Zeolites for fixation of cesium and strontium from radwastes by thermal and hydrothermal treatments

Several zeolites, minerals commonly used in radioactive waste decontamination, were heated in air up to 1000 °C for 4 h or heated hydrothermally at 300 °C for 4 weeks under a confining pressure of 300 bar after loading with Cs or Sr to determine the extent and nature of Cs and Sr fixation. Cs-loaded zeolites leached with 0.1 v KC I solution indicated that clinoptilolite, chabazite, erionite, and mordenite fixed only 7% to 26% of their Cs after heat treatment up to 800 °C. However, analcite and phillipsite fixed 84% and 81% of their Cs by heat treatments to only 200 and 500 °C, respectively. The fixation in analcite may be attributed to its dehydration, while in phillipsite the fixation is attributed to its transformation to Cs-feldspar-type phase at 500 to 800 °C. At 1000 °C, all the zeolites seem to be very effective in Cs fixation by forming cesium aluminosilicate minerals. Partially Cs-loaded phillipsites also fixed substantial fractions of their Cs by heat treatment in air as a result of the formation of the feldspar phase. Hydrothermal treatment of Cs-loaded zeolites resulted in the formation of pollucite with phillipsite, erionite, and chabazite and thus fixed Cs. All the Sr-loaded zeolites when leached with KCl were found to fix Sr by thermal treatment at 350 °C and fixed substantial fractions by treatment at 600 °C. For example, at 600 °C clinoptilolite, mordenite and erionite fixed 91%, 93%, and 73%, respectively, of the original Sr present in the samples. Analcite and phillipsite were also very effective in Sr fixation just as they were in Cs fixation. Analcite and phillipsite fixed 86% and 91% of their Sr by heat treatment to 420 and 600 °C, respectively. The fixation of Sr by phillipsite was probably due to its transformation to celsian. Hydrothermal treatment of Sr-loaded phillipsite, clinoptilolite, and mordenite also resulted in substantial Sr fixation. These results show that analcite and phillipsite fix the greatest fractions of Cs and Sr at the lowest temperature of treatment. Between analcite and phillipsite, the latter has the greatest capacity and selectivity for Cs and Sr when used in decontaminating radioactive waste solutions and, therefore, phillipsite may be ideally suited for radioactive waste decontamination followed by thermal or hydrothermal treatment to fix the sorbed Cs and Sr.