Coupling Redox-Active Prussian Blue Analogue Electrode with Membrane Bioelectrochemical System for Simultaneous Ammonium and Potassium Extraction from Wastewater

Recovering ammonia and potassium from wastewater is important for sustainable nutrient management and pollution mitigation. Membrane bioelectrochemical systems (MBESs) offer a promising approach for simultaneous recovery of ammonium (NH4+) and potassium (K+) from waste streams, although achieving distinct separation between these nutrient ions remains challenging. To address these limitations, this study aimed to improve reactor selectivity toward NH4+ and K+ to enhance nitrogen recovery while replacing the conventional oxygen reduction reaction with a solid-state electrode to reduce the aeration energy consumption. Here, we couple an ion-selective redox-active cathode based on Prussian blue analogues, potassium nickel hexacyanoferrate (KNiHCF), with a microbial fuel cell to evaluate ion intercalation behaviors and simultaneously recover NH4+ and K+ from synthetic wastewater. More electrons were produced to intercalate cations with the Na+-based catholyte than the Mg2+-based one. In most catholyte solutions, the electrode showed a preferential uptake of K+ and NH4+. The transport numbers indicated that the initial catholyte background cations (Na+ or Mg2+) played a critical role in facilitating transport of the target cations from the anolyte to the catholyte. When NH4+ was initially present in the catholyte, more than 60% of electrode occupancy corresponded to NH4+. These results demonstrate that catholyte composition strongly influences ion transport and selectivity in KNiHCF-coupled MBES systems.