Integration of charge repulsion and size exclusion effects into the skin layer matrix for enhanced Mg2+/Li+ nanofiltration separation

Nanofiltration (NF) is commonly recognized as one of the effective technologies for the separation of magnesium (Mg 2+ ) and lithium (Li + ) from salt-lake brines. Charge repulsion and size exclusion effects often play an important role, whose functions primarily occur at the top surface and the matrix, respectively, of the skin layer. In this study, a novel nanofiltration (NF) membrane is prepared that integrates both size exclusion and charge repulsion effects into the matrix of the skin layer for enhanced Mg 2+ /Li + separation. With the surface-modified hydrophilic polyethylene (PE) membrane as the supporting layer, the prepared NF membrane successfully incorporated Fe 3+ ions into the matrix of the skin layer during the interfacial polymerization (IP). Fe 3+ ions which chelate with amine groups of PEI (aqueous-phase monomers during IP) ensure abundant positive charges within the matrix of the skin layer, providing good hydrophilicity and sustained electrostatic repulsive force against the transmembrane transport of Mg 2+ , and thus the enhanced rejection. Results exhibited a remarkable 92.6 % rejection rate for MgCl 2 and a higher water permeance of 22 LMH/bar, which is 1.57 times greater than that of the traditional PEI-TMC membranes. The nanofiltration process effectively reduced the Mg 2+ /Li + ratio from 50 in the feed to 5.4 in the permeate solution. It is also interesting to find that although the pore size of the skin layer (0.53 nm in radius) exceeds both the hydrated radii of Mg 2+ and Li + , the positive charges within the matrix of the skin layer still provide good selective separation performance, thus effectively breaking the trade-off of permeability and selectivity. This study offers a fresh perspective on the preparation of nanofiltration membranes by systematically exploring the potential of integration of charge repulsion and size exclusion effects into one location, i.e., the matrix of the skin layer for enhanced Mg 2+ /Li + separation performance.