Isotopic abundance of uranium(IV) and uranium(VI) species in leachants of graptolitic argillites

The effect of graptolitic argillites' 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 (VI) that is most mobile and easily leached, especially when it does not isomorphously enter the mineral crystal structure. According to K. K. Roe and W. C. Burnett [6], uranium-234 (comprised in a mineral system with organic components) should be in a reduced state possessing thereby insignificant mobility. The samples of graptolitic argillites collected in Estonia, were studied. In the sample No. 1947, the content of uranium was 55 ± 5 mg/g; in the sample No. 2296 it was 200 ± 20 mg/g, the organic component made up 15 %- 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(IV) and uranium(VI) were separated. The alfa-sources were prepared by electrolysis from an isopropyl alcohol medium. Alfa-spectrometric measurements were performed with a silicon surface barrier detector. It has been shown that upon action of 11 m/1 HCl solution, the U-234/U-238 isotopic ratio in the uranium(IV) fraction is 1.09 ± 0,03 for the sample No. 1947, and 0.93 ± 0.02 for the sample No. 2296 (Table 2). More than 50 % of the uranium in the shale goes into solution, of uranium(IV) 60-70 %, as can be calculated from formula (4) and is seen in Table 2. Although no special investigation of uranium in kerogene was conducted, it may be assumed that at this stage the radiogenic uranium doesn't reduce at all or the reduction is insignificant and due to other reasons. In contrast, the uranium(VI) 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 presece of at least two different uranium-234(VI) species, one being accociated with mobile uranium and the other with one of the shale's phases. There are two corresponding mechanisms of the enrichment of leachants, one being realized in nature and the other under laboratory conditions.