Light-induced disappearance of nitrite in the presence of iron (III)

Understanding of rapid disappearance of nitrite in natural waters and its impact on nitrogen natural cycling has remained limited. We found that NO−2 disappeared rapidly in pH 3.2 aqueous Fe(III) solutions both in sunlight and in 356 nm light. Quantum yields of the NO−2 loss at 356 nm were 0.049–0.14 for initial levels of 10–80 μm NO−2 and 200 μm Fe(III). The NO−2 loss (at 356 nm) followed apparent first-order kinetics. The rate constants were 1.3 × 10–3 (40 μm NO−2) and 4.1 × 10–4 s–1 (80 μm NO−2) for 100 μm Fe(III), and 2.3 × 10–3 (40 μm NO−2) and 7.5 × 10–4 s–1 (80 μm NO−12) for 200 μm Fe(III) (t1/2=8.7, 27.9, 5.1, and 15.3 min, respectively). The rate constants were directly proportional to [Fe(III)]0 and inversely proportional to [NO−2]0. Agreement between the rate constants obtained experimentally and those calculated mechanistically supports the hypothesis that NO−2 was oxidized to NO2 by .OH radicals from photolysis of FeOH2+ complexes, and at high [NO−2]0 (e.g., 80 μm) relative to [Fe(III)]0, hydrolysis of NO2 or N2O4 to form NO−3 and NO−2 could be significant. This study showed that light and Fe(III)-induced oxidation of NO−2 (rate= 10–1–10–2 μm s–1) was more rapid than its direct photolysis (rate= 10–4 μm s–1), and the photolysis could be a significant source of .OH radicals only in cases where the Fe(III) level is much lower than the NO−2 level ([Fe(III)]/[NO−2] < 1/80). This study suggests that the light and Fe(III)-induced oxidation of NO−2 would be one potential important pathway responsible for the rapid transformation of NO−2 in acidic surface waters, especially those affected by acid-mine drainage or volcanic activities. This study also may be of interest for modeling certain acidic atmospheric water environments.