[en] Full text of publication follows: Actinide and lanthanide ions in solution present a wide variety of coordination environments which are very often unknown, even though a detailed knowledge of their molecular structures, dynamics and energetics in a given media is essential to predict their behaviour and to control a ligand affinity toward the metal ions. Quantum chemistry calculations on heavy elements ions in solution remain challenging but they can be an important tool to complete experimental measurements for obtaining structural and energetic information and for probing the nature of the metal-ligand bonds. In the present study, quantum chemistry calculations have been performed on lanthanide and actinide ions in the trivalent oxidation state in the presence of water molecules and nitrogen donor ligands. The geometrical structures of the complexes and the binding strength of the ligands have been determined in the gas phase and in solution. The calculations were first performed on lanthanide and actinide molecular compounds which have been well characterized experimentally in order to validate a theoretical approach and then extended to a larger number of actinide ions. The nature of the metal-ligand bonds within the series of cations and the specificity of the actinide-ligand bond will be discussed from a geometric, energetic and electronic standpoint and the role played by solvation effects will be analyzed by comparing gas phase and solution calculations. (author)

[en] Dissolution of spent nuclear fuel in nitric acid forms diverse octahedral complexes of the ruthenium fission product with one nitrosyl and a variable number of nitrate, nitrite, hydroxide and water ligands. Some of these ruthenium complexes can trouble uranium and plutonium extractions by TBP and their purifications. In this study, we analyze the effect of gamma radiation on the structure of ruthenium(III) nitrosyl complexes in 5 M nitric acid solution and highlight the necessity to take into account the nitrato-nitrite complexes [RuNO(NO₃)_x(NO₂)_y(H₂O)_5-x-y]^3-x-y. Accordingly, we performed a parametric study to analysis the influence of varying nitrous acid concentrations on ruthenium nitrosyl complexes in 1 M and 5 M nitric acid solutions. Raman spectra show that nitrites from nitrous acid replace nitrate ligands in ruthenium complexes. Resulting nitrite complexes were characterized by comparing the experimental Raman spectra to DFT calculations. This comparison supports a successive formation of mono-, cis- and potentially mer-ruthenium nitrite complexes with increasing nitrous acid concentrations. The degradation of such nitrite complexes has been followed by Raman measurements and the reverting to nitrate complexes occurs within several weeks. The radiolytic formation and the slow degradation shows a relevance of nitrite complexes to understand and hence control ruthenium in solvent extraction processes. (author)

[en] A new family of extractant containing the carbamide function (also called urea with the general formula R₁R₂NC(O)NR₃R₄) is currently studied in our laboratory for the separation of uranium(VI) and plutonium(IV) by solvent extraction from spent nuclear fuels. The objective of this study is to evaluate the effect of alkyl chains length on actinide extraction. Our results show that the chain length (from butyl to octyl chain) does not influence significantly uranium(VI) extraction while it has a strong impact on plutonium(IV) extraction. The U(VI) and Pu(IV) complexes formed in the organic phase were characterized by spectroscopic techniques. First, to gain information on the coordination sphere of U(VI) and Pu(IV) complexes with carbamide, single crystals were synthesized with short alkyl chain carbamide (R1 = R2 = R3 = R4 = iso-butyl). An octahedron complex and a bicapped dodecahedron complex are obtained for UO₂(NO₃)₂L₂ and Pu(NO₃)₄L₂ (with L = N,N,N',N'-tetra-iso-butylcarbamide) respectively. Then, the analysis of organic phases after uranyl extraction confirmed the participation of the carbonyl function and the nitrate anions denticity (bidentate) to uranyl coordination, and showed that the coordination sphere of uranyl is not modified by a change in the alkyl chains length of the carbamide. At the opposite, the speciation of plutonium(IV) depends on alkyl chain length of the carbamide and acidity of the solution. UV-Vis-NIR and EXAFS spectroscopy indicate that two different complexes are formed: a neutral complex Pu(NO₃)₄L₂ and an anionic complex Pu(NO₃)₆^2- and their proportion varies with the length of the alkyl chain. The organic phase containing the higher proportion of anionic complex is associated to the higher Pu(IV) distribution ratio. These results suggest that the anionic complex is more extracted than the neutral complex and that increasing alkyl chain length enhances its organic phase solubility.

