Graphene Oxide Membranes for Water Purification and Desalination

Abstract

This dissertation describes preparation and characterization of graphene oxide (GO) as a novel membrane material for water purification and desalination applications. The fundamental chemical, physical and transport properties of GO membranes were characterized. Also, the modifications of GO membranes were performed to improve their applicability for water purification and desalination. This dissertation is organized into six chapters, containing introduction and conclusion chapters, and the main topic of study can be classified as three primary areas: (a) the preparation and characterization of GO membranes, (b) the fundamental water and ion transport properties of GO membranes, and (c) modification of GO membranes for water purification and desalination applications. Chapter 2 describes basic physical and chemical properties of graphene oxide and GO membranes to know their pros and cons as a membrane material for water purification applications. GO is a class of carbon-based nanomaterials which has soft, unique two-dimensional structure decorated by various oxygen-containing functional groups. GO membranes prepared from stacking dispersed GO flakes have a layered structure with slit-like, two-dimensional transport channels. The interlayer spacing of GO can be changed due to not only the oxygen compositions but also the water sorption. They can reversibly sorb water vapor or liquid between interlayers. Also, the intercalated water molecules can rapidly pass through the GO membranes by pressure-driven process. Therefore, GO has a potential as membrane materials for water purification applications. In chapter 3, the fundamental water and ion permeability coefficients of GO membranes and the water/ion selectivity were analyzed, based on the solution-diffusion model to compare the transport properties of GO membrane with conventional polymeric membrane materials. By considering solubility and diffusivity, the fundamental transport properties of GO membranes can be revealed and compared to existing polymeric materials widely used in water purification and desalination applications. The effect of aqueous solution conditions on the transport properties of GO membranes were also studied, as well as the ion selective permeation properties of GO membranes. Chapter 4 demonstrates the preparation of ultrathin GO thin-film composite (TFC) membranes for the molecular separation. TFC membranes have been widely applied to commercial water purification and desalination membranes, such as nanofiltration and reverse osmosis membranes due to its excellent separation performance. GO has many advantages over TFC formation in terms of coating processing, thickness, and scalablity. On the other hand, GO membrane formation from aqueous dispersion is quite different with conventional membrane preparation methods. The effects of hydration and dehydration of GO membrane during the membrane preparation were carefully considered with separation performances. The separation performance of GO membranes were evaluated by filtration experiments, revealing the potential application of GO membranes in water purification and desalination applications. In chapter 5, GO membranes were modified by using physical and thermal methods to investigate the effect of physical and chemical structures on the mass transport properties of GO membranes. Both the permeability and selectivity of GO membranes can be tailored by changing the flake size, the interlayer spacing and the chemical compositions of GO. Chapter 6 describes how to further develop the GO-based membranes for water purification or desalination applications. Since GO membranes suffer from 1) low permeability, 2) low selectivity and 3) low stability, promising strategies to solve these problems were proposed in this thesis.