Equilibrium and Thermodynamic Studies of Cesium Adsorption on Natural Vermiculite and Optimization of Operation Conditions

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Removal of cesium from synthetic aqueous solution through adsorption on vermiculite, under batch equilibrium experimental condition at six initial values of pH (3, 4, 6, 9, 11 and 12) and five temperatures (25, 50, 75, 85 and 95 °C) has been investigated. It is necessary to propose a suitable model for a better understanding of the mechanism of cesium adsorption on vermiculite. For this propose the suitability of the Langmiur, Freundlich and Redlich-Peterson (R-P) adsorption models for equilibrium data were investigated. The parameters in the adopted adsorption isotherm models were determined by Eviews software. The study of equilibrium isotherm shows that the best model for analysis of experimental data is Redlich-Peterson model with correlation coefficient higher than 0.99(both for temperature and pH). The results showed that increasing of pH and temperature increased the adsorption ability of vermiculite. Optimum conditions for adsorption were determined as T=75 °C, pH=9, vermiculite dose=1.5 g and contact time of 24 hr. Finally the thermodynamic constants of adsorption phenomena, DH° and DS° were found to be 2.672 kJ/mol and 0.563 kJ/mol K in the range of 25-95 °C respectively. The negative value of the Gibbs free energy DG demonstrates the spontaneous nature of cesium adsorption onto vermiculite.