Ternary-coordination-regulated polyamide nanofiltration membranes for Li+/Mg2+ separation

Thin-film composite (TFC) membranes prepared by the interfacial polymerization (IP) of amines and trimesoyl chloride (TMC) have been widely utilized for Li+/Mg2+ separation from salt-lake brine. The relatively high lithium rejection hinders the achievement of high lithium extraction efficiency. In this study, a polyphenol-metal-assisted method was designed to regulate the structure of PEI-based polyamide nanofiltration (NF) membranes. Tannic acid (TA) was first deposited onto the substrate, followed by the impregnation of the Cu2+ ions-incorporated aqueous PEI monomer solution to establish ternary interactions among PEI, TA, and Cu2+ ions. Based on the synergistic effect of ternary interactions, the resultant PA-TA-Cu membrane possesses ultrathin thickness (≈11 nm), high positive charge, and relatively loose structure. It exhibited a high MgCl2 rejection (95.9 %) and a low LiCl rejection (17.1 %), with a pure water permanence of 4.87 L m−2 h−1 bar−1. Furthermore, the NF membrane exhibits a high Li+/Mg2+ separation factor (26.5) and excellent long-term stability. This study provides an efficient strategy for fabricating an excellent NF membrane for lithium extraction from salt-lake brines.