APATITE DISSOLUTION RATES AS A FUNCTION OF GRAIN SIZE

Permeable reactive barriers (PRBs) are a promising method for immobilizing uranium in groundwater. One material under evaluation for use in PRBs is apatite, a phosphorus bearing mineral [Ca₅(PO₄)₃(F,Cl,OH)], due to its high sorption capacity for trace elements, widespread availability, and low cost. Previous studies have shown that groundwater that interacts with uranium- and apatite-bearing formations contains a much lower concentration of uranium than expected, due to the formation of autunite [Ca(UO₂)₂(PO₄)₂·nH₂O], a uranyl phosphate mineral that is sparingly soluble. Autunite formation is thought to occur in the leached layer of apatite, as phosphate and calcium released from the dissolving apatite react with the uranium.

To aid in the design of PRBs for uranium immobilization, we are conducting batch experiments to determine apatite dissolution rates as a function of grain size. For the experiments we are using three grain sizes of apatite (149-250 μm, 250-420 μm and 420-841 μm). Calcium and phosphate concentrations are measured in solution at specific time intervals during the eight hour experiment. We also monitor pH and temperature. Using calcium as the reaction progress variable, we will determine apatite dissolution rates with the initial rate and the derivative methods. Calculated dissolution rates will then be regressed against grain size to determine if there is a relationship. Data collected from this project can be used to determine the most efficient size of apatite for constructing an effective PRB for uranium immobilization.