Leaching of structural Ca²⁺ ions from a chalcogenide adsorbent by H⁺ lifts Cs(I) uptake

Acidic wastewater containing radioactive ¹³⁷Cs is difficult to treat by selective adsorption. Abundant H⁺ under acidic conditions damages the structure of adsorbents and competes with Cs⁺ for adsorption sites. Herein, we designed a novel layered calcium thiostannate (KCaSnS) that contains Ca²⁺ as a dopant. The dopant Ca²⁺ ion is metastable and larger than the ions attempted before. The pristine KCaSnS demonstrated a high Cs⁺ adsorption capacity of 620 mg/g at 8250 mg/L Cs⁺ solution and pH 2, which is 68% higher than that at pH 5.5 (370 mg/g), a trend opposite to all previous studies. The neutral condition allowed the release of Ca²⁺ present only in the interlayer (∼20%); whereas the high acidity facilitated the leaching of Ca²⁺ from the backbone structure (∼80%). The complete structural Ca²⁺ leaching was made possible only by a synergistic interaction of highly concentrated H⁺ and Cs⁺. Doping a large enough ion, such as Ca²⁺, to accommodate Cs⁺ into the Sn-S matrix upon its liberation opens a new way of designing high-performance adsorbents.