One-step separation of oil-water emulsion with heavy metal ions by PVDF membrane modified with zwitterionic coating

Oily wastewater with heavy metal ions poses a substantial environmental risk to human and ecosystem. The one-step separation of oily wastewater with heavy metal ions through membrane technology remains challenging due to membrane fouling and limited multifunctionality. In this work, a one-step polymerization and co-deposition process was employed to coat polyvinylidene fluoride (PVDF) membranes with layer of polydopamine (PDA) and poly-(sulfobetaine methacrylate) (PSBMA), finally obtaining the PDA/PSBMA-coated PVDF membranes. The zwitterionic nature of PSBMA in this coating, characterized by high dipole moment and strong hydrophilicity, significantly enhanced the antifouling properties and hydrophilicity of the PVDF membranes, meanwhile enabling the adsorption of heavy metal ions. The optimized mass ratio of dopamine (DA) to sulfobetaine methacrylate (SBMA) showed in a modified membrane with significantly increased permeability (from 0 to 5017.9 L m−2 h−1 bar−1) and high separation efficiencies (above 99.0 % for oil-water mixtures and 96.0 % for emulsions). The PDA/PSBMA-coated PVDF membranes demonstrated exceptional one-step separation performance for oil-water emulsion with heavy metal ions, particularly for the Cu2+ adsorption efficiency and oil-water separation efficiency of 63.2 % and 94.2 %, respectively. The atomistic molecular dynamics (MD) simulations theoretically revealed that the excellent performance of membrane separation could be attributed to the synergistic electrostatic and hydrophilic effects of the charged groups in the PDA/PSBMA coating. This study provides a practical strategy for the development of advanced multifunctional membrane technology suitable for the treatment of complex oily wastewater.