Redox-active electrode architectures for selective nitrogen and phosphorus recovery via flow-electrode capacitive deionization (FCDI): a material-driven strategy

Environmental problems caused by excessive nitrogen and phosphorus discharge from wastewater, including eutrophication and resource depletion, have increased the demand for sustainable nutrient recovery technologies. This study presents a selective and energy-efficient nutrient recovery approach using a redox-active flow-electrode capacitive deionization (FCDI) system integrated with conventional methods. Unlike traditional capacitive deionization systems based only on electrostatic adsorption and limited by electrode saturation, the proposed system employs continuous operation, redox-active materials, and a flow-electrode architecture to improve ion selectivity and system stability. Ion exchange membranes were incorporated to regulate ion transport and minimize co-ion interference, while polarity reversal enabled controlled nutrient capture and release into concentrated recovery streams. The performance of the system was evaluated using electrochemical and separation parameters, including water recovery, current efficiency, productivity, energy consumption, energy-normalized removal score (ENRS), and thermodynamic energy efficiency (TEE). The developed system achieved 97% water recovery, 85% current efficiency, and a productivity of 90 L m⁻ 2 h⁻¹ with a concentration reduction of 30 mM at an energy consumption of 0.170 kWh m⁻ 3 . In addition, the ENRS of 49.0 μmol J⁻¹ and TEE of 29.4% demonstrated efficient energy utilization, highlighting its potential for sustainable wastewater treatment and circular economy applications.