Mineral-Armored Structure Enhanced the Stability of Polyethylene Microplastics Rather Than Polylactic Acid Microplastics: A Long-Term Natural Aging Study

The aging of microplastics (MPs) depends on their surrounding environment and has significant implications for their environmental behavior and ecological risks. However, there are limited data on the long-term aging of MPs in different natural environments. The natural aging characteristics of polyethylene MPs (PE-MPs) and polylactic acid MPs (PLA-MPs) exposed to air, soil surface, and subsurface conditions for 6 and 12 months, respectively, were evaluated. The results showed that PE-MPs and PLA-MPs exhibited distinct aging characteristics under identical conditions. Photolysis represents the primary aging mechanism for PE-MPs, and prolonged solar radiation significantly reduces their stability ( p < 0.05). Notably, soil minerals (e.g., Illite and quartz) formed armor-like coatings encapsulating PE-MP surfaces through chemical interfacial interactions (C–Si–O, Si–O–C, and Al–O–C)─a novel mechanism enhancing PE stability in soils. However, the stable interfacial adhesion between soil minerals and PLA-MPs is minimal. Microbial degradation as the primary aging mechanism renders PLA-MPs in soil more susceptible to aging compared to those in air, consequently exhibiting lower stability. This study highlights the mineral-mediated aging of MPs in soil and demonstrates how mineral coatings enhance the stability of PE-MPs. This underscores the necessity of incorporating mineral-mediated aging processes into MP risk assessments for soil ecosystems.