Nanoscale Degradation Characteristics of Biodegradable Bag-derived Microplastics in Soil

There is an increasing application of biodegradable plastics as sustainable alternative due to the serious pollution of traditional non-biodegradable plastics. However, their relatively slow degradation rates under natural environmental conditions still pose potential ecological and environmental risks. Until now, limited studies have been carried out to study the nanoscale degradation mechanism and fragmentation potential of biodegradable plastics in soil environment concerning global warming. In the present study, the biodegradable bag-derived microplastics (polylactic acid, PLA) were selected to analyze the changes of their physicochemical properties on nanoscale in soil environment at different temperatures (room temperature at 25 ℃ and high temperature at 50 ℃) for 0, 7 and 14 days aging based on an atomic force microscopy-infrared spectroscopy system (AFM-IR). The results show that the surface roughness of PLA gradually increased with aging time (7 d, 14 d) at both the two temperatures with highest roughness was observed at 50 ℃, indicating that the surface fragmentation of PLA was faster in soil environment at higher temperature. Nano-infrared spectrum (nanoIR) imaging analysis show that the signal of C—O functional group and its proportion of distribution area were much higher on aged PLA surface (37.9%-50.8%) than unaged ones (21.1%), and the signal of C—O was stronger at 50 ℃ than at 25 ℃ after 14 days aging. Similarly, compared to unaged ones, the C＝O signal and its distribution area on aged PLA surface were also enhanced, indicating an oxidization process during aging. The results of Lorentz contact resonance (LCR) analysis reveal that the frequency of the first vibration peak on PLA surface at 25 ℃ followed the sequence of 14 days >7 days > initial control, suggesting an increased rigidity of PLA surface with aging time. In addition, the rigidity of PLA surface was enhanced after 7 days but got weak after 14 days aging. Nano thermal analysis (nano-TA) show that the glass transition temperature (Tg) of 7 days-aged PLA surface increased compared to unaged ones. However, the Tg decreased and its distribution heterogeneity was enlarged at 50 ℃ after 14 days aging. These results implied an exfoliation corrosion of aged PLA surface. Future research should focus on the surface exfoliation corrosion process and the quantification of new-formed micro-nano plastics. This study provides technical theoretical and methodological supports for studying the fragmentation potential and assessing its environmental risk of biodegradable plastics in soil environment.