Selective Separation of Re(VII) and Tc(VII) by Xerogel Microcapsules Enclosing TOA Extractant

Special attention has been given to the separation and recovery of VII-group elements, Tc and Re, in relation to the partitioning of high-level liquid waste (HLLW) generated from the nuclear fuel reprocessing process. In this study, a tertiary amine (tri-n-octylamine, TOA), which is effective for the extraction of oxoanions, was encapsulated in Ca and H-types of alginate xerogel polymers (CaALG, HALG). The uptake behaviors of TcO4-and ReO4− (substitute of Tc) in the presence of HNO3 were examined by batch method using TOA-xerogel microcapsules (TOA-CaALG, TOA-HALG). The uptake of TcO4- in the presence of 0.1 M HNO3 was readily attained within 5 h, and a relatively large uptake(%) above 90% was obtained. The uptake(%) of Re(VII) for TOA-CaALG in the presence of 0.01∼0.1 M HNO3 was estimated to be about 90%, while gradually decreasing with HNO3 concentration, indicating that the extraction of HNO3 with TOA became dominant in this process: R3NH+NO3 (o) + ReO4 (aq) ↔ R3NH+ReO4 (o) + NO3 (aq). The order of the uptake(%) for different oxoanions in the presence of 0.01∼5 M HNO3 was Re(VII) > Zr(IV)> Se(VI) > Mo(VI) > Te(VI). The elution study of Tc(VII) revealed 95% and 99% of recovery with 5 M and 7 M HNO3, respectively. The chromatographic separation of Re(VII) from simulated HLLW (28 components of waste solution, SW-11E, JAEA) as well as from mixed solution was accomplished by the stepwise elution techniques using a column packed with TOA-MCs. The Re(VII) ions were effectively eluted with 5 M HNO3, and a relatively large recovery(%) of 98.60% was obtained. Other elements were eluted with H2O and 2 M HNO3. Thus the TOA-xerogel microcapsules are effective for the selective separation of Tc(VII) from HLLW.