Ryuji Nagaishi

Pulse radiolysis study of concentrated sulfuric acid solutions. Formation mechanism, yield and reactivity of sulfate radicals

In the pulse radiolysis of concentrated sulfuric acid solutions, the absorption spectrum and the molar absorption coefficient (1600 dm3 mol–1 cm–1) of the sulfate radical are unchanged up to 10 mol dm–3 H2SO4, suggesting that the sulfate radical exists in the dissociated form (SO–4). Two formation processes for the sulfate radical have been directly demonstrated in sulfuric acid and hydrogensulfate solutions: a fast one completed in the duration of the electron pulse, and a slow one occurring over a microsecond time range. For sulfate solutions only the fast formation process is observed. In sulfuric acid solutions the slow formation process is OH + HSO–4→ H2O + SO–4(4.7 × 105 dm3 mol–1 s–1) and OH + H2SO4→ HSO4+ H2O → SO–4+ H3O+(1.4 × 107 dm3 mol–1 s–1), and the fast formation process is the direct action of radiation on sulfuric acid with a G value of (2.7 ± 0.4)× 10–2 molecule eV–1. The yields of (OH + SO–4) and H can be quantified as: G(OH + SO–4)= 2.9fw+ 2.7fs and G(H)= 3.7fw+ 2.7fs. The yields of SO–4 have also been evaluated and the decay kinetics and reactions of the sulfate radical studied.

Luminescence study on solvation of americium(III), curium(III) and several lanthanide(III) ions in nonaqueous and binary mixed solvents

The luminescence lifetimes of An(III) and Ln(III) ions [An=Am and Cm; Ln=Nd, Sm, Eu, Tb and Dy] were measured in dimethyl sulfoxide(DMSO), N,N-dimethylformamide(DMF), methanol(MeOH), water and their perdeuterated solvents. Nonradiative decay rates of the ions were in the order of H2O > MeOH > DMF > DMSO, indicating that O-H vibration is more effective quencher than C-H, C=O, and S=O vibrations in the solvent molecules. Maximal lifetime ratios τD/τH were observed for Eu(III) in H2O, for Sm(III) in MeOH and DMF, and for Sm(III) and Dy(III) in DMSO. The solvent composition in the first coordination sphere of Cm(III) and Ln(III) in binary mixed solvents was also studied by measuring the luminescence lifetime. Cm(III) and Ln(III) were preferentially solvated by DMSO in DMSO-H2O, by DMF in DMF-H2O, and by H2O in MeOH-H2O over the whole range of the solvent composition. The order of the preferential solvation, i.e., DMSO > DMF > H2O > MeOH, correlates with the relative basicity of these solvents. The Gibbs free energy of transfer of ions from water to nonaqueous solvents was further estimated from the degree of the preferential solvation.

Aqueous Solutions of Uranium(VI) as Studied by Time-Resolved Emission Spectroscopy: A Round-Robin Test

Results of an inter-laboratory round-robin study of the application of time-resolved emission spectroscopy (TRES) to the speciation of uranium(VI) in aqueous media are presented. The round-robin study involved 13 independent laboratories, using various instrumentation and data analysis methods. Samples were prepared based on appropriate speciation diagrams and, in general, were found to be chemically stable for at least six months. Four different types of aqueous uranyl solutions were studied: (1) acidic medium where UO22+aq is the single emitting species, (2) uranyl in the presence of fluoride ions, (3) uranyl in the presence of sulfate ions, and (4) uranyl in aqueous solutions at different pH, promoting the formation of hydrolyzed species. Results between the laboratories are compared in terms of the number of decay components, luminescence lifetimes, and spectral band positions. The successes and limitations of TRES in uranyl analysis and speciation in aqueous solutions are discussed.

γ-Radiolysis study of concentrated nitric acid solutions

The formation of nitrous acid has been studied in the 60Co-γ radiolysis of concentrated nitric acid solutions. The yields of nitrous acid are proportional to the electron fraction of nitric acid, on the basis of which, an additional reaction path, NO–3 [graphic omitted]→ O(3P)+ NO–2 with a primary yield of 0.16 µmol J–1, has been verified for the direct action of radiation on nitric acid.

Thermochromic properties of low-melting ionic uranyl isothiocyanate complexes

Temperature-dependent yellow-to-red colour changes of uranyl thiocyanate complexes with 1-alkyl-3-methylimidazolium cations have been studied by different spectroscopic methods and this phenomenon is attributed to changes in the local environment of the uranyl ion, including the coordination number, as well as to cation-anion interactions.

Luminescence properties of tetravalent uranium in aqueous solution

Summary The luminescence spectra of U4+ in aqueous solutions were observed in the UV-VIS region at ambient and liquid nitrogen temperatures. The excitation spectrum indicates that the luminescence is arising from the deexcitation of a 5f electron at the 1S0 level and no other emissions of U4+ in aqueous solutions were detected for other f–f transitions. All the luminescence peaks were assigned to the transitions from 1S0 to lower 5f levels. To estimate the luminescence lifetime, luminescence decay curves were measured using time-resolved laser-induced fluorescence spectroscopy. At room temperature, the decay curve indicated that the lifetime was shorter than 20 ns. On the other hand, the frozen sample of U4+ in aqueous solution at liquid nitrogen temperature showed the same emission spectrum as at room temperature and its lifetime was 149 ns in H2O system and 198 ns in D2O system. The longer lifetime at liquid nitrogen temperature made it possible to measure the spectrum of U4+ at the concentration as low as 10-6 M. The difference in the anion species (ClO4-, Cl-, SO42-) affected the structure of the emission spectrum to some extent.

