Thermodynamics of the U(VI)-Ca2+-Cl−-OH−-H2O system: Solubility product of becquerelite

Abstract

SummaryThe solubility of synthetic becquerelite (Ca(UO2)6O4(OH)6·8H2O) was determined in 0.02, 0.1, and 0.5 M CaCl2solutions and at pCH+values ranging from approximately 4 to 11. The presence of becquerelite in equilibrated samples was confirmed by a combination of techniques involving X-ray diffraction, total chemical composition, and analyses of solubility data. The solubility data were interpreted using Pitzer's aqueous thermodynamic model and the thermodynamic data for U(VI) species available in the literature. The log of the solubility product for becquerelite [Ca(UO2)6O4(OH)6·8H2O + 14H+⇌ Ca2++ 6UO22++ 18H2O] was determined to be 41.4 ± 0.2. This value is similar to the values previously reported for other synthetic becquerelites, but is drastically different from a value reported for a natural sample.