When biodegradable plastic isn’t

The ubiquitous plastic bag is handy for transporting groceries and other items home from the store. However, this convenience takes a toll on the environment, with plastic debris littering land and waterways. Manufacturers offer biodegradable or compostable plastic bags, but in many cases, these claims have not been tested in natural environments. Now, researchers report in ACS' Environmental Science & Technology that the bags do not degrade in some environments any faster than regular polyethylene. 

According to the European Commission, about 100 billion plastic bags enter the European Union market each year. Many of these are used only once and then thrown away, tossed onto the ground or blown by the wind into lakes or oceans. Aside from being ugly, plastic debris can harm animals and ecosystems. Some bag manufacturers offer plastics that deteriorate through the actions of microorganisms, while others use oxo-biodegradable plastics containing pro-oxidant additives to hasten the process.

In addition, compostable plastics can be degraded by microorganisms under managed conditions. However, these potential solutions to the plastics problem haven't been well studied in different environments over long periods of time. Therefore, Imogen Napper and Richard Thompson decided to compare the degradation of different plastic bags in three natural environments over a 3-year period.

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The researchers placed plastic bags labeled as biodegradable, oxo-biodegradable or compostable, as well as conventional polyethylene bags, in the open air, buried them in soil or submerged them in seawater. Aside from the compostable bag in the marine environment, which degraded within 3 months, fragments or whole samples of each bag type remained after 27 months. After 3 years in the sea or soil, the conventional, biodegradable and oxo-biodegradable bags could still carry about 5 pounds of groceries without tearing.

The researchers concluded that none of the bags reliably deteriorated in all environments within 3 years, and the biodegradable bags did not consistently deteriorate faster than conventional polyethylene. Because biodegradable bags could have the unintended consequence of increased littering, and they are challenging to recycle in conventional waste streams, the researchers suggest that a better solution might be to encourage consumers to reuse shopping bags multiple times.