Selective reduction of nitrate to nitrogen by Fe0–Cu0–CuFe2O4 composite coupled with carbon dioxide anion radical under UV irradiation

Abstract

Zero-valent iron (Fe0) has been widely used for the reduction of nitrate, but the end reduction product is mainly ammonium. Here, a novel strategy for selective reduction of nitrate (NO3−) to nitrogen gas (N2) with high efficiency and N2 selectivity was investigated using Fe-based material (Fe0–Cu0–CuFe2O4) combined with citric acid (CA) and ultraviolet (UV) irradiation. In this strategy, the nitrate was firstly reduced to nitrite (NO2−) by Fe0–Cu0–CuFe2O4/UV process, and then the produced NO2− could be further reduced to N2 by carbon dioxide anion radicals (CO2•−) which was generated from CA that was added later. In this process, the selective reduction of NO3− to NO2− was a key step. For this purpose, we synthesized Fe0–Cu0–CuFe2O4 composite by simple chemical replacement and in-situ growth process, which made it have a delicate structure with good contact between Cu and Fe and CuFe2O4. The selective reduction of NO3− to NO2− in Fe0–Cu0–CuFe2O4/UV process was due to that the Cu0 was the electron enrichment center and the photo-generated hole could suppress the NO3− reduction to NH4+ by Fe2+. In this proposed strategy, 100% NO3− removal efficiency and 96.3% N2 selectivity were achieved when the initial NO3− concentration was 30 mg N/L and the reduction time was 60 min. The denitrification mechanism of the Fe0–Cu0–CuFe2O4/UV/CA system was proposed.