Dual-functionalized wood fibers with region-specific wettability for switchable oil/water separation systems.

The treatment of complex oily wastewater demands advanced separation materials capable of handling both oil-in-water (O/W) and water-in-oil (W/O) emulsions. Conventional membranes are often limited to a single separation function, or their Janus counterparts suffer from complex fabrication processes and a reliance on non-green substrate materials. This study develops the dual-functional wood fibers (WFs) with spatially defined hydrophobic and hydrophilic domains for on-demand, switchable separation. A facile two-step modification strategy is employed: initial hydrophobic treatment of lignin-rich regions with 1-dodecanethiol (DDT), followed by hydrophilic and hydrophobic reinforcement of cellulose and hemicellulose-rich zones with glutaraldehyde-crosslinked polyethyleneimine (PEI). This precise chemical patterning produces WFs/DDT@PEI exhibiting unique wettability-hydrophilicity, lipophilicity, underwater oleophobicity and underoil hydrophobicity. Consequently, the WFs/DDT@PEI achieves highly efficient simultaneous separation of diverse O/W and W/O emulsions, with high separation fluxes up to 2150 L m-2 h-1 and separation efficiencies up to 99.60 %. The WFs/DDT@PEI also demonstrates excellent long-term operational stability, enabling continuous separation for over 270 min for O/W emulsions and 156 min for W/O emulsions with corresponding treatment volume of 1809 mL and 920 mL, alongside remarkable anti-fouling performance. This work provides a simple and effective method for creating sustainable, dual-functional separation materials from natural fibers for complex emulsion separation.