Tailoring selective layer structure for high performance nanofiltration membrane fabrication

• Main Author: Yang, Yang
• Other Authors: Wang Rong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Environmental engineering::Water treatment; Engineering::Chemical engineering::Water in chemical industry
• Online Access: https://hdl.handle.net/10356/156872
• Institution: Nanyang Technological University

In recent years, fresh water scarcity has been intensified due to population growth and global climate change. Many researches have been devoted to transfer non-tradition sources of water into clean water. Among multiple choices of water treatment processes, membrane based technology is an energy-efficient separation process. However, current RO and NF membranes are still far from ideal in terms of filtration performance. Thus, this research work mainly focuses on the construction of the NF membrane selective layer with enhanced performance applied in water treatment process. In the literature review part, several fabrication methods applied on a porous substrate to provide separation ability are introduced. When the selective layer is successfully fabricated on the substrate, three mathematical models are reviewed to describe the process in different aspects. With the understanding of how the selective layers achieve separation, the optimization methods are summarized based on the above analysis. Based on the review of current study in NF membrane fabrication, inspired by the catechol chemistry, tannic acid (TA) was functionalized to tether PIP monomers and participated in the co-deposition via coordination bonds with Fe3+ ions on a substrate to form Fe3+/TA-PIP complex. A series of analyses confirmed the successful synthesis Fe3+/TA-PIP deposition and polyamide (PA) layer. The resultant membrane exhibited water permeability of 13.73 LMH/bar and 89.52% for MgSO4 rejection. The fabrication was further improved by embedding modified graphene oxide (GO) nanosheets into the Fe3+/TA-PIP complex. The optimized nanocomposite membrane achieved a water permeability at 21.66 LMH/bar, along with a well maintained MgSO4 rejection at 91.25% and NaCl/MgSO4 selectivity (α) at 10.02 under 2 bar operating pressure. The rapid co-deposition mediated by nanomaterials incorporation provides an effective approach to design high-performance thin film composite (TFC) membrane. However, the above presented method is still complicated with additional interlayer construction steps. Inspired by biomimetic membranes fabricated via incorporating water channel vesicles, another selective layer optimization method was presented. In this study, a fabrication method for thin film composite NF membrane was proposed via liposomes assisted construction without protein or water channels. The liposomes were first deposited on the surface of a membrane substrate and interfacial polymerization was carried out to form a polyamide skin layer over the adsorbed liposomes. The polyamide skin layer containing liposomes were found to be thinner and filled with wrinkles and nanovoids. As a result, compared with the control membrane, the liposome-assisted membrane prepared using optimized fabrication conditions was able to achieve an increased water permeability from 11.17 to 18.21 LMH/bar, alongside an excellent MgCl2 rejection of 95.87% and a monovalent/divalent (NaCl/MgCl2) ion selectivity (α) of 18.2. We found that the enhancement stems from an increase in effective membrane area, and a reduced surface hydrophobicity, which coupled with a thinner skin layer, reduces the hydrodynamic resistance of the polyamide layer. In addition to producing NF membrane with high permeability, increasing membrane’s selectivity is also important to achieve highly precise solute separation. The surface modification via the layer-by-layer self-assemble method has attracted great attention in precisely controlling the selective layer structure parameter. Therefore, we presented a novel polyelectrolyte multilayer as selective layer of NF membrane fabricated via ‘bio-glue’ dopamine (DA) self-assembly intercalation. Through alternative adsorption of PAH, DA and PSS molecules onto porous substrates, tailorable pore size and tunable surface structure could be achieved. The resultant (PAH/DA/PSS)4 PEM membranes with integrated DA self-assemblies exhibited excellent water permeability (more than 60% increase from 13.5 to 21.9 LMH/bar) and higher rejection to divalent ions (increases MgCl2 rejection from 91.0% to 95.5%) compared with the control group (PAH/PSS)4 membrane. Furthermore, the developed LbL method was applied on hollow fiber membrane modules and was able to achieve a higher Li+/Mg2+ selectivity from 15.6 to 37.8 and Na+/Mg2+ selectivity from 11.0 to 19.1 using different simulated brines as feed solutions. In conclusion, this thesis presents the novel design and construction method for the selective layer of NF membrane based on the deep understanding and analysis of ion transportation mechanism through the membrane. The NF membrane’s selective layer chemistry, structure, and surface properties could be tailored and optimized based on the demand in the filtration process. This work contributes to developing novel composite NF membranes, which facilitate the practical production of high performance NF membrane for water treatments.