Examination of the use of synthetic Zeolite NaA–X blend as backfill material in a radioactive waste disposal facility: Thermodynamic approach

The underground disposal of radioactive waste is based upon a multibarrier concept. For long-term performance assessment of radioactive repositories, knowledge concerning the sorption of radionuclides in backfill materials is required. As a part of the multibarrier system for effective isolation of radioactive waste in a repository, Zeolite NaA–X blend was prepared from fly ash, characterized, and evaluated to be used as a backfill material. In this concern, the sorption behavior of Cs+ on the prepared material as a function of pH, initial ion concentration and temperature was studied by batch technique. The sorption isotherm data was interpreted by Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models. The application of the Langmuir isotherm yielded monolayer capacity of 1546 mmol/kg at 298 K while the maximum sorption capacity predicted by D–R isotherm was of 2446 mmol/kg. Thermodynamic parameters for the sorption system were determined at three different temperatures. The enthalpy (ΔH°), entropy (ΔS°) and free energy (ΔG°) of sorption at 298 K were found to be 25.43 kJ/mol, 93 J/mol K and −2.316 kJ/mol, respectively. The positive value of ΔH° corresponds to the endothermic nature of the sorption process. The numerical value of ΔG° decreases with an increase in temperature indicating that the sorption was spontaneous and more favorable at higher temperatures.