Novel thin film nanocomposite membranes with improved properties for enhanced desalination performance

Abstract

Nowadays, polymeric thin film composite (TFC) membranes have received increasing applications for separation and purification processes such as desalination and wastewater treatment. The development of nanotechnology has opened new frontiers in the advancement of TFC membranes. The incorporation of nanomaterials into the thin film polyamide (PA) layer of TFC membranes, making a structure known as thin film nanocomposite (TFN) membrane, can offer the advantages of both inorganic nanofillers and organic polymeric membranes. However, the possibility of nanoparticles’ leakage and their low compatibility with organic membranes make the successful fabrication of TFN membranes challenging. Hence, investigating the structure-performance correlation of TFC/TFN membranes and their interactions with the incorporated nanomaterials is of great importance. In my PhD project, a serial of TFC membranes incorporated with variously sized, structured and surface functionalized silica nanoparticles have been developed and extensively characterized with the aim to advance the knowledge of interfacial interactions between inorganic nanomaterials and the thin film PA layer of TFC membranes. A statistical analysis was applied to study some key fabrication parameters of TFC membranes including PSF concentration in phase separation and aqueous phase soaking time and heat curing time in the interfacial polymerization course. Our findings highlighted the importance of considering interactions when devising a strategy to fabricate TFC membranes in order to optimize the overall desalination performance. After the study of the TFC fabrication parameters, fabrication of TFN membranes was performed to get a better understanding on the interfacial interactions between nanoparticles and TFC membranes. SN with different sizes (50 nm and 100 nm) and surface functionalities (hydroxyl, amine or epoxy) were synthesized and successfully incorporated into TFC membranes. Desalination performance evaluation of the TFN membranes showed that no matter which functional group was present on SN, the resulting TFN membranes had higher water flux and comparable or higher salt rejection compared with the TFC membrane without nanoparticles. In addition, nanoparticle surface functionalization using epoxy and amine moieties facilitated the chemical interaction between SN and the TFC membranes, resulting in TFN membranes with higher crosslinking density of their PA selective layer and improved performance stability. After studying the solid SN, hydrophilic hollow mesoporous silica nanoparticles (HMSN) have been used to fabricate novel TFN membranes in order to study the contribution of this peculiar porous structure on the properties and desalination performance of the resulting TFN membranes. The HMSN with an average particle size of ~ 68 nm were synthesized and incorporated into TFC membranes. The performance evaluation results revealed that with 0.03 wt % HMSN incorporation, water flux could improve up to 40 percent without obvious alteration of the salt rejection. Moreover, the compaction resistance of the resulting membranes was enhanced after HMSN incorporation. To enhance the compatibility of introduced SN with the PA layer, polyethyleneimine (PEI) modified SN (SN-PEI) were fabricated and incorporated into the TFC membranes. Introduction of SN-PEI to the TFC membranes could facilitate the formation of strong covalent bonds between SN-PEI and PA layer, improve the compatibility of SN with the organic PA and enhance the physicochemical properties of the resulting TFN membranes. The cross-flow filtration performance evaluation of the fabricated membranes showed improved water flux and salt rejection capability in addition to a higher compaction resistance for the developed TFN membranes. This thesis project has attempted to advance fundamental knowledge of the nanocomposite formation and fundamental processes governing water transport through reactive nanostructure to guide the development of nanoparticle-enabled multifunctional membranes.