Hydrophobic Porous Ceramic Membranes For Water Desalination And Oil/Water Separation

Membrane based separation process is considered as a cost-effective and energy-efficient technology. It has been experienced a rapid development in the past decades and found applications in various sectors such as petrochemical, energy, food and pharmaceutical industries. For successful applications, membranes are required to possess a large permeation flux, high separation efficiency and good stability. Our previous research has shown that ceramic membranes made from alumina and zirconia exhibit good water desalination performance comparable with polymeric membranes, and have better chemical, thermal stability than the latter. This thesis is thus devoted to the study of preparation, characterization and water desalination and oil/water separation performance of porous ceramic membranes.In Chapter1, porous ceramic membranes are first reviewed in terms of structures, processes, preparation and characterization methods. The conventional methods for water desalination and oil/water separation are next introduced, particularly the membrane based technologies. At last a scope and concept of this thesis is described.In Chapter2, a forming method based on phase inversion phenomenon is explored for preparation of porous ceramic membranes. The as-prepared asymmetric alumina membrane consists of a layered structure:a thin sponge-like layer with submicron-sized pores, which acts as separation function layer, and a thick finger-like layer with straight pores along thickness direction, which offers mechanical strength to the separation layer. The alumina membranes were grafted with fluoroalkylsilane (FAS), converting the surfaces of the membranes from hydrophilic with a contact angel (CA)～46°to hydrophobic with CA～135°. A superhydrophobic surface with water CA over150°was also created by hydrothermally growing ZnO nanorods on alumina membranes followed by grafting with FAS.In Chapter3, the grafting mechanism and stability of the FAS layer on ceramic surfaces are investigated. Most of the FAS are physically adsorbed to the ceramic surfaces as revealed from IR spectrum and TG analysis, while some FAS are chemically linked to the ceramic surfaces via Si-O-M (M=metal) bonds as identified by high resolution XPS analysis. The grafted FAS layer shows a good long-term stability in air at room temperature. The layer degrades gradually when exposed water and ethanol, resulting in the loss of hydrophobicity. Analysis also indicates that to maintain the desired hydorphobicity the working temperature of FAS-grafted membrane should not exceed～200℃.In Chapter4, the water desalination properties of the as-prepared hydrophobic planar membranes are reported. The porous alumina membranes were obtained through phase inversion tape casting-sintering-surface grafting method. The as-prepared membrane showed an high nitrogen permeability of1.0×106Lm-2h-1bar-1Owing to the surface hydrophobicity of the membrane, water vapor can pass through the membrane while liquid water and dissolved salts cannot permeate through it. The water desalination experiments were conducted in direct contact membrane distillation (MD) mode. A water flux of19.1Lm-2h-1and a salt rejection higher than99.5%were achieved by exposing one side of the membrane to a hot2wt.%NaCl solution (80℃), and sweeping the other side with a cold deionized water (20℃) to carry away the permeated water. These results were comparable with the best performances of polymeric membranes. In Chapter5, the anti-fouling properties of the superhydrophobic ceramic membranes for water desalination are presented. Superhydrophobic alumina hollow fibers were prepared using the method described in Chapter2, The MD experiments showed that the water permeation flux and salt rejection of the superhydrophobic membranes declined much more slowly than the conventional hydrophobic ones, demonstrating that the membrane exhibits much improved anti-fouling property desired for practical applications.In Chapter6, porous ceramic membranes for separation of oil/water are explored. Porous alumina disks were formed by phase-inversion tape casting process. The alumina disks contained finger-like through-pores with a gradient distribution of pore size along the thickness direction:the pore size being15-30μm at the top of the disks and80～100μm at the bottom. The surfaces of the alumina disks showed hydrophilicity (water CA of0°) in air and oleophobicity (octane CA of～130°) in water. The disk exhibited very high water permeability of12.6Lm-2s-1kPa-1, while are completely imperious to octane at pressure less than1.1kPa. By making use of The disk was applied for separation of octane/water (30:70in volume), and a separation efficiency higher than96%was achieved. In this study, the surface of an alumina disk was converted to hydrophobic (water CA of～146°) and oleophilic (octane CA of0°) via grafting with FAS. It found that octane could permeate through the oleophilic disk with a permeability as high as12.7Lm-2s-1kPa-1, while water was totally blocked at pressure less than3.2kPa. The oleophilic membrane exhibited desired water/oil separation property:a separation efficiency as high as99.5%was achieved a mixture of water and octane (70:30in volume). Owning to the extremely low mass transfer resistance presented by the finger-like pores, gravity-driven separation of water/oil mixture can be achieved, which is expected to be highly energy-efficient and cost-effective.In Chapter7, the summary of this dissertation is presented and further research is proposed.