Sequential extractions versus alpha autoradiography: Mutually complementary for the identification of U- and Ra-bearing minerals in mine tailings

To assess the long-term stability of uranium in mill tailings, the identification of radionuclide (238U and daughters)-bearing minerals is the first step. A combination of sequential extractions and alpha autoradiography (coupled to scanning electron microscopy) seems powerful for this purpose. Tailings coming from uranium mine sites in Gabon are composed mainly of quartz, feldspars, mica/illite, chlorite and kaolinite and many accessory minerals (siderite, Fe and Ti oxides, sulfate- or sulfide-bearing minerals, uraninite, zircon, etc.). Uranium was detected in minerals inherited from the ore, such as uraninite enclosed in quartz or bitumen and zircons (up to 18 wt% of U), and in newly formed minerals, such as siderite (up to 1.7 wt% of U) and Ti and Fe oxides (up to 3.5 wt% U). Sequential extractions indicate that 40 % of U is unextractable, which indicates U is trapped in an efficient way in the tailings, probably in zircon and in uraninite enclosed in quartz grains. U was mainly released during the acetate extraction step targeting the Ti and Fe oxides and siderite and during the H2O2 step that attacked the bitumen, allowing the dissolution of enclosed uraninite. Ba/Pb sulfates, which are well known to incorporate 226Ra through solid solution mechanisms, are identified as the main trap of 226Ra in the studied tailings. Barite, the predominant sulfate, occurs in two populations: an inherited one under grain form and a newly formed one under fibrous habits. Some fibrous habits are sometimes observed at the inherited barite grain surface, corresponding to barite recrystallization. Using alpha autoradiography, amounts of 226Ra ranging from 2 to 30 ppm were estimated in these sulfate minerals (corresponding to specific activities from 70 kBq to 1 MBq per gram of sulfate grain). Barite, the predominant and 226Ra richest sulfate mineral (30 ppm), is quite insoluble. Only the strongest leaching steps partially dissolved it; 40 % of 226Ra remained unextractable, highlighting its low mobility in environmental conditions.