Phase-dependent mechanisms in ferrihydrite photoreduction mediated by derivatives of polystyrene microplastics

The photoreductive dissolution of iron (hydr)oxides directly regulated the bioavailability of Fe(II), a process which was influenced by dissolved organic matter (DOM). Microplastic-derived DOM (MP-DOM), characterized by oxygenated aromatic structures, also mediated the photoreductive dissolution of iron (hydr)oxides. Oxidized derivatives of microplastics coexist in the environment as both dissolved and particulate phases, including particulate nanoplastics (NPs) and truly dissolved organic matter (DOM). These phases fundamentally differed in size, interfacial interaction mechanisms, and environmental persistence, potentially leading to distinct regulatory behaviors. However, the role of particulate microplastic derivatives remained largely underexplored, thereby constraining our capacity to predict how NPs may influence iron cycling in contaminated aquatic systems. To address this, this study investigated the photoreductive dissolution of ferrihydrite (Fh) mediated by both particulate polystyrene nanoplastics (PSNPs) and dissolved polystyrene-derived DOM (PS-DOM) under UVA irradiation. The results demonstrated that PSNPs significantly enhanced Fe(III) photoreduction on the Fh surface, increasing Fe(II) release 1.8 to 3.9-fold. Mechanistic analyses showed that PSNPs heteroaggregated with the mineral via electrostatic, hydrophobic, and hydrogen bonding interactions, with Fe-O-C bonds formed at the interface. These bonds altered the interfacial redox microenvironment, as indicated by a decreased redox potential of 0.618 V, an increased transient photocurrent of 0.11 nA, and a negative flat-band potential shift to -0.70 V. Collectively, these results suggest that Fe-O-C bonds act as efficient electron transfer channels, promoting interfacial charge transfer and subsequent photoreductive dissolution of the mineral. Concurrently, H₂O₂ generated during PSNPs photo-oxidation functioned as electron donors, efficiently driving Fe(III) photoreduction at the Fh interface. In contrast, PS-DOM promoted photoreduction of Fh primarily through enhanced ligand-to-metal charge transfer (LMCT) processes and photosensitized generation of reactive oxygen species (ROS). This study demonstrated that particulate PSNPs facilitated sustained Fe(II) generation with greater environmental stability, whereas dissolved PS-DOM exerted transient effects, yielding Fe(II) more susceptible to re-oxidation. Distinguishing the differential contributions of these phases is critical for accurately assessing Fe(II) supply and flux in iron cycling under microplastic pollution, providing novel insights into iron biogeochemical cycles in natural environments.