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Sonochemistry was used for the preparation of uranium oxide nanoparticles. In comparison to classical hydrolysis approach, larger, better defined and more crystalline UO2+xnanoparticles were observed under ultrasound.

International audience; The Pourbaix diagram of an element displays its stable chemical forms with respect to the redox potential and pH of the solution, whose knowledge is fundamental for understanding and anticipating the chemistry of the element in a specified solution. Unlike most halogens, the Pourbaix diagram in the aqueous phase for astatine (At, Z = 85) is still under construction. In particular, the predominant domains of two astatine species assumed to exist under alkaline conditions, At$^-$ and AtO(OH)$_2$$^-$, need to be refined. Through high-performance ion-exchange chromatography, electromobility measurements, and competition experiments, the existence of At$^-$ and AtO(OH)$_2$$^-$ has been confirmed and the associated standard potential has been determined for the first time (0.86 ± 0.05 V vs the standard hydrogen electrode). On the basis of these results, a revised version of astatine’s Pourbaix diagram is proposed, covering the three oxidation states of astatine that exist in the thermodynamic stability range of water: At(-I), At(I), and At(III) (as $_2$$^-$, $_2$$^+$, AtO(+), AtO(OH), and AtO(OH)$_2$$^-$

The mechanism of reaction of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) with ·CH

A spectroelectrochemical setup has been developed to investigate radioactive elements in small (0.7 to 2 ml) volumes under oxidation–reduction (redox) controlled conditions by X-ray absorption spectroscopy (XAS). The cell design is presented together with in situ XAS measurements performed during neptunium redox reactions., A spectroelectrochemical setup has been developed to investigate radioactive elements in small volumes (0.7 to 2 ml) under oxidation–reduction (redox) con­trolled conditions by X-ray absorption spectroscopy (XAS). The cell design is presented together with in situ XAS measurements performed during neptunium redox reactions. Cycling experiments on the NpO2 2+/NpO2 + redox couple were applied to qualify the cell electrodynamics using XANES measurements and its ability to probe modifications in the neptunyl hydration shell in a 1 mol l−1 HNO3 solution. The XAS results are in agreement with previous structural studies and the NpO2 2+/NpO2 + standard potential, determined using Nernst methods, is consistent with measurements based on other techniques. Subsequently, the NpO2 +, NpO2 2+ and Np4+ ion structures in solution were stabilized and measured using EXAFS. The resulting fit parameters are again compared with other results from the literature and with theoretical models in order to evaluate how this spectroelectrochemistry experiment succeeds or fails to stabilize the oxidation states of actinides. The experiment succeeded in: (i) implementing a robust and safe XAS device to investigate unstable radioactive species, (ii) evaluate in a reproducible manner the NpO2 2+/NpO2 + standard potential under dilute conditions and (iii) clarify mechanistic aspects of the actinyl hydration sphere in solution. In contrast, a detailed comparison of EXAFS fit parameters shows that this method is less appropriate than the majority of the previously reported chemical methods for the stabilization of the Np4+ ion.

New insights are provided about the formation mechanism of PuO

[H7 O3 ]4 [Tc20 O68 ] ⋅ 4H2 O [1] was prepared from an aqueous Tc2 O7 solution concentrated over anhydrous H2 SO4 . [Tc20 O68 ]4- is the first polyanionic species to be reported for Tc. The unit cell contains one centrosymmetric [Tc20 O68 ]4- polyanion as well as hydronium ions and water molecules. The core of the structure consists of four Tc(V)O6 octahedra that form a square Tc4 O4 ring. The four Tc(V)O6 octahedra are decorated by sixteen Tc(VII)O4 tetrahedra. Calculations show the bonding within the Tc4 O4 ring to consist of a 3-center bond formed between each neighboring pair of Tc atoms and their bridging oxygen. Calculations also indicate that a strong d→d electronic transition at 513 nm is the origin of the red color of [1]. The characterization of red HTcO4 solutions by X-ray absorption spectroscopy has complemented the description of this compound in aqueous solution. The formation mechanisms in solution, including the possible role of technetium's radioactivity in the formation of [1], are discussed.