[en] To separate minor actinides from lanthanides for used nuclear fuel reprocessing, solvent extraction processes combining neutral and acidic extractants are considered. Mixed systems present singular extraction properties where synergistic effects can be observed but information on chemical equilibria is lacking to properly model such behavior. Speciation of Eu(III) and Nd(III) complexes formed in an organic phase combining N,N'-dimeth-yl-N,N'-dioctyl-hexyl-ethoxy malonamide (DMDOHEMA) and di(2-ethylhexyl)phosphoric acid (HDEHP) was investigated through electro-spray ionization mass spectrometry (ESI-MS), time-resolved laser-induced fluorescence spectroscopy (TRLIFS), UV-Visible, nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and density functional theory (DFT) calculations. A method consists of adding one of the ligands to a solution containing Ln(III) complexes formed with the other ligand while following changes by each technique was used. In both cases, significant modifications in the metal coordination sphere were observed due to formation of mixed complexes containing both ligands: addition of DMDOHEMA to a Ln-HDEHP solution led to the formation of Ln(DEHP)₃(HDEHP)₂(DMDOHEMA) complexes while addition of HDEHP to a Ln-DMDOHEMA solution gave Ln(NO₃)_3-n(DEHP)_n(HDEHP)_m(DMDOHEMA)_x' (0 ≤ n ≤ 3; n + m ≥ 1 and 1 ≤ x' ≤ 3) complexes. (authors)

[en] DEHCNPB (butyl-N, N-di(2-ethylhexyl)carbamoyl-nonyl-phosphonate) is an amido-phosphonic acid that has remarkable properties for the separation of uranium from wet phosphoric acid. Despite previous studies, a detailed description of the DEHCNPB organic solutions at the supramolecular and molecular scales is missing. In the present work, we use classical Molecular Dynamics (MD) combined with SANS and SAXS experimental data in order to describe the aggregation of the bifunctional extractant DEHCNPB as well as the speciation of uranium(VI) in such systems. We provide a fine description of the molecular species in the organic solution and of the interactions within the aggregates formed, shedding light on solvent extraction mechanisms. Without uranium, the organic phase is highly composed of dimers and trimers H-bonded through phosphonate functions and without water molecules. With uranium, two to three extractant molecules coordinate directly the uranyl cation by their phosphonate groups. Uranyl is not fully dehydrated in this organic solution, and the amide groups of the extractants are found to form H-bonds with the water molecules bound to uranyl. These H-bond networks around the metallic cation stabilize the complexes and facilitate the extraction. These results underline the importance of considering weak interactions in the understanding of extraction processes and demonstrate how molecular simulations provide essential insights into such complex organic phase chemistry with a high number of species. (authors)

[en] Among the fission products present in the spent nuclear fuel, technetium exhibits a singular behavior in reprocessing operations performed by solvent extraction. Indeed, this strong acid readily dissociates to form the oxo-anion TcO₄^- that may interfere with uranium(VI), plutonium(IV), and zirconium(IV) in the extraction cycles of the PUREX process. This paper focuses on the uranium-technetium complex with TBP and on its non-radioactive rhenium surrogate. Despite the large set of distribution data available for rhenium and technetium extraction with TBP, the structures of the co-extracted complexes remain largely unknown. However, it is important to understand clearly the extraction mechanism of technetium with TBP in the PUREX process to optimize the separation process and to model its behavior during the extraction steps. Based on distribution data available in the literature, a thermodynamic model was developed for the extraction of technetium with TBP for a large excess of uranium(VI) in organic phase. A good representation of uranium and technetium distribution data was thus obtained when considering the formation of (HTcO₄)(TBP)_n complexes, as well as mixed UO₂(NO₃)(TcO₄)(HNO₃)_x(TBP)_n complexes. In the complex UO₂(NO₃)₂(HNO₃)_x(TBP)_n, one pertechnetate anion replaces one nitrate in the uranium coordination sphere. Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) with organic phases containing a large excess of technetium(VII) or rhenium(VII) enabled full characterization of the limit mixed uranium-technetium species and also of mixed uranium-rhenium species. Details on the coordination of the uranium-technetium complex are provided with the help of DFT calculations and XAS measurements. (authors)

