Multifunctional graphene oxide-TiO2 membrane for wastewater purification and oil-water separation

The severe scarcity of clean water is arousing concern worldwide. The development of clean water production heavily relies on membrane technology, although the performances of current membranes are significantly restricted by their monotonous function and membrane fouling. In this study, a novel multifunctional GO-TiO2 based membrane was delicately designed and fabricated for the first time to solve the drawbacks of current membranes. The novel membranes fabricated in this work exhibit the promising activities for wastewater purification and oil-water separation: (1) excellent mechanical flexibility because of the cross-linkers existed in the membranes; (2) eliminating organic fouling due to the strong photodegradation property; (3) withstanding various wastewater conditions (such as acidic and basic conditions); and (4) high oil-water separation efficiency towards oil-water mixtures, especially surfactant stabilized oil-water emulsions, due to the superhydrophilic and underwater superoleophobic properties.This study initiated from finding out the optimal TiO2 nanostructure for fabrication of the membranes. Various TiO2 nanostructures have been successfully synthesized, including one-dimensional (1D) TiO2 nanotube, 1D TiO2 nanowire, three-dimensional (3D) TiO2 sphere assembled by nanoparticles (TiO2 sphere-P) and 3D TiO2 sphere assembled by nanosheets (TiO2 sphere-S). The results of photodegradation activity indicate that the photodegradation efficiency of TiO2 sphere-S is the highest among the investigated TiO2 nanostructures. The best photodegradation activity of TiO2 sphere-S can be attributed to the highest light harvesting capacity resulted from multiple reflections of light, and hierarchical mesoporous structure. In addition, the combination of TiO2 sphere-S with graphene oxide (GO) sheets can further enhance the photodegradation and disinfection activities under solar light, which is more energy efficient. The promising photocatalytic activity of GO-TiO2 composites is originated from the enhanced light absorption and efficient charge separation. In the following work, GO-TiO2 membrane was fabricated through assembling as-synthesized GO-TiO2 composites on the surface of a polymer filtration membrane. This kind of membrane possesses the multifunction of concurrent water filtration and degradation of pollutants. GO sheets play double roles in GO-TiO2 membrane, including (1) cross linker for individual TiO2 microspheres; and (2) electron acceptor to enhance photocatalytic activity. Hence, this novel membrane shows sustainably high permeate flux due to the hierarchical membrane structure, high photodegradation activity and no membrane fouling. In order to further treat wastewater with different pH conditions, a novel GO-SO3H/TiO2 membrane was fabricated by chemical modification of synthesized GO sheets for the first time. This membrane shows enhanced strength and flexibility compared with pure inorganic membrane due to the special GO-SO3H/TiO2 heterojunctions. In addition, the strong coordination bonds between sulfonic group (-SO3H) of GO-SO3H and Ti4+ center of TiO2 endow this membrane with excellent adaptability in various wastewater conditions. This membrane possesses high efficiency for concurrent photodegradation and water filtration without organic fouling problem. Finally, considering the increasingly amount of oily wastewater, the fabricated multifunctional GO-SO3H/TiO2 (SGO-TiO2) membrane also exhibited high efficiency of oil-water separation. The SGO-TiO2 membrane was applied for the separation of a variety of oil-water mixtures, including free oil-water mixtures and surfactant stabilized oil-water emulsions. This is the first time to efficiently separate surfactant stabilized oil-water emulsions using a pressure driven SGO-TiO2 membrane, the separation of which remains a great challenge by conventional approaches. During the oil-water separation process, this membrane shows the combined advantages of high oil rejection rate and ultralow membrane fouling, thanks to its interconnected nanoscale network, superhydrophilic and underwater superoleophobic interface, and self-cleaning function. The results of this study indicated that this multifunctional membrane could be a good candidate for practical wastewater purification and oil-water separation.Doctor of Philosophy (CEE