Advanced Oxidation Processes Coupled to Nanofiltration Membranes with Catalytic Fe⁰ Nanoparticles in Symmetric and Asymmetric Polyelectrolyte Multilayers

The in situ synthesis of Fe⁰ particles using poly-(acrylic acid) (PAA) is an effective tool for fabricating catalytic membranes relevant to advanced oxidation processes (AOPs). Through their synthesis in polyelectrolyte multilayer-based nanofiltration membranes, it becomes possible to reject and degrade organic micropollutants simultaneously. In this work, we compare two approaches, where Fe⁰ nanoparticles are synthesized in or on symmetric multilayers and asymmetric multilayers. For the membrane with symmetric multilayers (4.0 bilayers of poly (diallyldimethylammonium chloride) (PDADMAC)/PAA), the in situ synthesized Fe⁰ increased its permeability from 1.77 L/m²/h/bar to 17.67 L/m²/h/bar when three Fe²⁺ binding/reducing cycles were conducted. Likely, the low chemical stability of this polyelectrolyte multilayer allows it to become damaged through the relatively harsh synthesis. However, when the in situ synthesis of Fe⁰ was performed on top of asymmetric multilayers, which consist of 7.0 bilayers of the very chemically stable combination of PDADMAC and poly(styrene sulfonate) (PSS), coated with PDADMAC/PAA multilayers, the negative effect of the Fe⁰ in situ synthesized can be mitigated, and the permeability only increased from 1.96 L/m²/h/bar to 2.38 L/m²/h/bar with three Fe²⁺ binding/reducing cycles. The obtained membranes with asymmetric polyelectrolyte multilayers exhibited an excellent naproxen treatment efficiency, with over 80% naproxen rejection on the permeate side and 25% naproxen removal on the feed solution side after 1 h. This work demonstrates the potential of especially asymmetric polyelectrolyte multilayers to be effectively combined with AOPs for the treatment of micropollutants (MPs).