Inorganic ion exchangers, because of their greater stability against high doses of gamma-radiations and due to various other physico-chemical properties, are the most promising materials for the separation and preconcentration of various radionuclides.
A new ion exchanger potassium copper nickel hexacyanoferrate(II) [KCNF] has been prepared and characterized thoroughly with respect to chemical analysis,. i.e., thermal decomposition, surface measurements and stability under various doses of gamma-radiations.
The adsorption of cesium from aqueous solutions on KCNF has been studied and optimized as a function of equilibration time, pH concentration of acids, salts and adsorbate using batch method and radiometric technique employing cesium-134 tracer. The adsorption of cesium on KCNF has been expressed in terms of percent adsorption and distribution coefficient. The exchange capacity of cesium has been found to be 2.25 m.mol-1 g-1 The equation for the determination of equilibrium constant of cesium adsorption from aqueous solution containing ammonium ions has been developed. The adsorption equilibrium shows the involvement of ion exchange process.
The temperature dependence of cesium adsorption on KCNF under optimum conditions for various cesium concentrations has been studied and various thermodynamic parameters such as Î”Ho, Î”So and Î”Go have been calculated. These values show an endothermic nature of adsorption process.
The uptake of cesium on KCNF at various temperatures has been interpreted in terms of Freundlich, Langmuir and Dobinin-Radushkevich (D-R) equations. various constants involved in the equations have been calculated and interpreted. The heat of adsorption is calculated to be 33.7 kJ mol-1 which agrees to the value calculated earlier.
The average value of mean free energy calculated in the D-R equation for the adsorption of cesium on KCNF is found to be 15.13 kJ mol-1 . This shows that adsorption of cesium on KCNF is an ion exchange process. This has been confirmed by the determination of the amounts of potassium, copper, nickel and iron released, after the adsorption of cesium on KCNF under optimum conditions.
Column experiments have been performed using KCNF in the column and breakthrough of cesium has been achieved under different operating conditions. Low value of cesium capacity of 1.1 m. mol-1 g-1 has been obtained at cesium concentration of 7.5x10-3 M. Desorption of cesium from the column has been achieved (45%) by 8M nitric acid solution at a flow rate of 0.5 cm3 min-1
The kinetic studies of cesium on KCNF have been carried out as a function of particle size and temperature. Two different cesium ion exchange mechanisms have been established by using the interruption test depending on the cesium I concentration in the solution. The data at cesium concentration of 7.5x10-7 M fit to the film diffusion model. Different physical iv parameters such as effective diffusion coefficient and activation energy have been calculated. The corresponding effective diffusion coefficient has been found to be 1.307x10-9 m2 sec-1. The activation energy from Arrhenius plot has been evaluated as 19.3 kJ mol-1 which confirms it to be a film diffusion process.
The ion exchange data at higher cesium concentration have not been interpreted by particle diffusion model. The experimental data have then been fitted to the fast chemical reaction model. The results obey the shell progressive reaction kinetics. The Arrhenius plot gives a straight line over the entire temperature range under investigation. The activation energy calculated by Arrhenius plot has been found to be 68.5 kJ mol-1. This confirms that the mechanism of cesium kinetics at higher concentration is controlled by fast chemical reaction process of shell progressive type.