Development of sulfur encapsulated zeolites for selective removal of radionuclides

Abstract

Up to date, various inorganic ion-exchange materials have been developed to eliminate radioisotope $^{137}Cs^+$ from contaminated water. Recently developed $Cs^+$ ion-exchange materials generally require expensive and complicated synthesis processes while conventional materials like aluminosilicate zeolites exhibit poor $Cs^+$ ion-exchange selectivity in the presence of competing cations. Here, we report a simple and economic strategy to substantially enhance $Cs^+$ ion-exchange selectivity of conventional zeolites. The strategy involves encapsulation of elemental sulfur within micropores of zeolites through vacuum sublimation at the elevated temperature. Several zeolite frameworks (NaA, NaX, chabazite, or mordenite) are selected as hosts for the synthesis of S-zeolites (S refers to sulfur). S-zeolites have exhibited significantly improved $Cs^+$ ion-exchange selectivity in excess competing cations ($Na^+$, $Ca^{2+}$) compared to bare zeolites, and even up to 99% $Cs^+$ removal rate for sulfur-chabazite (S-CHA) and sulfur-mordenite (S-MOR) in the simulated groundwater condition. The enhanced $Cs^+$ ion-exchange selectivity can be explained by Hard Soft Acid Base (HSAB) theory which describes that sulfur, a soft Lewis base interacts more strongly with $Cs^+$, a softer Lewis acid than other hard alkali and alkaline earth metal ions. We believe that the encapsulated sulfur does not provide additional adsorption sites for $Cs^+$ ions but raises $Cs^+$ selectivity by generating additional Lewis acid base interaction between $Cs^+$ and sulfur. Various material characterizations are carried out to prove that the sulfur within zeolites partially donates its pairs of electrons to the adsorbed $Cs^+$ cations. Due to heat tolerance nature of zeolites and improved $Cs^+$ ion-exchange selectivity from the encapsulated sulfur, we expect S-zeolites to be highly promising $Cs^+$ adsorbents in nuclear waste treatment.