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

ABSTRACT

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.

KEYWORDS:

• Covalent organic frameworks
• Li/Mg separation
• nonequilibrium molecular dynamics
• nanofiltration
• transport mechanism

Acknowledgements

The authors thank the High Performance Computing Centre of Nanjing Tech University for supporting the computational resources.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Natural Science Foundation of Jiangsu Province [grant number BK20190085], and the National Natural Science Fund for Distinguish Young Scholars [grant number 21825803].