Superhydrophilic TiO2 nanomaterials for efficient oil-water separation
Tan, Benny Yong Liang
Date of Issue2017-07-17
School of Civil and Environmental Engineering
The rapid growth of industrial activities and occurrences of environmental pollution such as contaminated oil spill evoke situations leading to water source contamination. Furthermore, surfactant-containing oil-water mixtures tend to emulsify the oil into small and stable droplet that are difficult to be separated from water. Currently, conventional techniques have shown their incapability of tackling such problem. Therefore, significant attention has been drawn to introduce advance materials or techniques that are suitable for effective oil-water separation process. This thesis explores the feasibility of TiO2-based material and its derivatives in separating oil from water. In general, the TiO2-based materials reported in this thesis are fabricated using hydrothermal method, and the membranes were synthesized via a 2 step process which include filtration and hot pressing. TiO2 materials has been the most promising candidate for oil-water separation membranes due to its intrinsic superhydrophilic and underwater superoleophobic properties. For the initial part of the work, the pristine TiO2 nanowire was investigated for its oil-water separation ability with the aid of a flexible scaffold. In this study, it is the first time that we fabricated a novel micro/nanowire hierarchical membrane with flexible, self-cleaning and underwater superoleophobic properties for efficient oil-water separation under vacuum filtration. This novel membrane composed of ultralong copper microwires (MW) and functional TiO2 nanowires (NW). The ultralong copper microwires act as a scaffold, as well as providing mechanical flexibility for the membrane, which allow it to be operated under vacuum-driven condition. Meanwhile, underwater superoleophobic TiO2 nanowires membrane provides nano-scale pore size, ensuring the filtration of smaller oil droplets with droplet size> 600nm under vacuum condition. Moreover, the possession of the self-cleaning function of TiO2 nanowires allows the degradation of foulants and contaminants under UV irradiation, which promote the facile recovery of the used membrane. In order to further understand the oil-water separation efficiency and photocatalytic effect corresponding to the TiO2 nanomaterial morphology, a comparative study of self-cleaning ability and oil-water separation efficiencies of 1D TiO2 nanostructure with that of 3D TiO2 nanostructure was conducted. Various TiO2 membranes formed by different nanostructures including 1D TiO2 nanotube, 1D TiO2 nanowire and 3D nanosheet-decorated TiO2 nanowire hierarchical structure were synthesized and compared. The oil-water separation results showed that the three membrane portrayed excellent oil-water separation ability due to the formation of water layers represented by their high water capture percentage (WCP). Studies from the self-cleaning experiment indicated that TSW (3D nanosheet-decorated TiO2 nanowire) membrane has the shortest time to self-clean the oleic acid accumulated on its surface and simultaneously recovers its superhydrophilicity property. In addition, the results for photodegradation activity showed that 3D nanosheet-decorated TiO2 nanowire has the highest photodegradation efficiency among the investigated TiO2 nanostructures to eliminate intermediate organic compounds. This is attributed to its high light harvesting capacity resulting from the multiple reflections of incident light and superior electron collection properties due to the hierarchical structure. Essentially, this study paves way for enhancing oil-water separation performance of other photocatalysts through the modification of its nanostructure. To reinforce the TiO2 nanomaterials with excellent oil-water separation ability, photocatalytic properties and mechanical flexibility, Fe2O3 nanoneedles were grown uniformly on TiO2 nanowire substrate to provide the clinching effect once compacted together. Hence, the subsequent study introduces a hierarchically nanostructure multifunctional TiO2/Fe2O3 composite membrane, which is capable of separating surfactant-stabilized oil-water emulsions with high separation efficiency. The experimental results showed that the membrane is capable of removing droplet size as small as 400nm. The high oil rejection rate is contributed by the acquisition of an interconnected delicate network and underwater superoleophobic interface. Meanwhile, its self-cleaning function promote the facile recovery of the contaminated membrane. Furthermore, the mechanical flexible characteristic of the multifunctional TiO2/Fe2O3 composite membrane widens its applicability in industrial employment. The separation performance of multifunctional TiO2/Fe2O3 composite membrane was further evaluated by operating the membrane in a two-step treatment process. The asprepared membrane was examined for its ability in photodegrading dye contaminants effectively without affecting its oil-water separation efficiency. Methyl blue (MB), Rhodamine B (RhB) and acid orange (A07) aqueous solution were the three types of common pollutants employed in this study. The results showed that the TiO2/Fe2O3 composite membranes demonstrated excellent oil-water separation ability under vacuum filtration process even after photodegrading the various types of dye contaminant. Ultimately, the introduction of this membrane provides an opportunity to eliminate dye contaminants and separating oil from water effectively. In sum, the results gathered from this thesis illustrated that TiO2-based membrane can be regarded as a practical option in managing polluted surfactant-stabilized oily wastewater.