Uranium Speciation in a Chalky Soil

In this article, the speciation and behavior of anthropogenicmetallicuranium deposited on natural soil are approached by combining EXAFS(extended X-ray absorption fine structure) and TRLFS (time-resolvedlaser-induced fluorescence spectroscopy). First, uranium (uranyl)speciation was determined along the vertical profile of the soil andbedrock by linear combination fitting of the EXAFS spectra. It showsthat uranium migration is strongly limited by the sorption reactiononto soil and rock constituents, mainly mineral carbonates and organicmatter. Second, uranium sorption isotherms were established for calcite,chalk, and chalky soil materials along with EXAFS and TRLFS analysis.The presence of at least two adsorption complexes of uranyl onto carbonatematerials (calcite) could be inferred from TRLFS. The first uranyltricarbonate complex has a liebigite-type structure and is dominantfor low loads on the carbonate surface (<10 mgU/kg(rock)). Thesecond uranyl complex is incorporated into the calcite for intermediate(& SIM;10 to 100 mgU/kg(rock)) to high (high: >100 mgU/kg(rock))loads. Finally, the presence of a uranium-humic substance complexin subsurface soil materials was underlined in the EXAFS analysisby the occurrence of both monodentate and bidentate carboxylate (or/andcarbonate) functions and confirmed by sorption isotherms in the presenceof humic acid. This observation is of particular interest since humicsubstances may be mobilized from soil, potentially enhancing uraniummigration under colloidal form. Weexplore the speciation and behavior of anthropogenicmetallic uranium deposited on natural soil by a multi-faceted technicalplatform, which combines EXAFS (extended X-ray absorption fine structure)and TRLFS (time-resolved laser-induced fluorescence spectroscopy).