Industrial wastewater, such as the effluents from textile and garment companies, may contain toxic organic pollutants, which resist conventional wastewater treatment. Their complete and environmentally friendly degradation requires innovative technologies. Photocatalysis, an advanced oxidation process, can serve this purpose. Since 1972, when the photocatalytic activity of titanium dioxide was first noticed, photocatalysis has drawn the attention of scientists and engineers but it has not yet been widely applied in industrial practice. This is mainly related to the challenges of up-scaling from laboratory experiments to large production sites.
The main goal of this thesis is to develop a concept of nanoparticle-based photocatalysis for the treatment of wastewater. Ideally, process parameters should be adjustable and process conditions should be well-defined. These constraints are prerequisite for establishing process models and comparing the photocatalytic efficiency of different photocatalysts or for different pollutants. More importantly, the configuration should be scalable, in order to cover a wide spectrum of applications.
In response to these requirements, this thesis introduces a new reactor concept for photocatalytic wastewater treatment, which relies on finely dispersed photocatalysts as well as uniform and defined process conditions with regard to illumination and flow. The concept was realized in a photocatalytic setup with an illuminated flow reactor. The flow channel has a rectangular cross section and meanders in a plane exposed to two dimensional illumination. Crucial process parameters, e.g., volumetric flow rate and light intensity, can be adjusted in a defined manner. This facilitates the study on the photocatalytic degradation of different organic pollutants in the presence of various photocatalytic materials under arbitrary illumination.
The thesis provides a comprehensive description of the operational procedures necessary to run photocatalytic reactions in the experimental setup. It includes three main steps: i) dispersion of photocatalysts, ii) equilibration with respect to pollutant adsorption and iii) accomplishing the photocatalytic reaction. Samples are collected in a mixing tank for online or offline analysis. The proceeding decrease in the concentration of organic pollutant is used to assess the activity of the photocatalytic materials.
A particular focus lies on the first of these steps, the dispersion of photocatalysts, because it is ignored in most studies. Typically, photocatalysts are in an aggregated state. The thesis demonstrates that type, intensity and energy of dispersion exert a crucial influence on size and morphology of the photocatalyst particles and, thus, on their optical properties and, accordingly, macroscopic photocatalytic behavior. Apart from this, a proper dispersion is necessary to reduce speed of gravitational solid-liquid separation, at best, to prevent catalyst sedimentation and to avoid misleading results.
The photocatalytic performance was intensively investigated for the color removal of a model dye substance, methylene blue. Commercial titanium dioxide nanoparticles, widely explored in literature, were used as a photocatalyst. Their characteristics (size, morphology, stability and optical properties) were determined. Photocatalytic experiments were carried out under UV irradiation. Influences of different factors, including the concentration of the photocatalyst, the concentration of the organic compounds, light intensity, optical pathlength and pH were examined. The degradation was quantified via the decrease of methylene blue concentration. This conversion is, however, an immediate result influenced by all process parameters, e.g., the volume, the light intensity, the optical pathlength. Hence, kinetic models on macroscopic and microscopic levels are established. Normalizations with respect to process conditions are proposed. The apparent reaction kinetics are traced back to volume- and intensity-related reaction rate constants, and the reaction rate constant at the illuminated surface of the reactor. Additionally, the model is modified to be used for time-variant UV intensities, as encountered for solar photocatalysis. These achievements allow for a comparison of the experimental results from different laboratories. Moreover, they are prerequisite for the translation of laboratory results into large scale plants.
Selected case studies for further applications are introduced. The photocatalytic degradation of different organic molecules (one antibiotic and two commercial dyes) with different photocatalytic materials (commercial nanomaterials and self-synthesized magnetic particles) under artificial or natural light sources was performed. Additionally, photocatalysis was studied in a realistic application. Preliminary tests with dye solutions of a textile company in Danang, Vietnam, impressively showed the feasibility of wastewater treatment by means of photocatalysis. Based on the reported capacity of wastewater in the current treatment plant of the company, the necessary process parameters were assessed. The rough estimation showed that photocatalysis can improve the working ability of the current wastewater treatment plant.
In conclusion, this thesis presents a concept for wastewater treatment by slurry photocatalysis. As the process conditions are adjustable and definable, the process can be ideally performed in laboratories for research purposes, where different materials need to be tested and the working volume can be lower than hundreds of milliliters. The photocatalytic configuration is expected to work with a capacity of hundreds of liters, although appropriate experimental evidences are reserved for further up-scaling studies
