Abstract
With the increasing global demand for energy storage solutions, particularly for electric vehicles, lithium has become a critical resource, driving the need for efficient extraction methods from salt lake brines. Simultaneously, the growing water scarcity crisis, intensified by population growth and industrialization, highlights the urgent need for advanced desalination technologies to ensure a sustainable freshwater supply. Layer-by-layer (LBL) nanofiltration (NF) membranes have demonstrated significant advantages in selectively separating monovalent and divalent cations, making them ideal materials for applications such as Mg2+/Li+ separation and seawater pretreatment. The separation process always involves highly saline solutions (HSS), especially at high water recovery. Due to the non-chemical weak binding of polyelectrolytes (PEs), LBL NF membranes tend to disassemble in HSS, limiting their use in such applications. This thesis focuses on the assembly mechanism and salinity stability of LBL NF membranes. It aims to address the instability of LBL NF membranes in highly saline environments.Firstly, a comprehensive understanding of the chemical physical basis for LBL assembly in correlation to salinity stability was provided in the literature review for the first time. The performance of LBL NF membranes in HSS was critically analyzed. Proposals to improve the salinity stability of LBL NF membranes were further concluded, by selecting PE pairs with higher binding strength and chemical crosslinking.
Then, the short/long term stability of LBL NF membranes in HSS was systematically investigated. Binding strength between polyelectrolytes was found to contribute significantly to salinity stability. Divalent Mg2+ ions would induce adsorption bridging with polystyrene sulfonate (PSS), altering its conformation and narrowing the pore size distribution, compared to monovalent Li⁺ ions. Dynamic crossflow filtration accelerated the loss of PEs from the LBL separation layer in HSS compared to static immersion. Additionally, chemical crosslinking demonstrated the most effective strategy to remain stable in HSS. This study provided chemical basis for understanding the behavior of LBL membranes in saline conditions and guiding the development of LBL NF membranes for potential wider applications.
Afterwards, the exceptional salinity stability of the poly(allylamine hydrochloride) (PAH)/PSS assembly, as evidenced by a critical salt concentration (CSC) of approximately 1.5 M, highlights the influence of hydrogen bonds in assembly. Furthermore, the poly(hexamethylene biguanide) (PHMB)/PSS assembly, which possesses dual hydrogen bonding capabilities, showed a CSC exceeding the maximum NaCl concentration studied, around 3 M. The presence of these hydrogen bonds was confirmed via advanced characterization techniques. The role of hydrogen bonds in stabilizing the membrane structure under saline conditions was further explored through molecular dynamics (MD) simulations.
Finally, experimental data is presented to demonstrate that neither electrostatic interactions nor hydrogen bonding alone can act as the sole driving force for LBL assembly. The findings ultimately reveal that the driving force behind LBL assembly— whether ionic bonds or hydrogen bonds—fundamentally stems from entropy increase.
This research provides valuable guidance for the application of LBL NF membranes in ion separation and resource recovery under highly saline conditions. They also demonstrate the potential of LBL assembly as a strategy for fabricating reverse osmosis (RO) membranes suitable for seawater desalination. Additionally, this work advances the understanding of LBL assembly mechanisms, offering a deeper and more fundamental insight into the underlying processes.
| Date of Award | 15 Aug 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Binjie Hu (Supervisor), Begum Tokay (Supervisor) & Tao He (Supervisor) |