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.

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