Molecular dynamics simulations of Li+/Mg2+ separation using hydroxyl modified PVDF under applied electric field

Abstract

The green and efficient separation of Li⁺/Mg²⁺ is a pressing challenge in the extraction of Li⁺ from salt lake brine. A transport model for Li⁺/Mg²⁺ in an –OH modified PVDF polyelectrolyte membrane was established. The effects of electric field strength, solution concentration, and the number of charging groups on the flux of Li⁺/Mg²⁺ and ion selectivity were calculated using non-equilibrium molecular dynamics. The migration mechanism for Li⁺/Mg²⁺ was elucidated by analyzing the parameters such as interaction energy, coordination number, and mean square displacement. The results showed that as the solution concentration increased, the fluxes of Li⁺/Mg²⁺ through the membrane decreased, while the selectivity for Li⁺ increased. It is necessary to increase the electric field strength to enhance the separation of Li⁺/Mg²⁺. The strong interaction energy between Li⁺/Mg²⁺ and the polyelectrolyte chains led to dechlorination and dehydration during transmembrane migration, especially in high concentration systems. The separation factor initially increases and then decreases as the number of –OH groups increases. Additionally, higher temperatures accelerate the diffusion of Mg²⁺ within the membrane, leading to a decrease in Li⁺ selectivity. Therefore, it is crucial to maintain the system temperature in the range of 298 K to 308 K. This work provides valuable theoretical insights for the design and separation strategies of high-performance ion exchange membranes.