The simultaneous removal of cationic and anionic radiotoxic nuclides, such as $Cs^{137}$ and $I^{129}$, from aqueous solutions has been studied. As an adsorbent material, the mixture of activated carbon and chabazite zeolite was used. Chabazite zeolite is selected due to its high selectivity for $Cs^+$, and activated carbon due to its strong affinity for anions. Taking into account release rates of these nuclides from defected fuel rods as well as the inventories of radiotoxic cesium and iodine isotopes at discharge, it would be the most conservative assumption that the concentration of $I^-$ is at most 1% of that of $Cs^+$ in spent fuel storage basin water or low-level liquid wastes.
Based upon this assumption, the mixture of 7:3 activated carbon-chabazite ratio in weight was found optimum for this reference concentration ratio f $Cs^+$ and $I^-$. Using this mixture of activated carbon and chabazite, he amount adsorbed until breakthrough points were estimated to be $0.62 \times 10^{-1}$ mol/kg for $Cs^+$ and $0.58\times 10^{-3}$ mol/kg for $I^-$. This conforms to the mole ratio of them in the feed solution.
In order to investigate the adsorption mechanism and process conditions, equilibrium and kinetic studies as well as the column study were carried out. Surface analyses (SEM-EDAX,EPMA surface image scanning, and XPS) of chabazite and activated carbon show that the adsorption mechanisms of $Cs^+$ and $I^-$ is not due to the formation of any precipitates or crystallines at the surfaces of chabazite and activated carbon. From the equilibrium study, it was noticed hat $Cs^+$ specifically adsorbed onto the activated carbon surface caused a synergy effect on the adsorption of $I^-$. This phenomenon was experimentally verified with the column runs according to the packing types of a fixed bed.
The effect of background electrolytes, namely Na+ and $Cl^-$,on the $Cs^+$ isotherm s minute over the whole range of equilibrium concentration, whereas that on the $I^-$ isotherm gets greater as the equilibrium concentration gets lower. Therefore, the dynamic model for $Cs^+$ is proposed based on the single-component isotherm and surface diffusion, whereas for $I^-$ based on competitive adsorption and surface diffusion. The lumped effective mass-transfer parameters of dedicated nuclides, external film mass transfer coefficients ($k_f$) and the effective intraparticle surface diffusion coefficients ($D_s$), was derived from the experimental concentration histories by an iterative two-parameter search technique predicted on the minimization of the sum of squares of residuals. With estimated $k_f$ and $D_s$, the adsorption behaviors in a batch reactor provided acceptable predictions for each nuclide. The breakthrough behaviors in a column were also predicted satisfactorily by he proposed dynamic models and estimated parameters.
Overall, adsorption characteristics investigated in the dedicated adsorption system will provide the valuable information on the practical implementation f the mixed adsorbent. How-ever, the relatively inferior predictions for aqueous iodine imply that more in-depth studies will be required on its adsorption mechanism and the proposed mass transfer model.
