Flexible Lignin-Based Carbon Nanofiber Membrane for Purification of Emulsified Oil Wastewater

Carbon nanofiber membranes (CNFMs) have broad application prospects in the field of oil–water separation due to their excellent stability, energy-saving properties, lack of secondary pollution, and high efficiency. However, the hydrophobicity, lipophilicity, and brittleness of carbon materials limit the application of CNFMs in oil–water separation. In this study, polyacrylonitrile (PAN), poly(lactic acid) (PLA), and lignin were used as raw materials for electrospun membranes. CNFMs were obtained by carbonizing the electrospun membranes, while PLA was thermally decomposed during heat treatment to form pores. The relationship between the pore structure and CNFM flexibility has been well discussed. When the ratio of micropores to mesopores approaches 1:1, Young’s modulus reaches 124.61 MPa after optimizing the heat treatment process. Lignin was introduced to improve the hydrophilicity of CNFMs due to its high hydrophilic group content. The contact angle test shows that CNFMs exhibited superhydrophilicity (water contact angle, WCA = 0) and excellent hydrophobicity (oil-in-water contact angle, OCA = 149.5°). The gravity-driven penetration test and total organic carbon (TOC) content analysis indicate that the separation efficiencies of the CNFMs for nonemulsified oil–water and emulsified water-in-oil mixtures were 99 and 97%, respectively. The TOC value of water separated by CNFM is 38.6 mg/L, which reaches the direct emission standards of the petrochemical industry in the integrated wastewater discharge standard (GB 8978-1996). This strategy has significance for designing simple, efficient, and green oil–water separation membranes.