Multifunctional PVDF-TiO₂ composite coating with semi-exposed structure for bidirectional oil/water selective separation

In view of the intensified pollution of oily wastewater caused by industrialization and urbanization, as well as the separation challenges posed by the coexistence of stable oil-in-water (O/W) and water-in-oil (W/O) emulsions, this paper proposes a multifunctional coating solution integrating bidirectional emulsion separation and antibacterial properties to address the problems of single function and susceptibility to microbial contamination in existing separation materials. Through a stepwise spraying strategy, a novel PVDF-TiO₂ semi-exposed composite coating was constructed. In this design, polyvinylidene fluoride (PVDF) serves as both the film-forming substrate and a binder, immobilizing TiO₂ nanoparticles to form a micro-nano rough structure. By controlling the process, TiO₂ is semi-embedded on the PVDF surface, endowing the coating with superhydrophilicity and superoleophilicity, thereby achieving unique bidirectional wetting behavior characterized by underwater superoleophobicity and underoil superhydrophobicity. Experimental results demonstrate that the coating not only efficiently separates O/W and W/O emulsions (separation efficiency >99 %) but also rapidly treats immiscible oil/water mixtures (with a flux as high as 5.6 × 10⁴ L·m^-2·h^-1 and efficiency >98 %). Simultaneously, the exposed TiO₂ imparts photocatalytic self-cleaning ability and long-lasting antibacterial activity to the coating, effectively addressing the issue of membrane fouling. Furthermore, the coating features a simple preparation process, environmentally friendly raw materials, broad substrate adaptability, and exhibits remarkable mechanical durability. This study achieves functional synergy and complementary advantages between the film-forming property of PVDF and the functionality of TiO₂ through the semi-exposed structure, offering a novel strategy for developing multifunctional separation materials for the treatment of complex oily wastewater.