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Prepartion of Novel Nanometer Sized Metal Oxides for the Treatment of Typical Organic Pollutants in Water

Author: LiuZuo
Tutor: YuHongBing
School: Nankai University
Course: Environmental Engineering
Keywords: Metal Oxides Nanomaterial Photocatalysis Adsorption Methylene Blue (MB) Pentachlorophenol (PCP)
CLC: X52
Type: PhD thesis
Year: 2012
Downloads: 41
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Abstract


With the development of the global society and economy, the environmentalpollution becomes more and more serious. Today, water pollution has become theNO.1killer to the world, causing many negative effects. According to the previousreports, the organic pollutants are the most important pollutants in the waterpollution. As a typical organic contaminant, textile dyes and other industrialdyestuffs are not only aesthetically unpleasant, but are also toxic to some organisms.And it also affects the human health by bio-accumulation. Pentachlorophenol (PCP)and sodium pentachlorophenate (PCP-Na), the most toxic representatives amongchlorophenols, have attracted great attention worldwide and have been listed aspriority pollutants by the U.S. Environmental Protection Agency owning to theirtoxicity, mutagenicity, carcinogenicity, bioaccumulation and endocrine disturbingeffect. Thus, it is urgent to to remove the dye and PCP from water, and it is of greatsignificance for protecting the environment, maintaining the ecological balance, andkeeping the sustainable development.Recently, for treating the organic pollutants in water, many studies have focusedon the adsorption and photocatalytic technology. The adsorption and photocatalysisare the main routes for the removal of such compounds, which have attracted muchattention. However, there are some problems to be solved.For the adsorption method, the dispersion and the adsorption capacity of theadsorbents are challenging. The recovery difficulty and long time required foradsorption equilibriums limit their practical applications. And the use of relativelyexpensive adsorbents such as activated carbons may pose a serious problem from theeconomic standpoint. Moreover, the adsorption mechanism is not yet available forremoving contaminants from water system. Furthermore, the adsorption method ismostly used to treat the heavy metal pollution, rather than the organic pollutants inwater.On the other hand, for the photocatalytic technology, titanium dioxide (TiO2) has been extensively studied as an effective photocatalyst for widespread application.However, there are several disadvantages in using TiO2to purify wastewater. Firstly,the wide band of TiO2(3.2eV) inhibits the utilization of solar energy in thephotocatalytic process. Secondly, the fixation and recovery of nanometer-sizedphotocatalysts are very difficult and tortuous. Therefore, novel materials with narrowband gap should be studied extensively to develop efficient visible-light-activephotocatalysts. For help recovering, it is also worthy to modify the morphologies orphysical properties of the photocatalysts.In our study, several nanometer-sized metal oxides were prepared and werecharacterized by various ways. Many experiments were carried out to remove themethylene blue (MB) and PCP in water. This dissertation mainly contains foursections as follow.1. The a-Fe2O3fibers have been prepared by electrospinning the correspondingsol-gel precursor. The characteristics of the fibers were investigated, andexperiments were conducted to study the formation mechanism of hierarchicalstructures. Photocatalytic degradation of MB in water was carried out underultraviolet (UV) light, showing that the fibers had better efficiency for removing MBthan other catalysts. Only59%of the degradation efficiency was observed after60min of continuous UV light irradiation without catalyst. However, in the presence ofa-Fe2O3fibers with the same UV light irradiation,99.2%of the dye was degraded,showing the excellent photocatalytic activity of a-Fe2O3fibers under UV irradiation.For comparison, the degradation efficiencies of the self-prepared a-Fe2O3nanoparticles and commercial Fe2O3powder were88.2%and80.5%respectively.Moreover, several process parameters have also been studied, which showed that theremoval of MB was influenced by the process parameters, such as the initial dyeconcentration, catalyst amounts, inorganic anions, UV intensity and so on. Finally,the mechanism of the photocatalysis process was analysed based on the UV-visspectra of MB and the energy band of a-Fe2O3.2. Cerium-doped SiO2/TiO2nanostructured fibers were prepared by a facilesol-gel and electrospinning technology. The fibers, with worm-like mesoporousstructure, were as long as several centimeters with diameters of0.4-1.0μm. The fibers were evaluated as efficient photocatalysts to degrade MB aqueous solutionunder simulated sunlight irradiation. In the absence of photocatalysts, thephotodegradation efficiency of MB was38.04%. The removed MB with P25, pureTiO2/SiO2fibers,0.2%Ce-doped TiO2fibers and0.2%Ce-doped TiO2/SiO2fibersunder irradiation for2h were62.18%,64.44%,38.06%and80.16%respectively.The0.2%Ce doping is proved to be the optimal concentration for the doping ofTiO2/SiO2, the highest photodegradation efficiency of which is92.6%. Severaloperative conditions were studied for their further practical application. Thephotodegradation efficiency decreased with the increase in dye concentration. Theoptimal solution pH was found to be equal to11. Negative ions (NO3-, SO42-and Cl-)showed inhibiting effects in MB degradation. The fibers can be easily recycled andtheir photocatalytic activity had good stability. Furthermore, electron acceptors andradical scavengers were used to study the mechanism of the photocatalytic process.We expect the as-prepared fibers to be utilized as promising photocatalysts in the dyeeffluents treatment and other wastewater treatment.3. Novel monodisperse magnetic pompon-like magnetite/chitosan (Fe3O4/CS)composite nanoparticles were successfully synthesized by a solvent-thermal method.The prepared Fe3O4/CS was used to remove PCP from aqueous solution. Themagnetic adsorbents can be well dispersed in the aqueous solution and be easilyseparated from the solution with a magnet after adsorption. The adsorptionequilibrium was achieved quite rapidly (within30min) and the prominent removal ofPCP (91.5%) was obtained at25℃and pH6.5. The PCP removal was stronglypH-dependent, and low concentration of NaCl hardly affected the adsorption. Thenegative values of ΔG and ΔH showed that the adsorption was a spontaneous andexothermic process. The adsorption process follows Langmuir isotherm andpseudo-second-order kinetics model. The adsorption mechanism can be summarizedas complex with electrostatic attraction, hydrogen bonding and π-π interactions. It isexpected that this new monodisperse magnetic Fe3O4/CS has great potential forremoval of contaminants from aqueous media.4. Novel visible-light-induced Ag2CrO4photocatalysts were successfullysynthesized via a facile and energy-efficient microwave-hydrothermal (MH) method. The photocatalysts prepared at different pH display diverse crystal structures andmorphologies. The Ag2CrO4samples prepared at natural conditions (pH=9.5) had thestrongest absorption in the visible light region and had obviously enhancedphotocatalytic activity in PCP-Na degradation. Moreover, the TOC reductiondemonstrated the mineralization of PCP-Na over Ag2CrO4photocatalysts. Severalprocess parameters have also been studied, showing that the removal of PCP wasinfluenced by the process parameters, such as the initial concentration, the initial pH,catalyst amounts, and so on. In addition, the possible photocatalytic mechanism wasproposed based on the energy band positions of the Ag2CrO4and the effects ofradical scavengers.

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CLC: > Environmental science, safety science > Environmental pollution and its prevention > Water pollution and its control
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