Positively Charged NF Membranes with Co-Enhanced Donnan and Size-Sieving Effects Fabricated Toward Efficient Li⁺/Mg²⁺...

Extracting lithium from salt-lake brines boasts distinct advantages including low production cost, low energy consumption and low environmental risks, and will serve as a primary supply source of lithium salts in the future. This trend raises higher demands for the efficiency and cost-effectiveness of lithium extraction technologies. Nanofiltration (NF) membranes, renowned for their superior discrimination between monovalent and divalent ions, have been extensively utilized to obtain Li⁺ from Mg²⁺-rich saline brines. In this study, positively charged NF membranes aimed at Li⁺/Mg²⁺ fractionation were fabricated via surfactant-interlayer-assisted interfacial polymerization (SIAIP). Catechol (CA) and polyethyleneimine (PEI) were utilized to construct the CA/PEI interlayer, and oil-phase dodecyl phosphate (DDP) was used as an additive for interfacial polymerization (IP). The strongly bonded CA/PEI nanoaggregates improved interlayer stability and preserved the positive charge of the double-layer membrane. DDP adsorbed piperazine (PIP) at the two-phase interface through electrostatic interactions, accelerating PIP diffusion and forming a thick polyamide (PA) layer with uniform pores. The combination of CA/PEI interlayer and DDP synergistically enhanced size-sieving and Donnan effects. With MgCl₂ and LiCl rejections of 97.9% and 36.2% respectively, the optimized membrane shows superior selectivity for Li⁺ over Mg²⁺. Moreover, the membrane exhibited weak electrostatic screening and concentration polarization, showing excellent operational stability under varied Mg²⁺-Li⁺ ratios and feed concentrations.