Waste-derived carbon fiber membrane with hierarchical structures for enhanced oil-in-water emulsion separation: Performance and mechanisms

A superhydrophilic and underwater superoleophobic membrane based on waste-derived carbon fibers was prepared with the assistance of tannic acid and (3-Aminopropyl)triethoxysilane. The surface morphology, chemistry, roughness and wettability of the membranes were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, atomic force microscopy, water contact angle (WCA) and underwater oil contact angle (UWOCA) measurements, respectively. Carbon fibers enabled the construction of micro-nano hierarchical structures on the membrane surface, which significantly increased the membrane surface roughness (2.1 times higher than the control) and effective surface area (1.6 times higher than the control). The carbon fiber membrane exhibited superhydrophilicity (WCA = 0°) with ultrafast water spreading in air and underwater superoleophobicity (UWOCA = 157.2°) with little oil adhesion. The carbon fiber membrane showed effective surfactant stabilized oil-in-water emulsion separation using various oils (permeation flux up to 713 Lm-2h-1 and separation efficiency up to 99.8%). We also proposed an integrated model to elucidate how the superhydrophilic and underwater superoleophobic property and surface roughness interact with the size exclusion mechanism by considering dynamic flows near the membrane surface, which further reveals the anti-fouling and self-cleaning properties of our carbon fiber membrane. Our model provides new insights into developing specially-wettable membranes with high fouling resistance for wide wastewater treatment applications based on surface wettability and roughness.