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

Sodium hexa-titanate (Na(2)Ti(6)O(13)) 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 TiO(6) 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 Sr(2+) and Ba(2+) ions, induces moderate structural deformation of the tunnels due to the stronger electrostatic interactions between di-valent ions Sr(2+) and Ba(2+) and the solid substrate. However, as the size of Ba(2+) ions (0.270 nm) is larger than the minimum width (0.240 nm) of the tunnel, the deformation can lock the Ba(2+) ions in the nanofibers, whereas Sr(2+) ions (0.224 nm) are smaller than the minimum width so the fibers can release the Sr(2+) ions exchanged into the channels instead. Therefore, the hexa-titanate (Na(2)Ti(6)O(13)) 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 Ra(2+) ions, which have a similar size and ion-exchange ability to Ba(2+) ions, from wastewater for safe disposal.