Synthesis, Characterization And Photocatalytic Activity Of Nonmetal Doped Semiconductor Minerals In The Degradation Of Macromolecule Organic Pollutants

Recently, the macromolecular organic pollutants have become a focus problem in the environmental remediation. As an effective advanced treatment technology, Heterogeneous phototcatalysis is a promising and innovative green purification technology. Photocatlaysis technology based on TiO2has provided an effective and promising means for remediation of environmental pollutants in air and water. However, the widespread technological used of TiO2is impaired by its wide band gap (3.2eV) which can only be activated under UV light. Iron and manganese oxide are abundant semiconductor minerals in soil and play a critical role in many chemical and biological processes due to their lower band gap energy. To our best knowledge, little has been done on the photocatalytic degradation of macromolecular organic pollutants with semiconductor minerals as catalyst and the mechanism of photocatalytic on the semiconductor minerals is not clarified. Therefore, considering the unsolved scientific and technological problems of semiconductor minerals and nonmetal doped semiconductor minerals for future application, valuable explorations have been carried out on photocatalytic mechanism, novel preparation methods of visible light photocatalytic materials, and the immobilization of catalysts in this dissertation. The study provides insights into the visible light photocatalytic mechanism, novel preparation methods for nonmetal doped semiconductor minerals photocatalytic materials and theoretical support for the application of nonmetal doped semiconductor minerals. The major research contents and results are listed as follows:(1) Goethite and cryptomelane were successfully synthesized by sol-gel method. In order to evaluate the photocatalytic activity of goethite and cryptomelane, a novel photodegradable polyethylene-goethite (cryptomelane) composite film was prepared by embedding the goethite (cryptomelane) into the commercial polyethylene. The degradation of PE-goethite (cryptomelane) composite films was investigated under ultraviolet and visible light irradiation. The photodegradation activity of the PE plastic was determined by monitoring its weight loss, scanning electron microscopic (SEM) analysis and FT-IR spectroscopy. The results show that the composite films have highly enhanced photodegradation, which decompose much faster and more completed than pure PE film. The main products of photocatalytic degradation of composite film are H2O and small molecular volatile materials with C=O and C-O under the UV light irradiation. The mechanism of the degradation of composite films could be attributed to the photodegradation and photocatalytic degradation of polyethylene. The reaction of polyethylene under UV light irradiation occurred via direct absorption of photons by the polyethylene macromolecule to create exited states, which resulted in some carbonyl and carbonxyl groups produced. The carbonyl and carbonxyl groups could be further destroyed by the reactive oxygen species generated on catalysts surface under the visible light irradiation.The degradation initially on the interface of PE and catalysis and then extended into polymer matrix. The weight loss was not detectable for the composite film and pure PE film under the visible light irradiation, which could be attributed to the stable of polyethylene under the visible light. The reactive oxygen species generated on the goethite and cryptomelane surface might not etch the polymer matrix.(2) Boron modified goethite and cryptomelane was successfully by sol-gel method using boric acid as boron source. Chemical composition analysis, BET, XRD, TEM, XPS, FT-IR and UV/Vis diffuse reflectance characterization were performed. In order to investigate the photocatalytic activity of nonmetal doped semiconductor minerals, a novel photodegradable polyethylene-boron-goethite (cryptomelane) composite film was prepared by embedding the boron doped goethite (cryptomelane) into the commercial polyethylene. The goethite (cryptomelane) catalyst was modified by boron in order to improve its photocatalystic efficiency under the ultraviolet and visible light irradiation. Solid-phase photocatalytic degradation of the PE-B-goethite (cryptomelane) composite film was carried out in an ambient air at room temperature under ultraviolet and visible light irradiation. The results showed boron doping changed the basic propersities of the goethite (cryptomelane), inhibit the grain growth, decreased the crystallize size, increased the BET surface area. UV-vis spectra revealed that the optical absorption edge of B-doped goethite (cryptomelane) was red shifted. Though there was no boron peak detected in the XPS results, the valence structure of other elements have been changed with B-doped. The photo-induced degradation of PE-B-goethite (cryptomelane) composite film was higher than that of pure films and the PE-goethite (cryptomelane) composite film under the UV light irradiation. SEM and FT-IR results showed that the PE-B-goethite (cryptomelane) composite film decompose much faster and more completed than pure PE film and PE-goethite (cryptomelane) composite film. The mechanism of degradation of PE-B-goethite (cryptomelane) composite film could be attributed to lower crystallize size, lager BET surface area, electronic characteristics of boron. There was also no degradation phenomenon occurred on the PE-B-goethite (cryptomelane) composite film, which due to the fewer reactive oxygen species generated on the B-doped goethite and cryptomelane surface under the visible light irradiation.(3) Heterogeneous photocatalytic degradation of microcystin-LR was demonstrated by visible light-activated carbon doped TiO2(C-TiO2) nanoparticles, synthesized by a modified sol-gel route based on the self-assembly technique exploiting oleic acid as pore directing agent and carbon source. The C-TiO2nanoparticles crystallize in anatase phase despite the low calcination temperature of350℃and exhibit a highly porous structure that can be optimized by tuning the concentration of the oleic acid surfactant. The carbon modified nanomaterials exhibited enhanced absorption in the broad visible-light region together with an apparent red shift of the absorption edge leading to an effective indirect bandgap of2.68eV, compared with3.18eV of a reference anatase TiO2. Carbon species were identified by XPS analysis through the formation of both Ti-C and C-O bonds, indicative of carbon substitution for oxygen atoms and the formation of carbonates, respectively. EPR spectroscopy revealed the formation of two carbon related paramagnetic centers in C-TiO2, whose intensity was markedly enhanced under visible light illumination, pointing to the formation of localized states within the anatase band gap, following carbon doping. The photocatalytic activity of C-TiO2nanomaterials was evaluated for the degradation of microcystin-LR (MC-LR) at pH3.0under visible light (λ>420nm) irradiation. The doped materials showed higher MC-LR degradation rate than reference TiO2, behavior that is attributed to the carbon incorporation into the titania lattice. (4) The powder of catalyst has has obvious drawbacks, such as agglomeration and not easy to recover, the possible mobility of suspended TiO2powders in the environment impose a health risk due to the possible toxicity of their nanoscale. Thus, it becomes a challenging to prepare materials immobilized on substrate. In this work, a carbon-based surfactant sol-gel method was employed to synthesize high surface roughness and visible-light-active C-N-TiO2films with borosilicate glass as surstrate. The enhancement of photocatalytic activity of C-N-TiO2films was evaluated for the degradation of the microcystin-LR (MC-LR) under the visible light irradiation in water. The results revealed that the physicochemical properties of the films, such as specific surface area, porosity, crystallite size and pore size distribution could be controlled by the calcination temperature. The higher surface area, smallest crystallite size and narrow pore size distribution were obtained for C-N-codoped TiO2films calcined at400℃, which exhibit very high surface roughness (360nm). UV-vis spectroscopy showed that as-prepared C-N-TiO2films exhibited stronger absorption in the visible light region and a red shift in the band gap transition due to C-N-codoping. C-N-TiO2films effectively degraded MC-LR under visible light compared to reference film. Especially, Similar MC-LR degradation rates under visible light after three cycles revealed high mechanical stability and no irreversible changes of the film during photocatalysis. The enhancement in visible light photocatalytic activities of the C-N-codoped TiO2films was attributed to the synergistic effects of carbon and nitrogen dopants, and high surface roughness of the prepared films.