STUDY OF URANIUM VALENCE SPECIES IN PHOSPHATE PHASES OF PRECIPITATE
A.E. Boguslavskii1,2, O.L. Gaskova1,3, S.M. Sofronova1, A.A. Saraev4, Z.S. Vinokurov4, A.V. Safonov5
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State Technical University, Novosibirsk, Russia
3Novosibirsk State University, Novosibirsk, Russia
4Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
5Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
Keywords: Uranium adsorption, uranium oxides, groundwater purification, valence state
Abstract
Since apatite is capable of adsorbing and retaining radionuclides, the experimental removal of uranyl ions (UO22+) from two Ca-containing technogenic and one model solutions was performed by neutralization with a solution of sodium hydrogen phosphate Na2HPO4. The chemical composition of the precipitate was analyzed by X-ray photoelectron spectroscopy (XPS), and its structure was refined by X-ray diffraction (XRD). The formation of hydroxyapatite was confirmed; two samples also contained CaH(PO4)∙(H2O)2 (brushite), in which up to 15% of calcium was substituted by uranium. Study of the uranium species revealed U4+, U5+, and U6+, with U5+ amounting to 30-35 at.% of the total uranium. The scanning electron microscopy (SEM) analysis did not identify precipitate phases with U > 18%. Uranium was detected in the grain rims, which indicates its adsorption. Thermodynamic computation showed the possible formation of individual uranium phases, such as β-UO2.333, β-UO2(OH)2, and NaUO2O(OH) (clarkeite), at the measured Eh-pH. The oversaturation of solutions and the spontaneous formation of solid hydroxyapatite and brushite particles lead to a change in the initial Ca/PO4 ratio; moreover, the P/Ca and Ca/O ratios, as well as the elemental composition in the near-surface layers of three precipitates, are not strictly constant. The newly formed phase contains segregations different in size, the degree of deformation, and sorption surface groups, which is reflected in the mechanism of capture of uranium compounds. The experimental results demonstrate the need for further studies of uranium precipitation during the crystallization of phosphates. The stability of phosphate phases ensures the high reliability of phosphate safety barriers for the uranium extraction facilities. In addition, uranium adsorption by phosphates can be used for remediation of aquifers polluted with radionuclides.
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