Multi-Type Microplastic Migration Model Driven by River Hydrodynamic Conditions

To address the difficulty in predicting the migration trajectories of microplastics in aquatic environments, this study develops a hydrodynamically driven migration model applicable to multiple types of microplastics. Based on hydraulic experiments, hydrodynamic thresholds are established to characterize transitions among drifting, suspension, and sedimentation. The model integrates hydrodynamic forces, gravity, buoyancy, and interparticle interactions, enabling accurate simulation of migration pathways and ultimate destinations. Compared with conventional models, the key innovation lies in incorporating differences in size, shape, and material, allowing differentiated representation and prediction of diverse microplastics. The pollutant accumulation patterns obtained by simulating microplastic migration in the Mulanxi River basin using this model are consistent with actual observational results, further demonstrating the model's reliability and applicability. Results from the Xianyou Section show that microplastics smaller than 0.5 mm account for 71.62%, dominated by fragmentary and fibrous types. There are significant differences in the migration behaviour of microplastics made from different materials; these differences are primarily attributable to variations in their density and physicochemical properties. Furthermore, transport rates at the downstream end are positively correlated with proximity to pollution sources and the abundance of lightweight microplastics. The total flux reaches 9.37 × 10¹¹ particles, with an overall transport rate of 68.34%. This study enhances the mechanistic understanding and predictive capability of microplastic transport in freshwater systems, providing new theoretical and methodological support for pollution control.