[en] The singular Pu^IV hexanuclear cluster [Pu₆(OH)₄O₄]¹²⁺ stabilized by 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) ligands was structurally characterized for the first time both in the solid state and in water solution by using X-ray diffraction and X-ray absorption and UV/Vis spectroscopy. The stability of this cluster in water and its high solubility over a large pH range are of upmost importance for plutonium environmental speciation with potential applications in a related migration model. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] The objective of this study is the analysis of Pu(IV) clusters in solution in the presence of carboxylate ligand: acetate is used as a surrogate of this family. Moreover, available data on speciation in solution of 'classical' monomeric complexes of plutonium (IV) acetate are rare, incomplete and sometimes contradictory. To detect all potentially formed plutonium-acetate species, solutions were first characterized by Vis- NIR absorption spectroscopy. Large variations of pH and acetate concentrations were used and six different species were identified: the Pu(IV) aquo cation (Pu⁴⁺), and five plutonium complexes with acetate. Those five acetate complexes were characterized by coupling experimental (Vis-NIR and EXAFS spectroscopies and ESI-MS spectrometry) and quantum chemistry. As a result, Pu₆O₄(OH)₄(AcO)₁₂(H₂O)₆ hexameric cluster has been identified: the missing block in the An(IV) series with formate and acetate ligands. The four other complexes are attributed to Pu(IV) acetate monomeric complexes. Species of Pu(IV) with acetate being described, their fractions in solution were evaluated and reported on a speciation diagram. The hexanuclear complex was not mentioned in the literature (despite chemical conditions equivalent to ours), this study allowed to define the new and more reliable Pu(IV)-acetate speciation diagram

[en] The complexation of 1,4,7,10-tetraaza-cyclodecane-1,4,7,10- tetraacetic acid (DOTA) ligand with two trivalent actinides (Am³⁺ and Pu³⁺) was investigated by UV-visible spectrophotometry, NMR spectroscopy, and extended X-ray absorption fine structure in conjunction with computational methods. The complexation process of these two cations is similar to what has been previously observed with lanthanides(III) of similar ionic radius. The complexation takes place in different steps and ends with the formation of a (1:1) complex [(An(III)DOTA)(H₂O)]^-, where the cation is bonded to the nitrogen atoms of the ring, the four carboxylate arms, and a water molecule to complete the coordination sphere. The formation of An(III)-DOTA complexes is faster than the Ln(III)-DOTA systems of equivalent ionic radius. Furthermore, it is found that An-N distances are slightly shorter than Ln-N distances. Theoretical calculations showed that the slightly higher affinity of DOTA toward Am over Nd is correlated with slightly enhanced ligand-to-metal charge donation arising from oxygen and nitrogen atoms. (authors)

[en] Diglycolamide extractants, and in particular N,N,N',N'-tetraoctyl diglycolamide (TODGA), are currently under investigation for use in nuclear fuel reprocessing by liquid-liquid separations. Several processes, such as ARTIST, i-Sanex, and EURO-GANEX processes, have been developed around the TODGA extractant. The solvent typically combines TODGA with a modifier in an alkane diluent. Due to the high radiation produced by the actinides and fission products present in the fuel, radiolytic degradation can greatly alter the chemistry of these solutions. The radiolysis and hydrolysis of the diglycolamide-based extractant was investigated. The effects of the presence of 1-octanol as phase modifier in organic phase and nitric acid concentration in aqueous phase were investigated. Nitric acid provides a slight protective effect, whereas the presence of 1-octanol seems to have a slight sensitizing effect. Fukui function calculations were done to supplement the experimental data and give an improved understanding of the behaviour of TODGA organic solutions after radiolysis. An increase in nitric acid concentration protects TODGA from radiolysis as nitrate absorbs the radicals. The slight sensitizing effect of 1-octanol is due to a combination of several competitive effects: a protective effect from TODGA-octanol adducts and a sensitizing effect from an increase in the water concentration in the organic phase, which produces OH^. radicals. Lanthanide extraction was performed with irradiated solutions to identify the degradation products that complex metal ions. Among the identified degradation products in solution, compounds retaining the diglycolamide skeleton and [2-(dioctyl-amino)-2-oxoethoxy]acetic acid participate in the lanthanide complexation to form mixed Ln-degradation product-TODGA complexes. In contrast, 2-hydroxy-N,N-dioctyl-acetamide acts more like a phase modifier rather than directly complexing lanthanide ions. (authors)