Settling velocity characteristics and model establishment of typical MPs under freshwater, saline and sediment conditions

The settling velocity of microplastics largely controls their vertical movement in aquatic environments. While horizontal diffusion depends on larger-scale hydrodynamics, vertical settling responds strongly to particle properties and local conditions. In natural waters, salinity changes and the presence of suspended sediments are two key factors that alter settling behavior. Therefore, developing a targeted predictive model to quantify their combined effects is essential for elucidating the mechanisms of microplastic vertical transport. In this study, we selected five typical microplastic polymers to conduct laboratory settling experiments. We systematically investigated the individual and combined influences of salinity gradients and suspended sediments on MP settling characteristics. Based on the results, we modified the standard settling velocity equation by adding a salinity correction coefficient (K) and a sediment interaction coefficient (K). This gave us a coupled salinity-sediment correction model. When tested against experimental data from freshwater, saline, and sediment-laden conditions, the model performed well, with high goodness-of-fit values that confirm its reliability and broad applicability. Our work provides a practical tool for assessing MP transport and managing pollution in complex water environments.