Cellulose Acetate Membranes with a Low COF Loading for Efficient Li+/Mg2+ Separation

Mixed-matrix membranes (MMMs) incorporating covalent organic frameworks (COFs) offer a promising strategy to overcome the permeability-selectivity trade-off effect in conventional polymer membranes for ion separation. However, agglomeration of COF fillers often leads to structural defects and performance degradation in MMMs. Herein, we report in situ synthesized CA@COF MMMs by integrating fibrous COF (TpPaCOOH) with cellulose acetate (CA) at a low loading of 1.63 wt%, achieving hydrogen bond-enhanced interfacial compatibility and uniform filler dispersion. In the binary-ion system for Li + /Mg 2+ separation, despite this minimal COF content, the resulting CA@TpPaCOOH membrane exhibits 21-fold and 16-fold enhancements in Li + /Mg 2+ selectivity compared with pristine CA and COF membranes, respectively, reaching a selectivity of ∼51 and a Li + flux of ∼0.3 mol m -2 h -1 . Moreover, the CA@TpPaCOOH membranes demonstrate a scalability with consistent performance across batch-to-batch synthesis and retention of >83.2% selectivity in pilot-scale (900 cm 2 ) preparations. This highly reproducible, low-dosage strategy provides a transformative pathway for efficient lithium recovery from brine.