Vertically Aligned Titanate Nanotubes Hydrothermally Synthesized from Anodized TiO₂ Nanotube Arrays: An Efficient Adsorbent for the Repeatable Recovery of Sr Ions

Vertically aligned titanate nanotubes (VA-TNTs) are prepared for the repeated and effective recovery of Sr ions. Two-electrode electrochemical anodization with a 50 V potential was used to prepare the TiO₂ nanotube arrays on a Ti metal foil. The growth and inner diameters of the TiO₂ nanotubes were measured as a function of the anodization potential and reaction time. VA-TNTs structures are synthesized via one-step alkaline hydrothermal (HT) reaction of the as-prepared anodized TiO₂ nanotube arrays (anodic TiNA). The titanate nanotube synthesis is successfully optimized, revealing that a low calcination temperature and a low HT reaction time allowed for the synthesis of a vertically aligned homogeneously dispersed "Grass"-type morphology among the titanate nanotubes. The equilibrium isotherm and kinetic data are used for model fitting. High-resolution transmission electron microscopy images and X-ray energy dispersive spectroscopy elemental characterizations provided a detailed chemical composition and surface analysis of the VA-TNTs. The recyclability of the VA-TNTs for repeated metal ion adsorption-desorption is demonstrated successfully. The crystallinity of the VA-TNTs after each repeated desorption cycle was dramatically improved by Na treatment. This structural reformation (Na treatment) step increased the number of possible high-yield metal ion recovery cycles. Also, the VA-TNTs have great potential for removing toxic heavy metal ions in an easy, economic, and environmentally friendly way.