Exploration of synergic solvent extraction and extraction chromatographic materials for the separation of 161Tb from enriched 160Gd targets

Abstract

One rare earth element (REE) radionuclide of considerable interest for nuclear medicine is ¹⁶¹Tb (t_1/2 = 6.95 day, β^–_max = 593 keV), which can be produced using nuclear reactors through an indirect neutron capture reaction, ${}_{64}^{160}\text{Gd}\left(n,\gamma \right){}_{64}^{161}\text{Gd}\left(t_{1/2}=3.7min,{\beta }^{-}\right){}_{65}^{161}\text{Tb},$ using enriched ¹⁶⁰Gd targets. A significant challenge in implementing ¹⁶¹Tb and new REE radionuclides into radiopharmaceutical agents is the efficient separation of the product and target materials to achieve a high specific activity. This study explored the use of synergic extractant combinations to enhance the extraction and separation efficiencies of Gd(III) and Tb(III). Three neutral organophosphorus extractants, octyl (phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO), dibutyl N,N-diethylcarbamylmethylenephosphonate (DBDECMP), and dihexyl N,N-diethylcarbamylmethylenephosphonate (DHDECMP), were investigated as synergic agents in solvent extraction (SX) when combined with one of two β-diketones, 2-thenoyltrifluoroacetone (HTTA) and 4-benzoyl-3-methyl-1-phenyl-2-pyrazolin-5-one (HP). The separation factors (SF_Tb/Gd) of Tb(III) and Gd(III) for each system are reported as a function of extractant concentration. The system with the highest SF_Tb/Gd was found to be 0.1 M HTTA and 0.1 M DBDECMP at pH 2.00, which displayed a SF_Tb/Gd of 3.7 ± 0.1. This extractant combination was sorbed onto a macroporous support at various extractant ratios using rotary evaporator methods, and their Gd(III) and Tb(III) uptake behavior was characterized. The prepared EXC resins exhibited excellent metal retention in column experiments, demonstrating promising potential for separating Gd(III) and Tb(III), while recovering approximately 96.6 % of ¹⁶¹Tb with <5 % of the initial Gd(III) mass. Thermodynamic studies revealed an endothermic adsorption progress with increased uptake and decreased Gibbs free energy (ΔG) with rising temperature. Overall, this work successfully demonstrated the conversion and characterization of synergic SX systems into novel synergic EXC resins for adjacent REE separations.