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How Biodegradable Are Compostable Plastics in Marine Waters?

According to a new study published in the open-access journal PLOS ONE by Sarah-Jeanne Royer and co-workers from the Scripps Institution of Oceanography at the University of California, San Diego, a widely used compostable plastic remains unmodified in marine environments for at least 14 months.

How Biodegradable Are Compostable Plastics in Marine Waters?
Lead author, Dr. Sarah-Jeanne Royer, holding a sample of microplastics. Image Credit: Iyvonne Khoo, CC-BY 4.0.

The study distinguishes between textile materials that can be composted in a controlled industrial setting (PLA) and those that can biodegrade in natural environments (cellulose-based textiles).

One of the greatest ecological issues confronting marine life is the build-up and persistence of oil-based plastic waste in the ocean. Macroscopic plastic products, such as discarded water bottles, can persist in the ocean for decades in their original form; even when broken up into microscopic pieces, known as microplastics, they are not biodegraded and instead remain undigestible pollutants that permeate the oceans.

In recent years, alternatives for oil-based plastics have been created with the goal of both lowering the use of fossil fuels in the production of plastic goods and delivering a more environmentally benign waste product when the item is abandoned through composting.

Polylactic acid (PLA), a lactic acid polymer generated from sugar and starch fermentation, is one of the most preferred replacements. PLA will break down into lactic acid at high temperatures observed in big compost piles. However, this degradation is not reliable in colder environments.

To investigate the fate of PLA in a natural marine environment, the scientists submerged PLA samples in cages in the coastal waters of La Jolla, California, with samples of oil-based materials, cellulose-based materials, and a blend of cellulose-based and oil-based materials. Weekly, samples were checked for signs of disintegration and returned to the ocean after a few hours.

According to the authors, the cellulose-based substance disintegrated swiftly in less than a month. The cellulose had been largely broken down by biological processes, not only mechanical wear, according to laboratory chemical analysis. During the 14-month experiment, neither the oil-based plastic, the mix, nor the PLA exhibited signs of degradation.

Our results indicate that compostability does not imply environmental degradation. Referring to compostable plastics as biodegradable plastics is misleading as it may convey the perception of a material that degrades in the environment. PLA-based plastics must be composted in appropriately controlled facilities in order to achieve their potential as compostable substitutes for oil-based plastics.

Sarah-Jeanne Royer, Scripps Institution of Oceanography

The authors say, “This work represents one of the few pioneer studies addressing the comparability between the biodegradability of different material types (natural to fully synthetic and bio-based materials) in natural environmental conditions and controlled closed systems.

They added, “This study shows the need for standardizing tests to see if materials promoted as compostable or biodegradable such as PLA actually do biodegrade in a natural environment. In this case, consumers who are concerned about microfiber plastic pollution should be informed, knowledgeable, and mindful of the materials they are buying.”

Journal Reference:

Royer, S.-J., et al. (2023) Not so biodegradable: Polylactic acid and cellulose/plastic blend textiles lack fast biodegradation in marine waters.


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