Advanced ion separation in polyamide nanofiltration membranes via interfacial polymerization regulated by dynamic polyphenol-metal coordination

Developing advanced nanofiltration membranes (NFMs) for inorganic salt ions separation remains a significant challenge in the value-added process of water treatment. Herein, we engineered a highly permeable-selective TA/Fe/Ca/PA-p NFM through a novel strategy for the interfacial polymerization (IP) regulated by the dynamic polyphenol-metal coordination. Interestingly, the dynamic coordination occurred after the substrate surface was first modified with tannic acid (TA) and Fe3+ and then coated with an aqueous phase solution containing piperazine (PIP) and Ca2+. In this process, the coordination competition between Fe3+ and Ca2+ for TA and the conversion of TA/Fe3+ bis-complex into tris-complex were conducive to forming more uniform TA/Fe/Ca complex on the substrate surface. Meanwhile, TA can also perform Michael addition with PIP to control the diffusion of PIP to the organic phase. Following the IP reaction, the nascent TA/Fe/Ca/PA NFM underwent post-treatment in a sodium citrate (SC) solution to dissociate the TA/Fe/Ca complex by further applying the dynamic coordination, resulting in a crumpled and thinner separation layer with desirable surface properties. The optimized TA/Fe2/Ca/PA-p NFM exhibited a pure water permeability of 36.1 L m−2 h−1⋅bar−1, nearly 2.4 times higher than that of the original PA NFM. Correspondingly, the Na2SO4 rejection reached 99.4 % and its Cl−/SO42− separation factor was 118.3. This outstanding permselectivity was mainly attributed to the decreased mass transfer resistance, the expanded effective permeability area, and the increased negative charge on the membrane surface. Additionally, the TA/Fe2/Ca/PA-p NFM was endowed with good operational stability, pressure resistance, and antifouling performance. This eco-friendly and cost-effective approach paves the way for fabricating high-performance NFMs and demonstrates the great potential of dynamic polyphenol-metal coordination in water treatment.