaks from the UV-Vis analysis show that all the 250˚ C experiments are being partially reduced, while the low temperature experiments are shown to remain in the oxidized form. We anticipate to present iodine partition coefficients (DI = Icrystal / Ifluid) and discuss mechanisms of its incorporation into calcium phosphates at different experimental conditions.