Taishi Kobayashi

Solubility measurement of zirconium(IV) hydrous oxide

The solubility of zirconium(IV) hydrous oxide was measured in the hydrogen ion concentration (pHc) range from 1 to 13 at 25 °C. The experiment was carried out with I=0.1, 0.5 and 1.0 mol/dm3 (M) of NaClO4 solution. The concentration of Zr was measured by ICP-MS after separating the aqueous phase from the solid phase by ultrafiltration methods. Some differences between the obtained values and literature data were observed due to the size distribution of chemical species. In the lower pHc region, the size distribution of filtrate species was investigated by different pore-size filtration in the range of 3 k to 100 kDa NMWL (nominal molecular weight limit) membranes. The solubility values of mononuclear species were obtained by the Flory model, and by using the specific ion interaction theory (SIT) for ionic strength corrections and the known literature value of the hydrolysis constant β1°, the tentative value of the solubility product (Ksp°) was determined to be −56.94±0.32.

Solubility of Zirconium(IV) Hydrous Oxides

The solubility of zirconium(IV) hydroxide and oxide was measured in the acidic, neutral and alkaline pH at 25°C by an undersaturation method. No influence of the formation of polynuclear and colloidal species on the apparent solubility was observed in the case of the ZrO2. By using the solubility data of the ZrO2 and by considering the contributions of the mononuclear species, Zr4+, ZrOH3+ and Zr(OH)4(aq), to the solubility curve in the lower pH region, the solubility product was obtained to be log K°sp = --62.46 ± 0.10. In the case of the Zr(OH)4, no equilibrium was observed during the experimental period up to four months.

Redox behavior of Tc(VII)/Tc(IV) under various reducing conditions in 0.1 M NaCl solutions

Abstract Redox behaviour of Tc(VII)/Tc(IV) was investigated in 0.1 M NaCl solutions containing different reducing agents in the pH range 2 to 13 at 22 ºC under inert Ar atmosphere. In several samples, the 1 × 105 mol/dm3 (M) initially added TcO4- was reduced to form a Tc(IV) oxide solid phase with low solubility. The observed Tc redox transformation processes are systematized according to Eh-pH conditions in solution, indicating that a borderline for the reduction of Tc(VII) to Tc(IV), TcO4- + 3e- + 4H+⇔TcO2· xH2O(coll, hyd) + (2-x)H2O exists, independent of the reducing chemical system. This experimentally derived borderline is about 100 mV lower than the equilibrium line calculated from the reported standard redox potential of TcO2· 1.6H2O(s). This behaviour can be related to the existence of more soluble solid phase modifications, i.e. nanoparticulate Tc(IV) oxide species (TcO2· xH2O(coll, hyd)). The reaction kinetics likewise correlate to the redox potential measured in solution. Slow reduction of Tc(VII) to Tc(IV) was observed when the redox potential in the system was slightly below the above mentioned reduction borderline. Fast reduction was observed in the systems far below the borderline, but also in those systems containing Fe(II) solids, suggesting a specific surface mediated effect in the reduction process. EXAFS analysis on two magnetite samples indicate reduced Tc(IV) species which do not remain adsorbed at the reactive mineral surface and are incorporated in the magnetite structure.

Hydrolysis Constant and Coordination Geometry of Zirconium(IV)

The solubility of zirconium was studied by comparing recent experimental data with predictions using an electrostatic hard sphere model. In the model, an antiprismatic structure was assumed for a mononuclear hydrolysis species of zirconium, and the formation of an oligomeric species such as Zr4(OH)8 8+ was considered by combining antiprismatic structure units, which was supported by the theoretical approach. By taking the known values of the solubility products, the hydrolysis constants of these mononuclear and polynuclear species were evaluated and were found to be consistent with the experimental solubility data of amorphous Zr(OH)4 and crystalline ZrO2.

Hydrolysis constants and complexation of Th(IV) with carboxylates

Abstract The hydrolysis constants of Th(IV) were determined using the solvent extraction method in a NaClO4 solution at I=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.

Systematics of Hydrolysis Constants of Tetravalent Actinide Ions

Systematic trends of mononuclear and polynuclear hydrolysis constants of tetravalent actinide ions were analyzed by using an electrostatic hard sphere model. The effective charges of actinide ions were introduced into the model by considering possible contributions of non-electrostatic interactions of actinide ions in addition to those of ordinary electrostatic ones. The systematic trends of hydrolysis constants were well fitted by the present model, and the parameter values such as the effective charges of actinide ions were determined. Some predictions were made and discussed in comparison with the solubility curves of tetravalent actinides.

