The surface structures and properties of a membrane largely determine its in-service performance during a filtration process. Here we report a facile hydrophilization method via co-deposition of mussel-inspired polydopamine (PDA) and polyethyleneimine (PEI) on a polypropylene microfiltration membrane. The deposition time is greatly shortened and the surface hydrophilicity is significantly improved compared to those membranes decorated only by PDA. The dopamine/PEI deposition solution can be reused several times with negligible effect on the surface hydrophilicity of membranes. Moreover, the PDA/PEI coating endows the membranes with ultra-high water permeability, allowing microfiltration separation of oil-in-water emulsions under atmospheric pressure.

Nanofiltration membranes are of great interest in brackish water desalination and drinking water purification. Improving the permeation and separation performance remains a big challenge in advanced nanofiltration membranes. Herein, we report triple-layered composite nanofiltration membranes constructed by the interfacial polymerization of diamine and acyl chloride on a cellulose nanocrystal interlayer supported by a microporous substrate. The cellulose nano-crystal interlayer plays a crucial role in the polyamide skin layer formation and the following nanofiltration process. It can store aqueous diamine monomers and slow down the interfacial polymerization for a relatively low cross-linking degree of the skin layer. This hydrophilic interlayer also facilitates water permeation through a “dragging effect”. The constructed membranes exhibit an ultra-high permeation flux up to 204 L m⁻² h⁻¹ under 0.6 MPa with a Na₂SO₄ rejection above 97%, which is the highest reported result to our knowledge. The ultra-high water permeation flux enables nanofiltration at low operating pressure, making it an energy-saving process. Moreover, the low cross-linking degree of the skin layer results in a high monovalent/divalent ion separation ratio.