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 H₂O₂ 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 NO₃⁻ concentration of 10 mg L⁻¹ NO₃⁻-N, the concentration of NO₂⁻ produced was above 0.1 mg L⁻¹ NO₂⁻-N, the Germany drinking water standard. When 25 mg L⁻¹ H₂O₂ was added, the NO₂⁻ level was decreased below the standard.