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Sedimentation behavior of aggregated microplastics: Influence of particle size and water constituents in environmental waters
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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.
Understanding the aggregation and settling behaviors of micro/nanoplastics (M/NPs) is essential for predicting their transport and fate in aquatic environments. However, gaps remain in comprehending how particle size and common environmental water constituents jointly influence M/NPs aggregation and sedimentation. This study investigated the effects of metal cations (Na+ and Ca2+) and humic acid (HA) on polystyrene particles of 100 nm (PS-100) and 500 nm (PS-500). The resulted indicated that larger PS-500 particles exhibited lower critical coagulation concentration (CCC) and a stronger tendency to aggregate, yet remained suspended even after aggregation ("aggregation without settling") due to their larger secondary energy minima, lower ion adsorption, and a lower density relative to water. In contrast, smaller PS-100 particles displayed a two-stage behavior of "settling following by floating", driven by stronger ion adsorption that increased their relative density and induced sedimentation ("settling without aggregation"). HA modified these processes differently in Na+ and Ca2+ systems: it increased CCC in Na+ solutions by enhancing electrostatic repulsion, but CCC in Ca2+ solutions through Ca2+-HA complexation and bridging effects. DLVO analysis confirmed that particle size and ion valence jointly controlled energy barriers and secondary minima, determining aggregate stability. Environmental water experiments further revealed that dissolved organic matter (DOM) composition significantly affected aggregation-settling patterns, with protein-like DOM enhancing particle aggregation and floating by competing with ions for adsorption sites. The findings indicate that as microplastics degrade into nanoplastics, their diffusion and settling behaviors change, thereby influencing their distribution in aquatic environments.
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This laboratory study investigated how microplastics behave when mixed with natural sediment particles and allowed to settle in water, finding that the combined aggregates settle at different rates than either material alone. Smaller microplastics were particularly prone to forming flocs with sediment, which can accelerate their sinking and burial in riverbeds and lake bottoms. Understanding these settling dynamics is important for predicting where microplastics accumulate in aquatic environments and how they interact with the food chain.
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Researchers conducted sinking experiments with diverse microplastic particles and found that sinking velocity depends not only on density and size but also on particle shape, and that biofouling and weathering can substantially alter sinking rates with implications for how microplastics distribute through the water column.
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.
Experimental study on parameterizing microplastic-sediment aggregation
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Researchers conducted laboratory flocculation experiments to parameterize microplastic-sediment aggregation, testing fibers, fragments, and spheres of varying sizes and densities to characterize how microplastics and sediment form flocs with enhanced settling velocity, with the goal of improving numerical transport models of microplastic fate in rivers and estuaries.
Modelling the sedimentation of macro-, micro- and nanoplastics in the ocean from surface to sediment
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