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Plastic bag floating in the ocean with seaweed

Plastic particles may clump and stick to coasts, never entering ocean

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Researchers from the University of Rennes 1, France, have investigated nanoplastics in estuaries and found that they tend to clump and stick to solid objects, rather than floating towards the ocean as with other plastic waste.

As plastic debris – from complete plastic objects to microplastics – is weathered in aquatic environments, it eventually fragments into nanoscale particles, known as nanoplastics. While scientists have a good understanding of how these particles are formed, it is not yet well understood how these tiny particles move in aquatic environments.

The French researchers from Rennes have now published an Environmental Science & Technology paper which demonstrates that most nanoplastics in estuarine waters can clump, gradually forming large clusters which either settle in estuaries or stick to solid objects. If this is the case, nanoplastics behave in notably different ways to larger items of plastic waste, which tend to be carried towards oceans.

This could help explain the enormous discrepancy between the millions of tonnes of plastic waste that enters rivers and streams and the amount which researchers estimate is actually in the oceans.

Previous research has demonstrated that nanoplastics congregate in well-mixed, stagnant saltwater. However, this does not apply when the particles are subjected to dynamic changes in salt content, such as in estuaries where freshwater from rivers meet tidal saltwater. The researchers mimicked this environment in the laboratory, using micro-sized chambers produced in order to study how nano-plastics interact and aggregate under these conditions.

They developed a lab-on-a-chip device just 1.7cm in length, in which they introduced 400nm crushed polystyrene beads along with freshwater on one side and saltwater on the other. At the opposite end, they collected the output. They used this tiny environment to test different flow rates replicating the salt gradient and water movement of an estuary on the island of Guadeloupe, in the French Caribbean.

Nanoplastic aggregates up to 10-μm wide were detected within the zone of highest salt concentration in the flow chamber, regardless of how fast the water was moving. At the highest flow rate, just 12 per cent of the nanoplastics were collected in the outlets; the remaining particles either clumped together and sank in the flow chamber or formed floating aggregates that stuck to the chamber's sides.

According to the researchers, their findings suggest that coastal environments like estuaries may filter out nanoplastics before they can enter the ocean, accounting for the discrepancy between the amount of plastic that enters aquatic environment and the amount found in oceans.

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