Selective separation of Re(VII) and Tc(VII) by xerogel microcapsules enclosing TOA extractant

摘要

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 TcO_4~-and ReO_4~-(substitute of Tc) in the presence of HNO_3 were examined by batch method using TOA-xerogel microcapsules (TOA-CaALG, TOA-HALG). The uptake of TcO_4~-in the presence of 0.1 M HNO_3 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 HNO_3 was estimated to be about 90%, while gradually decreasing with HNO_3 concentration, indicating that the extraction of HNO_3 with TOA became dominant in this process: R_3NH~+NO_3~-_((o)) + ReO_4~-_((aq))?R_3NH~+ReO_4~-_((o)) + NO_3~-_((aq)). The order of the uptake(%) for different oxoanions in the presence of 0.01 ~5 M HNO_3 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 HNO_3, 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 HNO_3, and a relatively large recovery(%) of 98.60% was obtained. Other elements were eluted with H_2O and 2 M HNO_3. Thus the TOA-xerogel microcapsules are effective for the selective separation of Tc(VII) from HLLW.