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Rapid flocculation and settling of positively buoyant microplastic and fine-grained sediment in natural seawater
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Laboratory experiments showed that positively buoyant microplastics rapidly flocculated with fine-grained sediment in natural seawater, causing particles that would otherwise float to sink quickly. The finding has important implications for predicting microplastic fate in estuaries, where plastic-sediment aggregates may settle to the seafloor rather than dispersing.
Interactions between microplastic (MP) and fine-grained suspended sediment in natural waters are important for the environmental fate of plastic particles. Estuaries are transitional areas between freshwater and open marine systems and are recognized as important accumulation zones for MPs. However, there is a knowledge gap on the processes driving the sedimentation of MPs in estuaries, especially with regard to positively buoyant MPs. Here we show from settling tube experiments that positively buoyant and non-spherical MP HDPE particles in different size-fractions (63-500 μm) and concentrations (1 and 5 mg l) rapidly flocculate and settle with natural fine-grained sediment in natural seawater. Our results demonstrate that flocculation is a key process for the vertical transport of MP in estuaries. The implication is that land-based sources of positively buoyant HDPE MP transported by rivers will likely settle and accumulate in estuarine environments and thereby increase the concentration of MP in the benthic zone.
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Laboratory experiments investigated whether microplastic particles flocculate with natural cohesive sediment in unfiltered seawater, testing the hypothesis that flocculation could cause microplastics to sink and accumulate in estuarine and marine sediments. The results have implications for understanding the fate and transport of microplastics in coastal environments.
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Researchers demonstrated that flocculation causes buoyant microplastics to settle 5-21 times faster in estuarine waters than in freshwater, supporting the concept that estuaries act as traps that reduce the overall microplastic load reaching the open ocean.
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Laboratory flocculation experiments showed that PVC microplastics form aggregates with fine cohesive sediment at environmentally realistic concentrations, with aggregate size and settling velocity influenced by salinity and mixing energy, affecting microplastic transport in estuarine environments.
Trapping of floating MP in estuaries, the importance of flocculation
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Laboratory experiments showed that when microplastics enter estuaries and encounter salt water, they can become entangled in flocs — aggregates of organic particles and sediment — causing them to sink and be trapped in estuarine sediments rather than reaching the open ocean. This flocculation process in estuaries may represent a major, previously underestimated sink for river-borne microplastics. Understanding this trapping mechanism is important for accurately predicting how much plastic actually makes it from rivers to the sea.
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