Coordination Structures of Uranium(VI) and Plutonium(IV) in Organic Solutions with Amide Derivatives

Abstract

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.

Evolution of Acid-Dependent Am3+ and Eu3+ Organic Coordination Environment: Effects on the Extraction Efficiency

“…Amidic extractants have recently gained considerable interest in the solvent-extraction community. Their potential uses include the separation of both uranium and plutonium from the bulk constituents of used nuclear fuel, − as well as the separation of trivalent lanthanides from the trivalent transplutonium actinides. These ligands are envisioned as advantageous to a TBP-based process for the separation of uranium and plutonium because of the ability to incinerate all components, no generation of insoluble phosphates, and no need to reduce Pu 4+ to Pu 3+ .…”

Piperazinyl-Based Diamide Ligand for Selective Precipitation of Actinyl (UO22+/PuO22+) Ions with Fast Kinetics

“…The U–O amide bond length is 2.338(19) Å, which is one of the shortest U–O amide bond distances reported. This short bond length is normally seen in the systems in which amide is selective to uranyl ion as reported in the case of [UO 2 (NO 3 ) 2 T i Bu], [UO 2 (NO 3 ) 2 (iso-C 3 H 7 CON­{iso-C 4 H 9 } 2 ) 2 ], [(UO 2 )­(adam) 2 (NO 3 ) 2 ·2­(adam)], and [UO 2 (NO 3 ) 2 (DHNRP) 2 ] . This particular amide U–O amide (CO) is shorter than U–O equatorial (PO) reported for the corresponding phosphine oxide complexes, which suggests that this amide CO bond is a strong donor similar to the phosphine oxide toward the uranyl ion.…”

Hydrophobic core formation and secondary structure elements in uranyl(vi)-binding peptides

“…Coordination of UO 2 2+ with various types of organic and inorganic ligands has been studied for decades and is still the subject of intensive investigations. [1][2][3] However, uranium interaction with peptides has been scarcely investigated. This is presumably because the synthesized peptides are very expensive materials and cannot compete with bulk chemical substances employed in conventional separation processes.…”