Guanidinium covalent organic framework modulated positively charged polyamide membranes toward superior Li+/Mg2+ selectivity

Abstract

Nanofiltration (NF) membrane separation technology offers significant potential for the effective Li⁺ extraction from salt-lake brines. Attaining high Li⁺/Mg²⁺ selectivity relies on increasing the positively charged density of the NF membrane surface. In this study, a guanidinium covalent organic framework (TbTG) nanosheet is designed to modulate positively charged polyamide (PA) thin-film nanocomposite (TFN) membranes. The TbTG nanosheets are synthesized and deposited on polyethersulfone ( PES) support alongside polyethyleneimine (PEI) to polymerize with trimesoyl chloride (TMC) and generate cross-linked PA-COF networks. The charge density of the membrane surface is increased to +1.48 mC m^-2, nearly twice that of the pristine PA membrane. The enhanced positive chargeability stems from the guanidine and amino groups of the TbTG framework, creating a stronger Donnan exclusion to repel Mg²⁺. Meanwhile, the hydrogen bond interaction between TbTG and PEI monomers brings about decreased pore size of the membranes. Therefore, the optimal PA-COF-3 membrane demonstrates an ultrahigh MgCl₂ rejection of 99.4%, as well as a superior separation factor ($$) of 76.5 at a LiCl/MgCl₂ mass ratio of 1:40, which exceeds the performance of most reported NF membranes.