Production Of Reactive Oxygen Species And Degradation Of Norfloxacin In Algae Liquid Induced By UV Irradiation

Algae can produce large amounts of reactive oxygen species under light radiation after a series of reactions. Thus, reactive oxygen species photogenerated by algae can be used in photochemical degradation of pollutants in water. However, the formation characteristics and mechanisms of typical reactive oxygen species such as ·OH and 1O2 are still unclarified. The lack of this part is not conducive to the overall understanding of photochemical degradation of organic pollutants by algae and the mechanism research and practical operation regulation. In this paper, three kinds of common freshwater algae(Chlorella vulgaris, Anabaena cylindrica,Microcystis aeruginosa) were chosen as the research subjects to explore the formation characteristics of ·OH and 1O2 in algae aqueous suspension induced by UV irradiation.Furthermore, the photodegradation characteristics and pathways of norfloxacin in suspension of algae were preliminarily analyzed. The main works and conclusions are as follows:Firstly, the mechanism of multi influence factors on the formation of ?OH in suspension of algae was studied. The results showed that considerable ?OH were detected in aqueous solution containing there kinds of algae respectively and the most suitable pH value was between 6 and 7 under which the degradation rate of organic pollutants could reach the maximum. Moreover, the amount of ·OH increased with increasing irradiation intensity and the optimum temperature was 25 oC. Algae pretreated with ultrasonic enhanced the production of ·OH, but algae pretreated with heat reduced the production. The yield of ·OH in single species algae suspension was the lowest and the mixed solution of three kinds of algae produced more ·OH than two kinds. Meanwhile, pH value of the mixed algae solution gradually increased with the production of ·OH and this process was accompanied by a certain amount of H+ consumption.Secondly, the mechanism of multi influence factors on the formation of 1O2 in suspension of algae was discussed. The results indicated that microalgae aqueous solution in dark environment didn’t yield any 1O2 and algae solution in natural light environment could form a small amount of 1O2, however algae solution under UV irradiation could produce a large number of 1O2. The amount of 1O2 increased with increasing irradiation intensity and the formation of 1O2 was positively correlated with the number of algal cells in a certain range.Furthermore, the optimum temperature was 25 oC and the production of 1O2 was at a high level when pH was 7. Algae treated with ultrasonic enhanced the production of 1O2, whereas algae treated with heat reduced the production. The kinetic models of 1O2 initial formation rate in different concentration of single species algae and mixed algae solution were in accordance with the power function relationship. The kinetic models were as followed respectively: v0 = 0.03337 Calgae0.1183(Chlorella vulgaris), v0 = 0.02005 Calgae0.1456(Microcystis aeruginosa), v0 = 0.06224 Calgae0.1112(Anabaena cylindrica), v0 = 0.03131 Calgae0.1318(mixed algae).Thirdly, the formation pathways of two kinds of typical active free radicals were preliminarily established. The results showed that the production of active free radicals had a relationship with the content of pigments(chlorophyll and carotenoid) in algae cells. After heattreatment, the physical and chemical structure of algal cells changed obviously, so that the amount of free radicals decreased. The secretions of algal cells were proved to contain sugars(glucose, xylose and arabinose) and small molecule acids(citric acid, malic acid, lactic acid).The formation pathways of typical active free radicals included a series of chemical reactions in suspension of algae. Reactants for producing 1O2 were algal secretions DOM, chlorophyll,and cytochrome P in algae, while reactants for producing ·OH were photosynthetic pigments and algal secretions DOM.Lastly, the mechanism of multi influence factors on the photodegradation of norfloxacin in suspension of algae was studied. The results showed that when the pH values were neutral or weak acidic and the temperature was 25 oC, microalgal cells had active metabolism. Under those conditions, the photodegradation rate of norfloxacin was maximum. After 60 minutes under UV irradiation, the degradation rates of norfloxacin were approximately 30.99%,23.78% and 19.01% in Chlorella vulgaris, Anabaena cylindrica, Microcystis aeruginosa aqueous solutions respectively. Oxidative degradation by active free radicals photoproduced in algae liquid was the main degradation pathways of norfloxacin solutions.