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 <i>in situ</i> 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.