Abstract:
Experimental studies on replacement of Na by Cs and Sr in analcime-albite bearing buffered assemblages were carried out at 250-300 degrees C and P-sat = 39.7-85.8 bars in pure water and in chloride solution. This investigation shows that such ion exchange of Na for Cs and Sr in analcime changes the Si/Al ratio. The compositions of end members for Cs and Sr analcimes at 250 degrees C are shown to be Cs0.95Al0.79Si2.17O6 and Sr0.645Al1.41Si1.62O6. Continuous solid solutions between Cs and Na and Sr and Na do not appear to exist. Based on the composition of the aqueous chloride solutions, the thermodynamic properties of Cs, Sr and Na analcimes were determined and used in thermodynamic simulations in Cs and Sr leaching from albite-(Cs, Sr, Na) analcime assemblages at 250 degrees C. In line with the above, the minimum leachability of Cs and Sr from Cs- and Sr-bearing analcimes occurs at pH values above 8 with an analcime-albite-paragonite buffer. The analcime-albite-paragonite assemblage can, therefore, act as a potential sorption barrier to the migration of Cs and Sr from radionuclide contaminated materials owing to the insolubility of Cs and Sr bearing analcimes. These findings have implications for underground High Level Waste storage facilities designed for maximum safety at moderately elevated temperatures.