Multiple Approaches to the Buildup of Asymmetric Polyelectrolyte Multilayer Membranes for Efficient Water Purification

The versatility of polyelectrolyte multilayer (PEM) coatings is very promising for their use as separation layers in nanofiltration applications. These membranes can for example be suited for the removal of micropollutants, such as medicines and pesticides, from water. The selectivity of PEM coatings can be further improved by so-called asymmetric coating. In this approach, the pores of the support membrane are filled with an open PEM layer to maintain a good water permeability, and subsequently a thin, dense layer is coated on top to determine the separation properties. Coating a dense top layer can be achieved in different ways. In this work, we systematically investigate the effectiveness of these different types of top layers. We show that coating a top layer at lower ionic strength than the bottom layer leads to a higher permeability and MgSO4 retention, compared with the reference, bottom-type layer coated with the same total number of layers. Also, other salt retentions can be improved with this approach. However, micropollutant retentions are hardly affected. Coating a top layer with a polyelectrolyte pair that forms denser layers at equal ionic strength, in contrast, leads to a significant change in separation properties with much higher MgSO4 and micropollutant retentions and improved water permeability compared with the reference layer. The concept of membrane optimization via asymmetric coating is thus most effective when using different polyelectrolyte pairs on top of each other. Moreover, we show that this approach allows us to selectively cross-link the top layer for further enhancement of the micropollutant retention, while water permeability is not much reduced. Asymmetric PEM coatings are therefore a promising method to optimize PEM membranes for micropollutant removal and other separation processes.