Development of graphene oxide hollow fibre membranes for solution based applications

Abstract

Graphene oxide (GO) is a chemical derivative of graphene which possesses interesting characteristics, such as atomic thickness, a 2D structure and robust mechanical strength, as well as a complex chemical structure constituted by various oxygen functional groups. GO flakes can be easily stacked on top of each other in order to form large area thin-film membranes. Since 2012, when the membrane potential of the GO was demonstrated for the first time, GO membranes have continued to attract high levels of interest from research communities which sought to continuously develop high performance membranes within the framework of various disciplines of separation. Nevertheless, the transport mechanism of the GO membrane in the separation process is not entirely understood, thus significant efforts have been made in order to further explore the unique characteristics the GO has to offer as a new generation of membrane material. The first part of this thesis explores the feasibility of GO membranes in the case of nanofiltration separation. GO membranes are unstable in dry air due to the shrinkage problems occurring under such conditions. When a membrane is exposed to air, a high tensile stress is imposed, which is far beyond what the membrane can withstand. In order to overcome the shrinkage problem, GO membranes were kept in water so as to preserve their intrinsic microstructures. Prior to depositing the GO membranes in water, controlled drying was required in order to avoid the redispersion of the membranes in water due to the hydrophilic nature of the GO. Depositing the GO membranes in water has successfully preserved their nanofiltration performance over a three-week interval, during which time the membranes that were exposed to dry air failed to achieve similar results. At the end of this first part, the transport mechanism in nanofiltration is discussed.

Although GO membranes have been found to be feasible in the case of nanofiltration separation, they have also been demonstrated to be incapable of removing smaller ionic species, such as magnesium chloride and sodium chloride, solely on the basis of their pristine nature. The second part of this thesis explores the feasibility of GO at the level of desalination applications. It is well known that the rejection of salts relies not only on the pore size of a membrane, but also on the charge of the membrane and the solute, thus creating electrostatic repulsion. Cross-linking GO membranes with Al3+ have successfully changed the negatively charge GO membranes to highly positive charge. On the basis of space charge model, the cross-linking has successfully increased the density of membrane charge, at the same time narrowing the effective channels for ion transport, simultaneously increasing the rejection towards salt solutions. The third part of this thesis further explores the feasibility of GO in the pervaporation dehydration of ethanol. For the pervaporation dehydration of ethanol, the surface of a GO membrane was modified with hydrophilic silane in order to turn it into a highly hydrophilic membrane. The performance of the modified GO membrane improved significantly with moderate separation factors. The transport mechanism in pervaporation is discussed at the end of this part. Moreover, the techno-economic analysis indicates that this modified GO membrane is environmentally benign and economically robust as a result of its small membrane area and energy requirements.