The Effect Of Hydroxyl Radical Preoxidation On Lignin Degradation By Fungi

Lignin is a renewable biomass energy that is not only widely distributed in nature but also has a huge amount of production.As a good indicator of terrestrial organic carbon,lignin has great research value for freshwater ecosystems and the global carbon cycle.The active oxygen in the environment is closely related to the source,the migration,transformation of lignin in aquatic environments.Hydroxyl radical is a kind of ROS.Its strong oxidation can promote the breakage of chemical bonds in plant cell walls,and destroy the binding of lignin to cellulose and hemicellulose.Therefore,hydroxyl radical have an important effect on lignin degradation.Studies have shown that hydroxyl radicals are involved in the degradation process of lignocellulose.However,there are no researchs and reports on the contribution rate of·OH.Based on this,the effect of hydroxyl radical preoxidation on lignin degradation by fungi has been studied.It is determined the lignin content of wheat straw,barley straw and rice straw.By using normal lignin?NL?and extracted lignin?EL?from rice straw as raw materials,this paper studies the two kinds of lignins structure,hydroxyl radical preoxidation on the structure of lignin,and the effect of hydroxyl radical preoxidation on fungal degradation of lignin.The Van soest method is used to determine the lignin content of wheat straw,barley straw and rice straw.And the results are 15.42%,11.69%and 17.88%,respectively.Spectral analysis of standard lignin?NL?and extracted lignin?EL?show that the main structures of NL and EL are similar,and the absorption bands also show the same vibration.The integrity and high purity of EL basic structural units all demonstrate that the extraction process works well in this experiment.The hydrogen spectra of NL and EL show a strong signal at 6.6 ppm,indicating that NL and EL contain springyl units.According to the signal intensity of syringyl units,guaiacyl units,p-hydroxyphenyl units,the result of content of syringyl units in NL is higher than that of guaiacyl units.Lignin forms a cohesive structure in the extraction process,therefore the tightness of EL is higher than NL.The optimal reaction conditions for Fenton reaction to generate hydroxyl radicals are 4 mmol/L H₂O₂,1 mmol/L Fe²⁺,40?and pH=2.For quenched normal lignin?QNL?,oxidized normal lignin?ONL?,quenched extracted lignin?QEL?and oxidized extracted lignin?OEL?,their infrared spectra at 1598 cm^-1,1508 cm^-1 and 1426cm^-1 don`t change,indicating that the benzene ring structure of lignin isn`t destroy by hydroxyl radicals.The ONL signals at 1712 cm^-1 and 1649 cm^-1 disappear and the signal at 1265 cm^-1 weaken.The absorption peaks at 1650 cm^-1 and 1263 cm^-1 of OEL disappear,indicating that non-conjugated and conjugated carbonyl groups are degraded by hydroxyl radicals generated by Fenton reaction.The O/C ratio of ONL is less than QNL,and the O/C ratio of OEL is also less than that of QEL,indicating that hydrogen is abstracted during the Fenton reaction,leading to the formation of unstable carbon radicals and the further formation of C-C bonds.Fusarium oxysporum produces higher lignin peroxidase?LiP?,manganese peroxidase?MnP?,and laccase?Lac?than Cladosporium during degrading lignins process.The degradation rates of Fusarium oxysporum to four kinds of lignins are higher than that of Cladosporium,which indicates that Fusarium oxysporum is more suitable than Cladosporium for lignin degradation.The enzyme activity of the two strains is MnP>LiP>Lac.MnP is about 2 times of LiP and 40 times of Lac,which indicates that manganese plays a major role on fungal degradation of lignin.It is in the EL degradation that the two strains produce more extracellular enzymes,and the degradation rate of EL is higher than NL,so EL is more easily degraded than NL.The activity of pre-oxidized lignin?ONL and OEL?is higher than that of the original lignin?NL and EL?,and the two strains have a higher degradation rate of ONL and OEL,thus that the preoxidation of hydroxyl radicals can destroy the structure of lignin and increase the degradation rate of lignin by fungi.