Speciation and mobility of stable chemical elements associated with uranium mineralization
Résumé
Uranium (U) ores extracted in France until 2001 can be associated to trace elements (TE) mineral phases. These TE can be potentially toxic according to their forms and/or contents. Some TE present in waste rock and tailings can be relatively mobile and redistributed in the various reservoirs of the critical zone such as soils and sediments. In the case of dredging lake sediments contaminated by mining inputs, the cocktail effect of TE presence, on the mobility of U, remains poorly understood.For that purpose, sediments of Saint-Clément Lake (Allier, France) were studied because they are potentially affected by two former mining sites: the U mine of the Bois Noirs Limouzat and the Cu-Snmine of Charrier. The geoaccumulation index and enrichment factor, calculated along a sediment core,highlight significant anthropogenic origin contaminations in Cu and Sn, the highest contamination levels resulting from the rehabilitation of the Charrier tailing pile. The objectives are thus to determine theTE’s solid speciation due to sediments re-oxidation and the geochemical processes governing their mobility.SEM, EPMA analyses and chemical extractions were performed to study the TEs’ bearing phases andleaching tests to understand their stability. First results show that the solid speciation is dominated by cassiterite (SnO2) for Sn and by sulfides e.g., chalcopyrite (CuFeS2) and adsorbed on organic matter for Cu. The standardized TCLP leaching test does not highlight Sn in the leachates but shows Cu concentrations up to 1020 µg/l, higher than the European norm for surface water (1,6 µg/l). These first results suggest a low risk of migration of Sn and a potential instability of Cu.