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Abstract
Desalination has become one of the most promising techniques of solving the global freshwater shortage problem. Enormous research effort has been invested to improve desalination technology, such as enhancing efficiency, reducing cost and limiting the environmental impact of desalination. One of the major environmental impacts of desalination process is the by-production of a large amount of highly concentrated brine, while the proper management of the brine is still a challenging task for researchers and the society. Treatment of the brine generally aims to further reclaim water and useful salt elements, and it is essentially a desalination process, though the feed contains a much higher concentration of salts and other contaminant contents compared to conventional seawater desalination. So, to suit the needs of brine treatment, not all desalination technologies can be applied, among which membrane distillation and pervaporation seem to have the potential because of their high permeate quality and capability of treating highly concentrated effluent. Membrane distillation has been widely studied, however, the wetting, fouling/scaling issues still limit its wide application. Unlike membrane distillation, the membrane used in pervaporation is dense and hydrophilic, which could effectively overcome the wetting and fouling/scaling issues. This dissertation presents a thorough investigation on pervaporation membranes for brine desalination, with the objective of obtaining high performance pervaporation membranes with both high permeate flux and good salt rejection for long-term operation.
In this study, a thin dense layer of polyvinyl alcohol (PVA) was coated on top of the supporting membrane to generate the composite pervaporation membrane. Based on a series of systematic experimental endeavours, how the crosslinking of PVA by glutaraldehyde and incorporation of graphene oxide (GO) nanosheets affected the pervaporation performance has been investigated. By further optimizing the crosslinking structure, membrane fabrication techniques and membrane compositions, the optimized PVA/PVDF composite membranes were further investigated regarding its performance encountering different brines as feed solution: they showed very stable performance for long-term operation with a nearly complete salt rejection (> 99.999 %). More importantly, the composite membranes also exhibited excellent storage stability, anti-fouling/scaling properties and cleaning efficiency.
Incorporation of defect engineered fumarate-based MIL-53(Al) metal-organic frameworks (MOFs) in the PVA matrix was further conducted recognizing its porous structure and good compatibility with PVA matrix. By optimizing the MIL-53(Al) porous structure and loadings in the PVA matrix, the obtained nanocomposite membranes showed improved fresh water productivity (up to 75 % improvement) for concentrated brine treatment, without compromising the membranes’ ultrahigh salt rejection, anti-fouling/scaling performance and long-term stability.
Finally, a nanocomposite membrane consisting of GO laminate deposited on top of porous supporting membrane was designed and fabricated for brine treatment by pervaporation. Because of the preferential water adsorption ability of GO membranes, molecular sieving by the limited d-spacing of GO laminate, fast water diffusivity through the GO membranes and the continuous dewatering effect caused by the vacuum, graphene oxide membranes exhibit high water permeability with negligible salt passage for long-term brine treatment. By further reducing the mass transport resistance from the supporting membrane, the optimized flux is comparable to microporous membrane distillation membrane.