The Overlooked Driver of Microplastic Chemical Oxidation in Cold Soils: Reactive Oxygen Species Generation Mediated by Freeze–Thaw Cycles

Freeze-thaw cycles (FTCs) are a pervasive geochemical force in cold regions, yet their mechanistic role in driving the oxidation of microplastics (MPs) in soil remains unclear. Here, we integrated a 122-day field study, laboratory experiments, and theoretical calculations to elucidate FTC-induced chemical oxidation of MPs in soil. Field observations revealed that only MP-containing conjugated aromatic structures, such as polylactic acid-polybutylene adipate-<i>co</i>-terephthalate (PPAT), polyethylene terephthalate (PET), and polystyrene (PS), underwent oxidative aging during freezing. Using PS MPs as a model, laboratory analyses demonstrated that this selective oxidation was driven by the generation of ¹O₂ and H₂O₂ during the initial freezing phase, which progressively altered soil properties over repeated FTCs. In contrast, no ¹O₂ was detected in soil systems containing MPs lacking aromatic structures (e.g., polyethylene or polyamide). This structural dependence is consistent with density functional theory calculations, which showed that PS possesses a lower excitation threshold and more efficient intersystem crossing than nonaromatic MPs. Notably, the complex reactive oxygen species transformation network within soil-PS systems under FTCs was systematically characterized here for the first time. These findings offer critical insights into freeze-thaw chemistry and open new avenues for decoding MP behavior and its ecological impacts in cold soil ecosystems.