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Two new zeolitic crystalline phases with stoichiometry,
CS2TiSi6O15, have been discovered.
CSTiSi2O6.5 has a crystal structure isomorphous to
the mineral pollucite, CsAlSi2O6, with Ti+4
replacing Al+3. This replacement requires a mechanism for charge
compensation. A combination of techniques including neutron diffraction,
single crystal x-ray diffraction and x-ray absorption spectroscopy have
revealed that eight extra oxygens are present per unit cell
Cs2TiSi2O6.5 as compared to pollucite.
As a result of the extra oxygen, the titanium coordination geometry is
five-fold. Pentacoordinate titanium and tetrahedral silicon form a network
structure with Cs residing in cages formed by the network. The crystal
structure of Cs2TiSi6O15 is unique, with
titanium octahedra and silicon tetrahedra forming an open framework
structure with the Cs residing in large cavities. The largest covalently
bonded ring opening to the Cs cavities in both compounds are smaller than a
Cs ion, revealing that the Cs ion has minimal mobility in the structure.
Cesium leach rates for both compounds are lower than or comparable to
The radioactivity of the Hanford site waste tanks is primarily from 137Cs and 90Sr, of which can both be selectively removed from solution using a crystalline silicotitanate (CST) ion exchanger. We are currently seeking waste forms alternative to borosilicate glass for Cs-CSTs. In order to obtain a fundamental basis for the development of an alternative waste form, we are investigating synthesis and characterization of CST component phases, namely Cs-Si-Ti-O phases. Two novel Cs-Ti-Si-O phases (one porous, one condensed) have been hydrothermally synthesized, characterized and evaluated as waste form candidates based on chemical and thermal stability, leachability, and ion exchange capabilities.
Crystalline silicotitanate ion exchangers are highly selective for separating Cs from Narich waste streams. However, use of these ion exchangers for removal of Cs from radioactive tank waste will result in large volumes of secondary wastes. Thermal conversion of silicotitanates produces a durable waste form with reduced volumes up to 40%. Leach tests (MCC-l and PCT) have shown that Cs leach rates of IE-91 1-Na (heat treated at 900°C for an hour) are extremely low, ranging from 0.1 to lwt% Cs loss in Cs fraction release, or 10-1 to 10-8g/m2day in normalized Cs mass loss. These are several orders of magnitude lower than that of borosilicate glass. In order to understand the interplay between the structure and high Cs durability, X-ray diffraction, 133Cs NMR, and thermogravimetric analysis have been used to identify phase(s) responsible for trapping Cs in these silicotitanates. Results indicate that Cs is likely to be contained in a crystalline silicate phase.