Effects of anions on the separation of Li+ and Na+ using ZIF-8 membranes

Abstract

In this work, we investigate the effects of anions (Cl⁻, NO₃⁻, SO₄²⁻) on the separation of Li⁺ and Na⁺ using ZIF-8 membranes through molecular dynamics (MD) simulations. Numerical results show that anions play a crucial role in separation processes. Large anions, such as SO₄²⁻, cannot go through the ZIF-8 membrane and completely prohibit the transport of Li⁺ and Na⁺. In contrast, smaller anions (Cl⁻ and NO₃⁻) can permeate the membrane more easily, facilitating the migration of Li⁺ and Na⁺ through the membrane. Compared to Cl⁻, the transport of NO₃⁻ enhances the flux of Li⁺ and improves the Li⁺/Na⁺ separation ratio. Temperature effects are also examined. The Li⁺/Na⁺ separation ratio decreases as the temperature is increased, while the fluxes of Li⁺ and Na⁺ are enhanced due to higher diffusion coefficients of Li⁺ and Na⁺. A case study using NO₃⁻ to improve Li⁺/Na⁺ separation for the system containing SO₄²⁻ is demonstrated. The findings in this work offer insights into the extraction of Li⁺ from seawater or salt lakes under complex environments.