Layer-by-layer assembly of TA-Fe (COF-LZU1)/PES membrane for dye/salt separation

Covalent organic frameworks (COFs) are effective materials for water treatment membranes due to their tunable pore structures and high compatibility. While composite modification strategies combining plant polyphenol tannic acid (TA)-Fe³⁺ and COFs show great promise for TFC membranes, they typically involve organic solvents or complex pre-modification of the fillers. In this study, a green, fully-aqueous layer-by-layer (LbL) self-assembly strategy was employed to prepare TA-Fe(COF-LZU1)/PES membranes. By exploiting the inherent synergy between the rigid crystalline channels of COF-LZU1 and the adhesive TA-Fe network, this approach directly utilizes pristine COF-LZU1 without secondary modification. Unlike conventional methods, the entire process avoids organic solvents, offering a highly sustainable technical alternative. The incorporation of COF-LZU1 significantly enhanced the membrane surface properties, with surface roughness increasing from 17.0 ± 2.2 nm to 43.8 ± 8.5 nm and the water contact angle decreasing from 77.0 ± 1.6° to 20.0 ± 1.0°, thereby substantially improving the membrane’s hydrophilicity. The TA-Fe(COF-LZU1)/PES membrane achieved a pure water permeance of 35.85 L·m - ²·h - ¹·bar - ¹, representing a 11.5% increase over the TA-Fe/PES membrane. It exhibited excellent separation performance, achieving high rejections of 98.3% for Direct Red 80, and further demonstrated robust stability in mixed dye/salt solutions. In five-cycle bovine serum albumin (BSA) tests, the membrane exhibited improved antifouling durability, yielding a flux recovery ratio of 75.5%. Additionally, it operated stably across a pH range of 4–8. This green self-assembly approach provides a practical strategy for preparing composite membranes for dye wastewater treatment.