Adsorption properties of technetium and rhenium for hybrid microcapsules enclosing Toa extractant

Special attention has been given to the separation and recovery of the 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 oxo anions, was encapsulated in a calcium alginate gel polymer (CaALG), and the adsorption behaviours of TcO4 and ReO4- in the presence of nitric acid and hydrochloric acid were examined by using calcium alginate microcapsules (M Cs) enclosing Toa extractant (Toa-CaALG M Cs). The order of the distribution coefficient Kd for different oxo anions at 0.1 M HNO3 was ReO4-> WO42-> CrO42- ∼ MoO42->> SeO32-. Toa-CaALG still exhibited high uptake ability for ReO4- even after irradiation with 60Co γ-rays (dose: 17.6 kGy). Uptake of TcO4- in the presence of 1 M HNO3 was readily attained within 3 h. Relatively large Kd values above 102 cm3/g were obtained for Toa-CaALG in the presence of 0.01 ∼ 1 M HNO3. All of the TcO4- was successfully adsorbed by Toa-CaALG from the simulated HLLW. The adsorbed TcO4- was then effectively eluted with 5 M or 7 M HNO3 solution. Further, the selective uptake of ReO4- (a chemical analogue of TcO4-) was confirmed by using actual HLLW (Fbr, Joyo, JAEA), and uptake (%) above 99% was obtained. Toa-CaALG was thus effective for the selective separation and recovery of TcO4- and ReO4- from waste solutions containing highly concentrated HNO3 and NaNO3. Microencapsulation techniques with alginate gel polymer can be applied to other ion exchangers and extractants, and the M Cs immobilizing these adsorbents are effective for the advanced separation of various radionuclides, rare metals and toxic elements. (Author)