Maintaining high magnesium/lithium selectivity in nanofiltration at high salinity: optimizing membrane charge distribution for synergistic Donnan and ion adhesion effects
Wenyan Ji, Ping Li, Feng Duan, Lulu Liu, Renqiang Cao, Jingya Yin, He Sun, Jianquan Luo, Hongbin Cao
Abstract
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 Mg 2+ /Li + up to 51. Notably, it maintained over 96.21 % MgCl 2 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 Mg 2+ /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.