Integrating Sulfate with Crown Ether To Construct Supramolecular Radium Traps in an Anionic Zirconium Metal–Organic Framework

Sequestration of radiotoxic ²²⁶Ra²⁺ is challenged by its complex coordination chemistry, which requires a precise molecular recognition strategy. Here, we report a supramolecular trap engineered within a stable zirconium metal-organic framework (ZJU-X102-SO₄) by integrating a size-selective 24-crown-8 ether with a proximal sulfate anchor. The formation of this preorganized trap and its cooperative binding mechanism were elucidated by single-crystal X-ray diffraction, solid-state NMR, XAFS, and DFT calculations. This dual-recognition design was first validated with the Ba²⁺ surrogate, affording rapid kinetics (equilibrium in 30 s) and a high capacity (455 mg g^-1). For the target ²²⁶Ra²⁺, ZJU-X102-SO₄ could remove 70% of ²²⁶Ra²⁺ within 10 min (C₀ = 10 Bq mL^-1) and demonstrate a capacity of 3.3 × 10⁴ Bq g^-1. In high-salinity media, a salting-out effect was observed and ZJU-X102-SO₄ showed a high distribution coefficient of 5.3 × 10⁴ mL g^-1. The sequestration of ²²⁶Ra²⁺ was also effective in 3 M HNO₃. This work presents a molecular engineering strategy for designing sorbents for the targeted sequestration of specific cations from aqueous media.