Complexation of neptunium(V) with aqueous phosphate using a dual experimental and quantum chemical approach

Understanding and quantifying the chemistry of actinides with strong complexing ligands found in the environment is of key importance for predicting their mobility in the subsurface, especially with respect to the safety of high-level nuclear waste disposal. Phosphate ligands are present in the environment as they originate from the natural decomposition or microbially mediated solubilization processes of phosphate containing rocks and minerals. They can also be produced by anthropogenic activities such as fertilizers spread on the fields or phosphate-based detergents. Furthermore, phosphates are constituents of glasses and ceramics that may be used to immobilize high-level wastes.

Current thermodynamic databases contain very little data on actinide-phosphate complexes [1]. For the trivalent actinide curium, a recent study combining luminescence spectroscopy, thermodynamics, and quantum chemical (QC) calculations could unambiguously establish the formation of 1:1 and 1:2 phosphate complexes with the H₂PO₄⁻ ligand [2]. Complexation constants for both species were derived and extrapolated to standard conditions using the specific ion interaction theory. By combining the data obtained in the luminescence spectroscopic investigations, such as the crystal-field splitting of the emitting excited states and the luminescence lifetimes, with quantum chemical calculations, the coordination number of the complexes could be determined. At room temperature, both the 1:1 and 1:2 complexes are coordinated by 9 ligands. At elevated temperature, only the 1:1 complex retains a coordination number of 9, while one water molecule is released from the 1:2 complex, thereby reducing its coordination number to 8.

In this study, a similar combined experimental and computational approach will be applied to bridge the gaps in the databases for neptunium(V)-phosphate complexes. The complexation reaction will be studied with UV-vis and infrared (IR) spectroscopies at varying ionic strengths and temperatures under acidic pH conditions. Thereby, complexation constants and thermodynamic parameters for Np(V) aqua ion with phosphate can be derived for the formed Np(V)-H₂PO₄⁻ complex(es) (Figure 1). However, the experimental data alone, do neither hold information on the coordination of phosphate to the NpO₂⁺ cation (mono or bidentate binding), nor on the overall coordination number. This calls for relativistic QC calculations that will help characterizing the stoichiometry and geometries of the complexes. Indeed, QC methods can quantify the relative stability of these structures as well as the complexation strengths with aqueous phosphate through potential change of the coordination number with increasing temperature. The calculated vibrational frequency supports the assignment of bands within the measured IR spectra. In addition, the electronic structures at fundamental and excited states will be computed in order to predict the absorption bands of Np(V) – phosphate complexes in acidic solution. Ultimately, by using the quantum theory of atoms in molecules (QTAIM), the topology of the neptunyl-ligand bonds can be scrutinized to i) discuss the evolution of its character as additional phosphate ligands bind neptunyl and ii) to complement the evolution of the complexation constants determined experimentally.

[1] Grenthe, I. Gaona, X. Plyasunov, A. Rao, L. Runde, W.H. Grambow, B. Konings, R.J.M. Smith, A.L. Moore, E.E. (2020). Second Update on the Chemical Thermodynamics of Uranium, Neptunium, Plutonium, Americium and Technetium. Chemical Thermodynamics Volume 14, OECD Publications, Paris.
[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.