Refinement of cesium ion separation via sub-angstrom precision vacancy of lamellar thiostannate

Abstract

The targeted recovery of cesium from high-salinity brines or radioactive wastewater is becoming increasingly vital due to its crucial function in cutting-edge technologies and environmental remediation efforts. Layered sulfides, especially thiostannate, have demonstrated strong selectivity in cesium extraction. Yet, studies on their selective mechanisms remain focused on traditional interlayer mass transfer and affinity coordination. The contribution of inherent structural properties, especially vertical ion transport channels, to selectivity has not been clarified. In this study, we report a novel ammonium-intercalated lamellar thiostannate, (NH₄)₂Sn₃S₇ (NTS), which exhibits remarkable selectivity for Cs⁺ over Na⁺ (separation factor: 608), K⁺ (370), Mg²⁺ (432), Ca²⁺ (823), and Rb⁺ (13) in real brine solutions. The dual ion-selective angstrom-scale channels were revealed via single-crystal structure analysis. Combined with the mechanism analysis of DFT calculation, it is further shown that Sn-vacancy channel (diameter: 4.53 Å) in the vertical direction, rather than the horizontal interlayer one (free-spacing: 3.91 Å), dominated the cation diffusion. The stable framework and matched size work together to accomplish Cs⁺ sieving with sub-angstrom precision. These findings provide an unprecedented insight into the transfer mechanism of layered metal sulfide materials. This work clearly demonstrates that precise sieving for extremely similar ions can be achieved through vacancy-engineered channels in two-dimensional materials.