(A) study on the removal of radionuclides from liquid radioactive waste using pre-formed ferrite method

Abstract

In this study, the feasibility of implementing the ferrite treatment method into the LRWPS has been proved in both theoretical and experimental ways. Currently limited applications of the ferrite process into the LRWPS can be ascribed to the followings : ① deficiency in knowledge about the reaction mechanisms, ② absence of adequate model, and ③ difficulty in removing non-transition metal elements. First of all, the overall reaction mechanisms involved in the $Co^{2+}/FeO· Fe_2O_3$ system were analyzed, and a new concept sorption model named ESCM were developed and proposed by extending the conventional SCM. The validity of the ESCM was verified by using a series of experimental data. As a result, two of the most important parameter values were determined as $log^*K_{Co2+}$ = -2.0 and $log^*K_{CoOH+}$ = -8.0, respectively. The optimal operating condition turned out to be above pH 8. The ion exchange of the $Co^{2+}$ with the structural $Fe^{2+} is dominant below pH 6, the surface complexations of $Co^{2+}$ and $CoOH^{+}$ play important parts in the range of pH 6 to 9, and the slight increase of the removal efficiency above pH 9 can be attributed to the surface or bulk precipitation of $Co^{2+}$ into $Co(OH)_2$. In addition, the removal efficiency of the Co species are hardly affected by the disturbing factors such as coexistence of chelating agent or competing cation, and high ionic strength. By performing a set of experiments with varying the ferrite composition, it turned out that $MnO·Fe_2O_3$ has the highest selectivity toward Sr, which has been known to be scarcely removed by the conventional ferrite process. The validity of the ESCM was reassured by using the experimental data attained from the $Sr^{2+}/MnO·Fe_2O_3$ system. The complexation constants $log^{*}K_{Sr2+}$ and $log^{*}K_{SrOH+}$ have the values of -0.5 and -13.0, respectively. The theoretical maximum Sr-removing capacity of $MnO·Fe_2O_3$ is about $0.28g·g^{-1}$. However, it turned out that t...