Yasuhisa Ikeda

Actinide Chemistry in Ionic Liquids

This Forum Article provides an overview of the reported studies on the actinide chemistry in ionic liquids (ILs) with a particular focus on several fundamental chemical aspects: (i) complex formation, (ii) electrochemistry, and (iii) extraction behavior. The majority of investigations have been dedicated to uranium, especially for the 6+ oxidation state (UO2(2+)), because the chemistry of uranium in ordinary solvents has been well investigated and uranium is the most abundant element in the actual nuclear fuel cycles. Other actinides such as thorium, neptunium, plutonium, americium, and curiumm, although less studied, are also of importance in fully understanding the nuclear fuel engineering process and the safe geological disposal of radioactive wastes.

Kinetic study on dissolution of UO2 powders in nitric acid

Abstract UO 2 powders of 90–150, 300–355, and 850–1000 μm were dissolved in HNO 3 solutions stirred at 300 rpm. Concentrations of HNO 2 in dissolvent solutions were held constant by supplying NaNO 2 solutions of appropriate concentrations at 0.2 ml/min. The dissolution reactions were analyzed on the assumptions that the UO 2 powders are spherical particles and homogeneously dissolved from their external surface. Dissolution rate constants (φ) in mol cm −2 min −1 were measured at various concentrations of HNO 3 and HNO 2 , and temperatures. Furthermore, the effect of UO 2 2+ and NO 3 − ions on the dissolution rate was also examined. It was found that the dissolution rate of UO 2 powders is not accelerated by UO 2 2+ ions and expressed as φ = ( k a + k b [HNO 2 ])[NO 3 − ] T 2.3 ([NO 3 − ] T = total nitrate concentration). Activation energies ( ΔE ≠ ) for k a and k b were evaluated to be 79.5 ± 6.7 and 36.8 ± 2.9 kJ/mol, respectively. The φ value at 80°C, [HNO 3 ] = 8M, and [HNO 2 ] = 0.024M (M = mol/dm 3 ) is 8.5 × 10 −6 mol cm −2 min −1 .

Molecular and crystal structure of bis(N-cyclohexyl-2-pyrrolidone)dioxouranium(VI) nitrate

Bis( N -cyclohexyl-2-pyrrolidone)dioxouranium(VI) nitrate, UO 2 (NO 3 ) 2 (NCP) 2 , has been prepared by mixing UO 2 (NO 3 ) 2 ·6H 2 O and N -cyclohexyl-2-pyrrolidone (NCP) in an aqueous nitric acid solution and characterized by NMR, IR spectroscopy, and single crystal X-ray diffractometry. The IR spectrum is consistent with typical absorption bands observed in other UO 2 (NO 3 ) 2 ·2L complexes (L=unidentate oxygen donor ligand), and supports that NCP molecule coordinates to uranium through carbonyl oxygen. The 15 N NMR spectrum shows that two structural isomers of the complex exist in inert solvents such as CD 2 Cl 2 . X-ray diffraction study reveals that average distances between uranium atom and uranyl-, nitrate-, and carbonyl- oxygen atoms are 1.748, 2.527, and 2.347 A, respectively.

Electrochemical Properties of Uranyl Ion in Ionic Liquids as Media for Pyrochemical Reprocessing

We have proposed a new reprocessing process by using ionic liquids (ILs) instead of molten salts of alkali chlorides in pyrochemical process. In the proposed process, spent nuclear fuels are dissolved in ILs by using Cl2 as an oxidant, and UO2 2+ and PuO2 2+ ions in ILs are recovered as UO2 and PuO2 by electrochemical reduction. In order to examine applicability of ILs as media for reprocessing, we have studied electrochemical behavior of UO2 2+ in 1-butyl-3-methylimidazolium chloride (BMICl), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4), and 1-butyl-3-methylimidazolium nonafluorobutanesulfonate (BMINfO). Electrochemical properties of uranyl chloride dissolved into ILs were examined by cyclic voltammetry. In BMICl, an almost reversible redox couple was observed, and the formal potential and the diffusion coefficient were evaluated as _0:758V vs. Ag/AgCl and 4:8 × 10−8 cm2s−1, respectively. On the other hand, the electrochemical reactions of UO2 2+ in BMIBF4 and BMINfO were irreversible. In BMINfO, some reduction peaks and one sharp oxidation peak were observed in the range of −0:6∼–0:2V and around 0.85V vs. Ag/AgCl, respectively. The reduction and oxidation peaks were assigned to multi step reduction of UO2 2+ to U(IV) via U(V) and/or direct reduction of UO2 2+ to U(IV), and the oxidative dissolution of the resulting U(IV) compounds, respectively. The electrochemical reduction of UO2 2+ in BMINfO at −1:0V vs. Ag/AgCl produced the deposits on a carbon electrode as a cathode. Analyses of the deposits with the scanning electron microscope and the energy dispersive X-ray spectrometer indicated that the deposits are compounds containing uranium, oxygen, and chlorine. As a result, it is expected that the UO2 2+ in IL can be recovered electrolytically as uranium compounds such as UO2 and uranium oxychlorides.

