Maintaining high magnesium/lithium selectivity in nanofiltration at high salinity: optimizing membrane charge distribution for synergistic Donnan and ion adhesion effects

Nanofiltration membranes are essential for lithium recovery from brine, but their selectivity declines under high-salt conditions due to electrostatic shielding and fouling. We introduce a novel strategy to enhance membrane performance by enhancing positive charge density on the membrane surface while creating negative charge microdomains within the membrane. Specifically, we functionalized a polyethyleneimine-based membrane with 1,4-butanesultone through a ring-opening reaction, which balanced ion-membrane affinity and optimized the transport kinetics of water and ions. The resulting membrane exhibited a significant enhancement in permeance up to 11.2 L m⁻² h⁻¹ bar⁻¹ and selectivity for Mg²⁺/Li⁺ up to 51. Notably, it maintained over 96.21 % MgCl₂ rejection across a wide salt concentration range of 1–20 g L⁻¹, outperforming the results reported in literature. The comprehensive testing and mechanistic study indicate that this improved performance is due to the synergy of Donnan effect and ion-attraction, facilitated by the designed charge distribution. A two-stage nanofiltration process employing this membrane reduced the Mg²⁺/Li⁺ ratio in brine samples from 43 to 0.25, highlighting its practical application potential. This work introduces a new paradigm for the design of highly selective membranes.