Breakdown of plastic waste in seawater exposed to sunlight.
Breakdown of plastic waste in seawater exposed to sunlight.

Trillions of tiny plastic fragments are afloat in our oceans but they represent a mere 1% of the total plastic waste dumped at sea. What happens to the rest? Scientists have suggested that some of the missing waste could be eaten by marine life or get mixed up with organic matter and sink to the ocean floor.

Now researchers from the US and China have found that plastic waste in seawater breaks down and dissolves into organic molecules much more quickly than expected when exposed to sunlight [Zhu et al., Journal of Hazardous Materials 383 (2020) 121065].

“We wanted to know what controls the fate of microplastics in the ocean,” says Aron Stubbins of Northeastern University, who led the work with Daoji Li of East China Normal University and colleagues at Florida Atlantic University. “Why do 98% of all floating plastics released into the ocean go missing each year?”

The researchers took samples from the North Pacific and created their own microplastic waste from common consumer product containers. Both types of microplastic were floated on seawater in a flask and exposed to simulated sunlight in the lab. Sunlight oxidizes the plastic, breaking down the polymers into highly soluble organic molecules called oligomers.

“Our results are the first report of the photo-solubility of a diverse range of plastics,” points out Stubbins. “Sunlight removes plastics more rapidly than expected. In fact, we show that sunlight may remove expanded polystyrene microplastics from the ocean in months to years, which may explain why we do not find polystyrene in open ocean surface waters.”

How quickly the plastics break down depends on their photoreactivity. Polystyrene, which dissolves quickly, possesses chemical groups called aromatics that absorb sunlight. Other types of non-aromatic plastic, like polypropylene and polyethylene, take much longer – from years to decades – to dissolve. Local bacteria can also ingest the soup of organic molecules produced by the dissolving plastics.

“Microbes were inhibited by organics leaching from polyethylene, suggesting this type of plastic may have an adverse effect on microbes,” explains Stubbins. “Why this happened is unclear and further experiments are required and to see if inhibition occurs when other plastic types dissolve.”

With funding from the National Science Foundation, Stubbins and Kara Lavender of the Sea Education Association are now working on this problem and improving their estimates of how quickly different sizes and chemistries of plastics photodegrade at sea.

“If we can stop dumping plastics in the ocean, the polymers that float will disappear in decades/centuries,” says Stubbins. “Ideally, we should stop using non-essential disposable plastic items and make sure waste is handled properly when it needs to be produced.”