Complexation of Cm(III) and Eu(III) with phosphates: an experimental and theoretical study

The environmental fate of radionuclides (RN), such as actinides and fission products, disposed of in underground nuclear waste repositories is a major concern. Long-term safety assessments of these disposal sites depend on the ability of geochemical models and thermodynamic databases (TDBs) to predict the mobility of RNs over very long time scales. One example where TDBs still have large data gaps is related to the complexation of trivalent actinides and lanthanides with aqueous phosphates. Indeed, solid phosphate monazites are one of the candidate phases for the immobilization of specific high-level waste streams for future safe storage in deep underground disposal facilities, therefore potentially and locally increasing the presence of phosphate at the final disposal site.

Recent work [1-3] obtained reliable complexation constants at 25 °C and at elevated temperatures and thus, closed some knowledge gaps. Laser-induced luminescence spectroscopy was used to study the complexation of Cm(III) and Eu(III) as a function of total phosphate concentration in the temperature regime 25-90 °C, using NaClO₄ as a background electrolyte. These studies have been conducted in the acidic pH-range to avoid precipitation of solid Cm and Eu rhabdophane. In addition to the presence of the CmH₂PO₄ ²⁺/EuH₂PO₄ ²⁺ species [1-3], the formation of Cm(H₂PO₄)₂ ⁺ [2] and Eu(H₂PO₄)₂ ⁺ [3] was unambiguously established from the collected luminescence spectroscopic data. The conditional complexation constants of all aqueous complexes were extrapolated to infinite dilution by applying the Specific ion Interaction Theory. Using the integrated van´t Hoff equation, both the molar enthalpy of reaction Δ_rH_m° and entropy of reaction Δ_rS_m° values were derived.

Depending on the concentration of phosphate, monodentate or bidentate Cm(III)/Eu(III)-phosphate complexes form with different overall coordination numbers (8,9), but obtaining such information from spectroscopic data only is often challenging. Thus, the structural properties, electronic structures, and thermodynamics of the 1:1 and 1:2 Cm(III) and Eu(III) phosphate complexes were solved using state-of-the-art relativistic quantum chemical (QC) calculations. In particular, the QC methods allowed i) to investigate the complexation strength of Cm(III) and Eu(III) with aqueous phosphate, ii) to understand the possible change of the coordination number with increasing temperature and iii) to investigate the nature (ionic/covalent) of the Cm/Eu bonds with water and phosphate.

Combining the information obtained from quantum chemical calculations with the observed spectral changes facilitates the decisive determination of the structures of the formed phosphate complexes and their overall coordination [2,3].

References
[1] N. Jordan et al., Inorganic Chemistry 57, 7015 (2018).
[2] N. Huittinen et al., Inorganic Chemistry 60, 10656 (2021).
[3] I. Jessat et al., Inorganic Chemistry (in preparation).