Effective nitrate removal with high N₂ selectivity by active-site-rich particle electrode of bentonite-based Cu-Fe LDH composite

A novel bentonite-based Cu-Fe layered double hydroxide (LDH) composite particle electrode (CuFe-LDH/BT) was fabricated and used as the catalyst to remove nitrate-nitrogen (NO₃^--N) in three-dimensional electrochemical (3D/E) system. The results showed that the prepared CuFe-LDH/BT exhibited the highest catalytic activity when the molar ratio of copper to iron was 3:1, the dosage of bentonite (BT) was 1 g, liquid-phase synthesis pH was 10, and liquid-phase synthesis temperature was 40 °C. The prepared composite particle electrode was characterized by X-Ray Diffraction (XRD), Scanning electron microscopy (SEM), Brunauer-Emmett-Teller method (BET), Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The characterization results indicated that LDH structure was successfully formed in CuFe-LDH/BT, and CuFe-LDH/BT had obvious layered structure, high specific surface area and excellent conductivity. Under the reaction conditions of CuFe-LDH/BT dosage of 3 g/L, current density of 8 mA/cm² and initial pH of NO₃^--N solution of 7, in the range of NO₃^--N concentration of 50∼200 mg/L, the maximum removal efficiency of NO₃^--N could reach 100% at reaction time of 240 min, and the maximum N₂ selectivity was 83.41%. The recycling test showed that CuFe-LDH/BT maintained high activity after 3 reuses. The possible reaction mechanism of NO₃^--N removal in the 3D/E system catalyzed by CuFe-LDH/BT was explored. In summary, the 3D/E system catalyzed by CuFe-LDH/BT can achieve the effective removal of NO₃^--N in water body.