Selectivity for Cs and Sr in Nb-substituted titanosilicate with sitinakite topology

The 25% mobium substituted crystalline titanosilicate with the composition Na1.5Nb0.5Ti1.5O3SiO4.2H(2)O (Nb-TS) was synthesized under hydrothermal conditions. Its selectivity for radioactive Cs-137 and Sr-89 was compared with the TS, Na2Ti2O3SiO4.2H(2)O, having sitinakite topology. The Nb-TS shows significantly higher uptake value for Cs-137 but lower for Sr-89 than the TS. To investigate the origin of selectivity, the ion exchanged Cs+ and Sr2+ forms with the composition, CsxNaHyNb0.5Ti1.5O3SiO4.zH(2)O(x = 0.1, 0.2 and 0.3, x + y = 0.5 and z = 1-2) and Sr0.2Na0.6H0.5Nb0.5Ti1.5O3SiO4 . H2O, respectively, were structurally characterized from the X-ray powder diffraction data using the Rietveld refinement technique. Simultaneously the kinetics of Cs-137 and Sr-89 uptake was investigated for the Nb-v free and doped samples. While the Cs+ and Sr2+ exchanged form of Nb-TS and the Cs+ exchanged form of TS retain the symmetry of the parent compound, the Sr2+ exchanged form of TS undergoes a symmetry change. The differences in the uptake of Cs+ and Sr2+ result from the different coordination environments of cesium and strontium in the eight-ring channel, that result from various hydration sites in the tunnel. The origin of selectivity appears to arise from the higher coordination number of cesium or strontium. Other effects due to Nb-v substitution are reflected in the increase of both, the a- and c-dimensions and thus the unit cell volume, and the population of water vs. Na+ in the channel to charge-balance the Nb5+ <----> Ti4+ substitution. (C) 2003 Elsevier Science (USA). All rights reserved.