Energy-efficient ammonium removal via low-voltage hybrid capacitive deionization using copper hexacyanoferrate electrode

Ammonium ion (NH₄ + ) contamination in water has emerged as a serious environmental issue due to its ecological toxicity and its role in eutrophication and particulate matter formation. Although conventional methods such as biological treatment and ion exchange have been employed, these methods suffer from operational complexity and high energy demands. In this study, we propose a hybrid capacitive deionization (HCDI) system employing Prussian blue analogue, copper hexacyanoferrate (CuHCF), as a redox-active electrode material for energy-efficient NH₄ + removal. Specifically, we evaluated the benefits in terms of energy consumption at low operating voltages by comparing the system with two representative CDI configurations (Membrane capacitive deionization (MCDI) and asymmetric MCDI (ACDI)). The HCDI system with CuHCF exhibited a high deionization capacity (13.4 ± 0.1 mg/g) and charge efficiency (76 ± 3 %) at 1.0 V, outperforming both MCDI and ACDI systems. Notably, even when operated at a reduced voltage of 0.8 V, the HCDI system with CuHCF not only exhibited superior deionization capacity compared with the MCDI and ACDI systems operated at 1.2 V but also demonstrated significantly lower energy consumption (0.46 ± 0.02 Wh/g) because of its low-voltage operation. These results highlight the advantages of integrating redox-active materials such as CuHCF into capacitive deionization systems for low-voltage and energy-efficient NH₄ + removal. • Ammonium ion contamination in water is a growing environmental concern. • We propose a HCDI system with a CuHCF electrode for energy-efficient NH₄ + removal. • The proposed HCDI system performs remarkably well in terms of energy consumption. • The proposed method has superior deionization capacity and low energy consumption. • It is a sustainable alternative for nitrogen management in wastewater treatment.