Study on the sorption and desorption behavior of La³⁺ and Bi³⁺ by bis(2-ethylhexyl)phosphate modified activated carbon

The separation of ²¹³Bi from its parent radionuclide ²²⁵Ac via radionuclide generators has proven to be a challenge due to the limited performance of the current sorbents. This study evaluated the separation performance of La³⁺ (as a surrogate for ²²⁵Ac) and Bi³⁺ using bis(2-ethylhexyl)phosphate monofunctionalized activated carbon (HDEHP/AC). The potential applications of phosphate groups as active sites and the carbon structure as a sorbent support were confirmed and validated. Various factors, including pH values, salt concentration, halide ions, contact time, solid-to-liquid ratio, initial La³⁺/Bi³⁺ concentration, and gamma irradiation were examined through batch sorption experiments in both single and binary systems. HDEHP/AC had a high sorption capacity for La³⁺via electrostatic attraction, with the sorption data fitting well to the pseudo-second-order kinetic equation and Langmuir model. The sorption performance of La³⁺ on HDEHP/AC was minimally affected as the NaCl/NaI concentrations increased at pH = 2, whereas the sorption capacity for Bi³⁺ decreased significantly. Additionally, selective desorption of La³⁺ and Bi³⁺ was achieved using HNO₃ and NaI solutions, respectively. These results backed up by a conceptual separation process point toward a potential use of these materials in a direct/inverse ²²⁵Ac/²¹³Bi radionuclide generator. Further optimization of the material and separation process will be required to bring this class of promising materials into an actual generator for medical applications.