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

Abstract

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.