Study On The Mechanism Of Enhancing Sulfadiazine Degraded By Methanotroph Under Light Irradiation

Methanotrophs can co-metabolize refractory organic matter through methane monooxygenase（MMO）while using methane for growth and metabolism.At the same time,photodegradation in water is also an important way of natural decay of refractory organic matter.However,as non-photosynthetic microorganisms,the influence of light on the growth and metabolism of methanotrophs has been neglected.To solve this problem,this study used sulfadiazine（SDZ）as the target refractory organic matter and Methylocysstis bryophila as the model methanotroph strain to explore the effect of light on the degradation process of SDZ by M.bryophila and relevant mechanism.The results of this study are as follows:（1）The methane oxidation activity and cell proliferation of M.bryophila were not significantly affected by SDZ below 30 mg/L,but the secretion of extracellular organic matters（EOMs）was stimulated by SDZ.With the increase in SDZ concentration,the EOMs produced by M.bryophila increased and its composition changed.SUVA254 and E2/E3 values increased,indicating that the aromatic degree increased while the molecular weight decreased;（2）With the increase in concentration,the degradation capacity of methanotroph to SDZ decreased significantly within 10 days:the degradation ratios of SDZ of 0.3 mg/L,3 mg/L,10 mg/L,20 mg/L,and 30 mg/L were 78%,25%,13%,8%,and 7%,respectively,indicating that the degradation capacity of methanotroph to SDZ was limited;（3）Degradation of SDZ by M.bryophila must be accompanied by methane oxidation,suggesting that the degradation process of SDZ is mediated by MMO co-metabolism.The low concentration of Cu²⁺in the reaction system indicated that s MMO was the key enzyme responsible for the degradation of SDZ;（4）The degradation ratio of 3 mg/L SDZ was increased from 28.9%to 80.6%and the mineralization ratio was increased from 9.9%to 36.5%within 8 days after adding light.Meanwhile,Ames experiment showed that the degradation product of M.bryophila co-metabolize SDZ alone had mutagenicity（MR=12.92）,while the mutagenicity disappeared after adding light,suggesting that light can significantly enhance the co-metabolic degradation process of M.bryophila to SDZ;（5）The degradation degree of SDZ was significantly different with the addition of quench agents to inhibit different active oxygen species in the indirect photodegradation process,indicating that EOMs-mediated indirect photodegradation is the key to SDZ degradation,in which ³EOMs*is the main active oxygen species,followed by·OH and ¹O₂;（6）Light changed the degradation pathway of SDZ in M.bryophila system.Under the co-metabolism of M.bryophila,the nitration and nitration of-NH₂ group,hydroxylation of the benzene ring and heterocyclic ring,breaking of sulfonamide bond（-NH-SO₂-）and cleavage of pyrimidine ring occur in SDZ to form transformation products with larger molecular weight.These transformation products were further degraded to formic acid,acetic acid and other small molecules under the action of reactive oxygen species after adding light,and finally achieved complete mineralization.