Rapid and Selective Uptake of Cs+ and Sr2+ Ions by a Layered Thiostannate with Acid–Base and Irradiation Resistances

137Cs+ and 90Sr2+ are highly soluble, highly environmentally mobile, and strongly radiotoxic. Efficiently sequestering 137Cs+ and 90Sr2+ ions from aqueous solutions remains a serious challenge, especially from acidic environments because of the effects of protonation and poor stabilities of materials. Here, efficient removal of Cs+ and Sr2+ ions is achieved by a layered thiostannate, namely [(EtNH3)1.68(Et2NH2)0.32]Sn3S7·0.68H2O (FJSM-SnS-4), which exhibits excellent acid–base and β and γ irradiation resistances. FJSM-SnS-4 possesses high maximum adsorption capacities and rapid kinetics for Cs+ and Sr2+ ions under neutral conditions. It can even capture Cs+ ions under acidic conditions (e.g., pH 1.6 and 0.4). It is interesting that FJSM-SnS-4 has excellent selectivity for Sr2+ ions in neutral environments, whereas it prefers capturing Cs+ from 1 M HCl solutions with coexisting K+, Na+, Ca2+, Mg2+, and Sr2+ ions. The Cs+ and Sr2+ adsorption mechanism has been revealed at the molecular level by single-crystal structural analysis coupled with XPS, EXAFS, and EA characterization, that is, ion exchange between Cs+ and Sr2+ ions and protonated organic amines in interlayer space owing to the flexibility of the current layered thiostannate framework and the strong affinity of the soft basic S2– of the framework for Cs+ and Sr2+ ions.