Dual resistance Janus PDA/PVDF membrane for removal and concentration of short- and long-chain perfluoroalkyl substances via membrane distillation

Membrane distillation (MD) has been noted for concentrating and separating both short- and long-chain PFASs to highly concentrated levels, facilitating the effective destruction of PFASs. However, MD has critical limitations, including membrane wetting and fouling, which can be disrupted over long-term operation with high-quality permeate water. This study developed asymmetric Janus polydopamine (PDA)/polyvinylidene fluoride (PVDF) membranes with a hydrophilic-hydrophobic dual-layer structure to effectively separate PFASs without any wetting and fouling from highly concentrated feed water. A direct current MD experiment was conducted using 0.1 M NaCl containing 20 mg L −1 short-chain (perfluoropentanoic acid, PFPeA) and long-chain PFASs (perfluorooctanoic acid, PFOA). For short-chain PFPeA, the Janus membranes achieved outstanding salt rejection (~99.50%) and a low PFAS concentration (~6 μg L −1 ) in the permeate water, with stable and enhanced flux for long-term periods. Otherwise, for long-chain PFOA, despite highly concentrated PFAS levels, they demonstrated significant membrane anti-wetting and anti-fouling with high salt rejection (~99.40%), low PFAS concentration (approximately 3.5 μg L -1) , as well as stable flux. In addition, they exhibited high concentrations of PFASs in the feed solution (more than 1.60–1.89 times the initial feed water concentration). However, PFAS substances were found to penetrate PVDF membranes and cause fouling, as confirmed by molecular dynamics simulations, making it difficult to use for MD operation to remove and concentrate PFAS compounds. Consequently, the Janus membrane is expected to be applied in practical applications, especially for effectively concentrating and separating PFASs prior to the destruction of these compounds.