Photocatalysis Of Titanium Dioxide Nanoparticles for Drinking Water Disinfection Under Vis-Leds And Sunlight Illumination
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Date
2017-10
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Addis Ababa University
Abstract
Response surface methodology, Box-Behnken experimental design, was applied to investigate and find optimum synthesis parameters for preparing visible-light active nitrogen doped titanium dioxide (N-TiO2) by sol-gel method. Nitrogen to titanium molar ratios, calcination temperature and calcination time have been selected as the study parameters. X-ray diffraction (XRD) crystal phase compositions, Brunauer-Emmett-Teller (BET) specific surface area and visible-light decolorization of methylene blue have been examined as experimental responses.
A total of 15 tests were conducted, and all the samples have demonstrated different photoactivity under visible light. Furthermore, the important synthesis parameters which affect the three selected responses were investigated using the analysis of variance (ANOVA). Calcination temperature was found to be the most significant parameter which has direct influence on the crystal phase compositions, the specific surface area and photoactivity of the synthesized catalysts. The model adequacy test and regression analysis have shown that the results were well fitted with quadratic model equations. Model predictions were in good agreement with experimental data with 96.68 %, 96.88 %, and 96.96 % variability. N/Ti molar ratio of 6, calcination temperature of 400OC and calcination time of 3 h was found to be the optimum condition.
The doped and undoped TiO2 prepared at the optimum condition were characterized using X-ray diffraction (XRD), nitrogen absorption-desorption analysis, Fourier Transformed Infrared spectrometer (FTIR), Raman spectrometer, X-ray photoelectron spectroscopy (XPS), UV/Vis/NIR Spectrophotometer, High-resolution transmission electron microscopy (HR TEM), field emission scanning electron microscope (FE SEM), thermo gravimetric analysis (TGA), differential scanning calorimeter (DSC) and fluorescence spectrophotometer. From all characterization techniques, the results show the successful preparation of the N-doped TiO2.N-doped TiO2 catalyst prepared under the optimum condition was immobilized on the surface glass beads using tetraethylorthosilicate (TEOS) as a binder. Simplified and low temperature sol-gel preparation method was investigated at different concentration of TEOS for better adhesive strength to glass support and photocatalytic decolorization activities on methylene blue. The physical and chemical characteristics of pure N-doped TiO2 and property changes due to addition of TEOS were studied using X-ray diffractometer (XRD), Brunauer Emmett Teller (BET) analyzer, UV-Vis spectrometer, Fourier transform infrared spectroscopy (FT-IR), Field emission scanning electron microscope (FE-SEM) and High-resolution transmission electron microscopy (HR-TEM). The results showed that the addition of TEOS significantly improved the BET surface area, surface hydroxyl group and adhesive strength of the catalyst. However, the photoactivity of the catalyst was suppressed at the higher concentrations of TEOS (20 and 30%). Based on these results 5% TEOS was found to be an optimum value for successful immobilization of N-doped TiO2 on the glass beads. The catalytic activity did not deteriorate, even after repeated application only 6 % loss in its photoactivity at the 5th cycle test.
In the selection and design of appropriate types of photoreactor system configuration in terms of total irradiated surface area of catalyst per unit volume, reaction kinetics and light distribution within the reactor are the most important factors in photocatalysis reaction. Though several photoreactor system configurations has been reported in the literature, most of them have been applied in degradation of hazardous organic compounds. In this thesis work, photocatalytic bacterial inactivation is studied with immobilized N-doped TiO2 using a novel multi stage fixed bed photoreactor. The photoreactor has been developed from N-doped TiO2 nanoparticles immobilized on glass beads using tetraethoxysilicate (TEOS) as a binder with low temperature synthesis route. The reactor was irradiated under visible light emitting diodes (vis-LED) and sun light. The photocatalytic bacterial inactivation efficiency was investigated by varying selected reactor operating parameters such as initial bacterial load (103 to 109 CFU mL-1), TiO2 concentration (0.3 to 0.6 gm), flow rate (40 to 60 mL min-1), and light incident photon flux (2.24 x 1016, 3.36 x 1016, and 5.23 x 1016 photons/s) using E. coli as test organism. The results demonstrated that, the lower percentage of surviving E. coli were found at initial bacterial concentration of 106 CFU mL-1, TiO2 concentration of 0.6 g, flow rate of 40 mL min-1 and light intensity of 5.23 x 1016 photons/s was selected as optimum operating parameters.
The photocatalytic inactivation efficiency was further investigated on different types of Gram-negative (E. coli, S. flexneri and S. typhimurium) and Gram-positive bacteria (B. subtilis and S. aureus) in water. The inactivation log unit reduction for E. coli, S. typhimurium, S. flexneri, S. aureus and B. subtilis were found to be 5.22, 4.69, 4.65, 4.03, and 3.08 in 120 min under vis-LED irradiation and 5.70, 5.05, 4.97, 4.25 and 3.90 in 40 min under sunlight irradiation, respectively. In both cases, the Gram-positive bacteria demonstrated the least log unit reduction. The rate of photocatalytic inactivation, calculated based on Chick-Watson disinfection kinetic model, indicated faster disinfection kinetics in the case of Gram-negative bacteria. The bacterial cell membrane damage, examined on S. aureus, and S. flexneri using field emission scanning electron microscope clearly showed the effects of TiO2 nanoparticle on bacterial cell structure.
Immobilized TiO2 recycling studies were performed using the proposed photoreactor with E. coli as test organism. The photocatalytic bacterial inactivation efficiency was evaluated and compared between the reuse cycles and results show that the log unit reduction of E. coli only decreased by about 0.89 after five-time reuse.
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Keywords
titanium dioxide nanoparticles, drinking water, Photocatalysis, vis-LEDs, sunlight illumination, drinking water disinfection