Engineering superwetting membranes through polyphenol-polycation-metal complexation for high-efficient oil/water separation: From polyphenol to tailored nanostructures

Utilizing membrane to deal with the issues of oily wastewater discharge from the processing of petroleum or gas products, oil field exploitation and oil leakage has been widely concerned. Superwetting membrane materials have hold great promising for oil/water separation in recent years. However, a facile strategy to design superwetting membranes with specific micro- and nanostructures is still challenging. In this study, we develop superhydrophilic polycation-polyphenol-metal composite membranes via a facile in situ polyphenol assembly strategy. The assembly of polyphenol (tannic acid, TA)-polycation (polyethyleneimine, PEI) polyelectrolyte complex into nanoconjugates aims to construct a nanostructured coating, and the further complexation of metal ions (M) aims to assemble metal-phenolic networks into TA-PEI nanoconjugates. The resulting dual cross-linked TA-PEI/M composite coatings on polyvinylidene fluoride (PVDF) membrane surfaces show nanopapillae-like structures and excellent superhydrophilicity. The superhydrophilicity of the PVDF/TA-PEI/M membranes greatly promote membrane permeability by more than 100%. The superhydrophilic PVDF/TA-PEI/M membranes are also underwater superhydrophobic. The opposite wettability towards water and oil for the PVDF/TA-PEI/M membranes promises their outstanding oil/water separation efficiency (flux above 3000 L/m2h and oil rejection above 99.5%) and anti-oil-fouling performance. Moreover, the representative PVDF/TA-PEI/Ti4+ membrane also exhibits outstanding chemical and long-term stability thanks to the robust dual cross-linking. This work provides a novel and effective polyphenol-polycation-metal complexation strategy to design and fabricate superwetting membrane for oil/water separation.