Book Description

(1) Na2Ti2O3SiO4 ·2H2O(CST). This compound is the most highly selective Cs+ exchanger known and is also selective for Sr2+. The structure of this compound was solved from powder data and found to have a framework structure enclosing unidimensional tunnels. For alkali metals the exchange sites were found to vary depending upon the size of the cation. Li+ and Na+ are small enough to fit into framework sites which accommodate half the required cations. The remainder lie within the tunnels. The larger Cs+ cannot fit into the framework sites and can occupy only half the tunnel sites. Thus, only 25% of the total exchanger sites are available to Cs+. However, the Cs+ -O bonds that form are exactly equal to the sum of the radii for Cs+ and O2-. All other cations have a poorer fit and tend to be displaced by Cs+. In the presence of Na+ dual exchange occurs simultaneously as Na+ fills the framework sites and a second site inside the tunnels. As the sodium ion concentration increases the uptake of Cs+ site decreases to 0.05 - 0.1 meq/g in tank waste simulants. (2) Trisilicates: M(I)2M(IV) Si3O9 ·H2O, (M(I) = Na+, K+, M(IV) = Ti, Sn Zr, Ce). We have prepared a family of trisilicates and solved the crystal structures of three of them. These compounds have framework structures enclosing alternating large and small cavities. The cavity sizes vary with the size of the M(IV) cation. For example, the Ti phase does not take up Cs+ but the Zr phase exhibits very high Kd values for Cs+ (105 ml/g in groundwaters) and is even 1500 ml/g in 6M NaOH. The mixed phase Na2Zr0.75Sn0.25(Si3O9)·H2O shows even higher Kd values. The interchangeability of the M(IV) ion changes the size of the cavities and governs the selectivity. This is an excellent example of crystal engineering where substitutions within the framework change selectivities. (3) Pharmacosiderites: We have prepared a family of compounds based on the pharmacosiderite mineral structure, solved their structures and determined the ion siting. The titanium silicate version, K3H(TiO)4(SiO4)3 · nH2O has a structure that is similar to that of the CST compound. It exhibits a high affinity for both Cs+ and Sr2+.7 The Kd values have been considerably improved by partial substitution of Ge for Ti, another example of controlling selectivity. The structure of the Sr2+ phase is under investigation. Sodium Nonatitanate: This compound, of composition Na4Ti9O20 · nH2O is layered and the interlayer spacing varies with the water content. It is selective for Sr2+ in highly alkaline systems and the strontium is easily eluted with mild acid solutions. It works well under column flow conditions and is stable to irradiation. The main interference is Ca2+. Summary of Additional Studies: (4) We have developed a simple technique to remove Sr2+ from tank wastes that contain high levels of complexants. The scheme is to add a cation that is preferentially complexed and so releases the Sr2+ to the solution that is readily removed with our strontium selective sodium nonatitanate or CST. This really works! (5) We have prepared sodium micas and alumina and zirconia pillared clays that exhibit extremely high Kd values (>105) for Cs+ in contaminated groundwater. They are superior to zeolites for Cs+ removal and may be used as a barrier to Cs+ movement in soils. The sodium mica traps the Cs+ permanently. (6) We have prepared a sodium niobium silicate that appears to have a pyrochlore structure. It is highly selective for Sr2+ in the presence of Ca2+, Mg2+ and Na+.