Search

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.

International audience

Under oxidizing conditions, the corrosion of spent nuclear fuel may lead to the leaching of radionuclides including soluble uranyl-based species. The speciation of the generated chemical forms is complex and the related potential formation of colloidal species appears surprisingly poorly reported in the literature. Their formation could however contribute significantly to the mobility of radionuclides in the environment. A better knowledge in the speciation and reactivity of these species appears particularly relevant. This study describes the preparation and characterization of intrinsic uranium(VI) colloids from amorphous and crystalline UO3 in pure water assisted by 20 kHz ultrasound. In the presence of carbon monoxide preventing the sonochemical formation of hydrogen peroxide, ultrasonic treatment boosts the conversion of UO3 powder into (meta-)schoepite precipitates and yields very stable and notably concentrated uranium(VI) nanoparticles in the liquid phase. Using HR-TEM, SAXS and XAS techniques, we confirmed that the colloidal suspension is composed of quasi-spherical nanoparticles measuring ca. 3.8 ± 0.3 nm and exhibiting a schoepite-like crystallographic structure. The proposed method demonstrates the possible formation of environmentally relevant U(VI) colloidal nanoparticles appearing particularly interesting for the preparation of reference systems in the absence of added ions and capping agents.

International audience; In this work we report on the electrochemical behavior of Ce(IV)/Ce(III) redox couple in pure N,N-dialkyl amides (N,N-DA), namely N,N-di(2-ethylhexyl)-n-butanamide (DEHBA), N,N-di(2-ethylhexyl)-iso-butanamide (DEHiBA), and N,Ndi(2-ethylhexyl)-3,3-dimethyl butanamide (DEHDMBA) equilibrated with nitric aqueous solutions as an entry to the direct electrochemical characterization of plutonium in these extractants. Ce(IV)/Ce(III) redox process was used as a model. Its potential (E 1/2  1.02 V/SCE) is not affected by the temperature and the nature of the N,N-DA and this clearly indicates that the functionalities of the these extractants produce the same relative effect on both +IV and +III oxidation states of the cerium cation. Linear variations of the current intensity of the reduction peak of Ce(IV) with the concentration of Ce(IV)/N,N-DAs/HNO 3 (5M) solutions were obtained from cyclic voltammograms recorded at 25°C and 40°C. Due to the poor definition of the voltammograms in DEHiBA and DEHDMBA, such characterization allows only the evaluation of the performances of the chemical extraction of Ce(IV) from aqueous nitric acid solution by the undiluted DEHBA. To our knowledge, the electrochemical behavior of Ce(IV)/Ce(III) in N,N-DAs was not previously studied and our findings will for sure open the door for further investigations in this field.

International audience; The influence of the sample morphology and experimental conditions towards the sonochemical dissolution of nanoscale ThO 2 samples in sulfuric acid media is described. Significant sonochemical dissolution rates and yields are observed at 20 kHz under Ar/O 2 atmosphere in dilute 0.5 M H 2 SO 4 at room temperature, contrasting with the generally-reported high refractory behavior for ThO 2. The dissolution of ThO 2 combines the physical effects driven by acoustic cavitation phenomenon, the complexing affinity of Th(IV) in sulfuric medium and the sonochemical generation of H 2 O 2. These sonochemical conditions further allow the observation of the partial conversion of ThO 2 into a scarce Th(IV) peroxo sulfate with 1D morphology resulting from one or both following processes: dissolution/ reprecipitation or formation of an intermediate Th(IV) surface complex.

Thorite, (ThSiO4) with a zircon type structure, is one of the most abundant natural sources of thorium on Earth. Generally, actinides are known to form nanoparticles in silicate medium, though no direct link between those colloids and the crystalline form of thorite was evidenced until now. Here we show the formation of thorite from colloids and nanocrystalline structures under experimental conditions close to environmental pH and temperature. Through in situ small and wide angle X-ray scattering (SWAXS) measurements, colloids with a few nanometers in size were first evidenced at a low reaction time. These colloids have elongated shapes and finally tend to aggregate after their size has reached 10 nm. Once aggregated, the system goes through a maturation step, ending with the emergence of nanocrystallites as thorite zircon structures. This maturation step is longer when the reaction temperature is decreased which highlights the kinetic considerations. These results have potential implications on the paragenesis of Th mineral deposits and also in the behaviour of Th and, by analogy, tetravalent actinides in the environment. The significant characteristics of this work are that Th-silicate colloids were demonstrated at low temperatures and a near neutral pH with long-term stability and a morphology in favor of high mobility in groundwater. If these species are formed in more diluted media, this could be problematic owing to the spreading of Th and, by analogy, other tetravalent actinides in the environment.

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.

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.

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.

