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How Microplastics cross the Buoyancy Barrier: A multi-scale Study
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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.
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How Microplastics Cross the Buoyancy Barrier
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How microplastics crosses the buoyancy barrier
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Researchers used Colloidal Probe atomic force microscopy (AFM) to study how the natural organic matter eco-corona on microplastic surfaces affects particle aggregation and buoyancy-relevant surface interactions, investigating the mechanisms by which microplastics cross the buoyancy barrier between water column and air.
How microplastics crosses the buoyancy barrier
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Researchers used Colloidal Probe-AFM to study nanoscale interactions between eco-corona-coated microplastic particles and surfaces under varying ionic conditions, finding that natural organic matter coatings substantially alter surface properties and aggregation behavior in ways that can allow buoyant plastics to sink.
Influence of typical clay minerals on aggregation and settling of pristine and aged polyethylene microplastics
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Researchers investigated how common clay minerals affect the aggregation and settling behavior of pristine and aged polyethylene microplastics in water. They found that high salt concentrations promoted the settling of microplastics when clay minerals were present, and that electrostatic repulsion was the dominant force governing interactions between plastics and clay particles. The findings provide new insights into how microplastics are transported and deposited in natural water systems.
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Laboratory experiments investigated how aggregation of microplastics with sediments and organic matter affects their sinking rates in water, finding that aggregate composition strongly influences settling velocity. These findings improve models predicting whether microplastics sink to the seafloor or remain suspended in the water column.
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