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-co-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.