Cm(III) and Eu(III) complexation with aqueous phosphates: an experimental, thermodynamic, and ab initio 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 reliably 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 (I = 0.5 to 3.0 M). These studies have been conducted in the acidic pH-range (−log₁₀ [H⁺] = 1.00, 2.52, 3.44, and 3.65) to avoid precipitation of solid Cm and Eu rhabdophane. For the first time, in addition to the presence of the 1:1 CmH₂PO₄²⁺/EuH₂PO₄²⁺ 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 found to increase with increasing ionic strength and temperature [1-3]. Extrapolation of the obtained conditional complexation constants to infinite dilution at 25 °C and elevated temperature was performed by applying the Specific Ion Interaction Theory (SIT). 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.

During complexation, depending on the concentration of phosphate, monodentate or bidentate Cm(III)/Eu(III)-phosphate complexes can form with different overall coordination numbers (8,9). Obtaining additional information about the complex structures from spectroscopic data only is often challenging, especially at the low metal-ion concentrations used in the experiments. Thus, relativistic quantum chemical (QC) methods can be considered as an additional tool to complement experimental observations. In this study, 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 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].

 

[1] Jordan, N., et al., Inorg. Chem. (2018), 57:7015−7024.

[2] Huittinen, N., et al., Inorg. Chem. (2021), 60:10656−10673.

[3] Jessat, I., et al., Inorg. Chem. (in preparation).