Engineering nanostructured selective layers for reverse osmosis membranes

• Main Author: Kovacs, Jason Richard
• Other Authors: Paula T. Hammond.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2015
• Subjects: Chemical Engineering.
• Online Access: http://hdl.handle.net/1721.1/98709

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2015. === Cataloged from PDF version of thesis. === Includes bibliographical references (pages 122-142). === A major challenge to communities across the world in the next century will be ensuring millions have access to adequate freshwater resources. Studies from the UN World Health Organization indicate that over 1.1 billion people currently lack access to reliable and secure freshwater supplies, with an estimated 2.5 million deaths per year from diseases associated with poor access and sanitation in 2007. Reverse osmosis (RO), a process through which water is desalted via pressurized flow past a salt-selective membrane, is an energy-efficient method to generate freshwater from oceanic, brackish, and waste water sources. However, there are a number of challenges to scaling up RO processes to large scale production, including the need to improve membrane selectivity and throughput. One method to assemble selective layers for RO membranes is layer-by-layer (LbL) assembly, which is a flexible, scalable assembly technique that enables the incorporation of a myriad of polyelectrolytes and inorganic nanoparticles into thin films. There is a gap in the scientific literature concerning the use of LbL to generate RO selective layers where previous approaches have not taken full advantage of the LbL process to incorporate nanomaterials that can generate ordered nanostructures for salt rejection. In particular, high-aspect ratio clay platelets are ideal for such a purpose; it was hypothesized that effective salt rejection could be achieved by hindering the diffusion of solvated ions through nano-channels formed by the platelets embedded within a polymer matrix. This body of work examines the application of spray layer-by-layer (spray-LbL) assembly with clay composite thin film architectures to generate nanostructured selective layers for use in RO membrane technology. First, appropriate substrates were identified as support layers for the deposition of spray- LbL assembled clay composite thin films. Both electrospun bisphenol-A polysulfone (PSU) mats of varying fiber diameter and polyethersulfone (PES) ultrafiltration (UF) membranes with varying pore diameters were examined. Second, a range of materials were investigated for the spray-LbL deposition of clay composite films. Laponite clay platelets were incorporated into several different film architectures including strong polyelectrolytes as well as cross-linkable weak polyelectrolytes to form both bilayer and tetralayer film architectures. The clay content was controlled via manipulating assembly conditions such as the pH and spray times of the film components. Assembled membrane architectures were tested at industrial RO operating conditions in dead-end permeation cells and evaluated for salt rejection, water permeability, and mechanical strength. Ultimately, it was determined the most uniform and robust films were those deposited on PES membranes with 30 nm pores, closely matching the characteristic length of the LAP clay platelets to reduce the impact of bridging. Although all the film architectures tested exhibited significantly greater water permeability than commercially available RO selective layers, the salt selectivity was found to be highly dependent on the film architecture and assembly conditions. The best performing film architecture consisted of a cross-linked clay composite tetralayer film, exhibiting salt rejection of 89% for aqueous 10,000 ppm NaCl solution with an order of magnitude increase in water permeability over a commercially-available thin film composite membrane. The key conclusion drawn from the studies indicate the presence of an optimal zone where the incorporation of clay platelets introduces additional salt selectivity via size exclusion, balanced with the cross-linked polymer component of the film to improve the mechanical strength and reduce the risk of critical defect formation during operation. Taken together, these investigations represent a new approach using structured nanomaterials to develop next generation clay composite RO selective layers. The increased water permeability of the clay composite selective layers offers an attractive advantage in desalting applications where high flux is desirable, such as with brackish water resources as well as in membrane unit operations near their thermodynamic limit. === by Jason Richard Kovacs. === Ph. D.