Enhanced ammonium removal and recovery from wastewater using an innovative redox-active flow electrode: The significant role of redox charge compensation and hydrogen bond network reconstruction

Flow-electrode capacitive deionization (FCDI) is an ideal electrochemical technology for NH4+ recovery but suffers from inefficient charge transfer, leading to high energy consumption and unsatisfactory recovery performance. In this study, we developed an innovative redox-active flow electrode (RAFE) comprising redox-active electrolyte ((NH4)3Fe(CN)6) and activated carbon (AC) to enhance the removal and recovery of NH4+. The ion capture and charge transfer mechanism of NH4+ within this electrode were further investigated. FCDI with RAFE cathode exhibited a rapid removal (31 mgN/m2/min) and recovery rate (26 mgN/m2/min), with about 32 % increment as compared to the FCDI with AC cathode. The lower removal (2.3 kWh/kgN) and recovery energy consumption (5.76 kWh/kgN) were also achieved. The CV curves and electrode characterization indicated that NH4+ was capacitive captured and released through electric double layer (EDL) and redox charge compensation. Dynamic charge efficiency analysis showed that EDL capacitive capture mechanism plays a dominant role during the charging stage while the redox compensation capture mechanism dominated the charge transfer during the discharging stage. Moreover, electrode suspension characterization demonstrated that the migration of NH4+ was facilitated by hydrogen bond network reconstruction. This work provided an energy-saving system for efficient NH4+ removal and recovery by coupling FCDI with RAFE cathode.