Title: Efficient Cs⁺–Sr²⁺ Separation over a Microporous Silver Selenidostannate Synthesized in Deep Eutectic Solvent

Abstract: Efficient Cs⁺–Sr²⁺ separation, highly desirable for radionuclide recovery in medical and industrial applications, was achieved by the ion exchange technique over a novel microporous silver selenidostannate, [NH₃CH₃]_0.5[NH₂(CH₃)₂]_0.25Ag_1.25SnSe₃ (AgSnSe-1). This material was synthesized in deep eutectic solvent (DES), where the alkylammonium cations play significant structure-directing roles in the construction of micropores that allow for selective ion exchange toward Cs⁺ against Sr²⁺. The much greater Kd^Cs (1.06 × 10⁴ mL g^–1) over Kd^Sr (87.7 mL g^–1) contributes to an outstanding separation factor SF_Cs/Sr of ∼121.4 that is top-ranked among inorganic materials. An ion exchange column filled with AgSnSe-1 exhibits a remarkable separation effect for 10 000 bed volumes of continuous flow, with removal rates of ∼99.9% and ∼0 ± 5.5% for Cs⁺ and Sr²⁺, respectively. AgSnSe-1 exhibits excellent β and γ radiation resistances and a chemical stability over a broad pH range of 1–12. The Se leaching level below the safe guideline value for drinking water highlights the environmental-friendly nature of AgSnSe-1. The high Cs⁺ exchange performance is almost unaffected by Na⁺, Mg²⁺, and Ca²⁺ cations. The Cs⁺-laden product AgSnSe-1Cs can be facilely eluted for recycling use, highlighting the great potential of open framework metal selenides in nuclear waste treatment and renewable energy utilization.