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

Abstract

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.

Wetting-Induced Polyelectrolyte Pore Bridging

“…The observed retentions are displayed in Figure 5c. As previously discussed, the maximum achievable ion rejection with the chosen coating solution composition was fairly low [44,45]. Hence, even the small retentions observed are an indication for a partially successful pore coverage with polyelectrolytes.…”

Asymmetric layer‐by‐layer polyelectrolyte nanofiltration membranes with tunable retention

“…To overcome this effect, asymmetric layer build‐up can be applied to materialize the specific characteristics of the individual PEs. Asymmetric membranes are formed by applying different PE couples in one membrane 36,43,44 . Several PE couples have already been researched, consisting of PDADMAC, PAH as PCs, and PAA, PSS as PAs 36,43,44 .…”

“…36,43,44 Several PE couples have already been researched, consisting of PDADMAC, PAH as PCs, and PAA, PSS as PAs. 36,43,44 When combined in one membrane assembly, these PE couples form asymmetric membranes with a relatively open, more permeable base layer and a thin dense top layer, which determines the separation properties. These combinations result in lower permeabilities compared to membranes with an open base layer only, but significantly higher retentions.…”

“…These combinations result in lower permeabilities compared to membranes with an open base layer only, but significantly higher retentions. 36,43,44 Ideally, a membrane has unchanged permeabilities, but higher retentions though. Therefore, contrary to what has been done before, here we develop an asymmetric membrane with an open base layer as well as an open terminating layer.…”

Polyelectrolyte Complex Tubular Membranes via a Salt Dilution Induced Phase Inversion Process

“…[4,5] The fabrication of tubular and hollow fiber nanofiltration composite membranes based on organic solvents and crosslinkers is still investigated and developed. [6][7][8][9][10][11][12][13] However, the use of organic solvents needs to be avoided and more benign fabrication procedures need to be focused on. Even national and international authorities such as the EU starts to limit the usage of organic solvents, [14] which are essential for the fabrication of polymeric membranes using the established nonsolvent-induced phase separation (NIPS) method.…”