Designing scalable dual-layer composite hollow fiber nanofiltration membranes with fully cross-linked ultrathin functional layer

Abstract The treatment of industrial wastewater containing complex foulants demands high efficiency yet cost-effective separation process. The composite hollow fiber nanofiltration membranes suffer from tedious fabrication procedures, impeding the large volume fabrication and application. Herein, we reveal a very simplified designing strategy by polyelectrolyte modification of the co-extruded polyimide/polyethersulphone dual-layer hollow fibers with hierarchical structure and chemistry. The active layer's thickness of the dual-layer hollow fiber substrate is only 800 nm, the thinnest in literature. Compared to integral active single layer hollow fiber, the ultrathin active layer not only leads to over 4 times increase in mechanical flexibility and tenfold reduction in material cost but also enables fully chemical transition from imide to cross-linked polyamide, minimizing the intrinsic hydrolysis tendency of active material. The optimized membranes with a well-defined pore radius of 0.23 nm have 93–95% rejections to Mg2+ and Ca2+ and PWP of 9.1 LMH/bar. Moreover, the composite membranes exhibit backwash ability and stability in the heated feed solutions, which meets the demand for direct treatment of industrial effluents. This work paves the way for the large volume fabrication and application of hollow fiber nanofiltration membranes for treating municipal water, industrial wastewater, and other applications.