Tetsushi Nagano

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.

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.

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.

Characterization of iron hydroxides in a weathered rock surface by visible microspectroscopy

In order to nondestructively characterize chemical forms of ferric hydroxides in weathered rock, charge-coupled device type visible microspectroscopy was applied to brown stains produced in weathered granite surfaces. The combination of visible microspectra and color parameters (a* and b*) was effective in examining chemical forms of ferric hydroxides in the analytical area. Color parameters in an a*–b* diagram of the brown stains, mostly lying between goethite and ferrihydrite trends, indicated that the brown stains contain ferrihydrite or hematite in addition to goethite. Similarity of the visible microspectra of the brown stains and their first derivatives to those of goethite or ferrihydrite suggests that goethite and/or ferrihydrite are the main weathering products of the granite. Occurrence of ferrihydrite as well as goethite in the brown stains implies that crystallization of ferrihydrite to goethite might be hindered during the granite weathering. This fact suggests the possibility of toxic metal retention in ferrihydrite by its long-term persistence during water–rock interactions at the earths surface.

A new method using titanium hydride for fabrication of Synroc

Synroc of high density and low leachability was formed by using titanium hydride which provides sufficiently reducing conditions during calcination and hot pressing without additional reducing gas and metal powder. The amount of titanium hydride sufficient to keep the reducing conditions was at the most30 wt% (in titanium dioxide equivalent). Omission of the gas flow into the calciner pot prevented powder carry-over up to the pot lid during calcination. The Synroc using titanium hydride consisted of three main phases, namely, hollandite, perovskite and zirconolite, plus Ti3O5 which formed due to dissociation-oxidation of titanium hydride. Leach rates of Ca, Cs and Na from this Synroc were similar to those from the Synroc in the conventional process. These results indicate that the method employing titanium hydride can be adopted for fabrication of Synroc, especially in a hot cell.

Continuous Liquid–Liquid Extraction of Uranium from Uranium-containing Wastewater Using an Organic Phase-refining-type Emulsion Flow Extractor

A previously reported emulsion flow (EF) extraction system does not equip the refining device for any used organic phase. Therefore, the processing of large quantities of wastewater by using the EF extractor alone could lead to the accumulation of extracted components into the organic phase, and a lowering of the extraction performance. In the present study, we developed an organic phase-refining-type EF system, which is equipped with a column for refining a used organic phase to prevent accumulation, and successfully applied it for treating uranium-containing wastewater.

Dissolution of Neptunium Dioxide in Aqueous Solutions Effect of Chloride Concentration on the Solubility of Amorphous Uranium Dioxide at 25 °C Under Reducing Conditions

Dissolution of Np0 2 (s) in NaC104 solutions of pH 2 to 6 was studied for durations up to 391 days at temperatures of 30 to 90 °C. Constant dissolution rates were observed following the initial non-linear dissolution that finished within the first two to three weeks. The obtained constant dissolution rates were 11 to 115 ppm/d at 30 °C, decreasing with the increase of pH between 2 and 6. Their temperature-dependence was small.