(A) study on the characteristics of activated carbon-zeolite mixed adsorbent for removal of aqueous radiotoxic nuclides

Abstract

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 $...