Capacitive deionization performance of porous carbon flow electrode for nitrogen, phosphorus, potassium and calcium ions

Flow electrode capacitive deionization (FCDI) is a promising technology for continuous desalination and energy-efficient water purification. However, the performance of FCDI is influenced by various factors, including the choice of electrode materials and the electrolyte composition. In this study, porous carbon was synthesized by activating biochar with different mass ratios of ZnCl2. We evaluated the physicochemical properties and capacitive deionization performance of the resulting porous carbon materials, assessed the deionization efficiency under varying electrolytes and initial ion concentrations, and analyzed the capacitive deionization process using kinetic models. The study identifies key factors affecting the FCDI process and explores its potential for application in environments with high ion concentrations. The results demonstrated that ZnCl₂ activation significantly enhanced the physicochemical and capacitive deionization properties of the porous carbon. When the mass ratio of ZnCl₂ to porous carbon was 1 (PC-1Zn-600), the specific surface area of the material increased to 1137.23 m² g⁻¹, and the pore diameter was reduced to 1.70 nm. Additionally, the concentration of oxygen-containing functional groups (e.g., C=O, O=C-O) increased. The specific capacitance of PC-1Zn-600 and PC-2Zn-600 improved to 72.30 F g⁻¹ and 169.98 F g⁻¹, respectively, compared to that of PC-600 (25.74 F g⁻¹), representing a 1.81-fold and 5.60-fold increase, respectively. Furthermore, the electrical resistance of PC-1Zn-600 was reduced to 2.53 Ω. Compared to PC-600 and PC-2Zn-600, the removal efficiencies of ammonia nitrogen, Phosphorus (P), Potassium (K), and Calcium (Ca) ions using the PC-1Zn-600 flow electrode reached 84.98%, 79.9%, 72.79%, and 91.21%, respectively, after 210 minutes. Among the three electrolytes tested (feed solution, K₂SO₄, and H₂O), the H₂O electrolyte was the most effective for removing ammonia nitrogen and phosphorus, with removal rates of 91.69% and 75.55%, respectively. Compared to the K₂SO₄ electrolyte, H₂O showed lower energy consumption for ion recovery and did not require the addition of chemicals, offering both low cost and high performance potential for practical applications. As the initial ion concentrations increased, the flow capacitive deionization (FCDI) process showed a slower removal rate in the short term. However, after 6–9 hours of operation, even with high initial concentrations of ammonia nitrogen (1000 mg/L), phosphorus (150 mg/L), potassium (1500 mg/L), and calcium (150 mg/L), removal efficiencies reached 99.33%, 98.15%, 98.50%, and 98.22%, respectively. These removal rates were comparable to those observed at lower ion concentrations (e.g., 100 mg/L ammonia nitrogen, 50 mg/L phosphorus, 500 mg/L KCl, and 50 mg/L CaCl₂), demonstrating the ability of FCDI to effectively treat high-concentration solutions. Therefore, it is recommended that the operation time for FCDI treatment of high-ion concentration solutions be extended to at least 6–9 hours. Additionally, the first-stage kinetic model (R² ＞ 0.96) was found to be well-suited for the FCDI deionization process, indicating that the process is primarily governed by electrostatic interactions. The migration process of ammonia nitrogen and potassium ions included three stages: a double-layer membrane stage, an ion-exchange membrane stage, and an equilibrium stage. For phosphorus and calcium ions, the process consisted of a double-layer membrane stage followed by an ion-exchange membrane stage, with the ion-exchange membrane stage acting as the rate-limiting step. Thus, optimizing the contact area between the influent solution and the ion-exchange membrane, as well as improving the properties of the ion-exchange membrane, are critical factors for enhancing the efficiency of FCDI in treating highly concentrated solutions.