Uranium(IV) Solubility and Hydrolysis Constants Under Reduced Conditions
- PDF / 268,240 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 51 Downloads / 201 Views
URANIUM(IV) SOLUBILITY AND HYDROLYSIS CONSTANTS UNDER REDUCED CONDITIONS TATSUYA YAJIMA, YUJI KAWAMURA,
AND SHINZO UETA
Naka Energy Research Center, Mitsubishi Materials Corporation. 1002-14 Naka-machi, Naka-gun, Ibaraki, 311-01 Japan. Abstract The solubility of U02 (s) was examined in dilute NaCI solutions at room temperature and in the pH range from 2 to 12. Dissolution equilibrium showed good agreements between the data obtained by oversaturation and undersaturation experiments. The dissolution reactions that control the solubility of U[IV] are estimated as U02 (s)+4H+- Ul++2H20 and UOz(s)+2H2O--U(0H) 4,. The log K at zero ionic strength are yielded 0.34±0.4 and -8.7±0.4, respectively. The hydrolysis constant (log ,4) is calculated -9.0±0.5 for the hydrolysis reaction of U'++4H2O- U(OH) 4 0 +4H+. Contribution of the other uranium species to the solubility are of minor importance. The crystallization of the precipitations of U02 (s) progressed gradually in the oversaturation experiments. Nevertheless the progress of crystallization had little effect on the solubility. Introduction Prediction of the aqueous behavior of uranium is a important matter for the safety assessments of radioactive waste isolation. The thermochemical data of U[IV] Is one of key data for the prediction of uranium behavior, because the valence of uranium is predicted tetravalent under the assumed redox condition in a deep groundwater. The solubility of U[IV] has been studied by many authors, however the reported data vary over several orders of magnitude[l-9].
The differences in
the reported data are probably based on the difference of keeping of redox potential, because U[IV] is readily oxidized to U[VII in the presence of 02. In this study, we have measured the solubility of U02 (s) and have estimated the hydrolysis constants. The valence of uranium was maintained as tetravalent by using sodium hydrosulfite (Na2 S2 04 ) and solvent extraction in a prepurified Ar atomosphere. Methods To eliminate the oxidation of U[IV] to U[VII, all experiments were conducted in a glovebox with prepurified Ar atomsphere (above 99.99% Ar, under one parts per milion of Oz). Water used was distilled water thoroughly sparged with high-purified Ar. Reagents U[IVI stock solution was prepared by electrolytic reduction from uranium nitrate solution. Uranium nitrate solution (-O.1M of U and 3N HNO 3 ) with 0.1M hydrazine was electrolyzed (5A,5V,TI catalyst) for 2 hours continuously. Just before the experiment, U[IV] stock solution was treated 3 times by solvent extraction with 10% TBP-CC1 4 repeatedly to eliminate the remained U[VI] in the stock solution. Ratio of U[VII to U[IVI was below 0.001%. It was measured by gravimetric analysis of U[VI] coprecipitation with fluorine. Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
1138
The U02 (s) sample was prepared by reduction of ammonium diuranate (ADU, (NH4 ) 2 U2 07 ). The reduction procedure was the roasting at 650'C for 2 hours in mixed gas (Hz50%, N2 50%) atomosphere. X-ray diffraction pa
Data Loading...
