可生物降解塑料袋膜在土壤中降解的纳米尺度特征

Nanoscale Degradation Characteristics of Biodegradable Bag-derived Microplastics in Soil

传统难降解塑料存在污染问题,因此可生物降解塑料的应用越来越广泛。但在许多自然环境条件下,可生物降解塑料降解速率依然缓慢,也存在一定生态环境风险。目前,在全球气温上升的背景下,可生物降解塑料在土壤环境中老化的纳米尺度机制和破碎化潜势研究还非常有限。因此,以聚乳酸(PLA)基生物降解塑料购物袋膜来源微塑料为研究对象,采用原子力显微镜-红外光谱联用系统(AFM-nanoIR)研究了不同土壤温度条件(常温25℃和高温50℃)下微塑料表面在纳米尺度上的老化特征。结果表明,两种温度条件下PLA表面粗糙度均随老化时间(7和14 d)增加而增加,且高温条件下粗糙度更高,表明高温土壤环境中PLA表面破碎化更快。纳米红外光谱(nanoIR)成像结果表明,与未老化样品相比,老化PLA表面C—O官能团信号明显增强且空间分布面积比例增加(由原始样品的21.1%提高到老化后的37.9%~50.8%);老化14 d时,高温条件下C—O信号比常温环境更强。相似地,老化PLA表面C=O信号增强,分布面积比例增加,表明PLA在老化过程中形成新的羰基化合键。洛伦兹接触共振技术(LCR)分析结果显示,常温土壤环境PLA表面第1振动峰所在频率变化趋势为14 d>7 d>对照,表明老化PLA表面第1振动频率升高,刚性增加;且高温条件下老化7 d处理PLA表面第1振动频率比常温环境更高,表明出现更硬而脆的表面,而老化14 d时刚性降低。此外,纳米热分析(nano-TA)结果显示未老化PLA样品表面玻璃化转变温度为(72.3±4.9)℃,老化7 d时,PLA表面玻璃化转变温度上升,表明耐高温性能增强。但老化14 d时,高温条件下PLA玻璃化转变温度下降且不同测试点位的玻璃化转变温度的异质性变大,表明PLA老化表面出现剥落现象。未来应进一步研究PLA表面剥落过程以及微纳塑料的释放和量化。研究结果为可生物降解材料在土壤环境中的破碎化潜势研究及环境风险评估提供技术支撑。

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.