Fouling of nanofiltration membranes based on polyelectrolyte multilayers: The effect of a zwitterionic final layer

In this work, we investigate the effect of membrane surface chemistry on fouling in surface water treatment for polyelectrolyte multilayer based nanofiltration (NF) membranes. The polyelectrolyte multilayer approach allows us to prepare three membranes with the same active separation layer, apart from a difference in surface chemistry: nearly uncharged crosslinked Poly(allylamine hydrochloride) (PAH), strongly negative poly(sodium 4-styrene sulfonate) PSS and zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine-co-acrylic acid) (PMPC-co-AA). Initially, we study foulant adsorption for the three different surfaces (on model interfaces), to demonstrate how a different surface chemistry of the top layer affects the subsequent adsorption of five different model foulants (Humic Acids, Alginates, Silica Nanoparticles, negatively and positively charged Proteins). Subsequently, we study fouling of the same model foulants on our polyelectrolyte multilayer based hollow fiber NF membranes with identical surface chemistry to the model surfaces. Our results show that nearly uncharged crosslinked PAH surface generally fouls more than strongly negatively charged PSS surface. While negative BSA adsorbs better on PSS, probably due to charge regulation. Overall, fouling was mainly driven by electrostatic and acid-base interactions, which led, for both PAH and PSS terminated membranes, to flux decline and changes in selectivity. In contrast, we demonstrate through filtration experiments carried out with synthetic and real surface water, that the bio-inspired zwitterionic phosphatidylcholine surface chemistry exhibits excellent fouling resistance and thus stable performance during filtration.