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Description
One of the most important tasks of the nuclear industry is to develop effective liquid radioactive waste treatment technologies with the aim to minimize their amount and immobilizу for future safe storage. Various sorption materials are used to extract radionuclides from aqueous solutions. The products of oil shale processing can be the promising sorbents, as their mineral components are principally clay-marly composition and the fine structure of kerogen distribution in the amount up to 30 % by weight.
Natural oil shale samples of Belarusian origin were processed under various conditions: thermal decomposition, steam treatment, γ-irradiation. Processing conditions for oil shale are presented in the Table 1.
Experimental study was carried out to determine the sorption properties of obtained material towards the model solutions containing radionuclides $^{137}$Cs, $^{60}$Co and $^{85}$Sr (neutral aqueous solution, 0,1 M NaCl and 0,05 M CaCl$_{2}$ solution). Distribution coefficient (K$_{d}$, ml / g) and sorption degree (S, %) are used for practical evaluation of sorption properties of the samples. The phase composition of the sorbents was studied by X-ray analysis. XRD patterns were collected on a DRON-3 powder diffractometer (CuKα radiation, 2θ = 10 – 70 grad). The sorption and texture properties of the sorbents were evaluated from isotherms of low temperature (– 196$^{\circ}$C) physical adsorption–desorption of nitrogen, measured by the volumetric method on an ASAP 2020 MP surface area and porosity analyser (Micromeritics, USA).The resulting values of distribution factor in aqueous solution are presented in the Figure 1.
The best sorption properties towards radionuclide $^{137}$Cs have shale samples 7, 8, 14. No influence of solution composition is shown on $^{137}$Cs sorption: high distribution factors were obtained as for solutions without electrolyte background as for solutions with competing stable cations Na$^{+}$, Ca$^{2+}$.
The effect of competing cations is significant towards radionuclides $^{60}$Co and $^{85}$Sr. The best sorption properties in the neutral aqueous solution (pH = 6.5) have: shale sample № 2 – towards $^{60}$Co; shale sample № 9 - towards $^{85}$Sr.
The specific surface area of sorbents varies between 12 – 29 m$^{2}$/g, the pore volume is 0.033 – 0.097 cm$^{3}$/g, the pore size is 11 – 17 nm.
Values of distribution coefficient (K$_{d}$) for samples of oil shale are 10$^{3}$ – 10$^{4}$. These values are rather high. Finally, based on these data, we can talk about high efficiency of natural sorbents on the basis of oil shale to extract radionuclides from solutions simulating liquid radioactive waste.
