Removal And Oxidation Kinetics Of Microcystins In Water By Chlorine Dioxide

The enhancement of eutrophication in superficial freshwater bodies had increased of blooms of toxic cyanobacteria (blue-green algae) which had drawn attention of environmentalists worldwide. Bloom of microcystis is the most harmful, and produced microcystins which is hepatotoxic peptides and tumor-promoting agent and abundant in surface water. The analysis of microcystins is classified as uncommon item and of 1μg/L limitation in the standard of life and drinking water(GB5749-2006) which was issued and carried out recently. It is necessary to find a new method to control the level of microcystins in water because conventional water treatment processes is noffective. In this thesis, the removal of microcystis and MCs by ClO₂ were researched systematically, and the the oxidation kinetics of MCs acting with ClO₂ were studied too.Removal of three typical microcystis including M. aeruginosa(905#å’Œ915#)and M. wesenbergii in aqueous solution by ClO₂ were investigated systematically, and the results were showed as follow, ClO₂ could effectively remove M. aeruginosa and M. wesenbergii. The removal efficiency of chlorophyll was in positive correlation to ClO₂ dosage and the reaction time and in negative correlation to initial concentration chlorophyll, whereas it was affected by temperature and pH value slightly. The removal ratios of M. wesenbergii and M. aeruginosa(905#å’Œ915#)could reach to their maximum as approximately 97.23%,96.2%å’Œ92.33% respectively under the conditions as follows: the ClO₂ dosage 2.5mg/L, reaction time15 min, and pH 7.17. The study of the mechanism of intracellular microcystins release showed that ClO₂ dosage 3.0mg/L, intracellular and extracellular microcystins had fallen to a very little level.Therefor, ClO₂ is a suitable oxidant for the degradation of microcystins in drinking water treatment processes.MC-LR, MC-RR and MC-YR were very typical among congeners of MCs, removal efficiency of them by ClO₂ was studied. Experimental results indicated that MCs were removed by ClO₂ effectively, at pH 6.48 and 10℃, dosage ClO₂ is 2.5 mg/L, and the initial concentration of MC-LR, MC-RR and MC-YR are 100μg·L-1,respectively, the removal efficiency are 94%, 96.03% and 94.9%. The efficiency of removal was in positive correlation to ClO₂ dosage and reaction time and in negative correlation to initial concentration of MC-LR, whereas it was affected by temperature(5-25℃) and pH (3.08-10.56) value slightly. The removal of mixed MCs by ClO₂ had no obvious difference with that of MC-LR, MC-RR and MC-YR. MC-LR, MC-RR and MC-YR were decreased from 10μg/L of to 0.9, 0.6å’Œ0.7μg/L by 2 mg/L ClO₂ in preoxidation process in 90min, respectively, and 0.8, 0.55å’Œ0.6μg/L in disinfection processes.The oxidation kinetics of MC-LR, MC-RR and MC-YR was investigated in detail using chlorine dioxide according to the method of the law of isolated parameter. Experimental results indicated that the oxidation process was second order overall and first order with respect to reactants in three actions of ClO₂ oxidation MC-LR, MC-RR and MC-YR. At pH 6.48 and 10℃, the rate constant k as follow, k²(MC-LR) 459.89 L/(mol·min)(R² = 0.9999) , k²(MC-RR) 583.15 L/(mol·min)(R²=0.9998) and k²(MC-YR) 488.43 L/(mol·min)(R²=0.9999), in the three reaction, the rate constant k of the actions could be increased through increasing temperature and decreasing pH value and ranged from 5.18×10² L/(mol·min) to 4.01×10² L/(mol·min) for MC-LR, 6.11×10² L/(mol·min) to 5.28×10² L/(mol·min) for MC-RR and 5.237×10² L/(mol·min) to 4.215×10² L/(mol·min) for MC-YR at 10℃. Activation energy of degradation by ClO₂ was 64.757kJ/mol for MC-LR, 53.01 kJ·mol-1 for MC-RR and 59.151 kJ/mol for MC-YR, it would be concluded that ClO₂ could remove MC-LR, MC-RR and MC-YR effectively on the condition of traditional water treatment. Study on removal efficiency of MC-LR, MC-RR and MC-YR by ClO₂ in preoxidation process and disinfection processes was carried out too, and the results was as follow, values of chlorine dioxide expose(CT) required for oxidation of MC-LR, MC-RR and MC-YR from 10μg/L to below 1.0μg/L process was 215.195,190.3 and 196.802 min·mg /L in preoxidation process and 156.638,137.1 and 141.263 min·mg /L in disinfection processes, respectively.Reaction mechanism of ClO₂ oxidation MCs were performed, analysis on structure and character of the products of ClO₂ oxidation MCs (MC-LR, MC-RR and MC-YR) by methods of UV(UV spectrophotometer), TOC (total organic carbon) and HPLC-MS was to make sure the possible path of reaction. The reaction products of MC-LR and ClO₂ were characterized by LC-MS and the main oxidation product which has m/z value of 1029(995+2×17=1029) could be determined, m/z value of 1072(1038+2×17=1072) for MC-RR and m/z value of 1079(1045+2×17=1079) for MC-YR. Results indicated that ClO₂ act with the the diene group of adda in MCs and change the structure of double bond between carbon and carbon in adda, the group of OH- were added the two carbon atoms to make the -C(OH)-C(OH)-. Products in the three action are all volatilizable and organics.Therefore, oxidation of ClO₂ could be taken as an effective technology for removing MCs from drinking water resources in traditional drinking water supplies in case of cyanobacterial toxin risk and the product was unharmful, this could be taken as basis of control of toxic cyanobacteria blooms and safe drinking water and has great theoretical and practical value .