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 N-2 hinders the application of this technology. In this study, a novel Cu@Fe-Cu-CuFe2O4-x photocatalyst was prepared by modifying CuFe2O4 with KBH4 and Cu(II), and used to selectively reduce NO3- to N-2 with a two-step reduction process. In step (1), with Cu@Fe-Cu-CuFe2O4-x/ultraviolet (UV) system, 91.0% NO3- was reduced to 52.3% NO2- and 39.4% N-2 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 center dot(-) 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 center dot 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% N-2 selectivity with initial NO3- concentration of 30 mg center dot N center dot L-1. In addition, the denitrification mechanism of the two-step reduction process was tentatively proposed.