Study On Microbial Aging Behavior And Environmental Effects Of Different Types Of Microplastics

In recent years,microplastic pollution has become a focus and research hotspot in the field of global ecological environment safety.It is reported that the surface properties and structure of microplastics will change significantly after migration and aging in the environment.However,sludge in sewage treatment plants is a potential storage of microplastics,and there are few studies on the microbial aging behavior of microplastics in sludge.In addition,as microplastics may adsorb pollutants as carriers,and China is a major producer and user of tetracycline,the change mechanism of tetracycline adsorption behavior of microplastics during anaerobic microbial action and the possible difference between the combined toxicity of aging microplastics and tetracycline and the original microplastics are still unclear.Therefore,the results of this study can provide a basis for understanding the microbial-driven decomposition process of microplastics,and provide data support for further analysis and mastering the environmental behavior and toxic effects of the change of surface properties of microplastics on co-existing pollutants.In this study,four microplastics,polypropylene（PP-MPs）,polyethylene（PE-MPs）,polyvinyl chloride（PVC-MPs）and styrene butadiene rubber tire particles（TSBR-MPs）,were taken as the research objects to study the change characteristics of their surface properties in the microbial process of anaerobic sludge.In addition,tetracycline was selected as a typical antibiotic,and Pseudokirchneriella subcapitata was selected as the test organism.Through adsorption test and toxicity test,the influence of microbial action process on the adsorption behavior of Microplastics to tetracycline and its compound toxicity was discussed.The main conclusions are as follows:（1）The degradation degree of the four kinds of microplastics was different under the process of anaerobic microorganisms.The absorption peak intensity of-C=O of PPMPs and PE-MPs showed an increasing trend.The surface C-H absorption peak intensity of PP-MPs increased,while the C=C absorption peak intensity of PE-MPs decreased.The surface functional groups of PVC-MPs did not change significantly after microbial process.TSBR-MPs with the same particle size showed different changing behaviors under the process of anaerobic sludge microorganisms.Among them,O-H absorption peak appeared in the TSBR-MPs of smaller particle,and its C=C absorption peak intensity showed a weakening trend.The results of contact Angle measurement show that the hydrophilicity of microplastics is generally enhanced after microbial process.（2）The adsorption of tetracycline on microplastics is a process of fast adsorption first,then slow adsorption to finally gradually reach adsorption equilibrium.In addition,this process can be well described by pseudo-second-order adsorption kinetics（the correlation coefficient is between 0.945 and 1）.Therefore,the influence of chemical processes such as surface charge,functional sites on the surface,liquid film diffusion,pore diffusion,electron transfer and electron sharing are dominant in the adsorption process.（3）The isothermal adsorption process of tetracycline on microplastics is affected by anaerobic microorganism.The isothermal adsorption process of original PP-MPs,PE-MPs,PVC-MPs and TSBR-MPs can be better described by Langmuir model（correlation coefficient between 0.889 and 0.995）.After anaerobic microbial process,Freundlich model（correlation coefficient between 0.869 and 0.994）is still generally lower than Langmuir model（0.949～0.997）.However,the correlation coefficient of Freundlich model of microplastics after microbial treatment（0.869～0.994）was higher than that of original microplastics（0.810～0.990）.This indicates that the isothermal adsorption process of tetracycline on microplastics after anaerobic microbial process is changing from single molecular layer to multi-molecular layer.（4）Single microplastics inhibited the growth of Pseudokirchneriella subcapitata.With the increase of microplastics concentration,the inhibitory effect of microplastics on Pseudokirchneriella subcapitata increased.At the same concentration,the inhibition effect of anaerobic microplastics on Pseudokirchneriella subcapitata was higher than that of the original microplastics.For example,when the concentration of microplastics was 2 g/L,the inhibition rate of anaerobic microplastics to s.chinensis ranged from 8.29% to 42.0%,which was higher than that of original microplastics（5.62%～40.23%）.（5）Under the compound system,the toxicity of tetracycline and microplastics to Pseudokirchneriella subcapitata was significantly enhanced,and the toxicity of anaerobic microplastics was stronger than the original microplastics.The toxicity factors of the composite system to Pseudokirchneriella subcapitata included concentration of pollutants,adsorption capacity of microplastics and toxicity of single pollutants to Pseudokirchneriella subcapitata.