Rates of Exchange of Cs+and Sr2+for Poorly Crystalline Sodium Titanium Silicate (CST) in Nuclear Waste Systems

The compound sodium titanium silicate, popularly known as CST, is highly selective for Cs+. It was synthesized for the purpose of removing 137Cs from basic nuclear waste systems. This compound has a tunnel structure in which the Cs+ ion just fits but diffusion through the tunnels is relatively slow. CST loses its ability to sequester Cs+ in the strongly basic nuclear waste solutions. However, replacement of titanium with 25 mol% of niobium increases the selectivity to a satisfactory level. It has been found that producing a less crystalline form of Nb-CST greatly improves the rate of Cs+ removal. Additionally, the non-niobium CST is selective for strontium both as Sr2+ and Sr(OH)+. It is suggested that both radioisotopes of cesium and strontium may be efficiently extracted by a combination of a mixture of poorly crystalline CST and Nb-CST.