Study On Photochemical Degradation Of Two Kinds Of Pesticides In Seawater

Marine organic photochemistry (MOP), as one of the important branches of marine chemistry, has a close relationship with many marine subjects such as marine biology and marine environmental science. The comprehensive study on MOP is of realistic significance for further understanding the removal patterns of organic pollutants in the ocean.The photodegradation of pesticides is an important pathway for their decomposition and transformation in natural environment. Pesticide molecules can absorb the light energy and be photodegraded themselves (direct degradation).The degradation may also be sensitized by other compounds, which referred to indirect degradation. It is of great significance to study the photodegradation of pesticides in seawater, because through the study, we can get the products and pathway for their photodegradation in natural environment and realize the safety of pesticides in order to instruct the use of pesticides and synthesize new products.Based on the former research work, the thesis focused on the photochemical degradation reaction of two representative pesticides in seawater by the simulative method in laboratory. Through instrument detection and experimental analysis, we systematically studied the photochemical degradation of triadimefon and rogor and obtained the following results:1. Photochemical degradation of triadimefon(1) Triadimefon could be significantly degraded under the irradiation of 300 W high pressure mercury lamp and showed the first-order reaction kinetic behavior. Under the different experimental conditions,the rate constants (k) of triadimefon varied from 0.0027 to 0.0607 min-1.(2) Many factors in the experiments would influence the photodegradation: a. Light source: Triadimefon was photodegraded a little under the irradiation of sunlight while it could be significantly degraded under the irradiation of 300 W high pressure mercury lamp. However, no photolysis of triadimefon was observed in the dark.b. Light intensity: The photolysis rate of triadimefon under 500 W high pressure mercury lamp was faster than that under 300 W high pressure mercury lamp, showing that light intensity is an important factor for the photodegradation of triadimefon.c. Aquatic media: The photodegradation of triadimefon in deionized water was the fastest among the three media-deionized water, natural seawater and artificial seawater. The photodegradation of triadimefon in artificial seawater was a little faster than that in natural seawater.d. Heavy metal ions: In all the reactions, different metal ions in different reaction systems displayed different effects. The results were obtained by changing the concentrations of five heavy metal ions in triadimefon solution. The effects of heavy metal ions were caused mainly by the interaction between ions and aquatic media.e. Photosensitizer: The selected photosensitizer - acetone could accelerate the reactions of triadimefon and the influence degree was determined by reactant and aquatic media.(3) pH: The pH of solutions reduced in all kinds of systems when pH was measured during the process of photodegradation of triadimefon, while the effects were different for the different systems.(4) Identification of degradation products: Three products have been identified using GC-MS detection. However, the fourth product, probably being the intermediate product of triadimefon, was not identified because of the lack of an authentic standard sample.2. Photochemical degradation of rogor(1) Rogor was hardly degraded under the irradiation of 300 W high pressure mercury lamp within 60 min, but was quickly degraded when H2O2 was added into the solution as a catalyst. Under the different experimental conditions, the photolysis of rogor showed the first-order reaction kinetic behavior and the rate constants (k) of rogor varied from 0.0213 to 0.0744 min-1.(2) Many factors in the experiments would influence the photodegradation:a. Aquatic media: The reaction in natural seawater was the slowest compared with the b. Amount of H2O2: The optimum dosage of H2O2 was 2.8 g/L.c. Heavy metal ions: In seawater, five metal ions (such as Cu2+, Zn2+, Cd2+, Hg2+, Pb2+) displayed different effects.d. Nitrate and nitrite: Both nitrate and nitrite showed photoquenching effects on degradation of rogor and the quenching degree was enhanced with increasing concentration of nitrate and nitrite.In brief, for the two pesticides in seawater, a series of results have been obtained in the respects of photochemical degradation, photocatalytic degradation, kinetic behavior and the factors influencing them. Based on the experimental results, we can have a further understanding of the photodegradation situations of these two kinds of pesticides in the marine environment.