Actinide +IV complexes (An

The coordination and redox chemistry of aqueous Ce

The redox chemistry of CeIIIDOTA in cage in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [CeIIIDOTA(CO3)]3- (or less plausible [CeIIIDOTA(HCO3)]2-) with the carbonate (bicarbonate) anion as the ninth ligand versus [CeIIIDOTA(H2O)]- present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple CeIV/III in the presence of carbonate versus its aqueous analogue. [CeIVDOTA(CO3)]2- and [CeIVDOTA(H2O)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [CeIVDOTA(H2O)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [CeIVDOTA(CO3)]2- species is produced radiolytically by oxidation of the trivalent species by CO3•- with a rate constant, measured using pulse radiolysis, of 3.3 × 105 M-1 s-1. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO3•- with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an outer-sphere electron transfer rather than the inner-sphere one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.

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.

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complex of UIV with H2 O, OH- , and F- as axial ligands was studied by using UV/Vis spectrophotometry, ESI-MS, NMR spectroscopy, X-ray crystallography, and electrochemistry. The UIV -DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The UIV /UIII redox couple was found to be quasi-reversible for the complex with both axially bonded H2 O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one-electron UV /UIV oxidation of the [UIV (DOTA)(H2 O)] complex quasi-reversible, which suggests the formation of the short-lived pentavalent form of the complex, an aqueous non-uranyl chelated UV cation.

International audience

Due to the increased attention given to actinide nanomaterials, the question of their structure-property relationship is on the spotlight of recent publications. Plutonium oxide (PuO

A new hexanuclear plutonium cluster has been stabilized in aqueous media with acetate ligands. To probe the formation of such a complex structure, visible-near infrared (vis-NIR) absorption spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) were combined. The presence of Pu

Despite being a member of the zircon type silicate family, the conditions allowing the hydrothermal synthesis of HfSiO4 were not well constrained. A multiparametric study was performed in order to follow the synthesis of this phase under soft hydro-thermal conditions and thus to determine the most efficient conditions to form single phase samples. Among the experi-mental parameters investigated, concentration of reactants, pH of the reactive media, temperature and duration of the hydrothermal treatment impacted significantly the formation rate of hafnon and its crystallization state. Pure HfSiO4 was obtained in acid reactive media with an acidity ranging from CHCl = 1.5 M to pH = 1.0 and for CSi ≈ CHf ≥ 0.21 mol·L 1. The silicate phase was prepared after a 24-hours treatment at temperatures ranging from 150°C to 250°C. However, rise of tem-perature and extension of the duration of the hydrothermal treatment favored the crystallization state of the final HfSiO4 samples.

The [AnIV(DPA)3]2− series with An = Th, U, Np, Pu has been synthesized and characterized using SC-XRD and vibrational spectroscopy. First principles calculations were performed, the total electron density is analyzed using the Quantum Theory of Atoms in Molecules. Crystal field parameters and strength parameters are deduced following a previous work on the LnIII analog series e.g. [J. Jung et al., Chem. – Eur. J., 2019, 25, 15112]. The trends in the parameters along the series are compared to the LnIII complexes. They evidence larger covalent interactions and larger J mixing.

International audience; Fundamental knowledge about plutonium intrinsic colloids is a key issue for the prediction of plutonium transport and release rates in the environment. Recent studies demonstrated that the particle size and surface properties of intrinsic Pu colloids are strongly influenced by the preparation method. In this work, a combination of synchrotron small angle X-ray scattering (SAXS) with X-ray absorption spectroscopy (XAS) is used to characterize two kinds of stable plutonium intrinsic colloids prepared by hydrolysis of Pu(IV) ionic species and sonolysis of PuO2 in pure water. Despite their similarities, the multi-scale structural properties of these colloidal suspensions are found to be strongly influenced by the synthesis route. The hydrolysis approach leads to spherical nanoparticles of ca. 2.0 nm, whereas sonolytic colloids show elongated structures measuring 5.7 nm of thickness and >30 nm long. This difference results from the synthesis mechanism and can be attributed to nanoparticle aggregation in the absence of capping-ions. The results obtained by both SAXS and XAS approaches converge in the description of Pu(IV) intrinsic colloids as core–shell nanoparticles made up of a PuO2 core covered with a disordered Pu–O shell characterized by a splitting of Pu–O and Pu–Pu distances and an associated strong increase of associated DWF parameters. The local distortions of both Pu–O and Pu–Pu coordination spheres observed by XAS seem to be correlated with the nanoparticle shrinking probed by SAXS rather from the contribution of higher Pu oxidation states. The stabilization of the hydrolytic colloidal particles is further suggested from SAXS simulation to result from interaction with counter-ions from the medium.

