Interference mechanism of cations on transport of lithium and magnesium inside COF nanofiltration membranes

Membrane technology is playing an important role in the separation of magnesium and lithium from brines, and high Li/Mg selectivity in membrane separation is constantly pursued. However, it has been found that there is always a selectivity variation when using individual salts to replace the salt mixture as the feed, implying that the interference mechanism of mixed cations remains unclear. Herein, by using covalent organic frameworks as an example, we investigate the ion transport behaviours through nanopores with the mixed and individual feeding of LiCl and MgCl2 via molecular dynamics simulations. There is an evident reduced Li/Mg selectivity while the mixed-salt feed is applied. It is revealed that Li+ interacts strongly with pore-wall atoms while Mg2+ with water molecules. Consequently, Mg2+ tends to flow with water molecules while Li+ is prone to adsorb to the pore wall. The presence of Mg2+ inside nanopores will bring down water flux and seriously reduce the transport of Li+ inside nanopores. The advantage of pre-adsorbing for Li+, which promotes the Li/Mg selectivity, is completely balanced out, thus reducing selectivity. Our results also suggest that impeding the entrance of Mg2+ into membranes will avoid the selectivity drop not only because of the sieving effecting.