Degradation Of Dye Wastewater By Photoelectrocatalytic Oxidation Processes

Environmental protection and sustainable development are important issues which humanity are confronting in the 21st century. Wastewater treatment is one important part of environmental protection.With the rapid development of textile printing and dyeing industry and technological progress in finishing, the quality of dye wastewater has also changed greatly due to the wide use of new auxiliaries, dyes, and finishing agents. The conventional method, such as physical chemical process, chemical process, biological process could hardly completely degraded these organic pollutants in dye wastewater.Photocatalytic oxidation and electrocatalytic oxidation are regarded as advanced oxidation processes (AOPs), which can produce a lot of strong oxidants, such as holes and hydroxyl radicals, under electromagnetic radiation or electric field. Hydroxyl radicals, in particular, could degrade almost any organic pollutant to CO2, H2O, or a mineral salt. Therefore, AOPs show a new application prospect wastewater treatment.In this thesis, dye wastewater samples containing dyes such as methyl orange (MO) or rhodamine B (RhB), as well as some real printing and dyeing wastewater ones, were chosen as objective decomposition substances for systematic investigations in electrocatalytic and photocatalytic oxidation. Based on the above investigations, a novel packed bed three-dimensional (3D) electrodes photoelectrocatalytic coupling reactor was designed, and the oxidation of dye wastewater with photocatalysis coupled with electrocatalysis in the 3D electrode system was explored. The optimum operating conditions for obtaining optimal coupling effect of photoelectrocatalysis were discussed. These studies provide a theoretical basis for the application of photoelectrocatalytic oxidation, and have been successfully applied to the treatment of real printing and dyeing wastewater samples. The main research work and results are summarized as follows:1) The electrocatalytic oxidation of dye wastewater was conducted in 3D electrodes reactor. The mechanism of the electrocatalytic oxidation in such a system was analyzed. The results show that both the electrocatalytic degradation efficiency of wastewater and the instant current efficiency (ICE) are much higher as compared with those obtained from 2D electrodes system, due to the particle electrodes effect in the packed bed. The electrocatalytic activity and the repolarization of particle electrodes are directly affected by material properties and structural shapes of the particle electrodes. Adding a certain proportion of insulating particles that are coated with polyimide films can reduce short circuit current and bypass current, thereby improving the repolarization of particle electrodes and the degradation efficiency. In 3D electrode system organic pollutants can be degraded by direct and indirect electrochemical oxidations, where the latter process plays the leading role in oxidation. The strong oxidants such as hydroxyl radicals generated during the oxidation process can degrade the organic pollutant to CO2, H2O, or mineral salts with no secondary pollution.2) Factors influencing the degradation efficiency were investigated in detail for optimizing process conditions. Computational models of current efficiencies were established and the regularity of the current efficiencies in oxidation process was explored. The functional relationship between electrochemical oxidation index (EOI) and initial COD concentration was disclosed by mathematical analysis. The results indicate that cell voltage, current density, working electrode space, and the initial concentration and pH value of the dye wastewater are the primary factors affecting the degradation efficiency. The results also indicate that the higher initial COD value is, the higher current efficiency will be. Very high ICE, high COD and color removal efficiency can be achieved in the first 20 minutes of electrocatalytic process, suggesting that the 3D electrode system has a high utility efficiency of electric energy. This result may provide a promising path for treatment of high concentration dye wastewater. In other words, high concentration dye wastewater could be treated with electrocatalytic oxidation process firstly for a short time and then treated with biochemical process, which will not only save energy but also obtain good degradation effect.3) Alkali metal ions, such as K+, Rb+, and Cs+, were selected as doping elements to improve the photocatalytic activity of TiO2 by ion doping. TiO2 nanoparticles doped with alkali metal ions were prepared by a modified sol-gel method, and characterized by various techniques, such as X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), UV-Vis absorption spectroscopy (UV-Vis), specific surface area (BET) measurement. The results