Experimental study on the motion characteristics and critical hydraulic parameters of microplastics in a freshwater environment

The migration behavior of microplastics in water is affected by many factors; in particular, the migration mechanism of microplastics in the terrestrial freshwater environment is more complicated than that in the marine environment. In order to understand the migration behavior of microplastics in the freshwater environment, the hydraulic parameter thresholds of different types of microplastics in water were identified based on hydraulic experiments and force analysis methods. The results show that the motion state of microplastics is affected by their own internal factors and external environmental factors, and the flow rate is the key external factor affecting the change of their motion state. In the vertical direction, the higher the density, the rougher the environment, and the closer the shape to the flake, the greater the critical starting flow velocity and the critical resuspension flow velocity. The settling velocities, critical initiation velocities, and critical resuspension velocities of microplastics range from 0.05 to 0.17 m s-1, 0.03 to 0.44 m s-1, and 0.251 to 0.83 m s-1, respectively. Horizontally, the bottom rolling velocities of microplastics vary significantly. These velocities are positively correlated with water flow velocity but are inversely proportional to the density of the microplastics and the roughness of the substrate. By combining experimental data, mathematical expressions for the critical hydraulic parameters of microplastics were derived, showing improved accuracy compared to traditional methods. This paper explores the trajectory of different types of microplastics after entering the water body and analyzes their migration mechanism in the river. The research results have certain theoretical guiding significance for revealing the migration law of microplastics in the freshwater environment.