International audience; Radiolysis of water adsorbed on PuO$_2$ surface has been the subject of several studies but still remains poorly understood. The present study brings new experimental results helping to further understand the phenomena occurring at PuO$_2$ surface. Radiolysis of adsorbed water on PuO2 surface leads to the reach of a steady state in hydrogen content when relative humidity is kept constant implying an equilibrium between generation and consumption of hydrogen. The influences of specific surface area, relative humidity, PuO$_2$ dose rate and composition of the atmosphere (Ar vs air) on hydrogen generation were investigated. A significant evolution of the hydrogen generation kinetics was observed upon PuO$_2$ surface aging. This result might be related to an evolution of the surface physicochemical state induced by a long-term storage in H$_2$O/H$_2$/O$_2$ atmosphere. The surface evolution modifies the equilibrium between generation and consumption of hydrogen. Several annealing thermal treatment were necessary to retrieve H$_2$ accumulations kinetics similar to the ones obtained with the same sample as freshly prepared. This indicates an important evolution of PuO$_2$ surface and subsurface. A mechanism is described to explain the surface evolution and its effect on H$_2$ accumulation through radiolysis of adsorbed water.

This review highlights recent data on the complexation of Pa(V) with inorganic (fluoride and sulphate) and organic (oxalate, nitrilotriacetate, diethylenetriaminepentaacetate) ligands in solution. New thermodynamic parameters relative to the complexation of Pa(V) with sulphate are presented. The review also includes gas phase and theoretical studies focused on the interaction of Pa(V) in the dioxo and oxo forms with water.

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.

International audience; The preparation and structural characterization of an original Th peroxo sulfate dihydrate, crystallizing at room temperature in the form of stable 1D polymeric microfibres is described. A combination of laboratory and synchrotron techniques allowed solution of the structure of the Th(O2)(SO4)(H2O)2 compound, which crystallizes in a new structure type in the space group Pna21 of the orthorhombic crystal system. Particularly, the peroxide ligand coordinates to the Th cations in an unusual μ3‐η2:η2:η2 bridging mode, forming an infinite 1D chain decorated with sulfato ligands exhibiting simultaneously monodentate and bidentate coordination modes.

The cointegration of optical and microfluidic functions on chemically resilient borosilicate glass to provide microfluidic chemical analyses is described. An evanescent wave sensing chip containing a fluid channel of 21 μL is implemented to carry out absorption spectroscopy measurements in the near-infrared range. Microchip packaging allows to perform remote analysis of harsh chemical solutions in confined environments. Detection of plutonium(VI) in aqueous 1 mol/L nitric acid solutions is achieved on solutions ranging from 0.05 to 0.13 mol/L. Results showcase the validity of the proposed approach by obtaining a sensor calibration curve and chip resilience to highly concentrated strong acids and radioactive elements. This proof-of-concept opens the path to optimized devices with a lower limit of detection.

Multiparametric study of the hydrothermal synthesis of thorite, ThSiO4, was performed with the aim to determine the most efficient conditions to form single phase thorite samples. Among the experimental parameters investigated, temperature of the hydrothermal process, concentration of carbonate ions, thorium and silicon reactants, and pH of the reactive media significantly affect the composition of the final system obtained. Single phase samples of ThSiO4 were prepared in weakly basic reactive media and at temperatures over 150 °C, for thorium and silicate concentrations higher than 8 × 10-3 mol L-1 and carbonate concentrations of at least 8 × 10-2 mol L-1. Although the synthesis of thorite in carbonate media was already described in the literature, this study gives new insights to explain the key role of carbonate ions in the preparation of thorite. Especially, beyond their simple role of pH buffer, carbonate ions are involved in the formation of thorium-carbonate complexes at high pH, increasing the apparent solubility of thorium in weakly basic media. The presence of carbonate ions has an important impact not only on the domain of formation of thorite but also on the morphology of the silicate phase.

The behavior of platinum and gold electrodes in the hydrophobic room-temperature ionic liquid (RTIL) N,N-Trimethylbutylammonium Bis(trifluoromethanesulfonyl)imide ([N4111][NTf2]) was investigated using transient electrochemistry e.g. voltammetry and chronoamperometry. As previously observed and contrary to what is often stated, these electrodes are not inert in the electroactivity domain of the solvent, between −3.0 and 2.7 V vs Fc/Fc+ for this RTIL. Indeed, a two-step oxidation of the platinum surface is observed at potentials higher than 1.0 V vs Fc/Fc+. The platinum oxide formed is reduced on reverse scan. Moreover, water being ubiquitous in ionic liquids, we can observe the oxidation of residual water and formation of O2. These processes both generate protons and lead to an activated state of the platinum surface. As a consequence, various signals around 0 V vs Fc/Fc+ can be attributed to the adsorption/desorption of protons and subsequent reduction to H2. In presence of water or HNTf2, this signal is enhanced. The behavior is very similar on a gold electrode but shows some specificity, particularly when it comes to the hydrogen evolution reaction.

