Implication of diffusion and significance of anodic pH in nitrogen-recovering microbial electrochemical cells

Microbial electrochemical cells (MXCs) using cation and anion exchange membranes (CEM and AEM) were tested in batch mode for ammonium recovery. Experimental results showed superior ammonium recovery of 61% in CEM-MXC. Migration effect accounted for 61% of the total ammonium recovery in CEM-MXC where other ammonium ions (39%) were transferred by diffusion. Current density was significant for improving the recovery rate of ammonium nitrogen, but proton accumulation deteriorated current density in CEM-MXC. The dissolution of ammonia hydrolyzed by urea produces hydroxyl ions, which can neutralize protons in the anode and maintain high current density in CEM-MXC. However, anodic pH increased over 9 at high current density (>2 A/m2) in CEM-MXC fed by urea, which indicates that the rate of ammonia dissolution becomes faster than proton accumulation rate at high current density. Balancing proton-accumulating rate with ammonia dissolution rate in the anode is significant for improving ammonium recovery in CEM-MXC.