Isotopic abundance of uranium(IV) and uranium(V I) species in leachant s of graptolitic argillite s

The effect of graptolitic argillite s' organic components on the valency of uranium isotopes and their fractionation has been studied. It is known that the leaching of uranium isotopes is dependent on their oxidation state. It is uranium-234(V I) that is most mobile and easily leached, especially when it does not isomorphously enter the mineral crystal structure. Uranium-234 (comprised in a mineral system with organic components) should be in a reduced state during processing, thereby reducing its mobility. The samples of graptolitic argillite s collected in Estonia, were studied. Uranium leaching was carried out with crushed ground samples of 5-20 g by water, sodium carbonate and hydrochloric acid solutions of various concentrations. After the HCl treatment uranium(I V) and uranium (V I) were separated. The α-sources were prepared by electrolysis from an isopropyl alcohol medium. The α-spectrometric measurements were performed with a silicon surface barrier detector. It has been shown that upon action of 11 m/l HCl solution, the U-234/U-238 isotopic ratio in the uranium (I V) fraction is 1.09± 0.03 for the sample No. 1947, and 0.93 ± for the sample No. 2296. More than 50% of the uranium in the shale goes into solution, of uranium (I V) 60-70%, as can be calculated from formula (4) and is seen in Table 2. In contrast, the uranium(V I) fraction is enriched in the daughter uranium in both water-carbonate or slightly acidic solution and in concentrated hydrochloric acid solutions (Fig. 2; Table 1). This is indicative of the presence of at least two different uranium-234(V I) species, one being associated with mobile uranium and the other with one of the shale' s phases. (author). 15 refs., 2 tabs., 2 figs