Sorption induced structural deformation of sodium hexa-titanate nanofibers and their ability to selectively trap radioactive Ra(II) ions from water

Sodium hexa-titanate (Na2Ti6O13) nanofibers, which have microporous tunnels, were prepared by heating sodium tri-titanate nanofibers with a layered structure at 573 K. The void section of the tunnels consist of eight linked TiO6 octahedra, having a quasi-rectangular shape and the sodium ions located in these tunnel micropores are exchangeable. The exchange of these sodium ions with divalent cations, such as Sr2+ and Ba2+ ions, induces moderate structural deformation of the tunnels due to the stronger electrostatic interactions between di-valent ions Sr2+ and Ba2+ and the solid substrate. However, as the size of Ba2+ ions (0.270 nm) is larger than the minimum width (0.240 nm) of the tunnel, the deformation can lock the Ba2+ ions in the nanofibers, whereas Sr2+ ions (0.224 nm) are smaller than the minimum width so the fibers can release the Sr2+ ions exchanged into the channels instead. Therefore, the hexa-titanate (Na2Ti6O13) nanofibers display selectivity in trapping large divalent cations, since the deformed tunnels cannot trap smaller cations within the fibers. The fibers can be used to selectively remove radioactive Ra2+ ions, which have a similar size and ion-exchange ability to Ba2+ ions, from wastewater for safe disposal.