Highly Efficient and Rapid Cs<sup>+</sup> Uptake by the Layered Metal Sulfide K<sub>2<i>x</i></sub>Mn<sub><i>x</i></sub>Sn<sub>3−<i>x</i></sub>S<sub>6</sub> (KMS-1) ManosManolis J. KanatzidisMercouri G. 2009 The details of the ion-exchange properties of the layered sulfide material K<sub>2<i>x</i></sub>Mn<sub><i>x</i></sub>Sn<sub>3−<i>x</i></sub>S<sub>6</sub> (<i>x</i> = 0.5−0.95) (KMS-1) with Cs<sup>+</sup> and Rb<sup>+</sup> cations are reported. X-ray photoelectron spectroscopy (XPS), elemental analyses, and powder and single-crystal diffraction studies revealed that the Cs<sup>+</sup> and Rb<sup>+</sup> ion exchange of KMS-1 is complete (quantitative replacement of K<sup>+</sup> ions) and topotactic. These data also revealed that the Cs<sup>+</sup> exchange is accompanied with a rare topotactic oxidation of Mn<sup>2+</sup> to Mn<sup>3+</sup> caused by atmospheric oxygen, while the Rb<sup>+</sup> ion exchange only slightly alters the oxidation state of the layer manganese atoms. The absorption of Cs<sup>+</sup> by KMS-1 follows the Langmuir model with a high exchange capacity of 226(4) mg/g (pH ≈ 7) and distribution coefficients as high as 2 × 10<sup>4</sup> mL/g. KMS-1 displays significant cesium uptake both under strongly acidic (pH 0.7−2.6) or basic conditions (pH 10−12). The kinetics of Cs<sup>+</sup> capture by KMS-1 is fast (>90% removal of ∼1 ppm of Cs<sup>+</sup> within only 5 min). KMS-1 was also found capable to efficiently absorb Cs<sup>+</sup> from complex solutions containing various competitive cations in large excess. KMS-1 (containing Mn<sup>3+</sup> ions) can be regenerated and reused for Cs<sup>+</sup> exchange with an exchange capacity very similar to that of the pristine KMS-1. The results indicate that layered metal sulfides with ion-exchange properties may be considered as highly selective and cost-effective sorbents for remediation of water contaminated with the radioactive <sup>137</sup>Cs isotope. The selectivity over other alkali ions for Cs originates not from a size effect but from the more favorable Cs···S soft Lewis acid/Lewis base interactions.