Time-resolved fragmentation pathways of expanded polystyrene microplastics: Intrinsic pathway modulated by sand morphology and degradation state

Microplastic fragmentation, driven by ultraviolet exposure, mechanical forces, and sand properties, remains poorly understood in natural settings despite its ecological significance. This study investigates temporal variation (6-240 h) in the shape, size, and number of EPS fragments (size distribution) and their dependence on sand morphology and parent microplastic degradation state based on pot mill experiments. Two experimental setups were employed: Time-Resolved Fragmentation (TRF) experiments using virgin EPS (∼5000 μm) with beach sand (TRF-VB), and virgin or degraded EPS with river sand (TRF-VR/DR). In the TRF-VB, two dominant size classes were identified: size class 1 (5-100 μm), appearing early (6-12 h), and size class 2 (200-1000 μm), emerging at 48-72 h and plateauing at 120 h due to hardened surface layer exfoliation of the parent EPS. The steep slopes of the size distributions (<-3) are explained by a combination of continuous-cascading and leap-cascading fragmentation mechanisms. In the TRF-VR experiment, only size class 1 persisted, whereas in the TRF-DR experiment, degraded EPS produced both classes by 120 h. The fragmentation pathway was influenced by both sand morphology and the parent degradation state. Volume balance analysis revealed the dominance of fine fragments (<5 μm) in both experiments, indicating their environmental relevance. These findings provide a conceptual framework for modeling EPS fragmentation and highlight the ecological risks associated with the rapid generation of fine microplastics. In the future, the continued integration of experimental, numerical, and theoretical approaches will be essential for advancing our understanding of plastic fragmentation processes.