Constructing Cationic Polyamide Membranes by Surface-Initiated Grafting for Precise Lithium Extraction from Salt Lake Brines

Efficient Li+/Mg2+ separation from salt-lake brines is vital for lithium recovery but is challenged by the limited charge selectivity of conventional polyamide nanofiltration (NF) membranes. Here, we engineer a stable, high-density positive charge on a PA membrane via a controlled "graft-from" strategy. A designed trithiocarbonate (TTC) moiety is first anchored covalently into the PA network as an immobilized reversible addition-fragmentation chain-transfer (RAFT) agent. Photoinduced RAFT polymerization then grafts a quaternary ammonium polymer from these sites. The resulting cationic membrane maintains a stable positive potential (pH 3-10), a pure water permeance of 15.8 L m-2 h-1 bar-1, and >90% rejection of divalent cations. It achieves high Li+/Mg2+ separation factors (19.2-24.8) under simulated brine conditions (Li+/Mg2+ mass ratio 1:5 to 1:100) with stable 7 days of operation. This work provides a precise "graft-from" platform for fabricating charge-engineered NF membranes, demonstrating great potential for selective ion separation in lithium extraction from complex brines.