Carbamide and monoamide derivatives are very promising molecules to achieve U­(VI) and Pu­(IV) extraction and separation from spent nuclear fuels through solvent extraction. Herein, coordination structures of U­(VI) and Pu­(IV) complexes with carbamide derivatives were characterized using X-ray crystallography as well as infrared, UV–visible, and EXAFS spectroscopies. Coordination structures are compared to those obtained for monoamide derivatives in order to better understand the role of coordination chemistry in extraction properties. Single crystals were first synthesized with a short alkyl chain carbamide analog. Carbamide complexation in the solid state is found analogous to that in the monoamide. In organic solution, upon solvent extraction from nitric acid aqueous solution, it is shown that both amide derivatives can bind in the inner and outer coordination spheres of uranium­(VI) and plutonium­(IV). The amount of outer sphere coordination complexes increases with the amount of nitric acid. With uranium­(VI), at a nitric acid concentration up to 5 mol·L–1, amide derivatives operate predominantly in the inner coordination sphere. In contrast, Pu­(IV) coordination geometry is much more sensitive toward acid concentration or ligand structure than U­(VI). Pu­(IV) changes from inner sphere complexation at 0.5 mol·L–1 HNO3 to mostly outer sphere complexation at 4 mol·L–1. The proportion of outer-sphere complexes is strongly influenced by the ligand structure. Higher Pu­(IV) extraction is found to be correlated with the amount of Pu­(IV) outer sphere species. Secondary interactions in the outer sphere coordination shell appear to be of primary importance for plutonium extraction.

In the course of nuclear fuel treatment, solvent extraction cycles separate uranium and plutonium from fission products and minor actinides. During these steps, small amounts of the fission product ruthenium may follow the uranium and plutonium products. Herein, we show by Raman spectroscopy how ruthenium complexes in the aqueous phase differ as a function of the nitric acid concentration. Furthermore, we compare ruthenium extraction by the industrially used solvent tri-n-butylphosphate to other extractants such as tetrabutyl urea, N-methyl, N-octyl ethylhexanamide, and N,N,N′,N′-tetraoctyl diglycolamide. Analysis by inductively coupled plasma atomic emission spectroscopy demonstrates that all of these solvents have different ruthenium distribution ratios. In contrast, similar ruthenium complexes were identified by Fourier transform infrared and extended X-ray absorption fine structure spectroscopy in all extractions from 4 M nitric acid. Hence, different distribution ratios of ruthenium are not due to different ruthenium complexes. By comparing ruthenium extraction with nitric acid and water extraction, we show a linear correlation between ruthenium and water concentration in the organic phase. This suggests an interaction between the solvent and water ligands during ruthenium extraction. To limit the ruthenium coextraction in nuclear fuel treatments, we suggest further investigation of solvents with low water coextraction.