Structural investigation of the efficient capture of Cs+ and Sr2+ by a microporous Cd–Sn–Se ion exchanger constructed from mono-lacunary supertetrahedral clusters

Visualization of the ion exchange mechanism for 137Cs and 90Sr decontamination is important for safe radioactive liquid waste reprocessing and emergency response improvement in the event of a nuclear accident. Here, the remediation of Cs+ and Sr2+ was achieved through ion exchange using a cadmium selenidostannate, [CH3NH3]3[NH4]3Cd4Sn3Se13·3H2O (CdSnSe-1), with rapid exchange kinetics, high β/γ radiation resistances, broad pH durability and facile elution. The framework constructed from mono-lacunary supertetrahedral clusters features a great negative charge density of 3.27 × 10−3 that accounts for the superhigh exchange capacities of 371.4 (Cs+) and 128.4 mg g−1 (Sr2+). Single-crystal structural analysis on the exchanger during the “pristine–ion exchange–elution” cycle supplies instructive information to elucidate the uptake and recycle mechanism for Cs+ and Sr2+. The broken symmetry of the cluster caused by a vacant site, combined with the co-templating effects of mixed methylammonium/ammonium, contributes to the formation of voids I and II that show adsorption activity for both Cs+ and K+ ions. In comparison, the divalent Sr2+ ions with higher hydration degree exchange with (alkyl)ammonium cations in a 1 : 2 molar ratio, resulting in its location at a new void (III) closer to the framework and thus a higher binding strength. The energy variation during the adsorption process based on a DFT calculation illustrates the high efficiency of CdSnSe-1 for capture of both Cs+ and Sr2+. This “visualized” ion exchange underlines the robustness and flexibility of CdSnSe-1 as a Cs+ and Sr2+ trapper, and reveals the deeper structure–function relationship from a new surface interaction viewpoint.