Robust stability and improved dispersion of lanthanum-based metal-organic frameworks in PVDF membrane via covalent interaction for phosphate removal

The practical application of lanthanum-based metal-organic frameworks (La-MOFs) with a high affinity for phosphate has been severely hindered by their powdery nature. To address this limitation, carboxyl-functionalized La-MOF (La-BTC) nanoparticles (NPs) were incorporated into a polyvinylidene fluoride (PVDF) membrane to develop mixed matrix membranes (MMMs) for phosphate removal. To enhance the dispersion and stability of NPs within the membrane matrix, the PVDF polymer was grafted with polyethyleneimine (PEI) to introduce -NH 2 groups. These -NH 2 groups subsequently formed covalent amide bonds with the -COOH groups of the La-BTC NPs via an amidation reaction. The grafting of PEI enlarged the finger-like pores of the membrane from approximately 20 μm to 75 μm, thereby reducing the diffusion resistance of phosphate within the membrane. The dispersion of La-BTC NPs in the PEI-PVDF matrix was also significantly improved due to their strong covalent interactions. Density functional theory (DFT) calculations verified that La-BTC exhibited stronger chemical bonding and more efficient charge transfer with PEI-PVDF compared to PVDF. Specifically, the modified system yielded a more negative binding energy of −23.44 eV and a narrower HOMO-LUMO gap of 3.86 eV. This strengthened interaction ensured the exceptional stability of the La-BTC NPs in the PEI-PVDF membrane. The resulting La-BTC/PEI-PVDF demonstrated faster phosphate adsorption and outstanding reusability, sustaining a removal efficiency of over 90% during five continuous regeneration cycles. This work presents an efficient method to incorporate MOF NPs into a membrane matrix, promoting the application potential of La-BTC in phosphate removal.