Dual activation of porous nitrogen-doped carbon materials for highly efficient mine water desalination by flow electrode capacitive deionization

Driven by stringent environmental regulations, coal mines are facing escalating challenges in recycling and treating highly saline mine water, so highly efficient deep desalination technologies are urgently needed to break through the industry’s dilemmas and enable the utilization of non-conventional water resources. Although flow electrode capacitive deionization (FCDI) technology is emerging within the field of desalination, there is still a barrier to desalination capability due to electrode material properties. In this study, a new method for the preparation of dual-activated porous nitrogen-doped carbon electrode materials is proposed. Using sucrose as the carbon source and melamine as the nitrogen source, the advantages of the dual template method, water vapor activation, acid-impregnated oxidation, and nitrogen doping were integrated, and the wettability and conductivity of the materials were improved. Furthermore, the electrode material has a unique neatly arranged columnar structure, larger specific surface area (1 039.76 m2/g), richer pore structure (0.32 cm3/g), more nitrogen-oxygen electronegativity sites, and better wettability, which can be used to realize highly efficient and low-energy consumption desalination of mine water. The material (H2O(g)-HNO3/CN) was applied in the FCDI device and desalted 1 g/L NaCl solution for 5 h after process optimization of voltage, active material content, initial concentration of treatment solution and pH, and the desalination rate was as high as 100%, and the average desalination rate, desalination amount, charge efficiency and standard desalination energy consumption were 0.00952 mg/(min·cm2), 37.08 mg/g, 24.02% and 481.98 J/mol, respectively. Even after seven cycles of suction-desorption experiments, the desalination rate could still reach 95.71%. In order to investigate the adsorption and desalination mechanism of H2O(g)-HNO3/CN, the electrochemical properties, adsorption kinetics and isotherms were analyzed. The results showed that its specific capacitance was as high as 99.38 F/g and the internal resistance was extremely small; the adsorption of Na+ ions was more consistent with the pseudo-second-order kinetic model and Langmuir isotherm model, and the saturated adsorption amount of Na+ ions was 4.30 mg/g, and the saturated adsorption time was 120 min.