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.

For the first time, stable intrinsic plutonium colloid was prepared by ultrasonic treatment of PuO2 suspensions in pure water at room temperature. The yield of Pu colloid was found to increase with PuO2 specific surface area and by using reducing carrier gas mixture. Kinetic data evidenced that both chemical and mechanical effects of ultrasound strongly contribute to the mechanism of Pu colloid formation. At first, the fragmentation of initial PuO2 particles accelerates the overall process providing larger surface of contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then reoxidizes yielding Pu(IV) colloid. Sonochemical colloid was compared with hydrolytic one using HRTEM, Pu LIII XAS, and STXM/NEXAFS (O K-edge) techniques. Both colloids are composed of quasi-spherical nanocrystalline particles of PuO2 (fcc, Fm¯3 m space group) measuring about 7 nm and 3 nm, respectively. Moreover, HRTEM revealed nanostructured morphology for initial PuO2. The EXAFS spectra of colloidal PuO2 nanoparticles were fitted using triple oxygen shell model for the first coordination sphere of Pu(IV). HRTEM and EXAFS studies revealed the correlation between the number of Pu-O and Pu-Pu contacts and atomic surface-to-volume ratio of studied PuO2 nanoparticles. The STXM/NEXAFS study indicated that the oxygen state of hydrolytic Pu colloid is influenced by hydrolyzed Pu(IV) species in much more extent than PuO2 nanoparticles of sonochemical colloid. In general, hydrolytic and sonochemical colloids can be described as core-shell nanoparticles composed of quasi stoichiometric PuO2 core and hydrolyzed Pu(IV) moieties at the surface shell.

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 long-term behavior of Th0.87Pu0.13O2 was examined in nitric acid concentrations. The normalized dissolution rates after 3380 days, range from (1.4 ± 0.2) × 10−6 g m−2 d−1 in 5 M HNO3 down to (3.2 ± 0.4) × 10−8 g m−2 d−1 in 10−3 M HNO3, which confirms the high chemical durability of this solid solution. The amounts of plutonium measured in solution lead to 0.9% and 2.1% of dissolved solid in 1 M and 5 M HNO3, respectively. In such conditions, the time required to reach the full dissolution of the material varies from 430 years (5 M HNO3) to 18,000 years (10−3 M HNO3). Moreover, the partial order related to the proton activity (n = 0.45 ± 0.03) suggests that the dissolution is mainly driven by surface reactions occurring at the solid/liquid interface. The characterization of the leached samples by SEM shows small microstructural modifications (i.e. detachment of crystallites) and the absence of neoformed phase while from PXRD, the unit cell parameter and crystallite size are not significantly affected.

In a previous paper, the influence of radioactive decay (α and β(-)) on magnetic susceptibility measurements by the Evans method has been demonstrated by the study of two americium isotopes. To characterize more accurately this phenomenon and particularly its influence on the Curie law, a new study has been performed on two uranium isotopes ((238)U and (233)U) and on tritiated water ((3)H2O). The results on the influence of α emissions have established a relationship between changes in the temperature dependence and the radioactivity in solution. Regarding the β(-) emissions, less influence was observed while no temperature dependence linked to this kind of radioactive emission could be identified. Once magnetic susceptibility measurements of actinide(iii) cations were corrected from radioactivity effects, methods of quantum chemistry have been used on free ions and aquo complexes to calculate the electronic structure explaining the magnetic properties of Pu(iii), Am(iii) and Cm(iii). The ligand field effect on the magnetic behavior (the Curie constant and temperature-independent susceptibilities) was analyzed by considering different solvation environments.

The influence of the ultrasonic frequency (20 kHz, 207 kHz, and 615 kHz) towards the formation kinetics of H2O2 under Ar and Ar/(20 vol.%)O2 atmospheres was evaluated in pure water and aqueous nitric solutions. Results obtained at low frequency ultrasound demonstrate that hydrogen peroxide formation is enhanced under an Ar/O2 gas mixture whatever the sonicated medium. Nevertheless, H2O2 yields are higher in aqueous nitric solutions whatever the nature of the saturating gas. These observations are consistent at high frequency ultrasound under Ar gas notwithstanding higher yields for H2O2. Surprisingly, an inverse tendency is observed for high frequency sonolysis carried out under an Ar/O2 atmosphere: higher yields of H2O2 are measured in pure water. Further studies in the presence of pure Ar revealed a more important decomposition of nitric acid under high frequency ultrasound leading to higher yields of both HNO2 in the liquid phase and NO in the gas phase. In the presence of Ar/O2 mixture, the intrabubble oxidation of NO causes cavitation bubble depletion in O2 leading to the drop of H2O2 yield. On the other hand, it was found that for Ar/(20 vol.%)O2 mixture there is no influence of oxygen on HNO2 yield whatever the ultrasonic frequency; this is most likely explained by two processes: (i) HNO2 formation results from nitrate-ion thermolysis in the liquid reaction zone surrounding the cavitation bubble, and (ii) effective intrabubble oxidation of NOx species by oxygen to nitrate-ion.

