Solubility measurements of the uranyl oxide hydrate phases metaschoepite, compreignacite, Na–compreignacite, becquerelite, and clarkeite

The mobility of uranium under oxidizing conditions can only be modeled if the thermodynamic stabilities of the secondary uranyl minerals are known. Toward this end, we synthesized metaschoepite (UO3(H2O)2), becquerelite (Ca(UO2)6O4(OH)6(H2O)8), compreignacite (K2(UO2)6O4(OH)6(H2O)7), sodium compreignacite (Na2(UO2)6O4(OH)6(H2O)7), and clarkeite (Na(UO2)O(OH)) and performed solubility measurements from both undersaturation and supersaturation under controlled-pH conditions. The solubility measurements rigorously constrain the values of the solubility products for these synthetic phases, and consequently the standard-state Gibbs free energies of formation of the phases. The calculated lg solubility product values (lg Ksp), with associated 1σ uncertainties, for metaschoepite, becquerelite, compreignacite, sodium compreignacite, and clarkeite are (5.6 −0.2/+0.1), (40.5 −1.4/+0.2), (35.8 −0.5/+0.3), (39.4 −1.1/+0.7), and (9.4 −0.9/+0.6), respectively. The standard-state Gibbs free energies of formation, with their 2σ uncertainties, for these same phases are (−1632.2 ± 7.4) kJ · mol−1, (−10305.6 ± 26.5) kJ · mol−1, (−10107.3 ± 21.8) kJ · mol−1, (−10045.6 ±24.5) kJ · mol−1, and (−1635.1 ± 23.4) kJ · mol−1, respectively. Combining our data with previously measured standard-state enthalpies of formation for metaschoepite, becquerelite, sodium compreignacite, and clarkeite yields calculated standard-state entropies of formation, with associated 2σ uncertainties, of (−532.5 ± 8.1) J · mol−1 · K−1, (−3634.5 ± 29.7) J · mol−1 · K−1, ( −2987.6 ± 28.5) J · mol−1 · K−1, and (−300.5 ± 23.9) J · mol−1 · K−1, respectively. The measurements and associated calculated thermodynamic properties from this study not only describe the stability and solubility at T = 298 K, but also can be used in predictions of uranium mobility through extrapolation of these properties to temperatures and pressures of geologic and environmental interest.