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The influence of sediment content and salinity in estuarine areas on the settling velocity of microplastics
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Laboratory settling experiments showed that both the concentration of suspended sediment and the salinity of water significantly affect how quickly microplastics sink in estuarine environments — factors that vary considerably as rivers meet the sea. The findings help explain the complex fate of microplastics in transitional coastal zones and improve models for predicting where plastics accumulate in ocean systems. This is important for identifying marine microplastic hotspots and understanding which species and habitats face the greatest exposure.
Since their emergence in the 20th century, plastics have been extensively utilized worldwide. Owing to their pronounced hydrophobicity, chemical inertness, and resistance to microbial degradation, plastics persist in the environment and are difficult to eliminate. Rivers serve as an important pathway for microplastics (MPs) to flow into the ocean. The mechanisms that affect the migration, transportation and fate of MPs in rivers are not yet clear. This poses obstacles to understanding the dynamics of MPs in the ocean, determining their main aggregation areas, and conducting sampling in the ocean. Through a comprehensive series of settling experiments, this study demonstrates that the settling dynamics of MPs are significantly modulated by suspended solid concentration (SSC) and salinity gradients-factors that are particularly characteristic of estuarine transition zones at the river-ocean interface. The results indicate that SSC significantly enhance the settling velocity of MPs, with a maximum increase of 15.6%. However, this promoting effect diminishes as sediment concentration increases, and an inhibitory effect emerges under high concentrations. Conversely, elevated salinity markedly suppresses settling velocity, with a maximum reduction of 33.96%, though the magnitude of this influence gradually decreases with further increases in salinity. Based on the experimental results, a modified settling velocity model that integrates the effects of SSC and salinity was developed. Validation indicates that the model reliably predicts MPs settling velocity under a wide range of environmental conditions. This study provides a theoretical framework for advancing the understanding of MPs transport processes in estuarine and coastal environments and offers valuable insights for marine environmental management and remediation.
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