| Marine organic photochemistry (MOP), as one of the important branches of marine chemistry, has a close relationship to other fields such as marine environmental science, marine geochemistry and marine biology. Studying on MOP would contribute to further understanding of the removal and transformation of organic matter in the ocean.With the development of dye industry, the textile effluent has become one of major sources of pollution. For which, photochemical reaction is an important pathway of decomposition and transformation. Although the reaction mechanism and influencing factors have been widely studied, how the dye is photodegradated in natural water was rarely discussed. Therefore, it is necessary to conduct research on its photodegradation in seawater and to understand its effect on natural environment.Based on the research work, the thesis focused on a representative dye: methylene blue (MB). We systematically studied the photochemical reaction of MB and assessed its ecotoxicity to environment, and the results were as follows:1. Reaction kinetic behaviorMost reactions showed first-order reaction kinetic behavior. Under different experimental conditions, the rate constants (k) varied from 0.0053 to 0.0452 min-1.2. Influencing factorsa. Light source: No photolysis of MB was observed in the dark and it was photodegraded a little under sunlight. Contrastively, it could be significantly degraded under the irradiation of high pressure mercury lamp.b. Light intensity: The photolysis rate of MB was faster under 500 W high pressure mercury lamp than under 300 W high pressure mercury lamp, due to higher light intensity.c.Oxygen: Without oxygen rarely MB could be degradated, and MB could only be degradated in the presence of oxygen. This justified that oxygen was absolutely necessary during photochemical reaction.d. Salinity: When the initial concentration of MB was 1 mg/L, the rate constant in the three media was in the order of: DW > ASW > NSW, salinity inhibited the photoreaction. When the initial concentration of MB was 5 mg/L, the rate constant in the three media was in the order of: ASW > DW > NSW, salinity accelerated the photoreaction.e. Heavy metal ions: Different ions of Cu2+, Zn2+, Cd2+, Hg2+[0] influenced the photolysis rate differently. In DW, the lower concentration of Cu2+, Zn2+ was, the more apparent the influence was and Cd2+, Hg2+could not influence the photoreaction. In ASW, the influence was more apparent..f. Humid acid: Humid acid inhibited the photoreaction. This may be a major reason why MB photodegradated much slower in NSW.g. Acetone: Acetone accelerated the photoreaction and all reactions with acetone showed zero-order reaction kinetic behavior.h. Algae: All three species of marine phytoplankton Isochrysis galbana8701, Chlorella pynenoidosa and Chaetoceros curvisetus accelerated the photoreaction. The maximum relative reaction rate could reach 2.42 with Chlorella pynenoidosa.3. Toxicity test(1) Acute toxicity: Four species of marine phytoplankton Heterosigma akashiwo Hada (Ha); Chlorella pynenoidosa; Skeletonema costatum and Phaeodactylum tricornutum were employed in this research, and the results showed that acute toxicity was decreased after photodegradation.(2) Chronic toxicity: After photodegradaion, chronic toxicity of MB was decreased, especially for Skeletonema costatum.In brief, this thesis has systematically studied the photoreaction kinetic behavior of MB and its influencing factors in seawater. Meanwhile, it combined photochemical experiment to biological toxicity test and justified that after photodegradaion, the toxicity of MB to marine phytoplankton was decreased. Based on the experimental results in this thesis, we can have a further understanding of the photochemical reaction of dye in the marine environment and offer theoretical basis to deal with the organic pollutants.
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