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

Abstract

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 simulations 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 diffusion of the ions. The mean square displacement of the ions revealed that Li+ ions exhibit superior diffusion 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 attraction 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.