Electrochemical Studies on Uranyl(VI) Chloride Complexes in Ionic Liquid, 1-Butyl-3-methylimidazolium Chloride

UV-visible absorption spectra of solutions prepared by dissolving Cs2UO2Cl4 or UO2Cl2.nH2O into 1-butyl-3-methylimidazolium chloride (BMICl) have been measured. As a result, it was clarified that uranyl species in BMICl exist as [UO2Cl4]2–. Cyclic voltammograms of [UO2Cl4]2– in BMICl were measured at 80 ± 1°C using a glassy-carbon working electrode, a platinum wire counterelectrode, and a Ag/AgCl reference electrode with a liquid junction filled with BMIBF4 in a glove box under Ar atmosphere. Peaks corresponding to one redox couple were observed around −0:73 (Epc) and −0:65V (Epa). The potential differences between two peaks (ΔEp) at scan rates in the range from 10 to 50mVs−1 are 70–80mV, and are almost consistent with the theoretical ΔEp value (67 mV) for the reversible one-electron-transfer reaction at 80°C. Furthermore, the (Epc + E pa/2 value is constant, −0:690 V, regardless of the scan rate. From these results, it is concluded that [UO2Cl4]2—in BMICl is reduced to [UO2Cl4]3—quasi-reversibly.

X-ray absorption fine structures of uranyl(V) complexes in a nonaqueous solution.

The structures of three different U(V) complexes, [U(V)O(2)(salophen)DMSO](-), [U(V)O(2)(dbm)(2)DMSO](-), and [U(V)O(2)(saldien)](-), in a dimethyl sulfoxide (DMSO) solution were determined by X-ray absorption fine structure for the first time.

Nuclear magnetic resonance study of the kinetics of the water exchange process in the equatorial positions of uranyl complexes

Abstract The kinetics of the water exchange process in the equatorial positions of the aqua uranyl ion, mono-DMSO, mono-chloro, and mono-bromo complexes, have been studied by the NMR line broadening method in water-acetone mixtures. The measurements were made in the temperature region from 25 to −100°C and it was found that the relaxation process was controlled by water exchange below −50°C. The first-order rate constants for the exchange of the coordinated water molecules measured at −70°C for the various complexes are: k aq = 2.99 × 10 2 sec −1 , k DMSO = 4.55 × 10 2 sec −1 , k Cl = 1.63 × 10 2 sec −1 , and k Br = 2.65 × 10 2 sec −1 . The values of the activation parameters are 9.9 ± 0.5 kcal mol −1 for ΔH ≠ and 2.1 ± 2.6 e.u. for ΔS ≠ for the aqua complex; considerably smaller values for ΔH ≠ and ΔS ≠ are obtained in the mono-substituted complexes.

Structural changes of uranyl moiety with reduction from U(VI) to U(V)

Summary Redox behavior of UVIO2(dbm)2DMSO (dbm = dibenzoylmethanate, DMSO = dimethyl sulfoxide, 2VI) and structural properties of UVO2+ complexes have been investigated with cyclic voltammetry and IR spectroelectrochemical technique, respectively. As a result, it was confirmed that 2VI is quasi-reversibly reduced to [UVO2(dbm)2DMSO]- (2V) without any successive reactions, and that 2V is stable in DMSO. The formal potential for the redox couple, 2V/2VI, is -1.362 V vs . the ferrocene/ferrocenium ion redox couple. The IR spectra measured at various applied potentials show a shift of the peak due to the asymmetric stretching (ν3) of O=U=O from 906 to 775 cm-1 with the reduction from 2VI to 2V. From the ν3 peak positions, U=O distances of 2V and 2VI were estimated as 1.82 and 1.76 Å by Badger´s rule.

New Approach to the Nuclear Fuel Reprocessing in Non-acidic Aqueous Solutions

A new nuclear fuel reprocessing method based on the anodic dissolution of spent fuels in aqueous alkaline solutions (Na2CO3-NaHCO3) has been proposed. Experiments of the anodic dissolution were performed by using a simulated spent fuel in a Na2CO3-NaHCO3 solution. Uranyl ions produced anodically were present in the solution as stable carbonato complexes, and at the same time, most of the simulated fission products (FP) were precipitated as hydroxo or carbonate compounds. Under this condition, Cs of an alkali metal group was dissolved in the solution and precipitated by adding sodium tetraphenylborate. Uranyl ion was recovered as hydroxo compounds by adding NaOH to the solution after removing precipitates of the simulated FP. In view of waste disposal, 99Tc having a long half-life should be removed. Precipitation behavior of Tc(VII) was examined by using Re(VII) as a simulant of Tc(VII). It was found that Re(VII) species are completely removed as a precipitate by adding tetraphenylphosphonium chloride. A large amount of Na used in the present method was recovered as NaHCO3 by blowing CO2 into alkaline solutions. As a result, it was clarified that the proposed method is fundamentally possible as a new reprocessing method.

Electrochemical Studies on [UO2(DMF)5](CIO4)2, UO2(acac)2DMF, and UO2(salen)DMF (DMF=N, N-dimethylformamide, acac=acetylacetonate, salen=N, N′-disalicylideneethylenediaminate) Complexes in DMF

Electrochemical studies on [UO2(DMF)5]2+, UO2(acac)2DMF, and UO2(salen)DMF (DMF=N, N-dimethylformamide, acac=acethylacetonate, and salen=N,N′-disalicylidene-ethylenediaminate) complexes in DMF containing tetrabutylammonium perchlorate as supporting electrolyte have been carried out by using cyclic voltammetry to compare the stabilities of the reduction products of U(Vi) complexes with unidentate or multidentate ligands. The [UO2(DMF)5]2+, UO2(acac)2DMF, and UO2(salen)DMF complexes were found to be quasi-reversibly reduced to U(V) species. The formal redox potentials (E°, vs. ferrocene/ferrocenium) for the U(Vi)/U(V) couple were determined to be -0.89 V for [UO2(DMF)5]2+, -1.47 V for UO2(acac)2DMF, and -1.67 V for UO2(salen)DMF. The U(V) species formed in the reduction of [UO2(DMF)5]2+ complex is not stable and further reduced to UO2, while those generated in the reduction of UO2(acac)2DMF and UO2(salen)DMF complexes can exist stably in DMF.