Insights into High Li+/Mg2+ Separation Performance Using a PEI-Grafted Graphene Oxide Membrane

Lithium extraction from brine or seawater using membrane technology has attracted extensive attention in recent years. Graphene oxide (GO), as one of the two-dimensional materials, has been proven as a competitive candidate for membranes. However, the GO membranes still suffer challenges for ion sieving due to the swelling in the aqueous solution. In this work, a GO–PEI membrane with positively charged channels was constructed by polyelectrolyte polyethyleneimine (PEI) molecular chain-grafted GO nanosheets. The GO–PEI membrane showed a high selectivity of 22.2 for Li+/Mg2+, together with a competitive Li+ permeation rate of 0.09 mol m–2 h–1 in a binary permeation test. In addition, the membrane showed excellent stability during the separation process. The enhanced Li+/Mg2+ separation performance of the GO–PEI membrane could be mainly attributed to the synergistic effect of size sieving and electrostatic repulsion. We further systematically studied the influence of various variables, such as PEI molecular weight, PEI content, membrane thickness, and ion concentration, on separation performance. This work deepens the understanding of membrane ion selectivity from multiple perspectives and puts forward a strategy to design a two-dimensional membrane structure from microscopic materials.