Complexation of trivalent actinides with DTPA (diethylenetriamine pentaacetic acid) was studied as a function of pcH and temperature in (Na,H)Cl medium of 0.1 M ionic strength. Formation constants of both complexes AnHDTPA(-) and AnDTPA(2-) (where An stands for Am, Cm, and Cf) were determined by TRLFS, CE-ICP-MS, spectrophotometry, and solvent extraction. The values of formation constants obtained from the different techniques are coherent and consistent with reinterpreted literature data, showing a higher stability of Cf complexes than Am and Cm complexes. The effect of temperature indicates that formation constants of protonated and nonprotonated complexes are exothermic with a high positive entropic contribution. DFT calculations were also performed on the An/DTPA system. Geometry optimizations were conducted on AnDTPA(2-) and AnHDTPA(-) considering all possible protonation sites. For both complexes, one and two water molecules in the first coordination sphere of curium were also considered. DFT calculations indicate that the lowest energy structures correspond to protonation on oxygen that is not involved in An-DTPA bonds and that the structures with two water molecules are not stable.

The data published on values of redox potentials E of heptavalent Np, Pu, Am, Ru and Mn and octavalent Ru and Pu were analyzed. The half-life (τ 1/2) for reduction reactions of heptavalent metal by water in aqueous alkaline solutions was also analyzed. The decrease in the half life stability of actinide in water with the redox potential for 3− charged anions of Np(VII), Pu(VII), and Am(VII) was found. At the same potential, a charge decrease from 3− to 1−reduces the stability approximately two fold. For the redox couple Pu(VIII)/Pu(VII), the estimated value is E > 1.15 V/NHE. If Pu(VIII) existed as PuO4, it would be very unstable (τ 1/2 ≈ × 102 s) in 1 M NaOH. For the anionic forms of Pu(VIII), PuO4(OH)2 2− and PuO6 4−, the half life was estimated to be very low (τ 1/2 < 1 s).

Single crystals of [Th(TMP)4(DMP)2]2[ClO4]4 and [UO2(TMP)(DMP)(NO3)]2 have been synthesized and their structures have been determined by X-ray diffraction analysis. The complexes of Th(IV) and U(VI) contain simultaneously both the molecular ligand trimethyl phosphate (TMP) and the anion dimethyl phosphate. The main structural motif in Th(IV) complex is centrosymmetric dimeric cation [Th(TMP)4(DMP)2] 2 4+ , and in U(VI) it is the neutral centrosymmetric dimeric complex [UO2(TMP)(DMP)(NO3)]2. Coordination polyhedron of Th(IV) is tetragonal antiprism, for U(VI) it is pentagonal bipyramid. The structures are compared with those of other Th(IV) and U(VI) complexes containing other trialkyl phosphates.

A new mixed valent neptunium compound [BuMeIm](5)[Np(NpO(2))(3)(H(2)O)(6)Cl(12)] was synthesized by evaporation of an ethyl acetate-acetone mixture where the salt [BuMeIm](2)[NpCl(6)] was dissolved. The crystal structure was determined using single crystal X-ray diffraction; it consists of four crystallographically unique Np centers with two different coordination environments in two different oxidation states. In addition, five crystallographically independent dialkylimidazolium cations stabilize the crystal through C-H...Cl hydrogen bonds. [Np(NpO(2))(3)(H(2)O)(6)Cl(12)] unit presents a highly symmetric structure with the formation of three Np(IV)-Np(V)O(2)(+) bonds. According to the crystal structure and density functional theory (DFT) calculations, the stability of the three Np(4+)-NpO(2)(+) interactions is due to the presence of chloride ions around NpO(2)(+) which brings electronic charge into the system and also to the presence of water molecules which create hydrogen bond with chloride ions.

