Modeling behaviors of permeable non-spherical micro-plastic aggregates by aggregation/sedimentation in turbulent freshwater flow

This study developed and evaluated a behavior model for permeable non-spherical micro-plastic aggregates in a turbulent flow of freshwater based on fractal theory, as conducting experimental and modeling studies. Laboratory-scale experiments evaluated attachment efficiency α to aggregation kinetics in an aquatic environment (pH 6, 20 ℃) of the electrolyte (Al³⁺). The experimental α was dependent on characteristics of plastics (type, size, and density) and ranged from 0.062 to 0.2772 (averaging 0.1) with a high correlation with the modeled α (R² > 0.92). Model validation was conducted under two simulation conditions: one drawn from a previously published study of impermeable spherical aggregates and the other based on fractal theory. The simulations verified the limited primary particle size with the lowest retention rate based on the previous study but it was difficult to determine the specific particle size with the lowest retention rate as a limiting factor. The sum of residual errors for aggregation/sedimentation between the two types of structures showed an overestimation of spherical structures. Such overestimation influenced the aggregate number concentration and distribution pattern. Therefore, the model needs to more detailed express the aggregation mechanism of permeable non-spherical aggregate structures in terms of surface growth.