Leave a cheap plastic bag in the sun long enough and it will eventually crumble into a powdery mess, its petrochemical fragments meant to be washed away by the elements.
Fragments of microplastic – seen on their own as a major ecological hazard – might not even be the worst thing to come out of this decay.
A study by researchers at the Woods Hole Oceanographic Institution reveals that sunlight is not only capable of breaking down plastics; it can convert their base polymers and additives into a soup of new chemicals.
The process isn’t exactly slow either, with the variety of plastic bags experienced when leaching a large mixture of soluble organic carbon compounds after being exposed to the sun for less than 100 hours.
While many communities around the world take the environmental cost of the convenience of a shopping bag seriously, it’s still all too easy to find a retailer willing to hand over a single-use polythene bag with your purchase.
Once used, there’s a good chance the bag will end up in a landfill rather than a recycling plant.
Or, worse, on display somewhere in the landscape, where it ends up making its way through streams or the ocean to join the astonishing 640,000 tonnes of plastic dumped each year by commercial fishing.
The ultimate fate of this debris is to be guessed. One group ends up in the bowels of wildlife, such as birds and whales. Eventually, it breaks down into smaller and smaller microscopic pieces.
But many details about its distribution and fragmentation have remained unclear.
Studies over the years have hinted at the possibility of more sinister transformations, with research finding that sunlight can chemically transform plastics and their additives into new polymers, as well as smaller chemical units that dissolve more easily. and are made airborne.
Individual discoveries are one thing. What was not known until now is the wide variety of chemicals that a plastic item could produce when baked in the sun.
Researchers gathered a sample of polythene bags intended for consumers from commercial companies such as Target and Walmart. The researchers also included a used bag from a CVS in a municipality with a plastic bag ban. A low density, additive-free film bag manufactured by Goodfellow served as a control.
The bags were characterized in terms of organic and metallic content and spectral qualities. The researchers placed samples from the bags into sterilized beakers filled with an ionized solution to simulate submersion in seawater.
Half of the beakers went into a dark draw for six days. The others were left in a temperature-controlled room for five days, bathed in a constant stream of radiation that mimicked the effects of sunlight.
Samples left in the dark were found to have released a tiny amount of organic compounds dissolved in the salt solution. Those left in the light, however, were swimming in new chemicals.
The used CVS bag had the greatest difference in concentrations between the darkened container and the one exposed to sunlight, a measurement that only increased as it remained in the light.
Separating this plastic soup into a list of its constituent molecules revealed tens of thousands of dissolved organic compounds, all produced on a time scale equivalent to a few weeks floating in the ocean in sunlight.
The whole process is at least ten times more complex than chemists previously understood, leaving plenty of room for toxic materials that we never even considered a problem.
“It’s amazing to think that sunlight can break down plastic, which is basically a compound that usually contains additives mixed together, into tens of thousands of compounds that dissolve in water,” says chemist Collin Ward.
“We have to think not only about the fate and impacts of the initial plastics that spread in the environment, but also about the transformation of these materials.”
What precisely these compounds do in the environment, or in the tissues of the organisms that live there, is now the big question. At low concentrations, there may be relatively little cause for concern.
But as plastic waste accumulates in a growing environmental disaster, these concentrations could rise to levels we wish we had paid attention to earlier.
“If the goal is to understand the fate and impacts of these materials, we need to study plastics representative of those that are actually released into the environment, as well as the weathering processes acting on them,” says Ward.
This research was published in Environmental sciences and technologies.