[H

The redox chemistry of Ce

Actinide +VI complexes (

Two new aqueous UIV complexes were synthesized by the interaction between the tetravalent uranium cation and the (1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid) (DOTP) macrocyclic ligand. Two distinct homonuclear complexes were identified; the first was characterized by X-ray crystallography as a unique "out-of-cage", [U(DOTPH6 )2 ] complex, in which the UIV cation is octa-coordinated to 4 phosphonic arms from each ligand in a square anti-prism geometry, with a C4 symmetry. The second is the "in-cage" [U(DOTPH4 )] complex, in which the tetravalent cation is located between the macrocycle O4 and N4 planes. With the help of UV-Vis absorption, 1 H/31 P NMR, ATR-IR, and MALDI-TOFMS analytical techniques, the chemical interchange between both species is presented. It is shown that the one-way transition is governed by the formation of a multiple number of soluble oligomeric species consisting of varied stoichiometric ratios of both characterized homonuclear complexes.

The [AnIV(DPA)3]2- series with An = Th, U, Np, Pu has been synthesized and characterized using SC-XRD and vibrational spectroscopy. First principles calculations were performed, the total electron density is analyzed using the Quantum Theory of Atoms in Molecules. Crystal field parameters and strength parameters are deduced following a previous work on the LnIII analog series e.g. [J. Jung et al., Chem. - Eur. J., 2019, 25, 15112]. The trends in the parameters along the series are compared to the LnIII complexes. They evidence larger covalent interactions and larger J mixing.

Attempts to synthesize plutonium(iv) silicate, PuSiO4, have been made on the basis of results recently reported in the literature for CeSiO4, ThSiO4, and USiO4 under hydrothermal conditions. Although it was not possible to prepare PuSiO4via applying the conditions reported for thorium and uranium, an efficient method of PuSiO4 synthesis was established by applying the conditions optimized for the CeSiO4 system. This method was based on the slow oxidation of plutonium(iii) silicate reactants under hydrothermal conditions at 150 °C in hydrochloric acid (pH = 3-4). These results shed new light on the potential behavior of plutonium in reductive environments, highlighting the representative nature of cerium surrogates when studying plutonium under such conditions and providing some important pieces of information regarding plutonium chemistry in silicate solutions.

International audience; Even though CeSiO4 was synthesized one time through a hydrothermal treatment, the conditions leading to its formation remain largely unknown. In order to define the optimized conditions of synthesis, a multiparametric study was developed by varying the pH of the solution, the temperature, and the nature of the reactants and of the complexing ions in solution. This study highlighted that CeSiO4 could not be obtained starting from Ce(IV) reactants. An optimal set of conditions was defined to prepare single phase samples. Pure CeSiO4 was obtained through a hydrothermal treatment at 150 °C using a starting mixture of 1 mol L−1 Ce(III) nitrate and Na2SiO3 solutions and by adjusting the initial pH to 8. The chemical limitations observed during the synthesis of CeSiO4 suggested that the formation of this phase may result from the slow in situ oxidation of a Ce(III) silicate complex during the hydrothermal treatment.

The complexation of Pu(iii) and Cm(iii) with a soft N-donor ligand was investigated using the van't Hoff method, microcalorimetry and DFT calculations. The studies revealed that the strength of the actinide-ligand bond as given by the enthalpic contribution drastically decreases on going from Pu(iii) to Cm(iii), while the complex stability remains nearly constant.

Separation processes based on room temperature ionic liquids (RTIL) and electrochemical refining are promising strategies for the recovery of lanthanides from primary ores and electronic waste. However, they require the speciation of dissolved elements to be known with accuracy. In the present study, Eu coordination and Eu

The influence of the sample morphology and experimental conditions towards the sonochemical dissolution of nanoscale ThO

