Method and mechanism for efficient radium isolation from bulk thorium based on anion exchange

•An efficient method for the isolation of radium from thorium was reported.•Separation behaviors in batch and column modes were systematically investigated.•Hot test demonstrated that 228Ra and 224Ra was easily separated from 232Th.•Separation mechanism was illustrated successfully by the species distribution. In recent years, targeted-α-nuclide therapy (TAT) has been widely concerned in the world. As one of the few representative TAT nuclides, 212Pb showed good efficacy in cancer treatment; however, the amount remains scarce. Separating 228Ra and 224Ra from 232Th is expected to fundamentally solve this issue, since they both are the parent nuclides of 212Pb and can be used in generators. In this work, an efficient method for trace radium isolation from bulk thorium was proposed and verified through a systematic investigation of the adsorption behaviors of an anion exchanger denoted IRA900 toward thorium and barium/radium. Batch experiments suggested that IRA900 had unique selectivity for thorium in nitric acid solution, with almost no barium adsorbed. The thorium adsorption performance kept improving with increasing concentration of nitric acid and decreasing temperature. The adsorption kinetics and adsorption isotherm could be well fitted by the pseudo-second-order kinetic model and the Langmuir model, respectively. Column experiments suggested that thorium could be effectively immobilized by a column packed with IRA900, while barium was directly penetrated. For desorbing the fixed thorium, 0.1 M HNO3 was optimal. A final hot test demonstrated the successful radium isolation from bulk thorium, with high chemical yield and purity. The selective separation mechanism was attributed to the formation of an anionic [Th(NO3)5]− complex, while barium or radium existed in the cationic form in any nitric acid concentration. This work provides a promising method for separating trace radium from thorium, which will be significant for promoting the large-scale utilization of 212Pb.