Highly Efficient and Rapid Cs~+ Uptake by the Layered Metal Sulfide K_(2x)Mn_xSn_(3-x)S_6 (KMS-1)

摘要

The details of the ion-exchange properties of the layered sulfide material K_(2x)Mn_xSn_(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 Mn3~+ 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≈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 ~(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.