"Biodegradable" plastic label may be misleading

A widely used compostable plastic has been found to persist unchanged in marine environments for at least 14 months, according to a study by Sarah-Jeanne Royer and colleagues from Scripps Institution of Oceanography at the University of California, San Diego.

The study highlights the distinction between textile materials that can be composted in a controlled setting (PLA) and the ones that can undergo biodegradation in natural environments (cellulose-based textiles).

One of the major ecological problems facing marine life is the accumulation and persistence of oil-based plastic waste in the ocean. Items such as discarded water bottles that enter the ocean may persist for decades in their original form; even when they break up into microplastics they are not biodegraded.

In recent years, substitutes have developed to replace oil-based plastics with the intention of both reducing fossil fuel use in creating plastic goods and providing a more environmentally benign waste product when the item is discarded through composting.

Polylactic acid (PLA) is one of the most popular substitutes. It is a polymer of lactic acid derived from fermentation of sugars and starches which will break back down into lactic acid at the high temperatures found in very large compost piles; however, it does not do so reliably or quickly in colder conditions.

The researchers submerged samples of PLA, along with samples of oil-based materials, cellulose-based materials and a blend of cellulose-based and oil-based materials, in cages in the coastal waters off La Jolla, California. Samples were examined weekly for evidence of disintegration and returned to the ocean after a few hours.

The cellulose-based material degraded quickly, in less than one month. Laboratory chemical analysis confirmed that the cellulose had been largely broken down by biological processes through CO₂ production, not simple mechanical wear. Neither the oil-based plastic, the blend nor the PLA showed signs of degradation throughout the 14 months of the experiment.

“Our results indicate that composability does not imply environmental degradation,” Royer said. “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.”

According to the authors, the research shows the need for standardising tests to see if materials promoted as compostable or biodegradable, such as PLA, actually do biodegrade in a natural environment.

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