Sorption studies of cesium and barium on magnesite using radiotracer and x-ray photoelectron spectroscopy

Abstract

As the consumption of the radioactive materials is continuously increasing, the problem of disposing the resulting v^^astes safely is becoming more challenging. One way through which these radioactive wastes could be isolated from the biological environment is by disposing them in deep geological formations. Clay minerals are proposed as backfill buffering materials that can delay the migration of the radionuclides and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. Up to now, several studies were carried out to examine the effect of such parameters on the sorption behavior of different radionuclides on various kinds of minerals. This study was conducted to investigate the effects of time, concentration and temperature on the sorption behavior of cesium and barium ions on magnesite. Cesium 137 140 and barium have the radioactive isotopes Cs (t^^^ = 30.1 y) and Ba (t^^^ = 12.8 d) 1/2 produced in high yields during the fission process which are important in radioactive waste considerations. Magnesite is a mineral composed mainly of magnesium carbonate 2+ together with minor amounts of quartz and has a single exchangeable cation. Mg . The radiotracer method and x-ray photoelectron spectroscopy, which is a powerful surface sensitive tool, were used in this study. The results obtained from both methods complemented each others and were in good agreement. Kinetic studies of the sorption process show that equilibrium was approached within one day of contact for both of cesium and barium ions. The data of the sorption of both cations using different concentrations at various temperatures were most adequately described by the Freundlich type isotherms which correspond to multilayer adsorption on heterogeneous surfaces. The values of the Freundlich constants k and n imply that barium ions have slightly larger adsorption affinity and adsorption intensity than cesium ions. The adsorption data at low concentrations were also observed to obey the Dubinin-Radushkevich type isotherms which describe monolayer adsorption on heterogeneous or homogeneous surfaces. The adsorption data were very poorly described by the Langmuir type isotherms. Thermodynamic parameters such as enthalpy change, AH°, entropy change, AS° and free energy change of adsorption, AG°, were calculated from the sorption data of cesium and barium ions at different temperatures. The values obtained for AH° and AS° were -37 kJ/moL, -0.09 kJ/moL.K and -13 kJ/moL, -0.009 kJ/moL.K for cesium and barium ions respectively. The negative AH° values indicate the exothermic nature of adsorption which means that low temperatures are favored. The decrease in entropy upon adsorption implied by the negative AS° values is indicative of the stability of adsorption for both cations. The values of AG° at different temperatures were all negative indicating the spontaneity of the adsorption process for both cesium and barium ions. The magnitudes of AG° were seen to be within the 8-16 kJ/moL range which is the energy range of ion- exchange type processes.