Common compostable plastic won’t break down after a year at sea

A common plant-based plastic marketed as compostable has been found to not degrade when it ends up in the ocean, remaining intact for over a year.

Compostables”bioplasticswere presented as a solution to plastic wastewhich enters the ocean at the rate of 12 million tons per year. Polylactic acid (PLA), a plant-derived material used in clothing, cups and single-use containers, is a leading alternative to traditional petroleum-based plastics. PLA can be composted in industrial facilities, but researchers didn’t know if the material would break down naturally once in the ocean.

Discover, Sarah Jeanne Royer at the University of California, San Diego, and his colleagues compared how several materials aged on the surface of the ocean and suspended 10 meters below in a fine-mesh cage. They used palm-sized textile samples made from petroleum-based plastics, bioplastics like PLA, and natural materials like cotton. Each week, they checked and photographed the samples, which were 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, their PLA sample remained as intact as petroleum-based plastics like polypropylene and polyethylene terephthalate (PET). In contrast, natural materials like cotton-based fibers completely disintegrate and decompose in about a month.

The team also replicated the experiment in the lab with seawater and bacteria to mimic the natural environment. Neither PLA nor petroleum-based fabrics repel carbon dioxide, confirming that plant-based plastics do not chemically degrade either. At sea and in the lab, “they don’t degrade at all,” says Royer.

The take home message says Frederik R. Wurm at the University of Twente in the Netherlands, is that “biodegradation must always take into account the end-of-life scenario”.

That’s why, when it comes to compostable plastics, descriptors 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, damp environment.

“Consumers in general are unaware of what they are buying,” says Royer. She recommends avoiding single-use plastics, opting for reusable containers and buying fewer clothes.