Uranium uptake by phosphate minerals from Ca-containing technogenic solutions: Experimental study

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. 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⁺ and U⁶⁺ was noticed with U⁵⁺ of up to 30–35 at. % from the total.

SEM-EDS did not allow to determine location of the phases with an uranium content over 14 %, for example Ca(UO₂)₂(PO₄)₂·11H₂O (autunite) or other uranium phosphates. Uranium is detected at the grain rims, indicating a sorption nature of its accumulation. At the same time, thermodynamic computations showed the possible formation of independent uranium phases, such as ß-UO_2.333, ß-UO₂(OH)₂ and NaUO₂O(OH) (clarkeite), at the measured Eh-pH. We believe that the supersaturation of solutions and the spontaneous formation of hydroxyapatite and brushite solid particles led to a change in the initial Ca/PO₄ 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. There have been no similar studies on uranium, and our results demonstrate the need for further studies on the influence of uranium on apatite crystallization. Stability of phosphate phases shows the high efficiency of phosphate safety barriers for the uranium recovery facilities and these processes can be used for the water purification of polluted aquifers.