Linking Natural Aging to Upcycling Potential: Fate and Thermochemical Valorization of Polyethylene Mulching Films from Farmlands

Mulching films are widely used to improve agricultural productivity; however, their residues can fragment during weathering and generate microplastics, posing potential environmental risks in farmland soils. Here, we collected polyethylene mulching films with well-documented natural exposure durations, i.e., fresh (Y0), and after 1 (Y1), 2 (Y2), and 3 (Y3) years in farmland, enabling a time-resolved assessment of in situ aging, and evaluated their thermochemical upcycling potential using Thermogravimetric-Fourier transform infrared spectroscopy (TG-FTIR) and Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Results showed that, during natural aging, the film surface became progressively rougher, developing numerous cracks and pores. Substantial amounts of oxygen were introduced into the film structure as O-containing functional groups (e.g., C═O, C–O–C, – OH), thereby increasing the carbonyl and hydroxyl indices. In addition, the contents of minerals such as Al, Si, Ca, Mg, and Fe within the films increased rapidly, leading to higher ash content and more residual solid after pyrolysis. Conversely, both the crystallinity and surface charge of the films generally decreased with prolonged exposure. Pyrolysis analysis revealed that, with aging, the proportion and amount of CO2 in the pyrolysis volatiles increased, while the contents of alkanes, alkenes, and cycloalkanes in the liquid products decreased; meanwhile, the yield of valuable oxygenates and other chemicals increased. This study reveals the natural aging process and potential resource utilization pathways of mulching film in farmland environments, providing a theoretical basis for addressing mulching film-related environmental issues.