The formation of autunite (Ca(UO2)2(PO4)2nH2O) within the leached layer of dissolving apatite: incorporation mechanism of uranium by apatite

We carried out experiments of U(VI) uptake by fluorapatite (FAp) plate samples with a U(VI) concentration of 4.0×10−4 mol·l−1, at final pHs ranging from 2.3 to 4.0 and at 25 °C to elucidate U(VI) uptake mechanisms. The U(VI) uptake by FAp increased with decreasing pH, which is in contrast with typical cation uptake by minerals where adsorption increases with increasing pH. Although the solutions were undersaturated with respect to autunite, Ca(UO2)2(PO4)2nH2O, a layer of autunite was in the FAp leached layer. If we assumed that pH was 1.3 times higher or U(VI) or P concentration was 40 times higher locally than the respective observed values in the bulk solutions, the local solutions were calculated to be supersaturated with respect to autunite. Our results strongly suggest that autunite formation is caused by local saturation in the leached layer even when a bulk solution is undersaturated with respect to autunite. Rutherford backscattering spectrometry (RBS) revealed that the thickness of the autunite layer increased with decreasing pH or with increasing time, i.e., with increasing dissolution of FAp, and that the concentration of U in the autunite layer was not significantly changed with time at constant pH. These indicate that the autunite layer becomes thicker without increasing the distribution density of autunite in the autunite layer as the reaction proceeds. RBS also revealed that a leached layer of FAp, where FAp was dissolving, was present between fresh FAp and the autunite layer, and characterized by a lower U concentration than that in the autunite layer and by higher and lower Ca concentrations than those in the autunite layer and FAp, respectively. Our results by RBS strongly suggest that the surface mineralization proceeds in the leached layer by local saturation and that autunite formed is finally added to and accumulated in the autunite layer. The surface mineralization in the leached layer explains the concurrent increases in U(VI) uptake, autunite-layer thickness and dissolution of FAp with decreasing pH or with increasing reaction time. Uranium(VI) uptake experiments were separately carried out on FAp powder samples with U(VI) concentrations of 4.0×10−4 and 4.0×10−6 mol·l−1, which resulted in autunite precipitation and U(VI) adsorption, respectively, for the uptake mechanisms. Surface mineralization should be taken into account for remediation and radioactive waste disposal in addition to adsorption and precipitation.