Makoto Arisaka

Chemical states of fallout radioactive Cs in the soils deposited at Fukushima Daiichi Nuclear Power Plant accident

The chemical states of radioactive Cs (caused by Fukushima Daiichi Nuclear Power Plant accident) in the contaminated soils have been characterized by the desorption experiments using appropriate reagent solutions and size fractionation of the contaminated soils. More than 65% of radioactive Cs remained in the residual fraction of the soil samples after treatment of 1 mole L−1 NH4Cl solution and 1 mole L−1 CH3COOH solution. Approximately 70% of radioactive Cs in the residual fraction were associated with the size fractions larger than the elutriated one, even though mica-like minerals were present in the elutriated one. These results strongly suggest that radioactive Cs was irreversibly associated with soil components other than mica-like minerals in the contaminated soil.

Thermodynamics and Mechanism Studies on Electrochemical Removal of Cesium Ions from Aqueous Solution Using a Nanoparticle Film of Copper Hexacyanoferrate

Nanoparticle (NPs) film of copper hexacyanoferrate (CuHCF(III)) was developed for electrochemically cesium separation from wastewater. Different form the electro- or chemical deposited films, CuHCF(III) NPs were firstly covered with ferrocyanide anions, so that they can be well dispersed in water and formed ink. Then CuHCF(III) NPs can be uniformly coated by simple wet printing methods, so it is feasible to prepare NPs film of any sizes, or any patterns at low cost. This process provided a promising technology for preparing large scale electrodes for sequential removal of Cs from wastewater in the columns. Cs separation can be controlled by an electrically switched ion exchange (ESIX) system. Effect of temperatures, and ionic strength on Cs removal was investigated. Thermodynamics results showed that Cs adsorption process was exothermic in nature and favored at low temperature. Ionic strength study indicated the CuHCF(III) film can selectively separate Cs in wide ionic strength range from 1 × 10(-4) to 1 × 10(-1) M Na(+). XPS results demonstrated that the electrochemical oxidation-reduction of Fe (II/III) made contributions to Cs separation.

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.

Direct evidence for enhanced inner-sphere chloro complexation of Eu(III) and Cm(III) in anion exchange resin phase studied by time-resolved laser-induced fluorescence spectroscopy

Summary In anion exchange resin systems with LiCl-aqueous and LiCl-(H2O + CH3OH) solutions, the extent of chloro complexation of M(III) (M = Eu, Cm) was estimated from inner-sphere hydration number (NH2O), i.e., the number of water molecules in the first coordination sphere of M(III) and from peak area ratio (A2/A1) of 5D0 → 7F2 transition (A2) to 5D0 → 7F1 transition (A1) in emission spectrum of Eu(III), which reflects the ligand environment both in the inner- and outer-spheres of Eu(III). The NH2O of M(III) both in the solution and resin phases decreased with an increase of LiCl and CH3OH concentrations, indicating the formation of an inner-sphere M(III)-chloro complex. From the comparison of NH2O between the both phases, it was found that the extent of the inner-sphere chloro complexation of M(III) in the resin phase is higher than that in the solution phase. The A2/A1 ratio in the both phases increased with an increase of interaction of Eu(III) with Cl−, which are well correlated with the results of NH2O. The sorption equilibrium of M(III) was discussed by comparing the extent of the chloro complexation with the distribution coefficient.

Sorption behavior and coordination state of Eu(III) and Cm(III) in hydrochloric acid-methanol mixture/cation exchange resin system studied by time-resolved laser-induced fluorescence spectroscopy

A role of methanol on the enhancement of the sorption of M(III) [M=Eu, Cm] in HCl-CH3OH mixture/cation exchange resin system was investigated by comparing the distribution coefficient of M(III) with its coordination states both in the solution and the resin phases. The inner-sphere hydration number (NH2O) of M(III), i.e., the number of water molecules in the first coordination sphere, was determined by the measurement of the luminescence lifetime of M(III) using Time-Resolved Laser-induced Fluorescence Spectroscopy. From the results showing a decrease of NH2O with an increase of methanol mole fraction (XCH3OH), it was found that the inner-sphere complexations of M(III) both with Cl- in the solution phase and with exchange group, SO3-, in the resin phase were promoted with an increase of XCH3OH. The variation of the coordination environment around Eu(III) estimated from the emission spectra was correlated well with the variation of the NH2O. It was concluded that the increase of the sorption is mainly resulted from the inner-sphere complexation of M(III) with SO3- in the resin phase.

