关键词: 纳滤, 氮烷基化反应, 锂镁分离, Donnan排斥效应, 选择性

Abstract:

Positive charge modification of nanofiltration membrane surface is expected to improve lithium-magnesium separation selectivity, but conventional surface positive charge modification methods are inefficient, the positive charge density of the membrane surface is low, the electrostatic shielding effect is difficult to eliminate, and the improvement of lithium-magnesium selectivity is limited. In this study, polyethyleneimine (PEI) molecules rich in amine groups were grafted onto the surface of a commercial polyamide nanofiltration membrane, creating a surface-modified membrane (PEI/PA). The surface amine groups were then quaternized in situ through alkylation, producing a membrane with significantly enhanced positive charge (N-alkyl@PEI/PA). After modification, the isoelectric point of the membrane surface increased from 4.4 to 7.9, indicating a successful enhancement of surface charge. The performance of the modified membrane was further evaluated using a Li⁺/Mg²⁺ mixture with a concentration of 1000 mg·L^-1 and a mass ratio of 10∶1. The results showed that surface modification had minimal impact on membrane flux. At pH 6.8, its Li⁺/Mg²⁺ separation factor reached 35.7. At pH 9.7, the reduced protonation and weakened positive charge of the PEI/PA membrane led to a 72% decrease in lithium-magnesium selectivity, whereas the N-alkyl@PEI/PA membrane, with its higher positive charge, exhibited a smaller decline in performance, maintaining a separation factor of 21.9. This demonstrates the membrane’s potential for efficient Li⁺/Mg²⁺ separation across a broad pH range.

Key words: nanofiltration, nitrogen alkylation reaction, Li⁺/Mg²⁺ separation, Donnan effect, selectivity