Preparation and characterization of CS/PDA-PVDF/PVP-modified membranes

In the purification of oily wastewater, polyvinylidene fluoride (PVDF) nanofiber membranes have attracted widespread attention for their excellent water–oil separation performance. And chitosan (CS), as a natural polymeric alkaline polysaccharide containing many functional groups interacting with heavy metal ions, such as –NH₂ and –OH, has been widely used to treat heavy metal ions in wastewater. However, a few studies have been conducted on superhydrophilic and underwater hydrophobic PVDF composite membranes for the simultaneous removal of heavy metal ions and oil from wastewater. In this paper, PVDF/PVP composite membranes were used as the substrate of separation membranes, and PVDF/PVP-modified membranes were prepared by surface coating with PDA and CS in turn. Among them, CS can be coated on the surface of PVDF/PVP composite membrane by cross-linking the amino group on the molecule with the quinone structure in the PDA molecule to form a stable CS/PDA gel layer. Compared with other modified membranes, CS/PDA-PVDF/PVP has better hydrophilicity and underwater oleophobicity. The underwater OCA of dichloromethane could reach 161°, and the OCA of other oils were above 140°. The water flux was 14,171 L m⁻² h⁻¹. After 15 mixture and emulsion separation tests, the separation efficiency was higher than 99% and 98%, respectively. The adsorption efficiency for Cu²⁺, Pb²⁺, and Cr³⁺ aqueous solutions with concentrations of 10–60 mg/L reached more than 80%. Therefore, this method has great potential in treating oily wastewater.