Influence of the Templating Amine on the Nanostructure and Charge of Layered Vanadates for Radioactive Wastewater Treatment

Abstract

Theoretical and experimental characterization methods have been combined to shed more light on the potential impact of the various templating amines that they may have on the formation of layered vanadates with a scroll morphology and their cation-exchange capacity. Eight branched- and straight-chain alkylamines were used as the structure-directing agents to make the V2O5 sheets roll up into a scroll structure during microwave-assisted hydrothermal synthesis. The as-synthesized materials were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), and elemental analysis in order to obtain the necessary information on their nanostructure and morphology as a function of the template. On the basis of these experimental results, the density functional theory (DFT) calculations completed with Monte Carlo simulation methods were applied to model the template conformation and the way it had been intercalated into the vanadate layers. It was then postulated that similar structuring mechanisms had been involved in the formation of different hybrid nanotubes with amine molecules forming tilted bilayers of overlapping hydrocarbon tails. In line with what one would expect from the hypothesis that the driving force of vanadate scrolling is due to a partial reduction of V(V) to V(IV) induced by interactions of amine template with the 2D V2O5 layers, the variations in the surface charge density were monitored by studying the cation-exchange capacity of the samples prepared previously by exchanging the amine units for ammonium cations. Then the isotherms for ion exchange of cesium onto such samples from aqueous solutions were measured by the solution depletion method. The use of cesium to probe the surface density of negative charge can also be regarded as viable in view of potential application of vanadate nanotubes in radioactive wastewater treatment.