Study On Preparation Of MWCNTs/TiO₂ Composite Photocatalyst And Degradation Of Persistent Organic Pollutants In Water

Persistent organic pollutants such as nitrophenols and dyestuffs wastewater are the main sources of industrial effluent. They are recognized as unmanageable industrial effluent, which can be ascribed to their large quantum, high concentration, complex component, dark chroma, high salinity, strong toxicity, and difficulty by biochemical degradation. Recently, environmental purification using TiO₂ as a photocatalyst under solar irradiation has attracted a great deal of attention due to its non-toxic, inexpensive and highly reactive nature. However, TiO₂ requires ultraviolet light to overcome the large band gap energy (E_g=3.2 eV), which considerably limits the utilization of solar light. The key for the improvement of light using efficiency lies in the breakthrough of the forbidden band width of catalyst and the expansion of the response spectrum to visible light. Taking advantage of the unique electronic properties and strong adsorbent activity of multi-walled carbon nanotubes (MWCNTs), it is expected that the combination of MWCNTs with TiO₂ may induce charge transfer and thus improve the photocatalytic activity of TiO₂. The thesis focused on preparation of MWCNTs/TiO₂ photocatalyst for treating persistent organic pollutants in the simulated wastewater. The research in this paper involves the following parts:(1)Functional material TiO₂ and MWCNTs/TiO₂ photocatalysts were prepared by sol-gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), ultraviolet-visible absorption spectra (UV-vis), Fourier transform infrared spectra (FT-IR) and thermo gravimetric-differential thermal analysis (TG-DTA). The results indicate that the prepared samples were sphericity, anatase phase with mean size of 20 nm. MWCNTs can slightly affect the agglomerated morphology and the particle size of the TiO₂. Compared with TiO₂, the UV-vis maximum absorbance of MWCNTs/TiO₂ shifted obviously to long wavelength.(2)Effective degradation of 2,4-dinitrophenol (DNP), 2,6-dinitro-p-cresol (DNPC) and 2-sec-butyl-4,6-dinitrophenol (DNBP), is possible by solar photocatalysis in the presence of MWCNTs/TiO₂. The MWCNTs/TiO₂ shows higher photoactivity than TiO₂. Strong acid or alkali environment are not available for degradation of dinitrophenols. Kinetic analysis reveals that photocatalytic reaction follows the Langmuir-Hinshelwood model. MWCNTs/TiO₂ exhibits much higher photocatalytic ability at high temperature. An optimal MWCNTs: TiO₂ ratio of was 0.05% (w/w) was found to achieve a higher efficiency on photodegradation of dinitrophenol. Real nitrophenols wastewater was also investigated using MWCNTs/TiO₂ under solar light and satisfying results were obtained. The color and COD removal is almost 100% and 50-65%, respectively.(3) The application of MWCNTs/TiO₂ in the treatment of reactive light yellow K-6G (K-6G) and C.I. Mordant black 7 (MB 7) wastewater were discussed. 31% K-6G and 42% MB 7 were adsorbed on the surface of MWCNTs/TiO₂ in the absence of light. The photodecolorization kinetics follows the pseudo-first order kinetics. An optimal MWCNTs: TiO₂ ratio of was 0.07% (w/w) was found to achieve a higher efficiency on photodecolorization of azo dyes. For a given initial concentration of azo dyes (C_[K-6G]=200 mg/L;C_[MB7]=100 mg/L), at acidic pH conditions, 8.0 g/L and 12.0 g/L of catalyst concentration were suitable for K-6G and MB 7 decolorization, respectively.In addition, the MWCNTs/TiO₂ was easy to recycle for photocatalytic purpose. Therefore, it was hopeful to be used in degradation of organic pollutants under sunlight. It opened an interesting field in tackling environmental issues like degradation of toxic pollution and industrial effluents at nanometer scale.