Tannic acid encountering ovalbumin: a green and mild strategy for superhydrophilic and underwater superoleophobic modification of various hydrophobic membranes for oil/water separation

Superhydrophilic membranes have attracted significant attention due to their desirable anti-oil-fouling performance for oil/water separation. Although many strategies have been developed to prepare superhydrophilic membranes, few of them are completely green and can be applied for various highly hydrophobic materials. Since most of the widely used membrane materials are usually hydrophobic, an environmentally friendly and mild strategy that can transform various hydrophobic membranes into superhydrophilic ones is highly desired. Herein, for the first time, based on edible ovalbumin (OVA) and tannic acid (TA, a natural and environmentally friendly plant polyphenol), we have developed a novel, mild and completely green strategy to realize the superhydrophilic transformation of various hydrophobic membranes including polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), copper mesh, and fabric. The water-soluble OVA can be adsorbed on hydrophobic materials via hydrophobic interaction, and the adsorbed OVA further acts as a pre-coating to facilitate the subsequent introduction of abundant TA molecules. On the other hand, TA can not only provide abundant hydrophilic groups, but can also induce the degeneration of the adsorbed OVA layer, realizing the formation and firm immobilization of hybrid coatings decorated with abundant nanonodes. The resulting membranes exhibit superhydrophilicity, underwater superoleophobicity (all underwater oil contact angles are above 155° and oil roll-off angles are below 5°) and favorable anti-oil-fouling performance and thus, high-efficiency oil/water separation can be realized. In addition, the membranes can maintain their superhydrophilicity and underwater superoleophobicity after continuous rinsing treatment with salt and surfactant solutions for 6 h or ultrasonic treatment for 5 min, demonstrating their favourable stability.