The Degradation Characteristics,mechanism And Bioenhanced Degradation Of Biodegradable Plastics In Aqueous Environment

With the widespread use of plastic products,a large number of plastic wastes continue to enter the environment.Because of its stable nature,it persists in the environment and adversely affects the ecological environment.In response to this problem,China’s"plastic restriction order"has significantly increased the use of biodegradable plastics.As an important storage repository for plastic waste,biodegradable plastics enter the water and are prone to property changes and degradation due to their bioavailability.However,there is a relative lack of research on the degradation behavior of biodegradable plastics in aqueous environment at present,and there is a lack of understanding of their degradation mechanism and potential risks.Based on this,two typical biodegradable plastics:poly（butyleneadipate-co-terephthalate）/polylactic acid/starch mixture（PBAT-H）and polylactic acid（PLA）were selected to explore their degradation characteristics in the natural water environment,and their degradation mechanism was analyzed by water contact angle,atomic force microscopy,thermogravimetric analysis,Fourier transform ion cyclotron resonance mass spectrometry and metagenomics analysis,and microbial enhanced degradation research was carried out on this basis.The main findings were as follows:1)In this study,the degradation characteristics of biodegradable plastics（PBAT-H and PLA）were explored by exposing plastics to natural water environment and analyzing changes of plastic surface properties and biofilm composition.The results showed that the biodegradable plastics PBAT-H and PLA had higher degradation rates than the three non-degradable plastics（PVC,PS and PP）.Among them,PBAT-H had no complete plastic sheets at 58 days,the surface was broken and flocculated,and the degradation rate of PLA reached 17.05%at 88 days.With the increase of exposure time,the hydrophobicity of plastic surface decreased,and the roughness increased,and the final water contact angle of PBAT-H and PLA surface was less than 90°,and the maximum roughness was 3.97 and 2.83 times the initial samples,respectively.The composition of plastic surface biofilms was further explored by metagenomics,and genes related to the metabolism of lactic acid,terephthalate,esterase,and hydrolase metabolism were discovered,showing that biofilms on plastic surfaces have the potential to drive the degradation of PBAT-H and PLA.2)In order to further explore the degradation mechanism of biodegradable plastics in aquatic environment,simulation experiments were carried out to analyze the contribution of abiotic factors of slurry water flushing and biological factors of microbial action to the degradation effect.The results showed that the main factors driving the surface characteristics’changes of PLA and PBAT-H were slurry water flushing and biological action,respectively.And the degradation effect of the two plastics was the best when slurry water flushing and biological interaction were combined.After 60 days of degradation,the mass loss of PBAT-H and PLA was 37.51%and 10.72%,respectively,the maximum surface roughness increased to 2.24 and 3.41times of the initial one,the water contact angle decreased to 82.6675°and 80.5725°,and the initial decomposition temperature shifted to low temperature by 14.2°C and12.6°C.The contribution of biodegradation was further confirmed by ¹³C isotope fractionation,and the degradation of PLA and PBAT-H was accompanied by the release of plastic oligomers（e.g.,lactic acid trimers,PBAT oligomers）and additives（e.g.,phthalates）.The results of metagenomic analysis showed that plastic oligomers and additives degraded microorganisms and functional genes existed in the degradable system,which confirmed that biodegradable plastics in aquatic environment underwent ester bond breakage,polymerization degree reduction,product and additive dissolution under biological action,and slurry water flushing had a promoting effect on the biodegradation process.3)In order to enhance the degradation efficiency of biodegradable plastics in aqueous environment,the efficient degradation bacteria of PLA and PBAT-H were screened and enriched,and their enhanced degradation effects were verified in the simulation system.The results showed that the enrichment of functional bacteria could significantly enhance the degradation of PBAT-H,holes were formed on the plastic surfaces and microplastics were produced.After 28 days of degradation,the hydrophilicity of PLA and PBAT-H increased under the action of enriched bacteria,the water contact angle decreased to 86.367°and 70.67°,the maximum roughness was 2.17and 3.93 times the initial,and the initial decomposition temperature shifted to low temperature by 14.9°C and 37.2°C,respectively.Community structure analysis showed that the species that could enhance PLA degradation were mainly Commonas and Brevundimonas,while Bosea and Caulobacter may be potential PBAT-H degrading bacteria,and the enhanced degradation of enriched bacteria depended on the synergy of multiple microorganisms.The presence of genes related to the metabolism of esterase and plastic monomer indicated that the degradation process of PLA and PBAT-H by microorganisms included reactions such as ester bond destruction and monomer degradation.In summary,degradable plastics（PBAT-H and PLA）in the aqueous environment can be naturally degraded,their surface properties changed and formed biofilms.The mechanism analysis showed that slurry water flushing and biodegradation both made important contributions to degradation,and the combined effect of the two significantly accelerated the degradation process,such as ester bond breakage,polymerization degree reduction,product and additive dissolution,etc.Degradable functional bacteria can be screened in the biofilm on the surface of plastics,and the addition of the latter can significantly enhance the degradation efficiency of plastics.This study reveals the environmental behavior and mechanism of biodegradable plastics under natural conditions,providing a theoretical basis for a comprehensive understanding of the attribution and risk assessment of degradable plastics and pollution control.