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 (NH₄ ⁺) removal from wastewater. However, NH₄ ⁺ removal performance by traditional pure carbon-based MCDI processes is still unsatisfactory due to the low electrosorption capacity and selectivity toward NH₄ ⁺. In this study, inspired by electrochemical NH₄ ⁺ storage, the application of activated carbon (AC)//MnO₂ hybrid MCDI for NH₄ ⁺ removal was systematically investigated. β-MnO₂ with a nanorod-like structure was prepared and applied as the MCDI electrode. Electrochemical analysis showed MnO₂ 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 NH₄ ⁺ storage. As expected, AC//MnO₂ MCDI exhibited enhanced NH₄ ⁺ removal performance as compared with AC//AC MCDI. Under the optimum conditions, the electrosorption capacity of AC//MnO₂ MCDI reached 37.34 mg/g at a voltage of 1.2 V in 10-mmol/L NH₄Cl solution. The electrosorption kinetics of NH₄ ⁺ 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//MnO₂ MCDI demonstrated preferential electrosorption selectivity toward NH₄ ⁺ over Na⁺. Finally, the applicability of the AC//MnO₂ MCDI was investigated with actual wastewater. This study helps to clarify the feasibility of MnO₂ electrodes for the removal and recovery of NH₄ ⁺ from wastewater.