In the framework of nuclear waste disposal, it is very important to well understand the behavior of actinides in the presence of the common environmental inorganic ligands such as sulfate and chloride. In this work, the AnO 2SO4 - and AnO2Cl 1-1 complexes have been evi- denced by capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICPMS) in perchlor- ate/chloride and in perchlorate/sulfate media for An ) Np and Pu. Their binding constants have been measured: logPuO2SO4 - 0 ) 1.30 ( 0.11, log � PuO2Cl 1 M NaCl ) -(0.40 ( 0.07), logNpO2SO4 - 0 ) 1.34 ( 0.12, and log � NpO2Cl 1 M NaCl )- (0.40 ( 0.07). These results are consistent with published values for Np(V). They confirm the expected analogy between Np(V) and Pu(V) for the weak bonding with chloride ligand, log10 � PuO2Cl ≈ log10 � NpO2Cl, attributed to mainly electrostatic interactions. Conversely, a slight shift is observed for the bonding with sulfate ligand, log10 � NpO2SO4 - > log10 � PuO2SO4 -, indicating that some covalency might stabilize the sulfate complexes. Actinides are usually chemical analogues when in the same oxidation state, i.e., forming cations of the same charge and similar sizes, which form complexes of the same stoichiometry and similar stabilities. This analogy is often used to predict the chemistry of actinides in oxidation states when they are difficult to study. It is typically the case for the actinides at the +5 oxidation state among which protactinium, neptunium, and plutonium can be stabilized in the environment. Indeed, the soluble Pu(V) can be quite stable at trace concentrations in environmental surface waters, and its aqueous chemistry is essentially known by analogy with Np(V), which is the most stable An(V).1 Conversely, Pa(V) is not a chemical analogue of the other An(V) ions since it has been recently shown that its aqua ion is not of the same form. 2-4 Therefore, it seems that such analogy rules need experimental verifications. Only few thermodynamical data are published for plutonium and protactinium in the +5 oxidation state (Pu(V) and Pa(V)), while more data are available in the literature for Np(V).1 Np(V) is stable as the linear NpO2 + neptunyl ion with solvent or ligand molecules in its equatorial plan. The Pu(V) aqua ion has a similar structure and a size close to that of Np(V), and the available complexing constants are similar for Np(V) and Pu(V). 1 Furthermore, Pu(V) is the second more stable An(V) after Np(V); therefore, their behavior versus complexation is compared here. The complexation can be driven by two components: electro- static interactions including possible outer sphere interactions and covalent bonding which exists only in inner sphere complexes. The electrostatic interactions between Np(V) and anionic ligands might be slightly weaker than those for Pu(V), since its density of charge is smaller. Conversely, Np covalent binding with anions might be stronger than for Pu since the 5f electrons of Np (electrons which participate in the bond) are less localized. The only binding constants selected by the NEA review1 for both Np(V) and Pu(V) are for carbonate complexes: log10 � AnO2CO3 - 0 ) 5.12 ± 0.06 and 4.96 ± 0.06 for An ) Pu and Np, respectively. The Pu complex is slightly more stable, but the difference is only a little more than the uncertainties. We recently measured similar values (log10 � AnO2CO3

Uranium monocarbide (UC) or ternary alloys are considered to be possible candidates for future nuclear fuels. Although the crystallographic and electronic structure of UC has been addressed in past investigations, discrepancies in the literature data have fostered a new investigation of the UC phase. We report here a reinvestigation of the UC phase by complementary X-ray spectroscopy and quantum chemical calculations. A combination of X-ray powder diffraction and extended X-ray absorption fine structure analysis at the uranium LIII edge led to the crystallographic determination of the UC phase of the NaCl type. For electronic structure investigation, a combination of uranium X-ray absorption near-edge spectroscopy at the LIII edge and at the NIV,V edges with quantum chemical calculations allowed us to define the evolution of the metal charge in comparison with metallic uranium on the one hand and uranium dioxide on the other hand.

The stability of the ionic liquid [MeBu3N][Tf2N], dry or after contact with water (where [MeBu3N]+ is the methyltributylammonium cation and [Tf2N](-) is the bistriflimide anion), was studied under 137Cs gamma irradiation in argon and in air. In a quantitative study with an absorbed dose of 2 MGy this ionic liquid was highly stable regardless of the radiolysis conditions. The radiolytic disappearance yields determined by ESI-MS were -0.38 and -0.25 micromol J(-1) for the cation and anion, respectively. ESI-MS, NMR, and liquid chromatography coupled with ESI-MS identified a large number of degradation products in very small quantities for the same dose. The cation radicals were formed by the loss of a Bu group, the Me group, or two H atoms to form a double bond with the butyl chain. Radiolysis of the anion produced mainly F and CF3 radicals. The anion radicals recombined with the cation to form a wide range of secondary degradation products regardless of the radiolysis conditions.

Visible/NIR spectrophotometry, solid-state NMR and spectroscopy reveal the hydrolysis of the PF6− anion in the room-temperature ionic liquid [BuMeIm]PF6, where BuMeIm+ is 1-butyl-3-methylimidazolium, in the presence of water and AnCl62−, where An(IV) is Th(IV), Np(IV), and Pu(IV). Slow absorption spectra evolution of AnCl62− solutions is finished by solid products' precipitation. Solids are attributed to the mixture of An(IV) phosphates and fluorophosphates.

