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Non-buoyant microplastic settling velocity varies with biofilm growth and ambient water salinity
Summary
Researchers investigated how biofilms (thin layers of bacteria that grow on plastic surfaces), water salinity, and suspended clay affect how fast microplastics sink in water, finding that biofilm growth alone increased sinking speed by up to 130% within just hours. These findings show that current models predicting where microplastics end up in rivers and oceans are too simplistic, and that biological and chemical conditions must be factored in for accurate predictions.
Abstract Rivers are the major conveyor of plastics to the marine environment, but the mechanisms that impact microplastic (<5 mm) aquatic transport, and thus govern fate are largely unknown. This prevents progress in understanding microplastic dynamics and identifying zones of high accumulation, along with taking representative environmental samples and developing effective mitigation measures. Using a suite of settling experiments we show that non-buoyant microplastic settling is influenced by a combination of biofilm growth, water salinity and suspended clay concentrations typically seen across fluvial to marine environments. Results indicate that biofilms significantly increased settling velocity of three different polymer types of non-buoyant microplastics (fragments and fibres, size range 0.02–4.94 mm) by up to 130% and significant increases in settling velocity were observable within hours. Impacts were both polymer and shape specific and settling regimes differed according to both salinity and sediment concentrations. Our results further validate previous statements that existing transport formula are inadequate to capture microplastic settling and highlight the importance of considering the combination of these processes within the next generation of predictive frameworks. This will allow more robust predictions of transport, fate and impact of microplastic pollution within aquatic environments.