Abstract
Current wastewater treatment is not able to efficiently treat trace micropollutants. These compounds include a wide range of biologically active compounds from various human sources, and are found in μg/L concentrations in domestic wastewater. Photocatalysis is an advanced oxidation process that can act as a polishing step after conventional treatment, which utilizes semiconductor materials that are activated by UV light to degrade these complex compounds. Currently most examples of commercial photocatalysts use them in slurry or powder form, however this requires post-treatment removal. Research increasingly focuses on the use of immobilized photocatalysts, while minimising disadvantages such as complexity of synthesis and reduced activity in comparison to systems using photocatalyst in suspension. Concentrating on the two most common semiconductors, TiO2 and ZnO, the scope of this research was to develop and optimise immobilised wire photocatalysts, where the oxide layer was grown on the surface of their parent metal via electrochemical anodization. Conventionally films are used as the support, however a metal wire aimed to increase the availability of the surface area and improve mass transfer in the reactor, in comparison to the film systems. Thus sustainable methods for wire anodization were investigated, which included the development of a tubular photocatalytic reactor, to ensure high surface illumination, and an in-flow anodization rig for the wire support. A ZnO layer could be successfully synthesised by anodizing a Zn wire using KHCO3 electrolyte, referred to as a ZnO wire, which demonstrated an increase in photocatalytic degradation of phenol, in comparison to an untreated Zn wire and increased quantum efficiency to ZnO films. The ZnO wire stability and photoactivity could be increase by the addition of oxygen to the reaction solution. Other parameters investigated, included reactor hydrodynamics, anodization time and the application of electricity to the metal core.TiO2 is the most widely used photocatalyst for water treatment. Producing immobilized TiO2 catalyst from Titania via anodization requires the use of hazardous fluoride electrolytes. Limited research exists on investigating non-fluoride based TiO2 anodization, specifically upon wires, and the photocatalytic activity of the produced oxide layer has not been studied. Different halide based electrolytes and additives were tested, and the optimum electrolyte solution was found to be potassium bromide with ethylene glycol. These materials were characterised using a variety of analytical techniques, such as scanning electron microscopy and X-ray diffraction. Subsequently annealing temperatures were refined to obtain TiO2 crystal modification (rutile to anatase ratio) that provides best photocatalytic performance. This study demonstrated TiO2 nanostructures could be successfully anodized with mild anodizing conditions upon a Ti wire, which demonstrated photoactivity to degrade micropollutants, carbamazepine and phenol.
Also included in this thesis is a study on the photocatalytic activity of different types of TiO2 nanoparticles that were produced via a novel membrane emulsification-precipitation process. The photocatalytic activity of the nanoparticles was investigated under UV light and under visible light for nitrogen doped varieties. Latter tests required the application of UV filter solutions. The nanoparticles demonstrated an increased degradation of the model contaminant, phenol in comparison to photolysis alone, however the overall photocatalytic was lower than that of a commercial photocatalyst used as benchmark.
It is common for a study to focus entirely upon either the syntheses or investigate photocatalytic materials that are already available, this thesis has concentrated on various aspects; production, material characterization and investigating the photocatalytic activity. To conclude, non-hazardous anodization of immobilized photocatalyst, TiO2 and ZnO wires were successfully developed and demonstrated, with annealing temperature highlighted as a key preparation parameter to tailor the properties of the photocatalyst. These wires demonstrated higher photoactivity and increased surface availability in comparison to the conventional films used. The development of a suitable photocatalytic reactor was required to optimise surface illumination of the wire photocatalyst. Reaction conditions, such as atmospheric conditions and flow rate, were shown to play a key role in photoactivity, and further investigation found that the key pathway for degradation is using hydroxyl radicals. The research presented in this thesis has led to three published papers. A research project during the initial stage of this integrated PhD contributed to two publications on membrane ozonation for water treatment and on-going research projects. Further work, regarding immobilized ZnO wires would be to identify inconsistences in the anodization process, to improve synthesis efficiency. The nitrogen-doped immobilized TiO2 photoactivity would be investigated using visible light, as well as optimising the nitrogen doping procedure. Finally, determine the relationship between physical properties and photoactivity of the TiO2 nanoparticles, in comparison to the photocatalytic benchmark, Evonik Aeroxide P25.
| Date of Award | 24 Jun 2020 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jannis Wenk (Supervisor) & Barbara Kasprzyk-Hordern (Supervisor) |
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