Efficient separation of dyes/salts using ionic liquid-modified covalent organic framework THPC-PEI/COFs-NH2PES composite nanofiltration membrane

Abstract

Covalent Organic Frameworks (COFs), with their high porosity, low mass density, excellent thermal stability, and tunable structures, exhibit immense potential in the field of membrane separation. However, the porous structure of COFs and their negatively charged nature result in suboptimal performance in dye/salt separation. Tetrakis(hydroxymethyl)phosphonium chloride (THPC), a zwitterionic liquid, contains a central phosphorus atom and numerous hydroxyl groups, offering the dual optimization characteristics of balancing membrane surface charge and optimizing membrane pore structure. In this study, we first modified polyethersulfone (PES) with p-phenylenediamine to prepare an amino-functionalized NH₂PES support membrane. Subsequently, we synthesized the COFs selective layer on the NH₂PES surface via interfacial polymerization to obtain the COFs-NH₂PES composite membrane. Then, we employed polyethyleneimine (PEI) to modify the COFs-NH₂PES membrane surface through a polyelectrolyte-mediated assembly strategy, forming the PEI/COFs-NH₂PES composite membrane. Finally, we conducted THPC modification to prepare the THPC-PEI/COFs-NH₂PES composite nanofiltration membrane. Zeta potential analysis indicated that the surface charge of the THPC-PEI/COFs-NH₂PES composite nanofiltration membrane was essentially neutral, favoring dye/salt separation. Dye/salt separation experiments demonstrated that the THPC-PEI/COFs-NH₂PES composite nanofiltration membrane exhibited excellent separation performance, with a CR rejection rate and dye/salt separation factor reaching up to 99.5 % and 181.2, respectively, and a flux of 211.36 L m⁻² bar^-1h⁻¹. Additionally, by adding THPC, the membrane surface was exposed to a greater number of hydroxyl functional groups, which improved the anti-fouling and stability of the THPC-PEI/COFs-NH₂PES composite nanofiltration membrane over time.