Efficient decontamination of radioactive cesium (137Cs+

) from nuclear wastewater is essential for the sustainable and safe 

use of nuclear power. Here, we report the effective applicability of sulfur-encapsulated mordenite (S-MOR), in which 

sulfur was introduced into the micropore of MOR via simple vacuum sublimation for the efficient removal of Cs+

 under 

high-salinity conditions. Our characterization and Cs+

 ion-exchange property in distilled water revealed that S-NaA with 

10 wt% sulfur (10 S-MOR) is the most suitable S-MOR candidate among various S-MORs because S-MOR prepared by 

vacuum sublimation with more than 15 wt% of sulfur nearly blocks its micropores, resulting in the poor Cs+

 ion-exchange 

performance. The optimized 10 S-MOR demonstrated superior Cs+

 selectivity, faster Cs+

 ion-exchange kinetics, and 

improved maximum adsorption capacity compared to both MOR and the well-known commercial Cs+

 adsorbent, Prussian

blue (PB), under high-salinity conditions including groundwater and seawater. These enhancements were attributed 

to the additional Lewis acid-based interaction between sulfur within the micropore of 10 S-MOR and Cs+

, as interpreted 

by the hard-soft acid-base theory. Due to its excellent kinetics and selectivity for Cs+

 removal, and simple procedure, our 

10 S-MOR holds significant promise for deployment in treating water contaminated with radioactive cesium.