Column study on electrochemical separation of cesium ions from wastewater using copper hexacyanoferrate film

Nanoparticle film of copper hexacyanoferrate (CuHCFIII) was developed and coated on the rolled sheet electrodes for column cesium separation from wastewater. Results indicated well balance of Cs adsorption and desorption can be achieved switching the potentials between anodes and cathodes. XPS and isotherm studies indicated the electrochemical oxidation–reduction of Fe(II/III) on the monolayer of the CuHCFIII film made contributions to reversible Cs separation. Minimization of secondary waste, simple regeneration and uniformly coating for any size, suggest a promising column technology for sequential removal of Cs from actual wastewater.

A remote valency control technique: catalytic reduction of uranium(VI) to uranium(IV) by external ultrasound irradiation

We here report the enhancement of a sonochemical effect (chemical reaction induced by ultrasound irradiation) by a Pt black catalyst; the sonochemical reduction of the highly stable U(VI) was demonstrated using this catalytic reaction.

Toward Innovative Actinide Separation Processes: Sequential Reduction Scheme of Uranium, Neptunium, and Plutonium in 3 M HNO3 by External Ultrasound Irradiation

An innovative remote valency control technique for actinide ions induced by external ultrasound irradiation was reported in the present study. It is known that ultrasound irradiation to water causes the oxidation and/or reduction of the solute by H• or OH• There were some reports on the redox behavior of actinide elements under ultrasound irradiation. Nevertheless, they showed that the ability of ultrasound is not sufficient for the use of the actinide separation processes. However, very recently we found that a noble metal catalyst drastically enhances the sonochemical effect and that even highly stable U(VI) is reduced to U(IV) by ultrasound irradiation. Employing this catalytic reaction, we are developing low-emission actinide ion separation schemes driven by external ultrasound irradiation. In the present work, U(VI), Np(VI) and Pu(VI) in 3 M HNO3 medium were chosen as target ions. Their valency was first adjusted to U(VI)/Np(V)/Pu(IV) by external ultrasound irradiation and, then, further sonochemical reduction to U(IV)/Np(IV)/Pu(III) was carried out. The present results confirmed the possibility to design the low-emission U/Np/Pu mutual separation scheme driven by external ultrasound irradiation.

Uranium(VI) Speciation at Elevated Temperatures and Pressures by Time-resolved Laser-induced Fluorescence Spectroscopy

A few experimental studies on uranium(VI) hydrolysis and complexation at elevated temperatures and pressures performed to date are obviously quite insufficient to depict a comprehensive picture of the species and the behavior in a wide variety of hydrothermal conditions in the geosphere. In this study, an optical cell system for spectroscopic speciation of metal ions in hydrothermal solutions was developed and combined with time-resolved laser-induced fluorescence spectroscopy for uranium(VI) speciation. Emission spectra and lifetimes of 1×10-4 M(=mol.dm-3) uranium(VI) in NaClO4 solutions in the presence and absence of sulfate or fluoride ion were measured as a function of pH, ligand concentration, temperature (25-200 °C), or pressure (0.1 or 40 MPa). The results were compared with speciation calculations at various conditions on the basis of thermodynamic model and data in the literature. The emission intensities and lifetimes of uranium(VI) decreased rapidly with increasing temperature. The pressure effect on the fluorescence properties could not be neglected, but it was rather small relative to the temperature effect. The temperature dependence of the lifetimes obeyed well the Arrhenius law, and the activation energy was characterized for uranyl(VI) ion and the hydroxide and fluoride complexes.

New alkyl-pyridinedicarboxyamides as extractants for separation of trivalent minor actinides from fission products by extraction chromatography

Abstract Four new alkyl-pyridinedicarboxyamides (R-PDA; R = butyl, octyl, decyl, dodecyl) were synthesized and used as extractants for partitioning minor actinides from high level radioactive waste using extraction chromatography. The R-PDAs were successfully impregnated into XAD resins (Amberlite). The prepared adsorbents, R-PDA/XAD, exhibited moderate adsorptions of both Am(III) and Eu(III), and separation between Am(III) and Eu(III), over a high HNO3 concentration range (3–5 M). Deterioration of the prepared adsorbent as a result of contact with HNO3 solution was prevented by using R-PDAs containing longer alkyl groups. The equilibrium data for Eu(III) were analyzed using Langmuir and Freundlich isotherm models. The adsorption of Eu(III) ions onto the R-PDA/XAD was fitted better to the Langmuir isotherm than to the Freundlich isotherm. The maximum adsorption capacity was determined to be 24.2 mg Eu/g (R-PDA/XAD).