The aqueous geochemistry of REE.: Part 8: Solubility of ytterbium oxalate and the stability of Yb(III)–oxalate complexes in water at 25°C to 80°C

The solubility of ytterbium oxalate (Yb2Ox3·xH2O) in aqueous solution was measured as a function of temperature (40°C, 60°C and 80°C), ionic strength (0.05, 0.1 and 0.2 m NaCl), and free oxalate concentration (10−7 to 10−2 molal). At high oxalate and/or NaCl concentration, the simple Yb–oxalate salt transforms into a double salt with the stoichiometry NaYb(Ox)2·yH2O. Our solubility data were used to obtain smoothed equilibrium constants (I=0, P=SVP) for the solubility products of both solid phases, and for the stepwise and cumulative association complexes of the aqueous Yb(III)–oxalate complexes, between 25°C and 100°C. Whereas the solubility of the simple Yb–oxalate salt changes little in this temperature range, the Na–Yb oxalate salt becomes less stable and significantly more soluble with an increase in temperature. The ytterbium oxalate complexes are strong, and may predominate over inorganic Yb(III) complexes (e.g., carbonate, chloride, sulfate, and phosphate) at geologically realistic ligand concentrations. Our results are relevant to researchers interested in the transport of ytterbium and other rare earth elements (REE) in near-surface waters and sedimentary basin fluids, and may also have engineering applications with regards to the precipitation of oxalate salts as a means of removing actinides and radioactive REE from aqueous nuclear wastes.