Facile fabrication of superhydrophilic and underwater superoleophobic nanofiber membranes for highly efficient separation of oil-in-water emulsion

Electrospun nanofiber membrane (ENM)-based filtration is an advanced technology that treats wastewater for reuse using gravity or low pressure as a driving force, thereby solving water scarcity in a way that is suitable for conditions of energy scarcity. The high hydrophobicity of ENMs remains the main challenge for their application in the treatment of oily wastewater. Although some techniques have succeeded in enhancing surface wettability, their disadvantages (e.g., difficulty, harsh operating conditions, harmful environmental effects, time consumption, or high costs) restrict the scalability and industrial application of those techniques. Herein, superhydrohilic and underwater superoleophobic ENMs were prepared using a two-step metal-phenolic network (MPN) coating process as a scalable, cost-effective, green, and powerful technique. The fabricated nanocoated-ENMs showed superhydrophilicity at air and even underoil, as well as underwater superoleophobicity. Thus, they could separate the oil-in-water mixture and surfactant-stabilized oil-in-water emulsion with outstanding flux values of 6.5 x 104 and >6.0 x 103 L/m2.h, respectively, and a remarkable recovery ability (up to 99.8%) and excellent oil rejection (up to 99.9%), using only gravity as a driving force. More importantly, the nanocoated-ENMs showed a stable and ultrahigh flux up to 13,756.7 L/m2.h, with a separation efficiency of 97.5%, for surfactant-stabilized oil-in-water emulsion in a continuous cross-flow separation system, at an ultralow transmembrane pressure of 5 kPa. Additionally, the nanocoated-ENMs exhibited excellent chemical stability, durability, and robust reusability under harsh environments. Interestingly, the obtained fluxes are more superior to most reported values, at the same conditions, and higher than that of the commercial membranes with one to two orders of magnitude, pointing to the significant applicability for energy-saving large-scale oily wastewater treatment process.