Little effect of further aging on soil-weathered plastics

Abstract Plastic waste accumulates in soils, impacting structure, biota, and ecosystem functioning. Understanding the persistence and aging of plastics in soil is therefore important. However, aging occurs over timescales which cannot be covered in conventional experiments. Many studies therefore focus on simulated weathering of pristine polymers over short timescales, but these experiments do not consider potentially different aging behaviour after extended environmental exposure. To address this, we investigated the continued aging of naturally weathered macroplastics sampled from agricultural soils by reburying them under field conditions for 19 months and comparing properties before and after. Samples of polyethylene, polypropylene, polystyrene, and polyethylene terepthalate were analysed for chemical structure, surface oxidation (carbonyl index), crystallinity, molecular weight, thermal stability, and surface morphology. All samples showed signs of oxidative aging and aging-related surface alterations before and after reburial. After reburial, most samples had little to no change in carbonyl index, with several showing significant decreases, likely due to loss of highly oxidised surface layers through fragmentation. Crystallinity, thermal stability, and molecular weight remained largely unchanged. Overall, our results suggest that the further aging of weathered polymers does not follow the same trajectory as the initial aging and that conventional plastics degrade very slowly in soil.