Synergistic enhancement of strontium extraction by open-chain polyether ligands in [Cnmim][NTf2] ionic liquids: a combined experimental and theoretical study

Abstract

The efficient separation of strontium from high-level liquid waste is crucial for nuclear waste management. This study investigates the synergistic enhancement of Sr²⁺ extraction using two open-chain polyether ligands, BPht-14 and BPPT-14, by replacing chloroform with imidazolium-based ionic liquids [C_nmim][NTf₂] (n = 4, 6, 8). The effects of pH, contact time, initial concentration, temperature, and IL alkyl chain length on extraction performance were systematically evaluated. The results demonstrate significant enhancement in IL systems, with BPPT-14 achieving 95.2% extraction efficiency in [C₄mim][NTf₂], compared to 84% in CHCl₃. The extraction followed pseudo-second-order kinetics and was spontaneous and exothermic. Selectivity studies revealed improved separation factors for Sr²⁺ over coexisting ions, with Sr/Cs increasing from 1.7 to 6.07. DFT calculations further suggested that the ionic-liquid systems favor Sr²⁺ extraction mainly through a thermodynamically favorable cation-exchange pathway, while differences in ionic-liquid polarity help explain the alkyl-chain-length dependence of extraction performance. This work provides a useful basis for the development of ionic-liquid-based systems for efficient and selective Sr²⁺ separation.