Synchronous Removal Of Heavy Metal Ions And Organic Pollutants In Aqueous Solution By TiO2Nanotubes Photocatalysis:Synergistic Effect And Mechanism

Recently, it has been demonstrated that photocatalysis is a promising technology applicable for the treatment of pollutants. Electron-hole pairs will be consistently generated from semiconducting TiO2 under irradiation, with electron'strong reducing power and the holes'strong oxidizing power, when electron-hole pairs are trapped by the contaminant (e.g., organic pollutants and/or metal ions) on surface, redox reactions will be initiated. Thus, the electron-hole synergy has been used to remove pollutants of oxidation and reduction state of the environment synchronously, becoming a hot topic in catalytic and environmental research. We have done some research work in the following areas, which provides a theoretical basis for the development of using TiO2 nanotubes photocatalysis to remove synchronously toxic organics and heavy metal ions in the wastewater.TiO2 nanotube was successfully synthesized by a hydrothermal method. The TiO2 nanotube photocatalytic material were characterized by using FT-IR spectrum, scanning electron microscope, X-Ray diffraction and BET surface area analysis etc. The results showed that the TiO2 nanotubes prepared by hydrothermal synthesis method are also primarily anatase and contain a small amount of rutile, not changing its crystal form. Compared to nano-TiO2, TiO2 nanotube grain sizes are smaller, while its surface area and pore volume are larger. Furthermore, TiO2 nanotubes are relatively good dispersion properties and can't easily reunion, which proves to prossess excellent photocatalytic activity.The experiments were carried out under the following conditions:magnetic stirrer; initial concentration.2×10-5mol/L; temperature, fixed at 26±0.5℃; TiO2 nanotubes concentration,2.0 g/L; the suspensions 500 mL; aerated at a flow rate of 300 mL/min; the average light intensity, about 8 MW/cm2; under 300W ultraviolet light irradiation. The persent study is targeted on using TiO2 nanotubes photocatalytic degradation of EDTA, citric acid, tartaric acid and DTPA. It was found that the photolysis of organics exsist to a certain extent, but the reaction rate is much faster and more efficient in the case of TiO2 nanotubes. An hour later, it's difficult to detect organics in the system through HPLC. Therefor, TiO2 nanotubes photocatalysis is very suitable for removing the wastewater containing EDTA, citric acid, tartaric acid and DTPA.The experiment using TiO2 nanotube photocatalysis to remove Cu (Ⅱ) and Ag (Ⅰ) was carried out under the same condition. An hour later, the removal efficiency of Cu (Ⅱ) and Ag (Ⅰ) were 83.7%,88.1% respectively. The results show that TiO2 nanotube photocatalysis is effective for removing dissolved transition metal ions from aqueous solution. Cu (Ⅱ) and Ag (Ⅰ) can be reduced to the corresponding metal elemental and its reductive products were characterized by XPS.The experiments were performed under the same condition using aqueous systems containing Cu(Ⅱ) with formic acid, acetic acid, methanol, ethanol and TiO2 nanotubes, respectively. The current work aims to know the correlation between organic standard redox potentials and the reduction rate of Cu(Ⅱ) and how the organics will affect the treatment of Cu(Ⅱ). It's found that not all organics can promote the photocatalytic reduction of metal ions. Carboxylic acids can catalyze the removal of Cu (Ⅱ), while it's not obvious in the case of alcohols. The influence of organic addictives on the reduction rate of Cu(Ⅱ) depends crucially on its potentials. Cu(Ⅱ) reductive product was analyzed by using XPS. The results indicated that organic addictives have no influence on Cu(Ⅱ) reductive product.UV/TiO2 photocatalysis of chelated copper in aqueous solutions has been performed starting from Cu (Ⅱ)-tartaric acid, Cu (Ⅱ)-citric acid, Cu (Ⅱ)-EDTA and Cu (Ⅱ)-DTPA, in the presence of oxygen and at acidic pH. The photocatalytic reaction obeys first-order kinetic equation. The influence of Cu (Ⅱ) on photocatalytic oxidation of organic ligands and how the various organics will affect the treatment of Cu (Ⅱ) were described. The result indicates that the stability constant, the photocatalytic oxidation rate of organic ligands and the solution pH are three major factors controlling the accelerating effect between the reduction of Cu (Ⅱ) and oxidation of organic ligands. It also proves that TiO2 photocatalysis is an effective approach for removing chelated copper from wastewater.The present study is targeted on the simulated wastewater containing Ag(Ⅰ), organics and TiO2 under UV illumination in the presence of oxygen and at acidic pH. The results indicated that photocatalytic oxidation of organics can catalyze the removal of Ag(Ⅰ), and conversely, Ag(Ⅰ) can also catalyze the photocatalytic oxidation of organics. A significant synergistic effect was observed between the reduction of Ag(Ⅰ) and simultaneous oxidation of organics. However, organic complexes with Ag(Ⅰ) would weaken the synergistic effect between Ag(Ⅰ) and organics.