Hybrid membranes based on electrospun nanofiber modified with PDA for selective oil-in-water emulsion separation

Separation membranes for water remediation require structural stability, efficient operation, and durability. In this work, we synthesize hierarchical nanofibrous membranes composed of polyacrylonitrile (PAN) reinforced with cellulose nanocrystals (CNC) and polydopamine (PDA). Hydrolysis endows the PAN nanofibers with rich carboxyl functional groups and forms a hydrogen-bonded cross-linked network with CNC. This process helped maintain the structural integrity of the membrane and developed swelling resistance, which was lacking in the CNC/PAN membrane. In a further PDA modification, PDA could still firmly adhere to the nanofiber surface even after sonication or homogenization treatment, showing a high surface roughness (Ra = 2.86 nm). Finally, the membrane displayed a tensile strength of 4.19 MPa, while the pore size was around 0.67 μm. The special wettability and pore structure of the membrane can trap more water, forming an oil/water/solid interface, thereby reducing the contact between oil droplet and membrane and effectively preventing the escape of tiny oil droplets, allowing water to permeate rapidly (oil-repellency in an aqueous). For example, in the phase separation test of three oil-in-water (O/W) emulsions (n-hexane, diesel, toluene), the separation efficiency of the O/W emulsion remained above 93% after 20 filtration cycles, which allowed regeneration and cyclic operation. The proposed method is a novel route to prepare non-swellable yet hydrophilic membranes comprising highly interconnected channels while exhibiting mechanical and structural integrity.