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

Flow electrode capacitive deionization (FCDI) is one of the most promising technologies for the continuous desalination and energy-efficient water purification. However, the performance of FCDI is confined to the electrode materials and the electrolyte composition. In this study, the porous carbon was synthesized to activate the biochar with different mass ratios of ZnCl₂. A systematic investigation was implemented to evaluate the physicochemical properties and capacitive deionization performance of the resulting porous carbon materials. An assessment was then performed on the deionization efficiency under varying electrolytes and initial ion concentrations. The capacitive deionization was analyzed using kinetic models. The key factors were determined on by the FCDI process. The potential application was given in various environments with the high ion concentrations. The results demonstrated that the ZnCl₂ activation was 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 1 137.23 m²/g, and the pore diameter was reduced to 1.70 nm. Additionally, there was the an increase in the concentration of oxygen-containing functional groups (e.g., C=O, O=C-O). The specific capacitance values of PC-1Zn-600 and PC-2Zn-600 were improved to 72.30 F/g and 169.98 F/g, respectively, compared with the PC-600 (25.74 F/g), indicating the 1.81 and 5.60 fold increase, respectively. Furthermore, the electrical resistance of PC-1Zn-600 was reduced to 2.53 Ω. 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 min, compared with the PC-600 and PC-2Zn-600. Among the three electrolytes (feed solution, K₂SO₄, and H₂O), the H₂O electrolyte was the most effective to remove ammonia nitrogen and phosphorus, with the removal rates of 91.69% and 75.55%, respectively. The H₂O showed the lower energy consumption for the ion recovery, compared with the K₂SO₄ electrolyte. No addition of chemicals was required to offer both low cost and high performance in practical applications. The FCDI shared the low removal rate in the short term, as the concentrations of initial ion increased. However, the removal efficiencies reached 99.33%, 98.15%, 98.50%, and 98.22%, respectively, even with the high initial concentrations of ammonia nitrogen (1 000 mg/L), phosphorus (150 mg/L), potassium (1500 mg/L), and calcium (150 mg/L), after 6-9 h of operation. The removal rates were comparable to those observed at the lower ion concentrations (e.g., 100 mg/L ammonia nitrogen, 50 mg/L phosphorus, 500 mg/L KCl, and 50 mg/L CaCl₂). As such, the FCDI can be expected to effectively treat the high-concentration solutions. Therefore, it is recommended that the operation time for the FCDI treatment of high-ion concentration solutions can 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. The process was primarily governed by electrostatic interactions. The migration of ammonia nitrogen and potassium ions included three stages: double-layer membrane, ion-exchange membrane, and equilibrium stage. In phosphorus and calcium ions, the process consisted of a double-layer membrane followed by an ion-exchange membrane stage, with the ion-exchange membrane stage acting as the rate-limiting step. Thus, the properties of the ion-exchange membrane were improved to optimize the contact area between the influent solution and the ion-exchange membrane. The efficiency of FCDI was then enhanced to treat the highly concentrated solutions.