(419h) Study of the Effects of Physicochemical Properties and Surface Crystalline Structure on the Electrosorption of Metal Ions By Titania Nanotubes

The separation and recovery of metal ions and ionic compounds from aqueous solutions is most challenging due to the stability of these charged species in aqueous environments.

The possibility of exploiting physicochemical properties of ions and specific interactions with separation media to trigger selective electrosorption of ions was explored in this work. The model materials used were titanium dioxide nanotubes, grown via anodization with well-defined diameters and lengths. The model ions used were sodium, lithium, cesium, and calcium in order to explore three physicochemical characteristics that could affect ion separation via electrosorption: (1) ion charge, (2) ion size and (3) ion solvation strength. Two different crystalline structures for the titanium nanotubes were explored: amorphous and anatase, achieved via annealing. Choronoamperometry experiments to quantify electrosorption were performed at applied potential windows between 0 mV and 600 mV, as to prevent the occurrence of any Faradaic reactions in the system. A three-electrode set-up was used, with titania nanotubes acting as the working electrode, a platinum wire acting as the counter electrode and a Ag/AgCl reference electrode.

In contrast with classical theory predictions, electrosorption of ions was achieved not solely in response to electrostatic interactions between ions and electrode surface. Selective separation of ionic species bearing the same charge was achieved most likely via desolvation and specific ion-surface interactions triggered by the difference in crystalline structure of the TiO₂-electrodes used in this work. For example, selective electrosorption of lithium ions over sodium and cesium ions (all monovalent) was achieved by changing the crystalline structure of titanium dioxide nanotubes via annealing.