Advanced Mg2+/Li+ separation nanofiltration membranes fabricated with Girard's reagent T based on functional end-capping strategy

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 MgCl2 at 97.2 %. Besides, the PEI/GRT NF membrane exhibited good Mg2+/Li+ selectivity (SMg2+/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.