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.