Uranium partitioning between apatite and hydrothermal fluids at 150-250 °C

This experimental study is focused on the immobilization of dissolved uranium (U) via its entrapment by apatite at hydrothermal conditions. Experiments on apatite crystallization were conducted in 0.5 M sodium chloride (NaCl) solutions at temperatures of 150, 200, and 250 degrees C. Uranium was introduced into the system as a solid compound (UO 3 ), which controlled the concentration of dissolved U in the hydrothermal fluids. The results showed that U contents in apatite exceed U concentrations in co-existing fluid by 2 - 3 orders of magnitude. Apparent Nernst partition coefficients ( D U ) and exchange coefficient ( K U/Ca ) were evaluated. Experimentally determined D U values were consistent with D U predicted by the Lattice Strain Model at 200 and 250 degrees C. The obtained K U/Ca values shows dependence on temperature. The results lay the foundation for further assessment of uranium immobilization in apatite and other minerals at repository conditions.