STRONTIUM UPTAKE BY CALCIUM PHOSPHATE MINERALS AT HYDROTHERMAL CONDITIONS

Advancements in nuclear fields and associated technologies are impressive. However, by-products of many nuclear feats and advancements include various radioactive wastes, though there are various explorations on nuclear waste containment and reduction. This research entails the facet of containment to the findings in the recent work, Jimenez-Arroyo, (2023), focusing on the uptake of strontium (Sr) by apatite (Ca₅(PO₄)₃OH), which radioactive isotope (⁹⁰Sr) is a fission product of uranium, and therefore, present in nuclear waste (U.S.NRC.) This project's objective is to evaluate the degree of Sr incorporation into calcium phosphate minerals at hydrothermal solutions experimentally aiding in modeling the hypothetical absorption rates of radioactive ⁹⁰Sr. Moreover, it will validate the brushite to apatite crystallization method for potentially immobilizing three (uranium, iodine, and strontium) of the radionuclides present in nuclear waste. Our experimental setup consists of synthesizing the mineral brushite (CaHPO₄·2H₂O), through a method used by Jimenez-Arroyo, (2023). Around 200 mg of brushite was placed in a 0.5M NaCl solution into three autoclaves. Aliquots of Sr standard was added to make a final concentration of 1, 5, and 10 ppm. The autoclaves were then placed into an oven at varying temperatures (40, 80, 120, 160, and 200 ˚C). After the experimental run, the fluids and solids were separated and analyzed using an ICP – OES to determine calcium and strontium concentrations. Solid samples were characterized using XRF and XRD. Results showthe uptake of Sr is favorable at high temperatures, from 120 to 200 °C. A decrease of Sr concentration in the fluid (Sr_Fl) from 5 to 3.22 ppm was detected at 120 °C. Yet only a minor decrease in Sr (from 5 to 4.44 ppm) was observed at low temperatures (40 – 80 °C). Sr concentrations in fluids and solids were used to calculate the Nernst partition coefficient of Sr as D^Sr = Sr _crystal / Sr _fluid. Calculations suggest a high uptake of Sr (D^Sr»297) at 120 – 200 °C compared to that of low-temperature (D^Sr»5 at 40-80 °C). The incorporation of Sr by Apatite exemplifies the potential applications of Calcium Phosphate minerals in the scene of nuclear waste containment.

Jiménez-Arroyo, Á., et. al., 2023. Uranium uptake by phosphate minerals at hydrothermal conditions. Chemical Geology, p.121581.

U.S.NRC. (2019). NRC: Backgrounder on Radioactive Waste.