Shinichi Nakayama

Diffusivity of U, Pu and Am carbonate complexes in a granite from Inada, Ibaraki, Japan studied by through diffusion

Abstract The diffusivities of uranium, plutonium and americium in Inada granite have been determined using through-diffusion method. Experiments were performed at (25±1)°C in a 0.1 mol l −1 NaHCO 3 solution where the actinides are present as carbonate or hydroxy–carbonate complexes. Effective diffusivity ( D e ) values of (1.42±0.24)×10 −13 m 2 s −1 and (5.1±2.0)×10 −14 m 2 s −1 were obtained for uranium and plutonium, respectively. Diffusion through 5 mm thick granite was not observed for americium within the experimental period of 366 days. The D e value for uranyl carbonate species obtained in these experiments was found to be four times higher than the previously obtained D e value for the uncomplexed uranyl ion.

Effective diffusivity of the uranyl ion in a granite from Inada, Ibaraki, Japan

The effective diffusivity of uranium(VI) in Inada granite has been determined by through-diffusion. Experiments were performed at room temperature (20–25°C) in a 0.1 mol 1−1 KCl solution where uranium is present predominantly as the poorly sorbing UO22+. An effective diffusivity (De) of (3.6 ± 1.6) × 10−14 m2 s−1 was obtained, close to that for uranine (nonsorbing organic tracer), but one order of magnitude lower than those obtained for Sr2+ and NpO2+, and two orders of magnitude lower than that obtained for I−. According to well established theory, a proportional relationship exists between De and the diffusivity in the bulk of the solution (Dv). The effective diffusivity obtained in granite was not proportional to Dv. This agrees with results obtained for effective diffusivity in a Swedish granite. The ratio DeDv was found to be not constant but increased with De or Dv. This result suggests a limit to the application of the theory.

The use of color to quantify the effects of pH and temperature on the crystallization kinetics of goethite under highly alkaline conditions

The crystallization kinetics of goethite were studied colorimetrically under highly alkaline conditions (pH 10.1–12.2) at temperatures from 40° to 85°C. Color changes during crystallization from fresh precipitates, plotted on a*-b* colorimetric diagrams, were used to discriminate between pure goethite and mixtures of goethite and hematite. Only the b* value increased as goethite crystallization proceeded, and even a minor increase in the a* value revealed the existence of hematite. The rate of goethite crystallization, estimated from the b* value, could be modeled by a pseudo-first-order rate law. This rate depended both on pH and on temperature. Apparent activation energies for the reactions of 56.1 kJ/mol at pH 11.7 and 48.2 kJ/mol at pH 12.2 were estimated from Arrhenius plots.

Color variations associated with rapid formation of goethite from proto-ferrihydrite at pH 13 and 40 degrees C

Color variations from brown to yellow of synthesized goethite have been studied colorimetrically and spectroscopically. Goethite with various colors was synthesized at pH 13 and 40°C by varying the incubation time. Colorimetry revealed that the b* value (yellowish chroma) in L*a*b* color space was a quantitative indicator of color variations of the diluted samples. From UV-VIS-NIR spectra, the increase in the b* value was found to be caused by the increase in crystal field absorptions due to goethite formation around 500 nm. The b* value was a good indicator of the relative proportion of goethite in the precipitates including ferrihydrite. X-ray diffraction patterns and infrared spectra revealed that crystallization of goethite was associated with loss of water from the proto-ferrihydrite.

XPS Study of Pb(II) Adsorption on γ-Al2O3 Surface at High pH Conditions

Lead(II) adsorption on γ-Al2O3 at high pH condition (11 <pH<13) was studied in NaNO3 solutions. With increasing pH from 11, Pb(II) uptake by γ-Al2O3 was decreased regardless to sodium concentration (0.1 and 1.0 M). XPS spectra showed no chemical shifts occurred in substrate Al (2s, 2p) and O (1s) binding energy by Pb(II) adsorption. Binding energy of adsorbed Pb (4f) had no dependence on pH and adsorption density and was higher than that of orthorhombic yellow PbO and Pb(NO3)2. Molecular orbital (MO) calculation revealed lowest unoccupied molecular orbital (LUMO) energy of Pb(OH)- 3 was about 6 eV higher than that of Pb(OH)2, and suggested Pb(OH)- 3 was less reactive than Pb(OH)2 with deprotonated surface hydroxyl ligand. These results suggest that Pb(II) is adsorbed on γ-Al2O3 as inner-sphere surface complex form at high pH condition. Although Pb(II) is adsorbed on γ-Al2O3 as inner-sphere surface complex, bonding between Pb(II) and γ-Al2O3 shows less covalent nature.

Sorption of Neptunium on Naturally-Occurring Iron-Containing Minerals

Sorption of neptunium on naturally-occurring hematite (aFe 2 0 3 ) , magnetite (Fe 3 0 4 ) , goethite (α-FeOOH) and biotite (K(Mg,Fe)3AlSi3O10(OH)2) in 0.1 Μ N a N 0 3 solution was studied at 30°C for pHs between 4 and 11. The mass to volume ratio was lg/1. The sorption-desorption reaction was reversible for all the minerals in the pH ranges studied. The dependence of neptunium sorption on pH differed between goethite and the other three minerals; goethite showed a strong sorption at pHs above 6 while the sorption on hematite, magnetite and biotite occurred at pHs above 9. Other oxyhydroxides, lepidocrocite (y-FeOOH) and boehmite (y-AlOOH), have a similar pH dependence of sorption to goethite. The sorption on hematite was similar to that on alumina (α-Α1203).

