Facile surface coating of metal-tannin complex onto PVDF membrane with underwater Superoleophobicity for oil-water emulsion separation

In this study, inspired by self-cleaning property of fish scale in water, a novel superhydrophilic and underwater superoleophobic tannin-metal complex@polyvinylidene fluoride (TA-Fe@PVDF) nanocomposite membrane was prepared by a facile and one-pot surface coating technology at room temperature using the ferric nitrate and tannic acid as the building blocks. The as-fabricated TA-Fe@PVDF nanocomposite membrane was characterized by scanning electron microscopy (SEM), attenuated total reflection fourier transform infrared spectroscopy (ATR-IR), atomic force microscope (AFM) and contact angle measuring instrument. It was found that the TA-Fe complexes improved the surface wetting, separation efficiency and repeatability. The optimal TA-Fe@PVDF-1 composite membrane was obtained at the concentration ratio of TA and Fe³⁺ of 1:3, which show underwater oil contact angle of 155°, and low oil adhesion. The separation efficiencies of TA-Fe@PVDF-1 composite membrane for various oil-water mixtures and oil-in-water emulsions are higher than 99.5%. The membrane flux can reach 2551 L m⁻² h⁻¹ bar⁻¹ for surfactant-stabilized petroleum ether-in-water emulsion separation, and 4505 L m⁻² h⁻¹ bar⁻¹ for petroleum ether/water mixture. This nanocomposite membrane exhibits excellent repeatability and good chemical stability (salt/acid/alkali resistance), to be potentially applied for the treatment of oily wastewater.