Long-Term Natural Aging Behavior of Polyvinyl Chloride and Polyamide Microplastics in Soil

ObjectiveMicroplastics are ubiquitous in environments, posing potential threats to ecosystems. As a persistent contaminant, they can undergo aging during prolonged environmental exposure. Aging alters the physicochemical properties of microplastics, thereby modulating their environmental fate and ecological risks. Considering that various environmental factors, including radiation intensity, medium type, humidity, and temperature, play a critical role in the aging process of microplastics, it is evident that the natural aging behavior of microplastics differs significantly from that observed under controlled laboratory aging simulations. However, research on long-term natural aging of microplastics remains limited.MethodIn this study, two typical microplastics, polyvinyl chloride (PVC) and polyamide (PA), were selected. A multi-environment exposure system encompassing atmospheric, topsoil (0 cm), and subsurface soil (10 cm) environments was established to conduct a 12-month natural aging experiment. The physicochemical properties of microplastics before and after aging were characterized using microscopic, spectros-copic, and thermogravimetric techniques, combined with chemical and thermal degradation approaches to evaluat- the stability of aged microplastics.ResultThe results showed that the natural aging behaviors of PVC and PA microplastics differed significantly and were closely dependent on environmental conditions. Specifically, sunlight-induced photo-degradation was the primary natural aging pathway for PVC microplastics. This process was characterized by significant changes in their physicochemical properties, including the formation of oxygen-containing functional groups, the generation of conjugated bonds, and the cleavage of the polymer backbone. These reactions resulted in notable alterations in surface morphology, color, and particle size. Concurrently, atmospheric-exposed PVC microplastics exhibited a significant decrease in stability, as demonstrated by a lower pyrolysis temperature and a higher oxidative degradation rate. In contrast, soil-exposed PVC microplastics (both topsoil and subsurface) exhibited comparable aging levels. While PVC microplastics on the soil surface were also exposed to sunlight, surface attachment to soil particles and the shading effect of plants significantly reduced the extent of their photoaging. Consequently, due to the absence of sunlight-induced photo-aging, the aging level of soil-exposed PVC microplastics was lower than that of atmospheric-exposed PVC microplastics. This suggests that PVC microplastics demonstrate greater persistence in soil environments over extended periods. Atmospheric-exposed PA microplastics underwent marked photodegradation, exhibiting a significant decrease in particle size (P ConclusionThis multi-environment aging study elucidates the divergent aging behaviors of PVC and PA in natural settings. These findings provide theoretical foundations for refining risk assessment frameworks and optimizing laboratory aging simulations for both microplastic types.