Highly efficient and rapid Cs ⁺ uptake by the layered metal sulfide K _2xMn _xSn _3-xS ₆ (KMS-1)

The details of the ion-exchange properties of the layered sulfide material K _2xMn _xSn _3-xS ₆ (x = 0.5-0.95) (KMS-1) with Cs ⁺ and Rb ⁺ cations are reported. X-ray photoelectron spectroscopy (XPS), elemental analyses, and powder and single-crystal diffraction studies revealed that the Cs ⁺ and Rb ⁺ ion exchange of KMS-1 is complete (quantitative replacement of K ⁺ ions) and topotactic. These data also revealed that the Cs ⁺ exchange is accompanied with a rare topotactic oxidation of Mn ²⁺ to Mn ³⁺ caused by atmospheric oxygen, while the Rb ⁺ ion exchange only slightly alters the oxidation state of the layer manganese atoms. The absorption of Cs ⁺ by KMS-1 follows the Langmuir model with a high exchange capacity of 226(4) mg/g (pH *ap; 7) and distribution coefficients as high as 2 × 10 ⁴ 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+ capture by KMS-1 is fast (>90% removal of ̃1 ppm of Cs ⁺ within only 5 min). KMS-1 was also found capable to efficiently absorb Cs ⁺ from complex solutions containing various competitive cations in large excess. KMS-1 (containing Mn ³⁺ ions) can be regenerated and reused for Cs ⁺ 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 137Cs 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.