Coexisting Ion Concentration Polarization in Ammonia Recovery via Flow-Electrode CDI: Impacts, Mechanisms, and Optimizations

Flow-electrode capacitive deionization (FCDI) offers an eco-friendly and energy-efficient approach for ammonia recovery from wastewater. However, the presence of coexisting ions significantly impedes recovery efficiency. Here, we investigate the interference mechanisms of coexisting ions on ammonia recovery using a gas-membrane-coupled FCDI system. Results reveal that a single FCDI cell with 10 wt % carbon mass loading achieves optimal performance (SEC_w = 1.10 kWh m^-3, SEC_N = 34 kWh kg^-1 N, and FE = 91.67%), yet these metrics decline markedly after 2 h due to ammonium back diffusion. This back diffusion is strongly correlated with coexisting ion concentrations and carbon loading in flow electrodes. Double current step tests demonstrate that ammonia transmembrane transport is governed by concentration polarization at the membrane/electrolyte interface induced by coexisting ions. To address this, a parallel FCDI cell is employed to discharge high-salinity brine while recovering energy from the salt gradient. Long-term operation (100 h) validates the system's robustness, achieving a concentrated ammonia extract of 954 ppm. This study elucidates critical ion interference mechanisms in FCDI-based ammonia recovery and presents a sustainable strategy for enhancing wastewater resource utilization.