In the framework of the French Environmental Nuclear Toxicology program, additional experiments related to the decorporation of actinides are planned. Decorporation is the removal or release from target organs (bones, liver, kidney …), tissues or cells of radioactive material previously incorporated in them, using chelating agents or other administrated pharmaceutical agents. The contradictory data on the neptunium complexation behaviour within blood and its transfer to target organs, as well as the inefficiency of therapeutic treatments, led us to study the complexation of this element with biological constituents. Within this purpose, the in vitro behaviour of Np(IV) and Np(V) in simple media simulating biological fluids was studied. This study was more specifically focused on the behaviour of neptunium with citrate ion, which is an essential component in a number of metalloenzyme active sites. In order to determine the speciation of this system, spectrophotometry was more particularly used. Concerning the complexation phenomenon, the existence of several complexes of Np(V) with various acido-basic forms of the citrate anion was observed; regarding Np(IV), complexes with Cit3- have been observed. From the quantitative study of these equilibria, the values of the absolute constants for the complexation of Np(IV) and Np(V) with citrate were determined. Concerning the stability of neptunium towards oxydo-reduction, it was confirmed that Np(VI) was very quickly reduced to Np(V) by the citrate anions, whereas Np(IV) was stable. In the case of Np(V), it was observed that, depending on the pH and the citrate concentration, Np(V) was unstable and was reduced to Np(IV). The E–pH diagrams, constructed using the stability constants determined in this study, showed that this instability was due to the Np(V) disproportionation.

1:1 and 1:2 complexes of americium(III) with a hydrophilic anionic SO3–Ph–BTP4− ligand were detected in acidic aqueous nitrate solutions by a solvent extraction method. The determined conditional stability constants of these complexes, logβ 1 = 4.35 ± 0.07 and logβ 2 = 7.67 ± 0.06, related to 1 M aqueous solutions, are much lower than the literature values for the analogous curium species, determined by TRLFS in very dilute aqueous solutions. There is also no evidence for the existence of the 1:3 Am3+ complex similar to the reported curium(III) complex. A hypothesis has been formulated to explain these discrepancies. It suggests the necessity to carefully check the equilibria in each phase of solvent extraction systems containing two competing ligands—lipophilic and hydrophilic.

The fate of radionuclides in the environment is a cause of great concern for modern society, seen especially in 2011 after the Fukushima accident. Among the environmental compartments, seawater covers most of the earth's surface and may be directly or indirectly impacted. The interaction between radionuclides and the marine compartment is therefore essential for better understanding the transfer mechanisms from the hydrosphere to the biosphere. This information allows for the evaluation of the impact on humans via our interaction with the biotope that has been largely undocumented up to now. In this report, we attempt to make a link between the speciation of heavy elements in natural seawater and their uptake by a model marine organism. More specifically, because the interaction of actinides with marine invertebrates has been poorly studied, the accumulation in a representative member of the Mediterranean coralligenous habitat, the sponge Aplysina cavernicola, was investigated and its uptake curve exposed to a radiotracer (241)Am was estimated using a high-purity Ge gamma spectrometer. But in order to go beyond the phenomenological accumulation rate, the speciation of americium(III) in seawater must be assessed. The speciation of (241)Am (and natural europium as its chemically stable surrogate) in seawater was determined using a combination of different techniques: Time-Resolved Laser-Induced Fluorescence (TRLIF), Extended X-ray Absorption Fine Structure (EXAFS) at the LIII edge, Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR) spectroscopy and Scanning Electron Microscopy (SEM) and the resulting data were compared with the speciation modeling. In seawater, the americium(III) complex (as well as the corresponding europium complex, although with conformational differences) was identified as a ternary sodium biscarbonato complex, whose formula can be tentatively written as NaAm(CO3)2·nH2O. It is therefore this chemical form of americium that is accumulated by the sponge A. cavernicola.

This work is aimed at a predictive description of the thermodynamic properties of actinide(III) salt solutions at high concentration and 25 degrees C. A new solution of the binding mean spherical approximation (BIMSA) theory, based on the Wertheim formalism, for taking into account 1:1 and also 1:2 complex formation, is used to reproduce, from a simple procedure, experimental osmotic coefficient variation with concentration for three binary salt solutions of the same lanthanide(III) cation: dysprosium(III) perchlorate, nitrate, and chloride. The relevance of the fitted parameters is discussed, and their values are compared with available literature values. UV-vis/near-IR, time-resolved laser-induced fluorescence spectroscopy experiments, and molecular dynamics (MD) calculations were conducted for dilute to concentrated solutions (ca. 3 mol.kg-1) for a study of the microscopic behavior of DyCl3 binary solutions. Coupling MD calculations and extended X-ray absorption fine structure led to the determination of reliable distances. The MD results were used for a discussion of the parameters used in the BIMSA.

