Investigation Of Characteristics And Mechanisms Of Nano Photocatalytic Stripping Of Reactive Dyes On Cotton Fabric

Photocatalytic color stripping is a method to amend the shade defects on fabric during the process of dyeing.Compared with conventional color stripping methods,photocatalytic color stripping has the advantages of simplity,short process,high efficiency,environmental protection and no pollution.Therefore,it is a great significance to develop a green and environmentally-friendly photocatalytic color stripping method for energy saving and emission-reduction production for textile industry.In this thesis,the photocatalytic color stripping of cotton fabric dyed with different structure reactive dyes was investigated by employing UV/nano-TiO2 system.The influences of pH value in stripping solution,concentration of TiO2,stripping temperature and stripping time on the photocatalytic color stripping performances of cotton fabric dyed with different structure reactive dyes were discussed during the process of photocatalytic color stripping by UV/nano-TiO2.The photocatalytic stripping process of cotton fabric dyed with different structures reactive dyes was optimized according to the photocatalytic color stripping properties.Simultaneously,the photocatalytic color stripping kinetic characteristics of cotton fabric dyed with different concentrations of reactive dyes were investigated with the optimum color stripping process for cotton fabric dyed with different structure reactive dyes,respectively.Furthmore,the effects of different capture agents or accelerants on the active species which generated during the photocatalytic reaction of nano-TiO2 were investigated by taking methylene blue as examples.The photocatalytic color stripping reaction mechanism for cotton fabric dyed with Reactive Red X-3B with monoazo structure was investigated based on the photocatalytic oxidation mechanism of nano-TiO2.Finally,the influence of photocatalytic color stripping with nano-TiO2 on the structure and properties of cotton fiber dyed with Reactive red X-3B was investigated by employing the Fourier transform infrared spectroscopy(FT-IR),X-ray powder diffraction(XRD)and thermogravimetric-differential thermogravimetric(TG-DTG).The results show that the pH value of the stripping liquid,concentration of TiO2,stripping temperature and stripping time had certain impacts on the photocatalytic color stripping for cotton fabric dyed with different structures reactive dyes.According to the classification of dye structure,the photocatalytic color stripping effects of cotton fabric dyed with azo-typed reactive dyes were better than those of anthraquinone-typed reactive dyes.Moreover,for the same type reactive dyes,the simpler molecular structure,the better photocatalytic color stripping effect on cotton fabric could be obtained.While the more complex molecular structure,the poorer photocatalytic color stripping effect on cotton fabric could be encountered.The investigation of the photocatalytic color stripping kinetics of reactive dyes with different structure on cotton fabric shows that the photocatalytic stripping kinetic reaction of reactive dyes with different structure conforms to the first order reaction characteristics,and the photocatalytic color stripping reaction is first order reaction.In the process of photocatalytic reaction of nano-TiO2,the dominant species were the strong oxidizing active ones which generated in the photocatalytic process,such as hydroxyl and superoxide radicals.In the process of reaction,the agent of tert-butanol can capture hydroxyl radicals,and benzoquinone can capture superoxide free radicals,while the accelerant of hydrogen peroxide can accelerate the whole photocatalytic reaction process.During the investigation of the photocatalytic color stripping mechanism for cotton fabric dyed with Reactive red X-3B involving monoazo structure,shows that the strong oxidizing free radicals(such as hydroxyl free radicals,etc.)which generated during the photocatalytic color stripping process,diffused to the surface and/or the internal of cotton fiber with water molecules,attacked the conjugated systems or chromogenic groups with high electron cloud density in the molecular structure of dyestuff on cotton fiber,oxidized the conjugated systems or chromogenic groups,and then destroyed the molecular structure of dyestuff to achieve the purpose of color stripping.The structure and properties of cotton fiber have been changed after the photocatalytic color stripping reaction by nano-TiO2.The results of FT-IR show that the content of hydroxyl groups in the macromolecule chain of cotton fiber increased over stripping time,which accounting for a shift to low wavenumber of the stretching vibration absorption from the hydroxyl group.The results of X-ray diffraction show that the intensity of main diffraction peak of cotton fiber was significantly reduced with the increase of stripping time,resulting in decrease of the crystallinity of cotton fiberd.The width of the diffraction peak decreased obviously,the diffraction angle of the main peak tended to a higher one,the grain size and crystal shape of cotton fiber might be affected.The results of TG-DTG analysis show that the one-step thermal decomposition temperature of cotton fiber did not shift significantly after photocatalytic color stripping at different time,and the thermal stability of cotton fiber decreased over the increasing of stripping time.