Sunlight can transform some plastics into water-soluble chemicals, new research has shown.
Researchers previously thought that sunlight would only ever break down plastics in the sea into microplastics – which then persist in the environment forever.
But analysis of real-world bags produced by retailers showed that the effect of sunlight is often very different.
Researchers found that the chemical reaction can produce tens of thousands of water-soluble compounds, or formulas, in just weeks.
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The researchers say this could mean manufacturers can produce plastics that are easier to break down in sunlight.
Co-author Collin Ward, assistant scientist in Woods Hole Oceanographic Institution’s Marine Chemistry and Geochemistry Department, said: “It’s astonishing to think that sunlight can break down plastic, which is essentially one compound that typically has some additives mixed in, into tens of thousands of compounds that dissolve in water.
“We need to be thinking not only about the fate and impacts of the initial plastics that get leaked into the environment, but also about the transformation of those materials.”
He added: “We don’t really know yet what impacts these products might pose to aquatic ecosystems or to biogeochemical processes such as carbon cycling.
"While plastics breaking down more quickly than expected may seem like a good thing, it’s unclear how these chemicals may affect the environment.”
The researchers write: “The growing evidence that photochemical transformation of plastics is an important transformation process in surface waters challenges a widely held assumption about the persistence of plastic in the environment.”
They say scientists and policymakers “tend to assume that sunlight exposure merely physically fragments macroplastics to microplastics, which subsequently persist forever in the environment.”
The new findings “fundamentally challenge this guideline and indicate that sunlight not only aids physical fragmentation of plastic, it chemically alters it, producing a suite of transformation products that no longer resemble the parent material," the researchers write.
The study examined the breakdown under sunlight of four different single-use consumer polyethylene plastic bags from three major American retailers that make a lot of plastic bags (Target, CVS and Walmart) and compared them to pure polyethylene film.
Most plastic bags are not just a pure base resin, but include a complex formulation of chemical additives to make the plastic behave or look a certain way.
Up to about one-third of the mass of each of the retailers’ plastic bags was inorganic additives.
The organic compounds produced by sunlight were analysed at the National High Magnetic Field Laboratory, which designed and developed a mass spectrometer equipped with a 21 tesla magnet that achieves the highest mass resolution and accuracy in the world.
Researchers found that under sunlight exposure, the four retailer bags from about 5,000 formulas (for the Target bag) to 15,000 formulas (for the Walmart bag), while the pure polyethylene film produced about 9,000 formulas.
Many previous studies of marine plastics have generally used pure polymers, which don’t reflect the reality of plastics released into the marine environment.
Ward said: “If the goal is to understand the fate and impacts of these materials, we need to study plastics that are representative of those that are actually leaked into the environment, as well as study the weathering processes acting on them,”
Co-author Christopher Reddy, senior scientist in WHOI’s Marine Chemistry and Geochemistry Department, said: “I am excited about this work because it provides actionable and attainable approaches for making less persistent plastics in the future.
“By simply modifying the ingredients in their recipes, the plastic industry can make their products more susceptible to breakdown once the product reaches its useful lifespan."
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