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How Microplastics cross the Buoyancy Barrier: A multi-scale Study
Summary
Researchers investigated how microplastics less dense than water overcome the buoyancy barrier to settle in sediments, using colloidal probe atomic force microscopy, microscale aggregation tests, sedimentation column experiments, and simulations to quantify eco-corona-mediated MP-sediment attraction across scales. They found that eco-corona coatings created attractive forces enabling heteroaggregation with suspended sediment, doubling MP settling frequency in bentonite suspensions and increasing sediment retention by 32%, with environmental shear forces too weak to disrupt the formed aggregates.
Microplastics (MPs), though less dense than water, are frequently recovered from sediments in aqueous environments, indicating they can cross the buoyancy barrier. We quantify eco-corona mediated MP-sediment attraction and MP transport from the nanoscale to the macroscale, linking all scales to a coherent mechanism explaining how MP overcome buoyancy and settle in sediments through interaction with suspended sediment. Colloidal probe atomic force microscopy (CP-AFM) detected attractive forces (0.15 - 17 mN/m) enabling heteroaggregation. Microscale tests confirmed aggregation and on larger scales sediment retention more than doubled with an eco-corona. Simulations showed that environmental shear force ($4 \cdot 10^{-4} mN/m$) cannot disrupt aggregates. In sedimentation columns, biofilm-covered MPs settled twice as often as plain MPs in bentonite suspensions. MP retention increased by 32 %. These results demonstrate that eco-corona/biofilm-mediated heteroaggregation is a robust pathway for MP sinking, accumulation, and retention in sediment beds. By identifying physical interaction thresholds and aggregation stability, we provide mechanistic insight into MP fate, highlight probable accumulation hotspots, and offer an evidence base for improved risk assessment and environmental modelling.