Modeling study on fate of micro/nano-plastics in micro/nano-hydrodynamic flow of freshwater

As the risk of micro/nano-plastics (MPs/NPs) on marine ecosystems is reported, there is a growing interest in behaviors of MPs/NPs in freshwater. Thus, this study aims at developing a plastics fate model linked with a flocculation kinetics model, PsFM/FKM, to understand the vertical behaviors of MPs/NPs in freshwater. Based on the Population Balance approach, the model numerically predicts vertical transport (sedimentation, resuspension, and burial) and transformation (degradation and aggregation) in water and sediments. This study performed model simulations to validate model accuracy and precision as estimating temporal changes in MPs/NPs concentration in water and sediments, based on the aggregation process to form homo-aggregates between plastics and hetero-aggregates between plastics and SS in water. It confirmed that a significant parameter of the aggregation was collision frequency in water and that attachment efficiency influenced the aggregation rate occurring in the collision. The model emphasized the importance of attachment efficiency with the size-dependence and confirmed that the formed hetero-aggregates promoted the sedimentation process to settle down to the sediments. The model validity was demonstrated by comparing experiments and simulations for attachment efficiency. A further study improves the model and extends its applicability to various types of MPs/NPs, SS, microalgae, and metal hydrate salts.