Uranium speciation in weathered granitic waste rock piles: an XAFS investigation

Investigation of uranium migration in the waste piles of granite rock in the Limousin region of France is vital for developing strategies which address related environmental issues. Despite the fact that the concentration of uranium is far below the lower end of the cut off level in these piles, the large volume of rocks - which measure in the hundreds of metric tons - and their conditions of repository make this type of waste a source of concern for the international community. In this work, X-ray absorption spectroscopy techniques (XAFS) were employed in order to identify the speciation of uranium in the different categories of samples collected from various regions of the rock piles which had undergone 50 years of weathering. The samples, such as weathered granite, arena and technosoils, were studied in order to probe the transformation of the U bearing complex. XANES indicates U(vi) valence with uranyl species in all samples. Using a linear combination analysis and shell fitting approach, distinct speciation of uranium was observed in the different categories of samples. In the weathered rock and arena samples with relics of magmatic U minerals, uranyl phosphates comparable to autunite are shown to be dominantly linked with monodentate PO₄ ^3-. However, the samples collected from technosoils are found to have a mixture of U-phosphate and U-clay minerals (phyllosilicates and silicates). Irrespective of the collection location, all the samples were found to contain U(vi)-oxo species The equatorial O ligands occur as two shells with an average separation of 0.14-0.21 Å. Moreover, all the samples have an Al/Si/P shell around 3.1 Å. A detailed EXAFS curve fit analysis shows that disorder afflicts the entire range of samples which can be attributed to either inhomogeneous binding sites on the disordered clay minerals or to the presence of a mixture of uranium-bearing minerals. XAFS investigations highlight the uranyl overriding forms of U (as U sorbed on clay minerals and secondary uranyl phosphates or silicates) contribute to the retention of U, even in oxidizing conditions known to enhance the mobility of U.