A thermodynamic database for the solution chemistry and solubility of europium(III) inorganic species: recent developments

Performance assessments of geological repositories for the underground disposal of high-level radioactive waste require a deep understanding of the phenomena influencing the mobility of radionuclides, e.g. sorption, redox immobilization, surface precipitation, incorporation, etc. Reliable thermodynamic databases (TDB) are essential in order to generate speciation calculations, surface complexation and reactive transport models to predict the aforementioned mechanisms. In this work, the focus was set on europium (Eu), a lanthanide used for decades as a chemical analogue of trivalent actinides (Pu, Am). However, a consolidated and internationally recognized Eu TDB does currently not exist.

Several reviews and reports [1-4] on the aqueous chemistry/geochemistry of europium were published, but had various drawbacks, for example:

→ Insufficient transparency about the selection procedure,

→ Lack of systematic screening to gather primary literature sources,

→ No original data but rather analogue values from other REE or compilations from secondary references,

→ Postulation of species not independently evidenced by means of advanced spectroscopic techniques,

→ Too high reliance on the analogy with trivalent actinides,

→ For weak complexes such as chloride and nitrate, changes in the activity coefficients due to the replacement of up to 100 % of the background electrolyte anion by Cl⁻ or NO₃⁻ was either completely overlooked or, if recognized, not handled properly,

→ Too high reliance on the charge analogy for the estimation of missing ion interaction coefficients when the Specific ion Interaction theory was applied.

This study aims at significantly improving the situation by carefully addressing all aforementioned issues in order to provide a reliable, robust, and internally consistent TDB for europium. For this, an extensive data survey of more than 350 peer-reviewed publications from around 1900 until June 2021 was performed. Furthermore, technical reports, scientific books, collected editions as well as various thermodynamic databases (Nagra/PSI [2], Thermochimie [5], etc.) were surveyed to identify their original Eu(III) data sources and references used therein. Data up to 90 °C and 5 M ionic strength were selected for screening, but all data dealing with hydrothermal conditions were out of the scope of this review. Thermodynamic data determined in non-aqueous solvents were explicitly not considered either. For the three ligands (SO₄²⁻, Cl⁻, PO₄³⁻) to which our attention was focused on in a first step, the result of the screening together with the number of selected data for aqueous complexation constants is shown in Table 1.

Table 1. Summary of the aqueous inorganic Eu complexation data records
System #total #selected
Eu – sulphate 244 34
Eu – chloride 186 0
Eu – phosphate 40 8

The complexation of Eu(III) with sulphate was investigated by ion exchange, solvent extraction, spectrophotometry, electrophoresis, and time resolved laser-induced fluorescence spectroscopy [6]. Despite the broad variety of methods used for the determination of the conditional complexation constants of the Eu(III) sulphate species in the literature, a consistent set of data was obtained for both EuSO₄⁺ and Eu(SO₄)₂⁻ aqueous complexes at 25 °C.
Chloride complexes with Eu(III) are very weak, and high chloride concentrations are required to form them. Related experiments (up to 4 mol∙L⁻¹ Cl⁻ at 25 °C) need to be considered with extra care because the perchlorate ion was systematically substituted by the chloride ion by more than 10 %, and in some cases even completely. However, the resulting changes in the activity coefficients due to such large compositional modifications of the background electrolyte were simply ignored. All our recalculations [7] based on the exclusive consideration of changes in activity coefficients without considering the formation of Eu-chloro complexes as proposed in the original studies, are able to describe the experimental data of the literature [6]. This means that the formation of Eu(III)-chloro complexes postulated in the literature was an artefact [6].
Unfortunately, very little is known concerning the complexation of Eu(III) with phosphate ions. The only experimental study was performed recently [8], by means of laser-induced luminescence spectroscopy at 25 °C and at different ionic strengths (0.6 – 3.1 mol∙L⁻¹) imposed by NaClO₄. The impact of temperature up to 80 °C on the formation of the EuH₂PO₄²⁺ complex was also investigated [8].

Recently, results of our critical evaluation for the chloride, sulphate, and phosphate ligands were published [6]. The recommended complexation constants and solubility products for further inorganic ligands, e.g. hydroxide and carbonate, will also be presented [9].

[1] Brown, P.L. and Ekberg, C. (2016) Hydrolysis of Metal Ions. Vol. 1, Wiley-VCH, Weinheim.
[2] Hummel, W. et al. (2002) Nagra/PSI Chemical Thermodynamic Data Base 01/01, Technical Report 02-16.
[3] Rard, J.A. (1985). Chemistry and thermodynamics of europium and some of its simpler inorganic compounds and aqueous species. Chem. Rev. 85(6): 555-582.
[4] Spahiu, K. and Bruno, J. (1995) A selected thermodynamic database for REE to be used in HLNW performance assessment exercises, SKB Technical Report.
[5] Giffaut, E, et al. (2014). Andra thermodynamic database for performance assessment: ThermoChimie. Appl. Geochem. 49: 225–236.
[6] Jordan, N. et al. (2022). A critical review of the solution chemistry, solubility, and thermodynamics of europium: recent advances on the Eu3+ aqua ion and the Eu(III) aqueous complexes and solid phases with the sulphate, chloride, and phosphate inorganic ligands. Coord. Chem. Rev. 473, 214608.
[7] Spahiu, K. and Puigdomènech, I. (1998). On weak complex formation: re-interpretation of literature data on the Np and Pu nitrate complexation. Radiochim. Acta 82: 413-419.
[8] Jordan, N. et al. (2018). Complexation of trivalent lanthanides (Eu) and actinides (Cm) with aqueous phosphates at elevated temperatures. Inorg. Chem. 57(12): 7015-7024.
[9] Jordan, N. et al. (2023). Coord. Chem. Rev. (in preparation).