Preparation of high-flux nanofiltration membranes for Mg2+/Li+ separation via amine monomer diffusion control and grafting modification

An amino-modified NF membrane is fabricated via integrating interfacial polymerization and chemical grafting. • Glycerol serves as a regulator to tailor the polyamide layer microstructure and surface amino group distribution. • Covalent grafting of AMH monomer endows the membrane surface with durable positive charge characteristics. • The membrane delivers a high water permeability of 14.0 L/m 2 ·h·bar and a MgCl 2 rejection rate of 96.21%. • A prominent Li + /Mg 2+ separation selectivity up to 34.27 is acquired under optimized preparation parameters. Polyamide nanofiltration (NF) membranes are generally hindered by the inherent trade-off between permeability and selectivity. To address the above drawbacks, this work constructs a high-performance positively charged NF membrane via a combined strategy of regulated interfacial polycondensation and covalent surface graft functionalization. Glycerol was blended into the aqueous monomer solution to tailor the growth process of polyamide (PA) selective layers; this treatment thinned the separation skin layer to boost water transport efficiency and meanwhile generated abundant hydroxyl reactive sites for subsequent grafting reactions. Under cerium ion-induced free radical polymerization, 2-[(2-methylacryloyl)oxy]ethanaminium (AMH) monomers were covalently immobilized on membrane surface via hydroxyl anchor points, endowing the membrane with persistent positive surface charges. A full set of material characterizations verified the successful grafting of AMH chains. The modified membrane exhibited a flattened surface structure with an average pore diameter of 0.61 nm, and the densified separation layer slightly weakened surface hydrophilicity. Under optimized fabrication parameters, the as-prepared membrane delivered outstanding separation performance: 96.21% MgCl 2 ion retention, a Mg 2+ /Li + separation factor of 34.27, and a water permeance of 14.0 L·m −2 ·h −1 ·bar −1 , which outperforms most NF membranes reported for brine lithium extraction. In addition, the membrane maintained steady separation capacity over a 72-h uninterrupted cross-flow filtration test, proving reliable long-term operational stability. This research provides an innovative fabrication route for high-throughput and high-selectivity NF membranes by coupling diffusion-controlled interfacial polymerization with directional surface functional grafting. Download: Download high-res image (190KB) Download: Download full-size image