Surface speciation of Eu3+ adsorbed on kaolinite by time-resolved laser fluorescence spectroscopy (TRLFS) and parallel factor analysis (PARAFAC)

Time-resolved laser fluorescence spectroscopy (TRLFS) is an effective speciation technique for fluorescent metal ions and can be further extended by the parallel factor analysis (PARAFAC). The adsorption of Eu(3+) on kaolinite as well as gibbsite as a reference mineral was investigated by TRLFS together with batch adsorption measurements. The PAFAFAC modeling provided the fluorescence spectra, decay lifetimes, and relative intensity profiles of three Eu(3+) surface complexes with kaolinite; an outer-sphere (factor A) complex and two inner-sphere (factors B and C) complexes. Their intensity profiles qualitatively explained the measured adsorption of Eu(3+). Based on the TRLFS results in varied H(2)O/D(2)O media, it was shown that the outer-sphere complex exhibited more rapid fluorescence decay than Eu(3+) aquo ion, because of the energy transfer to the surface. Factor B was an inner-sphere complex, which became dominant at relatively high pH, high salt concentration and low Eu(3+) concentration. Its spectrum and lifetime were similar to those of Eu(3+) adsorbed on gibbsite, suggesting its occurrence on the edge face of the gibbsite layer of kaolinite. From the comparison with the spectra and lifetimes of crystalline or aqueous Eu(OH)(3), factor C was considered as a poly-nuclear surface complex of Eu(3+) formed at relatively high Eu(3+) concentration.

Hydrogen production in gamma radiolysis of the mixture of mordenite and seawater

Hydrogen production by γ-radiolysis of the mixture of mordenite, a zeolite mineral, and seawater was studied in order to provide basic points of view for the influences of zeolite minerals, of the salts in seawater, and of rise in temperature on the hydrogen production by the radiolysis of water. These influences are required to be considered in the evaluation of the hydrogen production from residual water in the waste zeolite adsorbents generated in Fukushima Dai-ichi Nuclear Power Station. As the influence of the mordenite, an additional production of hydrogen besides the hydrogen production by the radiolysis of water was observed. The additional hydrogen can be interpreted as the hydrogen production induced by the absorbed energy of the mordenite at the yield of 2.3×10−8 mol/J. The influence of the salts was observed as increase of the hydrogen production. The influence of the salts can be attributed to the reactions of bromide and chloride ions inhibiting the reaction of hydrogen with hydroxyl radical. The influence of the rise in temperature was not significantly observed up to 60°C in the mixture with seawater. The results show that the additional production of hydrogen due to the mordenite had little temperature dependence.

Pulse radiolysis study of concentrated phosphoric acid solutions

In the pulse radiolysis of concentrated phosphoric acid solutions, in the direct action on radiation on phosphoric acid gives rise to the phosphate radical. The yields of the phosphate radical and the hydroxyl radical are proportional, respectively, to the electron fractions of phosphoric acid and water, and can be quantified as G(˙OH + H2PO˙4)= 0.30fw+ 0.33fsµmol J–1. The forward and reverse rate constants of the equilibrium ˙OH + H3PO4⇄ H2O + H2PO˙4 were evaluated as 4.2 × 104 and 2.5 × 103 dm3 mol–1 s–1 respectively, from which E0(H2PO˙4, H+/H3PO4)= 2.65 V was derived. The decay kinetics and the reactions of the phosphate radical have also been studied.

Instrumental development for spectroscopic speciation of f-elements in hydrothermal solutions: luminescence properties of lanthanide(III) ions

Summary An optical cell system for spectroscopic speciation of f-elements in hydrothermal solutions was developed and applied to study the luminescence properties of lanthanide[Ln](III) ions by time-resolved laser-induced fluorescence spectroscopy. The apparatus to maintain the hydrothermal conditions consists of a HPLC pump, an optical cell with three sapphire windows, an electric furnace, a back pressure regulator, etc. Temperature and pressure of sample solutions can be controlled independently in the range of ambient conditions to 723 K and 40 MPa, respectively. Emission spectra and lifetimes of Ln(III) in HClO4 solutions were measured as a function of temperature and pressure. The pressure effect on the emission spectra and lifetimes was not observed in the range of 0.1 to 40 MPa at a constant temperature. From the temperature dependence of the luminescence properties at 40 MPa, it was found that Ln(III) exist as a hydrated ion up to ca. 500 K and that the variation of the luminescence properties at higher temperature is mainly due to the hydrolysis of Ln(III). Thermodynamic parameters and isotope effects for the quenching of excited Ln(III) in H2O and D2O solutions were also estimated from the temperature dependence of the luminescence lifetimes.