In order to gain insights on the kinetics of the dissolution of Gen(IV) MOX fuels, sintered pellets of Th0.25U0.75O2 were prepared as surrogates. Particularly, the impact of nitrogen species on the kinetics of dissolution of Th1-xUxO2 in nitric acid must be evaluated. In this work, various experiments were performed to determine the influence of nitrate ions and nitrous acid on the dissolution rate and mechanism of Th0.25U0.75O2 in nitric acid. It appeared that the addition of 3 mol L−1 NH4NO3 to the 1 mol L−1 HNO3 solution led to a strong increase of the dissolution rate. Thus, it seemed that the greatest impact on the dissolution kinetics during the induction period was associated to undissociated HNO3 instead of free nitrate ions. What is more, addition of a constant HNO2 concentration of 2 × 10−3 mol L−1 to 1 mol L−1 HNO3 induced a strong increase of the dissolution rate. However, the dissolution kinetics remained much faster in 4 mol L−1 HNO3, which was associated to the presence of an equivalent concentration of HNO2 in the solution. It then appeared that HNO2 species was not the prevailing species responsible for the catalysis of the dissolution kinetics of Th0.25U0.75O2. However, microstructural evolutions of the pellets dissolved in the presence of HNO2 showed the existence of small intra-granular corrosion pits covering the entire surface of the grains. That confirmed the highly oxidizing nature of HNO2 regarding to tetravalent uranium. Nevertheless, the presence of large cavities also suggested the existence of heterogeneous progress of the dissolution, associated to the existence of preferential dissolution zones consequently to the accumulation of reactive species in confined areas. Such reactive species might be HNO2, but more probably another unknown catalyst present at equilibrium with HNO2.

The industrial fluorination of UO2 to UF4 is based on a complex process involving the manipulation of a large amount of HF, a very toxic and corrosive gas. We present here a safer way to accomplish this reaction utilizing ionic liquid [Bmim][PF6] as a unique reaction medium and fluoride source.

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complex of U

Actinide research at the nanoscale is gaining fundamental interest due to environmental and industrial issues. The knowledge of the local structure and speciation of actinide nanoparticles, which possibly exhibit specific physico-chemical properties in comparison to bulk materials, would help in a better and reliable description of their behaviour and reactivity. Herein, the synthesis and relevant characterization of PuO

Although the preparation of CeSiO4 has been already reported, the formation of pure cerium silicate from aqueous precursors appears as a challenge. An innovative way of synthesis has been identified in this study, allowing the formation of CeSiO4 after hydrothermal treatment starting from Ce(iii) silicate precursors. Among the experimental parameters examined, significant effects were found according to the nature of the precursor and of the reactive media considered, the pH of the reactive media and the temperature of the hydrothermal process. This study allows the determination of optimized conditions for the hydrothermal synthesis of pure CeSiO4 (A-Ce2Si2O7 or Ce4.67(SiO4)3O as starting precursors, nitric medium, pH = 7, 7 days at 150 °C). The in situ low oxidation rate of Ce(iii) into Ce(iv) was a key parameter to consider in order to avoid the presence of CeO2 in the final mixtures.

Even though CeSiO

Multiparametric study of the hydrothermal synthesis of thorite, ThSiO

A multiparametric study of the hydrothermal synthesis of ThSiO

Although thermodynamics of AnIVDTPA− (DTPA = diethylenetriaminepentaacetic acid) complexation have been reported for 50 years, reliable data at low ionic strength is still missing. Owing to the use of capillary electrophoresis coupled with an inductively coupled plasma mass spectrometer, it is possible to simultaneously detect at ultra-trace level all AnIVDTPA− species in various conditions (concentration of ligand, pH) and to determine the formation constants. New values were obtained for tetravalent actinides with DTPA as well as with nitrilotriacetate (NTA) used as a competitor. Besides, the formation constants of hydrolyzed An(OH)DTPA2− species (An = Np, Pu) were also obtained for the first time using the variation of electrophoretic mobility as a function of pH at a constant DTPA concentration. The use of these data in radiotoxicology indicates that two stable species, namely PuDTPA− and Pu(OH)DTPA2− are present at approximately the same concentration in blood. From ab initio molecular dynamic (AIMD), the metal–oxygen distances dAn–O were calculated, and a linear relation was shown between dAn–O and the formation constants. The interpolation of data allowed determination of K values along the tetravalent actinide series, e.g. KNpDTPA−, KPaDTPA− and KU(OH)DTPA2−.

