Oxide nanotubes formation by anodic process and their application in photochemical reactions for heavy metal removal

Abstract Research focused on synthesis, characterization, and applications of nanomaterials has resulted in nanomaterials with advanced morphologies suitable for the use in environmental remedies either as adsorbent or catalysts to remove pollutions. Among various nanostructures, nanotubular materials are known to have unique properties considering their high surface-to-area ratio and high surface curvature, rendering them to have high surface energy and active sites for absorption and catalytic reaction to ensue. Nanotubes can be produced by anodic process on suitable metal surfaces. For instance, when zirconium or titanium foil is anodized in fluoride electrolyte, the resulting anodic film is often found to consist of ZrO2 or TiO2 nanotube arrays. Anodization is an electrochemical oxidation process that relies on the migration of ions across a solid film for oxide growth to happen. To convert such a barrier-type film to nanotubular arrays, modification of the anodizing electrolyte with fluoride ions is essential. The alignment and aspect ratio of the nanotubes on the other hand are dependent on various other parameters including voltage applied. In this study, several aspects regarding the growth of oxide nanotube arrays on Ti, Zr, and Zr–Ti alloys are presented. Then, the possible applications of the nanotubular anodic film (TiO2, ZrO2, and ZrTiO4) grown on these substrates are described. One application discussed in this chapter is the removal of heavy metal ions, specifically chromium(VI), Cr(VI), by these oxides. Cr(VI) ions have high solubility in aquatic environments, and they can be easily absorbed by aquatic organisms. They are however toxic and carcinogenic; thus their exposure to the environment must be minimized. One way to achieve this is by reducing Cr(VI) to a more benign chromium(III), Cr(III), via a photocatalytic process on illuminated TiO2, ZrO2, and ZrTiO4 nanotubes at point of discharged [as a treatment for wastewater concentrated with Cr(VI) from industrial establishment].