The Soil–Air Interface of Iron-Bearing Clay Minerals as an Overlooked Hotspot for Microplastics Photoaging

Topsoil acts as a crucial long-term sink for microplastics via anthropogenic inputs and atmospheric deposition, providing venues for their light-driven photoaging. Here, we revealed that light-driven Fenton-like reactions of three representative iron (Fe)-bearing clay minerals (biotite, lizardite, and montmorillonite) at the soil-air interface accelerated photoaging and enhanced the toxicity of deposited polystyrene and polyethylene microplastics with diameters of ∼100 μm under simulated and natural sunlight. The severe photoaging was manifested by pronounced morphological damage with a higher structural Fe content and 2 times more carbonyl groups than those in reported aqueous systems. Although these structural alterations accounted for only ∼0.1% of the total carbon, they were predominantly localized on microplastic surfaces and significantly enhanced hydrophilicity and microbial affinity. The aggravated photoaging originated from boosted long-lived surface-bound hydroxyl radicals via light-driven Fenton-like reactions. The direct light exposure of clay surfaces and nanoconfinement within clay interlayers facilitated in situ oxidation of microplastics. Critically, photoaged microplastics exhibited severe toxicity toward luminescent bacteria, with acute toxic units increasing by up to 0.7. Similar microplastic photoaging and toxicity were observed after 7 days of natural sunlight exposure and wet-dry cycles. Therefore, Fe-bearing clay minerals at soil-air interfaces act as crucial yet overlooked hotspots, exacerbating microplastic photoaging and threatening soil health.