Tight ultrafiltration membranes based on engineered metal-monophenolic networks for efficient dye/NaCl separation

Tight ultrafiltration (TUF) membranes have gained prominence in the treatment of highly-saline textile wastewater because of their well-suited pore structure and molecular weight cutoff. However, widely used polyamide-based TUF membranes are made by environmentally unfriendly fabrication processes and suffer from the risk of selective layer delamination, inherent hydrophobicity and high fouling propensity. Herein, a green and facile surface engineering strategy was reported for fabricating non-polyamide TUF membranes by engineering a novel metal-monophenol coordination network, i.e., ferric ions (Fe³⁺)/vanillic acid (VA) network, as the selective layer onto a porous PES substrate for efficient dye/salt separation. The deposited metal-monophenol network endowed the membrane with a well-tailored pore structure and increased hydrophilicity, which can be precisely controlled by adjusting the duration of surface modification. Specifically, the optimal TUF membrane (Fe³⁺/VA-6 h) with a molecular weight cutoff (MWCO) of 2841 Da presented a significantly lower water contact angle of 38° and a pure water permeance as high as ~48.65 L·m⁻²·h⁻¹·bar⁻¹. Furthermore, filtration experiments revealed the exceptional dye/salt selectivity of the Fe³⁺/VA-based TUF membrane, which yielded >99.8 % dye rejection and < 4.8 % salt rejection for both reactive black 5/NaCl and reactive blue 2/NaCl mixtures. Additionally, the excellent fouling resistance and long-term stability of this Fe³⁺/VA coordination system were demonstrated by a 24-h continuous filtration test. Therefore, the proposed metal-monophenol coordination network presents a promising coating material, advancing the development of non-polyamide TUF membranes for sustainable textile wastewater management.