Although of major impact in terms of biological and environmental hazards, interactions of actinide cations with biological molecules are only partially understood. Human serum transferrin (Tf) is one of the major iron carriers in charge of iron regulation in the cell cycle and consequently contamination by actinide cations is a critical issue of nuclear toxicology. Combined X-ray absorption spectroscopy (XAS) and near infrared absorption spectrometry were used to characterize a new complex between Tf and Np (IV) with the synergistic nitrilotriacetic acid (NTA) anion. Description of the neptunium polyhedron within the iron coordination site is given.

Ultrasonic irradiation with the frequency of 20–22 kHz and absorbed acoustic power about 0.4 W ml–1 causes degradation of An(IV) tetrakis-β-diketonates, AnL4, where An(IV) is Th(IV), Np(IV), and Pu(IV), and HL is hexafluoroacetylacetone (HFAA) and dibenzoylmethane (HDBM), in hexadecane solutions in the presence of argon. The rate of the sonochemical process corresponds to a first-order kinetic law with respect to metal β-diketonate concentrations. The first-order rate constant of sonolysis increases with the increase in the volatility of the metal complexes. Solid sonication products consisted of a mixture of actinide carbides and partial degradation products, PDP, of initial metal β-diketonates. It is assumed that metal carbides are formed within the cavitating bubbles as a result of high-temperature process with participation of actinide(IV) β-diketonates and solvent vapours. PDP formation is attributed to the thermolysis of the complexes in a liquid reaction zone surrounding the cavitating bubble. To cite this article: S.I. Nikitenko et al., C. R. Chimie 7 (2004).

Seawater contains radionuclides at environmental levels; some are naturally present and others come from anthropogenic nuclear activity. In this report, the molecular speciation in seawater of uranium(VI) and neptunium(V) at a concentration of 5 × 10(-5) M has been investigated for the first time using a combination of two spectroscopic techniques: Time-resolved laser-induced fluorescence (TRLIF) for U and extended X-ray absorption fine structure (EXAFS) for U and Np at the LIII edge. In parallel, the theoretical speciation of uranium and neptunium in seawater at the same concentration is also discussed and compared to spectroscopic data. The uranium complex was identified as the neutral carbonato calcic complex UO2(CO3)3Ca2, which has been previously described in other natural systems. In the case of neptunium, the complex identified is mainly a carbonato complex whose exact stoichiometry is more difficult to assess. The knowledge of the actinide molecular speciation and reactivity in seawater is of fundamental interest in the particular case of uranium recovery and more generally regarding the actinide life cycle within the biosphere in the case of accidental release. This is the first report of actinide direct speciation in seawater medium that can complement inventory data.

The dissolution of Th1−xUxO2 solid solutions was studied by varying independently several parameters such as chemical composition, leachate acidity, leaching temperature, firing temperature, and densification state. The chemical durability of the samples was significantly affected by the uranium mole loading probably due to the oxidation of tetravalent uranium into uranyl during the dissolution process. The influence of nitric acid concentration and temperature also showed that the behavior of Th1−xUxO2 solid solutions with higher uranium incorporation fractions (x = 0.52, 0.75 and 1) differed significantly from that of Th0.84U0.16O2. Strong modifications of the dissolution mechanism occurring at the solid/liquid interface for the higher uranium loadings could explain this difference. It could result in the existence of the fast oxidation of U(IV) to U(VI) at the solid/solution interface then of the detachment of activated complexes formed with U(VI). Furthermore, initial normalized dissolution rate slightly depended on the elimination of crystal defects for firing temperatures below 900 °C but was almost independent of the crystallite size (T ⩾ 900 °C). Finally, dissolution tests on sintered Th0.84U0.16O2 samples showed that RL(Th) values decreased by an order of magnitude when the relative density increased from 79% to 89%.

The concept of simple solutions is applied to a quaternary system of U(NO3)4, HNO3, N2H5NO3 and H2O in order to determine fictive binary data for U(NO3)4. Hydrazinium nitrate solutions were used at low concentrations relative to the nitric acid and uranium(IV) nitrate concentrations, allowing this system to be considered as a ternary (U(NO3)4, HNO3, H2O) system. The simple behavior of the system allowed fictive binary data (osmotic coefficient and density) to be calculated from water activity and density measurements. The results were then used to calculate the coefficients related to Pitzer´s model and to specific interaction theory, used to model the deviation of the concentrated solution from ideality. The results were also used to calculate the activity coefficient of uranium(IV) nitrate in the presence of uranyl(VI) nitrate and nitric acid. The binary data obtained for uranium(IV) nitrate were found to differ significantly from the data for plutonium(IV) nitrate.

