Application of TiO₂ nanotubes as photocatalysts for decolorization of synthetic dye wastewater

Titanium dioxide (TiO2) nanotubes were synthesized using thin titanium sheets instead of conventional titanium foil by electrochemical anodization under voltages of 20, 30, 40, and 50 V. The higher anodization voltages increased the inner diameter and depth of the TiO2 nanotubes but decreased the wall thickness. The anodization influenced the oxidation of titanium to form TiO2 on the surface of the thin titanium sheets. The TiO2 nanotubes anodized at 50 V achieved the highest decolorization efficiencies of 74.14% and 65.71% for indigo carmine (IC) and reactive black 5 (RB5), respectively, under 180-min UVA irradiation and 4-mu M initial dye concentration. The transformation of dyes into structures with simpler by-products was observed. The kinetics of the process were characterized using the Langmuir-Hinshelwood (L-H) model. The highest specific reaction rates were 1.33 and 0.77 min(-1) W-1 for IC and RB5, respectively. The L-H reaction rate constants were compared with the first-order reaction rate constants.