Cesium incorporation and diffusion in cancrinite, sodalite, zeolite, and allophane
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At the US Department of Energy's Hanford site, high level nuclear waste has leaked from underground storage tanks. The waste consists of hyperalkaline solutions, which upon contact with the sediments, caused dissolution of silicate minerals and precipitation of new aluminosilicate minerals. Cancrinite, sodalite, LTA zeolite, and allophane have been identified as the new mineral phases in laboratory simulations. Cesium, the major radionuclide in the waste solutions, may be incorporated into the structural framework of the precipitates. The objectives of this study were to determine the resistance of incorporated Cs+ to ion exchange and the mobility and diffusion coefficient of Cs in the minerals. The minerals were synthesized in solutions mimicking the tank waste and were washed with deionized water. Two sets of experiments were conducted to test the resistance of Cs+ to ion exchange. In the first set, Cs+ was exchanged three times at 80 C by 0.5 N Na+, K+, or Ca2+. The Cs remaining in minerals was quantified after acid digestion. In the second set, we studied the Cs+ desorption kinetics using 0.1 M Na+ as the ion exchanger. Cesium concentration in the solution phase was measured as a function of time for 23 days. Cesium incorporated in sodalite and cancrinite was far more difficult to replace than that in LTA zeolite or allophane. Most of the incorporated Cs+ (94-99%) in LTA zeolite and allophane was readily exchangeable with Na+ or K+; less than 20% of Cs+ in sodalite and <55% of Cs+ in cancrinite could be exchanged. The fraction of desorbed Cs+ was also affected by the exchanging ions; the ion with lowest dehydration energy (K+) was the most effective in replacing Cs+. The ions had to partially dehydrate to access the cages of the minerals. The results of the desorption kinetics experiments showed that Cs+ desorbed quickly from LTA zeolite and the Cs+ diffusion coefficient was close to that in solution; i.e., about 10-9 m2/s. Solid-state NMR analysis also provided evidence for the high mobility of Cs+ in LTA zeolite. Cesium desorption from cancrinite, sodalite, and allophane, however, was slow, suggesting that Cs+ was trapped in cages and channels of these minerals. Effective diffusion coefficients for Cs+ in cancrinite and sodalite were near 10-14 m2/s. 2005 Elsevier Inc. All rights reserved.
