The Screening Of Polyvinyl Alcohol-degrading Strains Bacillus Cereus Sp.and Study On Its Mechanism Of Biodegradation Process

With the rapid development of polymer science,polymer materials have been widely used in all walks of life,which brought great convenience to people's lives.At the same time,related plastic waste also caused environmental pollution because of limited treatment methods and the secondary pollution produced in the degradation process of waste.To solve the pollution problem,researchers have made an extensive study on biodegradable polymers.Polyvinyl alcohol?PVA?,a kind of biodegradable polymers,is widely used in food packing,agricultural film,architectural paint industry,healthcare and so on because of its excellent adhesion,mechanical and antistatic properties.Though PVA can be degraded by microorganisms,its degradation rate is so limited that the undegraded PVA could gather in the water.Even though PVA is not toxic,it could decrease the dissolved oxygen,which would affect the activity of aquatic organisms.In order to solve these problems and accelerate the biodegradation of related PVA-based materials,researchers have undertaken a lot of researches based on PVA-degrading strains.In this study,PVA-degrading strains were screened from the surface of degraded PVA-based materials,and were identified further.Degradation capacity of these strains were compared.The degradation conditions of strain of which the degradation capacity is strongest were further optimized.The degradation kinetics and thermodynamics of PVA were studied.At last the functional groups,molecular weight distribution and stereoisomerism of PVA were characterized by Fourier Transform Infrared Spectrometer?FTIR?,Gel Permeation Chromatography?GPC?and Nuclear Magnetic Resonance Spectroscopy?NMR?.The degradation mechanism of PVA was further discussed.The main research contents and results of this paper are as follows:1.Three PVA-degrading strains CP01,DG01 and DG02 were screened from the surface of the PVA-based composites materials during the compost burial course.Combining the results of molecular biological identification and biochemical test,CP01,DG01 and DG02 were identified as Bacillus cereus sp.and named as Bacillus cereus.CP01,Bacillus cereus.DG01and Bacillus cereus.DG02 respectively.2.The PVA degradation capacity of the three strains was compared.The diameter of the transparent circle on the PVA plate,the degradation rate of PVA and the amount of CO₂released in the liquid medium by DG01 were higher than those of the other two strains,which indicated that the degradation capacity of DG01 was stronger than that of DG02 and CP01.Live bacteria concentration was also analyzed.The results showed that live bacteria concentration of DG01was much higher than that of CP01 and DG02 during the degradation process in the liquid medium,which indicated GD01 could grow well in environment containing PVA.3.The temperature,initial pH and yeast concentration of degradation conditions of DG01were optimized by complete randomalized design.The inorganic salt of medium were optimized by response surface method.Under optimum condition the degradation rate of PVA can reach 46.73%within 48 hours and the average degradation rate can reach 0.701 g/?d·L?.4.The kinetics and thermodynamics of PVA degradation by DG01 were studied respectively.The kinetic experiments results showed that the degradation process fitted well with the pseudo-first-order kinetic model.The kinetic constant reached maximum when the initial concentration is 1.5 g/L,which indicates that high PVA concentration would inhibit the degradation of PVA.On the analysis of thermodynamic experiments results,the activation energy of PVA degradation is 67023.3 J/mol.The activation energy is relatively low,which indicated that the degradation of PVA could take place easily under DG01.5.The degradation process of PVA was characterized by FTIR,GPC and NMR.According to the infrared spectra of the samples before and after degradation,obvious peaks at 1714 cm^-1,1372 cm^-1 and 1322 cm^-1 appeared,which indicated that carbonyl group was generated during the degradation process.The GPC results showed that the molecular weight of PVA decreased gradually during the degradation process.The results of NMR show that relevant amounts of PVA isotactic decreased gradually,which indicated that the isotactic portions of PVA molecule are easier for Bacillus cereus.DG01 to degrade.