Plant-derived plastics do not deteriorate easily even at sea
Shutterstock/Mike Dotta
Common plant-based plastics marketed as compostable have been found to remain intact for a year or more without breaking down in the ocean.
Compostable “bioplastics” are touted as a solution to the 12 million tonnes of plastic waste that enter our oceans annually. A leading alternative to traditional petroleum-based plastics is polylactic acid (PLA), a plant-based material used in clothing, disposable cups, containers, and more. PLA can be composted in industrial facilities, but researchers were unsure if the material would naturally degrade once it entered the ocean.
To investigate this, Sarah-Jeanne Royer of the University of California, San Diego and her colleagues studied how some materials age at sea level and are suspended 10 meters below in fine-mesh cages. A comparison was made to see how they aged under normal conditions. They used palm-sized textile swatches made from petroleum-based plastics, bioplastics such as PLA, and natural materials such as cotton. Each week, they checked and photographed samples next to a pier in La Jolla, California, and took a small portion of each sample for visual and chemical evaluation.
After 14 months at sea, the PLA samples remained intact, as did petroleum-based plastics such as polypropylene and polyethylene terephthalate (PET). In contrast, natural materials like cotton-based fibers completely decompose and break down in about a month.
The research team also mirrored laboratory experiments with seawater and bacteria to mimic the natural environment. It was confirmed that neither PLA nor petroleum-based fibers gave off carbon dioxide gas, nor did plant-based plastics chemically decompose. “They didn’t degrade at all,” says Royer, either in the ocean or in the lab.
The lesson, says Frederik R. Wurm of the University of Twente in the Netherlands, is that biodegradation always needs to consider end-of-life scenarios.
So statements like “biodegradable” can be misleading when it comes to compostable plastics. Just because bioplastics can be composted in a high-temperature, high-pressure facility doesn’t mean they will degrade in a cold, wet environment.
“Consumers are generally unaware of what they are buying,” says Royer. She recommends avoiding single-use plastics, choosing reusable containers, and buying less clothing.
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