Preparation of ferrocyanide molybdate and their selective uptake properties for palladium and cesium ions

The selective separation of heat-generating nuclide (Cs) and platinum group metal (Pd) containing in high-level liquid wastes (HLLW) is an important subject for the advancement of nuclear fuel cycle. Selective uptake of these nuclides was accomplished by using insoluble ferrocyanide molybdates (FeMo-1-4). The uptake properties of Pd²⁺ and Cs⁺ for MoFe 1-4 in the presence of 1 M HNO₃ were examined by batch method. Relatively high uptake percentages of Pd²⁺ and Cs⁺ above 90% were obtained within 30 min. The uptake percentage above 90% was kept in the presence of 0.1-3 M HNO₃. The uptake selectivity of Pd²⁺ was higher than that of Cs⁺; the separation factor of Pd²⁺ to Cs ⁺ increased with coexisting HNO₃ concentration and was estimated to be 15 at 3 M HNO₃. The uptake of Pd²⁺ and Cs⁺ on FeMo-1-4 followed a Langmuir-type adsorption equation, and the uptake capacities of Pd²⁺ and Cs⁺ were estimated to be 0.17-0.28 and 1.68-2.24 mmol/g, respectively. The uptake is mainly governed by the ion-exchange reaction between exchangeable cations (Na⁺; and K⁺) and target cations (Pd²⁺ and Cs²⁺). Further, the selective uptake of Pd²⁺ and Cs⁺ was confirmed by using simulated HLLW (28 components, SW-11, JAEA); the uptake equilibrium attained within 30 min and the uptake percentages of Pd²⁺ and Cs⁺ were 99 and 85 %, respectively. In order to granulate the fine powders of FeMo exchangers, the alginate gel polymer was used as an immobilizing matrix for the microencapsulation. The uptake percentages of Pd²⁺ and Cs⁺ for FeMo-3 microcapsule were above 80% even in the presence of 3 M HNO₃. Thus the molybdate can be converted to the ion-exchanger having high selectivity towards Pd²⁺ and Cs ⁺ in HLLW. This conversion method leads to the volume reduction of wastes and the utilization of useful nuclides.