Highly Selective Radioactive ¹³⁷Cs⁺ Capture in an Open-Framework Oxysulfide Based on Supertetrahedral Cluster

Removal of 137Cs+, one of the most hazardous radionuclides, from nuclear waste, is a challenging task because it requires simultaneously high capacity and high selectivity. Chalcogenides offer a great opportunity to design and create high-performance 137Cs+ absorbents. We report a new material (InSnOS) with facile ion-exchange properties. The anionic framework is based on corner-shared pseudo-T4 supertetrahedral oxysulfide [In8Sn12O10S34]16– clusters, resulting in the formula [In8Sn12O10S32]12–. The crystal structure features the interpenetration of two independent oxysulfide cluster frameworks which create pincer cavities based on sulfur atoms that prove highly effective for capturing Cs+ ions. The binding mode of the Cs+ ions by the material was determined by a single crystal structure refinement of a fully ion-exchanged single crystal. The structure determinations show that the small pores created by the two interpenetrating frameworks are the optimal size for capturing Cs+. This advantage makes the material very effective for the removal and recovery of 137Cs+ from aqueous solution. This framework shows not only extremely high exchange capacity (qm), 537.7 mg per g of anionic [In8Sn12O10S32]12– framework, ranking it among the best reported Cs+ sorbents, but also superior affinity and selectivity when using complex solutions simulating industrial and nuclear waste conditions.