Controlling Bicontinuous Polyelectrolyte Complexation for Membrane Selectivity: Redox-Mediated Electrochemical Separation of Volatile Fatty Acids

Fermentative volatile fatty acid (VFA) production is a sustainable approach for waste valorization. However, selective product recovery remains challenging due to the range of VFAs produced and their dilute concentrations, requiring energy-intensive purification. Membrane-based electrochemical separations comprise an energy-efficient and continuous platform for small molecule separations. At the same time, there is a lack of suitable ion-exchange membranes for separating between structurally similar organic acids. Here, bicontinuous polyelectrolyte complex (PEC)-layered nanofiltration membranes are designed for the selective recovery of VFAs using redox-mediated electrodialysis. Hydrophobic modification of polyelectrolytes via aza-Michael addition precisely tunes the complexation-induced phase separation behaviors and the assembled nanostructures. Surface-confined layer-by-layer complexation generates a nanoscale bicontinuous PEC active layer with tailored surface properties that is inaccessible through bulk complexation. Redox-mediated electrodialysis using the nanostructured membrane exhibits enhancement of both ion permeability and selectivity toward VFAs, with notable reduction of energy consumption by up to 80% compared to conventional electrodialysis. Treatment of synthetic and cow manure fermentation effluents showcases 2 to 4-fold enrichment of VFAs and simultaneous removal of co-existing organic acids, with an energy consumption as low as 1.5 kWh kg⁻¹. These findings advance the understanding of interfacial complexation-induced phase separation of polyelectrolytes and the development of next-generation nanostructured membranes for multicomponent separations.