Sustainable layer-by-layer assembled nanofiltration membranes with optimized pore size for lithium-magnesium selective separation

The main challenge of lithium (Li + ) extraction in salt-lake brine arises from the high magnesium (Mg 2+ ) to Li + mass ratio, further complicated by the similar chemical properties of Mg 2+ and Li + . To achieve effective separation of Li + from Mg 2+ , we developed novel nanofiltration (NF) membranes using a rapid layer-by-layer (LbL) assembly technique. Branched polyethyleneimine (PEI) as the polycation and lignosulfonate (LS) as the polyanion were sequentially sprayed over a polyamide-imide (PAI) substrate. Filtration of single salt (MgCl 2 and LiCl) and mixed salt solution revealed that double-layer polyelectrolyte coating resulted in optimal permselectivity. In a binary feed solution with Mg 2+ /Li + mass ratio of 20 and a concentration of 1000 ppm, the modified membrane rejected 80 % of Mg 2+ and 6 % of Li + , achieving a selectivity factor of 4.7 while maintaining a high permeate flux of 60 LMH. Our findings show that the interconnected polyelectrolyte network regulates the pore size of the modified membrane and enhances steric hindrance, significantly improving lithium and magnesium separation efficiency. Additionally, the internal positive charge, resulting from the dissociation of the PEI layer, further increases the membrane’s positive charge and enhances ion selectivity. In addition, the LbL modification significantly reduced the Mg 2+ /Li + ratio in the permeate from 20 in the feed to 6.5 and 8.7 in the permeate when using 1000 and 5000 ppm feed solutions, respectively. Long-term filtration over 24 h demonstrated consistent rejection for both cations. Moreover, utilizing LS improved the membrane antifouling characteristics, resulting in a 30 % reduction in flux decline. • A rapid, eco-friendly spray-assisted layer-by-layer method was used to fabricate lithium-selective NF membranes. • Mg²⁺/Li⁺ separation was achieved with a SF of 4.7 using double-layer polyelectrolyte NF membranes. • A high permeate flux of 60 LMH was obtained while rejecting 80 % Mg²⁺ and 6 % Li⁺ from a binary salt solution. • Lignosulfonate (LS) incorporation reduced fouling, achieving a 30 % reduction in FDR during long-term filtration. • Lower fabrication time and waste generation were achieved compared to conventional NF membranes, enhancing scalability.