Cation Adsorption in TiO₂ Nanotubes: Implication for Water Decontamination

TiO2 nanotubes constitute very promising nanomaterialsfor water decontamination by the removal of cations. We combined arange of experimental techniques from structural analyses to measurementsof the properties of aqueous suspensions of nanotubes, with (i) continuoussolvent modeling and (ii) quantum DFT-based simulations to assessthe adsorption of Cs+ on TiO2 nanotubes andto predict the separation of metal ions. The methodology is set tobe operable under realistic conditions, which, in this case, includethe presence of CO2 that needs to be treated as a substantialcontaminant, both in experiments and in models. The mesoscopic model,based on the Poisson-Boltzmann equation and surface adsorptionequilibrium, predicts that H+ ions are the charge-determiningspecies, while Cs+ ions are in the diffuse layer of theouter surface with a significant contribution only at high concentrationsand high pH. The effect of the size of nanotubes in terms of the polydispersityand the distribution of the inner and outer radii is shown to be athird-order effect that is very small when the nanotube layer is notvery thick (ranging from 1 to 2 nm). Besides, DFT-based moleculardynamics simulations demonstrate that, for protonation, the one-siteand successive association assumption is correct, while, for Cs+ adsorption, the size of the cation is important and the adsorptionsites should be carefully defined.