Pore-structure responses of polyethylene microplastic-amended Malan loess under different thermal regimes

Under the context of global climate change, increasingly frequent heatwaves and wildfires subject near-surface soils to substantial thermal disturbance. Microplastics, as emerging persistent pollutants, are ubiquitous in agricultural and urban soils as well as aeolian deposits; their melting, pyrolysis, and carbonization at elevated temperatures can markedly alter soil pore structure and connectivity. In this study, Quaternary Malan loess from Shaanxi Province, China, was used as the test material. A series of experiments was conducted by varying polyethylene microplastic contents (1%, 3%, 4%, and 5%) and heating temperatures (100, 200, 300, 400, 500, and 600 °C) to systematically investigate the evolution of loess pore structure under coupled high-temperature-microplastic effects. The results show that at the low-temperature stage (<200 °C), microplastics largely remain in a particulate state, and changes in porosity and specific surface area are limited. At the intermediate stage (200-300 °C), melted microplastics infill pores, causing the total porosity to decrease to 0.025, 0.020, 0.013, and 0.012 mm (for 1%, 3%, 4%, and 5% microplastic contents, respectively), representing decreases of 26.47%-63.64% relative to 100 °C; meanwhile, the specific surface area declines to 11.65, 6.17, 5.30, and 4.75 m²/g. At the high-temperature stage (300-500 °C), microplastics decompose and burn out, leading to a recovery in porosity; at 600 °C, porosity reaches the maximum values of 0.031, 0.030, 0.037, and 0.043 mm, increasing by 24%-258.33% relative to 300 °C, with a concurrent rebound in specific surface area. Pore-size distribution analysis indicates that mesopores (2-50 nm) dominate, whereas micropores and macropores account for smaller fractions. Samples with higher microplastic contents exhibit the lowest pore abundance during the intermediate-temperature stage, while the macropore proportion increases to 51.7%-55.2% at high temperatures. These findings suggest that the combined effects of microplastic melting, oxidation, and burnout, together with mineral thermal expansion, phase transitions, and thermally induced crack development, jointly govern the evolution of loess pore structure. This study provides an important reference for understanding how microplastic-loess interactions under wildfire and high-temperature conditions influence pore-network characteristics.