Safe trapping of Cs in heat-treated zeolite matrices

Cesium retention performances of two different natural zeolites (an Italian phillipsite-rich tuff and a Mexican erionite- rich tuff), pre-exchanged in Cs form and heat-treated, are compared. After thermal treatment at 1000 C both zeolites satisfactorily retain Cs during back-exchange tests with NaCl solution or by a prolonged contact with distilled water. The different mechanisms of Cs encapsulation are elucidated. The Cs-exchanged phillipsite, a less siliceous and less heat-stable zeolite, tends to form pollucite on heating. Pollucite is a naturally occurring mineral phase which contains and irreversibly traps cesium. The Cs-exchanged erionite tends to form an amorphous phase on heating and Cs is immobilized through glass formation.