Structural tailoring of poly(quaternary ammonium) thin-film composite membranes for efficient lithium recovery from concentrated salt-lake brine

The application of conventional polyamide nanofiltration (NF) membranes to lithium (Li) recovery from high-salinity salt-lake brine is challenging, despite their high separation efficiency, owing to their intrinsic negatively charged nature. Herein, we developed highly Li-selective, positively charged poly (quaternary ammonium) (PQA) thin-film composite NF membranes via an interfacial Menshutkin reaction by varying the structure of the linear tertiary amine monomers used. Less bulky tertiary amine monomers with shorter alkyl chain spacers and/or fewer amine groups produced a denser, more Li-selective PQA membrane with a strongly positive surface charge. Hence, the PQA membrane fabricated with the least bulky tertiary amine monomer exhibited the highest magnesium ion (Mg²⁺) rejection (∼99.4 %) and Li⁺/Mg²⁺ selectivity (∼90) under high-salinity mixed-salt conditions (10,000 ppm, Mg²⁺:Li⁺ mass ratio = 20:1). This performance considerably exceeded that of other reported Li-selective NF membranes, underscoring the potential of the PQA membrane for efficient Li recovery from concentrated salt-lake brine. The superior Li selectivity of the PQA membrane was attributed to its strongly positive surface charge and moderate molecular density. Our study provides a versatile means to fabricate advanced membranes for resource recovery or hazardous matter removal and elucidates their structure−property−performance relationship.