Selective separation and recovery of cesium by ammonium tungstophosphate-alginate microcapsules

A fine crystalline ammonium tungstophosphate (AWP) exchanger with high selectivity towards Cs⁺ was encapsulated in biopolymer matrices (calcium alginate, CaALG). The characterization of the AWP-CaALG microcapsule was examined using SEM/WDS, IR and DTA/TG analyses, and the selective separation and recovery of ¹³⁷Cs were examined by the batch and column methods using simulated (SHLLW) and real high-level liquid waste (HLLW). The free energy (ΔG°) of the ion exchange (NH₄⁺ ↔ Cs ⁺) for fine AWP crystals was determined at -13.2 kJ/mol, indicating the high selectivity of AWP towards Cs⁺. Spherical and elastic AWP-CaALG microcapsules (∼700 μm in diameter) were obtained and fine AWP crystals were uniformly immobilized in alginate matrices. Relatively large K_d values of Cs⁺ above 10⁵ cm³/g were obtained in the presence of 10^-3-1 M Ca(NO₃) ₂, resulting in a separation factor of Cs/Rb exceeding 10 ². The irradiated samples (⁶⁰Co, 17.6 kGy) also exhibited large K_d values exceeding 10⁵ cm³/g in the presence of 2.5 M HNO₃. The K_d values in the presence of 0.1-9 M HNO₃ for 67 elements were determined and the order of K _d value was Cs⁺ Rb⁺ > Ag⁺. The breakthrough curve of Cs⁺ had an S-shaped profile, and the breakpoint increased with decreasing flow rate; the breakpoint and breakthrough capacity at a flow rate of 0.35 cm³/min for the column (0.7 g AWP-CaALG) were estimated at 25.2 cm³ and 0.068 mmol/g, respectively. Good breakthrough and elution properties were retained even after thrice-repeated runs. The uptake (%) of Cs⁺ in SHLLW (28 metal components-1.92 M HNO₃, SW-11, JAEA) was estimated at 97%, and the distribution of Cs⁺ and Zr/Ru into the AWP and alginate phases, respectively, were observed by WDS analysis. Further, the selective uptake of ¹³⁷Cs exceeding 99% was confirmed by using real HLLW (FBR "JOYO", JAEA). AWP-CaALG microcapsules are thus effective for the selective separation and recovery of Cs⁺ from HLLWs.