In the context of nuclear fuel reprocessing, the recovery and purification of major actinides is achieved using a hydrometallurgical process known as PUREX (Plutonium Uranium Recovering by Extraction). Based on Liquid-Liquid extraction technics, this process requires a specific molecule to extract Pu and U, the Tri-n-butyl phosphate TBP. Monoamides are regarded as alternative family of extraction molecules to TBP, as they are well known for their strong extraction ability of Pu(IV) and U(VI) elements. In addition to this, they show some interesting features, such as the complete incinerability of the solvent degradation products, and a strong dependence of the extraction properties (distribution coefficient and selectivity) on the chemical conditions [1-3].

The strong influence of the structure of amide derivatives on their extraction properties has been demonstrated in several studies in the literature. In this study, we propose to investigate and rationalize the influence of the nature and length of the monoamide alkyl chains on Pu(IV) extraction/complexation, using the Density Functional Theory (DFT) method in the scalar relativistic framework. We will discuss the geometries of the inner/outer-sphere complexes and the interaction energies of [Pu(NO₃)₄] and [Pu(NO₃)₆]^2- with different monoamide and carbamide ligands have been calculated. Our investigation focuses on discriminating the influence of the bulkiness of the alkyl group, and that of the solvent polarity on the stability of inner and outer-sphere complexes. This study suggests key possibilities to tune the substituent effect by changing the polarity of the solution [4].

References
[1]G. Milanole et al. WO/2018/138441 (2018).
[2]C. Berger et al., Solvent Extr. Ion Exch., 37 , 111-125 (2019).
[3]E. Acher et al., Inorg. Chem., 55, 5558-5569 (2016).
[4]A. Failali et al., Phys. Chem. Chem. Phys., 23, 2229-2237 (2021).