Nitrite formation during low pressure ultraviolet lamp irradiation of nitrate

During ultraviolet light (UV) disinfection, nitrate (NO₃ ^-) present in raw water may transform to nitrite (NO₂ ^-) that can cause serious human diseases. In this study, the formation of NO₂^- from NO₃^- was studied at different experimental conditions under the irradiation of a low-pressure ultraviolet (LPUV) lamp at 253.9 nm. The investigated experimental variables included initial NO₃^- concentration, solution pH (6.2-9.5), and hydrogen peroxide (H₂O₂) dose (0-25 mg L^-1). Moreover, the effect of titanium dioxide (TiO₂) was determined. Results showed that the formation of NO₂^- was enhanced at a high initial NO₃^- concentration and a high pH, but was inhibited, to some different degrees, by introduction of H ₂O₂ or photocatalyst TiO₂. The effect of pH on NO₂^- formation was probably due to the impact of hydrogen ion on the stability of several intermediates such as peroxynitrite (ONOO ^-), N₂O₃, and N₂O₄. And the inhibiting effects of H₂O₂ and TiO₂ were attributable to production of additional hydroxyl radical (·OH) that scavenged NO₂^-. At pH 9.5 and an initial NO ₃^- concentration of 10 mgL^-1 NO ₃^--N, the concentration of NO₂^- produced was above 0.1 mgL^-1 NO₂^--N, the Germany drinking water standard. When 25 mgL^-1 H₂O ₂ was added, the NO₂^- level was decreased below the standard.