Physical degradation and element adhesion: The critical influence of sediment grain size on plastics

Plastic pollution in aquatic environments is an escalating global concern, with sediment interactions playing a critical yet underexplored role in the physical degradation of plastics. This study investigates how sediment grain size influences plastic fragmentation, surface roughness, and elemental surface composition under simulated bedload transport conditions. A rotating drum abrasion system was used to simulate high-energy environments typical of river flood events. Three plastic types, polycarbonate (PC) fragments, polyvinyl chloride (PVC) beads, and nylon (NY) fibres, were abraded with four sediment types ranging from fine sand to large gravel. Results show that larger grain sizes significantly accelerate plastic fragmentation and markedly rose surface roughness. Additionally, elemental analysis revealed increased silicon and trace element adhesion with increasing roughness and sediment coarseness, indicating a positive correlation between physical abrasion and surface chemical enrichment. These findings highlight the critical influence of sediment type on microplastic ageing, with significant implications for predicting their environmental fate, transport dynamics, and their enhanced potential to adsorb and transport other pollutants.