Preferential Transport of Mg2+ over Li+ in Negatively Charged Styrene Sulfonate-Regulated Graphene Oxide Nanochannels

Efficient lithium recovery from salt-lake brines is severely hindered by their high Mg2+/Li+ ratios. Recent nanofiltration technologies primarily directly extract lithium from brine, which incurs high energy consumption. Herein, using pressure-driven molecular dynamics simulations at 5 MPa (representative of practical operating pressures), we propose a strategy for preferential Mg2+ transport over Li+ to lower the Mg2+/Li+ mass ratios for further low-energy lithium extraction, based on intercalating styrene sulfonate inside graphene oxide nanochannels (SS–@GO). The results demonstrate a reversal in permeation selectivity from Li+ to Mg2+ upon negatively charging the lamellar channel through SS– insertion at an interlayer spacing of 1.8 nm (d). The Mg2+/Li+ selectivity exhibited a nonmonotonic dependence on negative charge density, first increasing and then decreasing, with the highest selectivity obtained at ρ = −2.07 × 10–3 e/Å2. The Mg2+/Li+ separation factor of the SS–@GO membrane can reach 16.4 at an Mg2+/Li+ mass ratio of 35. Notably, we observe that not all ions entering the channel successfully permeate, and some are reflected back to the feed side, particularly at higher negative charge densities. These findings provide molecular guidance for designing Mg2+-selective GO membrane to pretreat brines with high Mg2+/Li+ mass ratios prior to lithium recovery.