Selective photo-reduction of nitrate to nitrogen with a two-step process by a KBH₄/Cu(II) modified CuFe₂O₄ photocatalyst

Nitrate (NO₃⁻) removal by photochemical-reduction has received extensive attention. However, the low selectivity of NO₃⁻ reduction to N₂ hinders the application of this technology. In this study, a novel Cu@Fe-Cu-CuFe₂O_4−x photocatalyst was prepared by modifying CuFe₂O₄ with KBH₄ and Cu(II), and used to selectively reduce NO₃⁻ to N₂ with a two-step reduction process. In step (1), with Cu@Fe-Cu-CuFe₂O_4−x/ultraviolet (UV) system, 91.0% NO₃⁻ was reduced to 52.3% NO₂⁻ and 39.4% N₂ within 60 min. The rapid removal of NO₃⁻ was due to the synergistic effect of oxygen vacancies, Fe-Cu corrosion cell, and CuFe₂O₄ photocatalysis. In step (2), H₂C₂O₄ and H₂O₂ were introduced into the effluent of step (1) to promote CO₂·⁻ formation via Fe(II) and Fe(III) catalysis and UV radiation, which boosted the selective reduction of NO₂⁻ to N₂. When H₂C₂O₄ and H₂O₂ dosages were both 4.0 mmol·L⁻¹ and the reaction time was 30 min, the removal efficiency of NO₂⁻ achieved 100% and the selectivity of N₂ was 83.0%. Overall, the two-step reduction process achieved 95.0% NO₃⁻ removal efficiency and 90.1% N₂ selectivity with initial NO₃⁻ concentration of 30 mg·N·L⁻¹. In addition, the denitrification mechanism of the two-step reduction process was tentatively proposed.