Common anion and piperazine-based monomer chemistry tune Na+/Mg2+ selectivity in nanofiltration: A mechanistic mass-transport analysis

Selective separation of Na + and Mg 2+ is fundamental to softening and brine valorization, yet the joint roles of monomer chemistry and electrolyte environment in nanofiltration (NF) remain incompletely resolved. In this work, we report an exceptionally high Na + /Mg 2+ selectivity of 244, achieved using a thin-film composite membrane with a polyamide selective layer formed via interfacial polymerization. Systematic studies were conducted on membranes prepared with controlled variations in the aqueous-phase monomers, 1,4-bis(3-aminopropyl)piperazine (DAP) and 2-aminoethylpiperazine (AEP). The highest Na + /Mg 2+ selectivity was obtained for the membrane synthesized with 0.5 % w-w DAP in the aqueous phase. The variation of aqueous-phase monomer concentration allowed us to tune the polyamide layer structure and charge, which altered ion transport resistance. Ion separation performance was evaluated using single- and mixed-salt systems (NaCl + MgCl₂, Na₂SO₄ + MgSO₄), supported by Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE) analysis to quantify steric, Donnan, and dielectric partitioning and their coupling to advective, diffusive, and electromigrative fluxes. The common anion (Cl − vs SO₄ 2− ) critically governed Na + /Mg 2+ selectivity by shifting the balance between Donnan and dielectric exclusion. In Cl − feeds, tighter pores and smaller fitted dielectric constants strengthened dielectric exclusion of Mg 2+ , driving a steeper suppression of Mg 2+ transport and yielding the highest Na + /Mg 2+ selectivity of 244 for DAP (0.5 % w-w). In the SO₄ 2− system, higher fitted dielectric constants favored Donnan partitioning, with selectivity peaking at 66 for AEP at the highest concentration tested (1.0 %w-w). In positively-charged membranes, cation transport was diffusion-dominated (>80 % of flux), while anion fluxes show strong advection or diffusion–electromigration cancelation. These results establish a mechanistic basis linking monomer chemistry, common anion identity, and coupled ion transport, guiding system-specific NF membrane design for selective Na + /Mg 2+ separation.