Nanofiltration Membranes Containing a Metal–Polyphenol Network Layer: Using Casting Solution pH as a Tool to Tailor the Separation Performance

Thin-film composite (TFC) membranes containing metal-polyphenol network (MPN) selective layers were fabricated using a supramolecular self-assembly between tannic acid (TA) and ferric ion (Fe³⁺). The TA-Fe³⁺ thin film was coated on a porous polyacrylonitrile support using aqueous solutions of TA and FeCl₃ via a layer-by-layer deposition technique. The pH of the TA solution was used as a tool to alter the membrane characteristics. The surface porosity and water contact angle of the fabricated membranes gradually decreased as the pH of TA casting solutions was increased from 3 to 8.5 for both single-layered and double-layered TA-Fe³⁺ TFC membranes. This allowed us to tune the water permeance and the retentions of water-soluble neutral and anionic molecules by the MPN membranes by varying the pH of the casting solution. It has been shown that the water permeance decreased from 184 to 156 L·m^-2·h^-1·bar^-1 for single TA-Fe³⁺ layer coated membranes when the pH was increased from 3 to 8.5, while it declined from 51 to 17 for the double TA-Fe³⁺ layer. Anionic solutes in aqueous solutions were highly retained compared to neutral components as the TFC membranes had a negative surface charge. Retentions of 95 and 90% were achieved for naphthol green B and orange II dyes by a double-layered M4 membrane fabricated at pH 8.5, while only 13% retention was found for the neutral riboflavin. The neutral dye riboflavin permeated 30.8 times higher than the anionic dye naphthol green B during a mixed dye filtration test through the TFC membrane prepared by using a TA solution of pH 8.5. To the best of our knowledge, this is the highest selectivity of a neutral/anionic dye pair so far reported for a TFC membrane having an MPN selective layer. Moreover, fouling tests have demonstrated that the MPN separation layers exhibit robust stability and adequate antifouling performance with a flux recovery ratio as high as 82%.