Efficient capture of Cs⁺ ions by two supertetrahedral cluster-based microporous metal chalcogenides in neutral and...

The selective capture of ¹³⁷Cs⁺ from complex radioactive liquid waste presents a persistent and critical challenge due to its high solubility and strong mobility, particularly under harsh alkaline environments. Herein, two supertetrahedral cluster-based microporous metal chalcogenides (STC-MMCs) featuring two-fold interpenetrating frameworks, namely (C₂H₈NO)₆In₁₀S₁₈·(C₂H₇NO)·5H₂O (1) and (C₂H₈N)_4.5(C₃H₁₀N)_1.5In₁₀Se₁₈·4H₂O (2) were synthesized. They exhibit high adsorption capacity (q_m^Cs = 286.68 mg/g for 1, q_m^Cs = 276.85 mg/g for 2), rapid kinetics (equilibrium time t^Cs ≤ 5 min) and excellent selectivity and reusability for Cs⁺ capture in neutral environments. Supertetrahedral clusters as nodes and interpenetrating open frameworks confer their structural stability, providing the prerequisite for Cs⁺ capture in alkaline environments. They demonstrate high distribution coefficients (K_d^Cs > 10⁴ mL/g) and removal rates (R^Cs > 80%) even under pH of 12. Unprecedently, they retain excellent selectivity (K_d^Cs > 10³ mL/g) in the alkaline solutions (pH = 11 and 12) despite the presence of competing Na⁺, K⁺, Mg²⁺ and Ca²⁺. Notably, they maintain efficient Cs⁺ uptake in actual alkaline salt lake brine. Specifically, the single-crystal to single-crystal structural transformation for 2 reveals the basic mechanism at the molecular level. The excellent Cs⁺ capture is attributed to ion exchange between Cs⁺ and cations within the channels^, the high flexibility of the interpenetrating open framework with highly negative charge, and the strong interaction of Lewis soft basic sites Se^2- for Cs⁺. This work pioneers the systematic research on metal chalcogenides for radiocesium remediation in alkaline environments and provides new insights for treating alkaline radioactive liquid waste.