Crystallization of sodium titanium silicate with sitinakite topology: Evolution from the sodium nonatitanate phase

Titanium silicate (TS) with sitinakite topology and composition Na ₂Ti₂O₃SiO₄·2H₂O has received considerable attention because of its high ion-exchange selectivity toward cesium and strontium. In this paper we report the results of the studies of the crystallization process of TS with a combination of ex and in situ experiments. The effects of various parameters, such as gel composition, time, and temperature of the synthesis, on crystallinity and composition of the final product have been investigated. In situ X-ray powder diffraction studies carried out during the synthesis of TS revealed that the process begins with the formation of layered sodium nonatitanate (SNT) with chemical composition Na ₄Ti₉O₂₀·xH₂O, which is also an excellent ion exchanger, selective for strontium and actinides. The pathway of transformation of SNT to the final product is sensitive to alkalinity of the starting gels. At high hydroxide concentration the reaction resulted in the formation of sodium titanium oxide silicate, Na₂TiSiO₅, which has the mineral natisite topology. This study has revealed a pathway for the synthesis of a combined SNT-TS exchanger for removal of Cs, Sr, and actinides by a single-step in-tank process.