indicate that the samples consist of spherical nanoparticles with good dispersity and small size, and that the alkali metal ions have incorporated into the crystal lattice of TiO2. Alkali metal ions doping can evidently suppress phase transformation from anatase to rutile, inhibit the grain growth, decrease the crystal size of TiO2, and give rise to the bathochromic shift in the maximum absorption wavelength. Doping with a suitable amount of alkali metal ions also can increase the specific surface area of TiO2, and reduce the diminishing rates of specific surface area with increasing calcinations temperature.4) The photocatalytic degradation of the refractory organic compounds, MO, and RhB, was used as a probe reaction to evaluate the photoactivity of prepared nanoparticles. The doping mechanisms were discussed. TiO2 doped with K+, Rb+as well as Cs+ can reduce the recombination rate between electron-hole pairs and thereby significantly enhance the photocatalytic activity of TiO2, in which Cs+/TiO2 showed the best photocatalytic activity. The amount of ion doping and the calcination temperature have a great influence on, e.g., the crystal growth, crystalline structure, phase composition, specific surface area, and grain size, resulting in the activity changes of TiO2. The optimal doping amount of K+, Rb+, and Cs+ are 5%,1%, and 1% mole fractions, respectively. The optimal calcination temperatures for K+/TiO2, Rb+/TiO2, and Cs+/ TiO2 are 650℃,650℃, and 550℃, respectively. The dosages also influence the activity of catalyst. The optimal dosages of K+/Ti02, Rb+/TiO2, and Cs+/TiO2 are 1.2 g/L、1.2g/L, and 1.0 g/L, respectively, under this thesis experimental conditions.5) The TiO2/Ti photoelectrode with high catalytic activity was prepared by combining anodization and thermal oxidation. Particle photoelectrodes with high catalytic activity were prepared by using activated carbon and graphite as supports respectively with the method of combining TiO2 sols pressure impregnation and temperature programmed calcinations. The results of SEM, XRD, and EDS characterizations reveal that TiO2 films on TiO2/Ti working photoelectrode are even and compact, and the crystal phase is anatase. The catalyst Cs+/TiO2 is loaded and immobilized well on activated carbon and graphite, and the crystal phase is anatase.6) The 3D electrode photoelectrocatalytic reaction system was constructed, in which the TiO2/Ti photoelectrode was used as the working photoelectrode, and the Cs+/TiO2/CA or the Cs+/TiO2/ GR was used as the particle electrodes packed in bed. The material response and mass transfer were expanded from solid-liquid to solid-liquid-gas. Process and mechanism of coupling photocatalytic oxidation with electrocatalytic oxidation in the 3D electrode system were explored. Using MO as the objective decomposition substance, the main factors influencing photoelectrocatalytic oxidation were studied. Evaluation model for coupling effect of photocatalytic oxidation and electrocatalytic oxidation was established. The method to get the best coupling effect was investigated. The results reveal that the bias voltage is the key parameter affecting coupling effect. The application of a bias voltage leads to the tilt of energy band, and to promote the separation of photoinduced charge carriers on TiO2. The photoelectrocatalytic efficiency under anode bias voltage is higher than the efficiency under cathode bias voltage. The particle electrodes in bed play an important role. Under bias voltage, potential gradient arises on the whole 3D electrode field owing to the effects of particle electrodes. As a result, the mass transfer and reaction areas are increased significantly, and the yield of electron-hole pairs and hydroxyl radical formation is improved. Additionally, the recombination rate between electron-hole pairs is reduced. At a lower anode bias voltage, photoelectrocatalysis is more efficient than the sum of those by sole electrocatalysis and photocatalysis alone. On the basis of the experimental data, we designed a novel packed bed 3D electrodes photoelectrocatalytic coupling reactor, and applied it into the effective treatment of dye wastewater and real printing and dyeing wastewater.7) The related kinetic models were established in order to investigate the kinetics of dye wastewater degradation in electrocatalytic oxidation, photocatalytic oxidation, and photoelectrocatalytic oxidation, respectively. These degradations can be described by related kinetic equations. For low concentration dye wastewater, the degradations follow a pseudo first-order reaction under electrocatalytic oxidation, photocatalytic oxidation, or photoelectrocatalytic oxidation. The photocatalysis rate constants of TiO2 are enhanced significantly by doping K+, Rb+, or Cs+, and the rate constant of photoelectrocatalysis is much higher than that of photocatalysis or electrocatalysis.