Sunlight can break down polystyrene in just decades, instead of thousands of years as expected earlier, finds study
It was earlier thought that polystyrene took thousands of years to degrade, however, now the study has found that sunlight can do the job in just decades or centuries.
Microorganisms wear many hats. Some of them help us make bread, cheese, yogurt and wine while others play a vital role in our guts, metabolizing complex carbohydrates and defending us against pathogens.
Some microorganisms, meanwhile, address the pollution crisis by breaking down pollutants such as oil and plastics. However, they struggle to break down one particular kind of plastic that, until recently, was assumed to last forever.
Called polystyrene, the polymer is quite commonly found in food packaging and takeaway boxes. It was recently found that what microorganisms cannot pull off, sunlight can!
Building on this, scientists at the Woods Hole Oceanographic Institution (WHOI) have devised a method to degrade polystyrene. It was earlier thought that polystyrene took thousands of years to degrade, however, now the study has found that sunlight can do the job in just decades or centuries.
They arrived at this conclusion after they replicated sunlight's effect on the plastic in a controlled environment.
The idea that sunlight degrades plastics is nothing new, says Collin Ward, a marine chemist at WHOI and lead author of the study. He explains that plastics around us such as playground toys, park benches, or lawn chairs get rapidly sun-bleached.
"We understand a lot about how plastic behaves in landfills and compost piles, but we don't fully understand how it behaves in the natural environment."
Previous estimates on polystyrene degradation in landfills and compost piles, according to Ward, have been centered on microbial action. "Microbes don't readily degrade polystyrene because the polymer is too complex and too large for them."
"However, in the natural environment, there are other ways to degrade polystyrene, like sunlight. It turns out, that when you consider degradation by sunlight, the lifetime is much much shorter than when only considering microbes," he adds.
Ward explains that certain frequencies of sunlight act on the plastic and breaks it apart.
To investigate how sunlight breaks down polystyrene, the scientists used an instrument to create conditions in the lab that emulate sunlight with a solar simulator, a lamp that replicates the frequencies of sunlight.
Next, they selected five commercially available samples of polystyrene and dipped them in sealed glass containers of water. They then exposed these samples to the light emitted from the solar simulator.
They found that in addition to breaking down polystyrene, sunlight was performing another task: it was converting the plastic into dissolved organic carbon and carbon dioxide.
But carbon dioxide contributes to global warming. Ward assures that this is not a concern from a global climate change perspective because only traces of carbon dioxide is released.
He explains, "The amount of carbon in plastic that enters the ocean each year is a million times less than the amount of carbon dioxide added to the atmosphere each year from combustion of fossil fuels and land-use change."
They also examined the role of additives, which determine polystyrene's color, flexibility, and other physical features, in its degradation.
"Different additives seem to absorb different frequencies of sunlight, which influences how fast the plastic breaks down," says Christopher Reddy, co-author of the study.
In the future, Ward and his colleagues aim to explore other forms of plastics. "Currently my lab is interested in studying different types of consumer goods and understanding how they behave in the environment. This study is only focused on polystyrene," he explains.
"Ultimately, we'd like to apply the framework to a broader class of plastic types and consumer goods to test how general our findings are. Think about the numbers on the bottom of your recyclable consumer goods. We studied number 6. Now we need to learn more about the breakdown of numbers 1-5."