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Polyethyleneimine (PEI) nanofiltration (NF) membranes hold great application prospects in lithium extraction from salt lakes due to their high positive charge properties. However, the poor permeability of the membranes caused by the dense separation layer structure severely hinders their further application. Herein, a functional end-capping strategy was employed to address this problem by adding the effective end-capping reagent, Girard's Reagent T (GRT), to the PEI aqueous phase as the co-reaction reagent. Under the dual functions of the end-capping effect and the quaternary ammonium groups of GRT, the looseness and positive charge of the membrane were simultaneously optimized. After the modification of GRT, the looser structure of the PEI/GRT NF membrane improved its pure water flux (PWF) to 115 L m−2 h−1 at 6 bar, which was around 3.7 times higher than that of the original PEI NF membrane. Meanwhile, the enhanced positive charge (from −8 to 9 mV, pH = 7) of the membrane bottom surface allowed it to maintain the high rejection rate of MgCl 2 at 97.2 %. Besides, the PEI/GRT NF membrane exhibited good Mg2+/Li+ selectivity (S Mg 2+ /Li + = 22.7, 2000 ppm, Mg2+/Li+ mass ratio: 20:1). Therefore, this study provides a simple and feasible strategy for the preparation of high-permeable PEI NF membranes with good Mg2+/Li+ separation performance. [Display omitted] • A high permeable PEI-based NF membrane was prepared by the end-capping strategy. • The cross-linking degree of membrane was reduced by end-capping function of GRT. • The surface positive charge of membrane was enhanced by –N+ groups of GRT. • The optimized NF membrane exhibited high permeability and Mg2+/Li+ selectivity. [ABSTRACT FROM AUTHOR]

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 Na 2 SO 4 rejection reached 99.4 % and its Cl−/SO 4 2− 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. [Display omitted] • An advanced PA NFM was regulated by the dynamic polyphenol-metal coordination. • Dynamic coordination includes the competition, conversion and dissociation of complexes. • A crumpled and thinner separation layer with desirable surface properties was constructed. • The NFM exhibited high permeability and high Cl−/SO 4 2− selectivity. [ABSTRACT FROM AUTHOR]