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

The solubility of ytterbium oxalate (Yb(2)Ox(3) . xH(2)O) in aqueous soluti on was measured as a function of temperature (40 degrees C, 60 degrees C an d 80 degrees C), ionic strength (0.05, 0.1 and 0.2 m NaCl), and free oxalat e concentration (10(-7) to 10(-2) molal). At high oxalate and/or NaCl conce ntration, the simple Yb-oxalate salt transforms into a double salt with the stoichiometry NaYb(Ox)(2) . yH(2)O. Our solubility data were used to obtai n smoothed equilibrium constants (I = 0, P = SVP) for the solubility produc ts of both solid phases, and for the stepwise and cumulative association co mplexes of the aqueous Yb(III)-oxalate complexes, between 25 degrees C and 100 degrees C. Whereas the solubility of the simple Yb-oxalate salt changes little in this temperature range, the Na-Yb oxalate salt becomes less stab le and significantly more soluble with an increase in temperature. The ytte rbium oxalate complexes are strong, and may predominate over inorganic Yb(I II) complexes (e.g., carbonate, chloride, sulfate, and phosphate) at geolog ically realistic Ligand concentrations. Our results are relevant to researc hers interested in the transport of ytterbium and other rare earth elements (REE) in near-surface waters and sedimentary basin fluids, and may also ha ve engineering applications with regards to the precipitation of oxalate sa lts as a means of removing actinides and radioactive REE from aqueous nucle ar wastes. (C) 2000 Elsevier Science B.V. All rights reserved.