Asymmetric polyelectrolyte multilayer membranes with ultrathin separation layers for highly efficient micropollutant removal

New membrane materials are urgently needed to address the increasing concentrations of harmful organic micropollutants (e.g. pharmaceuticals, pesticides and plasticizers) in our surface and drinking water. Currently, the densest available membranes can remove micropollutants sufficiently, but only at very low permeabilities and by producing a highly saline, difficult to treat waste stream. We improve permeability 5–10 fold by producing an asymmetric polyelectrolyte multilayer (PEM) on a porous membrane, with a separation layer thickness of only 4 nm. This is achieved by first coating an open multilayer to prevent defects, and subsequently a thin and dense multilayer. This novel membrane shows a very high (98%) retention toward a mix of common micropollutants. Moreover, it only retains 10–15% of NaCl, preventing the formation of a saline waste stream. A detailed literature study shows that the asymmetric PEM membrane, with this unique combination of properties, significantly outperforms commercial membranes for micropollutant removal applications.