Revealing the selectivity source of di-2-ethylhexyl-phosphoric acid for trivalent Gd, Tb, and Dy separation

The outstanding performance of the radioactive nuclide ¹⁶¹Tb in tumor treatment has made it another emerging candidate following ¹⁷⁷Lu. Currently, it is mainly obtained through the irradiation of ¹⁶⁰Gd, with a small amount of ¹⁶¹Dy as a side product. Therefore, the efficient separation of Gd/Tb/Dy is a key technology for achieving large-scale production. The acidic phosphorus extractant P204 is widely used for trivalent lanthanide ion separation and has demonstrated robust performance. However, we do not fully understand its coordination mechanism and selectivity source towards lanthanides until now, which severely restricts improvements to this technique. Therefore, we performed ab initio calculations to explore the coordination properties of P204 with Gd/Tb/Dy and conducted static resin adsorption and liquid-phase extraction separation experiments to contrast with calculation results. It is found that although both the 4f and 5d orbitals participate in the coordination, selectivity is mainly determined by bond interaction between 4f and phosphate oxygens. In addition, we discovered that the O-P-O group has a precise size recognition ability towards lanthanide ions, which allows it to adopt two different coordination modes when extracting light and heavy lanthanide ions. Our results provide a clear understanding of the lanthanide separation mechanism of P204 and have guiding significance to the optimization of trivalent lanthanide separation extractants.