To evaluate the feasibility of developing an efficient electrochemical separation process for rare earth elements (REE) at room temperature, the electrochemical behavior of some representative REE ...was evaluated in butyl methylpyrrolidinium dicyanamide (BMPyr-DCA), a room temperature ionic liquid. Because of their chemical properties, their commercial availability and their technological applications, yttrium, lanthanum, neodymium and samarium were selected for this study. The differences in the electrochemical properties of these elements as trivalent salts in BMDCA solutions were evaluated by cyclic voltammetry using a Pt electrode under an Ar atmosphere. For the elements with chemical properties that predict only the reduction in one stage (Y, La and Nd), the results exhibited the simultaneous reduction of BMPyr-DCA and the REE cation and the corresponding oxidation of the reduced REE at anodic potentials that were dependent on the REE cation. For Sm, a two-step reduction process from Sm(III) to Sm(II) and from Sm(II) to Sm(0) was observed, and a clear cathodic peak was observed far from the cathodic electrochemical window limit (-1.38 and -2.25 V vs Fc/Fc+); only one anodic peak was observed at -0.93 V, a more negative potential than for the other studied REE. The results show the same behavior as observed in other media (aqueous and molecular media and molten salts) and allow for the evaluation of the possibility of selectively identifying the studied REE in the context of their differing electrochemical responses.
The behavior of U(IV) octahedral complexes cation2UCl6, where the cation+ is BuMeIm+ and MeBu3N+, is studied using UV/visible spectroscopy, cyclic staircase voltammetry, and rotating disk electrode ...voltammetry in hydrophobic room-temperature ionic liquids (RTILs) BuMeImTf2N and MeBu3NTf2N, where BuMeIm+ and MeBu3N+ are 1-butyl-3-methylimidazolium and tri-n-butylmethylammonium cations, respectively, and Tf2N- is the bis(trifluoromethylsulfonyl)imide anion. The absorption spectra of cation2UCl6 complexes in the RTIL solutions are similar to the diffuse solid-state reflectance spectra of the corresponding solid species, indicating that the octahedral complex UCl6 2- is the predominant chemical form of U(IV) in Tf2N--based hydrophobic ionic liquids. Hexachloro complexes of U(IV) are stable to hydrolysis in the studied RTILs. Voltammograms of UCl6 2- at the glassy carbon electrode in both RTILs and at the potential range of −2.5 to +1.0 V versus Ag/Ag(I) reveal the following electrochemical couples: UCl6 -/UCl6 2- (quasi-reversible system), UCl6 2-/UCl6 3- (quasi-reversible system), and UCl6 2-/UCl6(Tf2N) x -(3+ x ) (irreversible reduction). The voltammetric half-wave potential, E p/2, of the U(V)/U(IV) couple in BuMeImTf2N is positively shifted by 80 mV compared with that in MeBu3NTf2N. The positive shift in the E p/2 value for the quasi-reversible U(IV)/U(III) couple is much greater (250 mV) in BuMeImTf2N. Presumably, the potential shift is due to the specific interaction of BuMeIm+ with the uranium−hexachloro complex in ionic liquid. Scanning the negative potential to −3.5 V in MeBu3NTf2N solutions of UCl6 2- reveals the presence of an irreversible cathodic process at the peak potential equal to −3.12 V (at 100 mV/s and 60 °C), which could be attributed to the reduction of U(III) to U(0).
The tetrachlorouranium(VI) complex is formed in BmimTf2N and MeBu3NTf2N from a uranium(VI) solution in the presence of a stoichiometric quantity of chloride ions. The UVIO2Cl42- absorption and ...emission spectra show bands splitting in comparison with the UVIO22+ spectra, as observed in the solid state, organic solvents, and chloroaluminate-based ionic liquids. The fluorescence lifetime of UO2Cl42- in MeBu3NTf2N is 0.7 ± 0.1 μs. The reduction potential of this complex is −1.44 and −1.8 V vs Ag/Ag+ respectively in BmimTf2N and MeBu3NTf2N and does not depend on the chloride concentration. The mechanism proposed for the redox process is a monoelectronic reduction to form UVO2Cl43-, followed by a chemical reaction. The tetrachlorouranium(V) complex seems more stable in BmimTf2N than in MeBu3NTf2N. The electrochemical analysis put in evidence specific interactions of the ionic liquid cation with the uranium anionic species.
Laboratory scale 20
kHz sonochemical reactors with different geometries have been tested using thermal probes, the kinetics of H
2O
2 formation, and the kinetics of diphenylmethane (DPhM) ...sonochemical darkening. Results revealed that the overall sonochemical reaction rates in H
2O and DPhM are driven by the total absorbed acoustic energy and roughly independent the geometry of the studied reactors. However, the sonochemical efficiency, defined as
η
=
VG/
S, where
G is a sonochemical yield of H
2O
2,
V is a volume of sonicated liquid, and
S is a surface of the sonotrode, was proved to increase with the decrease of
S. This phenomenon was explained by growing of the maximum cavitating bubble size with ultrasonic intensity and its independence towards the specific absorbed acoustic power. For the cleaning bath reactor the kinetics of the sonochemical reactions in H
2O and DPhM depends strongly on the reaction vessel materials: the reaction rates decreased with the increase of the materials elasticity. Kinetic study of H
2SO
4 sonolysis using a sonoreactor without direct contact with titanium sonotrode showed that sulphate anion is an effective scavenger of OH
radicals formed during water sonolysis.
Protactinium occupies a key position in the actinide series between thorium and uranium. In aqueous acidic solution, it is stable at oxidation state (V), occurring either as an oxocation or as a ...naked ion, depending on the media. Very few structural information on the hydration sphere of Pa(V) in acidic medium is available, in particular in hydrofluoric acid. Combined EXAFS and theoretical calculations have been used in this work to characterize the protactinium coordination sphere at various HF concentrations. The correlation of the XAFS data with quantum chemical calculations provides complementary structural and electronic models from
techniques. At HF concentrations from 0.5 to 0.05 M, both theoretical calculations and EXAFS data suggest that the protactinium coordination sphere is mainly composed of fluoride ions. At the lowest HF concentration, the occurrence of a monooxo bond is observed with EXAFS, in agreement with the literature. A comparison of these data with related neptunium(V) and plutonium(V) diooxocations in perchloric acid is also presented.
The present work describes the first structural studies of protactinium(V) in sulfuric and hydrofluoric acid media using X-ray absorption spectroscopy. The results show unambiguously the absence of ...the trans-dioxo bond that characterizes the other early actinide elements such as U and Np. In concentrated sulfuric acid (13 M), Pa(V) is proved to exhibit a single oxo bond as postulated in the literature for species in more dilute media. In a 0.5 M HF medium, XANES and EXAFS spectra indicate the absence of any oxo bond: Pa(V) exists in the form of a pure fluoro complex.
The yields of the radiolytic oxidation of U(IV) and of the U(VI) formation, measured by spectrophotometry, are found to be the same (G(-U(IV))(N2O) = G(U(VI))(N2O) = 8.4 x 10(-7) mol J(-1)) and ...almost double the H(2) formation yield (G(H(2)) = 4.4 x 10(-7) mol J(-1)) in the (60)Co gamma radiolysis of N(2)O-aqueous solutions in the presence of 2 mol L(-1) Cl(-) at pH = 0 (HCl). According to the mechanism of U(IV) radiolytic oxidation, we show that under the conditions of our experiments the U(V) ions do not disproportionate, but undergo a stoichiometric oxidation into U(VI) by H(+) with forming H(2).
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