Spatially hierarchical hydrophilic coatings on PVDF membranes by vacuum deposition for efficient separation of oil-in-water emulsions and dye wastewaters

Oil scaling poses a significant threat to the stability of membrane separation flux during the process of oil-water emulsion separation. A single hydrophilic modified layer demonstrates the capability to effectively separate emulsions. However, it does not prevent severe water flux degradation. Although the “dual-defense” design has been proposed, the construction of a double hydrophilic anti-fouling process on the surface of the membrane is prone to clogging of the membrane, and the surface is difficult to regulate. Here, we designed a two-layer antifouling strategy using vacuum deposition method, in which the first barrier is formed by dopamine (DA) and polyethyleneimine (PEI)-coated carbon nanotubes (CNTs), and the second barrier is formed by a polyelectrolyte complex of phytic acid (PA) and polyethyleneimine (PEI) encapsulating the first barrier, which is deposited on the surface of AT-PVDF membranes. The spatial hierarchical structure provides a highly covered “double defense” barrier on the membrane surface, effectively preventing the adhesion of grease and dirt. In the separation of surfactant-stabilised oil-in-water emulsions, the emulsion separation efficiency reaches 99.8 %, and the water flux recovery rate is more than 90 % after cyclic separation. And the potential mechanism of underwater antifouling self-cleaning was analysed by density flood theory (DFT) calculations. In particular, the multilayer composite membrane was able to effectively adsorb cationic dyes in wastewater through the electrostatic effect of the first and second barriers, with a separation efficiency of more than 95 %. Therefore, the prepared superhydrophilic multilayer composite membrane has potential application prospects in the field of wastewater purification.