Studies On The Analytical And Removal Method And The Oxidative Degradation Mechanism Of Microcystins In Eutrophic Water

Cyanobacterial blooms in the eutrophic water could direct cause the concentration of Microcystin (MC) increases. MC is hepatotoxin and tumor promoter, its chemical property is so stable that conventional water treatment processes can not remove it effectively. This brought a great threat to the safety of drinking water. Around MC in Tianjin's eutrophic water , the research were carried out successfully which included analytical method, purification and identification means , removal technique, oxidative degradation kinetics and mechanism.The high performance liquid chromatography (HPLC) detection of MC and the solid phase extraction (SPE) enrichment of MC were optimized, and then the highly effective and sensitive SPE-HPLC method was established, which have good reproducibility, precision and returns-ratio. The minimum detection limit could achieve 0.1μg/L. So it adapts to analysis the trace MC in water. After the optimization of mass spectrum examination parameter according to the MC molecule properties, the organic combination of HPLC with MS formed the HPLC-ESI/MS analytical method which increased the sensitivity further, cut down the detecting limit to pg level. This method is also able effectively to carry out the qualitative analysis of MC and its degradation product, which lays the foundation for the in-depth study of MC.The research was carried on about the extraction, purification and identification of MC. Through the experimental study, the purification method of MC was optimized, which has the characteristics of high extraction rate, good reproducibility and low impurity content and can effectively purify MC from water blooms. The investigation discovered that the microcystis is major intergrant of cyanobacteria in Tianjin's eutrophic water and its content is over 90 percent. Using the HPLC and HPLC/ESI-MS method to analyze qualitatively and quantitatively of MC sample purified from cyanobacteria, it indicated that the alga in the raw water may produce two kind of microcystin such as MCRR and MCLR, and each intensity was higher, respectively 432.89 and 322.51μg / g dry weight of algae.Through the comparison of MC removal effects in unit technologies and their combination, several conclusions achieved as follow: MC could be only removed by less than 50 percent in conventional water treatment process, and coagulation even resulted in deficit removal efficiency because of the DMC content increasing. Three pre-oxidants such as chlorine, potassium permanganate and ozone could remove MC effectively with appropriate dosage, and increase of the dosage may improve the removal efficiency. Setting up pre-oxidation before conventional process can increase the removal rate of MC; it eliminates the phenomenon of MC's increase which due to the separate pre-oxidation or coagulation process. The adsorption of a single PAC was efficiency to the removal of DMC and inefficiency to IMC, while it was inefficiency to TMC for about 30 percent removal rate; The surface texture and chemical properties of PAC was modified by oxidation to enhance the removal of MC. The synergistic action of ozone and PAC was more conductive to the removal of MC than their application alone. The combined process of pre-oxidation of ozone + PAC adsorption + coagulation and sedimentation had over 80 percent removal rate of TMC to control the MC content in water effectively; three kinds of combined process such as pre-oxidation of ozone + conventional gas floatation, pre-oxidation of ozone + PAC adsorption + conventional gas floatation and pre-oxidation of ozone + modified PAC adsorption + conventional gas floatation could remove the MC effectively and the removal rate of TMC was 85.3, 91.6 and 96.2 percent in turn. After treated by combined processes, the MC in filtered water was very low even disappeared, and the two later could remove organic more effectively. In experiment system, the deep treatment followed conventional processes, such as medial oxidation, catalysis oxidation, biological active carbon, disinfection and so on, could ensure the water from plant safety because the removal effects to the residual MC were clear as to detecting limit below. Finally, through the comprehensive comparison of the removal effects of MC, alga and organic in unit and combined processes, it suggested a basis for the selection of water treatment processes suitable to eutrophia source water.Through the comparison of degradation properties of MCLR by chlorine, potassium permanganate and ozone under different conditions, it was found that the MCLR degradation processes of three oxidants complied with pseud-first-order kinetics mode as ln(C/C₁) =-kt. reaction rate had nothing to do with initial concentration of MCLR, while influenced greatly by oxidant initial concentration that were direct ratio to chlorine and KMnO₄ and 1.5 power direct ratio to ozone. Increase temperature did well to accelerating degradation but not outstanding. Reaction activation energy between MCLR and active chlorine, KMnO₄ and ozone were 15.86, 18.17, 25.38kJ/mol in turn. pH had great influence on the degradation of MCLR by ozone and increase of pH accelerated degradation.The analysis of the molecular weight and texture of degradation products discovered that mechanism of MCLR degradation by three oxidants were different. Active chlorine could take place additive reaction with MCLR to generate chlorohydrin-MCLR, and the halogen in the chlorohydrin may then undergo a nucleophilic substitution reaction with the solvent to form dihydroxyl-microcystin. While KMnO₄ could oxidize MCLR to dihydroxyl-MCLR directly, and under the condition of excessive KMnO₄, it could further oxidize to dicarbonyl-MCLR. Ozone could quickly oxidize MCLR to dihydroxyl-MCLR and further oxidize to alkone or aldehyde organic immediately. Under H₂O₂ catalysis action which generated during the course, above degradation products could further oxidize to carboxylic acid organic.