Effect of Solution Conditions and Applied Potential on Ion Transport in TiO2 Nanopores

This study investigated the material and ion transport properties of TiO2 nanopores as a function of solution conditions and applied electrode potentials. Zeta potential measurements revealed that the TiO2 surface charge was highly dependent on solution conditions, which was attributed to protonation/deprotonation of surface functional groups and adsorption of ions. Ion rejection followed the absolute magnitude of the membrane surface charge and was pH-dependent, reflecting the amphoteric nature of TiO2. The rejection of NaCl was approximately symmetrical about the point of zero charge of the membrane, with the highest rejection at acidic and basic conditions. Specific adsorption of SO42- and Mg2+ under acidic and basic conditions, respectively, neutralized the membrane charge and significantly reduced ion rejection. A mathematical transport model was fit to experimental data, and the model-determined membrane charge densities as a function of solution conditions agreed with experimental zeta potential measurements. Model results also revealed that rejection was primarily attributed to the Donnan exclusion mechanism. The application of both anodic and cathodic potentials directly to the TiO2 membrane caused permselective transport under specific solution conditions.