Investigation of NH₄⁺ Removal via Activated Carbon//MnO₂ Hybrid Membrane Capacitive Deionization

Membrane capacitive deionization (MCDI) technology has been regarded as a promising alternative for ammonia nitrogen (NH 4 + ) removal from wastewater. However, NH 4 + removal performance by traditional pure carbon‐based MCDI processes is still unsatisfactory due to the low electrosorption capacity and selectivity toward NH 4 + . In this study, inspired by electrochemical NH 4 + storage, the application of activated carbon (AC)//MnO 2 hybrid MCDI for NH 4 + removal was systematically investigated. β‐MnO 2 with a nanorod‐like structure was prepared and applied as the MCDI electrode. Electrochemical analysis showed MnO 2 had a high specific capacitance of 160 F/g and 89.67% of pseudocapacitance contribution. These properties can guarantee fast ionic reaction kinetics for electrochemical NH 4 + storage. As expected, AC//MnO 2 MCDI exhibited enhanced NH 4 + removal performance as compared with AC//AC MCDI. Under the optimum conditions, the electrosorption capacity of AC//MnO 2 MCDI reached 37.34 mg/g at a voltage of 1.2 V in 10‐mmol/L NH 4 Cl solution. The electrosorption kinetics of NH 4 + followed the pseudo‐first–order kinetics equation. Over 20 consecutive cycles, good operational stability with 90.21% capacity retention can be achieved. Furthermore, by treating a mixed solution, AC//MnO 2 MCDI demonstrated preferential electrosorption selectivity toward NH 4 + over Na + . Finally, the applicability of the AC//MnO 2 MCDI was investigated with actual wastewater. This study helps to clarify the feasibility of MnO 2 electrodes for the removal and recovery of NH 4 + from wastewater.