The role of uranium peroxide studtite on the retention of Cs, Sr and Se(VI)

The formation of uranyl secondary solid phases onto the spent nuclear fuel surface might influence the radionuclide concentration in solution via, among others, sorption processes. In this work, the incorporation of some radionuclides onto the uranium peroxide studtite, UO2O2·4H2O, has been tested. The study was carried out in batch experiments where a known amount of studtite (0.05 g) was put in contact with 20 cm3 of radionuclide solution. Once equilibrium was reached, radionuclide concentrations in solution were determined by ICP-MS. The radionuclide amount attached to the solid was calculated from the mass balance. The S/V values of the experiments were also determined from BET specific solid surface area measurements. In this work, data on sorption of caesium, strontium, and selenium as a function of pH are presented. The behaviour of caesium and strontium are similar: a relatively high amount of radionuclide is sorbed at neutral to alkaline pH while there is almost no sorption at acidic pH. On the other hand, in the case of selenium, the sorption maximum occurs at acidic pH and there is almost no sorption at alkaline pH. The different behavior of the radionuclides is related to the element speciation in solution and the surface charge of the solid. Strontium and caesium are sorbed at alkaline pH because they are positively charged in solution and the surface of the studtite is negatively charged (>O- groups) while selenium(VI) sorbs at acidic pH because the surface of the studtite is positively charged, and the predominant selenium(VI) species in solution is anionic. These preliminary data indicate that the sorption capacity of uranyl secondary solid phases such as studtite is an important process to be considered when establishing the migration of different radionuclides released from spent nuclear fuel.