Kinetic and mechanistic insights into the photo-Fenton oxidation of polystyrene nanoplastics in water

Microplastics and nanoplastics (NPs) are widespread in aquatic environments and readily accumulate along the food chain. Given their varied sizes in real systems, evaluating degradation processes at various scales is essential for a comprehensive understanding of their fate. In this study, the photo-Fenton degradation of polystyrene (PS) nanospheres with initial particle sizes of D 0 = 140, 252, 460, 909, and 1100 nm was investigated. Oxidation evolution and treatment efficiency were assessed using turbidity and Total Organic Carbon (TOC) measurements, while Transmission Electron Microscopy (TEM) provides insights into particle size and morphological changes. Pyrolysis–Gas Chromatography/Mass Spectrometry (Py-GC/MS) and Ion Chromatography (IC) were used to identify intermediate degradation products. The results demonstrated that smaller particles degraded more rapidly due to their higher surface-to-volume ratio, with complete TOC removal achieved for all particle sizes in relatively short reaction times (40–80 min). The degradation kinetics were accurately described using the Shrinking Core Model and the Prout-Tompkins Model, which revealed distinct stages of reactivity and sigmoid behavior. During the initial activation phase, oxygenated surface groups were incorporated into the PS NPs, followed by chain scission and oxidation into low-molecular-weight aromatic and aliphatic compounds. Finally, these intermediates were fully mineralized into CO 2 and H 2 O, leaving no detectable leached by-products or residual NPs. • PS NPs of all sizes were fully mineralized by the photo-Fenton process at 25 °C. • Higher surface-to-volume ratios significantly accelerate the degradation process. • Oxidation yields aromatic and aliphatic by-products and CO 2 and H 2 O as end products. • The SCM and Prout-Tompkins models accurately described the NPs degradation process.