Photodegradation of 2,4-D induced by NO2(-) in aqueous solutions: the role of NO2.

ABSTRACT

To elucidate the effect of nitrite ion (NO2(-)) on the photodegradation of organic pollutants, a 300 W mercury lamp and Pyrex tubes restricting the transmission of wavelengths below 290 nm were used to simulate sunlight, and the photodegradation processes of 2,4-dichlorophenoxyacetic acid (2,4-D) with different concentrations of NO2(-) in freshwater and seawater were studied. The effect of reactive oxygen species (ROS) on the photolysis of 2,4-D was also demonstrated using electron paramagnetic resonance (EPR). The results indicated that the 2,4-D photolysis reaction followed the first-order kinetics in freshwater and seawater under different concentrations of NO2(-). Meanwhile, the photochemical reaction rate of 2,4-D increased with increasing concentration of NO2(-). When the concentration of NO2(-) was lower than 23 mg/L, the photodegradation rate of 2,4-D in seawater was higher than that in freshwater. However, when the concentration of NO2(-) was reached 230 mg/L, 2,4-D degradation slowed down in seawater. It was important to note that EPR spectra showed NO2 radical was generated in the NO2(-) solution under simulated sunlight irradiation, indicating that 2,4-D photodegradation could be induced by NO2. These results show the key role of NO2(-) in photochemistry and are helpful for better understanding of the phototransformation of environmental contaminants in natural aquatic systems.