PuO2 is considered an important material for current and future nuclear fuel; however it is a very refractive compound towards dissolution. Among other techniques, its reprocessing can be performed via complexing dissolution in concentrated and boiling nitric acid containing hydrofluoric acid, or via oxidant dissolution in the presence of reagents with redox couples having high potentials such as Ce(iv)/Ce(iii), or Ag(ii)/Ag(i). Reductive dissolution can be performed under softer conditions and is considered an alternative to these methods which may suffer from several drawbacks (corrosion, effluent management, compatibility with nuclear waste disposal, etc.). In this study, a sonochemical and reductive approach is investigated for PuO2 dissolution under relatively mild conditions. At the first stage, the experiments are performed with CeO2 as an inactive surrogate for PuO2. The quantitative dissolution of both oxides can be achieved under ultrasound (20 kHz, 0.35-0.70 W mL(-1)) in 0.5 M HNO3/0.1 M [N2H5NO3]/2 M HCOOH sparged with Ar at 33-35 °C in the presence of Ti particles as a generating source of reductive species. Ultrasound enables the depassivation of the Ti surface (usually strongly passivated in nitric solutions) through acoustic cavitation which then allows further generation of the intermediate Ti(iii) reductive species. Dissolution rates and yields can be further increased with the injection of dilute fluoride aliquots (NH4F or HF) in the sonicated solution to favor Ti chemical depassivation. The rapid and complete dissolution of PuO2 under selected conditions is accompanied by Pu(iii) accumulation in solution.

This work investigates the electrochemical behavior of Ce(IV) complexes extracted in n-tributylphosphate (TBP) from nitric aqueous solutions as an entry to the direct electrochemical characterization of plutonium in organic extractants related to nuclear fuel reprocessing. Ce(IV) complexes displayed a reversible electrochemical reduction wave in TBP, which could be observed only at glassy carbon electrodes. At slow scan rates, that reduction wave showed a Nernstian dependence on the logarithm of both nitric acid and TBP concentrations in the organic phase, featuring the exchange of nitrates and TBP upon reduction to Ce(III). The presence of increasing amounts of a common degradation product of TBP, such as hydrodibutyl phosphate (HDBP) resulted in a shift of the Ce(IV) reduction wave towards more negative potential values, a behavior consistent with the formation of HDBP-Ce (IV) complexes.

Ionic liquids (ILs) are molten salts composed of an organic cation that are liquid below 100 °C. They demonstrate unique physico-chemical properties such as good conductivity, negligible vapor pressure and non-flammability. As a consequence, they are often regarded as green solvents and could become an alternative to the use of both high-temperature molten salts and volatile organic solvents in rare-earth elements processing and recycling. The diversity of interactions existing in an ionic liquid allows the solubilization of both polar and apolar compounds. Moreover, coordinating functions on their composing ions can lead to stabilization of some species. For instance, in non-aqueous media, europium exists in the (II) oxidation state which is not stable in aqueous solutions. Understanding the mechanism of Eu(III) electrochemical reduction to Eu(II) and the stabilization of Eu(II) species in IL media could be of major interest for the development of innovative recycling processes. Using transient electrochemistry and UV-Vis spectroscopy we could get valuable information on the redox behavior of the Eu(III)/(II) couple in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIm][NTf2]). Reduction of Eu(III) to Eu(II) and stability of Eu(II) was also studied thanks to a XAS-spectro-electrochemistry set up used to follow in situ the evolution of the XANES spectrum around the L3 edge of Eu during electrolysis.

A study of aqueous nitric acid solution alpha radiolysis was performed through experiments carried out at a cyclotron facility, where a helion beam with an energy of 13.5 MeV could be delivered into the solution. The effects of nitrate and hydronium ions on the formation yields of hydrogen peroxide and nitrous acid, G(H2O2) and G(HNO2), were studied. The results showed that G(H2O2) decreases linearly with increasing nitrate ion concentration. On the other hand, G(HNO2) increases with the nitrate ion concentration until it reaches a plateau for nitric acid concentrations higher than 2 mol L–1. It was also found that an increase of hydronium ion concentration has a favorable effect on G(H2O2) and G(HNO2). Furthermore, it appears that these effects are additive and that the variations of G(H2O2) and G(HNO2) can be described by two parametric expressions, as a function of the nitrate and hydronium ion concentrations.

This work investigates the electrochemical behavior of Pu(IV) and Pu(VI) complexes in n-tributylphosphate (TBP) as an entry to the electrochemical characterization of these complexes in organic extractants related to nuclear fuel reprocessing. Glassy carbon electrodes were used to show that Pu(IV) and Pu(VI) complexes display a reversible electrochemical reduction wave in TBP previously equilibrated with aqueous nitric acid solution. We investigated the reduction of Pu(IV) and Pu(VI) nitrato complexes extracted into TBP, with the aim to get thermodynamic (formal potential) and kinetic (diffusion coefficient) information about Pu(IV)/Pu(III) and Pu(VI)/Pu(V) redox couples in the TBP medium. The formal potentials of the two redox couples were respectively 0.510 ± 0.005 and 0.478 ± 0.005 V per SCE in TBP equilibrated with 3 mol L−1 nitric acid at room temperature. The diffusion coefficient values of Pu(IV) and Pu(VI) species were estimated to be 0.72 × 10−6 and 0.77 × 10−6 cm2 s−1 respectively. Also, the Pu(IV) reduction showed a Nernstian dependence on the logarithm of nitric acid concentration in the organic phase, featuring the exchange of nitrates upon reduction of Pu(IV).