A method is presented for calculating the density of concentrated ternary and quaternary mixtures of nitric acid, thorium(IV) nitrate or plutonium(IV) nitrate, and uranyl nitrate. The method is based on the concept of simple solutions and on the known binary data for each of the electrolytes. The deviation between the calculated and measured density values for ternary An(NO3)4/HNO3/H2O mixtures was less than 0.3% for densities of up to 1.797 g × cm-3 in the case of Pu(NO3)4. Density predictions for quaternary mixtures containing uranyl nitrate as the additional electrolyte were also investigated; the results show that the density of such mixtures can be predicted with less than 0.4% error, even at very high concentrations (1.836 g × cm-3) in the case of Pu(NO3)4.

A method is described for determining binary data for electrolytes from a study of a ternary mixture. The method is based on the concept of thermodynamically simple solutions and on water activity measurements. An application is described in which binary data for plutonium(IV) nitrate are determined from a ternary mixture of Pu(NO3)4, HNO3 and H2O. This novel application allows the thermodynamic behavior (activity coefficient) to be defined for a fictive solution of plutonium(IV) nitrate in water; the solution is ''fictive'' in that it does not correspond to physical reality, given that Pu(IV) is insoluble. The results can be used to calculate the activity coefficient of the electrolyte Pu(NO3)4 in any type of mixture containing nitric acid and other electrolytes for which the binary data are known.

The redox properties of uranium(IV) hexachloro complex have been investigated with a glassy carbon electrode in four bis(trifluoromethylsulfonyl)imide (Tf2N(-)) based ionic liquids (ILs): the 1-butyl-3-methylimidazolium ([BuMeIm](+)), the 1-butyl-2,3-dimethylimidazolium ([BuMe2Im](+)), the N-butylmethylpyrrolidinium ([BuMePyr](+)), and the tributyl-methylammonium ([MeBu3N](+)). The cyclic voltammetric analysis has revealed two main redox systems: U(V)Cl6(-)/U(IV)Cl6(2-) around 0.2 V and U(IV)Cl6(2-)/U(III)Cl6(3-) around -2 V. The formation of U(V)Cl6(-), a non-dioxo uranium(V) species, can then be observed at the electrode in [Tf2N](-) based ILs. This work also provides evidence of a specific interaction between the uranium anionic species and the IL cations because the standard potentials of both redox couples depend on the IL. The interaction extent has been evaluated by comparison of the IL cation number associated with the uranium anionic complex. For that purpose, the standard potentials of both systems have been measured in the less interacting medium [MeBu3N][Tf2N] as a function of the [BuMeIm](+), [BuMe2Im](+), and [BuMePyr](+) concentration. Predominance diagrams for uranium hexachloro complexes, analogous to the Pourbaix diagram, have then been built in [MeBu3N][Tf2N] depending on the IL cations concentration. The exploitation of these diagrams leads to the conclusion that the interaction is function of the charge of the uranium hexachloro complex (U(V)Cl6(-)U(IV)Cl6(2-)U(III)Cl6(3-)) and the IL cation ([BuMe2Im](+)[BuMePyr](+)[BuMeIm](+)). The influence of the IL cation could be correlated to the size and the electropositivity of the IL cation. The association would occur by H-bonding and electrostatic interaction. Ab initio calculations were also carried out to evaluate the strength of the interaction between the anionic uranium(IV) chloro complex and the IL cations. The results show that [BuMeIm](+) interacts the most and [MeBu3N](+) the least with U(IV)Cl6(2-), and the magnitude of the interaction is comparable for [BuMe2Im](+) and [BuMePyr](+).

International audience; In the spent nuclear fuel reprocessing, solutions of actinides and lanthanides are concentrated and cannot be assumed as ideal ones. Activity coefficients have then to be calculated to take into account the deviation from ideality. The aim of this study is to determine new neodymium individual contributions for the calculation of activity coefficients using the Bromley method. A comparison with experimental data demonstrates that, with the contribution values of this work, the Bromley method can be successfully applied in ternary mixtures for the calculation of activity coefficients of neodymium compounds in aqueous solutions for ionic strengths up to 6 mol kg(-1) as claimed by Bromley. The simple method described in this paper can be used to correct or to determine the contributions of any other ion.

Ultrasounds excite uranium: Uranyl ions exhibit an ultrabright multibubble sonoluminescence in cooled H3PO4 solutions exposed to high-frequency ultrasound. The mechanism of UO22+ excitation seems to vary with uranium concentration: sonophotoluminescence dominates in diluted solutions, and collisional excitation would contribute in concentrated UO22+ solutions