Ordinary compostable plastic does not decompose after a year in the sea

A common plant-based plastic marketed as compostable has been found to not degrade when it ends up in the ocean and remains untouched for more than a year.

Compostable “bioplastics” have been touted as a solution to plastic waste, which enters the ocean at a rate of 12 million tons per year. A leading alternative to traditional oil-based plastics is polylactic acid (PLA), a plant-derived material used in clothing, cups, and single-use containers. PLA can be composted in industrial facilities, but the researchers weren’t sure if the material would break down naturally once in the ocean.

To find out, Sarah-Jeanne Royer of the University of California, San Diego and her colleagues compared how various materials aged both on the ocean’s surface and suspended 10 meters below a fine-mesh cage. They used palm-sized samples of textiles made from oil-based plastics, bioplastics like PLA, and natural materials like cotton. Each week, they went through and photographed the samples, which were sitting next to a pier in La Jolla, California, taking a small portion of each sample for visual and chemical evaluation.

After 14 months at sea, their PLA sample remained as intact as oil-based plastics like polypropylene and polyethylene terephthalate (PET). By contrast, natural materials like cotton-based fibers completely disintegrated and decomposed in about a month.

The team also mirrored the experiment in the lab with seawater and bacteria to mimic the natural environment. Neither PLA nor oil-based fabrics expelled carbon dioxide gas, confirming that plant-based plastics didn’t break down chemically either. In the sea and in the lab, “they didn’t break down at all,” says Royer.

The bottom line, says Frederik R. Wurm of the University of Twente in the Netherlands, is that “biodegradation should always consider the end-of-life scenario.”

So when it comes to compostable plastics, descriptions like “biodegradable” can be misleading. Just because a bioplastic can be composted in a high-temperature, high-pressure facility doesn’t mean it will break down in a cold, wet environment.

“Consumers in general are not aware of what they are buying,” says Royer. He recommends avoiding single-use plastics, opting for reusable packaging, and buying less clothing.