Development of titania nanotube films for degradation of the recalcitrant organic pollutants in water

Abstract

This research work was designed to develop titania nanotube (TNT) films as photocatalysts for the destruction of recalcitrant organic pollutants in water and wastewater. In this study different TNT films were prepared by an anodic oxidation process under various experimental conditions. The morphology and structure of the prepared TNT films were examined by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The effects of nanotubular structures including tube length, tube wall thickness, and crystallinity on their photocatalytic activities were investigated with details. Photocatalytic activities of the TNT films were evaluated in terms of the degradation of 2,3-dichlorophenol (2,3-DCP) in aqueous solution under UV light irradiation. The experimental results showed that the large specific surface area and high pore volume, the thin tube wall, and the optimal tube length would be three important factors to achieve good performances. Moreover, the TNT films calcined at 500 oC for 1 h with the higher degree of crystallinity exhibited the higher photocatalytic activities than other TNT films calcined at 300 and 800 oC. These results indicate that the optimization of TiOi nanotube structures is critical to achieve the high performance of photocatalytic reaction. Research efforts were further focused on the effects of reaction factors including dissolved oxygen (DO), pH and anions on the photocatalytic activities of TNT films for the degradation of 2,3-DCP in aqueous solution. The results showed that the rate of 2,3-DCP degradation is a function of DO concentration, since DO acts as an effective electron acceptor to extend the hole's lifetime and to form the oxidizing species of .OH radicals, affecting the photocatalytic activities of the TNT film. The effect of solution pH demonstrated that an alkaline condition is favourable to 2,3-DCP degradation and dechlorination but an acidic condition is more conducive to further mineralization. The effects of individual anions NO3-, Cl-, SO42-, and H2PO4-) indicated that the inhibition degree of photocatalytic degradation of 2,3-DCP caused by these anions can be ranked from high to low as SO42- > Cl- > H2PO4- > NO3-. A comparison study to evaluate the photolytic and photocatalytic degradation of herbicide diphenamid (DPA) in aqueous solution was then investigated. It was found that at pH 7.2 the photolysis system with UVC (254 nm) irradiation alone was quite efficient in degrading DPA with 100% removal after 360 min, but poor in removal of dissolved organic carbon (DOC) only 8%. In contrast, the photocatalysis system with TNT/UVA (350 nm) showed 51% of DPA removal and 11% of DOC removal after 360 min, respectively. These results indicate that although the DPA degradation by the TNT/UVA photocatalysis was much slower than that by the UVC photolysis, the DOC removal was higher than that by photolysis. In the meantime, more than 20 intermediates from DPA degradation were identified by LC-MS and ^-NMR analyses, and the possible pathways of DPA degradation by direct photolysis and photocatalysis were proposed based on the identified intermediate products and the main reaction mechanism. Furthermore, a new type of TNT film, polythiophene sensitized-TNT (PTh/TNT) photocatalyst, responsive to visible light, was successfully synthesized by a two-step electrochemical method. The obtained composite PTh/TNT films were characterized by SEM, XRD, XPS and UV-Vis absorbance. UV-Vis absorbance analysis showed that these composites have a strong photoresponse in the visible region at 500 nm. The prepared PTh/TNT films revealed significant activities for 2,3-DCP degradation under visible light irradiation and also sunlight irradiation. The experiments also confirmed that the side-chains of polythiophene could influence its photocatalytic activities significantly. In summary, this research synthesized the highly ordered TNT films successfully and detailed the effects of nanotube structures on their photocatalytic activities. After the optimization of the TNT structures, effects of DO, pH and anions in aqueous solution on their photoactivities were investigated with details. Furthermore, a new type of polymer-sensitized TNT photocatalyst responsive to visible light was successfully developed. Based on the results obtained in this study, there is better understanding of the photochemical reactions using TNT films as a photocatalyst for the degradation of organic pollutants in water.