Controlled-potential coulometry is an assay method for determining the quantity of a given element from the measurement of a quantity of electricity. This method has the advantage of not being related to a chemical standard, having an extremely high trueness and reproducibility, and using low sample amounts. Nowadays, equipments intended to carry out coulometry with high trueness (typically uncertainties close to 0.1%) is seldom encountered. In order to arrive to such low uncertainties, the whole process, from the preparation of the sample test to thestorage of the material had to be refined. The qualification of the method was carried out thanks to the use of EQRAIN standard solutions. The coulometric proportioning of plutonium coming from pure solutions (without interfering elements) was extended to the proportioning of plutonium in the presence of uranium, which makes the technique not only a method of choice for standard solution certification but also appropriate for carrying out precise analyses assessments of different processes used in nuclear medium.

The complex formation of protactinium(V) with DTPA was studied at different temperatures (25-50 °C) and ionic strengths (0.1-1 M) with the element at tracer scale. Irrespective of the temperature and ionic strength studied, only one neutral complex with (1:1) stoichiometry was identified from solvent extraction and capillary electrophoresis coupled to ICP-MS (CE-ICP-MS) experiments. Density Functional Theory (DFT) calculations revealed that two complexes can be considered: Pa(DTPA) and PaO(H2DTPA). The associated formation constants were determined from solvent extraction data at different ionic strengths and temperatures and then extrapolated to zero ionic strength by SIT methodology. These constants are valid, regardless of complex form, Pa(DTPA) or PaO(H2DTPA). The standard thermodynamic data determined with these extrapolated constants revealed a very stable complex formed energetically by an endothermic contribution which is counter balanced by a strong entropic contribution. Both, the positive enthalpy and entropy energy terms suggest the formation of an inner sphere complex.

In this study, a thermodynamic model based on Mikulin and Sergievskii–Dannus equations to calculate activity coefficients in both organic and aqueous phases was used to model plutonium(IV), nitric acid and water extraction by 30% TBP in dodecane. The speciation of Pu(IV) in the organic phase was determined via extraction isotherms modeling. The best agreement between experimental and calculated plutonium distribution ratios was obtained by considering two Pu(IV) complexes in the organic phase: Pu(NO3)4(TBP)2 and Pu(NO3)4(TBP)3.

The measured redox potential of an actinide at an electrode surface involves the transfer of a single electron from the electrode surface on to the actinide center. Before electron transfer takes place, the complexing ligands and molecules of solvation need to become structurally arranged such that the electron transfer is at its most favorable. Following the electron transfer, there is further rearrangement to obtain the minimum energy structure for the reduced state. As such, there are three parts to the total energy cycle required to take the complex from its ground state oxidized form to its ground state reduced form. The first part of the energy comes from the structural rearrangement and solvation energies of the actinide species before the electron transfer or charge transfer process; the second part, the energy of the electron transfer; the third part, the energy required to reorganize the ligands and molecules of solvation around the reduced species. The time resolution of electrochemical techniques such as cyclic voltammetry is inadequate to determine to what extent bond and solvation rearrangement occurs before or after electron transfer; only for a couple to be classed as reversible is it fast in terms of the experimental time. Consequently, the partitioning of the energy theoretically is of importance to obtain good experimental agreement. Here we investigate the magnitude of the instantaneous charge transfer through calculating the fast one electron reduction energies of AnO2(H2O)n(2+), where An = U, Np, and Pu, for n = 4-6, in solution without inclusion of the structural optimization energy of the reduced form. These calculations have been performed using a number of DFT functionals, including the recently developed functionals of Zhao and Truhlar. The results obtained for calculated electron affinities in the aqueous phase for the AnO2(H2O)5(2+/+) couples are within 0.04 V of accepted experimental redox potentials, nearly an order of magnitude improvement on previous calculated standard potentials E(0) values, obtained using both DFT and high level multireference approaches.