Enhanced phosphate removal and recovery from wastewater by flow-electrode capacitive deionization (FCDI): Role of [Fe(CN)₆]^3–/4– redox couple

Flow-electrode capacitive deionization (FCDI) emerges as a promising technology for phosphorus (P) recovery from low/medium-strength wastewaters. Conventional activated carbon flow electrodes (ACFEs) face challenges in terms of poor dispersion and weak conductivity, which inhibits their cost-effectiveness, efficiency and stability. To address these issues, we introduce the [Fe(CN)₆]^3–/4– redox couple into ACFEs mixture to enhance the charge transfer rate and P recovery, achieving stable long-term operation. The results showed that the addition of 10 mM [Fe(CN)₆]^3–/4– significantly improved the average P removal rate (APRR) achieving 9.2 μg P min^–1 cm^–2 with low energy of 3.5 kWh kg^–1 P. This improvement was particularly evident over 10 successive cycles, where a consistent 90 % P recovery was obtained in each cycle. The improved performance can be attributed to the formation of continuous conductive channels, reduced internal resistance, as well as fast and stable electron transfer facilitated by [Fe(CN)₆]^3–/4–, leading to more efficient P adsorption and desorption. Futhermore, a highpurity CaP product was extracted from P-rich electrolyte by using simple filtration and crystallization process. The FCDI with [Fe(CN)₆]^3–/4– redox couple achieved 84.2 % P recovery from real domestic wastewater, demonstrating the potential of FCDI process for waste-to-resource applications. This study provides a proof-of-concept demonstration of enhanced FCDI by introducing [Fe(CN)₆]^3–/4– redox couple. Application of redox couple in FCDI process overcomes the limitations of traditional ACFEs, by enhancing P recovery and reducing energy consumption, making resource recovery and wastewater treatment more efficient, more economic and more sustainable.