Sulfur-modified chabazite as a low-cost ion exchanger for the highly selective and simultaneous removal of cesium and strontium

Highlights: • Low-cost ion-exchange materials for the simultaneous removal of Cs⁺ and Sr^2+. • The Cs⁺ selectivity increased as the sulfur loading in chabazite increased. • The Sr²⁺ selectivity increased only with a low content of sulfur in chabazite. • Chabazite with 5 wt% sulfur displayed excellent selectivity for both Cs⁺ and Sr^2+. • Excellent removal percentages of ¹³⁷Cs and ⁹⁰Sr from simulated groundwater. Low-cost inorganic ion-exchange materials with improved Cs⁺ and Sr²⁺ selectivities are of great interest for the treatment of various radioactive liquid wastes. Here the sulfur-modified chabazite (CHA) was formed by the simple encapsulation of elemental sulfur in the micropores of CHA for the cost-effective simultaneous removal of Cs⁺ and Sr²⁺. Our results showed that the Cs⁺ selectivity increased as the sulfur loading increased because of the enhanced Lewis acid − base interaction between sulfur and Cs⁺. On the other hand Sr²⁺ selectivity slightly increased with sulfur loading up to 5 wt% and decreased thereafter. Consequently distribution coefficients (K_d) of CHA containing 5 wt% sulfur (5S-CHA) are 3 × 10⁶ mL/g for Cs⁺ and 2 × 10⁵ mL/g for Sr²⁺ which are higher than those of previously reported Cs⁺ and Sr²⁺ ion-exchange materials. Moreover the 5S-CHA exhibited excellent simultaneous removal performances for ¹³⁷Cs and ⁹⁰Sr in simulated groundwater with K_d values of 9.1 × 10⁴ mL/g for ¹³⁷Cs and 7.8 × 10⁴ mL/g for ⁹⁰Sr which are much higher than or similar to those of commercial CHA and crystalline silicotitanate. Due to the easily scalable procedure using commercially available zeolites and very cheap elemental sulfur 5S-CHA has excellent potential for the treatment of a large volume of ¹³⁷Cs- and ⁹⁰Sr-contaminated water.