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Early thermal pulses drive interfacial pre-aging and plastisphere pre-shaping: An overlooked pre-conditioning pathway in PLA microplastic aging during composting
Summary
"Compostable" plastics like PLA don't always fully break down—they can leave behind tiny microplastic fragments that end up in soil used for farming. This study found that a short burst of extra-high heat early in the composting process makes these plastic bits break down faster and more completely by the end, without taking any extra time. That's good news, since it suggests a simple tweak to composting methods could help reduce the amount of lingering microplastic contamination in the food we grow and eat.
Polylactic acid (PLA), promoted as a "compostable" alternative to petroleum-based plastics, often persists as microplastics (MPs) in compost that re-enter the agricultural environment. However, how the earliest thermal signals of composting govern this aging trajectory remains untested. Here we show that the early thermal pulse of hyperthermophilic composting (HTC) leaves a lasting legacy on PLA-MPs aging through a continuous chain: early thermal pulse → interfacial pre-aging → plastisphere ecological pre-shaping → enhanced endpoint aging. Parallel HTC and thermophilic composting (TC) systems were established, and PLA-MPs aging was tracked across composting stages by combining surface and molecular characterization-electron microscopy with elemental mapping, infrared spectroscopy with two-dimensional correlation analysis, gel permeation chromatography, and contact-angle measurement-with high-throughput sequencing of the bacterial and fungal plastisphere. Aging was quantified using two entropy-weighted indices developed here: a Comprehensive Aging Index (CAI) and a Surface Aging Index (SAI). Least-squares regression isolated the independent contribution of early thermal history, while Boruta machine learning and structural equation modeling identified the bacterial-fungal consortia and environmental drivers of endpoint aging. The stronger early thermal pulse in HTC drove surface oxidation, roughening, and wettability change of PLA-MPs, triggering earlier bacterial-fungal community filtering and retaining aging-associated cross-kingdom consortia. HTC ultimately achieved a CAI of 0.95 versus 0.77 under TC (P < 0.05), a 23.38% enhancement without extending process duration. These findings establish the early thermal pulse as a key conditioning phase that couples interfacial and ecological processes, and provide new insights into the aging behavior of PLA-MPs in composting environments.