Conventional and biodegradable microplastics exhibit divergent photodegradation pathways modulated by iron oxide phases.

The photodegradation of microplastics is differentially regulated by specific iron oxide (FeOx) phases, but underlying mechanisms remain unclear. Here we examined how ferrihydrite (Fh) and hematite (Hem) mediated the phototransformation of conventional polystyrene (PS) versus biodegradable poly(butyleneadipate-co-terephthalate) (PBAT). The results indicated that degradation pathway was fundamentally determined by polymer chemical structure and finely regulated by oxide phase. PS underwent surface passivation during the photodegradation suppressing mass loss, showing 7.05% and 4.81% lower mass loss after 80 days under Fh and Hem mediation relative to the pure water control group. Conversely, PBAT experienced extensive backbone scission, with mass loss reaching 63.25% under Hem. Such dramatic divergence originates from their intrinsic chemical structures: PBAT is biodegradable polyester with vulnerable ester bonds, whereas PS is persistent aromatic polymer with stable benzene rings. Fh functioned as an oxidative buffer, releasing more less-aromatic soluble organic matter. By comparison, highly crystalline Hem acted as a catalytic driver, triggering rapid deep oxidation that elevated PS surface O/C ratio by 650% within 10 days, while simultaneously driving a dramatic 244% negative shift in the conduction band potential of PBAT. This phase-specific regulation extended to redox cycling, with Fh system continuously generating more •OH. These findings demonstrate that FeOx phases critically govern MPs fate by phase-specifically steering their degradation mechanisms: inducing surface passivation for persistent PS versus backbone fragmentation for reactive PBAT. Provides a molecular framework for predicting plastic pollution dynamics in complex natural environments.