Highly Efficient and Rapid Cs+ Uptake by the Layered Metal Sulfide K2xMnxSn3−xS6 (KMS-1)

Abstract

The details of the ion-exchange properties of the layered sulfide material K(2x)Mn(x)Sn(3-x)S(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 approximately 7) and distribution coefficients as high as 2 x 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 approximately 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 (137)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.