Tailored design of polyamide nanofiltration membrane interlayer via dynamic-polyphenol polymerization for complex inorganic salts separation

In this study, a novel dynamic polyphenol polymerization method for the preparation of nanofiltration membrane intercalations was proposed. In this method, TA, PEI and Al 3+ could successively form the first polyphenol-amine crosslink network and the second metal-phenol cross-linked network on the surface of the support layer. The dense crosslinked layer of TA-PEI-Al 3+ formed under dynamic conditions maintains high hydrophilicity compared to the crosslinked network formed under a single pH environment. Aluminum ions possess strong electronegativity since they have relatively high valence states and small ionic radius among transition metal ions. Therefore, this is the first time to incorporate Al 3+ into phenol-amine crosslinked networks to form the metal-phenol tri-complexes. The tri-complexes of TA and Al 3+ are more robust, promoting the coordination with more water molecules and forming more stable hydrophilic intermediate layers. When an interfacial polymerization reaction is performed on the intermediate layer, this hydrophilic intermediate layer can store aqueous-phase PIP and slow down the IP reaction rate through electrostatic interactions. This process helps to form a dense PA separation layer on the support layer surface. The prepared TFC-NF membranes can be used to separate dyes, antibiotics, and compound inorganic salts. The outstanding water permeance of the TFC-NF membrane was determined as 18.7 L/m 2 ·h·bar and the removal rate of mixed pollutants can reach 99 %. The firm intermediate layer enables the optimized modified membrane to maintain stable flux and high rejection rate in 72 h separation test. This modification method provides a useful reference for the preparation of high-performance nanofiltration membranes with plant polyphenols.