Impacts Of Different Types Of MPs On The Feeding Behavior And Excretion Characteristics Of Earthworms

Due to extensive use and the lack of reasonable regulatory disposal measures,micron-sized plastic waste-microplastics(<5 mm)were widely detected in the ocean,fresh water,sediment and soil.As a new type of environmental pollutants,the environmental fate and ecological risks of microplastics have become increasingly hot issue in the environmental field.At present,the research on microplastics mainly focuses on the water environment,and little is known about the terrestrial system closely related to human beings.The research about the feeding behavior and excretion characteristics of microplastics in the terrestrial environment by soil organisms are still rare,and their exposure risks under the feeding route need to be further studied.There are many kinds of microplastics,and most of them are nondegradable types such as PP,PE,PET,etc.,which become one of the sources of "white pollution" in the environment.Degradable plastics,as the substitute of traditional nondegradable plastics,have been widely applied in recent years,however,few researches have ever referred to degradable plastics as a potential source of microplastics.In this study,the feeding preference of two kinds of earthworms(Eisenia fetida and Pheretima guillelmi)on three different types of microplastics(including nondegradable microplasticspolycarbonate(PC)and polyethylene terephthalate(PET),partially-degradable microplastics-hydroxypropyl methyl cellulose(HPMC)and carboxymethyl cellulose(CMC)and fully-degradable microplastics-polylactic acid(PLA)and polyhydroxy fatty acid(PHA))were investigated and the factors affecting the ingestion of earthworms were discussed.Based on the results,the accurate mass spectrometry quantitative methods of microplastics preferentially ingested by earthworm were developed and improved,and applied to the study of earthworm excretion kinetics to assess the release risk of different types of microplastics in the soil environment.The main conclusions of this paper are as follows:The results of investigation on the feeding preference for earthworm showed that:(1)In the satiety state,Eisenia fetida preferred to ingest fully-degradable microplastics(PLA and PHA),and in the starvation state,Eisenia fetida showed suspicious feeding preferences for PET and CMC but no significant difference among three different types of microplastics;(2)Pheretima guillelmi showed a strong preference for nondegradable microplastic PET and fully-degradable microplastic PLA under the satiety and starvation state;(3)In the satiety state,Eisenia fetida tended to intake microplastics with small particle size and granular form(relative to fiber form).Furthermore,the mass spectrometry quantitative detection methods of PET and PLA,which were preferentially ingested by Pheretima guillelmi,were optimized and developed.The quantitative analysis method for PET was based on the method of alkali-assisted thermal depolymerization-liquid chromatography-tandem mass spectrometry established by our research group.Meanwhile,a rapid and reliable method based on the alkali-assisted alcohol phase thermal depolymerization(depolymerization at 135 ? for 20 min)-Oasis MAX ion exchange extraction-HPLCMS/MS detection,was applied for the determination of PLA microplastics in environmental samples with high sensitivity and accuracy.Through internal standard calibration,the recovery of PLA in environmental samples ranged from 65.3-84.7%,with the limit of quantification as low as 148 ?g/kg.The excretion characteristics of Pheretima guillelmi on the different types of microplastics(PET and PLA)were assessed for the first time,and the effects of intestinal digestion based on the microplastic content in fecal samples were compared.The results showed that PET and PLA(1% w/w)with an original particle size of 50-125 ?m could be degraded into the micron(0.8-125 ?m),submicron(0.1-0.45 ?m and 0.45-0.8 ?m)and nanometer(20-100 nm)levels through the intestinal digestion of earthworm.The degradation degree of PET and PLA microplastics by Pheretima guillelmi was different.Specifically,during the 10 days of clearance period,the content of micron-level PET in earthworm casts was higher than PLA,while the content of submicron and nanoscale PET was lower than PLA,indicating a higher intestinal digestion capacity of fully degradable microplastic PLA than nondegradable microplastic PET.The detection of nanoplastics in the casts of Pheretima guillelmi will lead to the further attention about the nano-toxicological effects of soil organisms on microplastics.