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Role of biophysical flocculation on microplastics and sediment interactions in a microtidal estuary
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Field surveys and lab experiments in a microtidal estuary found that microplastics flocculate with negatively charged clastic sediment through biophysical processes, influencing how microplastics are transported, aggregated, and deposited in estuarine environments.
Microplastics pose environmental risks in estuarine ecosystems due to their persistence and widespread distribution in water and sediment. This study investigates the role of biophysical flocculation in microplastics transport and accumulation in a microtidal estuary. Both field surveys and laboratory experiments were conducted to analyze microplastics aggregation with clastic sediment and their settling behaviors. The results shows that microplastics pervasively flocculate with negatively charged sediment particles, with fibrous flocs exhibiting higher aggregation tendencies than fragmented ones. Extracellular polymeric substance secreted by subaqueous microorganisms increased floc size and density, enhancing microplastics incorporation. Sediment compositions significantly influenced settling dynamics; sand-dominated sediments facilitated faster settling, while cohesive fine sediments retained microplastics longer. Controlled turbulence experiments showed moderate turbulence promoted flocculation, while higher turbulence disrupted large aggregates. Estuarine sediments served as temporary microplastics sink, but resuspension events driven by hydrodynamics prolonged microplastics persistence. These findings highlight the interplay of sediment composition, hydrodynamics, and biophysical processes in microplastics transport and deposition, providing insights for predictive models and management strategies to mitigate microplastic pollution in coastal and estuarine systems.
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Researchers investigated how biological flocculation — the aggregation of microplastics with organic matter driven by biological processes — affects the fate of microplastic pollution in river-estuary transition zones. The study found that bio-modulated flocculation significantly increases the sinking of otherwise buoyant microplastics, representing an important removal mechanism from surface waters in estuarine environments.
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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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