Effect Of NO₃^-/NO₂^-on2,4-D Photodegradation In Water

In this dissertation, the effects of the illumination, pH, nitrite ion (NO2-) and (or) nitrate ion(NO3-), humic acid (HA), extracellular organic matter (EOM) on the photodegradation of2,4-dichlorophenoxyacetic acid(2,4-D) under simulated sunlight were investigated. The resultsshowed:(1) Using revolve photochemical reaction device, in300W mercury lamp and500W xenonlamp light (λ＞290nm) as simulated sunlight sources, the2,4-D can direct photolysis. With theincrease of light intensity，the2,4-D photolysis rate was increased. And the result also showed thedifferent pH on photolysis rate of2,4-D in solution has no significant influence.(2) Using a300W mercury lamp to simulate sunlight, and in PYREX tubes restricting thetransmission of wavelengths below290nm, the photodegradation processes of2,4-D withdifferent concentrations of NO3-in freshwater and seawater were studied, respectively. At thesame time, the effect of hydroxyl free radical (OH) on the photolysis of2,4-D was also studiedusing EPR technique. The results indicated that the2,4-D photolysis reaction followed the firstorder kinetics in freshwater and seawater under different concentrations of NO3-. Meanwhile, thephotochemical reaction rate of2,4-D increased with the concentration of NO3-increasing. Whenthe concentration of NO3-was less than31mg·L-1, the photochemical reaction rate of2,4-D inseawater was slightly higher than that of in freshwater at the same experiment conditions.electron paramagnetic resonance (EPR) spectra indicated that OH radicals were generated in theNO3-solution under simulated sunlight irradiation, implying that2,4-D degradation could berelated to oxidation reactions caused by OH.(3) To elucidate the effect of NO2-on photodegradation of organic pollutants, a300Wmercury lamp and PYREX tubes restricting the transmission of wavelengths below290nm wereused to simulate sunlight, and the photodegradation processes of2,4-D with differentconcentrations of NO2-in freshwater and seawater were studied, respectively. At the same time,the effect of reactive oxygen species (ROS) on the photolysis of2,4-D was also demonstratedusing EPR technique. The results indicated that the2,4-D photolysis reaction followed thefirst-order kinetics in freshwater and seawater under different concentrations of NO2-. Meanwhile,the photochemical reaction rate of2,4-D increased with the concentration of NO2-increasing. When the concentration of NO2-was lower than23mg·L-1, the photodegradation rate of2,4-D inseawater was higher than that of in freshwater. However, when the concentration of NO2-wasreached230mg·L-1,2,4-D degradation decreased in seawater. It was important to note that EPRspectra showed NO2radical was generated in the NO2-solution under simulated sunlightirradiation, indicating that2,4-D photodegradation could be induced by NO2. These results showthe key role of NO2-in photochemistry and are helpful for better understandingphototransformation of environmental contaminants in natural aquatic systems.(4)The photolysis of2,4-D was investigated in HA solutions, and the influence ofcoexistence with NO3-and NO2-was also studied in seawater using a rotary photochemicalreaction device under the simulated solar light irradiation.We find the experimental resultsshowed that the photolysis process of2,4-D was accorded with the pseudo first order kineticsequation both in HA and in the coexistence solutions with different concentrations of NO2-andNO3-，and has the obvious inhibitory action, the exist of HA is competitive photon with2,4-D.HA and coexistence with NO3-and NO2-system, compare with the single exise or coexistencewith NO3-and NO2-system, the photochemical reaction rate of2,4-D in seawater reducedobviously. The exist of NO3-and NO2-reduced the inhibitory effect of HA on the photolysis of2,4-D.(5)The photolysis of2,4-D was investigated in Chlorella vulgaris or Nitzschia closteriumEOM solutions, and the influence of coexistence with NO3-and NO2-was also studied inseawater using a rotary photochemical reaction device under the simulated solar light irradiation.The experimental results showed that the photolysis process of2,4-D was accorded with thepseudo first order kinetics equation both in Chlorella vulgaris or Nitzschia closterium EOM andin the coexistence solutions with different concentrations of NO3-and NO2-. It has been foundthat the photolysis rate constant of2,4-D decreased with the Chlorella vulgaris or Nitzschiaclosterium EOM concentration increasing, which indicated that this two species of microalgaeEOM could inhibited the2,4-D photolysis. However, when NO3-or NO2-was added in themicroalgae EOM solution, the photolysis rate constant of2,4-D increased, which indicated thatthe inhibition of microalgae EOM decreased with the concentration of NO3-or NO2-increasing.Especially, the photolysis rate constant of2,4-D could be further promoted under the coexistenceof microalgae EOM with the higher concentration of NO3-and NO2-.