Electrospun hierarchically structured nanofibrous membrane for highly efficient oil-in-water emulsion coalescence separation

[Display omitted] •A hierarchical nanofibrous membrane was fabricated with enhanced roughness.•The skin layer with a nanofiber diameter of 59.1 nm improved separation efficiency.•The influence of various parameters on the separation performance was investigated.•Long-term separation showed the durability and effectiveness of the membrane.•Permeation flux up to 14,118 L m-2h−1 and a separation ratio of 99.9% were obtained. Oily wastewater poses a serious threat to the ecological environment due to the difficulty in efficiently and economically separating micron-sized oil droplets from the wastewater. Coalescence separation is an energy-efficient physical method used to separate oil from water and enable oil recycling. However, traditional coalescers are typically inadequate in effectively separating emulsified oil droplets in oil-in-water (O/W) emulsion. Herein, a hierarchically structured polyacrylonitrile (PAN) nanofibrous membrane was fabricated by depositing a skin layer with extremely fine nanofiber onto a common nanofiber substrate layer. The skin layer enhanced the surface roughness and provided hierarchical pore channels, which increased the effective collision positions for oil coalescence. The results revealed the PAN nanofibrous membrane with a skin layer exhibited a separation ratio of over 99.4% and a permeation flux of 7,059 L m-2h−1 for surfactant-stabilized emulsions (SSEs). Additionally, long-term separation experiments demonstrated that the PAN membrane could continuously separate 7.2 L SSEs in 3 h. Moreover, PAN membranes behaved with excellent separation ratio and permeation flux at different flow rates under a large range of initial oil concentrations. These findings highlight the potential of hierarchically structured membrane for separating emulsions and suggest its promise for the industrial separation of oily wastewater.