Relationship between zeolite structure and capture capability for radioactive cesium and strontium.

Zeolites are widely used for capturing radioactive Cs and Sr, but the important structural factors determining their performance have not been clearly understood. To investigate the structure-property relationship, we prepared thirteen zeolites with various structures and Si/Al ratios. Ion-exchange experiments revealed that Cs exhibited an enhanced affinity to zeolites with high Si/Al ratios, which could be explained by the dielectric theory. Notably, zeolites containing 8-membered ring (8MR) showed extra-high Cs selectivity. Structural analysis using X-ray diffraction proved that Cs with an ionic diameter of 3.6 Å was selectively coordinated within 8MR having a cavity diameter of 3.6-4.1 Å. Such unique size-selective Cs coordination is analogous to ion complexation by macrocyclic organic ligands (e.g., crown ethers). Divalent Sr showed decreasing affinity to zeolites as the Si/Al ratio increased, because of the increasing average Al-Al distance distribution. Sr exchange exhibited an insignificant dependence on zeolite structures due to its strong hydration, which inhibited close interaction with zeolite frameworks. In terms of kinetics, Sr exchange was significantly slower than Cs exchange because of the bulkiness of hydrated Sr ions. Therefore, the micropore channels with large apertures (e.g., 12-membered ring) were beneficial for achieving fast ion-exchange kinetics, especially in the case of Sr.