Pressure- and temperature-induced trapping of radionuclides using zeolites: Combined diamond-anvil cell and large-volume press experiments

Zeolite is one of the ideal hosts to absorb and store radionuclides via selective ion-exchange processes. Long-term storage of radionuclides using zeolitic host materials, however, is limited due to the possibility of back-exchange under changing environments. To enhance long-term storage capability of a zeolite, one needs to devise irreversible exchange mechanism for radionuclides or convert the radionuclides-containing zeolites into denser phases. Natrolite (Na16Al16Si24O80, 16H2O) is one of the small-pore zeolites that has recently been shown to have exchange capacity for various alkali and alkaline earth cations but not for trivalent cations under ambient conditions. It is also known to possess such a flexible framework as to absord foreign molecules under pressure by expanding its pores. By combining these unique chemical and structural characteristics of natrolite, we have investigated irreversible cation-exchange mechanism for trivalent cations such as europium and uranium and transformation of cesium- and strontium-exchanged natrolites into denser phases using pressure and heat. High-pressure and temperature experiments were performed in two-folds. First, diamond-anvil cell (DAC) experiments were performed to probe the structural changes of natrolites under various in-situ high-pressure conditions using synchrotron X-ray powder diffraction. Second, a series of ex-situ high-pressure and temperature experiments were conducted using a large-volume press (LVP) to produce recovered samples in suitable quantities to optimize and verify the DAC results. We show here that pressure and heat can induce irreversible exchange of natrolite for trivalent cations and transformation of cesium- and strontium-exchanged natrolites into dense feldpathoid materials.