Gel composite membrane with tannic acid/ferric ion separation layer for organic solvent nanofiltration

To address the permeability-selectivity trade-off of organic solvent nanofiltration (OSN) membranes, a gel composite membrane was fabricated by the complexation of tannic acid (TA) and ferric ions (Fe³⁺) on the surface of self-supported calcium alginate (CaAlg) gel membrane. The three-dimensional network of the CaAlg substrate serves as a channel for rapid solvent transport, while the dense TA/Fe³⁺ metal-phenol network achieves solute rejection. The chemical structure, morphology, pore size, zeta potential and mechanical properties of the CaAlg substrate and the gel composite membrane were characterized. Dye/solvent separation and solvent resistance were investigated. The results indicate that the TA/Fe³⁺ separation layer reduces the pore size, increases the negative charge and improves the mechanical properties of the gel membrane. By optimizing the TA/Fe³⁺ complexation conditions, the gel composite membrane has an ethanol permeance of 19.5 L m⁻² h⁻¹ bar⁻¹ and a congo red (CR) rejection of 99.0 %, and the separation performance is almost unchanged during 60 d of immersion in N, N-dimethlformamide (DMF). In addition, the composite membrane could separate CR from DMF and acetone, as well as cationic and anionic dyes in ethanol. This work provides a simple and environmentally friendly method to prepare an OSN membrane with high solvent permeance and harsh solvent stability, which is promising for solvent purification and recovery.