A STUDY OF THE U IN SEDIMENT PHOSPHATE ASES
А.E. Boguslavsky1, 2, O.L. Gaskova1,3, S.M. Sofronova1, A.А. Saraev4, Z.S. Vinokurov5, A.V. Safonov6
1 V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia,
2 Novosibirsk State Technical University, Novosibirsk, Russia
3 Novosibirsk State University, Novosibirsk, Russia
4 Boreskov Institute of Catalysis, Novosibirsk, Russia
5 Synchrotron Radiation Facility SKIF, Boreskov Institute of Catalysis, Koltsovo, Russia
6 Frumkin Institute of Physical Chemistry and Electrochemistry RAS, Moscow, Russia
Keywords: uranium adsorption, uranium oxides, groundwater purification, valent state.
Abstract
Since apatite is capable to adsorb and retain radionuclides, the removal of uranyl (UO22+) ions from two Ca-containing technogenic and model solutions (1-2 g/L of Ca) was studied when neutralized with sodium hydrogen phosphate solution Na2HPO4. Chemical composition of the sediments was analyzed by XPS and their structure was refined by XRD. The hydroxyapatite formation was confirmed using both XPS and XRD. XRD proves that the two samples contained additional CaH(PO4)•(H2O)2
(brushite), in which calcium changes to uranium of up to 15% in a model solution. When identifying the oxidation state of uranium, the presence of U4+, U5+ и U6+ was noticed with U5+ of up to 30-35 at% from the total amount.
SEM-EDS images of the sediment phases did not allow to determine location of the phases with an uranium content over 18%, for example Ca(UO 2) 2(PO 4) 2·11H 2O (autunite) or other uranium phosphates. Uranium is detected at the grain rims, indicating a sorption nature of its accumulation. Thermodynamic computations showed the possible formation of independent uranium phases, such as ß-UO 2.333, ß-UO 2(OH) 2 and NaUO 2O(OH) (clarkeite), at the measured Eh-pH. The supersaturation of solutions and the spontaneous formation of hydroxyapatite and brushite solid particles lead to a change in the initial Ca/PO 4 ratio, where the P/Ca and Ca/O ratios, as well as the
elemental composition in the near-surface layer of three sediments, are not
strictly constant. Newly formed phase contains isolations different in size,
deformation degree, presence of sorption surface groups, which is reflected in
the mechanism of uptake of uranium compounds. No similar study has been
performed with uranium, and our results demonstrate the need for further
studies on the influence of uranium on apatite crystallization. The stability
of phosphate phases shows the high reliability of phosphate safety barriers for
the uranium extraction facilities, and these processes can be used for
remediation areas of aquifers contaminated with radionuclides |
