Technology Demonstration on the Extraction Chromatography (EC) of Cesium Ion (Cs+) from Acidic High-Level Radioactive Liquid Waste

This study demonstrates the selective separation and recovery of Cs⁺ from high-level radioactive liquid waste (HLW) using 1,3-dioctyloxy-calix[4]arene-18-crown-6-impregnated XAD-7 (CC6/X7) and iso-decyl alcohol (IDA)-modified (CC6-IDA/X7) resins. Both resins were assessed for their adsorption capacity, selectivity, recovery, and reusability. The adsorption capacities of CC6/X7 and CC6-IDA/X7 for Cs⁺ were 56 and 55% from a 4.0 mol·L^–1 HNO₃ solution containing various radioactive and inactive components. CC6-IDA/X7 showed superior recoverability for Cs⁺ (38%) in 0.01 mol·L^–1 of HNO₃ compared to CC6/X7 (15%). Breakthrough and elution of Cs⁺ from CC6-IDA/X7 were evaluated, and reusability was tested over five adsorption–desorption cycles, maintaining >99% recovery in each cycle. The isotopic composition and concentration of Cs isotopes in HLW and product solutions were measured by using a thermal ionization mass spectrometer. Pyrolysis studies on CC6, X7, and CC6-IDA/X7 were conducted by using a simultaneous thermal analyzer (STA) under an argon atmosphere to illustrate spent resin management. The activation energy for the pyrolysis process was estimated by using Friedman and Vyazovkin isoconversional techniques. STA analysis revealed that Cs⁺ remained within the pyro-carbon residue of CC6-IDA/X7. These findings demonstrate that CC6-IDA/X7 is a reusable, Cs⁺-selective extractant with low-temperature pyrolysis of spent resin, offering effective management without Cs⁺ loss in the off-gas. The above findings make CC6-IDA/X7-based extraction chromatography a promising approach for the industrial-scale recovery of Cs⁺ from HLW.