Exploring the complexation of curium(III) and europium(III) with aqueous phosphates: a combined experimental and ab initio study

The environmental fate of radionuclides (RNs) such as actinides and fission products disposed in underground nuclear waste repositories is of major concern. Long-term safety assessments of these disposal sites rely on the ability of geochemical models and thermodynamic databases (TDB) to forecast the mobility of RNs over very long time periods. One example where large data gaps in TDB still exist is related to the complexation of trivalent actinides and lanthanides with aqueous phosphates. Indeed, solid phosphate monazites are one of the candidates for the immobilization of specific high-level waste streams for safe storage in deep underground repositories in the future, which could locally increase the occurrence of phosphate in the repository.
Recent works [1-3] have been carried out to partially close these gaps in order to provide reliable complexation constants at 298K and at elevated temperature. However, obtaining this information is challenging and requires the identification of the formed complexes by means of spectroscopic techniques, such as UV-Vis or TRLFS (Time-Resolved Laser Fluorescence Spectroscopy). Depending on the phosphate concentration, mono or bi-dendate phosphate complexes can be formed with various coordination numbers (8, 9). However, it is often a challenge to obtain further information about the complex structures from the spectroscopic data alone.
In this context, relativistic quantum chemical (QC) methods can be seen as an additional tool to complement the experimental observations. In this study, structural properties, electronic structures and thermodynamics of the 1:1 and 1:2 phosphate complexes of Cm(III) and Eu(III) (see below) have been extracted by state-of-the-art QC calculations. In particular, QC methods allowed i) studying the complexation strengths of Cm(III) and Eu(III) with aqueous phosphates, ii) suggesting a potential change of the coordination number with increasing temperature and iii) probing the character (ionic/covalent) of the Cm/Eu-water and Cm/Eu-phosphate bonds.

Combining the information obtained from the quantum chemical calculations with the observed spectral changes, facilitates a conclusive assignment of the phosphate complex structures and their overall coordination [2,3].

[1] Jordan, N., Demnitz, M., Lösch, H., Starke, S., Brendler, V., and Huittinen, N. (2018). Complexation of trivalent lanthanides (Eu) and actinides (Cm) with aqueous phosphates at elevated temperatures. Inorg. Chem. 57:7015−7024.
[2] Huittinen, N., Jessat, I., Réal, F., Vallet, V., Starke, S., Eibl, M., and Jordan, N. (2021). Revisiting the complexation of Cm(III) with aqueous phosphates: new insights from luminescence spectroscopy and ab initio simulations. Inorg. Chem. 60:10656−10673.
[3] Jessat, I., Jordan, N., Kretzschmar, J., Réal, F., Vallet, V., Stumpf, T., and Huittinen, N. (2023). Impact of temperature on the complexation of Eu(III) with phosphate ions: a spectroscopic and ab initio study. Inorg. Chem. (in preparation).