Carbon nanotube membranes for the separation of Li+ and Mg2+ ions: Effect of functional groups with charges

Non-equilibrium molecular dynamics simulations were employed to explore the Mg2+/Li+ separation efficiency of pristine and functionalized carbon nanotubes (CNT) for the MgCl2/LiCl solution. It is initially found that the dehydration degree of the first shell of Li+ near the mouth of pristine CNT membrane is greater than that of Mg2+, so the energy barrier for Mg2+ entering CNT membrane is generally smaller than the counterpart of Li+, emphasizing the potential of CNTs for the separation of Mg2+/Li+. Further, Mg2+ experiences greater attraction from the CNT membrane coated with carboxyl groups relative to Li+, which further enhances the permeability of Mg2+ over Li+. Therefore, the carboxyl-CNT membrane demonstrates enhanced Mg2+/Li+ selectivity compared to the pristine CNT membranes. Nevertheless, for the CNT membrane grafting with amino groups, the Mg2+ undergoes enhanced repulsion from the CNT mouth relative to Li+, so the retention time of Mg2+ at the fluid-CNT interface is increased, which makes the amino-CNT membrane to preferentially passing through Li+. Consequently, the Mg2+/Li+ selectivity is reduced in amino-CNT membrane, with respect to the pristine CNT membrane. It is also found that while the selectivity of Mg2+/Li+ increases with the external driving force for the pristine and carboxyl-CNT membranes, the amino-CNT membrane undergoes an opposite trend with the driving force. We note that in all the cases considered, the CNT membranes mounted with carboxyl groups deliver the best Mg2+/Li+ separation efficiency for carrying negative charges at the pore mouth. Our findings could facilitate the development in membrane design for efficient Mg2+/Li+ separation.