Transforming Cellulose Waste into Advanced Superhydrophobic Membranes for Effective Oil/Water Separation

Separation of oily wastewater and emulsions is essential for environmental protection and for curbing the associated health and economic consequences. Recently, there has been tremendous interest in developing materials with special wettability toward the oil and water phases and utilizing them for oil/water separation applications. These materials are designed by precisely tuning the surface chemistry, surface energy, and roughness. Herein, we report a novel two-step approach for upcycling cellulose-rich wastepaper into superhydrophobic membranes. Wastepaper first underwent femtosecond laser surface structuring inside a dilute aqueous solution of FeCl₃ to create controlled surface roughness features and air-trapping pockets for hydrophobicity enhancement. The laser-structured sample was then allowed to cross-link in the same FeCl₃ solution to enable surface nanoengineering that involved coordination of the Fe³⁺ ions with the polar functional groups on the nanofibrils of the wastepaper fibers. The optimized membrane exhibited a water contact angle of 153°, endowed by the laser-induced roughness features and the water repellency imparted by the cross-linked, self-assembled nanofibrils. The membrane achieved nearly complete separation of immiscible mixtures of water with n-hexane, n-dodecane, and diesel. In addition, separation of a water-in-diesel emulsion was possible with a separation efficiency of 93%. Our results demonstrate that femtosecond laser structuring plays a critical role in imparting high hydrophobicity and durability to the membrane and boosting the oil flux through the creation of surface microchannels.