The Hidden Crisis of Biodegradable Plastics: Polylactic Acid Microplastics Increase Soil Cd and Pb Bioavailability and Associated Human Health Risks

The increasing application of biodegradable plastics like polylactic acid (PLA) has raised concerns about their potential environmental impacts, especially in agricultural soils co-contaminated with heavy metals. However, the mechanisms by which PLA microplastics (MPs) regulate the environmental behavior of heavy metals remain insufficiently understood. A pot experiment was conducted to investigate the influence of PLA MPs on cadmium (Cd) and lead (Pb) dynamics, lettuce uptake, and associated human health risks in a soil-lettuce system. The results demonstrated that PLA MPs altered soil properties, dissolved organic matter composition, and microbial community structure, thereby enhancing Cd (13.0-73.8%) and Pb (8.7-60.9%) bioavailability. Consequently, PLA MPs promoted Cd accumulation and translocation in lettuce, which exhibited photosynthesis inhibition and oxidative damage. Medium and high levels of PLA MPs significantly increased non-carcinogenic (0.4-1.9-fold) and carcinogenic (0.8-2.6-fold) risks associated with heavy metals, with shoot Cd accumulation as the primary contributor. These findings highlight the ecological and health risks of biodegradable MPs, calling for a critical reassessment of their presumed environmental sustainability.