Molecular dynamics simulation of the fractionation of lithium and magnesium-ions on crown ether-grafted cellulose acetate polyelectrolyte membranes

Considerable amounts of lithium and magnesium are present in the sea. When properly harnessed, they could complement the existing extraction technologies and serve as a sustainable source for meeting up the rapidly growing demands. In the present study, we report using classical molecular dynamics simu-lations the formulation of new polyelectrolyte membranes comprising of cellulose acetate (CA) backbone and severally grafted crown ether macrocyclic pendant groups for the segregation and recovery of Li+ and Mg2+ ions. Highly compact hydrophilic polymeric membranes stabilized by both intra-and inter-molecular hydrogen bonds were successfully achieved with sufficient fractional free volumes for the dif-fusion of the ions. The mean square displacement of the ions revealed that Li+ ions exhibit superior dif-fusion on the 12-crown-4 grafted CA membrane, while the 15-crown-5 and 18-crown-6 grafted systems demonstrated stronger attraction for Mg2+ ions raising the diffusion energy barrier. The analysis of the interference of co-existing counterions in binary solutions revealed that the electronegative oxygen atoms on the macrocyclic rings on 12-crown-4 grafted CA consistently shielded the interfering ions resulting in the rapid diffusion of Li+ ions. The larger crown ethers in contrast exerted a minimal attrac-tion on the divalent Ca2+ ions, enabling the diffusion of Mg2+ ions. This study demonstrates the inherent potential of the crown ether-grafted polymeric membranes in the recovery of lithium and magnesium from seawater, a prospect that could be further explored in the stride toward effective brine mining.(c) 2023 Elsevier B.V. All rights reserved.