Coordination Structures of Uranium(VI) and Plutonium(IV) in Organic Solutions with Amide Derivatives Cle ́ mence Berger, Ce ́ cile Marie, Dominique Guillaumont,* Christelle Tamain, Thomas Dumas, Thomas Dirks, Nathalie Boubals, Ele ́ onor Acher, Marjorie Laszczyk, and Laurence Berthon CEA, DEN, DMRC, Université de Montpellier, Marcoule, 30207 Bagnols sur Cè ze, France * S Supporting Information 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 HNO 3 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. ■ INTRODUCTION Irradiated nuclear fuels can be reprocessed to recover uranium(VI) and plutonium(IV) and to reduce the volume of high-level radioactive wastes. Selective separation of actinides is achieved by solvent extraction where the spent fuel is dissolved into an acid aqueous solution and suitable organic ligands are used to extract actinide cations into an organic solution. The PUREX solvent extraction process (Plutonium Uranium Redox EXtraction) with tri-n-butyl phosphate (TBP) extractant is used on the industrial scale to separate and purify uranium(VI) and plutonium(IV) from fission products initially in highly concentrated nitric acid solution. TBP has been used for decades but shows some limitations such as its nonincinerable nature leading to the formation of solid phosphate residues during its destruction and the formation of degradation products by radiolysis which makes the management of the solvent complex. 1 Moreover, it shows strong affinity for actinides at high oxidation states +IV and +VI. As a consequence, the partition between uranium and plutonium requires the reduction of Pu(IV) to Pu(III) by the introduction of reducing and stabilizing agents such as uranium(IV) and hydrazinium nitrate. Other families of extractants are under investigation, and among potential candidates, N,N-dialkyl amides (or monoamides) are so far the most promising molecules to achieve U(VI) and Pu(IV) extraction. 2-15 They have received a large amount of interest, and solvent extraction studies with numerous monoamide derivatives have been reported. 5,12,16,17 Compared to mono- amides, carbamides (urea derivatives) have attracted much less attention, 18-21 but we have recently shown that they can also be extremely interesting candidates to achieve uranium(VI) and plutonium(IV) extraction from fission products. 22 Extraction properties of a series of tetra-alkylcarbamide derivatives with linear alkyl chains of varying lengths were investigated and compared to those measured with one promising monoamide derivative: N,N,N′,N′-tetra-n-butylurea (TBU), N,N,N′,N′-tetra-n-hexylurea (THU), and N,N,N′,N′- tetra-n-octylurea (TOU; Figure 1). Their strong capability to extract uranium(VI) and plutonium(IV) was demonstrated at a high nitric acid concentration (4 mol·L -1 HNO 3 in the aqueous phase), while uranium(VI)/plutonium(IV) separation can be achieved by decreasing nitric acid concentration (0.5 mol·L -1 HNO 3 in the aqueous phase). Moreover, it was shown that extraction strength and U/Pu selectivity can be tuned by changing the length of the alkyl chains. Among the various properties that are important for a comprehensive understanding of extraction strength and Received: October 15, 2019 Article pubs.acs.org/IC Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.9b03024 Inorg. Chem. XXXX, XXX, XXX-XXX Downloaded via QUEEN MARY UNIV OF LONDON on January 16, 2020 at 03:41:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.