Development of quality-assured Thermodynamic Database for Rare Earth Elements

New materials showing specific magnetic and/or electrooptic properties often incorporate Rare Earth Elements (REE). Due to their very specific technological application, it is necessary to separate and enrich the REE from each other. The optimization of physico-chemical conditions for the design of effective extraction and recycling processes of REE has to rely on accurate and reliable thermodynamic data. However, no fundamental, consolidated and internationally recognized Thermodynamic Databases (TDB) is currently available for REE.
This study aims at providing a reliable, quality assured and internally consistent TDB for Europium. The thorough evaluation of all available primary literature sources for Eu(III) complexation constants (log β) with inorganic ligands (OH−, Cl−, NO3−, SO42− and CO32−) enabled identifying several critical issues: i) inconsistencies between different sources ii) lack of accurate activity coefficient treatment in case of formation of weak complexes iii) absence of independent spectroscopic validation of the stoichiometry of the proposed complexes.
Thus, several actions have been undertaken for the Eu-chloro, -nitrato and -sulfato complexes:
a) recalculation of the log β of weak complexes by using an hypothetical reference state (at trace ligand concentration) [1]
b) use of advanced spectroscopic techniques (e.g. Time-resolved Laser-induced Fluorescence Spectroscopy), allowing to monitor on line the speciation evolution at micromolar range concentrations. This also enabled identifying the prevailing species as well as their stoichiometries. Finally, complexation constants were determined from the spectroscopic data sets.
c) The conditional log β were extrapolated to standard conditions (I = 0 M, T = 298.15 K) using the Specific Ion Interaction Theory.

[1] Spahiu, K. et al. (1998) Radiochim. Acta 82, 413-419.