Enhanced NH4+ Removal and Recovery from Wastewater Using Na-Zeolite-based Flow-Electrode Capacitive Deionization: Insight from Ion Transport Flux

Flow-electrode capacitive deionization (FCDI) is a promising electromembrane technology for wastewater treatment and materials recovery. In this study, we used low-cost Na-modified zeolite (Na-zeolite) to prepare a composite flow-electrode (FE) suspension with a small amount of highly conductive carbon black (CB) to remove and recover NH4+ from synthetic and actual wastewater (200 mg-N/L). Compared with conventional activated carbon (AC), the Na-zeolite electrode exhibited a 56.2–88.5% decrease in liquid-phase NH4+ concentration in the FE suspension due to its higher NH4+ adsorption capacity (6.0 vs. 0.2 mg-N/g). The resulting enhancement of NH4+ diffusion to the electrode chamber contributed to the improved performance of FCDI under both constant current (CC) and constant voltage (CV) conditions. The addition of CB to the FE suspension increased the conductivity and facilitated Na-zeolite charging for NH4+ electrosorption, especially in CV mode. NH4+-rich zeolite can be easily separated by sedimentation from CB in the FE suspension, producing a soil conditioner with a high N-fertilizer content suitable for soil improvement and agricultural applications. Overall, our study demonstrates that the novel Na-zeolite-based FCDI can be developed as an effective wastewater treatment technology for both NH4+ removal and recovery as a valuable fertilizer resource.