Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine

The structure of natural uranium-bearing, recently formed products arising from oxidative weathering of a U deposit (Peny, Massif Central, France) was studied to determine mechanisms of U-trapping by natural gels. Sampled waters are close to saturation with respect to amorphous silica and crystalline-hydrated U-hydroxides and silicates. All products are U-bearing Si/Al- and Fe-rich gels with minor Ca, Mg and water. Fourier Transform InfraRed spectrometry (FTIR) and X-ray absorption spectroscopy at Al–K, Fe–K and U–LIII edges show that these gels are composed of intimate mixtures of short-range ordered aluminosilicates and hydrous ferric oxides. In Si/Al-rich gels, Al is 6-fold coordinated to oxygens and Si tetrahedra are mostly isolated as in Al-rich allophane (Al:Si=2). Fe-rich gels exhibit a local structure dominated by edge-sharing octahedra, with d(Fe–Fe)=3.03 Å, likely resulting from hydrolysis and oxidation from Fe2+ solution, and poisoning by Si/Al during Fe3+ precipitation. The local Al environment is close to that measured in Al-substituted goethite, and the Si tetrahedra are poorly polymerized as in natural Si-bearing ferrihydrite. The local structure around U was solved up to 3–4 Å by Extended X-ray Absorption Fine Structure (EXAFS). Uranium is present in the hexavalent oxidized state, as uranyl complexes (UO22+) characterized by two axial oxygen atoms at d(U–Oax)=1.80 Å and four equatorial oxygen atoms, at two distinct distances (d(U–Oeq1=2.33 Å, d(U–Oeq2)=2.48 Å). The collinear Oax–U–Oax geometry explains the presence of multiple scattering (MS) events in the EXAFS spectra of the U-bearing gels. In Si/Al-rich gels, U–U pairings at 3.82 Å are consistent with polymers based on edge-sharing uranyls, in accordance to the speciation calculated in associated solutions. Alternatively to U–Oax MS, the contribution of Si neighbors at 3.3 and 3.7 Å accounts for structural data beyond uranyl polyhedra. It is consistent with the local structure of uranophane, suggesting a coprecipitation process of U and Si. Elements such as Al may have poisoned crystal growth. In contrast, no U–Fe, U–Si/Al nor U–U contributions are evidenced in the Fe-rich gels. Only a weak contribution due to U–Oax MS is present. The geometry of Oeq coordination shell suggests that uranyl is complexed or sorbed onto mineral surfaces. It is proposed that U has been complexed by low Z elements like Si, Al in a first step, then trapped within hydrous ferric oxides during iron precipitation in a second step, i.e., during the final oxidation of the solutions inside the mine galleries.