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

Abstract:

Positively charged nanofiltration (NF) membranes hold promise for enhancing Li⁺/Mg²⁺ separation selectivity. However, conventional surface modification methods are inefficient, resulting in low surface charge density and difficulties in eliminating the electrostatic shielding effect, which limits the enhancement of lithium-magnesium selectivity. 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 and a mass ratio of 10:1. 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 75% 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