Study On Degradation Mechanisms Of Different Oxidation Methods Based On Removal Of Microcystin-LR In Water

Microcystin-LR（MC-LR） is the secondary metabolite released by the toxic species of cyanobacteria and it is a monocyclic heptapeptide hepatotoxin. Because MC-LR has stable chemical property, it can not be effectively removed by the conventional water treatment processes. Therefore, the methods that used for enhancing the removal of MC-LR, the removal pathways, and the degradation products have become the focus in the water treatment field. UV, O₃, UV/H₂O₂, and UV/O₃ are the common methods to enhance the performance of the conventional drinking water treatment processes for the removal of pollutes. However, their performances for the degradation of MC-LR are still necessary to be further investigated. Thus far, the studies on the degradation pathways and degradation products of MC-LR by UV and O₃ and UV/H₂O₂ are not perfect, while the study on the oxidation intermediates, reaction sites and degradation pathways of MC-LR by UV/O₃ is limited. In addition, because different researchers used different experiment conditions, the results of the current study on the degradation of MC-LR usually have differences; therefore, it is difficult to directly compare the results. Based on above, it has the theoretical significance and the practical value to further investigate the removal efficiency, the influence factors, the degradation products and the degradation pathways of MC-LR by the above four methods, and then compare their differences and connections under the same experiment conditions.From the perspective of theoretical research, UV radiation, ozone oxidation, UV/H₂O₂ oxidation and UV/O₃ oxidation were investigated to the remove MC-LR from water in this study. The removal efficiency, influence factors and the mineralization of MC-LR were investigated. Then, the reaction intermediates of MC-LR were identified. According to the structures of the reaction intermediates, the reaction sites and degradation pathways of MC-LR were further investigated. Finally, the degradation processes of MC-LR by different methods were compared, and the advantages/disadvantages of these methods used for the degradation of MC-LR could be found.The degradation efficiency of MC-LR by UV radiation and influences factors were investigated. Results showed that the removal efficiency of MC-LR under UV radiation could be significantly affected by the UV intensity; however, p H（5-10）, HCO₃ˉ（0 – 240mg/L） and organic matters in the water had no significant influence on the degradation of MC-LR. Moreover, UV radiation has a low ability to mineralize MC-LR. The identification of the degradation intermediates of MC-LR showed that the major reaction intermediates were [4（E）,6（Z）-Adda5]-MC-LR and [tricyclo-Adda5]-MC-LR. Both of them were the isomers of MC-LR, which resulted from the isomerization of Adda side chain of MC-LR, and could be further degraded by UV radiation.The oxidation of MC-LR by ozone was investigated. Results showed that ozone oxidation was an effective method that could rapidly remove MC-LR from water. However, the oxidation efficiency of ozone was prone to be affected by p H and organic matters, but HCO₃ˉ（0 – 240mg/L）had no significant effect. MC-LR could be mineralized with high molar ratios of ozone to MC-LR. Reaction intermediates and aldehyde-based products with low molecular weight of MC-LR were identified. Results showed that the main reaction sites in the structure of MC-LR were the conjugated double bonds in the Adda side chain and the double bonds in Mdha amino acid. The cyclic structure of MC-LR could transform to linear polypeptides after the oxidative cleavage of double bonds in Mdha, but this process was more difficult than the oxidation of the Adda side chain. During the oxidation of MC-LR, the direct oxidation of ozone molecule was dominant compared to the indirect oxidation of hydroxyl radical（O₃ transformed to ·OH）.The removal efficiency of MC-LR by UV/H₂O₂ and UV/O₃, and the influence factors were investigated. Results showed that the mineralization efficiency of MC-LR by UV/H₂O₂ and UV/O₃ could be higher than 60%. During the oxidation of MC-LR by UV/H₂O₂, there was an optimum mole ratio of H₂O₂ to MC-LR. The removal efficiency of MC-LR by UV/H₂O₂ was affected by HCO₃ˉ and organic matters in the water, but the effect of p H was not significant. However, the removal efficiency of MC-LR by UV/O₃ could be affected by p H, HCO₃ˉ and organic matters in the water.The degradation intermediates and pathways of MC-LR during the oxidation by UV/H₂O₂ and UV/O₃ were investigated. It was indicated that the main degradation mechanism of MC-LR by UV/H₂O₂ involved radical（·OH） oxidation and direct UV photolysis. All of the reaction sites of MC-LR during UV/H₂O₂ oxidation located at the Adda side chain. The main reactions involved: the isomerization of the Adda side chain; the hydroxyl substitution（single hydroxyl and double hydroxyl） of the benzene ring; the oxidation of the methoxy group; the hydroxyl addition（dihydroxylation and tetrahydroxylation） and oxidative cleavage of the conjugated double bonds of Adda. Among these reactions, the hydroxyl substitution of the benzene ring and the hydroxyl addition of the conjugated double bonds were dominant. However, UV/O₃ had more reaction mechanisms to degrade MC-LR: ·OH oxidation, direct UV photolysis and the direct oxidation of ozone molecule. MC-LR could be degraded through more reaction sites and more reaction pathways: the isomerization, hydroxylation and oxidative cleavage of the Adda side chain, the oxidation and cleavage of Mdha amino acid, and the decarboxylation of Me Asp and Glu amino acids. The oxidative cleavage of Adda and Mdha could result from ·OH or O₃ oxidation, but the isomerization of Adda and the decarboxylation of Me Asp/Glu mainly resulted from UV radiation.The degradation processes of MC-LR by UV, O₃, UV/H₂O₂ and UV/O₃ were compared. Results showed that under the used experiment conditions, for the removal and mineralization of MC-LR, the efficiency of UV radiation was lowest, but the efficiency of UV/O₃ was highest. For the effect of the water quality, UV radiation had an advantage and its efficiency was not significantly affected by the water quality factors. For the reaction intermediates, MC-LR could be oxidized to the products with smaller molecular weights only by O₃ and UV/O₃. For the reaction sites of MC-LR, the reaction sites are located at the Adda side chain during treatments by UV and UV/H₂O₂. But during O₃ and UV/O₃ oxidation, the reaction sites of MC-LR located at both the Adda side chain and the peptide ring. For the number of the reaction sites of MC-LR, the oxidation by UV/H₂O₂ and UV/O₃ had more reactin sites. For the degradation pathways of MC-LR, UV/H₂O₂ oxidation involved the isomerization pathway of Adda side chain that found during UV radiation; it also involved the pathways of hydroxyl addition and oxidative cleavage of the conjugated double bonds that found during O₃ oxidation. In addition, the hydroxyl substitution of the benzene ring and the oxidation of the methoxy group were also found. However, during the degradation of MC-LR by UV/O₃, the above pathways were involved and at the same time, the pathway of the decarboxylation was also found.