Sodium hypochlorite activated dual-layer hollow fiber nanofiltration membranes for mono/divalent ions separation

The tailoring of nanofiltration membranes for the precise separation of mono/divalent ions often involves complicated procedures to modify the surface chemistry and structure. Herein, an efficient sodium hypochlorite (NaClO) activation method is developed to tune the selective layer of cross-linked polyimide (PI)/polyethersulfone (PES) dual-layer hollow fiber membranes. By oxidizing the hydroxyl and imine groups while hydrolyzing the amide groups in the cross-linked structure, the membranes are effectively tailored with the negatively charged surface and a more open pore structure with 0.47 nm mean effective pore size. The NaClO-activated membranes show over 97.85% rejection of Na₂SO₄ while the NaCl rejection decreases from 93.83% to − 19.85% during treating the mixed ions solutions. The fabricated membranes with high separation efficiency and long-term stability exhibit vast potential for the separation of mono/divalent ions in industries. Moreover, the elaborated NaClO activation mechanism potentially gives fundamentals for the modification of the PI membranes with cross-linked structures for other applications.