Anodic titanium dioxide nanotubes for greywater reclamation

Abstract

Photocatalysis has been seen by many as an invaluable eco-friendly tool for environment remediation. Of special interest has been the use of photocatalysis for wastewater treatment. Titanium dioxide (TiO2) has been the photocatalyst of choice for the last few decades. The combination of chemical stability, low-cost, non-toxicity and excellent optical properties have put it at the forefront of wastewater treatment research. Whilst other oxides, such as tungsten trioxide (WO3) and iron oxide (Fe2O3) possess lower band gaps and can thus be activated by visible light, their resistance towards photocorrosion is lower than that of TiO2. The inherent limitation of TiO2 is its wide band gap which means that it can only be activated by UV light. Interest in the material has been consistently growing with strategies being devised to extend its activity into the visible region of the spectrum. Similarly, the advent of LED lights with low power consumption has helped overcome this limitation. In this work TiO2 nanotubes were produced via anodic oxidation. Anodic oxidation or anodising, is a facile way by which nanotube array can be grown. The process also provides a high level of control on the morphology of the produced nanotubes. The effect of different process variables on the photocatalytic activity of the produced was assessed. The process parameters used were dependent on the type of electrolyte being used. In one instance, a solution of 0.5 wt% sodium fluoride in 1 M sodium sulfate was employed. The anodising was carried out at 20 V for 6 hours. An acidic electrolyte consisting of 0.5% wt% sodium fluoride solution in 1 M phosphoric acid was used with a potential of 20 V applied for 3 hours. A non-aqueous solution containing 0.5 wt% ammonium fluoride, 3 wt% water and ethylene glycol as the balance was used for a process at 70 V for 1 hour. One set of surfaces produced in glycol was decorated with silver nanoparticles. The different electrolytes and associated process parameters produced nanotube layers with different morphologies. All materials when irradiated with UVA light showed a high activity towards the degradation of both chemical and bacterial contaminants. The chemical contaminants were the dye methylene blue, the analgesic paracetamol and the surfactant sodium dodecyl sulfate. The ubiquitous E.Coli was used as the bacterial contaminant. All pollutants can be found in all wastewater streams including greywater. The materials produced in glycol had the highest photocatalytic activity towards all pollutants. This is attributable to the increased thickness, tube diameters and high anatase contents. These properties were the result of the high potential used (70 V) and the electrolyte which could support the high potential. The mechanical and chemical stability of the materials was tested using two different aging regimes. A lab-based reactor was constructed to expose the photocatalysts to flowing synthetic greywater and a light source similar in composition to that of the solar spectrum for 12 weeks. The second aging exercise aimed to better replicate the conditions encountered during field use. A second reactor was built where synthetic greywater was circulated over the nanotube array irradiated with solar light and exposed to the different climatic conditions encountered. The morphology of the surface governed the resistance of the materials against fouling. The arrays produced in the ethylene glycol were devoid of spaces between the tubes and this delayed the accumulation of debris and the resulting occlusion of the tube tops. Despite the surfaces showing extensive fouling in both aging exercises, a significant level of activity towards the degradation of both the dye methylene blue and the bacterium E.Coli were retained. The photocatalysts were also free from chemical dissolution and mechanical damage with the crystalline structure being similarly unaltered. A cleaning regime was implemented to determine the ability of the materials to recover their reactivity. Recovery rates were high for all materials with respect to the degradation of the dye and only marginal for the antibacterial activity. The material synthesised in the glycol without the silver decoration, was selected for upscaling. This material retained the highest activity after aging. The nanotube arrays were synthesised on 15 cm X 15 cm plates. These plates were installed in two different prototype flat plate photocatalytic reactors. The first reactor used UVA LED strips to activate the photocatalytic reaction. The effect of changing pollutant concentration, flowrate and the volume of solution were assessed in order to find the maximum productivity of the unit. A synthetic greywater mixture without E.Coli, paracetamol and methylene blue solutions were used as the water matrices to be treated. Another pilot reactor was installed on a rooftop to study the possibility of disinfecting greywater using sunlight. In this case the synthetic greywater mixture was seeded with E.coli. A noticeable level of degradation of the contaminants used was recorded in the UVA unit. The solar disinfection efficiency was only marginal. The efficiency of the reactors was hampered by an unfavourable ratio of photocatalyst surface area to solution volume. The nanotube arrays installed in flat plate reactor show potential for greywater treatment however further work is required before successful commercialisation of the units.