Study On The Characteristics And Mechanism Of Dye-Sensitized/Activated Visible-Light Photocatalytic Degradation Of Organic Compounds

Traditional dye-sensitized photocatalysis pays more attention to the suspension system with simple catalyst preparation process and high mass transfer efficiency.However,poor stability of sensitizers,lower separation efficiency of photoinduced hole-electron pairs,hard recycling of particles and easy cause of secondary pollution limit its practical application.In order to break through application bottlenecks and further improve the efficiency of dye-sensitized photocatalysis,in this study,the author established a high-stable visible-light photocatalytic system with dye-sensitized nanocrystalline film;constructed a dye-sensitization/persulfate coupling visible-light photocatalytic system with the features of co-absorption for quasi-full visible spectrum and synergistic electron transmission;systematically revealed the characteristics of organics degradation in the photosensitive dyes self-activating persulfate catalytic system.The whole study finally achieves self-decolorization of printing and dyeing wastewater as well as degradation of coexisting organics without catalysts addition,and this provides a“treating waste with waste”new thought for colored wastewater treatment.Main research contents and conclusions of this study are as follows:(1)D35 organic dye sensitizer,with strong photoelectric conversion ability and high stability,was employed for D35-TiO₂ nanocrystalline film preparation;based on this,built a three-electrode photocatalytic system like solar cell working anode.The system efficiently improves visible-light photocatalytic activity of TiO₂,which was investigated by the degradation of a typical endocrine disruptor bis-phenol A(BPA),and its mineralization rate by D35-TiO₂ is 15 times higher than that by TiO₂.Various characterization methods like UV-Vis,XPL,PL were employed to reflect the structure-activity relationship between the properties of D35-TiO₂ nanocrystalline film and its catalytic performance.Different reaction parameters were optimized to achieve higher pollutants degradation efficiency.Produced reactive oxidizing species as well as contaminants degradation pathway were explored to reveal the mechanism of the photocatalytic system.Results show that D35-TiO₂ has wide visible light adsorption range for 400-600 nm,due to the matched band structure,excited D35 dye can efficiently transfer electrons to TiO₂;there are three kinds of reactive oxidizing species generated and·O₂^-plays the most important role in pollutants degradation as well as acute biological toxicity reduction.D35-TiO₂ nanocrystalline film exhibits high stability,which remains ideal performance after multiple recycle.Moreover,film structure benefits for the catalyst separation and recovery from treated water.(2)To further improve the performance of D35-TiO₂,the D35-TiO₂/g-C₃N₄photocatalyst with the features of co-absorption for quasi-full visible spectrum and synergistic electron transmission was synthesized,and a dye-sensitization/persulfate coupling visible-light photocatalytic system was constructed.BPA was also chosen as the modal pollutant to test the performance of coupling system,which is rapidly degraded with 57 times higher rate than in TiO₂ visible-light photocatalytic system.Besides,the coupling system is efficient for various contaminants removal without selectivity.In order to explore the interaction relationship between each component of the composite,catalyst properties like morphology,light absorption and electron transmission were comprehensively characterized.Composition ratios and reaction conditions were optimized to achieve higher pollutants removal efficiency.By the identification of reactive oxidizing species and analysis of pollutants degradation pathway,internal mechanism of the coupling catalytic system was clarified.Results show that,due to the co-absorption ability of D35 and g-C₃N₄,the D35-TiO₂/g-C₃N₄ can absorb visible light with the range of 400-675 nm,closing to the full-visible spectrum(400-760 nm);because of the suited band structure,both of the excited D35 dye and g-C₃N₄ can inject electrons into the conduction band of TiO₂,through the single-electron-reduction process synergistically activate persulfate,subsequently,several kinds of free radicals and non-free radicals dominated by·OH are produced,achieving the ideal mineralization efficiency of pollutants.(3)Based on the principles of dye-sensitized photocatalysis as well as the single-electron-reduction of persulfate mentioned above,the degradation characteristics of organics in photosensitive dyes self-activated persulfate photocatalytic system were investigated.By optimizing the reaction conditions,characterizing the electron transfer process between reactants and analyzing the produced reactive oxidizing species,the persulfate activation basis and reaction mechanism were identified.Finally,a“printing and dyeing wastewater/persulfate”photocatalytic pretreatment process was constructed placed before biological treatment process,creating a new way for pollutants self-degradation without catalyst addition.Results show that,in the mixed dyes self-activated persulfate system,the maximum 100%chroma removal and co-degradation of other organics can be achieved under sunlight,meanwhile,the catalytic performance is hardly affected by the p H change and coexisting inorganic components.In the actual printing and dyeing wastewater/persulfate catalytic system,decolorization rate of the wastewater can reach 54%and the UV₂₅₄ is significantly reduced;the contained substances which have the similar structures as high cyclic aromatic proteins and tryptophan are efficiently degraded,and the acute biological toxicity of the treated sample is markedly weakened.Therefore,the pretreatment process can effectively reduce strikes which may be caused by the toxic substances contained in printing and dyeing wastewater to the biological treatment system,ensure its stable operation and cut the maintenance cost of the sewage treatment system.Significantly,it provides a“treating waste with waste”new route for colored wastewater treatment in industrial production.