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A spectroelectrochemical cell was fabricated for in-situ X-ray absorption spectroscopy (in-situ XAS). The XAS spectra of the uranium L III edge were monitored in electrolyte solutions during the electrochemical reduction. Tetravalent uranium, U4+, in 1 mol dm –3 (M) hydrochloric acid (HCl) was electrochemically prepared from hexavalent uranium, UO2 2+, by constant current electrolysis, and the extended X-ray absorption fine structure (EXAFS) was analyzed. The concentration ratio of UO2 2+ and U4+, which were formed via the disproportionation of pentavalent uranium, UO2 +, during the electrolysis, were calculated based on the intensity of the signal for the two axial oxygen atoms in the linear UO2 2+ unit, the U–O ax , bond that had a radial structural function. The apparent redox potential of the UO2 2+/U4+ couple in 1 M HCl was determined based on the Nernst equation using the concentrations of UO2 2+ and U4+. The electrode potential was shown to be close to the formal potential of the UO2 2+/UO2 + couple as reported previously. This result indicates that the UO2 + that was formed electrochemically at the electrode disproportionated to form UO2 2+ and U4+ in the bulk solution. The in-situ XAS of UO2 2+ in 0.1 M nitric acid was also performed. The U4+ that formed was partially re-oxidized to UO2 2+ by the NO3 – present in the solution. The formation of the UO2 + carbonato complex was observed by in-situ XAS in a 1 M sodium carbonate solution during the bulk electrolysis. The edge jump of the X-ray absorption near edge structure (XANES) spectrum shifted from 17.164 to 17.163 keV, and the bond distances of U–O ax and U–O for CO3 2– increased from 1.78 to 1.88 Å and from 2.42 to 2.53 Å, respectively, because of the reduction of the UO2 2+ to the UO2 + carbonato complex.

In order to enhance the understanding of redox reactions of plutonium ions in molten NaCl-CsCl eutectic, absorption spectrophotometry was performed for Pu3+, Pu4+, and PuO2 2+ in molten NaCl-CsCl at 923 K by controlling the flow-rate ratio of Cl2 and Ar or O2 in the ventilating mixture gas. Based upon the relations of the measured rest potentials to the concentration ratios of [Pu4+]/[Pu3+] and [PuO2 2+]/[Pu4+], the redox potentials of Pu4+/Pu3+ and PuO2 2+/Pu4+ couples in molten NaCl-CsCl eutectic at 923 K were estimated.

The hydrolysis constants of Th(IV) were determined using the solvent extraction method in a NaClO4solution atI=0.1 and 25 °C. A trace amount of thorium below the solubility limit was used in order to avoid the formation of polynuclear species and colloids in the experiment. The values were compared with the literature data. Considering the effect of competitive hydrolysis reactions under weak acid conditions, the complexation constants of Th(IV) with a series of dicarboxylic acids containing different alkyl chain lengths were determined.

SummaryThe solubility of uranium(IV) hydrous oxide was measured at 25 °C in NaClO4solution containing Na2S2O4as a reducing agent in the pH range from 12 to 14. The experiment was carried out from oversaturation atI= 0.5, 1.0 and 2.0 mol/dm3(M). The solubility data were analyzed to obtain the hydrolysis constants (βm) of U(OH)m(4-m). By taking the specific ion interaction theory (SIT) for ionic strength corrections and by using the solubility product logKsp° = −53.93 ± 0.20, the hydrolysis constants atI= 0, log β5°6° = 48.95 ± 1.01, were determined together with the ion interaction coefficients of U(OH)5−and U(OH)62−.

SummaryThe hydrolysis constants of tetravalent uranium were determined by a solvent extraction method using thenoyltrifluoroacetone(TTA) and233U. The distribution ratio of U(IV) was measured as a function of the pHcvalue in the aqueous phase atI=0.1, 0.5 and 1.0, and was analyzed to obtain the hydrolysis constants (βm) of U(OH)m(4-m)+at the standard state (I=0). By taking the specific ion interaction theory (SIT) for ionic strength corrections, the hydrolysis constants atI=0 were determined to beβ1°=13.71±0.31,β2°=26.12±0.21 andβ3°=36.85±0.36, together with the ion interaction coefficients of UOH3+, U(OH)22+and U(OH)3+. The solubility product (Ksp) for the reaction of UO2·xH2O=U4++4OH-+ (x-2)H2O was also calculated to be logKsp° =-53.93±0.20 by considering the hydrolysis constants.

The solubility of Pu(VI) hydrous oxide was measured in the pHcrange from 4 to 5.5 at 25±1°C in a NaClO4solution containing O3gas. The experiment was carried out by oversaturation and undersaturation methods at ionic strengthI=0.1, 0.5 and 1.0. The concentration of Pu(VI) was measured byα-spectrometry and UV/visible spectrophotometry, and the equilibrium constant of the reaction, PuO2(OH)2=PuO22++ 2 OH-, was obtained. From the obtained results, the solubility products (Ksp) of Pu hydrous oxide atI=0.1, 0.5 and 1.0 were determined. The experimental values were extrapolated to the standard state (I=0) by using the specific interaction theory (SIT), and the solubility product of Pu hydrous oxide was determined to be logKsp°=-22.88±0.39. The ion interaction coefficient (PuO22+, ClO4-) was also evaluated to be 0.30±0.09.

Summary Extraction equilibrium of lanthanides between tri-butyl phosphate solution and molten calcium nitrate hydrate, which is generally called a hydrate melt, was studied radiochemically. The dependence of the extractions on the water content of the melt was studied at 70 °C in the range of R from 3 to 12.6 for Ca(NO3)2 · RH2O. From the observed distribution ratios, variation of the activity coefficients of lanthanide nitrates in hydrate melts was analyzed as a function of water content. It was found that, in a concentration region of R greater than about 5.7, the increase of the activity coefficients with decreasing R shows a different tendency compared with the more water-abundant region. This is discussed in conjunction with the characteristics of the hydrate melt, as well as with the difference of the ionic size of lanthanide cations.

SummaryThe solubility of PuO2·xH2O was measured at 25°C in a NaClO4solution containing Na2S2O4as a reductant. The experiment was carried out by an oversaturation method atI= 1.0 and by an undersaturation method atI= 0.5, 1.0 and 2.0. The concentration of Pu3+was measured by α-spectrometry and UV/Visible spectrophotometry, and the equilibrium constant of the reaction, PuO2·xH2O + e−= Pu3++ 4 OH−+ (x–2) H2O, was obtained. From the obtained results, the solubility product (Ksp) of PuO2·xH2O, for PuO2·xH2O = Pu4++ 4 OH−+ (x − 2) H2O atI= 0.5, 1.0 and 2.0 was determined. By using the specific interaction theory (SIT), the solubility product of PuO2·xH2O at the standard state (I= 0) was determined to be logK°sp= −57.97 ± 0.24, and the ion interaction coefficient ε(Pu4+, ClO4-) was also evaluated to be 0.99 ± 0.20.