The crystal structures of strontium exchanged sodium titanosilicates in relation to selectivity for nuclear waste treatment

The exchange of Sr2+ into the proton form of the ion exchanger Na2Ti2O3(SiO4)·2H2O was examined by means of X-ray powder diffractometry. This material has a sitinakite framework structure enclosing a tunnel parallel to the c-axis. At the 25% Sr2+ exchange level, the strontium ion was found near an oxo-framework oxygen midway between two adjacent faces of the tetragonal unit cell. It is bonded to five framework oxygen atoms and five water molecules. The Sr coordination number (CN) is reduced to nine in a sitinakite phase for which 16 mol% of the Ti (IV) was replaced by Nb (V). It has been previously shown that at a higher level of Nb substitution (25 mol%), the CN was further reduced to seven. This change in CN is opposite to what is observed for Cs+ where the CN is eight for the Ti phase and 12 for the Ti1.5Nb0.5 phase. The changes in CN are accompanied by a series of changes in the space group to accommodate occupancy of the exchange sites by the cations and water molecules. The selectivity of exchanger is governed by the strength and number of bonds formed by the exchange reactions.