Piperazine concentration-driven effect fabricated NF membrane with ambidextrous selectivity for SO₄²⁻/Cl⁻ and Mg²⁺/Na⁺: Mechanism and efficient water purification

Nanofiltration (NF) membranes with efficient selectivity are critical for addressing water source pollution from toxic heavy metals and emerging micropollutants. Herein, we engineered a polyamide (PA) NF membrane with ambidextrous selectivity by leveraging the concentration-driven effect of piperazine (PIP) during interfacial polymerization (IP). This PIP concentration-driven effect was verified by modulating the PIP concentration gradient from low to high, which enhanced IP efficiency via both an intensified alkaline reaction environment and concentration-driven diffusion. The optimized NF membrane features an ultrafine pore size distribution and reduced electronegativity, exhibiting a remarkable shift from sole SO₄²⁻/Cl⁻ selectivity to impressive ambidextrous selectivity, encompassing both SO₄²⁻/Cl⁻ (α ≈ 48, 98.3 % for Na₂SO₄) and Mg²⁺/Na⁺ (α ≈ 37, 98.2 % for MgCl₂). Moreover, it membrane also achieved exceptional removal efficiencies of 94.8–98.9 % for heavy metals and 77.6–99.3 % for antibiotics, while maintaining a water permeability of 19.3 L/m² h bar and mitigating the perm-selectivity trade-off. When purifying surface water containing heavy metals and antibiotics, the optimized NF membrane exhibited superior multi-pollutant removal and improved water production quality compared to the NF270 and NF90 membranes, and especially with its water permeance increased by ∼27 % to 179 %. Our work uncovers a fundamental correlation between the PIP concentration, IP reaction, PA nanostructures and NF performance, providing valuable insights into customizing high-performance NF membranes to address complex water pollution challenges.