Synthesis and Evaluation of Cd-MOFs Materials for Adsorption of Simulated Radioisotopes ¹³⁷Cs and ⁹⁰Sr

In medical science and industry field, radioisotopes ⁹⁰Sr and ¹³⁷Cs have extensive utilization, whilst large amounts of them require separating in the vitrification process of high level radioactive liquid waste from spent fuel reprocessing. Through a solvothermal process, anionic metal-organic frameworks, DMA2Cd₃(FDC)₄·2H₂O, have been synthesized, whose structure has been characterized via powder X-ray diffraction, fourier-transform infrared spectroscopy and thermogravimetric analysis. Scanning electron microscope analysis has been carried out to compare the morphologies of the structures before and after the adsorption of Cs and Sr ions, indicating Cd-MOFs stability during the removal process. In addition, fluorescence spectroscopy has evidenced that europium ions have been sensitizated by Cd-MOFs adsorption, enabling the materials to emit fluorescent red. Furthermore, the adsorption experimental studies of Cs and Sr ions adsorption demonstrate that over 50% of adsorption has been achieved within the first 50 min contact time, obtaining the maximum adsorption of Cs⁺ and Sr²⁺, 63.94 mg/g and 53.06 mg/g, respectively. Additionally, kinetic analysis points out that the adsorption of Cs⁺ and Sr²⁺ ions on Cd-MOFs obeys the pseudo-second-order kinetics, suggesting that the rate-determining step is chemisorption. Theoretically, ions’ hydrated radius, helix pitch of the organic frameworks, and their interactions have effects on adsorption capacities and kinetics.