TiO2 nanofiber composite membrane for water purification
Date of Issue2016-06-01
School of Civil and Environmental Engineering
With the emergence of treatment technologies such as the polymeric membrane, we can now micro manage the specific type of pollutants and particles to be excluded according to the sizes of the membrane selective layers – be it Micro Filtration (MF), Ultra Filtration (UF), Nano Filtration (NF) or Reverse Osmosis (RO). However the application of the membrane technologies in water treatments are highly subjected to operation conditions such as temperatures, pH and foulants. In the use of membrane for water and wastewater treatment, the fouling problems are irreversible due to the growth of microbiological layer or biofilms, cake layer, internal pore blocking and concentration polarization which limit the membrane throughputs. Constant membrane cleanings not only reduce the efficiency, shorten membrane lifespan but also generate secondary wastewater and increase the membrane water treatment costs. The development of inorganic/ceramic membranes however not only enables the end users to overcome the problems faced by the polymeric membranes such as short lifespan, this new class of membrane is able to withstand harsh environment and together with reasonable good flux. Tougher built material ensures the lifespan of such membrane even under frequent cleanings. However, unit area cost, rigid structure and small surface area greatly limit the wide application of such inorganic membranes. Therefore there is an urgent need to develop a new flexible multifunctional membrane which can overcome the obstacles faced by both types of membranes and extend its new functions which enables the membrane to be able to concurrently attain filtration efficiency and disinfection as well as self-cleaning. In this studies, the author first carried out experimental works for the evaluations of nano structured TiO2 photocatalysts in various forms for its respective photocatalytic performance in terms of organic pollutant degradation and bacteria disinfection. In the later part of the experiments, the author has also successfully counter the disadvantages of current commercially available membranes by fabricating a novel inorganic nanocomposite multifunctional membrane which is flexible and able to perform concurrent filtration and disinfection as well as self-cleaning. The 20 µm thick functional layer was assembled using TiO2 nanowire composite with ZnO nanorod and carbon nanotube (CNT). By performing the molecular weight cut off (MWCF) test using PEG 35,000 and PEO 60,000, It was indicated that the as assembled nanocomposite multifunctional TiO2/ZnO/CNT membrane is a tight microfiltration (MF) / loose ultrafiltration (UF) membrane that has flux achieved over 1000 measured in L·M/H under 5 bar which is much higher compared with conventional polymeric and ceramic membrane. The photocatalytic function of this as assembled membrane is determined with the degradation of AO7, achieving near 100% (Co = 50mg/L) in 30 min. Compared with microfiltration (MF) polymeric membrane and ceramic membrane, it is clearly indicated that the as assembled membrane has great advantages to overcome the obstacles faced by polymeric and ceramic membranes. In the later part of the studies, further work has been carried out to investigate the intrinsic disinfection capability of some TiO2 based engineered nanomaterials (ENMs). Comparisons have been done among P25 nanoparticles (NPs), TiO2 nanofiber, TiO2 nanotubes, TiO2 microsphere and 3 types of 3 dimensional (3 D) dendritic TiO2 microsphere with engineered nanoribbons evenly integrated on the surface. It was found that the microspheres with nanoribbons possess the highest antibacterial capability which achieved 49% of bacterial inactivation rate in 2 hrs with initial sample concentration of 100 μg/ml and E. coli as the model bacterial at 107 cfu/ml from the beginning. The mechanisms of this disinfection is found to be physical puncture of cell membranes by the nano pointy tips and possibly enhanced by the charge accumulated on the tips of the ENMs upon approaching the bacterial cells which were supported in other articles not directly linked with such studies. The next step of the research shall then be focusing on the further studies of stabilizing the ENMs onto the as synthesized membranes and the investigations of the interactions of the ENMs with membrane and/or with bacteria after integration with the as synthesised membrane for concurrent disinfections even without the presence of UV or visible light and also to study the tertiary effect under the UV exposure with the presence of simulated organic pollutants.