Formation of goethite and hematite from neodymium-containing ferrihydrite suspensions

The effects of neodymium (Nd) on the transformation of ferrihydrite to iron oxides was studied. The possible isomorphous substitution of Nd3+ for Fe3+ in iron oxides was examined also. Nd was used as an inactive substitute of trivalent radioactive actinide elements. Hydrolysis of ferric nitrate solution containing 0–30 mole % of Nd formed Nd, Fe-rich ferrihydrite as initial precipitates, which were poorly crystalline. Aging of the Nd-containing ferrihydrite in 0.3 M OH− at 40°C and at pH 9.2 at 70°C formed Nd-free goethite and Nd-substituted hematite. The abundance of these crystalline phases was related to Nd in the parent solutions. Phase abundance, unit-cell parameters, and peak width were estimated by use of the Rietveld method.

Effect of Ionic Strength on the Solubility of Neptunium(V) Hydroxide

The effect of ionic strength on the solubility of neptunium(V) hydroxide was studied under the oxidizing, C02-free argon atmosphere in the pH range of 7 to 13.5 at room temperature (20—25° C). The solubility was mainly approached from oversaturation direction. The ionic strength was adjusted by NaC104 to 0.012, 0.05, 0.10, 0.40 or 0.80. Estimated values for the solubility product (K¡ρ) and the first and the second hydrolysis constants (ßu β2) were: log Ksp = -10 .21 ± 0.02, log ßt = 2.91 ± 0.73, log ß2 = 5.50 ± 0.06 at / = 0.80, log Κsp = 9 . 6 1 ± 0.04, log jSj = 3.49 ± 0.25, log β 2 = 4.7 ±1 .0 at 7=0.40, log = 8 . 9 4 ± 0.01, log β i = 2.67 ± 0.34, log β 2 = 5.74 ±0.07 at 7 = 0.10, log Ksp = 8 . 9 1 ± 0.05, log ßi = 3.31 ± 0.30, log ß2 = 5.74 ± 0.30 at 7 = 0.05 and log Ksp = 8 . 0 4 ± 0.05, log ß2 = 5.16 ± 0.94 at 7 = 0.012, where 7 is the ionic strength in the molarity unit. By using these values, the specific-ion-interaction theory gives the constants at infinite dilution, 7 = 0:log A?p = 8 . 6 8 ±0.26, log ßl = 3.30 ± 0.35 and log ß°2 = 5.58 ± 0.31. Neptunium hydroxide precipitates showed no X-ray diffraction peaks from 1 day to 3 months after preparation. This result indicates that the precipitates remained non-crystalline for up to 3 months.

Leaching behavior of a simulated bituminized radioactive waste form under deep geological conditions

The leaching behavior of a simulated bituminized waste form was studied to acquire data for the performance assessment of the geologic disposal of bituminized radioactive waste. Laboratory-scale leaching tests were performed for radioactive and non-radioactive waste specimens simulating bituminized waste of a French reprocessing company, COGEMA. The simulated waste was contacted with deionized water, an alkaline solution (0.03-mol // KOH), and a saline solution (0.5-mol// KCl) under atmospheric and anoxic conditions. The concentrations of Na, Ba, Cs, Sr, Np, Pu, NO3, SO4 and I in the leachates were determined. Swelling of the bituminized waste progressed in deionized water and KOH. The release of the soluble components, Na and Cs, was enhanced by the swelling, and considered to be diffusion-controlled in the swelled layers of the specimens. The release of sparingly soluble components such as Ba and Np was solubility-limited in addition to the progression of leaching. Neptunium, a redox-sensitive element, showed a distinct difference in release between anoxic and atmospheric conditions. The elemental release from the bituminized waste specimens leached in the KCl was very low, which is likely due to the suppression of swelling of the specimens at high ionic strength.

Modeling of Neptunium(V) Sorption Behavior onto Iron-Containing Minerals

Sorption behaviors of neptunium (V) on naturally-occurring magnetite (Fe 3 O 4 ) and goethite (α-FeOOH) in 0.1M NaN0 3 electrolyte solution under aerobic conditions were interpreted using the surface complexation model (SCM). The surface properties of these materials were experimentally investigated by C0 2 -free potentiometric titration, and SCM parameters for the constant capacitance model, such as protonation/deprotonation constants of the surface hydroxyl group, were determined. The number of negatively charged sorption sites of goethite rapidly increased with the increase of the bulk solution pH compared with that of magnetite and this tendency was similar to the pH dependence of neptunium sorption. This implies that the neptunyl cation, NpO 2 + , plays a dominant role in possible sorption reactions. Assuming that the dominant surface complex is XO-NpO 2 , modeling by means of SCM was carried out, and the results were found to agree with experimental data.