(233c) Utilization of the Layer-By-Layer (LBL) Method and Interfacial Polymerization (IP) for Fabricating Polyelectrolyte-Polyamide Based NF Membranes for Mg+2/Li+ Separation

The demand for lithium has increased rapidly driven by both increased penetration of electric vehicles (EVs) and increased storage capacity from renewable energy. Nanofiltration (NF) membranes, with their selectivity for monovalent over multivalent ions, could be utilized for direct lithium extraction (DLE) from salt-lake brines with reduced water and energy use [1]. Positively charged polyethyleneimine (PEI)-based polyamide membranes and layer-by-layer (LBL) based polyelectrolyte membranes offer an opportunity for enhanced separation due to their enhanced Donnan exclusion [2], controllable pore size [3,4] and increased Mg⁺² rejection [1]. In this study, porous PSF based substrates were prepared and then coated with a polyelectrolyte (PE) bi-layer of polystyrenesulfonate (PSS)/PEI using the LBL method. Subsequently, PEI reacted with trimesoyl chloride (TMC) via interfacial polymerization (IP) reaction to form a PE-polyamide layer which was further grafted with PEI. The combination of LBL and IP aimed to leverage the advantages of positively charged polyamide membranes and the unique benefits of PE NF membranes. The fabricated membranes were characterized by ATR-FTIR, XPS, AFM and water contact angle measurements supporting the successful formation of the PE-polyamide layer, while the morphology of the membranes was investigated via SEM. The membrane performance was studied via filtration experiments of pure water and of 1000 ppm salt solutions (i.e. MgSO₄, MgCl₂, Na₂SO₄, NaCl and LiCl) in a dead-end stirred cell. The results of these experiments revealed that pH of PEI (LBL and grafting) plays a crucial role in the successful formation of a dense polyamide layer. Specifically, at alkaline pH (pH=10), the resulting membrane displayed an increased salt rejection for MgCl₂ (over 70%) compared to the corresponding membrane before grafting. This highlights the potential of the combination of the LBL method with IP for the preparation of NF membranes for Mg⁺²/Li⁺ separation.

Literature

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