Solubility and stability of liebigite, Ca2UO2(CO3)3·10H2O(cr), in dilute to concentrated NaCl and NaClO4 solutions at T = 22–80 °C

Abstract

Abstract The solubility and thermodynamic stability of a synthetic liebigite was investigated in NaCl and NaClO4 solutions within a wide range of ionic strength (0.03\u202fm\u202f≤\u202fIm\u202f≤\u202f5.61\u202fm), pH (7\u202f≤\u202fpHm\u202f≤\u202f9, with pHm\u202f=\u202f–log [H+]) and temperature (22\u202f°C\u202f≤\u202fT\u202f≤\u202f80\u202f°C) conditions. A comprehensive characterization of the synthetic solid phase using XRD, quantitative chemical analysis, TG–DTA, SEM–EDS, IR and Raman spectroscopy confirmed the stoichiometry of Ca2UO2(CO3)3·10H2O(cr). At room temperature, liebigite remains stable and controls the solubility of U(VI) in the investigated NaCl and NaClO4 systems with Im\u202f≤\u202f0.51\u202fm. For the same temperature but high ionic strength (5.61\u202fm NaCl), liebigite transforms into andersonite (Na2CaUO2(CO3)3·6H2O(cr)). This solid phase transformation results in a decrease in solubility of approximately 2 log10-units at pHm\u202f≈\u202f8. Solubility data in combination with solid phase characterization (XRD, quantitative chemical analysis) likewise confirm the transformation of liebigite into CaU2O7⋅xH2O(cr), Na2U2O7⋅xH2O(cr) and/or other sub-stoichiometric Na-uranate compounds in all systems investigated at T\u202f=\u202f80\u202f°C. On the basis of solubility data at room temperature determined in this work, in combination with thermodynamic and activity models available in the literature for the aqueous speciation in the system Ca–U(VI)–carbonate, solubility products for liebigite and andersonite are derived: Ca2UO2(CO3)3·10H2O(cr)\u202f⇔\u202f2 Ca2+\xa0+\xa0UO22+\xa0+\xa03 CO32–\xa0+\xa010 H2O(l) log K°s,0\u202f=\u202f–(32.3\u202f±\u202f0.3) Na2CaUO2(CO3)3·6H2O(cr)\u202f⇔\u202fCa2+\xa0+\xa02 Na+\xa0+\xa0UO22+\xa0+\xa03 CO32–\xa0+\xa06 H2O(l) log K°s,0\u202f=\u202f–(31.8\u202f±\u202f0.5) These results complement previously reported thermodynamic data, now allowing complete thermodynamic and geochemical calculations for the system UO22+–Ca2+–Na+–H+–CO2(g)–HCO3––CO32––H2O(l), including U(VI) aqueous species and solid compounds, in the context of environmental uranium chemistry and nuclear waste disposal.