Understanding the processes of uranium migration and sequestration is an important issue for the prediction of radionuclide retardation in the vicinity of uranium mine tailings sites or for the safety assessment of potential high-level nuclear waste repositories. Uranium speciation, controlled by biotic and abiotic factors, represents a key parameter for the control of uranium transfer in the environment. This study firstly deals with uranium speciation in opals from the Nopal I uranium deposit (Mexico). Microscopic observations of opals at the nano-scale revealed the occurrence of vorlanite, cubic CaUO4. This was the first time this rare calcium uranate has been found displaying a cubic morphology, in agreement with its crystal structure. Nopal I opals have been further investigated through time-resolved laser fluorescence spectroscopy. The opals spectra and their comparison with those of experimentally produced standards indicate occurrence of mono- or polymeric uranyl complexes (associated or not with calcium or phosphate) sorbed onto internal surface of opal around pH 7-8. Finally, the speciation of uranium was studied in mill tailings from Gunnar (Canada). In the first tailings site, uranium primarily occurs as monomeric, inner-sphere uranyl complexes sharing edges with Fe(O,OH)6 octahedral sites of iron-oxyhydroxides and chlorite. Our results suggested that U(VI) coprecipitates with iron (oxyhydr)oxides predominate in the second tailings sites. Therefore uranium mobility in Gunnar is governed by sorption/desorption and dissolution/(co)precipitation processes
