Enhancement in Li⁺/Mg²⁺ separation from salt lake brine with PDA–PEI composite nanofiltration membrane

Extraction of lithium from high Mg²⁺/Li⁺ ratio salt lake brine with the nanofiltration (NF) membrane is significantly challenging. Interfacial polymerization was utilized for the facile modification of NF membranes with polydopamine (PDA) and polyethylenimine (PEI) to enhance lithium separation efficiency. Comparing permeability and salts rejection (Li⁺ and Mg²⁺) of three NF membranes before and after PDA/PEI deposition, it was observed that separation efficiency was not only dependent on steric hindrance but also affected by Donnan exclusion mechanism. In the case of NF270 membrane after facile polymerization, due to small pore size distribution and low charge density confirmed by zeta potential measurements, Li⁺ permeability was reached about 95% at a flux of 21.33 L·m⁻²·h⁻¹. Although with the DK membrane, separation factor SF_Li/Mg was also increased up to 60 after modification, the pore narrowing effect significantly decreased lithium permeability and flux. Experimental results showed that facile modification not only enhanced stability and hydrophilicity but also reduced the high Mg²⁺/Li⁺ ratio from 30 to 4.1 in single-stage separation.