Optimization of Anodizing Parameters on Photo Decolorization of Textile Dye Solution Using N-Doped Titanium Nanotubes with Response Surface Methodology

Abstract

Titania nanotube arrays are of great interest due to their nanotubular structure and size-dependent properties. Self-organized TiO₂ nanotube arrays could be simply synthesized using anodization method. Many researchers had synthesized TiO₂ nanotube arrays with various pore-sizes and lengths through changing the electrolyte type and concentration, pH, applied voltage, and electrolyte composition. Despite the relatively large bandgap of TiO₂ (3.2 eV), rapid recombination of electron hole generated during photocatalytic reaction has limited the efficiency of the photocatalytic reactions under the visible light. Many researchers have been conducted to improve the photocatalytic efficiency of the TiO₂ nanotube arrays by doping with metal and nonmetal ions. In this work, TiO₂ nanotubes were synthesized using the anodization in the variety conditions such as electrolyte concentration, anodization voltage, and time. The photoreactivity of the TiO₂ nanotube before and after doping was investigated. The morphology, microstructure, photo micro kinetics and photo absorption of the doped–TiO₂nanotube were characterized using SEM/EDX, XRD, and UV–Vis spectrum. The decolorization of C.I. acid red 14 dye (AR 14) by Photocatalytical process based on TNT films was calculated in a batch type photoreactor. Response surface methodology (RSM) was used to recognize single and multi-effects of the main independent parameters (anodizing time, anodizing voltage, N₂ treatment time, and catalyst surface area) on the decomposition rate. A central composite design (CCD) was used to maximize the photoreaction of AR 14 efficacy. A second-order empirical relationship between the response and independent variables was extracted. The results of the analysis of variance (ANOVA) demonstrated a high coefficient of determination value (R²=0.980). Optimum decolorization rate was calculated and experimentally conformed. The optimum anodizing time, anodizing voltage, N₂ treatment time, and catalyst surface area were found to be 200 s, 20 V, 2077 h and 5 cm², respectively. Under the optimal estimated conditions, the highest bleaching efficiency of AR 14 (>99%) was obtained at the lowest reaction time. The results clearly proved that, the response surface methodology could be the suitable method to optimize and scale up the photo catalytically conditions in lab and industrial cases. Also, graphical 2-3 D plots were employed to obtain the optimum point and continuous response.