Zirconium Solubility in Ternary Aqueous System of Zr(IV)-OH-Carboxylates

The solubility of zirconium(IV) hydroxide in the presence of oxalic and malonic acids was measured for a wide range of pH values and organic acid solutions prepared by the oversaturation method. In nearneutral pH solutions, the apparent solubility increased with the concentration of the organic acid due to the formation of soluble carboxylate complexes. On the other hand, under acidic conditions, a precipitate of carboxylate salts was formed and the solubility was controlled by the precipitate. The complex formation constants and solubility products of zirconium oxalate and malonate were obtained by analysis of the solubility results.

Analysis of Sorption Behavior of Cesium Ion on Mineral Components of Granite

The influence of pH and ionic strength on the sorption of Cs by quartz, biotite, and microcline was investigated to understand the Cs sorption process onto the granite host rock. Cesium sorption by each mineral increased with increasing pH, and reached a constant value at high pH. The Cs sorption decreased in the presence of sodium perchlorate as a matrix ion, and the apparent Kd decreased with an increasing initial concentration of Cs. In the analysis, a two-site model either giving or not giving consideration to the presence of the triple layer was applied to the results of each mineral, and the model parameters were determined. By taking the obtained parameters, the model was used to predict the distribution coefficient Kd of cesium on granite. The simulated Kd values were discussed by comparing themselves with the reference ones for granite.

Asymmetric divertor plasma distribution observed in Heliotron J ECH discharges

Abstract An asymmetric divertor plasma distribution observed in the standard configuration of Heliotron J is reported. The divertor plasma profiles were investigated with two Langmuir probe arrays, which were installed at the geometrically up–down symmetric positions, for three different heating schemes of ECH with 53.2 or 70 GHz microwaves. Although the position of the divertor plasma flux was almost consistent with the footprint position of the divertor field lines, the existence of two types of up–down asymmetry was revealed in the divertor plasma density and floating potential profiles. The first type of asymmetry was mainly observed near the boundary to the ‘private region’. This asymmetry seems to be independent of the heating schemes of the toroidal position of the heating source. As the direction of the confinement field was reversed, the feature of the plasma profile on the top array came to appear on the bottom array, and vice versa. This field-direction dependence indicates that the asymmetric B ×∇B drift motion of charged particles might cause this type of asymmetry. The second type of asymmetry was observed in the region away from the boundary and seemed to depend on the heating schemes.

Redox chemistry of Tc(VII)/Tc(IV) in dilute to concentrated NaCl and MgCl2 solutions

Abstract The redox behaviour of Tc(VII)/Tc(IV) was investigated within the pHc range 2–14.6 in (0.5 M and 5.0 M) NaCl and (0.25 M, 2.0 M and 4.5 M) MgCl2 solutions in the presence of different reducing agents (Na2S2O4, Sn(II), Fe(II)/Fe(III), Fe powder) and macroscopic amounts of Fe minerals (magnetite, mackinawite, siderite: S/L = 20–30 g L–1). In the first group of samples, the decrease of the initial Tc concentration (1 · 10–5 M, as Tc(VII)) indicated the reduction to Tc(IV) according to the chemical reaction TcO4– + 4H++ 3e– ↔ TcO2 · 1.6H2O(s) + 0.4H2O. Redox speciation of Tc in the aqueous phase was further confirmed by solvent extraction. A good agreement is obtained between the experimentally determined Tc redox distribution and thermodynamic calculations based on NEA–TDB (Nuclear Energy Agency, Thermochemical Database) and ionic strength corrections by SIT or Pitzer approaches. These observations indicate that experimental pHc and Eh values in buffered systems can be considered as reliable parameters to predict the redox behaviour of Tc in dilute to highly concentrated NaCl and MgCl2 solutions. Eh of the system and aqueous concentration of Tc(IV) in equilibrium with TcO2 · 1.6H2O(s) are strongly affected by elevated ionic strength, especially in the case of 4.5 M MgCl2 solutions. In such concentrated brines and under alkaline conditions (pHc = pHmax ∼ 9), kinetics play a relevant role and thermodynamic equilibrium for the system Tc(IV)(aq) ↔ Tc(IV)(s) was not attained from oversaturation conditions within the timeframe of this study (395 days). Tc(VII) is reduced to Tc(IV) by magnetite, mackinawite and siderite suspensions at pHc = 8 – 9 in concentrated NaCl and MgCl2 solutions. Sorption is very high in all cases (Rd ≥ 103 L kg–1), although Rd values are significantly lower in 4.5 M MgCl2 solutions. XANES (X-ray absorption near edge spectroscopy) evaluation of these samples confirms that Tc(VII) is reduced to Tc(IV) by Fe(II) minerals also in concentrated NaCl and MgCl2 brines.