Photo-reduction of nitrate to nitrite in aqueous solution in presence of CaO: Selectivity, mechanism and application

Nitrate (NO3−) photolysis leads to various nitrogenous products, which depends on the reaction conditions and solution properties. How to regulate the composition of products is an important issue. Herein, we found that the involvement of CaO in the NO3− photolysis could selectively reduce NO3− to nitrite (NO2−). Almost 100 % NO2− selectivity was achieved in CaO/UV/NO3− system at initial pH (3–10), initial NO3− concentration (5–45 mg N/L) and CaO dosage (1–6 g/L). The high NO2− selectivity in CaO/UV/NO3− system was attributed to the rich basic sites and paramagnetic surface species of CaO, which captured nitrogen oxides (NOX) and promoted the conversion of NOX to NO2−, respectively. The two-step reduction strategy of CaO/UV coupled with H2NSO3H was used to remove NO3− in wastewater, in which NO3− was firstly reduced to NO2−, then the produced NO2− was further reduced to N2 by H2NSO3H. As for secondary effluent of municipal wastewater with initial NO3− concentration of 30 mg N/L, 93 % removal of NO3− and 100 % of N2 selectivity were achieved at CaO dosage of 2 g/L and H2NSO3H dosage of 0.4 g/L.