Alkali-assisted metal-phenolic network assembly for structural regulation of nanofiltration membranes

Metal-phenolic networks (MPNs) have been widely used in the preparation and modification of nanofiltration (NF) membranes due to their rapid reaction, good film-forming ability, and excellent hydrophilicity. Nevertheless, NF membranes incorporating MPN selective layers often suffer from reduced rejection performance, owing to the challenge of controlling the selective layer's porous architecture during synthesis, which restricts their separation efficacy of small organic compounds. In this study, the substrates were pretreated with a NaOH solution prior to the coordination synthesis of the Fe-tannic acid (Fe-TA) selective layer to achieve three key objectives: Firstly, the application of NaOH facilitates the enrichment of Fe³⁺ on the membrane surface in the form of Fe(OH)₃. Secondly, this process allows for the gradual release of Fe³⁺, as it reacts with TA, thereby enabling a controlled coordination reaction that forms the Fe-TA network. Finally, the presence of OH⁻ helps to maintain an alkaline environment throughout the reaction, enhancing the crosslinking degree of the MPN network. Compared to conventional Fe-TA NF membranes, the Fe-TA membrane pretreated with NaOH solution (pH = 10) exhibited a reduction in average pore size from 0.91 nm to 0.66 nm, and an increase in Na₂SO₄ rejection from 87.6 % to 96.6 %. Moreover, the Fe-TA membrane demonstrated enhanced rejection performance for various dyes, notably achieving a rise in the rejection rate for metanil yellow from 94.6 % to 99.4 %. This study not only provides alternative for the fabrication of high-performance non-polyamide NF membranes, but also introduces a new strategy for regulating MPN structures.