Mechanisms Of Photosensitive Degradation Of Microcystin-LR By Dissolved Organic Matter (DOM) In Water

Eutrophication frequently occurs in lakes and reservoirs in the worldwide.Its secondary metabolism production is microcystin-LR（MC-LR）,which will contaminate water bodies.MC-LR is difficult to decay in natural environment and predisposes human beings to liver cancer,which then threat the safety of water and human health.Dissolved organic matter（DOM）is a photosensitive substance,commonly being found in natural waters.Thus,DOM can be used for photodegradating organic pollutant.In this study,FA solution,as the representative of DOM,was divided into five fractions by ultrafiltration method,and then analyzed the molecular structure by spectroscopy.Afterward,the photodegradation kinetics,products and pathways,reaction mechanisms of MC-LR in water with different fractions of FA were further discussed under simulated sunlight.The results indicated that:The TOC content of different fractions of FA were5.48-59.99 mgC·L^-1.Fractions of FA contain several kinds of functional groups such as hydroxyl group,carboxyl group,carbonyl group,aromatic ring phenols and aliphatic substances,among which the smaller molecular weight components contain more active oxygen functional groups,aromatic structures and fluorescent active groups.The degradation rates of MC-LR in pure water and various fractions of FA solution were in the range of 51-59%.The degradation process was consistent with the second-order reaction kinetics,and the degradation rates constant ranges from 0.0663 to 0.0973mg^-1·L·h^-1.The contribution rates of ¹O₂ and·OH in the self-sensitized photolysis process of MC-LR were 33.4%and 37.6%respectively.The concentration of ¹O₂ and·OH produced by photoexcitation of different fractions of FA were 10^-15-10^-14 M and10^-18-10^-1717 M,respectively,and the contribution rates to MC-LR degradation were in the range of 7.4-44.8%and 12.5-35.6%,respectively.The photosensitivity degradation rates of different FA fractions to MC-LR were as follows:（5-10 kDa）>（<1 kDa）>undivided solution>（3-5 kDa）>（1-3 kDa）>（>10 kDa）.The results of LC-MS/MS indicated that the photolysis process of MC-LR involves the ring opening reaction of C-C and C-N single bond cleavage,photooxidation,deamination and decarboxylation,in which photooxidation can destroy the cyclic structure of MC-LR and the conjugated double bond,and the cleavage and isomerization of the Adda conjugated double bond plays a leading role in the detoxification of MC-LR.