Effects On The Growth And Microcystin-LR Production Of Microcystis Aeruginosa And Mechanistic Study Of Microcystin-LR Degradation Induced By UV₃₆₅/NaNO₂

Discharge of production and domestic wastewater to lakes and other natural water bodies has caused the increasing concentrations of nutrient salts in lakes and other natural water bodies. Occurrences of eutrophication have been reported worldwide and they are nowadays in a growing trend. The corruption of organic matter after the algal bloom results in water quality deterioration and consumes large amount of dissolved oxygen. Degradation of organic matter produces nitrite, which poses a serious threat to drinking water sources and the ecological environment. Major harmful algae produce microcystin, a strong liver tumor promoter. In the process of decaying of algal cells, the toxin will be released into the water. Considering its hazardous effect on human health and ecological environmental, the World Health Organization recommend 1μg/L as the detection limit of MC-LR in drinking water.The MC-LR used in the experiments was extracted and purified from the laboratory culture of Microcystis aeruginosa. Efficiency in the extraction and purification method of MC-LR was evaluated. The effect of nitrite and UV/nitrite on MC-LR degradation was studied by HPLC and MS analysis, and the influence of UV/nitrite on the growth and toxin production of Microcystis aeruginosa was also investigated. The main conclusions are as follows:1. Methanol content in the sample of MC-LR has a significant impact on the results of MC-LR in HPLC analysis. Different proportions of methanol in the MC-LR sample will result in different HPLC detections. When methanol content is below 80%, HPLC response of MC-LR (peak area) will increase slowly as the methanol content increase. When methanol content is above 80%, HPLC response of MC-LR (peak area) will decrease drastically as methanol content further increase, with a maximum peak area at the content of 80%. Therefore, the methanol content in the sample has a significant impact on HPLC analysis. To get an accurate and comparable result of MC-LR, the methanol content in the samples should be strictly controlled.2. The degradation of MC-LR by HNO2 was observed for the first time. Pseudo-first-order kinetics characteristics of the reaction and effects of paremeters including reaction solution pH, NaNO2 concentration and MC-LR concentration were studies. The reaction mechanisms was analysed according to the LC-MS results. 94.89% of MC-LR was observed within 5 h under optimum initial condition of 5 mmol/L sodium nitrite, pH 1.73,9.78 mg/L MC-LR. The pseudo-first-order kinetics constant k increased when the pH value lowered. When the NaNO2 concentration was less than 5 mmol/L when, MC-LR degradation reaction rate constant k increased with the NaNO2 concentration increased, while the NaN02 concentration was greater than 5 mmol/L time, k decreased when the concentration of NaN02 increased. The concentration of MC-LR had little effect on its degradation. The major reaction pathways are:the destruction of the conjugated double bonds of the Adda group, electrophilic addition reaction on the benzene ring end and the diazo reaction happened between the-NH2 terminal and nitrous acid.3. In the degradation of MC-LR by combination of UV365/nitrite, the change of MC-LR concentrations could well fit the pseudo-first-order kinetic equation and the second-order kinetic equation, but the coefficient of the pseudo-first-order equation was higher. The degradation rate of MC-LR was positive correlated with NaNO2 concentration and UV365 intensity, and negative correlated with pH value and MC-LR concentration. LC-MS was used for analysis of intermediates and reaction pathways. The conjugated double bonds and benzene ring groups at the end of the Adda side chain are the major reaction sites and degradation reaction were initiated by NO racidal and OH radical through electrophilic addition reactions. The methoxyl group and the double bonds in the cyclic MC-LR molecule structure are also the major reaction sites. Under the irradiation of UV, nitrite and MC-LR can be removed from the water at the same time.4. A tentative research has been carried out to study the effect of UV365/NaNO2 on the growth of Microcystis aeruginosa and MC-LR production. Under laboratory conditions, cultured Microcystis aeruginosa grew into stable phase within 15 days. The results showed that with sufficient nitrate in the culture medium, when higher concentrations of nitrite (30 mg/L) was added to the medium, its concentration decreased over time, when lower concentration of nitrite (10 mg/L and 20mg/L)were added, their concentrations would increase. Nitrite had little effect on the growth and MC-LR production of Microcystis aeruginosa, while UV365 had notable effect on the photosynthetic rate and intracellular MC-LR content of Microcystis aeruginosa.