Dual Regulation of Monomer Transport Enables Ultrahigh Mg2+/Li+ Selectivity in Polyamide Nanofiltration Membranes

Nanofiltration membranes are promising for lithium recovery from salt-lake brines, yet their Mg2+/Li+ separation performance can be further improved. Here, we present an organic-phase engineering strategy that systematically tailors the carbon-chain length of alkane solvents, from n-hexane (C6) to 2-methylundecane (C12), to simultaneously regulate piperazine enrichment at the water–organic interface and trimesoyl chloride diffusion kinetics. This dual regulation transforms conventional rapid and random interfacial polymerization into a more confined and homogeneous growth process. The resulting polyamide membranes exhibit a reduced negative surface charge and a narrower and more uniform pore-size distribution. These structural features substantially enhance Mg2+/Li+ separation, with the C12 membrane achieving a Mg2+/Li+ selectivity of up to 306.6 in the mixed-salt solutions. To assess performance under realistic conditions, a two-pass nanofiltration process was evaluated using simulated salt-lake brine. The C12 membrane increased lithium purity from 2.4% to 97.7% while reducing the Mg2+/Li+ mass ratio from 40 to 0.024. This study provides new insight into the kinetic regulation of interfacial polymerization through organic-phase engineering and demonstrates an effective membrane design strategy for high-selectivity lithium extraction from complex brines.