Bedload transport rates of microplastics on natural sediments under open channel flow: The role of exposure in acceleration

Microplastic (MP) pollution in aquatic environments has emerged as a significant environmental concern. This study pioneers the development of a semi-theoretical bedload transport rate model specifically for exposed MPs by employing a combination of computational fluid dynamics coupled with the discrete element method (CFD-DEM) and experimental methods. Our results show that, while the dimensionless bedload transport rates for exposed MPs and natural sediments are close, indicating that density and particle diameter remain key influencing factors for their bedload transport, detailed analysis reveals that exposed MPs follow distinct transport rates compared to natural sediments, and the proposed bedload transport model of MPs quantifies the different transport rates of exposed MPs compared to surrounding sediments, providing a metric to quantify the acceleration from exposure. In particular, our findings indicate that exposed MPs exhibit higher transport rates, potentially causing wider spatial distributions and posing greater ecological risks than buried MPs. To facilitate the application of our findings in engineering contexts, we adopt a simplified modeling approach (eq 15 in the main text) that employs the mean critical Shields parameter of MPs corresponding to the average MP diameter. The accuracy of this simplified model is validated through a series of laboratory experiments, ensuring its practical relevance. Overall, this research provides pioneering insights into the transport dynamics of MPs in natural aquatic systems, offering a foundation for improved predictive models and enhanced strategies for forecasting the ecological impacts of MP pollution.