[en] The redox behavior of the Np(VI)/Np(V) couple was the subject of a spectrometric study of the Np(VI) reduction reaction in nitric acid solutions (4 to 5 M) containing variable concentrations (1.5 to 3.5 x 10^-3 M) of nitrous acid. A low nitrous acid concentration and a high nitric acid concentration were found to favor the stabilization of Np(VI). The stoichiometric coefficients of nitrous acid and nitric acid in the Np(VI) reduction reaction were determined thermodynamically, although only the reaction order with respect to HNO₂ could be calculated from a kinetic analysis. Adding nitrate ions to a HNO₃/HNO₂ solution enhanced the stability of neptunium at oxidation state +VI, but also increased the reduction rate. When uranium(VI) was added to the HNO₃/HNO₂ solutions, the total quantity of neptunium at oxidation state +V (either free or as a Np(V)-U(VI) complex) remained practically unchanged, as did the Np(VI) reduction rate. The electrochemical behavior of the Np(VI)/Np(V) couple was investigated in a weak acidic medium by voltammetry with an ultra-micro-electrode (UME). The oxidation wave limiting current variation was a linear function of the Np(V) concentration when a gold UME was used, but not with a platinum UME; the reduction wave limiting current variation versus the Np(V) concentration was linear with either gold or platinum UMEs. The presence of the Np(V)-U(VI) complex in the neptunium solutions was characterized by a shift in the normal apparent potential of the Np(VI)/Np(V) couple toward anodic potentials consistent with the previously determined values of the complexation constants. (author)

[en] 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, N-di(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 ≅ to 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 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/HNO₃(5 M) solutions were obtained from cyclic voltammograms recorded at 25 degrees C and 40 degrees 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. (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)

[en] 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-III/Eu(II)electrochemical behavior as a function of water content in 1-ethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide ([EMIm][NTf2]) was investigated using UV-visible spectrophotometry, time-resolved laser fluorescence spectroscopy, electrochemistry, and X-ray absorption spectroscopy.In situ measurements were performed in spectroelectrochemical cells. Under anhydrous conditions, Eu(III)and Eu(II)were complexed by NTf2, forming Eu-O and Eu-(N,O) bonds with the anion sulfoxide function and N atoms, respectively. This complexation resulted in a greater stability of Eu-II, and in quasi-reversible oxidation-reduction with an E-0' potential of 0.18 V versus the ferrocenium/ferrocene (Fc(+)/Fc) couple. Upon increasing water content, progressive incorporation of water in the Eu(III)coordination sphere occurred. This led to reversible oxidation-reduction reactions, but also to a decrease in stability of the +II oxidation state (E-0'=-0.45 V vs. Fc(+)/Fc in RTIL containing 1300 mm water). (authors)