Popular Compostable Plastic Not So Biodegradable

A new study published in the open-access journal PLOS ONE by Sarah-Jeanne Royer and her team from Scripps Institution of Oceanography at the University of California, San Diego, reveals that a commonly used compostable plastic remains unchanged in marine environments for at least 14 months.

This study sheds light on the distinction between textile materials that can be composted in controlled industrial settings (such as polylactic acid or PLA) and those that can biodegrade naturally in the environment (like cellulose-based textiles).

The accumulation and persistence of petroleum-based plastic waste in the ocean pose significant ecological challenges for marine life. Large plastic items, such as discarded water bottles, can endure for decades in their original form, and even when they break down into microscopic fragments called microplastics, they do not undergo biodegradation. Instead, they persist as indigestible pollutants that pervade our oceans.

In recent years, alternative materials have been developed as substitutes for petroleum-based plastics. These alternatives aim to reduce the consumption of fossil fuels in plastic production and provide more environmentally friendly waste products when discarded, particularly through composting.

One of the most widely adopted substitutes is polylactic acid (PLA), a polymer made from lactic acid derived through the fermentation of sugars and starches. PLA is designed to break down into lactic acid when exposed to high temperatures typically found in large composting facilities. However, the study highlights that PLA does not reliably or rapidly degrade in colder conditions.

To investigate the degradation of polylactic acid (PLA) in a natural marine environment, a study was conducted by the researchers. In this experiment, various materials including PLA, oil-based materials, cellulose-based materials, and a blend of cellulose-based and oil-based materials were submerged in cages placed in the coastal waters off La Jolla, California. Weekly observations were made on the samples to assess their disintegration, and after a few hours, the samples were returned to the ocean.

The results of the study revealed that the cellulose-based material underwent rapid degradation, taking less than one month to break down. Chemical analysis conducted in the laboratory confirmed that the breakdown of cellulose was primarily facilitated by biological processes, resulting in the production of CO2, rather than simple mechanical wear and tear. In contrast, the oil-based plastic, the blend of cellulose-based and oil-based materials, and the PLA exhibited no signs of degradation throughout the entire 14-month duration of the experiment.

“Our results indicate that compostability does not imply environmental degradation,” Royer adds. “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.”

The authors further emphasize this research stands out as a pioneering investigation that examines the comparability of biodegradability across various material types, encompassing natural, fully synthetic, and bio-based materials, under both natural environmental conditions and controlled closed systems.

The findings from this study underscore the necessity of establishing standardized testing protocols to determine the true biodegradability of materials marketed as compostable or biodegradable, such as PLA, in real-world environmental settings.

Consequently, consumers who are genuinely concerned about the issue of microfiber plastic pollution should be well-informed, knowledgeable, and discerning when making purchasing decisions, taking into consideration the materials used in the products they choose.

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