Microplastics in aquatic environments: interaction with sediments and transport dynamics

Microplastics in aquatic environments represent a growing global concern due to their persistence and ecological consequences, as these particles accumulate in ecosystems, altering and threatening the integrity and functioning of aquatic communities. The fate and behaviour of microplastics in these environments are controlled not only by particle characteristics (e.g., shape, density) and hydrodynamic conditions, but also by interactions with fine sediments such as clays, which influence aggregation, settling, and transport, ultimately determining their spatial distribution and concentration. The understanding of the processes that control microplastic transport and deposition in aquatic environments still suffers from several knowledge gaps, which are further complicated by the absence of standardised sampling and analytical methods, which hampers cross-study comparisons and the establishment of long-term monitoring strategies. To address these gaps, this research employs a combined approach integrating: i) methodological optimisation, ii) controlled laboratory experiments, and iii) field observations. A density-separation protocol was refined, achieving high recovery of microplastics across various sediment types, including mud-rich deposits. Laboratory experiments have demonstrated that microplastic fibres actively influence sediment aggregation, promoting floc formation and accelerating the deposition of fine-grained sediment. Experiments further revealed that suspended sediment concentrations strongly regulate microplastic transport: under clay-rich conditions, fibres settle efficiently and accumulate on the bed, whereas in the absence of suspended sediments, they largely remain suspended, facilitating downstream transport. Field investigations along the Arno River (Italy) during a flood confirmed that microplastics are widespread in both the sediments and the water column. The study also revealed highly variable deposition patterns, which are difficult to predict using most existing models. Overall, these findings advance our understanding of microplastic behaviour in aquatic systems, emphasising the critical roles of sediment interactions and hydrodynamic forces in determining their fate. Moreover, this study underscores the importance of standardised methodologies to ensure consistent quantification and enable meaningful comparisons of microplastic concentrations across diverse studies and environmental contexts.