Preferential permeation of Li+ over Mg2+ through homogenously negatively-charged nanochannels

Abstract

Positively-charged nanofiltration (NF) membranes have become popular in membrane separation for lithium extraction from salt lake. However, negatively-charged NF membranes hardly achieve a satisfying Li+/Mg2+ selectivity due to the lack of the understanding on the mechanism of the effect of the electrostatic attraction on the Li+/Mg2+ permselective permeation. To this end, classical molecular dynamics simulations were performed to explore the effect of charge density of the negatively-charged nanochannels on the structure and dynamics of the confined Li+ and Mg2+ ions. Surprisingly, an appropriate modification by negative charges could simultaneously achieve a knock-on hopping for efficient Li+ permeation and a remarkable retardation to the Mg2+ self-driven diffusion, which realized a higher preferential permeation to Li+ over Mg2+ through the nanochannels. The underlying mechanism was explored by a series of analysis, including ion occupancy, mean square displacement (MSD), loading/exiting time, and trajectory tracking, etc. It was found that a combination of the elevated ion occupancy and an appropriate attraction by the negatively-charged channel generated an efficient ionic collision between the confined Li+ ions, thus producing a transition from the self-driven diffusion to the knock-on hopping permeation. Due to the higher positive charge, the confined Mg2+ ions hardly allowed a high ion occupancy and strongly interacted with the negatively-charged channel, which significantly retarded the Mg2+ transmembrane permeation. The novel understanding of the mechanism of the Li+/Mg2+ permselective permeation in the negatively-charged nanochannels will guide an effective development of high-performance membranes for Li+/Mg2+ separation.