Activated carbon fibers with different hydrophilicity/hydrophobicity modified by pDA-SiO₂ coating for gravity oil–water separation

Three-dimensional (3D) superwetting materials have attracted significant interest in oil–water separation due to their high selectivity and notable flux. However, complex preparation procedures and material contamination severely limit their practical application in oily wastewater. This study was inspired by the mussel strategy to prepare hybrid coated pDA-SiO₂ by simultaneous polymerization of dopamine (DA) and hydrolysis of tetraethyl orthosilicate (TEOS) under weak alkaline conditions. They were firmly deposited on hydrophilic activated carbon fibers (ACF₁) or hydrophobic activated carbon fibers (ACF₂) by a simple dip-coating method to form pDA-SiO₂-ACF₁ or pDA-SiO₂-ACF₂ with excellent superwetting, to be used for oil–water separation under gravity drive. The pDA-SiO₂-ACF₁ and pDA-SiO₂-ACF₂ exhibited excellent hydrophilicity and underwater lipophobicity due to the synergistic effect of hydrophilic groups and nanostructure roughness, their water contact angles were 0° (WCA), and the underwater oil contact angles (UWOCA) of different viscosities were greater than 150°. Furthermore, it provided ultra-high permeation flux (pure water flux ≥ 2.7 × 10⁵ L·m⁻²·h⁻¹) due to numerous pores and super-large pore size. Meanwhile, the interweaving of the fibers inside the material made it possible to separate the oil droplet emulsion smaller than its particle size. Based on the superwetting and the material's properties, the composite material had superior oil rejection efficiency and separation performance for oil–water mixtures, sodium dodecyl sulfate (SDS) stabilized oil–water emulsion, and actual oily wastewater. More interestingly, the pDA-SiO₂ decorated ACF₁ and ACF₂ had remarkable oil resistance stability and recyclability after the water washing treatment and 25 cycles of emulsion separation. Therefore this 3D material, with its simple manufacturing method, high flux, and super oil resistance, offers significant potential for treating oily wastewater.